Session C1: Poster Session I (2:00-5:00PM)

Partition function zeros and phase transitions of a square-well polymer

The zeros of the canonical partition functions for flexible square-well polymer chains have been computed for chains up to length 256 for a range of square-well diameters. We have previously shown that such chain molecules can undergo a coil-globule and globule-crystal transition as well as a direct coil-crystal transition [1]. Here we show that each of these transitions has a well-defined signature in the complex-plane map of the partition function zeros. The freezing transitions are characterized by nearly circular rings of uniformly spaced roots, indicative of a discontinuous transition. The collapse transition is signaled by the coalescence of roots onto an elliptical horse-shoe segment pinching down towards the positive real axis. For sufficiently small square-well diameter the elliptical collapse ring merges with the circular freezing ring yielding the direct coil-crystal transition. The root density of all rings increases with increasing chain length and the leading roots move towards the positive real axis, implying a divergence of the specific heat in the thermodynamic limit (as originally proposed by Yang and Lee). \\[4pt] [1] M.P. Taylor, W. Paul, and K. Binder, J. Chem. Phys. \textbf{131}, 114907 (2009).   

Modeling Amphiphilic Solutes in a Jagla Solvent

Methanol is an amphiphilic solute whose aqueous solutions exhibit distinctive physical properties. The volume change upon mixing, for example, is negative across the entire composition range, indicating strong association. We explore the corresponding behavior of a Jagla solvent, which has been previously shown to exhibit many of the anomalous properties of water. We consider two models of an amphiphilic solute: (i) a ``dimer'' model, which consists of one hydrophobic hard sphere linked to a Jagla particle with a permanent bond, and (ii) a ``monomer'' model, which is a limiting case of the dimer, formed by concentrically overlapping a hard sphere and a Jagla particle. Using discrete molecular dynamics, we calculate the thermodynamic properties of the resulting solutions. We systematically vary the set of parameters of the dimer and monomer models and find that one can readily reproduce the experimental behavior of the excess volume of the methanol-water system as a function of methanol volume fraction. We compare the pressure and temperature dependence of the excess volume and the excess enthalpy of both models with experimental data on methanol-water solutions and find qualitative agreement in most cases. We also investigate the solute effect on the temperature of maximum density and find that the effect of concentration is orders of magnitude stronger than measured experimentally.   

Localized entrapment of green fluorescent protein within nanostructured polymer films

Protein entrapment within ultrathin polymer films is of interest for applications in biosensing, drug delivery, and bioconversion, but controlling protein distribution within the films is difficult. We report on nanostructured protein/polyelectrolyte (PE) materials obtained through incorporation of green fluorescent protein (GFP) within poly(styrene sulfonate)/poly(allylamine hydrochloride) multilayer films assembled via the spin-assisted layer-by-layer method. By using deuterated GFP as a marker for neutron scattering contrast we have inferred the architecture of the films in both normal and lateral directions. We find that films assembled with a single GFP layer exhibit a strong localization of the GFP without intermixing into the PE matrix. The GFP volume fraction approaches the monolayer density of close-packed randomly oriented GFP molecules. However, intermixing of the GFP with the PE matrix occurs in multiple-GFP layer films. Our results yield new insight into the organization of immobilized proteins within polyelectrolyte matrices and open opportunities for fabrication of protein-containing films with well-organized structure and controllable function, a crucial requirement for advanced sensing applications.   

Time-resolved WAXD/SAXS Characterization on the Crystallization of Silica Filled HDPE Nanocomposite

The isothermal crystallization behavior of high density polyethylene/silica (HDPE-SiO2) with different SiO2 loading of 2{\%} and 5{\%}, along with the pure HDPE sample was studied by using the time-resolved wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) techniques. For isothermal crystallization at 120 $^{\circ}$C, WAXD profiles show HDPE-Si2{\%} has the highest ending crystallinity index, while HDPE-Si5{\%} has the lowest value. Avrami exponent of pure HDPE is about 3.9, while HDPE-Si2{\%} has a value of 3.2 which is typically heterogeneous nucleation behavior due to the addition of silica in HDPE. SAXS patterns show that the silica inside HDPE has mass fractal structure. The mass fractal dimension is determined by using the fitting method and the value is less than 3 for all HDPE-SiO2 samples. The structure of HDPE-SiO2 is sketched based on the obtained results.   

The CHX Beamline at NSLS-II: a Tool to probe Structure and Dynamics in Soft-Condensed Matter

The Coherent Hard X-ray (CHX) beamline currently under construction at NSLS-II (Brookhaven National Laboratory) will serve as an optimized tool for the study of structure and dynamics in soft condensed matter. The unprecedented coherent flux will enable the study of dynamics in soft matter systems down to microsecond time scales via X-ray Photon Correlation Spectroscopy (XPCS). The available scattering geometries such as (GI)SAXS and (GI)WAXS can be used in a simultaneous fashion to collect static and dynamic scattering information on length scales ranging from supramolecular assemblies to atomic distances.   

Ionic conductivity of imidazole-functionalized liquid crystal mesogens

Imidazole has been investigated as a novel anhydrous proton conducting functional group that could enable higher temperature operation ($>$ 120 $^{\circ}$C) of polymer electrolyte fuel cells. Its amphoteric behavior can support Grotthuss-like proton transport; however molecular mobility and a high concentration of imidazole groups are needed to achieve high ionic conductivity. Our hypothesis is that liquid crystal ordering, particularly in layered smectic phase, can facilitate formation of 2D proton transport and promote proton conductivity. We have designed and synthesized two imidazole-terminated liquid crystal mesogens, and the ionic conductivities in the liquid crystalline and isotropic states have been measured. Here we report on synthesis and characterization of diacylhydrazine liquid crystals bearing imidazole terminal groups. The proton conductivity of products is compared to pure liquid imidazole and to liquid crystal mesogens without imidazole groups.   

New Soft Matter Soft X-ray Scattering Facility and the Advanced Light Source with Real Time Analysis

We present the development and first experiments at a new user scattering facility at the Advanced Light Source at Lawrence Berkeley National Laboratory designed to elucidate the structure of polymer thin films, including a number of systems important for understanding organic devices. These experiments utilize enhanced contrast mechanisms available due to resonance effects to gain sensitivity to morphology which other probes and harder X-ray beams cannot probe. We present the development of new characterization and calibration techniques as well as procedures to mitigate the unique problems that come with scattering in this energy range. These allow us to obtain quantitative scattering profiles and thus ensemble morphological information of these devices. In addition, we present a live analysis package which does initial data reduction in real time while scattering data is being collected.   

Phase behavior of silica-polypeptide colloidal particles immersed in liquid crystal forming mesogens

Silica-polypeptide composite particles (Silica-PCPs) are core-shell colloids. The size and function of the silica core are easily controlled. Established synthesis methods ensure uniformly sized cores and provide the ability to have magnetic or fluorescent inclusions within them. The interactions of the polypeptides in the shell with those unbound in solution can be explored by small/wide angle x-ray scattering (SAXS/WAXS) and polarized optical microscopy (POM) to identify the concentration at which the solution changes from isotropic to ordered. Silica-PCPs/untethered polymer/solvent systems are studied to determine the phase behavior in a non-aqueous environment, free from complications arising from electrostatic effects. The tendency of poly(gamma-benzyl-L-glutamate) to form liquid crystalline phases on its own is affected by the presence of Silica-PCPs.   

A Novel 3D-Lattice Model of Fibrillar Polymeric Material

To elucidate a possible mechanism for simple material properties of fibrillar polymeric bulk material containing cross-links between constituent components, we introduce a 3D-lattice model that depends on cross-link number density ($\rho$) and the ratio ($\chi$) of cross-link bond strength to thermal energy. The model predicts a phase transition in specific heat capacity occurring for $\chi$ between approximately 0.5 and 1.5, dependent on $\rho$. We present evidence that the properties of the represented phases are consistent with those of a solid phase and a liquid phase. These results indicate that variations in $\rho$ or $\chi$ alone may provide a convenient basis for Nature to provide a range of material properties with limited resources.   

PolyGraph: a Polymer Visualization system

Rapid advances in computational hardware and parallelization have made complex simulations of large polymers increasingly ubiquitous. However, visualizing such simulations remains a challenge. Here we present PolyGraph, a Blender-powered visualization system for complex polymer simulations. As a specific example, we study molecular dynamics simulations of condensing polymers. We illustrate our initial simulation results, suggesting that formation of local beads is an initial step in the condensation process. (This finding is consistent with earlier conjectures about polymer condensation.) PolyGraph makes it possible to create precise and visually appealing clips of polymer simulations. *contributed equally   

Confinement of Block Copolymer Nanocomposites within Nanoporous Templates

This research investigates the impact of surface-functionalized nanoparticles (NPs) on self-assembly of confined block copolymer (BCP) systems. Lamellar and cylindrical BCP morphologies were explored with variations in NP diameters and functional groups. The bulk and templated microphase-separated structures were characterized. The addition of addition of NPs resulted in a variety of morphological transitions due to a combination of selective localization of NPs and confinement.   

Surface Modification for Controlling the Orientation of Block Copolymers in thin film and in Cylindrical Nanopores

A series of benzocyclobutene-functionalized random copolymers of styrene and 4-vinylpyridine were synthesized by nitroxide-mediated controlled radical polymerization with BPO and TEMPO. Our research was to use these random copolymers of P(S-r-BCB-r-4VP) to control the orientation of microdomains in block copolymers(BCPs) of poly(styrene-$b$-4-vinylpyridine)(PS-$b$-P4VP) thin films and in cylindrical nanopores of anodized aluminum oxide (AAO) membranes. On P(S-r-BCB-r-4VP)-modified substrate,we found that in some particular compositions of random copolymer ,the parallel orientation of the microdomains is switched to be perpendicular in PS-$b$-P4VP thin film. We also introduced P(S-r-BCB-r-4VP) solution into the nanopores of the AAO and nanotubes formed after solvent evaporation and pyrolysis. And then BCPs of PS-$b$-P4VP were drawn into the P(S-r-BCB-r-4VP)-modified nanopores in the melt via capillary action to form P(S-r-BCB-r-4VP) coated nanorods of PS-$b$-P4VP.Similarly,in some particular compositions of random copolymer, we observed that the interactions of the blocks with the walls are not strong or if the interactions are balanced, then the orientation of the microdomains will change from being parallel to being perpendicular to the confining walls.   

Fast Fabrication of Hierarchically-Structured Nanocomposites in Thin Films

Significant attention has been given to developing bottom-up routes to direct hierarchical assemblies of nanoparticles in thin films for applications such as novel electronics, energy harvesting modules, and photonic materials. It remains challenging to control the self-assembly of the nanocomspites in a short period of time using methods compatible with scalable manufacturing. Here, we report a facile method utilizing solvent vapor annealing to precisely control the morphologies of nanocomposite thin films (PS-b-P4VP(PDP)$_{x}$ with nanoparticles) in the time scale of minutes. By controlling the solvent vapor pressure during the annealing process, the orientations of the microdomains and the nanoparticles can be tailored. Long range-ordered lateral morphologies of the nanocomposites can be obtained. In-situ GISAXS experiments show that the fast assembly process may be attributed to the nature of the supramolecules. The rate of solvent evaporation has been identified as a critical factor in controlling the final morphologies of the nanocomposites.   

Fabrication of Three-Dimensional Bilayered Nanostructures via Block Copolymers

The self-assembly of block copolymers (BCPs) has been received wide attention because of their great potentials in various advanced lithographic applications. For specific applications, one of the most significant is controlling the orientation of the microdomains. In this study, we synthesized ketene-based cross-linkable cylinder forming BCPs, PMMA-b-P(S-r-ketene), in which 3.0 mol% of cross-linkable functional groups were incorporated. Then we manufactured 3-D bilayered BCP films consisting of underlying cross-linked cylinder and top lamellar layers. Interestingly, it was found that the top lamellar layer exhibited perpendicular orientations, regardless of size of domain spacing of underlying cylinder layer and thickness of top lamellar layer. In monolayer lamellar BCP films, neutral layer makes perpendicular orientation near the 1L0. But in 3-D bilayerd BCP films, thickness of perpendicular orientation, by comparison, became significantly broader. This may be due to the entropically driven nematic interactions between these two layers, and the total free energy of this system was theoretically considered. In the work, the orientation of top lamellar films as functions of film thickness and domain spacing of underlying cylinder layers was systematically investigated.   

Fabrication of square arrays of inverted pyramids using ABC triblock terpolymer

Nanolithography using Self-assembly of block copolymer thin film is promising technique to fabricate a wide range of useful devices. Previously, we have reported that we could achieve square array which is one of most important device geometry by using Polyisoprene-b-polystyrene-b-polyferrocenylsilane triblock terpolymer. In this presentation, self-assembled PI-b-PS-b-PFS triblock terpolymer thin film was used as an etching mask to fabricate array of silicon inverted pyramids. Solvent annealed thin film PI-b-PS-b-PFS triblock terpolymer forms a square array of PFS and PI alternation cylinders in a PS matrix with a period of 44 nm. When this square arrayed polymer film immersed into hexane, a good solvent for PI and poor solvent for PS and PFS, an ordered square array of holes was produced by PI phase coming out from its cylindrical post and covering the surface. By using this hole patterned polymer film as an etching mask, KOH anisotropic silicon etching produce square array of inverted pyramids of period 44nm etched into silicon substrate. These square arrays of inverted pyramids are used to template the dewetting of metal film to form metal nanoparticle arrays. Produced ordered metal nanoparticles can be used as magnetic memory arrays and also useful as catalysts for nanowires/nanotube growth.   

Hierarchical buckling of block copolymer thin films on PDMS substrates

Buckling of thin films on low modulus substrates such as polydimethylsiloxane (PDMS) is the well-known phenomenon in buckling instability originating from moduli mismatch between the substrate and a top film. Recently, many studies on the microstructure created by the buckling have been reported but most of the work has typically employed either metal or semiconductor thin films and few utilized the block copolymers (BCP) as the covering films. Here, we present the buckling of oriented BCP thin films, placed by a novel off-set printing, on PDMS substrates. Buckling instability was induced by either swelling of the PDMS substrate with chemical vapor or the mechanical strain, resulting in the hierarchical structure of BCP microdomains. Due to the buckling of the BCP thin films, we observed the structural change or transitions of the films depending on the alignment of BCP domains with respect to the buckled direction of the substrates.   

Surface Properties of Bottlebrush Copolymer Thin Films

Bottlebrush polymers are novel macromolecules with polymeric side-chains. Due to their large size and densely crowded side-chains, bottlebrush polymers are candidates for a number of potential applications, including rheological modifiers, drug-delivery vehicles, and polymeric photonics. However, the structural details of bottlebrush polymers in solution and in the bulk are poorly understood. For example, while the overall size of bottlebrush polymers has been measured, the polymer stiffness and side-chain conformation have not been quantified. Here, we present a study of the surface properties of well-defined bottlebrush polymer and copolymer thin films. Surface property measurements provide a method to investigate polymeric side-chain conformation and flexibility. Bottlebrush polymers with mixed and copolymer side-chains are prepared via living ring-opening metathesis polymerization of norbornene-functionalized macromonomers. Contact angle measurements, x-ray photoelectron spectroscopy, and atomic force microscopy measurements on bottlebrush polymer thin films show that polymeric side-chains have significant conformational flexibility and that bottlebrush polymers with mixed and copolymer side-chains can be used to prepare stimuli-responsive surfaces.   

Self-assembled nanostructures in a cross-linkable block copolymer

The self-assembly of block copolymers in films 50$\sim$100 nm thick provides an attractive approach to patterning nanoscale features. Chemical and thermal stability of the morphology in thin films is critical for the generation of robust templates for subsequent fabrication processes, and can be improved by cross-linking the copolymer domains. Atom transfer radical polymerization was used to synthesize PS/PMMA block copolymers with cross-linkable units capable of reacting through an acid-catalyzed mechanism or by photoinitiation with UV exposure. The self-assembly behavior of lamellar-forming block copolymers with or without cross-linkable units were compared in thin films through top-down characterization. We have developed approaches to decouple the self-assembly process from the cross-linking process, and characterized the cross-linking density and reaction rates within the nanostructured domains. The cross-linked nanostructures exhibit enhanced solvent and thermal stability, and have been demonstrated for the fabrication of three-dimensional block copolymer nanostructures in thick films using a layer-by-layer approach.   

Tuning the Morphology of Solvent Annealed Thin Films of Polystyrene-b-Polyethylene Oxide with Controlled Saturations of Water and Toluene Vapors

Solvent annealing can be used to facilitate the self-assembly of block copolymer thin films and has several advantages over thermal annealing including room-temperature processing, domain orientation control and the ability to anneal polymers not amenable to thermal processing. We have developed a controlled process design for performing solvent annealing that incorporates continuous flows of solvent-saturated carrier gas, multiple simultaneous co-solvents and in-situ metrology. This new method is modular and applicable to a wide range of block copolymer and solvent systems. The control over annealing and quenching conditions afforded by this new technique allows us to reproducibly control the domain orientation and periodicity in thin films of cylinder-forming polystyrene-b-polyethylene oxide (PS-b-PEO) annealed in environments with high saturations of water and toluene vapors without modifying the block copolymer or substrate. By adjusting the humidity of the quenching gas flow we are able to control the orientation of the PEO cylinders, and by adjusting the humidity during annealing we are able to tune the domain spacing of PEO cylinders oriented perpendicular to the substrate.   

Laterally confined diblock copolymers

With the help of cell dynamics simulation we investigate the self-assembly of cylinder-forming diblock copolymer thin films laterally confined within square and non-square geometries. The size of the confinement affects the ordering of the block copolymer domains and their symmetry. We found hexagonally packed cylinders and square packed cylinders by changing the box size of the system. In particular we performed several simulations to find that if the size of the pattern is comparable with the natural bulk period of the copolymer the packing symmetry changes from hexagonal to square. In this case the ordering induced by the pattern edges becomes dominant allowing the square lattice to be more stable than the hexagonal one.   

Self-Assemble Behavior of Polystyrene-tethered Hydrophilic POSS Nanoparticle in Thin Film

Recently, block copolymer thin films are of great interest in their applications of surface patterning. However, due to the restriction of the polymer material properties, the etch contrast between two phases is usually low and the smallest pattern block copolymer can create is around 10 nm. By taking the advantage of the hybrid property of POSS nanoparticle, we can produce the thin film pattern with great etch contrast with only a few nanometers in size. PS-tethered hydrophilic POSS can self assemble to form nano-pattern in both bulk and thin film state. As the PS molecular weight increases, the thin film morphology can transfer from lamella to cylinder then to sphere as block copolymer does. But because of the nature of the POSS particle, we discover a new packing symmetry in the cylinder phase that has not been discovered in block copolymer thin film system. Unlike block copolymer which has higher order-disorder transition temperature in thin film, PS-POSS thin film also shows some unique phase transition behavior during heating, and this transition behavior also affected by the different functional group attached on POSS particles.   

Morphology of ABC linear triblock polymer melts: self-consistent-field theoretic simulation approach

It is well known that block copolymer morphologies resulting from microphase separation finds wide spread use in many applications, including nanolithographic templates, membranes, and electronic arrays. In this study, we have performed self-consistent field theoretic based simulations to examine the plethora of self-assembled morphologies obtained in symmetric linear ABC Tri-block polymer melts in bulk and confined films. Specifically in bulk systems, the conditions leading to lamellae, tetragonal cylinders, sphere, perforatedlamellae, and network morphologies have been elucidated at various volume fractions of middle (B) block. In general, the predicted morphologies are in good agreement with the limited number of published experimental studies. The effect of confinement on the microphase separation is also studied by performing simulations of two neutral parallel walls. Overall, we observe lamellar and cylindrical domains that are oriented parallel to the walls; however, lamellar domains could become perpendicular to the neutral walls when the natural period of lamellae is incommensurate with the film thickness. Our calculations form the basis for understanding the stability of parallel and perpendicular orientations in thin films of ABC tri-block polymeric systems.   

Hierarchical silver concentric ring patterns of block copolymer microdomains for high efficient SERS

We fabricated the half-onion like microdomains of polystyrene-block-poly(methyl methacrylate) copolymer (PS-b-PMMA) when it was confined within hemi-spherical anodic aluminum oxide (AAO) template. The ring number of the concentric ring patterns was easily controlled by changing the molecular weights of the block copolymers, which was verified by scanning and transmission electron microscopes and atomic force microscope. We also deposited silver with 6nm height selectively on the PS microdomains. A number of silver ring patterns were varied from 5 to 8 depending on the molecule weights of the block copolymer. This silver ring structures showed high surface enhanced Raman scattering (SERS) with a maximum enhancement factor of 10$^{7}$.   

Effect of tacticity on the interfacial structural properties of adsorbed polystyrene thin films

We have carried out atomistic Molecular Dynamics simulations to investigate interfacial structural properties of thin films of polystyrene (PS) adsorbed onto solid substrates. The films are made of PS stereoregular chains, that are, isotactic PS or syndiotactic PS. Three types of surfaces of different phobicity and roughness were considered in the present work: hydroxylated silica, graphite and amorphous silica. The structural properties were studied in terms of side chains, end groups and backbone concentration and orientation in the interfacial regions (substrate/film and film/vacuum). Moreover the effect of temperature was investigated by adsorbing the films at different temperatures, below and above the glass transition temperature. Our results were compared to results obtained previously by our group on the adsorption of films made of atactic polystyrene.   

Memory Effects of Multistage Crosslinking and Scission on Stress in Polymer Networks

While the independent network model (INM) typically used for polymers is excellent for networks that truly are independent, the model has to be modified for networks that are not independent. Such dependent networks can be the result of scission occurring in strain states where crosslinking also occurs. The overall material retains a memory of the strain/crosslinking/scission history because of the bias in how the crosslinks are introduced. These memory effects mean that the resulting zero-stress state (the shape to which the sample will return at equilibrium) varies depending on the sample preparation history, in contrast to the INM where only the number of crosslinks in each network should matter. In this talk, I will present recent results of molecular dynamics simulations where coarse-grained polymer models undergo sequential crosslinking and scission in multiple strain states. The effects of entanglements will also be considered.   

Reduction of Dielectric Hysteresis in Multilayered Film via Nanoconfinement

Micro-/nano-layer coextrusion was used to fabricate polycarbonate (PC)/poly(vinylidene fluoride) (PVDF) layered films with significantly reduced dielectric losses while maintaining high energy density. The high-field polarization hysteresis was characterized for layered films as a function of PVDF layer thickness (6000 to 10 nm) and composition (10 to 70 vol.{\%} PVDF), and was found to decrease with decreasing layer thickness and PVDF content. To gain a mechanistic understanding of the layer thickness (or nanoconfinement) effect, wide angle X-ray diffraction, polarized Fourier transform infrared spectroscopy, and broadband dielectric spectroscopy were employed. The results revealed that charge migration, instead of dipole flipping, was responsible for the hysteresis in multilayered films.   

Fullerene-based shape amphiphiles towards hierarchical supramolecular assemblies

Self-assembly is an elegant strategy which is used to create assemblies with tunable properties in nature as well as the artificial supramolecular systems. The self-assembly of [60]fullerene (C60) has drawn an tremendous amount of interest due to the exceptional optical and optoelectronic properties of C60. By constructing hierarchical supramolecular structures of C60 derivatives, the molecular functionality of C60 can be accumulated, amplified, and then transferred to bulk material properties. This research focuses on manipulating the balance of non-covalent interactions among C60 derivatives into molecular alignments in the supramolecular structure. The aliphatic tails and spacers in the C60 derivatives will improve the solubility and provide sufficient mobility for the effective packing of C60s. The differences in shape and intermolecular interaction between C60s and alkyl chains allow these molecules to arrange themselves in microphase-separated mesostructures. The hierarchical structure built up by C60 derivatives can be obtained via the highly-ordered packing of C60 in mesostructures. By adding an additional bisamide group to the molecular structure, the balance of the intermolecular interactions and the spatial packing can be tuned to exhibit supramolecular polymorphism.   

Chain Packing and Trajectory of Isotactic Polypropylene $\alpha $ Crystals studied by Solid-State NMR

Isotactic polypropylene (ipp) is one of the simplest polyolefins and the crystalline structures, have been extensively studied. Ipp crystallizes as $\alpha $ form via isothermal crystallization from the melt state. Packing structures of $\alpha $ form has been used as structural markers of crystallization process. With the development of high resolution solid-state NMR (SS-NMR) technique, it becomes a powerful tool to investigate order-disorder of chain packing in the crystalline regions. We performed a series of experiments on different samples crystallized at different temperatures and studied the formation of $\alpha $ crystals influenced by different parameters, such as molecular weight, catalyst type and stereo, region regularity. Using selectively $^{13}$C enriched ipp samples we detect inter-nuclear correlations between the neighboring stems. This information provides chain reentry information. We figure out relationship between the chain reentry and chain packing of ipp $\alpha $ crystals. The recent discovery of the ipp $\alpha _{2}$ single crystal offers a great opportunity to understand this topic also.   

Tube Visualization and Properties from Isoconfigurational Averaging

We introduce a simulation method to visualize the confining tube in polymer melts and measure its properties. We studied bead-spring ring polymers, which conveniently suppresses constraint release and contour length fluctuations. We allow molecules to cross and reach topologically equilibrated states by invoking various molecular rebridging moves in Monte Carlo simulations. To reveal the confining tube, we start with a well equilibrated configuration, turn off rebridging moves, and run molecular dynamics simulation multiple times, each with different initial velocities. The resulting set of ``movies'' of molecular trajectories defines an isoconfigurational ensemble, with the bead positions at different times and in different ``movies'' giving rise to a cloud. The cloud shows the shape, range and strength of the tube confinement, which enables us to study the statistical properties of tube. Using this approach, we studied the effects of free surface, and found that the tube diameter near the surface is greater than the bulk value by about 25\%.   

Unfolding of Collapsed Polymers in Shear Flow Enhanced by Colloidal Suspensions: Effect of Colloid Banding Structures in Confining Channels

Very recently, using hydrodynamic simulations we have demonstrated that colloidal suspensions can greatly enhance the unfolding of collapsed single polymers in shear flow. Furthermore, we have shown that the enhancement may be suppressed if the colloid size is commensurate with the confining channel height, where the colloids form well-defined banding structures. In this study, we analyze the colloid banding structures in details and their relations to the polymer unfolding. We find that, for the colloid volume fractions up to 30\%, the colloids form simple cubic (sc), hexagonal (hex), or the mixture of sc + hex structures depending on the commensurability of the colloid size and the channel height. By directly changing the height of the confining channels, we show that the collapsed polymers have the highest unfolding when the channel height is non-commensurate with either sc or hex structures.   

Slippage of polymers: Influence of the chemical structure

The continuing miniaturization of microfluidic devices causes a growing importance of the solid/liquid interface for the flow dynamics. Our experiments probe slippage using the dewetting process of thin polymer films on hydrophobic substrates. As hydrophobic coatings we use amorphous polymers (AF1600, AF2400) and different types of highly ordered self-assembled silane monolayers on top of ultraflat silicon substrates. Polystyrene (PS) of low molecular weight shows slip lengths between several hundreds of nanometers and even micrometers on silane surfaces [1], whereas on AF1600 nearly no slip is observable. However, slip can be induced by increasing the molecular weight of the PS [2]. Recent studies using scattering techniques showed an ordering effect of PS at the solid/liquid interface depending on the structure of the substrate [3]. Will the situation change if, instead of PS, polymethyl methacrylate (PMMA) is used? To probe the influence of the polymers composition on slippage, we show very first results of the dewetting dynamics of PMMA on AF2400.\\[4pt] [1] R. Fetzer, et. al., Europhys. Let., 75, no. 4, 638 (2006)\\[0pt] [2] O. B\"aumchen, et al., PRL, 103, 247801 (2009)\\[0pt] [3] P. Gutfreund, et. al., arxiv.org 1104.0868v1 (2011, April 5)   

Glass transition in ultra thin polymeric films measured by differential AC-Chip calorimetry

The film thickness dependency of glass transition in polymer films is still controversially discussed. For different experimental probes different dependencies are observed and a generally accepted link to molecular mobility is not yet established. Calorimetry has proven to provide useful information about glass transition, because it establishes a direct link to energetic characterization [1]. In several cases a direct comparison with results from other dynamic methods like dielectric spectroscopy is possible giving further insights. For thin films in the $\mu $m{\ldots}nm range standard calorimetric methods are mostly not applicable. We set up a differential AC-chip calorimeter capable to measure the glass transition in nanometer thin films with pJ/K sensitivity in a relative broad frequency range [2]. Changes in heat capacity can be measured for sample masses below one nanogram as needed for the study of the glass transition in nanometer thin polymeric films. The glass transition in thin films was determined at well defined experimental time scales. No thickness dependency of the glass transition temperature was observed within the error limits - neither at constant frequency nor for the traces in the activation diagrams. [1] C. Schick, Glass transition under confinement-what can be learned from calorimetry, Eur. Phys. J. Special Topics \textbf{189} (2010) 3-36. [2] H. Huth, AA. Minakov, and C. Schick, Differential AC-Chip Calorimeter for Glass Transition Measurements in Ultrathin Films, \textit{J. Polym. Sci. B Polym. Phys. }\textbf{44} (2006) 2996-3005.   

Viscoelastic behavior of polymer in confined system

Materials under nanoconfinement have been studied to exhibit interesting static and dynamic behaviors deviated from those in bulk state. In this study, we analyzed viscoelastic behavior of polymer confined to mesoscopic space. Polystyrene was adopted as the material to be probed, and anodized alumina was used as the substrates providing mesoscopic confinement. Controlling the confining conditions, the viscoelastic behavior of polystyrene was examined. Analyzing the surface tensions of the materials, we also considered the effect of interfacial tension on the thermodynamic stability of polymer in the confined geometry. In this presentation, we qualitatively explain the effects of confinement dimension and interface in comparison with that not in confined system.   

Confinement effects on thermal frontal polymerization

Frontal polymerization systems belong to the broader class of reaction-diffusion systems. In thermal frontal polymerization, the interplay of heat diffusion and Arrhenius reaction kinetics gives rise to a moving reactive front; propagation occurs through a positive feedback mechanism. We seek to understand how spatial confinement can change the system dynamics and cause to deviate the system from bulk behavior. In highly confined systems, the front propagates slower and the front profile is flatter than in bulk. The polymerization product shows a decrease in polydispersity index ($<$1.5) for high confinement. The molecular weights of the samples do not exhibit simple unimodal distributions, unlike bulk. In specimens at the highest degree of confinement the molecular weight of the resultant polymer decreases with increasing distance from the external heating source. In highly confined systems a smaller number of radicals are generated, resulting in a slower propagation step and polymer chains of more uniform length; front propagation is slow as it is reaction-limited. In contrast, for bulk or unconfined systems, higher heat generation rates, leading to faster polymerization with higher polydispersity in molecular weight; front propagation is fast as it is diffusion-limited.   

Temperature Dependence of Polymer Diffusion in MWCNT/PS Nanocomposites

Temperature dependence of homopolymer diffusion can be explained by the WLF equation. Here, we explore whether the WLF equation applies to polymer diffusion in nanocomposites. Previously, we found the diffusion coefficient shows a minimum with increasing MWCNT concentration. By studying the temperature dependence of polymer diffusion in this system, we will investigate the relative importance of entropic barriers or enthalpy interactions between polymer chain and fillers. Our composites contain MWCNT and polystyrene and are fabricated by a coagulation method. Using forward recoil elastic scattering (FRES), we probe the depth profile of tracer polymer (dPS) and obtain the diffusion coefficients by fitting the profile with Fick's second law.   

Effect of confinement on reaction rates within polymer nanotemplates

The most efficient catalysts have been developed and optimized by living systems. Indeed, in vivo enzyme-catalyzed reactions are several orders of magnitude more efficient than platinum based catalyzed reactions. However, the rate of reaction and equilibrium interactions are considerably reduced when the biological systems are studied in vitro. This phenomenon is largely attributed to the effect of confinement or macromolecular crowding present in the cell. Confinement can also be observed in an aqueous solution containing surfactants (amphiphilic copolymers). For example, copolymers can self-assemble into well defined ordered structures such as micelles, nanotubes, vesicles; and the geometries and shapes of a given copolymer can be controlled by their solvent affinity. The hollow nanoarchitectures obtained by self-assembly can be used as a model template to study confinement within a soft shell system to mimic biosystems. These systems provide a very controlled environment for the study of confinement. In this paper we will present the effect of confinement on polymerisation reactions combining both simulation and experimental characterisation for a comprehensive study of the effect of confinement on the interactions among confined molecules.   

Influence of Thermal History in Crystallization of Physically Confined 2-Decanol

Bulk 2-decanol exhibits substantial hysteresis between its melting (-3$^{o}$C) and freezing (-23$^{o}$C) temperatures. When it is physically confined, the melting and freezing temperature are lowered and still exhibit hysteresis. Any presence of partially melted crystals, however, can trigger crystallization at higher temperatures upon subsequent cooling, suggesting that the hysteresis is thermal history dependant. The history, however, can be erased at temperatures higher than melting point. The lowest temperature at which the thermal history can be erased is physical size dependent. For example, 2-decanol physically confined in 100 nm has to be heated to a temperature between -10$^{\circ}$C and -5$^{\circ}$C to erase its history while 2-decanol confined in 300 nm has to be heated to a temperature between -2$^{\circ}$C and 2$^{\circ}$C.   

Polymer Diffusion in Nanocomposites having attractive particle- polymer interactions

The addition of fillers into polymeric materials has drawn tremendous attention because of the remarkable mechanical and functional properties exhibited by these composites. Previously, we studied diffusion in a weakly interacting system and found that the reduced diffusion coefficient (D/D$_{0})$ scaled with the confinement parameter, defined as the inter-particle distance relative to the tracer size. Using elastic recoil detection, tracer diffusion of deuterated poly(methacrylate) is studied in a poly(methyl methacrylate) matrix containing silica nanoparticles with number average diameters of 12.8 and 28.8 nm. Because the silica contains surface hydroxyl groups, PMMA nanocomposites are a model system for studying diffusion in strongly attractive polymer-nanoparticle systems. In contrast to the weakly interacting system, the reduced diffusion coefficient in the matrix with smaller nanoparticles is less than that with larger nanoparticles and this difference increases as the confinement parameter decreases (i.e., more crowded system). The reduced diffusion coefficient is also analyzed in terms of the surface to volume ratio of the nanoparticles.   

Anomalous yet Brownian

The items on the growing list of exceptions to Gaussian statistics have been given system-specific interpretations that fail to provide a universal picture of how Fickian diffusion can be non-Gaussian for diffusion over distances that much exceed the size of the diffusing object. Here, based on experiments in four separate systems, we present a general reasoning which indicates that the measured dynamics can be decomposed into a wide set of diffusivities that reflect slowly-varying, heterogeneous microscopic fluctuations. The identification of non-Gaussian yet Fickian diffusivity with long-lived environmental fluctuations allows us to conclude that non-Gaussian diffusivity should characterize much mobility in soft matter.   

Glass Transition Temperature Gradient evidenced by NMR and Calorimetry

Polymer, when confined, exhibit a dynamics different from the bulk one. However, measurements on unique films are rather difficult. One of the commonly accepted hypothesis is that there is a gradient of glass transition temperature in the vicinity of the solid surfaces. We have developed since ten years model nano-composite systems consisting in monodisperse spherical particles dispersed in an elastomer matrix. From Neutron Scattering, we can deduce the density of polymer located at any distance from any silica surface. We have then measured by NMR the magnetization relaxation at various temperatures above the glass transition temperature and its vicinity. From these measurements we were able to fit the whole set of data at various temperatures by a unique relation for the glass transition temperature Tg as a function of the distance $z$ from a solid surface Tg(z)=Tg(1+$\delta $/z), with a unique parameter $\delta $. In addition, the same law holds with the same parameter $\delta $ in the presence of solvent. Moreover, the parameter measured by NMR allows predicting quantitatively the Differential Scanning Calorimetry response, even after an aging step.   

End Group Effects on the Hydrogel Formation of PEO-PPO-PEO Triblock Copolymers

Pluronic F108, a triblock copolymer consisting of outer polyethylene oxide (PEO) chains and an inner polypropylene oxide (PPO) chains, has been shown to be an effective hydrogel matrix for DNA separation by capillary electrophoresis using single-stranded conformation polymorphism. This presentation will discuss a new pathway to potentially enhance the separation abilities of F108 by altering the chain end groups of the block copolymers. F108 is believed to form a micelle in aqueous solutions with the hydrophobic group in the interior, thus we expect considerable interaction between the DNA sample and the end groups found at the hydrophilic brush layers of the micelle. The rheological properties of end group derivatives of F108, in combination of small angle x-ray scattering, can reveal structural differences in the micelles. In particular, gelation temperature of the end group derivatives can be linked to differences in the micelle structure. Dynamic light scattering can also be used to determine the effects of chain end groups on the hydrodynamic size of the block copolymer micelles in dilute solution.   

Self-assembly of polystyrene-$b$-poly(4-vinylpyridine) and polystyrene-$b$-poly(ethylene oxide) in small molecules melt

It is well known that amphiphilic block copolymers in selective solvents self-assemble into micellar structures, where solvophilic blocks tend to contact with solvents while solvophobic blocks are shielded from the solvents. Different from the conventional micellization in liquid systems, we report that block copolymer poly(styrene-block-4-vinylpyridine) (PS-$b$-P4VP) and poly(styrene-block-ethylene oxide) (PS-$b$-PEO) can self-assemble in melted small molecules at high temperature and the structures can be retained in ``solid state'' after being cooled down to room temperature. Transmission electron microscopy (TEM) was used to probe the structures and we found that a series of self-assembled structures, including spherical micelles, wormlike micelles and vesicles can be obtained by varying the length of block copolymers and the morphologies are dependent on annealing temperature and time. Since these nano-structures can be retained in solid state, we also demonstrate to extract the nano-structures by removing small molecules using appropriate solvents. These extracted structures, especially the vesicles, which are loaded with solid molecules, are potential applications of nanocapsules and controlled release   

Nanoparticle-Loaded Multifunctional Block Copolymer Micelles

We have studied the incorporation of pre-synthesized hydrophobic inorganic nanoparticles within the cores of amphiphilic polystyrene-block-poly(ethylene oxide) (PS-PEO) diblock copolymer micelles formed through solvent-evaporation-induced interfacial instabilities of emulsion droplets. Using iron oxide, gold, and cadmium selenide nanoparticles coated with native alkane ligands, highly uniform encapsulation is obtained for cylindrical micelles, while spherical micelles can be enriched to $\sim $ 90 {\%} of loaded micelles through simple magnetic or centrifugal purification steps. Multiple different types of nanoparticles can easily be incorporated into each micelle, yielding multi-functional micelles. The ability to encapsulate both spherical and rod-like particles of different core chemistries and sizes ranging from $\sim $ 1 to 20 nm, without the necessity of coating particles with specially designed ligands, makes this a versatile route to prepare hybrid micelle structures.   

Phase Behavior of Star-shaped polystyrene-\textit{block}-poly(methyl methacrylate) Copolymers

Star-shaped polystyrene-\textit{block}-poly(methyl methacrylate) copolymer (PS-$b$-PMMA) was synthesized by utilizing $\alpha $-cyclodextrin ($\alpha $-CD) as a junction point of the star-shaped block copolymer. Eighteen hydroxyl groups on $\alpha $-CD were substituted with bromine by the reaction with $\alpha $-bromoisobutyryl bromide for atom transfer radical polymerization. We found that the number of bromine substituted arms per one $\alpha $-CD was higher than 16 measured by nuclear magnetic resonance and Matrix-assisted laser desorption/ionization. We could control molecular weight of this unusual kind of block copolymer depending on polymerization times. Those polymers were characterized by gel permeation chromatography and nuclear magnetic resonance. Phase behavior of these star-shaped block copolymers were investigated.   

Kinetics of micellization for diblock copolymers in selective solvents studied using self-consistent field theory

The kinetic barriers to association and dissociation of diblock copolymers in various selective solvents are calculated using self-consistent field theory. The variation of these kinetic barriers for both crew cut as well as hairy micelles are studied. The kinetic barriers are found to be very sensitive to temperature and become prohibitive except in a modest range of temperature near the critical micelle temperature. The dependence of kinetic barriers upon the chain and block lengths, and solvent quality are also studied.   

Blending of Diblocks and Triblocks with identical hydrophilic block for multicompartment and multigeometry nanostructures

Unique micellar morphologies, such as toroids, disks, and helices, have been obtained from a single triblock copolymer PAA-PMA-PS poly(acrylic acid)-b-poly(methyl acrylate)-b-poly(styrene)~(PAA-PMA-PS) through a self-assembly process in dilute water/THF(tetrahydrofuran) solvent mixtures in the presense of organic multiamine molecules. Aiming to better understand their formation and explore novel structures, diblock copolymers, e.g. PAA-PMA and PAA-PS, were mixed with PAA-PMA-PS to co-assemble at desired solution conditions to produce known nanostructures (e.g. toroid, disk, helix fomations). By taking advantage of the kinetic pathway of assembly and mulitamine-PAA complexation, the additional diblock copolymers can be trapped in the same micelle with the triblock PAA-PMA-PS. Interesting transitions were found in toroid/disk/helix fomations by changing the amount of the added diblock copolymers. The morphologies of the blended nanoparticles was characterized with cryogenic and conventional transmission electron microscopy, dynamic light scattering, and small angle neutron scattering.   

Kinetic control of block copolymer self-assembly into multicompartment and novel geometry nanoparticles

Micelles with the segregation of hydrophobic blocks trapped in the same nanoparticle core have been produced through co-self-assembly of two block copolymers in THF/water dilute solution. The dissolution of two block copolymer sharing the same polyacrylic acid PAA blocks in THF undergoes consequent aggregation and phase separation through either slow water titration or quick water addition that triggers the micellar formation. The combination and comparison of the two water addition kinetic pathways are the keys of forming multicompartment structures at high water content. Importantly, the addition of organic diamine provides for acid-base complexation with the PAA side chains which, in turn, plays the key role of trapping unlike hydrophobic blocks from different block copolymers into one nanoparticle core. The kinetic control of solution assembly can be applied to other molecular systems such as dendrimers as well as other block copolymer molecules. Transmission electron microscopy, cryogenic transmission electron microscopy, light scattering have been applied to characterize the micelle structures.   

Dynamic Processes in Diblock Copolymer Micelles with a Semi-Crystalline Core

Amphiphilic diblock copolymers, which form micelle structures in selective solvents, offer a great advantage of tunability in physical characteristics as compared to low molecular weight surfactants. Their micelles in aqueous solvents have been a subject of great interest in drug delivery applications for their high loading capacity and targeted drug delivery. The aim of this work is to understand the dynamic processes underlying the self-assembly of diblock copolymer micelle systems which have a semi-crystalline core. The present work focuses on amphiphilic diblock copolymers containing blocks of poly(ethylene oxide) (a hydrophilic polymer) and polycaprolactone (a hydrophobic polymer), which spontaneously self-assemble into spherical micelles in water. Polycaprolactone is a semi-crystalline polymer. A variety of experimental techniques are used to probe the kinetic processes occurring during micelle self-assembly, including time-resolved neutron scattering, dynamic light scattering, pulsed field gradient nuclear magnetic resonance, and fluorescence resonance energy transfer experiments.   

Long Circulating Micelles based on Helix Bundle-Forming Peptide-Polymer Conjugates

Stable, multi-functional organic nanoparticles that combine long in vivo circulation, the ability to cross vessel walls to reach tumor tissues and controlled disassembly for eventual clearance will have a significant impact in nanomedicine. Although current self-assemblies of amphiphiles provide a versatile platform to generate modular organic nanoparticles, it remains a significant challenge to simultaneously control nanoparticle size in the range of 10-30 nm, enhance particle stability and tailor disassembly within suitable timescales. We have advanced this goal by designing a new family of amphiphiles based on coiled-coil 3-helix bundle forming peptide-polymer conjugates. By attaching a polymer chain to the middle of a helical peptide, the protein tertiary structures are used to position entropic forces of compressed polymer chains comprising the headgroups so as to effectively slow down the subunit desorption rate and enhance the in vivo stability. The resultant monodispersed nanoparticles are composed of subunits, $<$ 4 nm in size, that form highly stable 15-17 nm diameter particles and demonstrate an in vivo circulation half life-time of 28 hrs, minimal accumulation in the liver and spleen and effective urinary clearance. By uniquely combining the configurational entropy of a polymer chain with a common protein structure, i.e. coiled-coil helix bundle, and a lipid core, self-assembled nanoparticles have been engineered with tunable stability and disassembly.   

Physics of Polymersomes: lateral segregation experiments and raft simulations

Coupling between the inner and outer leaflets of a bilayer plays an important role in biomembrane function, particularly in inducing and registering rafts across leaflets for various cellular signals. However, mechanisms of raft registration remain elusive and several alternatives have been proposed, ranging from electrostatic coupling to chain interdigitation. A general mechanism has been suggested by recent experiments on Polymersomes in which binary mixtures of diblock copolymer amphiphiles exhibit domain registration upon ligand-induced segregation. Using coarse grained molecular dynamics (CGMD) simulations rooted in atomistics, raft registration arises spontaneously in bilayers with a calcium- or ligand-crosslinked ordered phase segregating from a liquid disordered phase. When rafts are not registered, a thickness mismatch between phases induces a ``bump'' in the apposing liquid phase leaflet, and the associated localized curvature guides rafts together stabilizing the registered state. The absence of explicit charge in the model and the fact that domain size modulates transmembrane coupling demonstrate that collective interactions are sufficient for raft registration. References: (1) D.A. Christian, et al. Spotted vesicles, striped micelles, and Janus assemblies induced by ligand binding. Nature Materials 8: 843--849 (2009). (2) D. Pantano, P.B. Moore, M.L. Klein, D.E. Discher. Raft registration across bilayers in a molecularly detailed model. Soft Matter 7, 8182-8191 (2011).   

Controlling the Morphology of TiO2 Nanorods/Polythiophene Composites for Bulk Heterojunction Solar Cells Using H-Bonding

We demonstrate how the morphology of solution-processable hybrid bulk heterojunction solar cells, within an active layer consisting of modified poly(3-hexylthiophene) (P3HT) and TiO2 nanorods, can be controlled by H-Bonding. The hybrid bulk heterojunction solar cells suffer from the problems of the aggregation of inorganic nanocrystals and the interface between nanocrystals and the polymer matrix. To address these issues, we utilize P3HT-based block copolymer (BCP), in which one block is P3HT and the other block is a P3HT derivative containing a poly(ethylene glycol) (PEG) oligomer side chain. In the mean time, we functionalized the TiO2 nanorods with dyes having multiple COOH groups. This design both enables self-assembly of the devices via micophase segregation into well-defined morphologies and provides a means for establishing strong preferential interaction between TiO2 nanorods and the PEG side chain. This strong, preferential H-bonding limits the aggregation of the TiO2 nanorods and modifies the interfacial properties between donor and acceptor. TEM showed the self-assembly structure of the TiO2 nanorods in polymer matrix. Using this modified thiophene copolymer, hybrid devices are made with power conversion efficiencies 50\% higher than that of conventional P3HT homopolymer.   

Magnetic Nanoparticle/Block Copolymer Hybrid Materials for the Fabrication of Electromagnetic Devices

Significant efforts have been directed towards incorporating inorganic nanoscopic materials into well-defined, phase-separated block copolymer systems to create hybrid materials with intriguing optical, electrical or magnetic properties. Lin et al. recently reported the use of strong interactions between NPs and one segment of weakly segregated BCP systems to drive the assembly of well-ordered morphologies while confining the NPs specifically in the desired spherical, cylindrical or lamellar domains. Here we use this approach to assemble magnetic nanoparticles of high permeability into well ordered systems. Magnetic nanoparticles (MNPs) were synthesized and subsequent surface functionalization and/or ligand exchange reactions were carried out to decorate the NP surfaces with hydrogen-bonding donating groups and good particle dispersibility in polar solvents. The ligand attachment was confirmed by TGA and FTIR. The morphology evolution of BCP/MNPs composites was examined by SAXS. Such ferromagnetic structures with precise geometric control in nanoscale would enable the cost-effective fabrication of more advanced devices for AC electromagnetic applications such as miniaturized antennas with extended bandwidth, integrated microwave electronics and efficient power transformers.   

Novel Approach to Quantify Dispersion of Spherical Nanoparticles in Polymer Nanocomposites

Nanocomposites have emerged as promising materials due to improved conductivity, toughness, and permeability when compared to conventional bulk polymers. Controlling the dispersion of the nanoparticles in a polymer matrix is still one of the greatest challenges that limit our ability to achieve the aforementioned property enhancements. Nanoparticles tend to agglomerate due to strong interparticle interactions. The dispersion of nanoparticles in polymers is generally determined by transmission electron microscopy (TEM) or scanning electron microscopy (SEM). However, in these studies quantification of dispersion depends on the visual observations and is prone to subjective conclusions; thereby hampering possible comparison between samples. Considering its importance, little effort has been put forward to quantify the dispersion of nanoparticles in polymers. In the current study, we have applied network theory approach to quantify the dispersion of spherical particles. A network is a collection of nodes, which are entities (such as nanoparticles), and edges that connect pairs of nodes (distances between nanoparticles). By employing nodes and edges, we constructed interaction networks, which were then analyzed in terms of global topological measures: degree distribution, clustering coefficient, and average distance. We show that this approach is a powerful tool to quantify dispersion of spherical nanoparticles in a polymer matrix.   

Mesoscopic Iron-Oxide Nanorod Polymer Nanocomposite Films

Dispersion of nanostructures in polymer matrices is required in order to take advantage of the unique properties of the nano-sized filler. This work investigates the dispersion of mesoscopic (200 nm long) iron-oxide rods (FeNRs) grafted with poly(methyl methacrylate) (PMMA) brushes having molecular weights (MWs) of 3.7K, 32K and 160K. These rods were then dispersed in either a poly(methyl methacrylate) or poly(oxyethylene) (PEO) matrix film so that the matrix/brush interaction is either entropic (PMMA matrix) or enthalpic and entropic (PEO matrix). Transmission electron microscopy (TEM) was used to determine the dispersion of the FeNRs in the polymer matrix. The results show that the FeNRs with the largest brush were always dispersed in the matrix, whereas the rods with the shorter brushes always aggregated in the matrix. This suggests that the brush MW is a critical parameter to achieve dispersion of these mesoscopic materials. This work can be extended to understand the dispersion of other types of mesocopic~particles   

Additive Driven Self Assembly in Poly(ethylene glycol ) monomethyl ether monomethacrylate-block-Poly(ethyl methacrylate) Copolymers

Recent work in our labs has demonstrated the concept of additive driven assembly by hydrogen bond interactions between poly(ethylene oxide) segments of a disordered PEO containing block copolymer and a nanoparticle or organic additive induces strong segregation in the composite to yield well ordered morphologies. In some cases, these interactions were introduced by light-activated deprotection of functional groups on the additive to enable regioselective, photo-induced order in block copolymer films. While PEO crystallization in these polymers is suppressed by strong interaction between the additive and the PEO segments at high additive loadings, crystallization of the PEO block with no loading or at low additive loadings is highly undesirable for many applications. To remedy this issue, we prepared PPEGMEMA-PEMA using RAFT living polymerization. This block copolymer phase mixed, noncrystallizable at room temperature. The incorporation of organic additives with multiple carboxylic acid groups or functionalized nanoparticles induces phase segregation in these systems. Furthermore, the use of additives in which the hydrogen bond donating group is protected with an acid labile group in combination with a photo acid generator enables photo-induced order of the composite films.   

Correlating nanoscale structural changes to macromolecular gel formation in Laponite containing Pluronic F127 photogeling systems

Polymer nanocomposites has been a growing scientific field over the last 20 years. Recently, there has been increasing interest on nanocomposite systems with active responses to external stimuli such as heat, magnetic fields and light. The focus of this work is on a reversible thermoresponsive system, Pluronic F127 with added inorganic disk-shaped nanoparticles of Laponite. We further modify this system with the addition of a photoacid generator to enable photogelation. However, the nanoscale particle-particle and polymer-particle interactions within this Laponite/ block copolymer system are not well understood. We report on the photogelation kinetics of this system and further probe the interactions and rearrangement kinetics with heat and light exposure using in-situ synchrotron small angle x-ray scattering.   

Gold-Decorated Supraspheres of Block Copolymer Micelles

Gold-decorated supraspheres displaying various surface morphologies were prepared by infiltration of gold precursor into polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) supraspheres under acidic condition. The supraspheres were fabricated by emulsifying PS-b-P2VP polymer solution into surfactant solution. Selective swelling of P2VP in the suprasphere by gold precursor under acidic condition resulted in the formation of gold-decorated supraspheres with various surface structures. As evidenced by TEM and SEM images, dot pattern was formed in the case of smaller supraspheres than 800 nm; whereas fingerprint-like pattern was observed in larger supraspheres than 800 nm. Gold nanoparticles were located inside P2VP domains near the surface of prepared supraspheres as confirmed by TEM. The optical property of the supraspheres was characterized using UV-vis absorption spectroscopy and the maximum absorption peak at around 580 nm was observed, which means that gold nanoparticles densely packed into P2VP domain on the suprasphere. Our approach to prepare gold-decorated supraspheres can be extended to other metallic particles such as iron oxide or platinum nanoparticles, and those precursors can be also selectively incorporated into the P2VP domain.   

Self-Assembly of CdSe Nanorods by Controlled Solvent Evaporation and Applied Electric Field

We report the synthesis of CdSe nanorods (NRs) having variable aspect ratios and their vertical alignment into a hexagonal arrays over a large area. NRs with a constant diameter of 6 nm and lengths, tunable to over 100 nm, were obtained by controlling the concentration of ligands in solution and injection of Se at defined time intervals. Hexagonal arrays of aligned, alkane covered CdSe nanorods within a polymer matrix were achieved by controlled solvent evaporation in the presence of an applied electric field. The nature of the solvent identity, substrate hydrophobicity, and strength of applied voltage were optimized such that close packing of nanorods was achieved over a large area. Such organized nanocomposites hold potential application in organic-inorganic bullk heterojunction photovoltaic devices.   

Emulsions of Polymer Blends Stabilized by Janus Particles

Particle-stabilized emulsions of both immiscible and partially miscible polymer blends have recently received renewed interest. In particular, bicontinuous stabilized emulsions are attractive for their three-dimensional expression of the properties of each component, but a true incarnation of this structure has yet to be demonstrated in polymer systems, due to the difficulties in preparing particles that neutrally wet both polymer phases. Janus particles, which possess different surface chemistries on two halves of the particles, afford a way to bypass the necessity of neutral wettability. Both theory and experiment have shown enhanced interfacial adsorption energies for Janus particles, in comparison to homogeneous particles. To investigate these concepts, silica particles were homogeneously and anisotropically functionalized and dispersed in fluid mixtures; interfaces were created by thermally induced phase separation or mechanical mixing. The resulting structures were characterized by laser-scanning confocal microscopy and transmission electron microscopy. The results elucidate the role of particle wettability on the structure of stabilized emulsions.   

Crystallization of Polymers at liquid/liquid interface templated by single-walled carbon nanotubes

Nanosized single-walled carbon nanotube rings were fabricated by using a Pickering emulsion-based method. By tuning a water/oil/SWNT miniemulsion system, SWNT rings with a diameter of $\sim $200 nm can be readily achieved. The formation mechanism is attributed to the bending force induced by the curved liquid/liquid interface. Crystallization of polyethylene homo- and copolymers using this unique SWNT rings as the nucleation agent was conducted at the curved liquid/liquid interface. Crystal structure, hybrid morphology and crystallization kinetics were systematically studied. The structure of controlled alternating patterns on SWNT rings has great potential in various applications in large-scale integrated circuits and single-electron devices.   

Effect of Nanoparticle Size on Nanoparticle Spatial Distribution in a Diblock Copolymer Supramolecular Thin Film

The self-assembly of nanoparticles (NPs) opens many pathways towards generation of functional nanostructured materials with desirable optical, mechanical and electrical properties. A great challenge in this field is the effective control of NP spatial distribution within the block copolymer matrix, which is crucial in tailoring the macroscopic properties of the polymer/nanoparticle composites. We systematically investigated the effect of NP size on the spatial distribution of nanoparticles upon blending with a diblock copolymer based supramolecule in thin film. The spatial distribution of NPs in thin film was observed to be strongly dependent on NP size. These observations can be explained by the increase in entropic penalty of incorporating larger NPs associated with the deformation of the BCP block to accommodate the NPs. This effect is observed for NPs with different chemistries and could serve as a promising route to creating multifunctional thin film nanocomposites.   

Direct Nanorod Assembly Using Block Copolymer-Based Supramolecules

One-dimensional nanomaterials with high aspect ratios, such as nanorods, exhibit unique and useful anisotropic optical, magnetic, and electrical properties. The collective properties of 1-D nanomaterials depend on their spatial arrangements, interparticle ordering, and macroscopic alignment. Developing routes to control their organization with high precision is critical to generate functional materials. We have investigated the co-assemblies of nanorods and block copolymer (BCP)-based supramolecules that self-assemble into spherical, lamellar and cylindrical morphologies. By varying energetic contributions from the rod-rod interactions and the deformation of the supramolecule, a wide library of nanorod assemblies including highly aligned arrays, continuous networks, and clusters can be readily accessed. Since macroscopic alignment of BCP microdomains can be obtained by application of external fields, present studies open up a new route to manipulate macroscopic alignments of nanorods. Fundamentally, these studies have demonstrated that in these blends, the energetic contributions from the polymer chain deformation and rod-rod interactions are comparable and can be tailored to disperse nanorods with control over inter-rod ordering and their relative alignment.   

Magnetically aligned polymers and nanocomposites for energy harvesting and energy storage applications

The realization of anisotropic, nanostructured, functional materials by self-assembly is impaired by the persistence of structural defects which render the properties of the system isotropic on macroscopic length scales. We present three distinct systems including ZnO nanowire-semiconducting polymer composites, Li-ion conducting block copolymer membranes, and perylene-based block copolymers where self-assembly under a magnetic field yields alignment and global anisotropy of their physical properties. The resulting aligned nanostructured systems are attractive for ordered heterojunction photovoltaics, high performance solid polymer electrolyte membranes and electro-optical devices, respectively. Our results demonstrate that magnetic fields offer a viable route for directing the self-assembly of certain soft functional materials. The ready scalability of this approach makes it potentially important from a technological standpoint.   

Absorption of nanoparticles onto curved surfaces

We study the adsorption of gold nanospheres onto cylindrical and spherical glass surfaces from stagnant aqueous particle suspensions. The curved surfaces were obtained as tapers and microspheres fabricated from optical fibers and were coated with a nm-thick layer of the polycation polyallylamine hydrochloride, causing irreversible adsorption of the negatively charged spheres. Our results fit well with theory, which predicts that the rates of particle adsorption will depend strongly on the surface geometry. In particular, {\em adsorption is significantly faster on curved than on planar surfaces} at times long enough that the particle diffusion length is large compared to the surface curvature. This is of particular importance for plasmonic sensors and other devices where particles are deposited from a suspension onto surfaces which may have non-trivial geometries.   

Thermodynamics of ligand-coated nanoparticle complexation with lipid bilayers

Recently, nanoparticles (NPs) coated with a mixed alkanethiol surface monolayer were observed to spontaneously penetrate cell membranes via a non-endocytotic mechanism. Penetration seems to depend on the structure of the ligand monolayer; particle uptake was greatest when the surface self-assembled into a striped morphology consisting of alternating domains of hydrophilic and hydrophobic ligands. Furthermore, these `striped' particles form stable complexes with single component lipid bilayers, implying that cellular penetration may be related to bilayer interactions. Bilayer complexation is surprising, however, because the width of the striped domains is less than the thickness of the bilayer core, implying that charged ligands are exposed to hydrophobic lipid tails. In this work, we provide a thermodynamic analysis of NP-bilayer complexation supported by implicit bilayer simulations. Our results show that complexation is related to the deformation of both the ligand monolayer and the bilayer to maximize favorable hydrophobic interactions while minimizing unfavorable insertion of charges into the bilayer core. This study will improve our understanding of bilayer interactions and enable the design of ligand-coated nanoparticles for drug delivery and biosensing applications.   

Morphological robustness in polythiophene/fullerene mixtures

The morphology of the photoactive layer of organic solar cells evolves differently under different processing conditions such as annealing temperature, annealing time and casting solvent. Hence, characterizing it is crucial in understanding its effect on device performance. In our study, we used Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Energy-Filtered Transmission Electron Microscopy (EFTEM) to characterize the in-plane morphology of poly(3-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) mixtures. We found that the characteristic length scale determined through GISAXS did not vary significantly for different processing conditions thus making P3HT/PCBM a robust system. For example, different spin-casting solvents did not significantly affect lateral phase separation, and consequently, device performance was similar once thickness effects are accounted for.   

Thin-film fabrication and electric field induced poling of an azo dye-doped cyclic olefin copolymer

Cyclic olefin copolymers (COC) exhibit high transparency, low birefringence, and a high glass transition temperature, which make them promising as host materials for nonlinear optical chromophores. However, the non-polar COC environment limits chromophore loading, resulting in potentially weak nonlinear optical functionality. In this research, we fabricated COC films doped with the azo dye Disperse Red 1 (DR1), and corona poled the films to establish dipolar chromophore order. Second-harmonic generation experiments were used to probe chromophore orientational order during the poling process and to monitor the subsequent temporal relaxation. Preliminary experiments indicated a maximum loading of 5 wt\% DR1 in COC, and that the effective second order nonlinear optical coefficient decayed by 50\% within two weeks following poling.   

Temperature Dependent Study of Regioregular and Regiorandom Poly (3-hexyl thiophene)s

Structure evolution of bulk regioregular (rr) and regiorandom (rra) poly (3-hexyl thiophene)s (P3HT) with temperature has been characterized by wide angle X-ray diffraction (WAXD). Different thermal behaviors associated with the inter- and intra-molecular packing were shown as regioregularity changes. Transition temperature of $\sim $100$^{o}$C for crystalline rr-P3HT has been assigned to side chain melting point where the thermal expansion along the edge-on direction shows a turning point separating solid and molten hexyl side chains. Meanwhile, $\pi -\pi $ stacking distance was observed to initially decrease and then increase upon heating, with a minimum at $\sim $100$^{o}$C. The average inter-molecular distance of rra-P3HT was independent of temperature while the average intra-molecular distance increased upon heating. Molten rr-P3HT and rra-P3HT show same inter- and intra-molecular distance. Temperature UV-Vis absorption and photovoltaic device performance were also measured.   

Additive-Driven Assembly of Block Copolymer-Nanoparticle Hybrid Materials for Solution Processable Floating Gate Memory

The preparation of well-ordered hybrid materials at nanoscale is not only fundamentally interesting but also of significant importance for the development of next generation functional devices. In this study, we present a simple approach for the preparation of well-ordered polymer/NP composites through the concept of additive-driven assembly, and its application for the fabrication of floating gate organic FET memory devices. The addition of gold NPs that selectively hydrogen bond with pyridine in poly(styrene-$b$-2-vinyl pyridine) is shown to induce an ordered structure. This enables the fabrication of well-ordered hybrid materials with lamellar domains at Au NP loadings of more than 40 wt{\%}. The fabrication of floating gate memory devices was demonstrated by the ordered Au NPs / block copolymer hybrid film as a charge trapping layer, which is sandwiched between a SiO$_{2}$ dielectric layer and a poly(3-hexylthiophene) semiconductor layer. This approach enables us to fabricate well-ordered charge storage layers by solution processing and to achieve facile control of the memory windows by changing the density of gold NPs. The devices show high carrier mobility ($>$ 0.1 cm$^{2}$/Vs), controllable memory windows (0$\sim $50V), high \textit{on/off} ratio ($>$10$^{5})$ between memory states and long retention times ($>$10$^{4}$ s). This approach is potentially suitable for roll-to-roll printing techniques to make flexible, large area and high density devices.   

Classification of Semiconducting Polymeric Mesophases to Optimize Device Post-Processing

Semiconducting polymers form a variety of phases and mesophases that respond differently to post-deposition solvent or thermal treatments. Here it is shown that classification of these materials into their appropriate mesophases can be a useful tool to optimize their post-deposition treatments. Calorimetry is used to quantify differences between very similar materials, using a well-established framework based on the kinetics and thermodynamics of phase changes. By way of example, this classification scheme is used to identify differences in the presence and distribution of mesophases in three polymers, poly(3-hexylthiophene) and two isomeric bithiophene-thiophene copolymers (pBTTT and pATBT). The diverse phase structure is notable in light of the molecular similarity of the three polymers, and it has impact on optimum post-processing conditions for maximum electrical performance in thin film transistor devices.   

Conformation Effects on the Photoluminescence Behavior of Anchored MEH-PPV Pancakes and Brushes

Single molecular layer of poly[2-methoxy-5-(2'-ethylhexyl)oxy)-1,4- phenylenevinylene] (MEH-PPV) grafted on primed silicon wafer were synthesized, forming brushes (chain spacing ~0.54 nm via graft-from) or pancakes ($\sim$ 7nm to 34 nm via graft-to). For the tight-packed brushes, the PL emission peak, residing in the range from 434 nm to 550 nm depending on the chain length, was generally unchanged when transferring between the dry and solvent immersion states. However, for the pancakes, the emission peak blue-shifted dramatically (up to 100 nm) when dried in the air relative to that in the solvent. These shifts were fully reversible in the dry-wet cycles. The large blue shifts of the anchored pancakes were attributed to the mechanical stretching of entangled MEH-PPV segments in contact with substrate upon solvent loss. In contrast, the blue shifts disappeared and small red shifts emerged instead for extremely slowly drying (24 hrs drying time), revealing the stress-relaxation pathways in the equilibrium conditions. The drying-induced blue shift was also observed in the un-anchored drop-casting samples but the reversibility vanished. Finally, a large enhancement of PL intensity was accompanied with the blue shifts, manifesting the effect of the molecular constraints.   

Properties of Alq$_{3}$ waveguides containing embedded metal layers

We study the properties of aluminum-quinoline (Alq$_{3})$ waveguides with embedded thin (few 10 nm thick) metal layers using the m-line technique at a wavelength of 633 nm. Our goal is to investigate how the guided TM and TE modes and their effective refractive indices are affected by the metal layers. The layered waveguides are fabricated on a glass substrate by organic molecular beam deposition (OMBD). A Mg$_{0.9}$Ag$_{0.1}$ alloy is used for the metal layers. Pure Alq$_{3}$ waveguides serve as reference samples. Our experiments show that TM modes in an Alq$_{3}$ waveguide with a single centered metal film are nearly unaffected, except for a slight increase of the bulk refractive index in the sample. TE modes are more strongly affected. Compared to our reference sample, we do not observe the TE$_{0}$ and TE$_{2}$ mode. Other TE modes whose electric field nodes are at the location of the metal film are visible. A similar behavior is also found in waveguides with embedded multiple metal layers. Our experimental data is compared to a multi-layer model simulation and to an effective-medium model. The results indicate that strategically placed metal layers can potentially be used to tailor waveguides structures.   

Mechanical deformations of the metal cathodes of polymer light emitting devices during stability tests

The polymer light emitting devices (PLEDs) include materials with substantially different mechanical properties - oxides as anode, organic polymers as light emitter, and metals as cathode. The typical expectation is that this mix of materials would be a problem if there is an exposure to large temperature variation. Our studies on the stability of the PLEDs give evidence that even without substantial heating of the devices, mechanical deformations appear. They are manifested in delamination and loss of contact area with all associated negative impact on the light emission from the devices. Our optical and scanning electron microscopy images show round shaped cathode deformations with enhanced degradation of the polymer along their periphery. In order to check the hypothesis of build-up of mechanical stress in the metal films during their deposition, we have used thermally evaporated aluminum cathodes with different thickness. We have used anodes different than the traditionally used ITO film in order to verify if an oxygen evolution is responsible for the formation of the mechanical deformations. We will show results on how a buffer layer between the light emitting polymer and the cathode influences the formation of the mechanical deformations.   

N-type Organic Field Effect Transistor Using Densely Aligned Carbon Nanotube Array Electrodes

We present fabrication of n-type organic field effect transistors (OFETs) using densely aligned array carbon nanotube (CNT) electrodes. The CNTs were aligned with a high linear density via dielectrophorosis (DEP) from an aqueous solution. In order to fabricate the CNT electrodes, aligned CNTs were cut by electron beam lithography (EBL) and precise oxygen plasma etching. The OFETs were fabricated in a bottom-contact configuration by depositing a thin film of C60 molecules between the CNT source and drain electrodes, and compared against a controlled OFET with gold electrodes. The room temperature electron transport measurements of the OFETs using CNT electrodes show better transistor characteristics compare to OFETs using gold electrodes due to improved charge injection from densely aligned and open-ended nanotube tips.   

Thermal Analysis of defects in Organic Light Emitting Diodes using Thermoreflectance imaging technique

Organic light emitting diodes (OLEDs) have emerged as a next generation technology for flat panel displays. One of the factors inhibiting commercialization of this technology are their shorter life spans. Various defects induced during fabrication and operation tend to grow with time causing catastrophic failure of the device. In our work, we use thermoreflectance imaging (TR) to study defects in OLEDs. TR is based on measuring the change in reflectivity of the device as a function of temperature. We want to study the defects before catastrophic failure. We're interested in the physical origin of the defects, under what conditions failure occurs, and how defects affect current injection uniformity. Several defects including dark spots and bright spots on the surface of OLEDs are visible through the TR imaging. In our experiment, we are making controlled defects in OLEDs to study specific thermal maps of various kinds of defects. Understanding local temperatures and heat spread via TR imaging will provide better insight into formation of defects, which could be used to increase the lifetime and efficiency of OLEDs.   

Correlation of interfacial width with device characteristics in all-polymer thin film transistors based on P(NDI2OD-T2)

The interface between a conjugated polymer and a dielectric is critical to the performance of an organic thin-film transistor device, since charge transport occurs essentially along a 1 nm deep accumulation layer in the semiconducting polymer at the interface with the dielectric. Utilizing resonant soft x-ray reflectivity, we measure the interfacial widths of poly([N,N'-bis(2- octyldodecyl)- 11 naphthalene- 1,4,5,8- bis(dicarboximide)- 2,6-diyl]-alt- 5,5'-(2,2'-12 bithiophene)) and differentially cast dielectric layers of polystyrene (PS), poly(methyl methacrylate) (PMMA) or CYTOP, respectively. We demonstrate that devices with PMMA as the dielectric layer show the sharpest interface, whereas CYTOP has the largest. The measured widths correlate with the onset voltages of the thin-film transistor devices and anticorrelate with the activation energies in these three systems. A comparison to the surface roughness prior to differential casting furthermore indicates that deposition of the top dielectric layer affects the thus-formed interfaces to a different extent for various dielectric materials. Overall, the results suggest that further control of the interfacial properties during fabrication is required for ultimate performance.   

Effect of bridging atom identity on the morphology of solution-processed small molecule bulk heterojunction photovoltaics

Organic bulk heterojunction photovoltaics have gained much attention based on their potential to be low-cost, large area modules. Solution-processable small molecules are attractive materials due to their well-defined nature and purity. We examine two small molecules that differ by the identity of one atom: 5,5'-bis{\{}7-(4-(5-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine{\}}-3,3'-di-2-ethylhexylsilylene -2,2'-bithiophene, (\textbf{SM}$_{Si})$ and its carbon-bridged analog, (\textbf{SM}$_{C})$. Despite having similar absorption spectra and energy levels, \textbf{SM}$_{Si}$ and \textbf{SM}$_{C}$ have different thermal transitions and device performance. \textbf{SM}$_{Si}$ melts at 215\r{ }C, while \textbf{SM}$_{C}$ melts at 145\r{ }C, and similarly annealed devices based on blends of \textbf{SM}$_{Si}$ or \textbf{SM}$_{C}$ with [6,6]-phenyl-C$_{71}$-butyric acid methyl ester (PC$_{71}$BM) show power conversion efficiencies of $\sim $3.4{\%} and $\sim $1.0{\%}, respectively. To determine the origin of this difference, techniques including grazing incidence wide angle X-ray scattering, UV-Visible spectroscopy, optical microscopy, and \textit{in situ} current-voltage measurements were employed to monitor morphological changes during thermal annealing. \textbf{SM}$_{C}$:PC$_{71}$BM devices exhibit a greater propensity to crystallize into large domains upon annealing, which is detrimental to performance. These results highlight the connections among structure, morphology, and performance and provide insight into the design and characterization of photovoltaic materials.   

High Optical Quality Organic Thin Films for Nonlinear Photonics Fabricated by Molecular Beam Deposition

We use small donor-acceptor substituted organic molecules that sublimate without decomposition to fabricate organic thin films by organic molecular beam deposition. These thin films have thicknesses of the order of micrometers and are well adapted for integrated nonlinear optics. These films are essentially amorphous, without the formation of microcrystals. They combine a high third-order susceptibility of the order of 1000 times that of fused silica with a high optical quality. Surface roughness is below +/- 5 nm for micrometer thick films. The films have been shown to be durable and robust, with a long shelf life (>2 years). We successfully integrated such films with silicon-on-oxide waveguides which have been used to demonstrate ultrafast all-optical switching on the silicon photonics platform, successfully demultiplexing a 170 GBit/s signal to 42 GBit/s.   

Nanoscopic Assembly of Organic Semiconductors for Organoelectronic Devices

Small molecule organic semiconductors have many advantages over their polymer counterparts, including high purity and well-defined electronic properties. To fabricate organoelectronic devices, they need to form smooth thin films with control over their spatial organization to tailor the electronic percolation pathway. A quaterthiophene (4T) containing alkyl and phenolic moieties was hydrogen-bonded to the 4-vinylpyridine groups of a block copolymer, polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) or a homopolymer, P4VP. These supramolecules can be readily cast into uniform films where the 4Ts form oriented nanostructures without hindering the charge mobility. We also extended this supramolecular approach to blends of symmetrically and asymmetrically-functionalized organic semiconductors at various mixing ratios. Fundamentally, present studies shine light on how to synergize two self-assembly processes, i.e. molecular ordering of organic semiconductors and microphase separation of BCP, and provide useful guidance toward directed hierarchical assemblies in multi-component systems. Present studies may also open a new route for the fabrication of nanostructured thin films of organic semiconductors using solution processing.   

Connecting the Nanodots: Programmable Nanofabrication of Fused Metal Shapes on DNA Templates

We present a novel method for producing complex metallic nanostructures of programmable design. DNA origami templates, modified to have DNA binding sites with a uniquely coded sequence, were adsorbed onto silicon dioxide substrates. Gold nanoparticles functionalized with the cDNA sequence were then attached. These seed nanoparticles were later enlarged, and even fused, by electroless deposition of silver. Using this method, we constructed a variety of metallic structures, including rings, pairs of bars, and H shapes. Due to the flexibility of the design these structures may offer great promise for electronic and plasmonic applications.   

$\pi -$Conjugated Copolymers of Thiophene: Effect of Chain Architecture on the Physical and Optoelectronic Properties for Photovoltaic Applications

We found that polymer chain architecture strongly influences phase separation capabilities of the donor-acceptor blend in bulk heterojunction organic photovoltaic devices. Ni-catalyzed controlled polymerization was utilized to access new conjugated copolymers of 3-hexylthiophene and 3-(hexyloxy)methylthiophene, two donor polymers. Monomer sequence was controlled along the copolymer chain by the rate of addition of the comonomers, to achieve diblock, random and gradient copolymer chain architectures. This allowed us to study the effect of copolymer sequence of polythiophene based copolymer/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend on the structure, nanoscale morphology and local charge transport properties using conductive and photoconductive atomic force microscopy. The gradient configuration showed the largest phase separation behavior with PCBM.   

Organic-Inorganic Nanocomposites by Placing Conjugated Polymers in Intimate Contact with Quantum Rods

Semiconductor organic-inorganic nanocomposites were synthesized by directly grafting conjugated polymer poly(3-hexylthiophene) onto cadmium selenide nanorods surface (i.e., P3HT-CdSe NR nanocomposites). The direct grafting was accomplished by two simple yet robust coupling reactions: Heck coupling of vinyl-terminated P3HT with bromobenzylphosphonic acid functionalized CdSe NRs (i.e., BBPA-CdSe), and a newly developed catalyst-free click reaction of ethynyl-terminated P3HT with azide functionalized CdSe NRs. Such rationally designed nanocomposites possessed a well-defined interface between P3HT and CdSe NRs, thereby promoting the effective dispersion of CdSe NRs within the nanocomposites and facilitating their electronic interaction. The success of grafting was confirmed by nuclear magnetic resonance spectroscopy and dynamic light scattering. The occurrence of charge transfer at the P3HT/CdSe interface was evidenced by UV-Vis absorption, photoluminescence (PL), and time-resolved PL studies. Notably, the nanocomposites prepared by the catalyst-free click reaction exhibited a faster charge transfer from P3HT to CdSe. These nanocomposites offer a maximum interfacial area between the constituents for efficient exciton dissociation. As such, it represents a significant advance in rational design and fabrication of organic-inorganic hybrid solar cells with improved power conversion efficiency.   

Studies on Morphology of Three-component Polymer Solar Cells

We prepared solar cells with an active layer that contains three components, including poly(3-hexylthiophene) (P3HT), a low bandgap polymer poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). P3HT and PCPDTBT have complementary light absorption spectra such that, in parallel, the two cover a very broad range of the solar spectrum. It has been demonstrated that the power conversion efficiency of the P3HT/PCPDTBT/PCBM system is enhanced over that of either P3HT/PCBM or PCPDTBT/PCBM. Understanding the morphology developed for this system, therefore, can provide valuable insight into enhancing the performance of these technologically relevant and providing a fundamental challenge in controlling the phase behavior of such mixtures that undergo ordering. Morphological studies showed that the crystallization dynamics of P3HT was influenced by the presence of PCPDTBT. There are two domains existing in the thin films, one is pure P3HT domains and another is PCBM-rich domains mixed with amorphous PCPDTBT and P3HT.   

Diketopyrrolopyrrole (DPP)-based low band gap polymers for efficient solar cells

For bulk heterojunction (BHJ) organic photovoltaic (OPV) devices, effective strategies to maximize the performance have to be developed and fundamentally understood. In BHJ-type solar cells, the ability to control and optimize the active layer morphology is critical. A BHJ OPV of diketopyrrolopyrrole (DPP)-based low band gap polymer with phenyl-C71-butyric acid methyl ester (PCBM) was studied. DPP-based polymers are highly crystalline. The use of mixed solvents was critical in developing an optimal BHJ morphology, that was characterized by GISAXS and GIWAXS. In thin films, the polymer adopts an edge-on orientation and the blends are phase separated without thermal annealing. The diffusion of PCBM into the DPP polymer is markedly different from that observed with P3HT. The development of the morphology during solvent evaporation was studied in real-time by x-ray scattering and diffraction.   

From carbon dioxide to organic compounds: Novel cathode materials for microbial electrosynthesis

Electrode materials play an important role in the production of organic compound during microbial electrosynthesis, which uses bacteria as a catalysts to reduce carbon dioxide to organic compounds and stores electric energy in carbon-carbon bonds. An ideal electrode generally has high surface area, high electric conductivity, physical and chemical stability, and biocompatibility with bacteria. Based on these criteria, three types of materials were considered for the electrode design: CNTs, metal, and conductive polymers. Through a combination of these three materials, different properties were incorporated onto the electrodes for microbial electrosynthesis. The morphologies of the electrodes were characterized by high resolution TEM, SEM, and laser scanning confocal microscopy. The productivity of organic compounds was also verified.   

Dynamics of Poly(ethyleneoxide) in the Presence of LiTFSI: Neutron Spin Echo and Dielectric Spectroscopy Study

Rechargeable lithium-ion batteries based on solid polymer electrolytes (SPEs) offer many advantages over their liquid counterparts. In comparison to a liquid electrolyte, the solid polymer is less flammable and more environmentally friendly. Issues such as microscopic dynamics of lithium ions and their dependence on their polymeric matrix are known to play a key role in determining the ionic conductivity in SPEs. Therefore, understanding of the microscopic dynamic characteristics of the lithium ions in relation with the dynamics of surrounding polymeric matrix is a crucial step for the interpretation of their transportation behavior and ultimately toward the control of their properties. Here, we investigated dynamics of poly(ethylene oxide) (PEO) in the presence of LiTFSI using neutron spin echo and dielectric spectroscopy techniques. Experimental results suggest that the dynamics of PEO is dramatically slowed down and strong correlation between ion transportation and alpha-relaxation of PEO.   

Effect of Solvating Plasticizers on Ion Conduction of Polysiloxane Single-ion Conductors with Lithium Tetraphenyl Borate and Cyclic Carbonate Side Chains

We synthesize polysiloxane single-ion conductors containing two side groups: cyclic carbonates as a polar group and weak-binding tetraphenyl borate anions with Li$^{+}$ counterions as an ionic group. With increasing ion content, the ionomer T$_{g}$ increases because of ion aggregation, making the conductivity drop greatly. To enhance segmental mobility and decrease the tendency for ion aggregation, this ionomer was plasticized with poly(ethylene glycol) (M$_{n}$ = 600, PEG600) to various extents. The room temperature conductivity of the plasticized ionomer is 3 orders of magnitude higher than that of the neat ionomer. Addition of PEG600 increases ion mobility by lowering T$_{g}$ and increases conducting ion content due to raising the dielectric constant. This suggests that PEG600 plays an important role by solvating Li$^{+}$ so as to lower T$_{g}$ by dissolving ionic aggregates, consistent with X-ray scattering, which shows the ionomer aggregation peak decreases in intensity as PEG600 is added.   

Fine-tuning Structures from Molecules to Nanophases: Insight into the Origin of Superior Organic Photovoltaic Efficiency

Organic photovoltaics (OPV) represent one of the most promising technologies for next-generation solar energy conversion due to their low-cost and scalability. To realize this potential, efficiencies must be improved for which a deeper understanding of the nanoscale morphology and molecular organization is required. Using the state-of-the-art PTB series of conjugated copolymers synthesized at the University of Chicago, we probed the internal structure of these materials both in solution and in films containing polymer/fullerene blends using a suite of tools spearheaded by neutron and x-ray scattering and, thereby, conceive the structural evolution from solution to thin films. Fine-tuning molecular structures via selective atomic replacement on the main chain of PTB copolymers, we gained unique insights into the structure-performance relationships, especially key features such as intermixing of polymers with fullerenes. Progress established in the course of these structural and morphological characterizations outline above will serve as the foundation for further improving the efficiency of polymer solar cells to realize their large-scale commercial use.   

Tuning Ion Conducting Pathways Using Holographic Polymerization

While much research has demonstrated repeatable characteristics of electrolyte membranes, the fundamentals behind the interactions during ionic diffusion in solid polymer electrolyte membranes for battery applications are not well understood, specifically the role of nanostructures, which hold the key to improving performance of energy storage devices such as fuel cells and Lithium ion batteries. The challenges in fabricating highly controlled model systems are largely responsible for the interdependent ambiguities between nanostructures and the corresponding ion conducting behavior. In this work, Holographic Polymer Electrolyte Membranes (hPEM) volume gratings comprised of alternating layers of crosslinked polymer resin and lithium ion salt were fabricated using holographic polymerization with an average d-spacing of approximately 200 nm. These one-dimensional confinement structures were used to quantitatively study the anisotropic ionic conductivity between the directions of in-plane and normal to the layers, and the unique ion conducting behavior was correlated with nanoscale phase separation. These volume gratings also offer an exciting route to fabricate multifunctional gratings for optic and sensing applications.   

Tuning interfacial interactions between conjugated polymers and inorganic nanocrystals for control over bulk thermoelectric properties

Electronic transport in conjugated polymers is highly dependent on doping level and chain conformation. Recent advances in next-generation thermoelectric devices rely on the fabrication of conjugated-polymer:inorganic-nanocrystal composites, but little is known about the how the thermoelectric properties are influenced by the doping level or morphology of the conjugated polymer at the organic-inorganic interface. A thorough understanding of the coupling between the molecular orbitals of conjugated polymers and the energy states of inorganic materials in such systems is, therefore, paramount for achieving improvements in these devices. In this study, we present our results on the coupling between Poly-(3-hexylthiophene) (P3HT), a well-studied conjugated polymer, with solution-processed lead selenide (PbSe) nanocrystals of controlled shape and morphology. Conjugated oligomeric ligands are used to tailor the organic/inorganic interface between P3HT/PbSe. Our results address the effects of polymer conformation on the nanocrystal surface, polymer doping levels, and organic-inorganic energy level alignment on the bulk thermoelectric properties of these hybrid materials.   

Donor-acceptor block copolymer self-assembly in polymer-based photovoltaics

Polymer-based photovoltaics represent potentially low-cost, solution-processable devices for achieving sustainable energy generation. The optimal polymer-fullerene bulk heterojunction photovoltaic relies on a phase-separated microstructure in which domains of each component exist to allow for exciton dissociation at the interface and transport of each free electron (hole) through the n-type (p-type) domain to the cathode (anode). Due to the kinetically trapped, complex microstructure and phase-impure domains formed in this multi-component system, the development of a single-component material with pure n- and p-type domains is of great interest. Here we explore the microstructural evolution of a diblock copolymer of donor and acceptor segments, P3HT-b-DPP, due to various thermal treatments. Atomic force microscopy (AFM), resonant soft X-ray scattering (RSoXS), and grazing incidence wide-angle X-ray scattering (GIWAXS) are exploited to demonstrate that thermal treatments above the melting transition of each segment lead to an ordered domain structure containing both types of crystals. The effect of cooling rate from the melt on domain size and crystalline structure within those domains is investigated.   

Synthesis and Characterization of a Systematic Series of All-Conjugated Diblock Copolymers

All-conjugated block copolymers can potentially self-assemble into nanoscale structures beneficial for charge separation and transport, but due to synthetic challenges a comprehensive investigation of all-conjugated block copolymers has not been carried out . Here we detail a novel synthetic approach to all-conjugated block copolymers and characterize the structure of a systematic series of materials. The materials are prepared via copper-catalyzed azide-alkyne click chemistry followed by selective solvent removal of homopolymer impurities. This allows us to readily vary the molecular weight and type of each block in order to systematically study the properties of a family of block copolymers. As a system relevant to organic photovoltatics, we investigate a series of diblock copolymers based on poly(9,9-dioctyl-fluorene) and poly(3-alkylthiophene). This series of block copolymers is characterized with respect to phase behavior, including micro-phase segregation and crystallinity, optical properties, and charge mobilities.   

Incorporation of Functionalized Metal Oxides into Poly(Ethylene Oxide) Based Solid Polymer Electrolytes for Lithium-Ion Batteries

A detailed study on the influence of size and shape as well as surface properties of metal oxides incorporated into poly(ethylene oxide) based solid polymer electrolytes for lithium-ion batteries is explored. The morphology of the solid polymer electrolyte is determined using small angle x-ray scattering and transmission electron microscopy. The surface properties of the metal oxide are modified through silane chemistry. Surfaces are modified to be hydrophobic, hydrophilic, high dielectric constants, as well as single-ion conductors. The electrochemical properties of the solid polymer electrolyte systems are investigated by electrochemical impedance spectroscopy and cyclic voltammetry. These functionalized metal oxide particles not only reduce crystallinity of poly(ethylene oxide) based solid polymer electrolytes, but also increase the conductivity.   

Morphology Study of Bulk Heterojuction Solar Cells Based on PCDTBT

To achieve high efficiency, the processing of conditions bulk heterojunction photovoltaic are important so that the desired morphology favoring efficient charge separation, electron and hole transportation can be generated. Solar cells based on poly[N-9"-hepta-decanyl-2,7-carbazole-alt- 5,5-(4',7'-di- 2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT) as the donor system have been made. The effect of solvent, additives and annealing temperature have been systematically investigated. Grazing incidence small angle scattering (GI-SAXS), wide angle scattering (GI-WAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were utilized to study the morphology.   

Comb-branched Polymer Electrolytes: Architectural Changes That Promote Increased Li$^{+}$ Transport

The use of solid polymer electrolytes (SPEs) in the batteries of next generation technology applications is promising due to their safety and stability, yet is also hindered by low conductivity and high temperature requirements. Recent simulations of comb-branched poly(epoxide ether)-based SPEs have shown the comb-branched architecture to be able to overcome some of these hindrances. While providing the advantage of preventing crystallinity and allowing the optimization of the backbone to be decoupled from that of the side chain, the comb-branched SPE studied suffers from slow Li$^{+}$ cation dynamics due to coordination with the backbone. To improve Li$^{+}$ transport, we have modified the architecture of the comb-branched poly(epoxide ether) by attaching the side chains to the backbone with non-coordinating, flexible spacers. Inclusion of these spacers has resulted in a five-fold increase in the diffusion of Li$^{+}$ cation effected through 1) an increased side chain flexibility, 2) a decreased interaction of the cation with the dynamically slow backbone, and 3) quick conformational dynamics of the entire side chain. Additionally we report on other desirable architectural changes, such as including carbonate solvating moieties, to comb-branched SPEs allowing enhanced mobility of Li$^{+}$.   

Fluid Properties in Electromagnetic Fields

We study the effect of electromagnetic fields on the fluid properties to see if the modifications in the fluids properties are causing the effect of magnetic fields on the bacterial growth. For this purpose we design a simple experiment to study the changes in spectrum of light through the nutrient broth that is the fluid used to study the bacterial growth.   

Electro-coflow as a route to drop generation

We couple electric and hydrodynamic forces in glass-based microfluidic devices to generate droplets in steady state. The size of the drops can be larger, comprable and smaller than the tip diameter, which is the smalles geometric feature of the device.   

Water nano-hydrodynamics: The interplay between interfacial viscosity, slip and chemistry

The understanding and the ability to manipulate fluids at the nanoscale is a matter of continuously growing scientific and technological interest. Fluid flow in nano-confined geometries is relevant for biology, polymer science and geophysics. The applications range from gene sequencing to protein segregation, cell sorting, sensors, nanotribology and diffusion through porous media. Here, we present experiments which show how the interfacial viscosity of water strongly depends on the wetting properties of the confining surfaces. This dependence is fully explained by considering water slippage at the stationary solid surface. The interfacial viscous forces as a function of six surfaces with different wettability are fitted with a modified form of the Newtonian definition of viscosity, which takes into consideration the fluid slip. This simple relationship can explain the viscosity measurements and permits us to extract a ``slip parameter'' for each investigated surface. This slip parameter is found to increase with the static contact angle of the solid surface as expected from previous work, bringing clear evidence of the relationship between viscosity and slip.   

Investigating the hydrodynamics of asymmetrically driven polymer brushes in micro-channels

The interfacial hydrodynamics between a polymer brush, under the influence of an asymmetric external driving field, and the surrounding solvent is explored across varying grafting densities, field strengths, and polymer chain lengths. A soft core model based on dissipative particle dynamics is used along with a 2-body FENE potential and a 3-body harmonic potential which define the internal mechanics of the polymer chains. The morphology of the polymer chains and the shearing produced at the interface layer between the polymer and solvent regimes is investigated as a function of hydrodynamic coupling to the external driving field. The momentum transfer and drag forces at the interface are shown to be the primary factors which lead to coupling and a directed non-zero net flow of the surrounding solvent through the coplanar nano-channel. The aim of the model is to introduce a novel mechanism to circumvent the large pressure gradients necessary to sustain fluid flow through micro/nano-pores as a consequence of Poiseuille's law.   

Solid Polymer Electrolytes by Self Assembly of Multi-Ionic Janus POSS Li-Salts in Polyethylene Oxide

Solid polymer electrolytes (SPEs), of which the most investigated has been polyethylene oxide (PEO), have low room temperature (RT) ionic conductivities and low lithium ion transport numbers. PEO is semicrystalline, with a glass transition temperature, T$_{g}$, of -60 $^{o}$C and a melt temperature, T$_{m}$ of $\sim $ 65 $^{o}$C. Since lithium ion conduction occurs in the amorphous phase of PEO, conductivities $>$ 10$^{-4}$ S/cm are only obtained after the crystalline domains have melted. Although high concentrations of lithium salts can eliminate the crystallization that causes the low conductivity at RT, the result is an amorphous liquid, not a solid. In order to impart structural rigidity to a noncrystalline PEO at RT, we have developed Janus-like multi-ionic lithium salts, in which one half of the salt is ionic in character, and the other half is hydrophobic. These salts are highly dissociative, imparting RT conductivities $>$ 10$^{-4}$ S/cm and the hydrophobic ends aggregate. A phase separated morphology is formed in which the hydrophobic moieties form crosslink sites, and impart a solid state morphology, and the Li$^{+}$ ions dissociate into the conductive, amorphous PEO phase.   

Molecular-based polyhedral crystalline colloids

We fabricate a new family of colloids with polyhedral morphology by controlled crystallization of metal ions and organic bridging ligands in the presence of capping regents. The size and morphology of the crystalline colloids can be tuned by changing the ratio of the metal ions to the organic ligands and the amount of the capping regents. Unlike spheres that isotropically interact along a curved surface, the polyhedral particles in suspension associate in a directional facet-to-facet fashion, forming clusters whose elemental units are orderly not only in interparticle distance but also mutual orientation. We also present unique supraparticle structures obtained by mixing different particles.   

Molecular dynamic simulations of hydrophilically decorated poly-dots

The structure and interfacial characteristics of nanoparticles formed by collapsed single conjugated polymer chains decorated with hydrophilic groups, poly \textit{para} phenylene ethynylene (PPE) with substituted carboxylate side chains, have been studied by molecular dynamic simulations. These particles constitute a new type of light emitting/harvesting entities. Their interactions with the surrounding will determine their use where for example, directed interactions with membranes are essential for penetrating into organisms as bio markers. Trapping pre-decorated polymer chains into the nano dimensions with the expectations that some of the decorated groups will statistically reside at the nanoparticle interface has opened the way to control the surface decoration and hence the interactions of these particles. The effects of the distribution of the decorating groups along the polymer backbone on the conformation of the polymer chain within the poly dot and the resulting interfacial distribution of the COONa groups will be presented. The distribution of the hydrophilic groups at the surface of the particles will direct their interactions with the environment.   

Optical Force Probe Microscopy of the Pericellular Matrix

The pericellular matrix is a microns-thick grafted polymer film on the surface of cells. Its structure and mechanics influence processes as diverse as filtration, cell adhesion, proliferation, migration, cancer metastasis and possibly mechanotransduction. Optical force probe microscopy enables dynamic and equilibrium measurements of this polymer film on living cells. We show that equilibrium force measurements can be related to the osmotic pressure in the pericellular matrix, leading to a prediction of a spatially varying correlation length (mesh size) profile ranging from $\sim $100 nm at the cell surface to 1000 nm near the edge of the cell coat. Assuming the film is brush-like, comparison to polymer brush theory provides estimates of the equilibrium brush length and the grafting density at the surface. The equilibrium length is consistent with that observed during dynamic force measurements, and the grafting density is close to that of maximal packing for the large, space filling molecules in the system - i.e. tethered polymers populated by semi-flexible side chains with cross sections~$\sim $80 x 400 nm$^{2}$.   

Mapping optical trapping energy of nanoparticles via confocal microscopy

Optical traps are highly focused laser beams that can hold and manipulate objects of microscopic scale. They are used to study the motion and energy of particles such as colloids or DNA molecules. In order to achieve this purpose we first propose to determine the energy of optical traps. We use a fluorescent nanoparticle ensemble within the optical trapping volume along with confocal microscopy to map the fluorescence intensity distribution of nanoparticles. This mapping allows us to calculate the trapping energy profile in three dimensions. We repeat this process with different trapping powers to find the depth of the trapping potential well as a function of trapping power. Trapping energy per trapping power thus measured is consistent with results obtained through previous methods such as fluorescence correlation spectroscopy conducted by our research group. With this technique of mapping trapping energy we can further study particle-particle interactions.   

Structural analysis of three-dimenstionl photonic crystals in nature

We studied the structural origin of the color and photonic band structure in exoskeletons of Eupholus weevils and dorsal wings of lycaenids butterflies. The internal structures of the insects were systematically investigated using focused ion beam (FIB) milling, and the optical response of the insects was observed by optical microscopy and a microspectrophotometer. A series of sequential SEM images were obtained during the FIB milling process and 3D structures were reconstructed by image processing. The correlation of the structures and the optical responses were studied by theoretical modeling. Diamond-based 3D photonic crystal lattice existed in Eupholus weevils, while gyroid structure was in lycaenids butterflies. The calculated photonic band structures matched the measured optical response. Aluminum oxide and titanium oxide were deposited on the weevils and the butterflies in order to study the effect of refractive index contrast to the photonic band structure and the optical response.   

Structure and Dynamics of Polymer-Coated Nanoparticles in Ionic Liquids Studied by In-Situ Electron Microscopy

Ionic liquids (ILs) have unique solvent properties, including extremely low vapor pressure and high conductivity, which makes IL-solvated soft matter systems suitable to investigation by electron microscopy. ILs, as two-component solvents, may themselves organize into nanostructures, and these organizations can affect the behavior of dispersed polymers/particles. To understand these effects, the structure and dynamics of nanoparticles IL systems have been studied via multiple-particle tracking with electron microscopy. Several systems consisting of different polymer-coated nanoparticles, different ILs, and different substrates were prepared and analyzed with fluorescent microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Spatial and temporal imaging information affords insight into particle-IL and polymer-IL interactions.   

Polymer Conformation near the Critical Demixing Point of a Binary Solution

We have used Contrast Matching Small Angle Neutron Scattering (CMSANS) to probe directly the conformation change of polyethylene glycerol (PEO) chains in the critical demixing region of Acetonitrile-d3 in (D2O + H2O) at concentration of the components corresponding to zero-average contrast condition. The d-PEO and h-PEO were mixed to match the scattering length density (SLD) of the critical liquid solution, which allowed us to extract single-chain dimension of polymer molecules in the aggregates near the critical point of the solvent. A non-monotonic variation of Rg was detected as temperature approached the critical temperature of phase demixing of acetonitrile- water solution, which was attributed to the interaction asymmetry of the solvent molecules with polymers predicted by Brochard and de Gennes two decades ago. To our best knowledge, this is the first direct experimental evidence supporting this prediction.   

Fluorescent Polystyrene Sulfonate for Polyelectrolyte Studies

The slow-mode decay found by dynamic light scattering for polyelectrolytes in low-salt conditions has perplexed investigators since its first observation. Many characterization methods have suggested temporary or transient aggregation, although there is still no consensus on the cause. Many different polyelectrolytes demonstrate the slow-mode decay, but the sodium salt of polystyrene sulfonate (NaPSS) is the most popular choice for study. Commercially available NaPSS may have hydrophobic patches due to incomplete sulfonation leading to associations apart from any putative ionic mechanisms. Therefore, essentially full sulfonation, or ``patchless'', NaPSS should be synthesized. To facilitate fluorescence measurements, which can provide new insights to the slow-mode phenomenon, the material must be rendered fluorescent (F-NaPSS). Several approaches to F-NaPSS have appeared; some labeled a previously synthesized NaPSS without concern for its hydrophobic patches. Other strategies include a free radical copolymerization of styrene sulfonate and a vinyl amine to provide side chains viable for labeling. This method is successful, but yields only small amounts of nearly monodisperse polymer after fractionation. In this presentation, a high-yield synthesis of fully sulfonated, low-polydispersity, fluorescently tagged polymer will be discussed.   

Electromagnetic Propagationg of Waves in Helical Stochastic

We develop a model for studying the axial propagation of elliptically polarized electromagnetic waves in a spatially random helical media. We start by writing Maxwell equations for a structurally chiral medium whose helical angle contains both a stochastic contribution and a deterministic one, this latter corresponding to an uniform rotation. We write the electromagnetic equations into Marcuvitz Schwigner representation to transform them afterward by using the Oseen transformation. We exhibit that in the Oseen frame, Marcuvitz Schwigner equations turns out to be a linear vectorial stochastic system of equations with multiplicative noise. From this result and utilizing a well known formalism for treating stochastic differential equations, we find the governing equations for the first and second moments of the field amplitudes for a general correlation model for the slope angles, and calculate their corresponding band structure for a particular spectral noise density. We show that the average resulting electromagnetic fields exhibit dissipation and the appearance of a new reflection band whose chirality is the opposite of the one obtained for a simple cholesteric liquid crystals.   

Flow Instability of Soft Gels from Pluronic F108 Aqueous Solution Under Steady Shear

Nonionic surfactants of Pluronic tri-block copolymers have received special interest during the past decades because of the temperature dependent self-assembly characteristics that would lead to the formation of hydrogels upon heating. Here, we investigate the gelation behavior of Pluronic F108, (PEO)132-(PPO)50-(PEO)132, aqueous solution with an aim to elucidate how the shear affects the thermo-reversible transitions between micellar liquids and hydrogels. Specifically, we have studied the rheological characteristics of soft gels as an intermediate state between liquid to hard gels. From steady shear experiments, we found that there exists a shear rate window, where the flow instability of soft gels is observed. On the contrary, non-Newtonian behaviors following power-law are still observed at the shear rates above and below the shear rate window showing the flow instability. Small angle x-ray scattering and dynamic light scattering experiments had been performed to reveal how the temperature dependent rheological behavior correlates with the structural changes in the micellar aqueous solutions of F108.   

Fourth-order Fluctuations near the Colloidal Glass Transition

Fourth-order statistical fluctuations are probed in order to determine the size of cooperative regions as the colloidal glass transition is approached. The experiment makes use of the electrical conductance of a colloidal fluid containing KCl measured through a micropore in a membrane placed in the colloid. The diameter of this micropore is adjusted to be at least ten times the diameter of the colloidal particles. The dynamics of the particles near and within the pore are revealed in the fluctuations of the conductance. For the largest particles, a polydisperse sample of 106 to 125 microns, the dynamics are driven by a mechanical shaking, while for the smaller particles, $<$10 microns, Brownian motion drives the dynamics. Using a spherical probe, we mapped the conductivity change near the pore which enabled us to determine that the effective volume for the fourth-order fluctuations was $\sim $1.5D$^{3}$, where D is the diameter of the membrane pore. Thus, near the glass transition at a volume fraction of .58, the number of particles probed is about 1700 which is small enough to see the growth of fourth-order fluctuations. It is hoped that this technique can be applied to very small, $\sim $100 nm, particles for which the cooperative length can be studied over a wide range of volume fractions.   

Migration of Surfactant Vesicles during spontaneous emulsification

In aqueous AOT solutions, micrometer sized vesicles appear above the critical micelle concentration. These vesicle move spontaneously towards an oil interface and form an emulsion under the condition of ultra low interfacial tension. We discuss the probable mechanisms that lead to the migration behavior of these vesicles.   

Controlling Droplet Impact with Polymer Additives

When a water drop falls on to a hydrophobic surface, such as the waxy leaf of a plant, the drop often bounces off leading to wasted agrochemicals which harm the environment. However, adding small quantities ($\sim$100 $\mu$gml$^{-1}$) of a flexible polymer can completely prevent rebound. This is surprising since the shear viscosity and surface tension of such drops are almost identical to those of pure water. The effect has for some time been explained in terms of the stretching of polymer chains by a velocity gradient in the fluid, resulting in a transient increase in the so-called ``extensional viscosity.'' We have developed an epi-fluorescent microscope system, to visualise the flow of fluid inside an impacting drop using tracer particles at 2000 fps. Analysis of the velocity as a function of radius showed negligible differences between water and polymer drops except near the edge, indicating that the extensional viscosity cannot be responsible for the anti-rebound effect. To probe the true mechanism, fluorescently labelled ?-DNA was used to visualise the edge of an impacting drop. During the retraction phase, DNA was shown to be stretched by the retreating droplet providing an ``effective friction'' at the contact line. \\[4pt] [1] M.I Smith and V. Bertola, Phys. Rev. Letts. 104, 154502 (2010).   

Glints of light reveal properties of coiled electrospinning jets in flight

Optical observations of jets provide information needed to control features of polymer nanofibers produced by electrospinning. Quantitative information about the location, vector velocity, and rotation of selected segments of the multiply coiled path of an electrospinning jet of polymer solution was provided by a combination of videography, stereography, and stop motion flash photography. Visual observations of a jet depend largely on glints of incident light that are specularly reflected from particular places on the multiply coiled jet path. The traces of moving glints bifurcate when a new turn is added to a coil, at an observed rate of 600 turns per second. The polarization of light, in glints reflected at Brewster's angle, allows measurement of the index of refraction of the fluid in a jet, in flight. Strategic placement of lights to either illuminate, or leave dark, parts of the coils, make both the handedness and changes in handedness of the electrical bending coils apparent to visual observation. The changes occurred at intervals of a few seconds in our experiments. Evidence was found, in the form of ribbon-like glint traces that were polarized, for the occurrence of undulations on the surface of some jets.   

Digital Reconstruction of 3D Polydisperse Dry Foam

Dry foam is a disordered packing of bubbles that distort into familiar polyhedral shapes. We have implemented a method that uses optical axial tomography to reconstruct the internal structure of a dry foam in three dimensions. The technique consists of taking a series of photographs of the dry foam against a uniformly illuminated background at successive angles. By summing the projections we create images of the foam cross section. Image analysis of the cross sections allows us to locate Plateau borders and vertices. The vertices are then connected according to Plateau's rules to reconstruct the internal structure of the foam. Using this technique we are able to visualize a large number of bubbles of real 3D foams and obtain statistics of faces and edges.   

Asymptotic Behavior in Liquid Drop Coalescence

During coalescence, two drops first touch and then merge, as a liquid bridge grows from initially microscopic scales to a macroscopic size comparable to the drop diameter. The initial dynamics of coalescence are expected to be universal, owing to a singularity in the Laplace pressure, which diverges when the curvature of the liquid interface is infinite at the point where the drops first touch. Conventionally, this process has been thought to have just two regimes: a highly viscous one dominated by macroscopic flows pulling the two drops together and an inertial one described by local deformations near the growing neck. We use high-speed imaging, electrical measurements and full Navier-Stokes simulations to reveal a new regime that dominates the asymptotic dynamics of coalescence for any finite viscosity. The character of this new regime improves our understanding of the unexpectedly late viscous-to-inertial crossover [1]. An argument based on force balance and an appropriate choice of length-scales allow the construction of a new phase diagram of coalescence.\\[4pt] [1] J. D. Paulsen, J. C. Burton, S. R. Nagel, PRL 106, 114501 (2011).   

Carbon nanotube-induced chirality and macroscopic helical twist in achiral liquid crystals

A small quantity of carbon nanotubes was dispersed in an achiral liquid crystal, and the mixture was found to exhibit a weak degree of chirality both in the smectic and nematic phases. The induced chirality in the LC was probed by means of the electroclinic effect in the liquid crystal's smectic-$A$ and nematic phases, which showed significant pretransitional behavior on approaching the smectic-$A$ -- smectic-$C$ and the nematic -- smectic-$A$ transition temperatures, respectively, from above. The carbon nanotubes also were found to induce a bulk twist over macroscopic dimensions in an achiral nematic matrix. The nanotube-induced chiral pitch length $P$ was determined as a function of average nanotube concentration by measuring the radii of curvature of reverse twist disclination lines in 90\r{ } twist nematic cells. The results reveal information about the nanotubes' spatial distribution inside the cells. A concentration for the onset of significant aggregation of the nanotubes can be quantified from the apparent saturation of $P^{-1}$ at higher concentrations. The macroscopic helical twisting power of the nanotubes has been estimated from the results. The results indicate that there is a net chirality associated with the carbon nanotubes, which is transmitted into the achiral liquid crystal.   

Pretransitional Clusters in Multicolor Liquid Crystalline Honeycombs

X-shaped tetraphilic molecules consisting of a rod-like core with two hydrogen-bonding terminal groups and two mutually incompatible side-chains A and B form a range of honeycomb-like structures in which the rods act as bricks in the walls of polygonal cylinder cells containing the fluid side-chains. Some of these systems exhibit a 2nd-order transition from the high-temperature mixed (``1-color'') phase to a low-temperature phase in which the side-chains are separated in A and B cells (``2-color''). This is the situation with triangular, rectangular and square honeycombs. Strong pre-transitional 2-color domains formation is observed above the transition temperature. Particularly interesting is the case of the hexagonal honeycomb, where no fully phase-separated ground state can exist. Here the 2-color ``ordered'' phase consists of [A] cells and [A(1/4)B(3/4)] cells. The situation is similar to frustrated ferro- and antiferromagnets on a kagome lattice. Instead of the spins flipping, it is the molecules that undergo 180 degree rotations about the axis of their rod-like cores [Science 331, 1302 (2011)].   

Compaction of DNA with Lipid Modified Silica Nanoparticles

There is an increasing interest in modified inorganic nanoparticles, polymers or hybrid polymer-inorganic nanoparticles for use in DNA transfection, rather than viral vectors or liposomes. Adsorption of the DNA to the nanoparticles prevents enzymatic degradation of the DNA, although the reason for this protection is not completely understood. In order to compact the negatively charged DNA, a positively charged surface is required, and for transfection applications, the nanosystems must remain stable in suspension. It is also useful to minimize the amount of cytotoxic cationic lipid needed for DNA compaction in delivery applications. Here we investigate the colloidal stability of supported lipid bilayers (SLBs) composed of mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC, 14:0 PC) and 1,2-dimyristoyl-3-trimethylammonium-propane (DMTAP, 14:0 TAP), and their ability to compact plasmid DNA. Ionic strengths and DMPC/DMTAP ratios that resulted in SLB formation, no excess small unilamellar vesicles (SUVs) in the suspensions, and colloidal stability, were determined. DNA/SLB/lipid ratios that resulted in compaction were then investigated.   

Tunable and regenerative DNA zipper based spring

We report a DNA zipper based actuator device termed `DNA- spring' with tunable and repeated cycles of extension and contraction ability. DNA zipper is a double-stranded DNA system engineered to open upon its specific interaction with appropriately designed single strand DNA (ssDNA), opening of the zipper is driven by binding energy differences between the DNA strands. The zipper system is incorporated with defined modifications to function like a spring, capable of delivering approximately 9 pN force over a distance of approximately 13 nm, producing approximately 116 kJ/mol of work. Time-lapse fluorescence and fluorescent DNA gel electrophoresis analysis is utilized to evaluate and confirm the spring action. A second zipper incorporated into the spring provides the ability to couple/decouple to an object/substrate. Such devices would have wide application, including for conditionally triggered molecular delivery systems and as actuators in nano-devices. zippers.   

Single particle tracking for a particle in a thermo-osmotic trap

Recently Jiang et al. (PRL 102, 2009) showed that colloidal particles in a polymer solution can be trapped by imposing a local temperature gradient. It is believe that the thermophoresis of the polymers leads to a polymer concentration gradient that drives the colloids to get trapped. We investigate the trapping force further by performing single particle tracking of a probe colloidal particle. Our system consists of Polystyrene beads immersed in PEG solution. We impose a temperature gradient by shining an IR laser to a chromium coated surface. We found the particle position distribution to have non-Gaussian tails but quiet symmetric for the radial positions. However we found quiet asymmetric distribution for the axial positions where the particle likes to stay very close to the heated surface. From the distribution we read out the trapping potential. We will present results for the dependence of the trapping potential to particle size and polymer concentration.   

Controlled Transport of Functionalized Nanochannel though Lipid Membrane

Via the Dissipative Particle Dynamics approach, we study the directed transport of a transmembrane nanochannel to a desired location within a lipid bilayer. Each nanochannel encompasses an ABA architecture, with a hydrophobic shaft (B) with two hydrophilic ends (A). One of the ends of the nanochannel is functionalized with hydrophilic functional groups, or hairs. The hydrophilic hairs serve a dual role: (a) control transport across the membrane barrier, and (b) enable the channel relocation to a specific membrane site. Our system comprises a lipid membrane with an embedded transmembrane nanochannel with the hairs extending into solution. First, we hold a suitably functionalized pipette above the membrane while the nanochannel freely diffuses within the membrane. For an optimal range of parameters, we demonstrate that the hairs find the pipette and spontaneously anchor onto it. We then show that by moving the pipette for a range of velocities, we can effectively transport the channel to any location within the membrane. This prototype assembly can provide guidelines for designing a number of systems for biomimetic applications.   

Molecular Dynamics Simulations of Phosphatidylinositol Bisphosphate (PIP2)

We are interested in the dynamics of membranes containing the highly charged phospholipid phosphatidylinositol bisphosphate (PIP$_2$ or PtdIns\emph{P}$_2$). We performed a geometry optimization at the Hartree-Fock 6-31+G* level of theory to determine the biological conformation of the phospholipid headgroup in the presence of water and partial charge distribution. The angle between the headgroup and the acyl chains that form an anchor in the membrane is $94 ^\circ$, indicating that the inositol ring may lie flat along the surface of the inner plasma membrane. Next, we employed hybrid quantum mechanics/molecular mechanics simulations to investigate the protonation state of PIP$_2$ and its interactions with physiological divalent cations such as magnesium and calcium. Based on preliminary data, we propose that the binding of magnesium to PIP$_2$ is mediated by a water molecule that is absent when calcium binds. These results may explain the ability of calcium to induce the formation of PIP$_2$ clusters and phase separation from other phospholipids.   

Divalent cation-induced cluster formation by polyphosphoinositides in model membranes

Phosphatidylinositol-(4,5)-bisphosphate (PI4,5P2) binds with variable levels of specificity to hundreds of intracellular proteins in vitro. Such restricted targeting of proteins to PIP2 in cell membranes is thought to result in part from the formation of spatially distinct PIP2 pools. The hypothesis that PIP2 forms nanodomains in the membrane by electrostatic interactions with divalent cations is tested using lipid monolayer and bilayer model membranes. Competitive binding between Ca2+ and Mg2+ to PIP2 is quantified by surface pressure measurements and analyzed by a Langmuir competitive adsorption model. Addition of Ca2+, but not Mg2+, Zn2+ or polyamines to PIP2-cotnaing monolayers induces surface pressure drops coincident with the formation of PIP2 nanodomains visualized by fluorescence, atomic force and electronic microscopy. Studies of bilayer membranes using probe-partitioning fluorescence resonance energy transfer (FRET) and fluorescence correlation spectroscopy (FCS) also reveal Me2+-induced domain formation or diffusion retardation which follows the trends: Ca2+ $>>$ Mg2+ $>$ Zn2+, while polyamines have minimal effects. These results suggest that divalent metal ions have substantial effects on PIP2 lateral organization under their physiological concentrations.   

Driven Brownian transport through arrays of symmetric obstacles

The transport of a suspended overdamped Brownian particle driven through a two-dimensional rectangular array of circular obstacles with finite radius is numerically investigated [P. K. Ghosh et. al., \textit{Phys. Rev. E}, submitted (2011)]. Two limiting cases are considered in detail, namely, when the constant drive is parallel to the principal or the diagonal array axes. This corresponds to studying the Brownian transport in periodic channels with reflecting walls of different topologies. The mobility and diffusivity of the transported particle in such channels are determined as functions of the drive and the array geometric parameters. Prominent transport features, like negative differential mobilities, excess diffusion peaks, and unconventional asymptotic behaviors, are explained in terms of two distinct lengths, the size of single obstacles (trapping length) and the lattice constant of the array (local correlation length). Local correlation effects are further analyzed by continuously rotating the drive between the two limiting orientations.   

Hydrophobic Drug Encapsulation Mechanisms of an Injectable Self-Assembling Peptide Hydrogel

We examined a beta-hairpin peptide network that is a shear thinning injectable solid with immediate rehealing behavior. These rheological properties result from the entangled and branched fibrillar nanostructure of the hydrogel networks. The fibrils are formed by the intramolecular folding and subsequent intermolecular assembly of the self-assembling peptides. Taking advantage of the nanofibrillar peptide structures, the hydrogel can be used to encapsulate curcumin, a hydrophobic, natural anticancer agent and indian spice. The hydrogel shields curcumin from the environment while depositing it exactly where it is intended through syringe injection, taking advantage of the hydrogel shear thinning and rehealing behavior. How the network envelopes and interacts with the curcumin is examined using fluoresence and electron microscopy methods to better understand the exact mechanisms and behaviors of the gel itself and the gel-curcumin construct.   

PNiPAM: an amphiphilic polymer probing the mean energetic state of water

We find that the effect of alcohol on the thermodynamic properties of aqueous solutions of poly-N-isopropyl acrylamide (PNiPAM) directly relates to the mixing enthalpy of the water/alcohol mixture itself. This correlation between solution and solvent thermodynamics indicates that the thermodynamics of aqueous solutions of PNiPAM is primarily determined by the mean energetics of the solvent. Such behavior sheds light on the nature of hydrophobic hydration, which we will discuss in this contribution.   

Single molecule discrimination as the sequential hypothesis testing, maximum information gain measurement

Detection of single molecule and single molecule complexes is used widely in the field of molecular biology, biochemistry, physical chemistry. The task is often to discriminate among several distinct possibilities such as which molecule (A,T,C,G) out of the set is present, is the excitation transfer pair in bound or loose state. At present the approach is to accumulate the sufficient statistics for reliable discrimination among the possible outcomes. We suggest the different approaches that adjust the experimental conditions on every step of the measurement in order to maximize the expected information gain of the experiment. The approach is illustrated on the task of single molecule discrimination. Our simulation shows that approach based on sequential hypothesis testing and sequential experimental planning outperforms traditional maximum likelihood hypothesis testing.   

Force extension response of single self-associating polymer chains

The structure and dynamics of polymeric molecules plays a crucial role in a number of synthetic and biological processes. Great progress has been made in using force spectroscopy methods, such as optical tweezers and atomic force microscopy, to probe these properties of single molecules in novel ways. While there has been significant advances at using analytical theory to model the behavior of, for example, single domain unfolding or multi-domain unfolding of identical domains, we consider for the first time the force-extension behavior of self-associating homopolymers. We use a Brownian dynamics simulation with a Bell-like association model that represents, in a coarse-grained fashion, biological polymers such as von Willebrand Factor that use domain-domain interactions to modulate a larger quaternary structure. We also present a theoretical description of pulling that utilizes a master equation description of the relevant coordinate of the polymer network's ``shortest chain.'' We can accurately reproduce the force-extension features, notably the appearance of a dissipative plateau upon the increase of the association time scale, and provide a clear conceptual description of the appearance of this feature. Qualitative comparison to von Willebrand pulling in experiment is shown.   

Modeling the Strength of $\beta$-sheet Structures in Silk Crystals and Protein Molecules

The mechanical response of $\beta$-sheet structures to a tensile force directed along the axis of one chain can be modeled as an array of elastic springs. The \mbox{3-D} potential of H-bonds in $\beta$-sheets gives a shear stiffness of 4.5Nm$^{-1}$ and the chain repeat stiffness is 60Nm$^{-1}$. Nanocrystals $>$3.5nm long with $\geq$20 \mbox{H-bonds/chain} are the strong component of spider silk. They behave much like macro-scale objects, and two conditions must be met for pull-out failure: (1) the load on the most stressed H-bond exceeds the bond strength. (2) the energy of the system is lower after failure. (1) is the critical condition, and the predicted pull-out load is 3-4 times the H-bond strength. An energetically favorable `stick-slip' process is kinetically forbidden. Arrays within a single molecule such as titin have fewer bonds and can fail at low loads by the `stick-slip' process. The logarithmic rate dependence of failure load observed in AFM is \mbox{50pN/decade} and the stick-slip prediction is \mbox{30pN/decade}. Simulations at short times and high loads give slopes $>$10$\times$ higher, matching the prediction for failure at a single bond.   

Searching DNA by Brownian Motion of Transcription Factors

Transcription factor proteins are able to locate their DNA operator binding site by slide-skip motion along DNA without getting trapped by the large binding energy fluctuations associated with partial recognition. Mirny and Slutsky proposed that this can be avoided by assuming that partial recognition involves increased strain energy of the protein. We present an analytical model for such a search in the form of a 2D random walk with DNA arc-length and an internal configurational parameter as coordinates. This leads to a relation between the nature of the DNA randomness and the optimal choice for the internal energy spectrum.   

Anomalous Freezing Behavior of Nanoscale Liposomes

Experiments have shown that the melting transition of small liposomes is broadened when compared to large vesicles or planar membranes. Despite their significant biological and biomedical importance, theoretical and computational studies of the phase behavior and structural properties of small liposomes have been limited. Presented here is a systematic computational study of the phase behavior and structural properties of liposomes using a recently developed coarse-grained particle-based model. Great assiduity was given on the effect of liposome diameter on their thermal and structural properties. Below the melting transition, liposomes are faceted with the gel facets separated by ``grain'' boundaries that are in the fluid phase. In agreement with experiments, we found that the melting transition is significantly broadened as the liposome diameter is decreased and that the heat capacity exhibits two distinct peaks for diameters less than 33 nm, an indication of a decoupling of the melting transition of the two leaflets. This decoupling is clearly demonstrated by the chain order parameters of the two leaflets, which show that the upper leaflet undergoes a melting transition before the inner leaflet.   

Desalination membranes from functional block copolymer via non-solvent induced phase inversion

Commercially available reverse osmosis (RO) and forward osmosis (FO) membranes are most commonly derived from materials such as polysulfone, polyimide, and cellulose acetate. While these membranes have improved the efficiency of the desalination process, they suffer from mechanical and chemical stability, fouling issues, and low fluxes. In this study, we combine a well-established membrane formation method, non-solvent-induced phase separation, with the self-assembly of a functional amphiphilic block copolymersAn amine and acid functional polystyrene-block-poly(ethylene oxide-co-allyl glycidyl ether) were chosen for the membranes. Membranes were formed by casting a concentrated polymer solution (12 to 25 wt\% polymer) on PET fabric followed by immersion in a non-solvent bath. Scanning electron microscopy revealed an asymmetric porous structure consisting of a dense skin layer on top of a highly porous layer. Membrane performance was investigating using an FO test cell under the seawater condition.   

Morphological study on a phospholipid mixture and their Dependence of Temperature, Concentration and Chemical Composition

A variety of morphologies, such as nanodiscs (bicelles), bilayered ribbons, unilamellar vesicles (ULVs), multi-lamellar vesicles (MLVs) and perforated lamellae exist in phospholipid mixtures composed of a long chain phospatidylcholine (PC), its charged counterpart (i.e., phosphatidylglycerol, PG with the same hydrophobic chain length) and a short-chain PC. Here, we present a comprehensive the structural characterization of such mixtures with various combinations of long-chain (e.g., di-14, di-16, di-18 PC) and short-chain (e.g., di-06, di-07 PC) lipids at a constant charged density using small angle neutron scattering (SANS), dynamic light scattering (DLS) and transmission electron microscopy (TEM). A time-resolved DLS study is also carried out to understand the kinetics of the structural transformation as a function of temperature, lipid concentration and composition. The preliminary data indicate that uniform nanodiscs and/or bilayer ribbons generally exist at low temperature, while at high temperature ULVs or MLVs are obtained. Moreover, the nanodiscs coalesce with each other over a period of time. The fundamental understanding of the structural formation mechanism and kinetics can lead to potential application of this system to bionanotechnology, such as drug carrying and therapeutic imaging.   

The Effect of Short-Chain Lipid on the Morphology of Bicellar Mixtures

Lipid bicellar mixtures, composed of long-chain phospholipids (usually dimyristoyl phosphatidylcholine (DMPC) and charged lipid dimyristoyl phosphoglycerol (DMPG)) and a short-chain phospholipid (e.g. dihexanoyl phosphocholine (DHPC) or (3-[(3-Cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO)), are ideal substrates for structural characterization of the membrane protein, because they provide the natural lipid bilayer environment and furthermore can be aligned in the magnetic field.Recently, structural phases of zwitterionic DMPC/DHPC and charged DMPC/DMPG/DHPC bicellar mixtures have been studied extensively.However, the effect of the short-chain lipid on the morphology is still unknow. Here we report the phase diagram of DMPC/CHAPSO and DMPC/CHAPSO/DMPG mixtures by SANS and NMR. Compared with DMPC/DHPC and DMPC/DHPC/DMPG, different temperature dependence of morphology is observed in the counterpart of CHAPSO by SANS. Also, The PFG-NMR result shows different diffusion behaviors of polyethylene glycol associated with the membranes composed of DMPC/CHAPSO and DMPC/CHAPSO/DMPG, which are magnetically alignable. Both SANS and NMR results suggest tha bilayered ribbon the is formed in the case of DMPC/CHAPSO, DMPC/CHAPSO/DMPG.   

Rheology of Vimentin Intermediate Filament Networks

A cell's ability to function is highly dependent on its structure and material properties - its capacity to withstand and respond to forces in its environment. The cytoskeleton, which largely determines the cellular mechanical properties, is comprised of biopolymer networks, including filamentous actin, microtubules, and intermediate filaments (IF). Intermediate filaments are much less studied than actin and microtubules. They are much more varied and specialized as well, and have been suggested as being an important platform in mechanotransduction processes in cells. It is thought that they can withstand very high strains and exhibit strain stiffening behavior. We are characterizing vimentin, a type III IF that is found in all vertebrate cells, using rheological techniques. Vimentin elasticity increases upon addition of multivalent cations, which act like molecular crosslinkers. By varying the concentration of cations, we can extract valuable information about how the networks assemble and function.   

Force Dependent Changes in Non-Erythroid Spectrin and Ankyrins

Mechanotransduction in cells describes the process by which external physical stimuli are converted into biochemical activity and plays an important role in many biological functions on both the cell and tissue level. However, the specific mechanisms by which mechanical forces lead to particular molecular and cellular responses are much less understood. We investigate the changes in non-erythroid spectrin and ankyrins as a result of equi-biaxial strain application to live cells in culture. Specifically, we focus on the spectrins' role in the ubiquitination process - a vital process in the regulation of protein degradation- of spectrin and ankyrins. We utilize immune-fluorescence staining and fluorescent fusion proteins for quantitative fluorescence imaging as well as biochemical methods to measure changes in of cell's spectrin and ankyrin content. Protein expression levels and localization between cells exposed to mechanical stimuli of different temporal and spatial profiles are compared. In addition, the threshold behavior of cell proliferation - as measured by number densities - of a variety of cell types as a function of mechano-stimulation is investigated.   

Dynamics and Structure of Disordered Peptides from Two-Dimensional Infrared Spectroscopy

Two-dimensional infrared (IR) spectroscopy is a powerful tool for investigating the ultra-fast dynamics and association of complex biological macromolecules such as proteins and DNA. In addition to the improved spectral discrimination afforded by a two-dimensional spectrum, the ultra-fast time-resolution inherent to the technique provides unique insight (unobtainable by standard linear IR measurements) into the time-scales of macromolecular conformational fluctuations, particularly for intrinsically disordered systems. Here we discuss the use of accurate line shape modeling of peptide amide I vibrations as an advanced method for extracting structural and dynamic information from experimental spectra. The mixed quantum-classical model makes use of standard MD trajectories and a parametrized site energy and coupling map as inputs for excitonic calculations of the delocalized amide I vibrations. We present examples of the application of this method to extract site-specific structural information (such as hydrogen bond number and turn conformation) as well as insight into conformation dynamics and time-scales from experimental data for disordered peptides.   

Vibrational Coherence Spectroscopy Investigation of Cytochrome c equilibrium Unfolding

Using vibrational coherence spectroscopy (VCS) we studied guanidinium hydrochloride (GdHCl) induced equilibrium unfolding of ferric cytochrome c excited at 412 nm. Upon unfolding, we observed that the strong 50cm$^{-1}$ mode, which dominates the VCS spectrum of native cyt c, loses its intensity relative to the 80cm$^{-1}$ mode. The 224 cm$^{-1}$ mode ($\gamma _{24})$ also shifts to 205 cm$^{-1}$, reflecting the heme configuration change associated with unfolding. We also compared the amplitude of these unfolding sensitive modes at different GdHCl concentrations using a cyt c-imidazole complex as a model system. The peak of the Soret band does not shift when the cyt c- imidazole complex is unfolded. Because the resonance conditions are invariant, the relative intensities are a direct probe of the heme structural changes. Our results show that the 50 cm$^{-1}$ mode dramatically loses amplitude, while the 80 cm$^{-1}$ mode stays nearly the same. When compared to other Raman studies, which suggest that the heme adopts a more planer structure when cyt c is unfolded, the 50cm$^{-1}$ mode may reflect a similar structural change as the 569 cm$^{-1}$ ($\gamma _{21})$ mode. We suggest that these modes are diagnostic of a protein-induced ruffling distortion.   

Protein Dynamics Studied via a Disclination Network Based Approach

Protein dynamical transition at ca. 200 K is an elusive collective process by which the time-scale distribution of different relaxations is strongly altered (Okan, Atilgan, Atilgan, Biophysical J. 2009, 97, 2080; Atilgan, Aykut, Atilgan, Biophysical J. 2008, 94, 79). It is now confirmed that the backbone topology during this dynamic transition remains intact and there is no structural phase transition during such a dynamic transition. However, the dynamics of backbone torsional jumps shows a freezing behavior below the transition temperature (Atilgan et al., 2008). In the current work, we map the heavy atoms of proteins onto a disclination network and probe temperature dependent dynamics on such a construct. Using a series of 40 ns molecular dynamics cooling simulations spanning the temperature range of 320 K to 160 K with 10 K increments, we track the dynamics of orientational defect networks. The size distribution and defect lifetimes in the disclination network are shown to be central for the dynamical transition in proteins.   

Accoustic Force Microscopy in Aqueous Environments to Determine Protein Folding Dynamics

Characterizing the dynamic mechanical response of (bio)molecular thin films can give insight into the dynamics of biomolecules on surfaces. Here, acoustic atomic force microscopy (AFM) methods are promising tools since they enable sensitive mapping of the mechanical properties of samples by introducing high frequency modulation while imaging the topography. We are among the first to show that it is possible to utilize acoustic AFM methods in aqueous environments. Accounting for the indentation dependent, off-resonance cantilever dynamics, we are able to obtain spatially resolved elasticity maps of organic thin films in water. Encouraged by these results, we are testing the hypothesis that the rapid unfolding/refolding cycles of the five B domains of SpA-N, a 291 residue protein, composed of five nearly identical 56 to 61 residue domains, confer a unique form of flexibility to the molecule. To this end we employ a non-resonant, acoustic AFM method in a frequency spectroscopy mode, and measure the viscoelastic properties of SpA-N monolayers quantitatively under physiological conditions. We present evidence that the apparent, frequency-dependent stiffness of SpA-N coincides with the kinetics of individual domain folding cycles.   

Analysis of Soybean Microtubule Persistence Length; New Evidence on the Correlation between Structural Composition and Mechanical Properties

Recent studies on microtubules composed of different $\beta $ tubulin isotypes indicate their different functionality in terms of their dynamical behavior or the mechanism of their interaction with chemotherapeutic drugs. Along these lines, the result of our recent study measuring the rigidity of neural and non-neural samples of microtubules with different $\beta $ tubulin isotype compositions suggests that the distinguished mechanical properties of microtubules, such as rigidity, may also be associated with the different distribution of their $\beta $ tubulin isotypes. In our current study, we have reported the persistence length of a single soybean microtubule. This plant microtubule has a structural composition different from that of mammalian microtubules. Under the same experimental methods of measurement, the soybean microtubules showed a different persistence length as compared to the value of the persistence length that we estimated in the study of both single Bovine Brain and MCF7 microtubules.   

Conformational response of a clay binding protein (EGF) by a coarse-grained Monte Carlo simulation

Biofunctionalization of montmorillonite (MMT) clay platelets with epidermal growth factor (EGF) appears to play a critical role in tissue regeneration (cell growth and migration) [1]. How the protein (EGF) binds to clay platelet and conforms is very important in its ability to activate the epidermal growth factor receptor. It is however difficult to monitor such structural response systematically in a current laboratory setting. We investigate the structural response of the protein EGF as it binds to the clay platelet with a coarse-grained model already used to investigate binding of short peptides. Both the EGF protein and the clay platelets are described by nodes tethered together by fluctuating covalent bonds. Each residue interacts with a phenomenological interaction (based on its hydropathy index). Protein and platelet perform their stochastic motion with the Metropolis algorithm. A number of local (e.g. mobility and structural profiles) and global physical quantities such as gyration radius are examined as a function of temperature. We are able to identify the immobilized segments of protein and the variation of its size as a function of temperature. [1] C.A. Vaiana et al Biomacromolecules xxx (2011)   

Biocompatibility of Titanium

Titanium is the material of choice for orthopaedic applications because of its known biocompatibility. In order to enhance osteogenic properties of the Ti implants, it is necessary to understand the origin of its biocompatibility. We addresses the origin of Ti biocompatibility through (1) theoretical modeling, (2) the precise determination of Ti surface chemistry by X-ray photoelectron spectroscopy (XPS), (3) and the study of fibronectin adsorption as a function of Ti (near) surface chemistry by Enzyme-linked immunosorbent assay (ELISA). We compare the protein adsorption on Ti with the native oxide layer and the one coated by TiO2 in anatase phase using ion beam assisted deposition (IBAD). We show that the thin native sub-stoichiometric titanium oxide layer is crucial for biocompatibility of Ti surface. This is due to the enhancement of the non-specific adsorption of proteins which mediate cell adhesion. Improving the surface oxide quality, i.e. fabricating stoichiometric TiO2 (using IBAD) as well as nanoengineering the surface topology that matches its dimensions to that of adhesive proteins, is crucial for increased protein adsorption and, as a result, further increases biocompatibility of Ti implant materials.   

Partially-Functionalized Isotactic Polystyrene with Blocky Comonomer Segments

Isotactic polystyrenes (iPSs) have been functionalized in solution, while the accessibility of functionalizing agent is limited by the formation of crystalline domains at various temperatures. The chemical system used is the borylated isotactic polystyrene system, and we investigated the temperature effects on reaction kinetics to ultimately control the blockiness of borylated segments in the resulting copolymer. The chemical composition of partially borylated iPS reaches a steady state that is dependent on temperature. This synthesis has been performed at many different temperatures, with different steady states being reached at different temperatures. Further analysis by differential scanning calorimetry (DSC) has shown that the higher temperature reactions have greater effect on breaking down the crystal lattice structure of the isotactic polystyrene. As a result, the lower temperature reactions affect the crystalline structure less, and the resulting copolymer has more blockiness.   

Rigid amorphous fraction of Nylon 11 determined from TMDSC

High precision, high accuracy heat capacity measurements were used to study both neat Nylon 11 and Nylon 11 nanocomposites which had been prepared by different processing procedures. The heat capacity step at the glass transition temperature was characterized from the reversing flow using temperature modulated differential scanning calorimetry, and this allows us to determine the mobile amorphous fraction. Heat fusion was obtained from endotherm area of the total heat flow curve, and was correlated with the degree of crystallinity determined from X-ray diffraction. Based on three phase model of the semicrystalline polymer structure, the rigid amorphous fraction (RAF) in Nylon 11 could be calculated. Studied Nylon 11 samples include solution cast, liquid quenched, and isothermally crystallized films, solution cast films containing multi-walled carbon nanotubes, and electrospun fibers. We observed that a rigid amorphous fraction exists in all Nylon 11 samples, and the amount of RAF is strongly dependent upon the crystalline fraction and the nanofiller content.   

Structure of Oriented PLA/Graphene Nanocomposite Fibers

Highly-aligned polylactic acid (PLA)/graphene nanocomposite fibers were successfully electrospun. Through a combination of thermal analysis and X-ray scattering, the phase structure, molecular orientation, and fiber shrinkage of the oriented PLA fibers were investigated to evaluate the molecular chain confinement. Calorimetric studies were performed to identify the molecular origin of the post-$T_{g}$ exothermic peak. We found that the shrinkage of the oriented amorphous polymer serves as a precursor for the cold crystallization revealed by the post-$T_{g}$ exotherm. Using real-time 2-D wide angle X-ray scattering and molecular retraction tests, we further quantified the orientation level and the oriented amorphous fraction in the as-spun amorphous fibers, and investigated the subsequent formation of oriented crystals during heating under ``frozen-in'' tension. The preferentially oriented amorphous region that possesses a degree of medium-range order has high similarity with the concept of the rigid amorphous phase that has been widely studied in thermal analysis area, and a new phase structure model was established. Graphene filler has a significant influence on molecular orientation, crystallization behavior, and electrical conductivity of PLA fibers.   

The Role of Surface Charge of Nucleation Agents on the Crystallization Behavior of Poly(vinylidene fluoride)

The effect of the surface charge of nucleation agents on the crystallization behavior of poly(vinylidene fluoride) (PVDF) has been investigated. Ion-dipole interaction between the positive surface of nucleation agents and the partially negative CF2 dipoles of PVDF was considered as a main factor for further lowering free energy barrier for nucleation, and thus increasing significantly the crystallization kinetics. This is in contrast to the behavior observed for nucleation agents possessing either negative surface or neutral charges. Positive nucleation agents led to a remarkable increase in the crystallization temperature (lower supercooling), the melting point and degree of crystallinity of PVDF as compared with that of neat PVDF. The dispersion of each type of nucleation agents is also important. The melting temperature needs to be higher than the melting temperature of PVDF. The detailed crystallization behavior and its kinetics, including the conformational changes of the PVDF chain during the crystallization of neat PVDF and PVDF with specific nucleation agents, have also been investigated. With the addition of positive nucleation agents, the $\gamma $ and $\beta $ chain conformations, instead of the $\alpha $ phase, dominate.   

Characterization of aging behavior associated with multi-layered polymer laminates

To predict the physical stability of multilayered polymer laminates film, the underlying segmental motion of each layer needs to be characterized as a function of time and temperature. We focused on the dimensional stability of various poly(vinylidene fluoride) (PVDF) and its blends. Because of the significant density difference between the crystals formed and the amorphous phase, dimensional stability or the presence of residual stress is difficult to avoid. We have used a combination of techniques to characterize the effects of various comonomers, nucleation agents or blends on the crystallization behavior, the morphology formed and thus dimensional stability. The stress changes in polymer films were investigated as a function of temperature with the cantilever deflection approach. The volume relaxation with time is observed using density gradient column. The morphological features evolved were also analyzed. The magnitude of the residual stress was quantitatively evaluated. The time evolution reaching a stable structure was also investigated in terms of the local glass transition temperature. We further discussed the changes by adding nuclear agents in order to force the polymer films to reach equilibrium states achieving stability stable in a short time scale.   

Optically Fixable and Optically Elastic Photomechanical Responses in Azobenzene Liquid Crystal Polymer Networks

Photoresponsive behaviors in azobenzene functionalized polymers have seen widespread interest for a variety of applications as adaptive materials. In a series of recent works, we have distinguished the photomechanical response of polydomain, monodomain and twisted nematic glassy azobenzene liquid crystal polymer networks (azo-LCNs). Interestingly, these materials primarily exhibit optically fixable shape memory to blue-green irradiation, but in some instances can exhibit ``optically elastic'' (muscle-like) responses. Towards this end, this contribution will summarize the photomechanical responses of these materials and illustrate some of the underlying contributors that dictate fixable or elastic responses upon removal of the irradiation.   

Effect of an external field on nematic order in semiflexible polymers

Semiflexible polymers of sufficient stiffness exhibit liquid crystalline order at sufficient polymer concentrations. In this work, we investigate blends of flexible and semiflexible polymers under a uniform external field with the aid of Monte Carlo simulations of a bond-fluctuation model. The model is an extension of Shaffer's bond-fluctuation model, where chain stiffness is controlled by including different forms of bending penalties, and includes attractive interactions between monomers. From simulations for a range of values of the bending energy, density, and temperature, we determine the effect of an external field on liquid crystalline order and domain formation.   

The evaluation of the dynamic data near Tg from a new aspect

In this work, we compare and analyze the G$_{g}$ obtained by the KWW\footnote{G. Williams and D. C. Watts, Trans. Faraday Soc. 66, 80 (1970); F. Kolrausch, Pogg. Ann. Phys. 12, 393 (1847).} and the BSW\footnote{M. Baumg\"{a}rtel, A. Schausberger, and H. H. Winter, Rheol. Acta 29, 400 (1990).} functions for small molecule, polymer and colloidal glass formers in the context of the Dyre shoving model.\footnote{J. C. Dyre, N. B. Olsen, and T. Christensen, Phys. Rev. B 53, 2171 (1996).} The Dyre shoving model relates G$_{g}$ with temperature by attributing the free-energy barrier for a ``flow event'' near to T$_{g}$ to the work done by shoving aside the surrounding molecules; the free-energy barrier is proportional to G$_{g}$, which increases as the temperature decreases. Importantly, the model gives a non-singular growth of the relaxation time or viscosity with decreasing temperature and does not invoke ideas related to dynamic heterogeneity and growing length scales, both of which are commonly used in the description of glassy dynamics. To the extent that the material classes investigated seem to be reasonably described by the shoving model, it is suggested that divergence of time scales and length scales may not be essential to the glass transition phenomenon.   

Formation of Stable Polymer Glasses via Matrix Assisted Pulsed Laser Evaporation

Via Matrix Assisted Pulsed Laser Evaporation (MAPLE), we are able to form amorphous polymer films that exhibit significant changes in material properties and structure. In the MAPLE method, a pulsed laser ablates a target, consisting of a frozen dilute solution of the desired polymer, in order to produce films of the material. By carefully controlling the growth rate of film formation and the substrate temperature during deposition, we are able to form glassy films with structures that are either less or more dense compared to the standard glass. Interestingly, the morphology of the low and high-density amorphous films is significantly different. The low-density glasses are nanostructured and the high-density glasses are not. In this poster, we discuss how MAPLE can be used to tune the morphology and hence, the properties of polymer glasses.   

A configurational entropy theory for the non-Arrhenius relaxation in disordered systems

Here we develop a novel configurational entropy theory to explain the non-Arrhenius slowdown relaxation behavior in disordered systems on approaching the glass transition. The theory explicitly shows that this intriguing slowdown behavior originates from the growth in the configurational entropy loss (and the structural order degree) of the system, associated with the attractive interaction among relaxing units (RUs). A thermodynamic/kinetic equilibrium state with order singularity at the finite temperature $T_{c}$ due to the attractive interaction among RUs is predicted. An expression is derived for characterizing the average relaxation time of RUs relaxing towards such equilibrium state. The resultant relaxation time expression offers a novel connection between kinetics and thermodynamics, different from that of the $A-G$ entropy equation, and it is shown to be a generalization of several well-known relations in current use. The theory also implies that the diverging relaxation time at finite temperature arises from the existence of an underlying long-range order phase transition at $T_{c}$. Our results could provide a novel understanding of the non-Arrhenius slowdown relaxation behavior in disordered systems on approaching the glass transition.   

Purification and Fractionation of PEO-PPO-PEO Triblock Copolymers

PEO-PPO-PEO triblock copolymers are produced on a commercial scale for nonionic surfactant applications. Anionic polymerization has been employed to produce the block copolymers and it is likely that the triblock copolymer contain low molecular weight contaminants that would interfere with the micellization and gelation in aqueous solution. We have taken advantages of the recent developments on the interaction chromatography (IC) technique to separate the neat triblock copolymers from the as-received triblock copolymers. Because the IC relies on the adsorption of polymers, we were able to purify and fractionate the triblock copolymers on a larger scale than the prep scale SEC. Upon purification of the triblock copolymers, the influence of contaminants in the as-received triblock copolymers will be discussed on the micellization and gelation of the triblock copolymer solution in water.   

Size Interplay between Polymer and Nanopores for the Band Broadening of SEC

The size interplay between polymer chains and nanopores plays a key role in governing the retention time of polymer chains in liquid chromatography. These nanopores also contribute to the band broadening of the resulting peaks seen in most liquid chromatography systems including size exclusion chromatography (SEC). We have studied how the relationship between the size of the nanopores and the hydrodynamic radius of the polymers affects the band broadening during SEC. This related to Brown random motion of polymer chains in solution, whose motions are restricted by the presence of nanoporous stationary phase for the SEC. We have prepared model polystyrene samples with extremely narrow polydispersity (PDI $<$ 1.0001) using temperature gradient interaction chromatography. Those model samples allow us to directly measure the band broadening of SEC using different size pore columns at various solvent conditions.   

SANS Mapping of Orientation and Relaxation of Polymer Chains upon Uniaxial Extension

Small angle neutron scattering (SANS) has been used to measure entangled chains in polymer melts upon uniaxial stretching. The degree of anisotropy and the coil dimension are discussed in the light of the applicability of the tube model.   

X-ray scattering investigation of structural relaxation in an ordered block copolymer melt subjected to uniaxial extensional flow

The structural dynamics of an ordered styrene-ethylene butylene-styrened triblock copolymer have been studied in uniaxial extensional flow using in situ x-ray scattering. Experiments were performed in a custom instrument consisting of an SER extensional flow fixture housed in a convection oven designed to facilitate x-ray access. Use of synchrotron radiation provided sufficient time resolution to study the structural response during inception of uniaxial flow, and as a function of time following flow cessation. The sample studied here exhibits hexagonally packed cylindrical microdomains of polystyrene embedded in a poly(ethylene butylene) matrix. Application of extensional flow produces multiple structural effects, including deformation of the microphase-separated morphology, and a complex reorientation process in which elongated PS microdomains progressively orient along the stretching axis. A series of experiments was run in which samples were stretching to varying Hencky strains, allowing investigation of the nature of structural relaxation from a variety of flow-induced structural states induced during extensional flow. Significant differences in structural relaxation are found depending on the total applied extensional strain.   

Comparative study of polymer chain dynamics in aqueous solutions by FPR and DLS

Self diffusion of tagged polymer chains in aqueous solutions of hydroxypropylcellulose (HPC) was measured by Fluorescence Photo-bleaching Recovery (FPR) and compared to mutual diffusion of scattering species in the same solutions measured by Dynamic Light Scattering (DLS). The effect of the dye presence on thermodynamic concentration fluctuations observed by DLS was also studied. The observed multimodal spectra in DLS and FPR were analyzed with CONTIN and stretched exponential fits. A set of consistent dissimilarities in the modal distributions of FPR and DLS spectra was found. This indicates a comparative limitation or sensitivity in range of detectable diffusive processes between FPR and DLS in this complex system. In addition, it was found that the fluorescent tag and/or tagging process seem to alter the mutual diffusion processes seen by DLS. In particular, a slower mode which is apparent in the non-tagged sample does not appear in the tagged sample. It seems likely that the dye chemically affects the polymer chains keeping them from clustering with each other, altering the solvent environment preventing formation of polymer clusters responsible for the slow mode usually seen in HPC.   

Silicone Elastomer Networks with Tunable Modulus for Cell Mobility Studies

Silicone elastomer networks provide a versatile platform for studying the effects of compliance and surface chemistry on cell movement. One major advantage of these materials over more commonly used poly(acrylamide) surfaces or hydrogel networks is that they do not swell with water; therefore, the physical properties measured in the dry state are representative of those present under aqueous conditions. In this work, we tuned the moduli of poly(dimethylsiloxane) (PDMS) networks by manipulating the cross-link density\textit{ in-situ} during network formation. This regulation of cross-link density was accomplished by cross-linking at polymer chain ends with both difunctional and multi-functional cross-linking agents. The difunctional cross-linkers serve as chain extenders whereas the multifunctional cross-linkers facilitate formation of a chemically cross-linked network. Networks with moduli ranging from $\approx $10 kPa to $\approx $1 MPa were fabricated in this fashion by adjusting the polymer molecular weight, the ratio of difunctional to multifunctional cross-linker, and the ratio of total cross-linker to number of polymer chains. The same approach can be applied to fabricate poly(vinylmethylsiloxane) networks, which can be further tailored through side-chain cross-linking or surface functionalization of pendent vinyl groups.   

New Insights into Chain Order Dynamics and Structural Development in Sulfur-Vulcanized Natural Rubber Latex using Multiple Quantum NMR and Synchrotron X-Ray Diffraction

Network structure, chain dynamics, and structural development in sulfur-vulcanized natural rubber latex were studied by Multiple-Quantum (MQ) NMR and synchrotron x-ray scattering. Three important processes that can influence rubber network structure and its overall mechanical properties were the main focus and analyzed by both of these techniques: pre-vulcanization, drying, and post-vulcanization. MQ NMR experiments can provide quantitative information regarding networks at very small length scales, including network defects, number of cross-links, and spatial distribution of cross-links. Structural development in natural rubber was studied under uniaxial deformation by in-situ synchrotron x-ray diffraction, which can provide information on network structures at much larger length scales. Molecular orientation and strain-induced crystallization was analyzed by both stress-strain relations and wide-angle x-ray diffraction (WAXD). The morphology of the latex rubber particle during deformation was analyzed by small-angle x-ray scattering (SAXS). The combination of these techniques can provide a considerable amount of information regarding rubber network structure.   

Chemotaxis of active, self-oscillating polymer gels in solution

Fighting, fleeing and feeding are hallmarks of all living things; all these activities require some degree of mobility. Herein, we undertake the first computational study of self-oscillating polymer gels and show that this system can ``communicate'' to undergo a biomimetic, collective response to small-scale chemical changes. In this study we harness unique properties of polymer gels that undergo oscillatory Belousov-Zhabotinsky (BZ) reaction. The activator for the reaction is generated within these BZ cilia and diffuses between the neighboring gels. In order to simulate the dynamics of the BZ gels in surrounding fluid we have developed a nonlinear hybrid 3D model which captures the elasto-dynamics of polymer gel and diffusive exchange of BZ reagents between the gel and the fluid. We illustrate that multiple BZ gels in solution exhibit a distinct form of chemotaxis, moving towards the highest activator concentration in the solution. Similar ability to sense and move in response to chemical gradients constitutes a vital function in simple organisms, enabling them to find food and flee from poisons.   

Effect of Thickness on the Thermal Properties of Hydrogen Bonded Layer by Layer Assemblies

Layer by layer (LbL) assemblies have attracted a lot of attention for their functional versatility and ease of fabrication. However characterizing thermal properties, especially for ultra thin LbL assemblies, has remained a challenging topic. We have investigated the role of the film thickness on the glass transition temperature ($T_{g})$ for poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) and (PEO)/poly(methacrylic acid) (PEO/PMAA) hydrogen bonded LbL assemblies in both bulk as well as in confined thin films using modulated differential scanning calorimetry (MDSC) and temperature-controlled ellipsometry. PEO/PAA LbL assemblies exhibit a well-defined $T_{g}$, both in bulk and thin films. For films less than 100 nm thick, the $T_{g}$ increased slightly as film thickness decreased. On the other hand, PEO/PMAA LbL assemblies displayed clear glass transitions only after thermal treatment, which produces anhydride crosslinks. Also, the thickness dependence on $T_{g}$ was less pronounced for PEO/PMAA LbL films. It was also seen that the thermal expansion coefficient ($\alpha ) $ increased for film thickness below 200nm. We speculate that interactions between the film and substrate likely influence the thickness-dependent $T_{g}$   

Layer-by-Layer Assembly of Polyelectrolyte Chains and Nanoparticles on Porous Substrates: Molecular Dynamics Simulations

We performed molecular dynamics simulations of a multilayer assembly of oppositely charged polyelectrolyte chains and nanoparticles on porous substrates with cylindrical pores. The film was constructed by sequential adsorption of oppositely charged species in a layer-by-layer fashion from dilute solutions. The multilayer assembly proceeds through surface overcharging after completion of each deposition step. The substrate overcharging fraction fluctuates around 0.5 for nanoparticles-polyelectrolytes systems and around 0.4 for polyelectrolytes-polyelectrolytes systems. The surface coverage increases linearly with the number of deposition steps. The rate of surface coverage increase as a function of the number of deposition steps changes when the pore is closed. The closing of the pore occurs from the pore entrance for nanoparticles-polyelectrolytes systems. In the case of polyelectrolytes-polyelectrolytes systems the pore plug is formed inside the pore and then spreads towards the pore ends.   

Interaction between Brush Layers of Bottle-Brush Polyelectrolytes: Molecular Dynamics Simulations

Interactions between tethered layers composed of aggrecan (charged bottle-brush) macromolecules are responsible for the molecular origin of the cartilage biomechanical behavior. To elucidate the role of the electrostatic forces in interaction between bottle-brush layers we have performed molecular dynamics simulations of charged and neutral bottle-brush macromolecules tethered to substrates. In the case of charged bottle-brush layers the disjoining pressure $P$ between two brush layers in salt-free solutions increases with decreasing the distance $D$ between substrates as $P\propto D^{-1.8}$. A stronger dependence of the disjoining pressure $P$ on the surface separation $D$ was observed for neutral bottle-brushes,$ P\propto D^{-4.6}$, in the same interval of the disjoining pressures. These scaling laws for dependence of the disjoining pressure $P$ on the distance $D$ are due to bending energy of the bottle-brush macromolecules within compressed brush layers. The weaker distance dependence observed in polyelectrolyte bottle-brushes is due to interaction between counterion clouds surrounding the bottle-brush macromolecules preventing strong brush overlap.   

Synthesis and Physical Behavior of Model Polymer Electrolyte Membranes for Alkaline Fuel Cells

Alkaline fuel cell (AFC) technology holds significant promise for portable power supplies because AFCs are very efficient at temperatures under 200\r{ }C, but also because AFCs can use relatively inexpensive, non-noble metals (Ni, Fe, Co) as the catalyst material. Wide-spread use of the AFC has been prevented by the use of aqueous KOH liquid as the electrolyte, which is easily poisoned by the formation of K$_{2}$CO$_{3}$. Development of an semipermeable polymeric alkali anion exchange membrane (AEM) would significantly improve the usefulness of AFCs by eliminating carbonate poisoning and the engineering problems associate with a liquid electrolyte. We have been exploring model copolymers containing phosphonium cations as candidate materials for AEMs. Recent findings on the transport properties and stability of random copolymers of styrene and p-vinylbenzyl-trimethylphosphonium chloride will be presented, as well as ongoing efforts to study the effect of polymer morphology on transport and stability in ionomers based on both phosphonium and ammonium cations.   

In-depth Analysis of Proton Mobilities in Sulfonated Block Copolymers

Polymer electrolytes are an important component of a wide variety of electrochemical devices such as battery, fuel cell, and chemical sensor owing to their ability to provide a pathway for ion transport between electrodes. Considerable efforts have been devoted to a subject of ion transport mechanisms in polymer electrolytes since the ion mobility in the polymer electrolytes plays a central role in determining the efficiency of the devices. In present study, we carried out an in-depth analysis of proton mobilities in model ionic block copolymers. The system of interests is a series of sulfonated poly(styrene-b-methylbutylene) (PSS-b-PMB) copolymers. Dilute solutions of PSS-b-PMB copolymers in methanol, which indicate highly uniform spherical ionic micelles, were examined yields. In particular, on virtue of the self-assembly nature of block copolymers, the system revealed well-defined ionic PSS domains with different thickness ranging from 3.0 to 7.8 nm. The proton transport in PSS-PMB copolymers was found to be facilitated by the decrease in the ionic domain sizes, which was rationalized by the different proximity of acid groups at the surfaces of ionic domains.   

Functionalizing Silica Nanoparticles Designed for Improved Ion Transport

Silica nanoparticles have been functionalized with polyethylene oxide and sulfonated polyester to synthesize improved single ion conducting polymer nanocomposites. Our motivation is to minimize ionic aggregation at room temperature by anchoring the sulfonate anion to the surface of a nanoparticle via polymer grafting. The tethered anion's mobility should be restricted, allowing the cation to conduct more easily through the short chain PEO matrix, thereby increasing the free ion content. Secondary benefits of nanoparticle dispersion include suppressed crystallinity and increased viscosity at room temperature in low molecular weight systems where ionic conductivity is elevated.   

Effect of water uptake on morphology of polymerized ionic liquid block copolymers and random copolymers

Dynamic studies of polymer morphology probe how the physical properties of polymerized ionic liquids are affected by the environment, such as temperature or moisture. For a series of poly(methyl methacrylate-$b$-1-[2-(methacryloyloxy)ethyl]-3-Butylimidazolium X$^{-})$ block and random copolymers with hydrophilic counterions (X$^{-}$ = Br$^{-}$, HCO$_{3}^{-}$, OH$^{-})$, the introduction of water vapor to the system can swell the ionic liquid block, causing enlarged hydrophilic domains and swollen channels for ion conduction. This expected expansion of ionic liquid domains in humid environments can be used to intelligently design these copolymers for use in technological applications. The effect of water vapor exposure in these imidazolium-based acrylate polymers is studied by small-angle X-ray scattering. These morphology results will be discussed alongside complementary studies of water uptake and ion conductivity.   

Block Copolymer Exhibiting Simultaneous Electronic and Ionic Conduction

Poly(3-hexylthiophene)-block-Poly(ethylene oxide) (P3HT-PEO) allows for the simultaneous conduction of electronic and ionic charges at the nanometer length scale needed for lithium battery electrodes. In order to study the charge transport properties of P3HT-PEO, we characterized a series of P3HT-PEO block copolymers with and without the addition of lithium bis(trifluromethanesulfonyl) imide (LiTFSI). We specifically looked at the relationship between morphology and the transport of both electronic and ionic charges. Previously reported work has primarily focused on transport of one charged species. In particular, the results of the study shed light on the effects of LiTFSI on electronic conduction and the intrinsic electronic conduction of the P3HT microphase.   

Discontinuous Changes in Ionic Conductivity of a Block Copolymer Electrolyte through the Order-Disorder Transition

Simultaneous small angle X-ray scattering and ionic conductivity measurements of a block copolymer electrolyte as it transitions from an ordered lamellar structure to a disordered phase reveal a discontinuous increase in conductivity at the phase transition. A simple framework for understanding this result is presented, incorporating both morphology factor corrections that account for constraints imposed by the geometry of the conducting phase and `vehicular' transport of coupled polymer chains and ions.   

Conformation of polystyrene sulfonate (PSS) and its origin of counterion distribution

The hydrodynamic radius and electric potential of single chains of polystyrene sulfonate (PSS) is studied, as a function of the molecular weight, by single molecule fluorescence techniques - fluorescence correlation spectroscopy (FCS) and photon count histogram (PCH). The results show different scaling law for low and high molecular weight. When the chain is short (N$<$300), the scaling power index is about unity, indicating a rod-like conformation, while the index changes to 0.58 when the chain gets longer (N$>$300), indicating a random coil conformation. The electric potential of the single PSS chains is determined and the results clearly show the decrease of the effective charge density with the increase of molecular weight. The results indicate that the charged chain has increased extent of counterions binding at longer chain length, and therefore takes different conformation.   

Tuning Thermal Transitions in Dry and Hydrated Polyelectrolyte Layer by Layer Assemblies with Ionic Strength and pH

Layer-by-layer (LbL) assemblies are of significant interest for their potential applications in diverse fields such as energy and drug delivery. However, characterizing their thermal properties has remained a challenge. Here, we present the characterization of dry and of hydrated LbL films containing strong polyelectrolytes poly(diallyldimethylammonium chloride) (PDAC) and poly(styrene sulfonate) PSS) using modulated differential scanning calorimetry (MDSC) and temperature controlled quartz crystal microbalance with dissipation (QCM-D). Our results suggest that hydrated exponentially growing (assembled from 0.25-1.25 M NaCl solutions) PDAC/PSS LbL films have glass transition temperatures ($T_{g}$'s) between 48-51 $^{o}$C, while linearly growing (assembled from 0 M NaCl) films did not. Other systems explored include poly(allylamine hydrochloride)/poly(acrylic acid)(PAH/PAA) LbL assemblies, which demonstrate linear/exponential growth depending on assembly pH conditions. These results support a standing hypothesis in that linear (or exponential) growth is observed for glassy (or rubbery) LbL films. We have also demonstrated for the first time, thermal transitions in thin PDAC/PSS LbL films using QCM-D by monitoring fluctuations in film hydration and viscoelasticity by probing the film's internal structure as a function of film depth.   

Anomalous $\chi $ for Polydisperse Polystyrene-$b$-Poly(octyl acrylate)

We have performed small-angle neutron scattering (SANS) measurements on a disordered block copolymer from deuterated polystyrene (dPS) and self-adhesive poly(octyl acrylate) (POA) in order to elicit the effective Flory-Huggins $\chi $, which carries the essence of the copolymer phase behavior. The sample for the measurement was prepared by blending two polydisperse dPS-$b$-POAs of different molecular weights for the purpose of adjusting the average size to a proper value. The SANS profiles for the copolymer were fitted to Leibler's scattering function for a polydisperse copolymer system described by Schulz-Zimm distribution. It was shown that the resultant $\chi $ as a function of inverse temperature has a strong entropic contribution and a weak enthalpic contribution. By adopting a simple analysis for specific interactions, it was found that the entropically dominated $\chi $ for dPS-$b$-POA arises from the steric hindrance of long alkyl side groups of POA.   

Thermal Rupture Of Linear Polymer Chains Under Tensile Stress

The thermal rupture of a linear alternating copolymer fixed at one end and pulled by a constant force at the other end has been studied using molecular dynamics simulation. The dependence of the first breakage time distribution on the mass ratio of the constituent beads has been studied. The Arrhenian nature of the scission process has been confirmed and an estimate of the effective energy barrier has been made.   

Phase Behavior of Hydrogenated Derivatives of Linear ABC Block-Random Copolymers of Styrene and Isoprene

The capacity to synthesize block-random copolymers, block copolymers with one or more random copolymer blocks, allows for continuous tuning of the inter-block average segmental interaction parameter, $\chi $, through the composition of the random copolymer without changes to system chemistry. By lithium-initiated anionic polymerization in a mixture of cyclohexane and triethylamine, we synthesized an asymmetric near-monodisperse linear ABC block-random copolymer of styrene and isoprene: S$_{30}$-I$_{12}$-SrI$_{12}$, where SrI denotes a random copolymer block with 50 wt. {\%} styrene, and block molecular weights are 30-12-12 kg/mol. Upon complete hydrogenation, the VCH-hI-VCHrhI triblock exhibits microphase separation into a well-ordered two-domain lamellar structure with an order-disorder transition between 180-185\r{ }C via small-angle x-ray scattering. This two-domain lamellar structure is confirmed using electron density modeling of the SAXS peak intensity and domain spacing arguments. A three-domain lamellar structure is expected in the diene-selective hydrogenated derivative, S-hI-SrhI, due to increased $\chi $ between the middle and end blocks. Additional S-I-SrI and S-SrI-I triblock copolymers are being synthesized and the effects of block sequence, end block molecular weight, and hydrogenation (S vs. VCH) on phase behavior are being explored.   

Fast field theoretic simulation of block copolymers with the mode Monte Carlo method

We explore the self assembly of block copolymers by using a field theoretic simulation method. Former studies on field theoretic simulation methods update the fields by Langevin-like dynamics which is a local update of the fields in real space. Our simulation method updates the mode structure (k-space) of the fields and transfers them to real space. By using a Monte Carlo scheme for updating fields, we decrease simulation time by nearly an order of magnitude. We also consider new self-learning searching strategies using this method. We tested our algorithm by simulating the phase diagram for thin films of diblock copolymers in select graphoepitaxial templates and by simulating the self-assembly of nanoparticles and block copolymers.   

Fluctuations/Correlations in Symmetric Diblock Copolymers: Simulations and Theories

Modeling symmetric diblock copolymers as discrete Gaussian chains with soft, finite-range repulsions as commonly used in dissipative-particle dynamics simulations, we have performed fast off-lattice Monte Carlo (FOMC) simulations1 in a canonical ensemble with variable box lengths to study the thermodynamic and structural properties of both the disordered and lamellar phases. Our FOMC results for the disordered phase are further compared, without any parameter-fitting, to those from the reference interaction site model (RISM) and the polymer reference interaction site model (PRISM) theories, as well as the Gaussian fluctuation theory, based on the same model system. Such direct comparisons unambiguously and quantitatively reveal the consequences of various theoretical approximations and the validity of these theories in describing the fluctuations/correlations in disordered diblock copolymers. \\[4pt] [1] \textit{Q. Wang and Y. Yin}, \textbf{J. Chem. Phys., 130}, 104903 (2009).   

Transformation kinetics of BCC sphere morphology of diblock copolymer under electric field

We performed Dynamic Self-Consistent Field simulation of BCC sphere morphology of diblock copolymers under an electric field. Several distinct kinetic pathways were determined. Different pathways were found depending on the strength of the applied electric field and orientation of the BCC domain. The fastest growing mode was found in 111 directions. Dynamic critical exponents were deducted from the simulation data. The results are compared with the previous works on different morphologies and on BCC morphology by different techniques.   

Genistein Modified Polymer Blends for Hemodialysis Membranes

A soybean-derived phytochemical called genistein was used as a modifying agent to polyether sulfone/polyvinyl pyrrolidone (PES/PVP) blends to produce multi-functional hemodialysis membranes. With the aid of phase diagrams of PES/PVP/genistein blends, asymmetric porous membranes were fabricated by coagulating in non-solvent. Both unmodified and genistein modified PES/PVP membranes were shown to be non-cytotoxic to the blood cells. Unmodified PES/PVP membranes were found to reduce reactive oxygen species (ROS) levels, whereas the genistein modified membranes exhibited suppression for $\sim$60$\%$ of the ROS levels. Also, the genistein modified membranes revealed significant suppression of pro-inflammatory cytokines: IL-1$\beta$, IL-6, and TNF-$\alpha$. Moreover, addition of PVP to PES showed the reduced trend of platelet adhesion and then leveled off. However, the modified membranes exhibited suppression of platelet adhesion at low genistein loading, but beyond 15 wt$\%$, the platelet adhesion level rised up.   

Solubilization of Genistein in Poly(Ethylene Glycol) via Eutectic Crystal Melting

Genistein (5,7,4'-trihydroxyisoflavone) is a phytoestrogen found in soybean. It possesses various biological/pharmacological functions, e.g., tyrosine kinase inhibitory, anticarcinogenic, antioxidant, anti-inflammatory, and anti-microbial activities. However, genistein has poor water solubility and skin permeability, which have seemingly prohibited the progress to preclinical evaluation. Eutectic melting approach has been performed as a means of solubilizing genistein in poly(ethylene glycol) (PEG). Eutectic phase diagrams of blends containing genistein and PEG having three different molecular weights, i.e., 44k, 7k, and 500 g/mol, were established by means of DSC and compared with the theoretical liquidus and solidus lines, calculated self-consistently by taking into consideration all interactions including amorphous-amorphous, crystal-amorphous, amorphous-crystal, and crystal-crystal interactions. The eutectic temperatures were found to decrease with decreasing molecular weight of PEG. Guided by the phase diagram, it was found that genistein can be dissolved in PEG500 up to $\sim $7 wt{\%} at room temperature. More importantly, the solubility of genistein in PEG can be improved to meet the end-use criteria of the PEG/genistein mixtures.   

Topographically Uniform but Chemically Heterogeneous Nanostructures by Nanoimprinting Demixed Polymer blends

This study examined the coarsening process of phase-separated polymer blends under physical confinement. The physical confinement was realized by nanoimprinting phase-separated PS (polystyrene)/PMMA (polymethylmethacrylate) blend thin films. The influences of the imprint temperature, blend composition and film thickness on the morphological evolutions were systematically investigated. All the patterned PS/PMMA films showed topographically uniform structure after nano-imprinting, regardless of the surface roughness caused by the initial stage of the phase separation. The morphologies, or the phase structures, of the PS/PMMA patterns were found to be dictated by the preferential wetting of PMMA onto silicon oxide surface. The interplay of this preferential wetting and the domain coalescence resulted in a range of complex and unique encapsulated structures. Furthermore, by inhibiting such preferential wetting of a blend component using a neutralized surface, non-capsulated morphology can be achieved.   

Properties of PET/PLA Electrospun Blends

Electrospun membranes were fabricated from poly(ethylene terephthalate), PET, co-spun with poly(lactic acid), PLA. The PLA contained 2{\%} of the D-isomer, which served to limit the overall degree of crystallinity. Membranes were deposited from blended solutions of PET/PLA in hexafluoroisopropanol. The PET/PLA composition ranged from 0/100, 75/25, 50/50, 25/75, and 100/0. Electrospun membranes were made using either a static flat plate or a rotating wheel as the counter electrode, yielding unoriented mats or highly oriented tapes, respectively. We report on our investigation of the crystallinity, crystal perfection, and mechanical properties of these materials using differential scanning calorimetry, wide and small angle X-ray scattering, and dynamic mechanical analysis. In particular, we study the ability of one blend component (PET) to crystallize in the presence of existing crystals of the second blend component (PLA) which crystallizes first and at a lower temperature than PET.   

Reactive Poly(Amic Acid)/ Poly(Glycidyl Methacrylate-r-Poly(ethylene Glycol) Methyl Ether Methacrylate) Blends as Gas Permeation Membranes

Polymers containing polar moieties, such as ether groups show an affinity for acidic gases, such as CO$_{2}$ due to dipole-quadrapole interactions. Polymer blends in which one of the components is poly(ethylene glycol) (PEG) have been studied extensively in literature as a CO$_{2}$/light gas permeation membrane, but due to the crystallization and poor mechanical properties have been difficult to incorporate PEG above 60wt{\%}. In this study, a series of random copolymers containing both glycidyl methacrylate and poly(ethylene glycol) methyl ether methacrylate in different ratios are blended with a poly(amic acid) prepolymer made from 4, 4'-oxydianiline and pyromellitic dianhydride to create gas permeation membranes. By using a reactive blend PEG loadings above 70{\%} have been realized with sufficient mechanical properties, and since the side chain on the PEGMA is short these blends do not suffer from crystallization.   

Evaluation of Flame Retardancy, Mechanical Properties, and Bicompatibility of HIPS/PBrS Blends

Our research focused on thermal and mechanical properties of High Impact Polystyrene (HIPS) system. Brominated Polystyrene (PBrS) was incorporated to replace halogenated Flame Retardant (DB) in HIPS blends. We have previously shown that ditallow functionalized clays could become nearly universal class of compatiblizers [si-2006]. Here we show that a new type of surface with Resorcinol bis (biphenyl phosphate) (RDP) could achieve the same goals. We demonstrate the strong compatibilization on the highly immiscible systems of HIPS/PBrS. Furthermore, we show that this system also works well, when a third component, Antimony Trioxide (AO) is added to provide flame retardant properties. Tensile test, dynamic mechanical analysis, and UL-94 flame test were applied to investigate this system. We found that the amount of AO used in flame retardant formulations could be minimized by addition of RDP clay, which could also increase some mechanical properties that Cloisite 20A clay couldn't. Besides, we evaluated the toxicity of Cloiste 20A and RDP clay. Langmuir-Blodgett trough and atomic force microscopy were used to make and check monolayer clay. Confocal Microscopy was used to assess cell morphology. The results showed RDP clay has potential for biomaterial applications.   

PVDF/PVIm polymer blend films for fuel cell membranes

We report the preparation and characterization of binary blend films of poly(vinylidene fluoride) (PVDF) and poly(1-ethyl-3-vinylimidazolium trifluoromethylsulfonimide) (PVIm+TFSI-). The potential utility of such materials in proton exchange membrane fuel cells is of particular interest. Thin PVDF/ PVIm+TFSI- films were fabricated from solutions of dimethly formamide by doctor blading. The nature of the PVDF crystalline polymorph and degree of crystallinity was evaluated as a function of the volume fraction of imidazolium polymer and thermal treatment. The morphology, thermal and mechanical characteristics of the blend films was studied by wide angle X-ray diffraction, thermogravimetry, calorimetry, and Fourier transform infrared spectroscopy. In these materials, conditions such as choice of solvent, drying conditions, and thermal treatment affect the crystal phase, crystallite size, and degree of crystallinity of PVDF as well as the distribution of PVIm+TFSI-. The beta phase of PVDF crystals dominates in as-cast films, while the alpha phase is observed after cooling from the melt. PVDF imparts mechanical strength and chemical stability to the composite films, and because of its high crystal melting point (Tm $>$ 160 C), serves to improve the high temperature stability of resulting films.   

Polydispersity Effects on Scaling Behavior of Polymers Grafted on Surfaces of Varying Curvature

Nanoparticle assembly can be tuned by grafting nanoparticle surface with polymers and in turn manipulating the interactions between the particles and medium. Despite past studies showing the importance of graft molecular weight on effective inter-particle interactions in monodisperse polymer grafted nanoparticles, and evidence of non-trivial polydispersity effects in systems containing polymers grafted on flat surfaces, not much work has been done to explore polydipsersity effects in polymer grafted nanoparticles. Using Monte Carlo simulations we elucidate how polydispersity (PDI) in grafted chain lengths affects chain conformations, and in turn the scaling behavior in systems of polymer grafted particles, as a function of grafting density and particle size. With increasing PDI the scaling exponent of the grafted chains at intermediate and high grafting density approaches that of a single chain grafted on the nanoparticle of same size. This is because longer chains in polydisperse systems have to stretch less due to reduced crowding from neighboring shorter chains as compared to monodisperse system. Also, due to reduced crowding effects at high PDI, in densely grafted particles the differences in chain conformations arising from varying curvature becomes negligible.   

Size Controlled Polymer Coated Nanoparticles as Efficient Compatibilizers for Polymer Blends

Polymer-coated gold nanoparticles (Au) with controlled size and surface were successfully synthesized and applied to tailor the structures and properties of polytriphenylamine (PTPA) and polystyrene (PS) blends. Two different polymer-coated Au NPs with sizes of 5.9 nm (Au1) and 20.7 nm (Au2) were designed to be thermally stable above 200 $^{\circ}$C and neutral to both PS and PTPA phases. Hence, both Au NPs localize at the PS/PTPA interface and function as compatibilizers in the PS/PTPA blend. To show the compatibilizing effect of the particles, the morphological behaviors of PS/PTPA blends containing different particle volume fraction of Au NPs were observed using cross-sectional TEM, and for quantitative analysis, the size distribution of PTPA droplets in the PS matrix was obtained for each sample. The number-average droplet diameter (Dn) of the PTPA domain in the blend was dramatically reduced from 1.4 $\mu $m to 500 nm at a small volume fraction of 1.0 vol{\%} Au1. The same trend of decreasing Dn was also observed with the addition of larger Au2, but a higher volume fraction was required to obtain the same amount of reduction in the PTPA droplet size. To demonstrate the effectiveness of Au NPs as compatibilizers, PS-b-PTPA block copolymers were also synthesized and used as compatibilizers.   

Remote fluorescence-based sensing of elastic thin-film elastic moduli

The fluorescence lifetime of dyes embedded within thin polymer films is sensitive to the local mechanical environment. Here we show that this enables direct measurement of elastic properties of thin films and small samples whose investigation by conventional macroscopic mechanical characterization would not be possible. The specific example of poly(ethylene oxide) (PEO) and its nanocomposite thin films is highlighted. The method is also validated by comparison to a family of other polymers of known macroscopic moduli. Being simple, rapid, and reliable, we propose that this analysis can in principle apply generally to a broad class of soft materials and other polymer multilayer films.   

Macroscopic Ordering of CNTs in a Liquid Crystalline Polymer Nano-Composite by Shearing

We present a series of complimentary experiments exploring the macroscopic alignment of carbon nanotubes (CNTs) in a liquid crystalline polymer (isotactic polypropylene - $iPP$) nano-composites as a function of temperature, shear, and CNT concentration. The phase behavior of $iPP$+CNT, studied by Modulated Differential Scanning Calorimetry, revealed the evolution of the $\alpha$-monoclinic transition and its dynamics, which are dependent on CNT content and thermal treatment. These results indicate that the CNT nucleates crystal formation from the melt. Spectroscopic ellipsometry reveals a change in the optical constants that are connected to the ordering of CNTs when the $iPP$+CNT is sheared. This anisotropy is also exhibited in measurements of the electrical and thermal conductivities parallel and perpendicular to the shear direction. The amount of order induced into the dispersed CNTs is relatively low for these low concentration samples ($< 5$~wt\%).   

Self-assembly Nano-filled Polymer Blends in a Photovoltaic Thin-film Device

Engineering heterodyne junction solar cells requires a well-defined morphology of the photoactive polymers and the PCBM conductors such that maximum current reaches the electrodes with minimal resistive scattering. One possible method for accomplishing this may be to use polymer phase segregation in combination with the nanoparticles' natural segregation to the interfaces. In this manner, large-scale devices can be formed using self-assembly methods, rather than fixed methods. We have used Molecular Dynamics simulation to predict the morphology of polymer blends and determine which combination of factors would yield the optimal morphology that would contact the electrodes, while producing the largest number of interfaces. Secondly, we were also able to determine the conditions that would cause the particles to segregate and template along the interfaces, which would provide direct conductivity to the electrodes. Using thin film and bulk structures and by manipulating particle size, the attraction between the particle and the polymer component, and the amount of filler within the material, we can explore the formation of cheaper, more effective and efficient networks.   

Transformative Skin: From Anti-biofouling to Reliable Power Cable

This talk will discuss the fundamental physics and potential applications of transformative skin, an electroactive polymer system recently developed at Duke Soft Active Materials Laboratory (SAMs Lab). The working mechanism of the transformative skin is based on the creasing-to-cratering instability in polymers under electrical voltages. The instability can induce failures in power cables and polymer capacitors. By suppressing the instability, one can greatly enhance the reliability and energy density of the cables and capacitors. Surprisingly, the same instability can generate a rich variety of on-demand patterns on polymer surfaces in response to voltages. The feature size of the pattern can be tuned from millimeter to nanometer. The pattern formation and surface deformation can dynamically debond biofilms formed on polymer surfaces, giving extraordinary capability of active antibiofouling.   

Molecular Dynamics Simulation of Diffusivity and Mobility of Ionic Charge Carriers in Model Battery Polymers

Molecular dynamics simulations of poly(ethylene-co-acrylic acid) monomers doped with Lithium ions were conducted using LAMMPS. The drift velocity of the ions resulting from a range of static electric fields were used to compute the ionic mobility. The frequency dependent mobility was also studied by using oscillatory electric fields.   

Hypersonic properties of polymer films and multi-layers

Picosecond acoustic measurements were performed on ultrathin films of polymers and thin film polymer multilayers supported on silicon (Si) substrates using a state of the art THz acoustic technique. In these experiments, a high power laser is used to excite picosecond duration strain pulses in an aluminium film evaporated on the reverse side of the Si substrate. These strain pulses then propagate through the substrate and interact with the polymer film/multi-layer. Vibrations in the film are detected optically using the same (pump-probe) beam which is passed through an optical delay line and reflected from the surface of the polymer film/multi-layer. Ultrathin films of polystyrene and a styrene-butadiene-styrene block copolymer were found to exhibit quantized closed-pipe organ like modes in the 0- 50 GHz regime that were attributed to vibrations of the entire polymer film. Thin film polystyrene/polyvinylpyrrolidone multilayer structures were found to display folded phonon dispersion curves that are characteristic of super-lattice structures. These structures have potential applications in GHz and THz optical switching and biosensing applications.   

Local stress in thin polystyrene films

Thin polymer films have received significant attention due to the deviation of material properties from bulk values. Part of the difficulty in describing the underlying physics is an incomplete understanding of how internal and surface stresses affect thin films. Here we exploit ``wrinkling'' to create a simple model system in which local stress can be easily tuned. When a thin polymer film bound to an elastic substrate is subjected to a compressive stress the film buckles out of plane. Due to the curvature at each crest and trough of the wrinkles, there arise local stresses in the polymer film. The curvature of the wrinkles is controlled by the modulus of the film and substrate, the film thickness and the applied stress, allowing us to apply an arbitrary local stress. After wrinkling, films are annealed above their glass transition temperature which allows flow and relaxes any stress. The local stress is then transferred to that of a thickness variation in the thin film. Importantly the flow of thin polymer films can be modeled using the well established lubrication theory, resulting in a simple scaling model. Our model allows us to investigate the response of thin films where deviations from bulk behavior are expected, as well as more complex thin diblock polymer films.   

Cracking in thin films of colloidal particles on elastomeric substrates

The drying of thin colloidal films of particles is a common industrial problem (e.g paint drying, ceramic coatings). An often undesirable side effect is the appearance of cracks. As the liquid in a suspension evaporates, particles are forced into contact both with each other and the substrate, forming a fully wetted film. Under carefully controlled conditions the observed cracks grow orthogonal to the drying front, spaced at regular intervals along it. In this work we investigated the role of the substrate in constraining the film. Atomic force microscopy, was used to image the particle arrangements on the top and bottom surfaces of films, dried on liquid and glass substrates. We present convincing evidence that the interface prevents particle rearrangements at the bottom of the film, leading to a mismatch strain between upper and lower surfaces of the film which appears to drive cracking. We show that when the modulus of the substrate becomes comparable to the stresses measured in the films, the crack spacing is significantly altered. We also show that cracks do not form on liquid substrates. These combined experiments highlight the importance of substrate constraint in the crack formation mechanism.\\[4pt] [1] M.I. Smith, J.S. Sharp, Langmuir 27, 8009 (2011)   

Reaction Kinetics at the Interface between Immiscible Polymers: Competition between Diffusivity and Reactivity

Reactive blending processes at the interface between deuterated bisphenol-A polycarbonate ($d$PC) and amorphous polyamide ($a$PA) bilayer film were characterized by Fourier transform infrared (FTIR) and neutron reflectivity (NR). It was found that the aminolysis occurred during thermal annealing at 160 -- 180 $^{\circ}$C, inducing simultaneously scission of $d$PC chains and formation of $d$PC-aPA copolymer chains. Two or three stages of reaction kinetics as a function of time were probed by FTIR, depending on the competition between chain diffusivity and chemical reactivity for sample annealing at different temperatures. The late stage was controlled by potential barrier arising from previously formed copolymer, and it appeared earlier when annealing at 160 $^{\circ}$C than that at higher temperatures. A phenomenon of transient interfacial instability which origin was ascribed to the mismatching in mobility of the polymer chains on either side of the interface was observed by NR. The copolymer once formed, remains localized at the interface and inhibits the diffusion of other reactive polymer chains still present in the bulk phase toward the interface.   

Thermal Behaviors of PS Thin Films on Grafted PS Layer with Varied Grafting Density

The thermal behavior properties (especially glass transition) are the key parameters for determining the mechanical properties of polymer system. In thin film system, the interfacial interactions at the substrate/polymer and polymer/air influence the transition behavior. In this study, we investigated the inter-relationship between the interfacial interactions arising from grafted polymer layers and the glass transition behavior (Tg) of thin polymer films with same chemical identity. We controlled grafting density of hydroxyl-terminated polystyrene (HO-PS). For polymer chains on the brushes of the same chemical identity with high density, the autophobic dewetting behavior can be attributed to the entropic effects of the polymer and the grafted polymer chains.   

Instability of Surface-initiated ATRP Polyelectrolyte Brushes in Aqueous Environments

Surface-bound macromolecules have been produced using a number of polymerization schemes, including free-radical polymerization (FRP), reversible addition-fragmentation chain transfer polymerization (RAFT), and atom transfer radical polymerization (ATRP). In order to prove useful in any technology, the tethered polymer chains must remain stable in a variety of environments over relatively long timescales. We have investigated the dependence of the pH and ionic strength of aqueous solutions on the stability of surface-bound polyelectrolyte chains (strong and weak) with varying molecular weights and grafting densities. Our findings suggest that the ester bond in the most common form of ATRP surface initiator (BMPUS) will hydrolyze over a broad pH range, leading to chain degrafting. We further compare the stability of a BMPUS derivative which has had the ester bond replaced with an amide bond, as well as a free-radical initiator containing only aliphatic carbons. Results related to the effect of chain tension on brush stability will also be presented. Finally, we discuss the likely mechanism of degrafting, and ways in which to improve stability.   

Lateral phase separation of mixed polymer brushes on planar and spherical surfaces

A mixed polymer brush consists of two (or more) polymer species grafted to a surface at a high density, inducing the polymers to highly stretch to maximize favorable solvent interactions while minimizing polymer overlap. The enthalpic and entropic interactions between the different polymers give rise to lateral phase behavior on the surface. Understanding this phase separation behavior is interesting for applications in nanotemplating and controlled protein adsorption. In this work, we present a novel theoretical model to quickly predict lateral phase separated morphologies of mixed polymer brushes on planar, cylindrical and spherical surfaces. The model combines a Flory-Huggins model for enthalpic interactions between the polymer components with an Alexander-de Gennes model for the entropy of the brush layers. When there is a length difference between the polymer components, these two interactions along with the conformational entropy of the system lead to a range of morphologies including stripes, dimples, mixing, and complete phase separation. The computational efficiency of this model allows for phase diagrams to be generated with great accuracy. The results of our model thus allow for the fast prediction of lateral morphologies on different geometries.   

Photonic multilayer sensors from photo-crosslinkable polymer films

Photo-crosslinkable copolymers containing pendent benzophenone (BP) groups provide a convenient means to fabricate multilayer polymer films. We describe the preparation of alternating multilayers of photo-crosslinkable poly(N-isopropylacrylamide) (PNIPAM), a water-swellable, temperature sensitive polymer, and poly(para-methylstyrene) (PpMS), a non-swellable polymer, by sequential spin-coating and photo-crosslinking. This route provides well-defined layered structures with minimal interfacial broadening between layers and uniformity of thickness from layer to layer as determined by dynamic secondary ion mass spectrometry (d-SIMS). Appropriate choices of layer thicknesses yield 1-D photonic gel sensors. The reflectance peak is shifted through the visible spectrum upon swelling or de-swelling of the PNIPAM layers in water, providing an accessible means for colorimetric temperature sensing.   

Template polymerization using a controlled reaction scheme

We employ a Monte Carlo simulation scheme based on the bond fluctuation model to simulate template polymerization via controlled scheme (i.e., termination and chain transfer reactions are neglected) involving co-polymerization of free monomers and monomers bound to a template that consists of four linear substrates with equispaced sites occupied by bound monomers. A new macromolecule is initiated by activation of an initiator; any monomer (free or bound) that is within the reaction distance (nearest neighbors) of the initiator can be incorporated into the chain. As the chain propagates, it adds new monomers to the macromolecule. Those monomers can either be bulk (i.e. free) monomers or those that are placed on the predefined template. We analyze the effect of the number of spacing of the bound monomers on the composition and monomer distribution in the resultant co-polymer. Our results reveal that the larger the total number of bound monomers in the system and the more dense spacing, the greater is the likelihood of those getting incorporated in the growing chains. In addition, a greater number of bound monomers on the linear template promotes polymerization of most/all of the bound monomers to form a linear array attached to the template.   

Wrinkling of Inhomogeneously Strained Thin Polymer Films

Surface wrinkles have received much attention recently due to their potential importance in applications. Wrinkles occur due to a mechanical instability when sufficient strain applied to an incompressible thin film. For wrinkles made with a polymer film supported on a soft elastomer, it has been well-established that the amplitude is directly proportional to the wavelength and the square root of the applied strain. Importantly, the wavelength is largely insensitive to the applied strain and defined by the properties of the film and substrate. These dependences have been confirmed with ideal substrates where the global strain is homogeneously distributed, but the influence of strain inhomogeneity has not been considered previously. Will regions that wrinkle first lead to strain localization, or will the system try to homogenize strain globally? We use a recently developed adhesion contact line method to prepare polystyrene thin films with periodic regions of different wrinkle amplitudes, hence strains, on soft substrates. We find that surfaces with multiple wrinkle amplitudes will approach a single amplitude globally upon the application of sufficiently large strains. We derive relationships to describe this process.   

Comparison of surface mobility on low and high molecular weight glasses

The glass transition temperature of thin polymer films and the related issue of mobility at glass surfaces have attracted considerable interest in the last twenty years. We have recently conducted experiments on both polymeric and low molecular weight glass formers that provide information about mobility near the free surface. For several polymers, measurements of dye reorientation on freestanding films show a fast population that can be consistently assigned to the near-surface region. For indomethacin (a low molecular weight glass former), we have directly measured surface diffusion by following the decay of imprinted gratings of different periods. For both high and low molecular weight glassformers, we observed enhanced mobility near the free surface (by factors up to 10$^{4}$ and 10$^{7}$, respectively) relative to the bulk dynamics. In both cases, surface mobility has a substantially weaker temperature dependence than bulk mobility near the bulk Tg.   

Molecular Bottlebrushes as Tensile Machines for Probing Specific Bonds under Tension

Significant tension on the order of 1 nN is self-generated along the backbone of bottlebrush macromolecules due to steric repulsion between densely grafted side chains. The intrinsic tension is amplified upon adsorption of bottlebrush molecules onto a substrate and increases with grafting density, side chain length, and strength of adhesion of the substrate. This allows us to employ these molecular bottlebrushes as miniature tensile machines to probe the mechanochemistry of specific bonds. For this purpose, bottlebrush macromolecules with a disulfide linker in the middle of the backbone were synthesized by atom transfer radical polymerization (ATRP). Two processes, (i) homolytic cleavage of disulfide and (ii) scission of disulfide due to reduction by dithiothreitol were monitored through molecular imaging by atomic force microscope (AFM). In both cases, the corresponding rate constants increase exponentially with mechanical tension along the disulfide bond. Moreover, the reduction rate at zero force is found to be significantly lower than that in bulk solution, which suggests an acidic composition of the water surface with pH=3.7. This opens a new application of brush-like macromolecules as surface pH sensors.   

Simultaneous Vapor Deposition and Phase Separation of Polymer Films

Initiated chemical vapor deposition (iCVD) is a solventless, free radical technique used predominately to deposit homogeneous films of linear and crosslinked polymers directly from gas phase feeds. The major goal of this research is to force and arrest phase separation of deposited species by co-depositing non-reactive molecules (porogens) with reactive monomers and crosslinkers. We introduce these species during iCVD to force and quench polymer induced phase separation (PIPS) during film growth as a step toward tunable pore-size, density, and morphology. Polymerization, crosslinking and PIPS are intended to occur simultaneously on the substrate, resulting in a vitrified microstructure. Cahn-Hilliard theory predicts that the length scale of phase separation depends on the polymer-porogen interaction energy, the polymerization rate and the species' mobility. A series of films were grown by varying deposition rate, porogen type, and reagent flowrates. Crosslinkers were introduced to limit the growth of phase separated domains and to provide mechanical support during porogen removal. To elucidate how phase separation competes with polymerization and film growth, deposited films were studied using a combination of electron microscopy, profilometry and spectroscopic techniques.   

Controlled viscous and viscoelastic instabilities of polystyrene film on topographically patterned poly(methyl methacrylate)

We report the spontaneous morphological evolution of polystyrene (PS) films spin-cast on pre-patterned poly(methyl methacrylate) (PMMA) substrates, at annealing temperatures above the glass transition temperatures of both polymers. The influences of the molecular weights and spin-cast volume of PS on the morphological development were systematically examined. Both were found to modulate the instabilities and result in distinctive morphologies. In particular, thick PS films that fully covered the topographically patterned PMMA substrate would dewet through nucleation and growth of randomly formed holes. However, the rupture of thinner PS films were manifested through formation of arrays of non-axisymmetrically confined threads and subsequent capillary instabilities of these threads. Low and high molecular weights of PS gave rise to two different modes of thin-thread instabilities, identified as varicose and peristaltic mode, respectively.   

Novel Synthesis of Surface-Grafted Radical Initiator With Improved Stability and Yield

Polymers grafted at interfaces are attractive for applications including antifouling coatings, biologically functionalized materials, and responsive surfaces. The ``grafting from'' approach to form densely packed brushes involves functionalizing a substrate with initiating moieties, and carrying out polymerization, e.g., surface-inititated free-radical polymerization (SI-FRP). Azo initiators for SI-FRP are commonly synthesized by a low-yielding route requiring large amounts of potassium cyanide. Those initiators are linked to a substrate by means of an ester group, rendering tethered polymers susceptible to degrafting by hydrolysis. We present a novel synthetic route to an asymmetric azo initiator, whose yield is nearly double that of previous syntheses. Cyanide usage in the new method is reduced by 72 percent. The spacer linking the initiator to a substrate contains only carbon-carbon bonds, resulting in stable brushes. Results are demonstrated for SI-FRP as well as reverse ATRP and RAFT.   

Impact of adhesion on thermal boundary conductance at organic-metal interface

The thermal boundary conductance (TBC) at the interfaces between a small molecular organic semiconductor (copper phthalocyanine CuPc) and several metals (Al, Mg, Au, Ag) has been measured using the 3-omega method at room temperature. The TBC increases with the interfacial bonding strength of the two dissimilar materials. Our measurements of adhesion between the metal and organic films agree qualitatively with the distances between the metal surface and CuPc molecules measured by Stadtm\"{u}ller et al. [1]. In contrast to the trend observed for organic-metal interfaces, the TBC of organic-organic interfaces (e.g. CuPc/C60) is insensitive to interface bonding due to the large overlap between materials' phonon density of states. Conventional acoustic and diffuse mismatch models fail to describe the observed trend in TBC. Modifying the acoustic mismatch model (AMM) with an effective spring constant describing interfacial bonding, an accurate trend is obtained. To better understand the influence of the molecular structure on TBC, we performed molecular dynamics simulations of interfacial heat transfer and interfacial bonding. These simulations reveal quantitative discrepancies in the simulated TBC relative to that predicted by the modified AMM model in the regime of poor interfacial adhesion, which we attribute to a greater contribution of anharmonicity. The bonding strength for vacuum deposited films is $\sim $2 GPa for CuPc/Ag and CuPc/Au, well within the regime in which anharmonic processes play an important role in interface thermal transport. [1] Stadtm\"{u}ller et al. \textit{PRB} 83 085416   

Ion Correlations at an Electrified Liquid/Liquid Interface

Ion correlations have been suggested as the underlying mechanism of a number of counterintuitive phenomena such as like-charge attraction. Here we present the first molecular-level tests of density profiles predicted by an ion correlations model. Synchrotron x-ray reflectivity reveals ion condensation at the liquid/liquid interface, when polarized with an electric field.\footnote{N. Laanait et al. J. Chem. Phys., \textbf{132}, 171101, 2010} Tuning the density of this ionic layer allows for a detailed study of ion correlations as a function of the Coulomb coupling strength in the system. We propose a parameter-free density functional theory that describes ion-ion correlations within a weighted density approximation and explicitly treats ion-solvent effects through a solvent interaction potential simulated by molecular dynamics. Agreement with the x-ray reflectivity and the interfacial excess charge is found over the entire experimental range of ion-ion correlation energies up to nearly 4 $k_{B}T$. These results suggest that ion correlations in the electrical double layer can be accurately described by mapping to a simple Coulomb system, in this case a one-component plasma.   

Fluctuations of one-dimensional interface in the directed polymer formulation: role of a finite interface width

An elastic interface living in a disordered medium always exhibits geometrical fluctuations, characterized in particular by the distribution of its relative displacements as a function of the lengthscale $r$, whose variance defines the interface roughness $B(r)$. Those fluctuations are the manifestation of the probability and associated effective free-energy of the different configurations of the interface, in presence of disorder and at finite temperature. Focusing specifically on the one-dimensional interface, we use the exact mapping of the static interface on the directed polymer in random medium in order to explore both analytically and numerically the role of a finite interface width $\xi>0$, assuming a short-range elasticity and a random-bond quenched disorder. Confirming the existence of a low-temperature regime where the finite microscopic width plays a crucial role, as predicted by previous Gaussian-Variational-Method predictions [Phys.Rev.B 82, 184207 (2010)], we propose a coherent picture of the physics at stake, compatible both with numerical computations and generic scaling arguments.   

Wang-Landau simulations of polymer adsorption on diluted surfaces

We consider a single linear lattice homopolymer in three dimensions that interacts with a diluted planar surface. A fraction $p$ of the total number of the sites on the substrate is attractive, while the remaining $1-p$ remains neutral. Our focus is on the conformational transitions the polymer can experience under different environmental conditions, for instance, the surface dilution and the strength of the substrate attraction, compared to the intensity of the monomer-monomer interactions. To get insights on the phase diagram we have performed extensive Monte Carlo simulations, by using the Wang-Landau sampling, for different values of the surface attraction $\epsilon$ and the concentration of attractive sites $p$, specially near the surface percolation threshold $p_c$.   

The Behavior of Water at the Interface with Polystyrene

Solid-aqueous interfaces are of great importance in many industrial applications ranging from oil recovery to biotechnology. The behavior of interfacial water differs drastically from that of the bulk liquid and strongly depends on the atomistic details of the surface itself. Molecular dynamics simulations have been used extensively to study the structure and dynamics of the interface between a polymeric thin film and water. Using a fully atomistic molecular dynamics simulation, we have examined the structure and dynamics of water and atactic polystyrene (aPS) chains near the aPS-water interface. In this talk, we present results for the contact angle of water and the interfacial surface tension at the aPS-water interface.   

MD study of polymer melts confined in thin films and nanopores

The structure of chains is strongly affected if they are confined to films or pores smaller than the radius of gyration [1,2]. We report new molecular dynamics simulations of polymer melts confined between structureless walls comparing the thin film and pore confinement. It is shown that the form factor which is measured in scattering experiments is affected in a subtle way by the corrections to ideality [2]. The strong confinement reduces the number of overlapping chains and thus reduces entanglements. This leads to a clear acceleration in thin film confinement. For pore confinement, an acceleration is also found for intermediate pore diameters, but for extreme confinement, chains are segregated and they will block each other. [1] H. Meyer et al Eur. Phys. J. Sp.Top. 141 (2007) 167. [2] N. Lee et al EPL 93 (2011) 48002.   

Adsorption of Derivatized Dextran Polyelectrolytes onto Nanocrystalline Cellulose

The adsorption of a series of cationically derivatized dextran polyelectrolytes onto anionic nanocrystalline cellulose (ANC) has been studied using quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR). Samples of dimethylaminoethyl-dextran (DMAE-Dex), diethylaminoethyl-dextran (DEAE-Dex), and diisopropylaminoethyl-dextran (DIAE-Dex) had degrees of substitution (DS) ranging from 0.06-0.90. DMAE-Dex, DEAE-Dex, and DIAE-Dex all showed decreasing adsorption onto ANC and decreasing water content of the adsorbed film with increasing DS. Additionally, DEAE-Dex films adsorbed onto ANC had lower water contents than DMAE-Dex films with the same DS. Interestingly, QCM-D results for DIAE-Dex with high DS revealed mass loss, while SPR results clearly showed DIAE-Dex adsorbed onto ANC. These observations were consistent with dehydration of the ANC substrate. This study indicates that by controlling the DS and hydrophobic content of the polyelectrolyte, the water content of the film can be tailored.   

Engineering Novel Thermoreversible Hydrogels with Applications in Regenerative Medicine and Delivery Systems

A major concern in regenerative medicine is the increasing need for effective biomaterials for scaffolds, cell delivery vehicles, and drug delivery systems. In this study, we engineered a thermo reversible composite hydrogel of hard, medium and soft stiffness by blending Pluronic F127 (F127) with biocompatible hyaluronic acid (HA) and bioadhesive gelatin. Rheological analysis demonstrated that hard gel produced the highest elastic modulus in both HA-F127 and Gelatin-F127 hydrogels. It was found that increasing the concentration of HA and gelatin increased the critical solution temperature (CST) at which the solution gels. Glucose and sodium chloride, additives commonly found within the body, were analyzed to have minimal effect on the mechanical properties but caused a decrease in CST. Adult human dermal fibroblasts were plated on the composite hydrogels to demonstrate scaffolding and cell delivery. The highest growth was observed in hard Gelatin-F127 hydrogels. Cells also showed the best response to hard Gelatin-F127 gels in shear modulation force microscopy and were found to be homogenously distributed in the three-dimensional matrix of the gels. Our novel composite hydrogel displayed synergistic properties of its individual components and had the necessary characteristics for effective use in the medical setting: mechanical strength, cell adhesion and viability.   

Synchrotron SAXS and WAXD Studies of Cellulose Nascent Crystals: Experiment and Structure Analysis

Cellulose nascent crystals extracted from biomass (wood pulp, jute and cotton)by combined chemical and mechanical treatments are low cost, environmentally friendly and high performance materials to form the barrier layer in ultrafiltration membranes. This research project is aimed at using the synchrotron X-ray scattering methods to characterize the nascent crystalline nanofibers in different formats. The SAXS (Small Angle X-ray Scattering) data of cellulose nanofiber suspensions was analyzed and the polydisperse ribbon model with rectangular cross section fit the data well. The 2D and 3D simulations of WAXD (Wide Angle X-ray Diffraction) pattern of jute cellulose fibers solved the contents ratio of cellulose I-alpha and I-beta and Hermans' orientation parameter P2.   

Friction between Brush Layers of Charged and Neutral Bottle-Brush Macromolecules. Molecular Dynamics Simulations

We used MD simulations to study lubricating properties of neutral and charged bottle-brush coatings as a function of the compression and shear stresses, and brush grafting density. Our simulations have shown that in charged systems under shear there is a layer with excess of counterions found in between brush bearing surfaces. The main deformation mode of the charged layers is associated with the backbone deformation resulting in the backbone deformation ratio, $\alpha $, and shear viscosity,\textit{$\eta $}, being universal functions of the Weissenberg number, $W$. In the case of neutral systems in addition to the backbone deformation there is also side chain deformation. The coupling between backbone and side chain deformation violates universality in $\alpha $ dependence on $W $and results in scaling exponents varying with compression stress and brush grafting density. Existence of different length scales controlling deformation of neutral bottle--brushes manifests itself in shear viscosity,\textit{$\eta $}, dependence on the shear rate, $\dot {\gamma }$. Shear viscosity,\textit{$\eta $}, as a function of the shear rate, $\dot {\gamma }$, has two plateaus and shear thinning regimes. The low shear rate plateau and shear thinning regime correspond to the backbone deformation while the second plateau and shear thinning regime at moderate shear rates is due to side chain deformation. For both systems the value of the friction coefficient increases with increasing the shear rate.   

Dynamics of Nanoparticle Adhesion

We have performed molecular dynamics simulations of peeling of nanoparticles from substrate to understand the dynamics of nanoparticle adhesion. In our simulations we have calculated the potential of mean force characterizing the strength of the nanoparticle interaction with the substrate as a function of the particle-substrate separation. These simulations have shown that the detachment of the nanoparticle from substrate occurs through neck formation. The neck height decreases with increasing nanoparticle shear modulus (crosslinking density). Furthermore our simulations have established that the detachment time t$_{R}$ scales with the applied force as $f^{-5}$. This strong force dependence is a result of the fine interplay between nanoparticle surface energy, elastic energy and its adhesion to the substrate that controls the shape of the nanoparticle.   

Measuring Adhesion of Freestanding Polymer Nano-fibers

A novel method is used to measure directly adhesion between two freestanding polymer nano-fibers. A single fiber is attached to two microspheres readily glued to an atomic force microscope (AFM) cantilever. Another freestanding fiber is similarly prepared on a mica substrate. The fibers are arranged in orthogonal crossed-cylinder geometry. External load is applied to deform the two fibers into complementary V-shapes, and the force response allows determination of elastic modulus. At a critical tensile load, ``pull-off'' occurs and the adhering fibers spontaneously detach from each other, yielding the interfacial adhesion energy. Loading-unloading cycles are performed to investigate fiber degradation and repeated adhesion-detachment.   

Differential AC chip calorimeter for in situ investigation of vapor deposited thin films

Physical vapor deposition (PVD) can be used to produce thin films with particular material properties like extraordinarily stable glasses of organic molecules. We describe an AC chip calorimeter for in-situ heat capacity measurements of as-deposited nanometer thin films of organic glass formers. The calorimetric system is based on a differential AC chip calorimeter which is placed in the vacuum chamber for physical vapor deposition. The sample is directly deposited onto one calorimetric chip sensor while the other sensor is protected against deposition. The device and the temperature calibration procedure are described. The latter makes use of the phase transitions of cyclopentane and the frequency dependence of the dynamic glass transition of toluene and ethylbenzene. Sample thickness determination is based on a finite element modeling (FEM) of the sensor sample arrangement. A layer of toluene was added to the sample sensor and its thickness was varied in an iterative way until the model fits the experimental data.   

Effective Temperature: from Red-Blood-Cell Membrane Fluctuations to trapped active particles

Biologically driven nonequilibrium fluctuations are often characterized by their non-Gaussianity or by an ``effective temperature'', which is frequency dependent and higher than the ambient temperature. We address these two measures theoretically by examining a randomly kicked particle, with a variable number of kicking motors, and show how these two indicators of nonequilibrium behavior can contradict. Our results are compared with new experiments on shape fluctuations of red-blood cell membranes, and demonstrate how the physical nature of the motors in this system can be revealed using these global measures of nonequilibrium. Furthermore, oure concept of The concept of the effective temperature is extended to active particles trapped in a potential well. We calculated the average escape time and find that Kramers' reaction-rate theory is found to quite hold in this system. Using this calculated escape time, together with the exact calculation of the active diffusion coefficient for the escaped (free) active particle; one can attempt to fully describe the long-time random-walk of the tracer particle in the active gel.   

Structural and phase transitions of one and two polymer mushrooms

A polymer mushroom here refers to a group of $n$ homopolymer chains end-grafted at the same point on a flat, impenetrable and homogeneous substrate. Using lattice self-consistent field (LSCF) calculations with the Kronecker $\delta $-function interactions (instead of the commonly used nearest-neighbor interactions), we have studied the structures of one and two polymer mushrooms in an explicit solvent as a function of the polymer volume fraction, the solvent quality characterized by the Flory-Huggins \textit{$\chi $} parameter, and the distance between the two mushrooms. We have constructed phase diagrams of these systems showing the coil-globule transition (CGT) of one mushroom and how it is coupled with the fused-separated transition (FST) of two mushrooms. Since LSCF results are exact only in the limit of $n\to \infty $, we also use the newly proposed fast lattice Monte Carlo (FLMC) simulations$^{1}$ with the same Hamiltonian as in LSCF theory to examine how this limit is approached with increasing $n$. Direct comparisons between LSCF and FLMC results without any parameter-fitting quantify the fluctuation/correlation effects neglected in LSCF theory. We also find a second-order symmetric-asymmetric transition (SAT) for one-mushroom system in the globule state, and examine its coupling with CGT and FST. [1]~\textit{Q. Wang}, \textbf{Soft Matter, 5}, 4564 (2009); \textbf{6}, 6206 (2010).   

Quantification of Complex Topologies in Macromolecular and Nanoscale Structures using Small-Angle Scattering

Polymers are characterized by molecular weight distribution, tacticity, block copolymer content and branch content and chain topology. The branch structure and particularly the topology of branched chains has remained a difficult characterization problem. Recently we have developed a scaling model that can be coupled with small-angle scattering to measure the average branch length, number of branches and branch-on-branch structure in macromolecules of complex topology. This method has been extended to understand the structure of two dimensional structures and crumpling in these macromolecular systems. We have explored a wide range of materials in this regard. This poster will give an overview of the current uses for the scaling model for macromolecular topology. References pertaining to this poster can be found at http://www.eng.uc.edu/$\sim$gbeaucag/BranchingPapers.html.   

Microfluidic Insights into Filter Design

Nearly every application involving fluid relies heavily upon filtration, yet filter design is not well understood. Previous studies show that hard sphere clogging in microfluidic channels is well described by a probabilistic model that also reveals information about the clogging material's proclivity to aggregation. Design features, such as pore size distribution, can be modeled in two dimensions using soft lithographic techniques to fabricate microscale pores. We then test the efficacy of variations in pore design by clogging these pores with polystyrene microparticles. The clogging behavior of these fabricated pores is then compared to the aforementioned probabilistic model to elucidate the function of various features of filter design.   

Photoelastic study of sound waves in grain packings

By means of photoelasticity, we success in visualizing in real time the propagation of acoustic waves in a granular packing of cylinders. Our experimental procedure allows an access to the local state of stress of individual grains as a function of time with a good accuracy. We first present results on 1d chains of cylinders that are constrained by a static confinement force. Both linear and nonlinear regimes are presented. We emphasize the role of the grains roughness on the propagation properties, and also of the solid friction with the walls. Our results concerning the wave velocity as a function of the amplitude, and of the confinement force, are compared to the theory and to the spherical beads case. We then present experimental results on 2d pilings, in particular for the wave speed.   

Dense granular flow around a rigid or flexible intruder

We experimentally studied the flow of a dense granular material around an obstacle (rigid cylinder or flexible plate) placed in a 2 dimensional confined cell at a packing fraction near the 2D jamming threshold. In the case of the rigid obstacle, the displacement field of grains as well as the drag force experienced by the obstacle were simultaneously recorded and a parametric study was done by changing the cell size, the intruder diameter or the packing fraction. The drag force experienced by the intruder and the formation of a wake behind the obstacle were very sensitive to the approach to jamming. The same experimental set-up was adapted to a flexible intruder and coupling between the granular flow and fibre deflexion were imaged. The deformation of the fibre could be compared with theoretical predictions from elastica.   

Mechanisms of defect motion in hexagonal systems on sinusoidal substrates

In this work we have studied through a Landau's free energy functional approach the dynamic and equilibrium configurations of two-dimensional hexagonal systems constrained to lie on a substrate with sinusoidal topography. Similarly to previous studies, we have found a strong coupling between defects and geometry, where the regions with the highest local Gaussian curvature act as sink for disclinations. We have studied the influence of the local curvature of the substrate on the diffusional dynamics of dislocations and disclinations. We have found a novel mechanism of defect diffusion where linearly correlated arrays of dislocations (scars) move according to the gradient of curvature.   

Colloidal cluster array formation by weak ac electric fields

We study experimentally the influence of weak alternating electric fields on dilute colloids with a free surface open to the air. When the experimental conditions minimize the evaporation of the solvent, a well-defined one-dimensional cluster array is formed along the straight contact line. The cluster array evolves due to a hydrodynamical instability which can be controlled by the applied field [1]. Once the clusters are formed, if the evaporation is favored, the contact line recedes. Under these conditions, the capillary flows towards the clusters strengthen. As a consequence, elongated columns of particles are deposited on the substrate. The characteristic length between these columns is controlled during the initial stages of the cluster formation [2]. To shed light on the cluster formation mechanisms, further work is currently carried out for identifying similar phenomena without the existence of a free surface.\\[4pt] [1] M. Pichumani et al. Phys. Rev. E 83 (2011), 047301\\[0pt] [2] R. Aslam et al. In preparation   

Melting of Colloidal Crystals

We experimentally studied the melting and freezing behaviors of colloidal crystals composed of diameter tunable poly-N-isopropylacrylamide (NIPA or pNIPAM) microgel spheres by bright-field and confocal video microscopies. The melting behaviors of three-dimensional (3D), two-dimensional (2D) and multilayer thin films of both single crystals and polycrystals were systematically studied with single-particle dynamics. Thick films ($>$4 layers) melt heterogeneously, while thin films ($<$5 layers) melt homogeneously even in polycrystals. A novel heterogeneous melting at dislocation is discovered in 5- to 12-layer films. The equilibrium phase behaviors are different in three thickness regimes: thick films have a liquid-solid coexistence regime which decreases with the film thickness and vanishes at 4 layers, thin films melt into the liquid phase in one step, while monolayers melt in two steps with an intermediate hexatic phase. These results provide new challenges in theory.   

Formation of Uniform Hollow Silica microcapsules

Microcapsules are small containers with diameters in the range of 0.1 -- 100 $\mu $m. Mesoporous microcapsules with hollow morphologies possess unique properties such as low-density and high encapsulation capacity, while allowing controlled release by permeating substances with a specific size and chemistry. Our process is a one-step fabrication of monodisperse hollow silica capsules with a hierarchical pore structure and high size uniformity using double emulsion templates obtained by the glass-capillary microfluidic technique to encapsulate various active ingredients. These hollow silica microcapsules can be used as biomedical applications such as drug delivery and controlled release.   

Multiblock Janus particles with tunable patch size

Micro-printing methods are used to systematically tune the size, position and number of functionalized patches on colloidal particles with excellent monodispersity. This study introduces two new members of the Janus particle family: triblock particles with two small poles, and eyeball Janus particles. Without the intervention of any external field, the former form chain, ring, coil and knot structures. In principle, dynamic and reconfigurable colloidal bilayers can be built from eyeball Janus particles.   

Effective static and high-frequency viscosities of concentrated suspensions of soft particles

We obtain an analytic expression that allows determining the static and high-frequency viscosities as function of the volume fraction of a concentrated suspension of soft spherical particles. The particles consist of a hard core of radius a covered by a porous layer of thickness d. The proposed expression incorporates the results for the intrinsic viscosity obtained on the basis of a cell model into a recently obtained relation for the effective viscosity of concentrated colloidal suspensions [J. Chem. Phys. 130, 044904 (2009)]. In this model, the correlations between the particles due to crowding effects are introduced through an effective volume fraction which is then used as integration variable in a differential effective medium procedure. The final expression is simple, accurate, and allows collapsing all the data in a universal master curve. The only difference between the static and high-frequency cases is that in the last case the effective volume fraction also incorporates hydrodynamic interactions arising from the so-called relaxation term. We have tested the accuracy of our model with experiments and simulations. In all cases the agreement with the data is extremely good.   

Nanoparticle adsorption from a crowded solution at a solid/liquid interface

Adsorption of nanoparticles at solid-liquid interface is of great importance in the field of colloidal science and biophysics. Protein adsorption is one of the most significant processes and can be mimicked by colloidal systems. We will present results of our studies of the kinetics of adsorption of gold nanoparticles from a crowded polyvinyl alcohol (PVA) polymer solution on a solid/liquid interface using phase-modulated ellipsometry. The experimental system mimics many biological processes, where the adsorption of particles or proteins takes place in the presence of many other components.   

Responsive Poly ($\varepsilon$-carbobenzyloxy-L-lysine)-based Colloidal Particles: Exploring and Characterizing the Inverse $\alpha $-Helix to Random Coil Transition in m-Cresol

Like other synthetic polypeptides, poly ($\varepsilon $-carbobenzyloxy-L-lysine), PCBL, exhibits unique properties that make it a good candidate for a broad range of applications and fundamental investigations. This work explores one particular feature found in PCBL in the single solvent, m-cresol: a sharp, reversible coil-to-helix transition at 27$^{\circ}$C. In nature, such polypeptides undergo similar transitions while attached to living cells. Tethering PCBL polymers to spherical silica particles enables the study of effects such as polypeptide chain length, grafting density and core particle curvature in a fundamental way. This can be accomplished in a single, organic solvent without interference from strong pH and salt effects. This presentation will concern steps taken to make such studies a reality. Methods to characterize grafting density have been established, good control has been exerted over the core size and, by some synthetic routes, also the molecular weight of the PCBL chains in the shell. The coil-to-helix transition is observed for some particles but not all.   

Evidence for localized surface plasmon polaritons in a liquid crystal containing gold nanoparticles

We report an observation of the localized surface plasmon polaritons (SPPs) in a nematic liquid crystal containing 14 nm diameter gold nanoparticles (Au NPs). We observe attenuated total reflection (ATR) of p-polarized laser beam incident upon a high-index prism/liquid-crystal-Au-NPs/glass structure used in the Kretschmann configuration.$^{1}$ Unlike the ubiquitous ATR configuration, in which the prism base is coated with a noble metal thin film, our experimental set up does not utilize any such coating. The ATR observed at a specific incident angle and only for p-polarized laser reflects the excitation of localized SPPs at NP/liquid-crystal interface. We discuss possible SPPs related effects, which can significantly change the electro-optical properties of polymer-dispersed liquid crystals.$^{2}$   

Novel Ordering of Soft Matter Using Chromonic Lyotropic Liquid Crystals

Chromonic lyotropic liquid crystals (CLLCs) are a unique and powerful system for governing self-organization in soft materials due to their temperature- and concentration-dependent ability to form nematics in (bio-compatible) aqueous suspensions. We present preliminary observations on several novel soft matter phases governed by the ordering of CLLCs. We first examine the phase behavior of low-concentration chromonic aggregates in surfactant-based lyotropic lamellar, hexagonal and nematic phases, and observe phase separation and mutual ordering. We also discuss the placement of colloidal particles at the interface of a thermotropic nematic liquid crystal and a chromonic nematic liquid crystal, and present initial results on defect coupling across the interface and colloidal self-organization from elastic interactions. We also present additional preliminary work examining the structure of CLLCs in confinement and their interactions with biologically inspired materials.   

Segregation by Complementarity of nanoDNA based on Liquid Crystal Ordering and Centrifugation

Nanometer length DNA segments ( $<$20 base pair long) that are complementary can duplex and condense to make liquid crystal phases at concentrations $>\sim $500 mg/mL This nanoDNA duplexing combined with order-disorder phase separation offers a means of sequestering molecules in mixtures of different DNA sequences based on their degree of complementarity. Here we show that isotropic and liquid crystalline phases, comprising respectively single strands and duplexes in multi-component nanoDNA solutions, can be physically separated by liquid crystal condensation followed by centrifugation.   

Ferromagnetic viscoelastic liquid crystalline materials

Novel ferromagnetic liquid crystalline materials were designed by mixing ferromagnetic nanoparticles with glass forming oligomers and low molar mass liquid crystals. The matrix in which nanoparticles are embedded is highly viscous that reduces aggregation of nanoparticles and stabilizes the whole composition. Mechanical and optical properties of the composite material are studied in the broad range of nanoparticle concentrations. The mechanical properties of the viscoelastic composite material resemble those of chemically crosslinked elastomers (elasticity and reversibility of deformations). The optical properties of ferromagnetic cholesteric materials are discussed in detail. It is shown that application of magnetic field leads to the shift of the selective reflection band of the cholesteric material and dramatically change its color. Theoretical model is suggested to account for the observed effects; physical properties of the novel materials and liquid crystalline elastomers are compared and discussed. [1] P.V. Shibaev, C. Schlesier, R. Uhrlass, S. Woodward, E. Hanelt, Liquid Crystals, 37, 1601 (2010) [2] P.V. Shibaev, R. Uhrlass, S. Woodward, C. Schlesier, Md R. Ali, E. Hanelt, Liquid Crystals, 37, 587 (2010)   

Modeling Mechanotunable Transmembrane Transport in Lipid Vesicles

Using Dissipative Particle Dynamics approach, we study the effects of applied stress on transmembrane transport in lipid vesicles. The lipids comprising the vesicle are composed of a hydrophilic head group and two hydrophobic tails. The vesicle is immersed into the hydrophilic solution and initially contains a number of amphiphilic species inside its cavity. We show that such enclosed species can be released ``on demand'' by vesicle's stretching. We find that the magnitude of the external force required to release the vesicle's content depends on the chemical nature and volume fraction of the enclosed species. Furthermore, we isolate the scenarios where the stretching of the lipid vesicle depleted of the enclosed species results in its ``refilling'' with the fresh species from the outer solution. Our results illustrate that applied mechanical stress provides an effective means to fine tune the transmembrane transport in lipid vesicles.   

Simulations of buckling of gel-phase lipid bilayers

The structure of lipid bilayer vesicles below the main chain transition temperature is believed to be approximately polygonal, based on evidence of faceting from electron microscopy. The structure and mechanics of the interfaces between facets, (ridges) are issues that may be pertinent to the rate of trans-bilayer permeability as well as vesicle melting. Here we present evidence from atomistic molecular dynamics simulations that the preferred angle between facets is strongly dependent on the tilt orientation of lipid tails with respect to the ridge vector.   

Investigation of the Physics of Flocculation in Algal Systems

Algae biofuel production has gained a great deal of interest in recent years due to the high photosynthetic efficiency of various algae strains and the ability of stressed algae populations to produce large quantities of lipids within their cells. Separation of the algae from the background aqueous medium engenders large energetic costs for standard separation techniques including filtration, centrifugation, and dissolved air flotation since algae cells are small (microns to 10s of microns), have densities similar to the surrounding fluid, and normally occur at low volume fractions (1E-4 -$>$ 1E-3). Flocculation is one possible route to reducing the cost of collecting the algae biomass, since large algae flocs can easily be removed from the aqueous environment through either differential settling or standard filtration. To this end, We model flocculating systems of algae cells using discrete particle dynamics techniques which incorporate a recently developed adhesive granular potential to govern the cell interactions. This potential is shown to reproduce morphological characteristics, kinetics, and size distributions that agree well with known results for flocculation in the diffusive regime (DLCA). We further investigate flocculation under steady shear and compare our results to both experiment and predictions from various orthokinetic models.   

Nonlinear Dynamic Heat Capacity of a Simple Chain Polymer

Molecular dynamics simulations were run on a simple bead-spring polymer system, known to be a glass former, with varying amplitude sinusoidal temperature oscillations. Small amplitude sinusoidal temperature produces a small amplitude sinusoidal energy, and dynamic heat capacity is the complex-valued transfer function between the two. For large amplitude temperature oscillations, the energy is no longer sinusoidal, and linear response theory breaks down. Instead, the resulting energy is can be written as a Fourier series. From the Fourier coefficients, we derived the entropy generated per cycle, which is related to the imaginary part of the dynamic heat capacity in the linear limit. Using this, we formulated a nonlinear extension of the dynamic heat capacity.   

High-Pressure Dynamic Light Scattering Implemented in a Diamond Anvil Cell

In recent years full-spectrum analysis in light-scattering has been utilized to explore the liquid-glass transition at variable temperatures and ambient pressure. Our lab is currently working on developing dynamic light scattering in a diamond anvil cell to extend this characterization method to high pressures. There are many inherent complications in the implementation of dynamic light scattering in a diamond anvil cell such at distinguishing between self-beating and homodyne contributions to the signal. However, if successful, it would give rise to a several decade window for analysis of glass-forming dynamics not attainable through Brillouin or Raman alone. Progress toward an effective method for obtaining DLS in a diamond anvil cell for the purposes of such analyses will be presented.   

Wet granular walkers and climbers

We have observed that when a bidisperse mixture of glass beads is moistened by a fluid and shaken sinusoidally in a vertical container, small clusters of beads take off from the surface of the pile and rapidly climb up the container walls against gravity. These self-organized clus- ters are held together and against the wall by liquid capillary bridges, and are led by one large grain with one or more small grains trailing behind. When similar clusters are placed on a horizontally vibrating substrate they self-align and travel horizontally along the axis of vibration with a ratchet-like motion. We report a detailed experimental study performed for the simplest walker system consisting of one large and one small bead, and present a simple model that accounts for the observed behavior. Reference: Z.S. Khan {\it et al.},{\it New J. Phys} {\bf 13}, 053041 (2011).   

Surface melting of wet granular matter in two dimensions

The transition from the solidlike to the liquidlike state of a monolayer of wet glass beads under horizontally swirling motion is investigated experimentally. Due to the cohesion arising from the formation of capillary bridges, the wet particles initially form a crystal like structure at moderate driving. As the driving frequency increases, this structure is found to melt with two steps: A rearrangement into a hexagonal packing sheltered by a premelted layer, followed by a melting from the surface. This process is characterized by means of Voronoi tessellation and bond orientational order parameters, and discussed within the scenario of KTHNY theory that accounts for crystal melting in two dimensions.   

Coefficient of restitution for wet impacts

As the experience of playing football in the rain may tell, wetting could influence the coefficient of restitution (COR) dramatically. This is due to the extra energy dissipation from the wetting liquid, for instance viscous damping. To unveil the underlying mechanisms accounting for the influence, we study experimentally the COR by tracing free falling particles bouncing on a wet surface. The dependance of the COR on the impact velocity, various particle and liquid properties will be presented and discussed in terms of dimensionless Stokes' and capillary numbers.   

Characterizing the movement of grains in a 2D rotating drum with imposed vibrations

We study particle trajectories and surface behavior of photoelastic grains in a 2D circular rotating drum subjected to imposed vertical vibrations. Granular materials are ubiquitous, from industrial processing and pharmaceutical powder mixing to avalanches and sinkholes, yet general equations of flow are not known for these materials. For granular materials, flow is characterized by sudden avalanches, large shear gradients, and history dependence and these characteristics make it difficult to form general equations of flow. This jamming transition is general, occurring in foams, emulsions, and traffic, and can vary significantly with vibration (the ``granular temperature'').To quantitatively measure the flow, jamming, and mixing properties of the grains, we analyze images to determine each particle's position and velocity for each frame. External vibration leads to increased compaction of the grains, larger rearrangements, and a narrower shear band. Particle tracking allows us to closely analyze the velocity profiles, trajectories of individual grains, and separation and diffusion of originally neighboring grains. We will present results on the stability of the pile as well as the effects on mixing as external vibration is varied.   

Simulating Topological Defects in Twisted Fiber Bundles

Twisted bundles are a common motif found in naturally occurring structures of self-assembled fibers, such as collagen and fibrin. By understanding the general principles governing such organizations, new synthetic materials--from the nano to the macroscale--may also be realized. Recently, continuum elasticity theory has been applied to describe generic twisted fiber bundles. This has revealed a relation between a bundle's twist and the presence of topological defects in the cross-sectional packing of the fibers. Here we employ numerical simulations to examine this interdependence. We model a bundle's cross-section as beads confined to a plane. The interactions between beads is governed by a modified Lennard-Jones potential that accounts for the effects of twist. We observe configurations that range from perfect hexagonal packing for cases of no twist, to defect populated structures above a critical amount of twist. For small bundles of less than $\sim$100 beads, there exists a discrete spectrum of energy ground states corresponding to integer numbers of five-fold disclinations. For larger bundles, we hope to uncover what types of defect arrangements effectively screen the stresses caused by twist, and compare these to current predictions of the internal organization of collagen fibrils.   

Molecular Adhesion of Curved Filaments: Self-Assembly of Bacterial Flagellar Bundles

The self-assembly of chiral filaments, such as filamentous proteins, introduces an unavoidable frustration between inter-filament and intra-filament geometries. We study the self-assembly of bacterial flagella with a model that focuses on three geometric features: a preferred distance of closest approach; a preferred relatively-parallel orientation; and a mechanical cost for changes of helical shape. As a result, intrinsically curved filaments cannot simultaneously minimize all three driving forces, even in the limiting case of 2-filament contacts. Using a model of depletion-driven and electrostatically-stabilized interactions we present a phase diagram for two filament bundles depicting the complex thermodynamic behavior of adhesive contact using three ratios: one describes the shape of the filaments, and the other two capture the competition of the modulus associated with mechanical deformations and with the cost of having a non-zero twist vs the modulus associated to changes in inter-filament spacing. Finally, we describe the dependence of these ratios on experimentally-adjustable variables and predict critical concentrations of depletant molecules for known polymorphic configurations which may favor the filaments to undergo a transition from non-adhesive to adhesive state.   

Self-assembly of Janus Ellipsoids

The self-assembly of particles into a desired mesoscopic structure and function are considered as a bottom-up strategy to obtain new bulk materials that have potential applications in broad fields including drug delivery, photonic crystals, biomaterials and electronics. We propose a primitive model of Janus ellipsoids that represent particles with an ellipsoidal core and two semi-surfaces coded with dissimilar properties, for example, hydrophobicity and hydrophilicity, respectively. We investigate the effects of the aspect ratio on the self-assembly morphology and dynamical aggregation processes using Monte Carlo simulations. We find that the size and structure of the aggregates can be controlled by the aspect ratio, which should be an interesting result from a design viewpoint.   

Electron Shock Waves with a Significant Current behind the Shock Front

Electrical breakdown of a gas in a strong electric field is carried out by a wave with a strong discontinuity at the wave front, traveling with speed comparable to speed of light. For theoretical investigation of electrical breakdown of a gas, we apply a one-dimensional, steady profile, constant velocity, three component fluid model. For current bearing breakdown waves, in addition to the set of electron fluid dynamical equations, the shock conditions on electron velocity and temperature need to be modified as well. For breakdown waves, considering a significant current behind the shock front, we will derive the boundary conditions on electron velocity and temperature at the shock front. The modified set of boundary conditions has made the integration of the set of electron fluid dynamical equations through the dynamical transition region of the wave possible. The wave profile for electric field and electron velocity, temperature and number density will be presented.   

X-ray Studies of Rapidly Evolving Interfacial Nanostructures

Interfacial nanostructures represent a class of systems that is of great interest for studies of quasi-2D systems, chemical self-assembly, surfactant behavior and biologically relevant membranes. During the compression process, the self-assembled nanoparticle films at the air-liquid interface exhibit rich mechanical behavior, undergoing a rapid structural evolution which is marked by the transition from monolayer to multilayer and/or the formation of periodic wrinkles and folds. Due to the compression rate of the barrier, the timescale of this evolution is typically several minutes, which is much shorter than the time the conventional X-ray Reflectivity (XR) takes for a liquid surface measurement. Therefore we explore the ability of Grazing Incidence X-ray Off-Specular (GIXOS) scattering to capture the elastic properties, structures and surface fluctuating modes of Au nanoparticle films during the rapid structural evolution. We present here the detailed analysis of GIXOS data from the self-assembled Au nanoparticle films and show how we obtain quantitative, Angstrom-resolution details of electron density profile normal to the surface with a temporal resolution that allows us to study in-situ the rapid evolution of Au nanoparticle films structure in response to the compression.   

Frequency Enhancement in Coupled Noisy Excitable Elements: The effect of excitability, system size and topology

The oscillatory dynamics of coupled noisy excitable FitzHugh-Nagumo elements is investigated as a function of the coupling strength~$g$. For a system consists of coupled noisy excitable elements, the synchronize frequency will be higher than the uncoupled frequencies of each element. As~$g~$ increases, there is an unexpected peak in the mean of frequency distribution before reaching synchronization at the optimal coupling strength. This enhancement level is investigated systematically on depending on~hypercubic lattices in different dimensions, Erdos-Renyi random graphs and random networks with fixed coordination numbers. It is found that the maximal enhancement coupling and enhancement level depend on the connection topology and spatial dimensions, the enhancement level and the frequency distribution widths follow scaling laws as verified by different lattices(honeycomb, square, and hexagonal) in two-dimensions, suggesting some sort of universality in frequency enhancement.   

Phase Diagram of the mixed spin-2 and spin-5/2 Ising system with two different single-ion anisotropies

In the last five decades the Ising model has been one of the most largely used to describe critical behavior of several systems in the nature. In particular, in the physics of the condensed matter it is important to describe critical behavior and other thermodynamics properties of a variety of physic systems (disordered system, spins glass, random field Ising model, etc.). Recently, several extensions have been made in the spin$-1/2$ Ising model to describe a wide variety of physic systems. For example, the models consisting of mixed spins of different magnitudes are interesting extensions, which are so-called mixed-spin Ising models. In this work we present a study of the effects of two single-ion anisotropies in the phase diagram and in the compensation temperatures of the mixed spin-2 and spin-5/2 Ising ferrimagnetic system. We employ the mean-field theory based on the Bogoliubov inequality for the Gibbs free energy, and the Landau expansion of the free energy in the order parameter to describe the phase diagrams. In the plane critical temperature versus single-ion anisotropie the phase diagram display behavior tricritical (second-order transition separated of the first-order transition lines by a tricritical point).   

Monte Carlo Study of a Mixed Spin-1 and Spin-3/2 Ising Ferrimagnet with Random Anisotropies

The interest in studying magnetic properties of systems of mixed spin Ising has been grown in last years. This is due to such systems have less translational symmetry than their single-spin counterparts. This property is very important to study some type of ferrimagnetism, which are of current interest. One of the most interesting features of the ferrimagnetism is that they can exhibit compensation temperature. Spin systems mixed with spins in the two sublattices and with random anisotropies are not commonly studied. Monte Carlo method have been proven to be reliable and relatively simple technique to analyze mixed Ising model. In this work, we performed Monte Carlo simulations to study a mixed system ferrimagnetism on a square lattice. The model system consists of two interpenetrating sublattices with spins spin-1 and spin-3/2 in the presence of random anisotropies. The magnetic properties such as magnetization, susceptibility and Binder cumulant, were determined to obtain the critical temperature of the system in various situations. We also determined the phase diagram in the plane temperature versus anisotropy strength for several values of the randomness anisotropy probability.   

Measurement of organization in complex and co-evolving networks

We apply a new method for measurement of organization of complex and co-evolving networks using the quantity of physical action. We consider simple arrangements of elements in a network and constraints to their motion along paths and calculate the amount of organization in each system using the following measure: organization is the inverse of the average sum of physical actions of all elements in a system per unit motion multiplied by the Planck's constant. The meaning of quantity of organization here is the number of quanta of action per one unit motion along a path of an element. A unit motion along a path for a network, such as internet, is the transmission of one bit of information. The calculation can be expanded to systems consisting of many elements and constraints and also can be followed as a function of time with improvement of the organization of a system or connected systems and networks. Thus, the principle of least action becomes the driving force, and the least action state of the system, the attractor for all of the paths of its elements and states of its constraints. We consider also the rate of constraint minimization, or decrease of action per element and motion, as a function of the number of elements i.e. quality as a function of quantity. Increase of quantity, within specified limits, leads to increase of level of organization and vice versa.   

Direct Method in Determining the Direction of Action in the Coupling Map Ring

Recently, Hung and Hu provided a genuine way to carry binary data on a coupling map ring (CMR) for the chaotic communication (Phys. Rev. Lett., 101, 244102 (2008)). They use the temporal transfer entropy (TTE) to determine the direction of action, which tells bit 1 or 0, between two neighboring chaotic maps of the CMR. A direct method based on the correlation between the changes of two chaotic data is not only capable to resolve the carried bits but more effecient than the scheme of TTE.   

Jamming in complex networks with degree correlation

We study the effects of the degree-degree correlations on the pressure congestion $J$ for a diffusive transport process on scale free complex networks. Using the gradient network approach we find that the pressure congestion for disassortative (assortative) networks is lower (bigger) than the one for uncorrelated networks which allow us to affirm that disassortative networks enhance transport through them. This result agree with the fact that many real world transportation networks naturally evolve to this kind of correlation. We explain our results showing that for the disassortative case the clusters in the gradient network turn out to be as much elongated as possible, reducing the pressure congestion $J$ and observing the opposite behavior for the assortative case. Finally, we apply our transportation process to real world networks, and the results agree with our findings for model networks.   

Stretch-induced stress patterns and wrinkles in hyperelastic thin sheets

Wrinkles are commonly observed in stretched thin sheets and membranes. This paper presents a numerical/experimental study on stretch-induced wrinkling of hyperelastic thin sheets. The model problem is set up for uniaxial stretching of a rectangular sheet with two clamped ends and two free edges. A two-dimensional stress analysis is performed first under the plane-stress condition to determine stretch-induced stress distribution patterns in the elastic sheets, assuming no wrinkles. As a prerequisite for wrinkling, development of compressive stresses in the transverse direction is found to depend on both the length-to-width aspect ratio of the sheet and the applied tensile strain. Next, an eigenvalue analysis is performed to find the potential buckling modes of the elastic sheet under the prescribed boundary conditions. A nonlinear post-buckling analysis is performed to show evolution of stretch-induced wrinkles. The wrinkle wavelength decreases with increasing strain, in good agreement with the prediction by a scaling analysis. However, as the tensile strain increases, the wrinkle amplitude first increases and then decreases, eventually flattened beyond a moderately large strain, in contrast to the scaling analysis. Finally, experimental measurements with polyethylene sheets will be presented in comparison with the numerical results.   

Random fields at an absorbing state transition

We investigate a nonequilibrium phase transition in the presence of disorder that locally breaks the symmetry between two equivalent absorbing states. Such ``random-field'' disorder is known to have dramatic effects on equilibrium phase transitions; in low dimensions it can completely destroy the phase transition. In contrast, we demonstrate that the absorbing state transition of the one-dimensional generalized contact process persists in the presence of random fields. However, the dynamics in the inactive phase becomes ultraslow. We illustrate our theory by means of large-scale Monte-Carlo simulations.   

Fluctuation-driven Turing patterns in predator-prey dynamics

Models of diffusion-driven pattern formation that rely on the Turing mechanism are utilized in many areas of science. However, many such models suffer from the defect of requiring fine tuning of parameters or an unrealistic separation of scales in the diffusivities of the constituents of the system in order to predict the formation of spatial patterns. In the context of a generic model of predator-prey population dynamics, we show that the inclusion of intrinsic noise in Turing models leads to the formation of ``quasipatterns'' that form in generic regions of parameter space and are experimentally distinguishable from standard Turing patterns. The existence of quasipatterns removes the need for unphysical fine tuning or separation of scales in the application of Turing models to real systems. We also exhibit numerical simulations of the formation of quasi-patterns deep in the quasi-pattern phase.   

The Explanation of the Photon's Electric and Magnetic Fields; and its Particle and Wave Characteristics

Using the principles of the Vortex Theory, the creation of the photon's electric and magnetic components are explained: the condensed region of space is responsible for creating the photon's electric component and its particle effect; its expansion and contraction is responsible for its frequency; its motion through three dimensional space creates a wave in the surrounding space. This wave is responsible for the photon's magnetic component and wave characteristics. The simultaneous expansion and contraction of both the dense region of space that is the photon and the surrounding space it passes through explains why the electric and magnetic effects are at right angles to each other. Also the photon's particle and wave characteristics are explained.   

Pure Quantum Gravity Simulation in 1+1 Dimensions using Causal Dynamical Triangulation

Causal Dynamical Triangulation (CDT) is a conservative approach to quantizing gravity. It involves decomposing spacetime into ``triangular'' building blocks. In computer simulations, we implement CDT in 1+1 dimensions without matter. We compute the ratio of the corresponding universe's mean size to its spread and compare with analytical results.   

Contesting the paradigm of chirality

In 1893 Lord Kelvin coined the term chirality, and stated what is to become the elementary paradigm of chirality: 'I call any geometrical figure, or any group of points, chiral, and say it has chirality, if its image in a plane mirror , ideally realized cannot be brought to coincide with itself'. While the notion of chirality has greatly advanced our understanding of the structures of molecules and crystals, it has been shown to be inconsistent with every pseudo-scalar quantification. In this talk I will present a tabletop demonstration of a chiral structure which is constructed through the achiral summation of identical elementary units which are symmetric under reflection. The seeming contradiction to the definition of chirality is reconciled by proposing an alternative definition, relying on the physicist interpretation of the right hand rule.   

Phase-Space Networks of Frustrated Spin Models

We propose a complex-network approach to study phase-space structures of frustrated spin models and lattice gas models. Their highly degenerated ground states are mapped as discrete networks such that the quantitative network analysis can be applied to phase-space studies. The resulting phase spaces share some common features and establish a class of complex networks with unique Gaussian spectral densities. A one-to-one correspondence is discovered between the six-vertex model (jigsaw puzzle) and sphere stack.   

Common features in phase-space networks of frustrated spin models and lattice-gas models

We mapped the phase spaces of the following four models into networks: (1a) the Ising antiferromagnet on triangular lattice at the ground state and (1b) above the ground state, (2) the six-vertex model (i.e. square ice or spin ice), (3) 1D lattice gas and (4) 2D lattice gas. Their phase-space networks share some common features including the Gaussian degree distribution, the Gaussian spectral density, and the small-world properties. Models 1a, 2 and 3 with long-range correlations in real space exhibit fractal phase spaces, while models 1b and 4 with short-range correlations in real space exhibit non-fractal phase spaces. This result supports one of the untested assumptions in Tsallis's non-extensive statistics.   

Simple Design Rules for Spike Neural Network Based General Purpose Networks

It has been much lamented over the past decade that, although spiking neural networks (SNNs) have exciting proven computational properties, there are no design rules for assembling networks for specific purposes. Here we offer design approaches for creating three general purpose networks namely, a temporal pattern (serial channel) detector, sequence detector (parallel channel), and any specific mapping of input to output spike patterns on a serial channel. Central pattern generators are instances of this last design. These design rules are based on synchrony detection which SNNs do so well. Here we also introduce a modification to the basic SRM0 model which not only reduces the computational cost, but also enables us to develop these design rules. We discuss how these designs may be combined into fairly general spatio-temporal pattern detectors. Finally, by adding a capability for feature discovery/extraction, we envision an approach to learning spatio-temporal pattern classifiers.   

Vulnerability and Vaccination Strategies on realistic complex network

The general understanding of non-equilibrium stochastic processes evolving on complex networks become an important challenge. In the framework of epidemiology, one of the key challenges is the identification and the understanding of the role of critical and vulnerable nodes in the diffusion process. Considering the SIER model evolving on a large realistic complex networks, we present a study of the vulnerability for different viral strength. Our result are used to evaluate the possible vaccination strategies.   

Transitions in Physiologic Coupling: Sleep Stage and Age Dependence of Cardio-respiratory Phase Synchronization

Recent studies have focused on various features of cardiac and respiratory dynamics with the aim to better understand key aspects of the underlying neural control of these systems. We investigate how sleep influences cardio-respiratory coupling, and how the degree of this coupling changes with transitions across sleep stages in healthy young and elderly subjects. We analyze full night polysomnographic recordings of 189 healthy subjects (age range: 20 to 90 years). To probe cardio-respiratory coupling, we apply a novel phase synchronization analysis method to quantify the adjustment of rhythms between heartbeat and breathing signals. We investigate how cardio-respiratory synchronization changes with sleep-stage transitions and under healthy aging. We find a statistically significant difference in the degree of cardio-respiratory synchronization during different sleep stages for both young and elderly subjects and a significant decline of synchronization with age. This is a first evidence of how sleep regulation and aging influence a key nonlinear mechanism of physiologic coupling as quantified by the degree of phase synchronization between the cardiac and respiratory systems, which is of importance to develop adequate modeling approaches.   

Levels of complexity in scale-invariant neural signals

Many physiological systems exhibit complex scale-invariant and nonlinear features characterized long-range power-law correlations, indicating a possibly common control mechanism. It has been suggested that dynamical processes, influenced by inputs and feedback on multiple time scales, may be sufficient to give rise to this complexity. Two examples of physiologic signals that are the output of hierarchical multiscale physiologic systems under neural control are the human heartbeat and human gait. We show that while both cardiac interbeat interval and gait interstride interval time series under healthy conditions have comparable scale-invariant behavior, they still belong to different complexity classes. We compare results from empirical findings and stochastic feedback modeling approaches to cardiac and locomotor dynamics, which provide new insights into the multicomponent neural mechanisms regulating these complex systems.   

Characteristics of group networks in the KOSPI and the KOSDAQ

We investigate the main feature of group networks in the KOSPI and KOSDAQ of Korean financial markets and analyze daily cross-correlations between price fluctuations for the 5-year time period from 2006 to 2010. We discuss the stabilities by undressing the market-wide effect using the Markowitz multi-factor model and the network-based approach. In particular we ascertain the explicit list of significant firms in the few largest eigenvectors from the undressed correlation matrix. Finally, we show the structure of group correlation by applying a network-based approach. In addition, the relation between market capitalizations and businesses is examined.   

Multifractal Analysis of Asian Foreign Exchange Markets and Financial Crisis

We analyze the multifractal spectra of daily foreign exchange rates for Japan, Hong-Kong, Korea, and Thailand with respect to the United States Dollar from 1991 to 2005. We find that the return time series show multifractal spectrum features for all four cases. To observe the effect of the Asian currency crisis, we also estimate the multifractal spectra of limited series before and after the crisis. We find that the Korean and Thai foreign exchange markets experienced a significant increase in multifractality compared to Hong-Kong and Japan. We also show that the multifractality is stronge related to the presence of high values of returns in the series.   

Rural-urban migration including formal and informal workers in the urban sector: an agent-based numerical simulation study

The goal of this work is to study rural-urban migration in the early stages of industrialization. We use an agent-based model and take into account the existence of informal and formal workers on the urban sector and possible migration movements, dependent on the agents' social and private utilities. Our agents are place on vertices of a square lattice, such that each vertex has only one agent. Rural, urban informal and urban formal workers are represented by different states of a three-state Ising model. At every step, a fraction $a$ of the agents may change sectors or migrate. The total utility of a given agent is then calculated and compared to a random utility, in order to check if this agent turns into an actual migrant or changes sector. The dynamics is carried out until an equilibrium state is reached and equilibrium variables are then calculated and compared to available data. We find that a generalized Harris-Todaro condition is satisfied [1] on these equilibrium regimes, i.e, the ratio between expected wages between any pair of sectors reach a constant value. \\[4pt] [1] J. J. Silveira, A. L. Esp\'indola and T. J. Penna, Physica A, {\bf 364}, 445 (2006).   

Non-equilibrium vortex relaxation in disordered type-II superconductors

We study the non-equilibrium steady states and relaxation properties of driven vortex lines in the presence of both randomly distributed point and columnar pinning centers. We model the vortices as interacting elastic lines and employ a Langevin molecular dynamics algorithm to extract steady-state and non-stationary time-dependent behavior. In order to characterize the relaxation properties towards thermal equilibrium, we investigate transient two-time correlation functions. In particular we compare results obtained for systems with randomly distributed point pins and parallel columnar defects.   

Dynamics of Sheared Polydisperse Emulsions

Polydispersity is an important parameter in the jamming transition of soft materials which has not been well understood yet. In our work, we study the elastic response of a high volume fraction polydisperse emulsion to a periodic shear stress. The droplets' dynamics is imaged by confocal microscopy. Our results reveal that most of the droplets in the emulsion exhibit an elastic and periodic motion under the shear stress. However, the smaller droplets often move with anomalously large or small amplitudes compared to the mean motion. Some droplets also move nearly perpendicular to the mean flow field. The broad distribution of the amplitudes and phases of the smaller droplets' motions can be attributed to the motion the larger droplets and the distribution of the droplet sizes.   

The rigidity (unjamming) transition of disordered solids is caused by non-affinity

I will present a theoretical framework which allows one to account for the non-affinity of particle displacements due to disorder within a statistical mechanical description of the elasticity of disordered solids (1). A few important results can be derived analytically from first principles for disordered harmonic packings/lattices. First of all, the theory successfully recovers the unjamming or rigidity transition G ~(z-2d), where G is the shear modulus and z the coordination number, in excellent quantitative agreement with the numerical simulations of Ref. (2). Secondly, the theory explains this scaling law, which was hitherto enigmatic, in terms of the competition between the elastic (bonding) energy of the solid and the non-affine relaxations which are a consequence of structural disorder and contribute a dissipative term to the free energy of the solid. Potential applications to unsolved issues related to transport and vibrational properties of disordered solids will be briefly discussed. References (1) A. Zaccone and E. Scossa-Romano, Phys. Rev. B 83, 184205 (2011) (2) C.S. O'Hern, et al. Phys. Rev. E 68, 011306 (2003).   

Amended tunneling model to explain the anisotropy of the glassy properties of crystals and quasicrystals

The low temperature acoustic and thermal properties of amorphous, glassy materials are remarkably similar and they can be explained to a large extent by assuming that the material contains a large number of dynamic defects. These dynamic defects are tunneling systems and are modeled by an ensemble of two-level systems (TLS). Crystals with defects--with a large enough amount of disorder--exhibit also glass-like properties, but these properties are not so universal and, even more, they are not isotropic. In Phys. Rev. B {\bf 75}, 064202 (2007) we proposed an amended model for the description of the interaction of two-level systems with arbitrary strain fields. Here we show how this model explains the anisotropy of the glass-like properties of disordered crystals and quasicrystals.   

Dynamical Mechanism of Superdiffusive Processes and Multifractals

The diffusive process and the multifractal property are investigated in the Baduk. We ascertain that the difference of position between black and white stones undergoes the superdiffusion. In our study, we mainly estimate and analyze the generalized Hurst exponent, singularity spectrum, and multifractal strength in the tipping points of the Baduk. We also discuss the multifractal property of three segments, after analyzing the multifractality. In multifractal structures, it is found that segment $5$ has shown a stronger multifractal behavior than the other segments for the tipping points of the Baduk.   

On the metal-insulator-transition in vanadium dioxide

Vanadium dioxide (VO$_2$) undergoes a metal-insulator transition (MIT) at 340 K with the structural change from tetragonal to monoclinic crystal. The conductivity $\sigma$ drops at MIT by four orders of magnitude. The low temperature monoclinic phase is known to have a lower ground-state energy. The existence of the $k$-vector ${\mathbf k}$ is prerequisite for the conduction since the ${\mathbf k}$ appears in the semiclassical equation of motion for the conduction electron (wave packet). The tetragonal (VO$_2)_3$ unit is periodic along the crystal's $x$-, $y$-, and $z$-axes, and hence there is a three-dimensional $k$-vector. There is a one-dimensional ${\mathbf k}$ for a monoclinic crystal. We believe this difference in the dimensionality of the $k$-vector is the cause of the conductivity drop.   

On Van Hove Singularities in Pure Cubic Crystals

Pure elements form crystals of various lattices, cubic, tetragonal and others. At very low temperatures the lattice heat capacities in three dimensional crystals obey Debye's $T^3$-law, where $T$ is the absolute temperature. X-ray scattering experiments and lattice dynamics calculations reveal van Hove singularities when the density of states is plotted as a function of the phonon frequency. A physical origin of the singularities, jumps in the derivative of the density of states, is clarified. The singularities occur in three and two dimensions when the constant-frequency plane touches the Brillouin zone boundary {\it and} undergoes a curvature inversion. The face-centered cubic lattice is composed of two simple cubic sublattices and one tetragonal sublattice. The first (second) major peaks in the observed density of states in aluminum (Al) are shown to arise from the transverse phonons associated with the cubic (tetragonal) sublattices. We predict that the density of states has one major peak with a shoulder (two peaks with shoulders) for a body-centered cubic (face-centered cubic) crystal.   

Aging processes in reversible reaction-diffusion systems

Reaction-diffusion systems with reversible reactions generically display power-law relaxation toward chemical equilibrium. In this work we investigate, through numerical simulations, aging processes that characterize the non-equilibrium relaxation. Studying a model which excludes multiple occupancy of a site, we find that the scaling behaviors of the two-time correlation and response functions are similar to that discovered previously in an exactly solvable version with no restrictions on the occupation numbers. In particular, we find that the scaling of the response depends on whether the perturbation conserves a certain quantity or not. Our results point to a high degree of universality in relaxation processes taking place in diffusion-limited systems with reversible reactions.\\[0.3cm] [1] N. Afzal, J. Waugh, and M. Pleimling, J. Stat. Mech. P06006 (2011).   

Numerical master equation approach for complex transport processes

Applicability of the numerical master equation analysis in studies of complex non-equilibrium systems is discussed. For small systems (up to $\sim10^6$ states), solving of master equations directly is found very useful especially when studying parameter sensitive and elusive properties, such as drifts caused by the ratchet effect. Also various types of optimization methods can be applied. Efficient numerical methods for solving large master equation systems are discussed. Properties that can be readily computed include mean values and fluctuations of trajectory and state dependent observables, such as velocity, diffusion coefficient and shape deformations of the object. To further study the microscopic mechanisms of complex transport processes, we apply a graph optimization method to master equation sets. This allows one to identify the dominating processes that are responsible for the transport. We present detailed case studies for reptating polymers and metal-on-metal atomic islands in non-homogeneous potentials. These models represent complex many-particle systems with rich nonlinear behavior, such as inversions and increasing of velocity and large shape deformations in external potentials.   

String Theory (Knot Really)

Knots can make one string out of two, without adhesive, simply by entanglement. We present a computer simulation of knots untying. A series of spherical masses connected by spring forces and excluded by normal forces approximate the strings. We tie the simulated strings into knots and tug on their ends with constant forces. Plotting time to untie versus a static coefficient of friction reveals critical frictions above which the knots hold. The critical frictions depend on the knot type (including square, thief, and surgeon's thief knots) and might be used to classify practical knots.   

Metal Insulator transition in Vanadium Dioxide

MAR12-2011-000262 Abstract Submitted for the MAR12 Meeting of The American Physical Society Sorting Category: 03.9 (T) On the metal-insulator-transition in vanadium dioxide AZITA JOVAINI, SHIGEJI FUJITA, University at Buffalo, SALVADOR GODOY, UNAM, AKIRA SUZUKI, Tokyo University of Science --- Vanadium dioxide (VO2) undergoes a metal-insulator transition (MIT) at 340 K with the structural change from tetragonal to monoclinic crystal. The conductivity {\_} drops at MIT by four orders of magnitude. The low temperature monoclinic phase is known to have a lower ground-state energy. The existence of the k-vector k is prerequisite for the conduction since the k appears in the semiclassical equation of motion for the conduction electron (wave packet). The tetragonal (VO2)3 unit is periodic along the crystal's x-, y-, and z-axes, and hence there is a three-dimensional k-vector. There is a one-dimensional k for a monoclinic crystal. We believe this difference in the dimensionality of the k-vector is the cause of the conductivity drop. Prefer Oral Session X Prefer .   

Neutrosophic Physics as a new field of research

Neutrosophic Physics describes collections of objects or states that are individually characterized by opposite properties, or are characterized neither by a property nor by the opposite of that property. Neutrosophic Physics means a mixture of physical concepts/ideas/spaces/laws/theories $<$A$>$ with their opposite $<$antiA$>$ or with their neutral $<$neutA$>$ {\{}where $<$neutA$>$ means neither $<$A$>$ nor $<$antiA$>$, but in between, i.e. the neutral part{\}}, and it is a combination of heterogeneous contradictory things which hold together. There are many cases in scientific fields (and in humanistic fields) that an item $<$A$>$ and its opposite $<$antiA$>$ or their neutral $<$neutA$>$ are simultaneously valid. - Several examples of neutrosophic physics: (1) unmatter, which is formed by matter and antimatter that bind together (Smarandache, 2004); (2) neutral Kaon, which is a pion {\&} anti-pion composite (Santilli, 1978) and therefore a form of unmatter; (3) neutrosophic cosmological model (Rabounski-Borissova, 2011); (4) among possible Dark Matter candidates there may be exotic particles that are neither Dirac nor Majorana fermions; (5) mercury (Hg) is a state that is neither liquid nor solid under normal conditions at room temperature; (6) non-magnetic materials are neither ferromagnetic nor anti-ferromagnetic; (7) quark gluon plasma (QGP) is a phase formed by quasi-free quarks and gluons that behaves neither like a conventional plasma nor as an ordinary liquid; (8) neutrosophic methods in General Relativity (Rabounski-Smarandache-Borissova, 2005); (9) neutrosophic cosmological model (Rabounski-Borissova, 2011); etc.   

Superconductors \textit{(History {\&} Advanced Research)}

\textit{Superconductors are} materials that have no resistance to electricity's flow; they are one of the last great frontiers of scientific discovery. In 1911 superconductivity was first observed in mercury by Dutch physicist Heike Kamerlingh Onnes When he cooled it to the temperature of liquid helium, 4 degrees Kelvin (-452F, -269C), its resistance suddenly disappeared. It was necessary for Onnes to come within 4 degrees of the coldest temperature that is theoretically attainable to witness the phenomenon of superconductivity. The next great milestone in understanding how matter behaves at extreme cold temperatures occurred in 1933. German researchers Walther Meissner and Robert Ochsenfeld discovered that a superconducting material will repel a magnetic field. A magnet moving by a conductor induces currents in the conductor. This is the principle on which the electric generator operates. But, in a superconductor the induced currents exactly mirror the field that would have otherwise penetrated the superconducting material - causing the magnet to be repulsed. This phenomenon is known as strong diamagnetism and is today often referred to as the ``Meissner effect'' (an eponym). In 1941 niobium-nitride was found to superconduct at 16 K. In 1953 vanadium-silicon displayed superconductive properties at 17.5 K. And, in 1962 scientists at Westinghouse developed the first commercial superconducting wire, an alloy of niobium and titanium (NbTi).   

Teaching: the Wave Mechanics of McLeods' Stringy Electron, Explicit Nucleons, and Through-the-Earth Projections of Constellations' Stick Figures

This shows how Hooke's law, for electron, proton and neutron, 2D and 3D, strings, builds electromagnetic string-waves, extending, and pleasing, Schr\"{o}dinger. These are composed of spirally linked, parallel, north-pole oriented, neutrino and antineutrino strings, stable by magnetic repulsions. Their Dumbo Proton is antineutrino-scissor cut, and compressed in the vicinity of a neutron star, where electrostatic marriage occurs with a neutrino-scissor cut, and compressed, electron, so a Mickey Neutron emerges. Strings predict: electron charge is - 1/3 e, Dumbo P is 25 {\%} longer than Mickey N, and Hooke says relaxing springs fuel three, separate, non-eternal, inflations, after Big Bangs. Gravity is strings, longitudinally linked. Einstein says Herman Grid's black diagonals prove human vision reads its information from algebraically-signed electromagnetic field distributions, (diffraction) patterns, easily known by ray-tracing, not requiring difficult Spatial Fourier Transformation. High-schoolers understand its application to Wave Mechanics, agreeing that positive-numbered probabilities do not enter, to possibly displease God. Detected stick-figure forms of constellations: like Phoenix, Leo, Canis Major, and especially Orion, fool some observers into false beliefs in things like UFHumanoids, or Kokopelli, Pele and Pamola!   

Properties of Neutrinos and Leptonic Processes

We study the effect of the modifications in the properties of neutrinos to the scattering crossections of purely leptonic processes. Neutrinos change their behavior in the hot and dense media and the properties like mass, induced magnetic moment, number density and the refractive energy are also modified as a result. We investigate that how these properties affect the leptonic decays and scattering processes. Some of the important applications of these results are also indicated in this study.   

Measurement Setup for Characterizing Thermoelectric Materials at High Pressure

Several investigations have recently resulted in the development of methods for studying thermoelectric materials at high pressure. However, these investigations have focused primarily on probing the electrical resistivity and Seebeck coefficient. While these are important to the effective operation of thermoelectric materials, they are only part of the whole picture. In an effort to address this on a bulk materials scale, facilities have been developed in the High Pressure Laboratory of the Mineral Physics Institute to investigate these properties, along with investigations of the thermal conductivity and ultrasonic sound velocity. With these facilitites, it is likely that further studies of these materials will help advance the theoretical understanding of thermoelectrics and further the development of new, more effective thermoelectric materials.   

X-ray imaging of electrospinning polymer solutions

The study of electrospinning polymer solution jets, and the evolution of the polymer entangled network during electrospinning, is of interest in clarifying nanofibers microstructure. We used fast X-ray phase-contrast imaging to investigate the flow of electrospinning PEO and PMMA semidilute solutions. The jet profile, velocity and absorbance were measured at high resolution ($<$ 1 $\mu $m/pixel) for the first 10 mm of jet length, at various electrospinning conditions. Jet radius measurements demonstrate a viscosity-dominated flow, with common power-dependence on the distance from the orifice. The flow field was analyzed by imaging solutions with micron-size silica particles, revealing axial velocities of 0.5-1 m/s and strain rates of 200-300 1/s, as well as radial and rotational velocity components. The measured axial velocity provides a direct indication for rapid solvent evaporation as early as 2-3 mm from the jet start. X-ray absorption measurements reveal substantial polymer concentration rise along the jet and at the jet boundaries, evidence for rapid evaporation. Additionally, at high stretching conditions, the polymer concentration rises at the jet center, implying polymer network lateral contraction as predicted by theory.   

Nano-indentation experiments: from viruses to cells

Over the last years AFM imaging and nanoindentation have become an indispensable tool for biophysical studies in liquid at the nano- and micro-scale. We look at both these length scales, at the cellular as well as the sub-cellular level. In particular, we perform combined imaging and force spectroscopy experiments on viral particles to elucidate their structure and mechanics [1]. These studies revealed that Noro virus has found an intriguing way to increase its mechanical strength. These self-assembling, natural nanoparticles incorporate a pre-stress during assembly, consolidating the structure of its protein shell that protects the genome [2]. Next, we studied whole cells. Mechanical loading is increasingly recognized as an important stimulus to cells. Establishing the local viscoelastic properties within a cell is vital to the understanding of the underlying mechanisms of cytoskeletal changes in response to these stimuli. We study the mechanical response of mammalian bone forming (osteoblast-like) cells on a substrate of physiological stiffness using spherical, $\mu$m-sized probes and we compare these results to the properties of bone forming cells originating from fish (teleosts).\\[4pt] [1] Roos et al. Nature Physics (2010), 6:733\\[0pt] [2] Baclayon et al. Nano Letters (2011), 11:4865   

Study of the interactions of magnetosomes inside magnetotactic bacteria (MTB) through modeling and experimentation

Since the discovery in 1970s, magnetotactic bacteria (MTB) have enjoyed great interest to biomimetic and biophysical research, due to their fine-tuned synthesis of magnetosomes (magnetic nanocrystals enclosed in organelles) and challenges in understanding the precise mechanism of magnetotaxis via aligning the magnetosomes. We investigate magnetosomes inside an MTB cell as a chain of magnetic crystals and explore their internal interactions as well as the influence from an external magnetic field. Applying both analytical and simulation approaches, we are able to identify the threshold external magnetic field for effective magnetosome alignment depending on crystal configurations. The results agree well with high resolution X-ray diffraction data. They also provide a simple and quantitative model for elucidating the mechanical stability of the magnetosome chain, thus the energetically favorable states of MTB in the presence of an external magnetic field.   

Tayloring the local electronic and structural properties of colloidal nanocrystals

Recent advances in wet-chemical synthesis techniques allow unprecedented control of the composition, size, shape, and surface chemistry of colloidal nanocrystals. The structural, opto-electronic and magnetic properties of these materials can be tailored to enable new quantum phenomena with applications in biology, energy harvesting, and fundamental physical studies. Moreover, sophisticated understanding of colloidal nanocrystals requires local probes of individual particles, such as scanning tunneling microscopy (STM) and spectroscopy (STS), that can measure the local density of states (LDOS) and particle wave-functions in real space with atomic resolution. Here, we present our STM/STS studies of the structural and electronic properties of individual CdS, Cu2S and binary CdS/Cu2S heterostructure nanocrystals. Detailed local study of electronic properties of the nanocrystals could bridge the existing knowledge gap between bulk and nanoscale. Such understanding is crucial for the design of novel materials based on colloidal nanocrystals.   

Non-uniform forcing of a magnetic soap film

The effects of a non-uniform magnetic field on a suspension of magnetic nanoparticles in a soap film are investigated. Experiments show that a strong non-uniform field will form a two-dimensional structure of magnetic nanoparticles within the soap film. A model is developed to compare with experimental results for several different magnetic field configurations. The system is modeled with a two-dimensional thin film equation with no-flux boundary conditions. We show qualitative agreement between the model and experiments.   

The cryogenics magnetocaloric effect in the blocking state superparamagntic nanocapsules

The large cryogenics magnetocaloric effect was obtained in superparamagnetic nanocaspules when they are in blocking state. Interestingly, the entropy change of superparamagnetic nanocapsules (Lanthannide-transition metal intermetallic compound), including GdAl$_{2}$/Al$_2$O$_3$, TbAl$_{2}$/Al$_{2}$O$_{3}$, DyAl$_{2}$/Al$_{2}$O$_{3}$ nanocapsules, shows an unusual sharp increase, when the nanocaspules go into their blocking state. Combining the structural and magnetic analysis, we find the high moment density and low anisotropy energy play crucial role in exciting and hindering the rotation of the moment process, which finally decide the magnitude of the entropy change. The entropy change dependence of the temperature change and applied magnetic field was obtained according to the langevin theory.   

Following the Dynamics of DNA in Living Cells

Cells are brimming with molecular activity, but the cellular interior is much more than a test tube for biochemical reactions. The intracellular environment imposes a variety of mechanical constraints and engenders interactions from molecular crowding to a range of motor-driven activity responsible for transcription, replication, cargo transport, cytoskeletal rearrangement, chromosomal remodeling, and so on. We have developed a two-color correlational microscopy technique to follow the dynamics of DNA interacting with the \textit{in vivo} cellular environment. Substantial differences between live cells and dead, yet structurally intact, cells point to a strong coupling of active, motor-driven fluctuations in the cell. This suggests that the motion of native, cellular DNA may similarly be driven by active processes, instead of relying on purely thermal, passive fluctuations. We also note that the correlations provide a sensitive measure for the effective length of the DNA probe on a length scale around one persistence length ($\sim $ 50 nm). This paves the way for experiments with more complex DNA probes that can bind transcription factors to form protein-mediated DNA loops, the dynamics of which could be observed through this method.   

Enhanced resonant magnetoelectric coupling in frequency-tunable composite multiferroic bimorph structures

We report on the enhanced resonant magnetoelectric (ME) coupling in composite multiferroic magnetostrictive/piezoelectric composites bimorph structures. The approach was shown to provide more than ten-fold gain in ME coefficient and broad-band range magnetic and electric field assisted stress-configurable resonance frequency tuning up to 100{\%} in these ME structures. The ME structures were investigated using optical spectroscopy. It was shown that this principle of continuously tuned resonance could be used to improve selectivity, signal to noise ratio and universality and sensitivity of ME magnetic sensors.   

Strongly interacting lattice bosons in disorder potential: a strong coupling approach

We use a strong coupling canonical transformation to study the phase diagram of strongly interacting bosons in an optical lattice in the presence of one-body disorder potential. Our strong coupling approach treats the disorder potential non-perturbatively and can be applied to moderately high disorder potentials as long as the on site repulsion energy scale for the bosons (U) is larger than the scale of the disorder potential (V). Within the strong coupling approach, we systematically derive the low energy effective Hamiltonian, and, using variational Gutzwiller type wavefunctions, study the phase diagram of the disordered Hubbard model, identifying the Mott insulator, superfluid and Bose glass phases.   

Imaging of nearby NV centers beyond diffraction limit

Nitrogen-Vacancy (NV) centers in diamond have been widely applied in physical and biological study. Single NV center has shown its good quality as a single-photon source. More importantly, when two NV centers are close to each other within tens of nanometers, the strong dipole-dipole interaction can be applied in quantum information techniques. Diamond nanocrystal with NV center has been applied for imaging biological processes. Therefore, to image and distinguish two nearby NV centers is becoming more and more important. In experiment, NV centers are usually detected with scanning confocal optical fluorescence microscopy where the single-photon counts of spontaneous emission are measured to describe their optical imaging. Also, the method of stimulated emission depletion fluorescence microscopy was used to distinguish two nearby NV centers. Here, we proposed a new technique based on our home-built confocal scanning microscopy to image two nearby single NV centers. With the new imaging technique, we have experimentally imaged and distinguished two NV centers with the distance about 40nm, which is well beyond the diffraction limit. This imaging technique can, in principle, distinguish particles with any overlapping.   

Phase transition kinetics and critical phenomena of the site dilute Ising model with long range interactions

We consider the effect of quenched site dilution on the phase transition kinetics and critical phenomena of the Ising model with long range interactions. First, we generalize the Harris criterion to mean-field and near mean-field systems with long range interactions and show that the critical exponents of both the critical point and spinodal do not change regardless of the dimensionality. This is supported by measurements of the isothermal susceptibility near the spinodal line from Monte-Carlo simulations. We also generalize the Coniglio-Klein method by mapping the dilute Ising critical point onto an equivalent site-bond percolation problem. In addition, using the Hubbard-Stratanovich transformation, we are able to write down a coarse-grained field theory for a specific dilution realization and use it to interpret simulations of the defect density dependent local growth of the stable phase after an instantaneous quench.   

Computation of Shock Waves in Binary Inert Gas Mixtures of Diatomic Gases Using the Generalized Boltzmann Equation

This paper describes the methodology for computing non-equilibrium shock waves in a binary inert mixture of monoatomic and diatomic gases using the Generalized Boltzmann Equation (GBE). For solving the classical Boltzmann equation for a mixture of monoatomic gases or the Generalized Boltzmann equation for a mixture of diatomic gases, these equations are formulated in impulse space, instead of velocity space. A binary mixture of two inert gases results in four coupled GBEs. The computational framework available for the classical Boltzmann equation is extended by including the rotational and vibrational degrees of freedom in the GBE for a mixture of diatomic gases in impulse space. The problem including both rotational - translational (RT) and vibrational -- translational (VT) energy transfers is solved by applying a two-stage splitting procedure to the four coupled GBEs. The two stages consist of free molecular transport and RT -VT relaxations. Computations are presented for the shock structure in a binary mixture of N$_{2}$ and Ar, and N$_{2}$ and O$_{2}$ with 1:1 density ratio at Mach 5. This is the first time, such complex computations are being reported in the literature. The methodology can be easily applied to an inert mixture containing an arbitrary number of species.   

Optomechanically Induced Transparency and Slow Microwaves in Circuit Nano-electromechanics

Using a low-mass ($\sim$ 15\,pg), high-Q ($>$ 100 000) nanomechanical oscillator coupled to a Nb superconducting quarter wave cavity, we realize a circuit nano-electromechanical system coupling microwaves to mechanical motion oscillating at 1.45\,MHz. By exciting the system on the lower motional sideband with a strong drive tone, a transparency window for a probe field is created originating from the effect of optomechanically induced transparency (OMIT). This phenomenon, analogous to electromagnetically induced transparency in Atomic Physics, arises from the interference of different excitation pathways for an intracavity probe field. We utilize the transparency window to demonstrate slow microwave propagation. A tunable delay up to 4\,ms is demonstrated experimentally for a microwave pulse on resonance with the cavity. Furthermore, we systematically investigate the temporal dynamics of this transparency window when the drive tone is modulated, and the effect of the oscillator's Duffing nonlinearity on the OMIT window.   

Performance of Multi Walled Carbon Nanotubes Grown on Conductive Substrates as Supercapacitors Electrodes using Organic and Ionic liquid electrolytes

In this work we will present the use of Multi Walled Carbon Nanotubes (MWNT) directly grown on inconel substrates via chemical vapor deposition, as electrode materials for electrochemical double layer capacitors (EDLC). The performance of the MWNT EDLC electrodes were investigated using two electrolytes, an organic electrolyte, tetraethylammonium tetrafluoroborate in propylene carbonate (Et$_{4}$NBF$_{4}$ in PC), and a room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF$_{6})$. Cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy measurements to obtain values for the capacitance and internal resistance of these devices will be presented and compared.   

Measure of nonclassical correlation in coherence vector representation

We consider the coherence vector representation (CVR) of a bipartitestate and obtain a necessary and sufficient condition for a zero-discord state. Based on this, a measure of quantum, classical and total amount of correlations in bipartite states is proposed in this representation. Analytical expressions for this measure are available for any bipartite states. Our measure of nonclassical correlation coincides with the geometric measure of quantum discord for some particular states. References: T Zhou, J. Cui and G L Long, PRA A84, 062105 (2011)   

Law of Universal Repulsion

All objects in the universe repel each other. Repulsion between two objects is directly proportional to the external energy (mv$^{2})$ of their relative motion and indirectly proportional to their relative motion radius (one object is in relative rest, while the other one is in relative motion). \textbf{Application examples} Suppose a man whose mass is 100 kg, runs on the earth at a speed of 10 meters per second. The radius of the earth is 637100 meters. The repulsion between the earth and the man is: F=mv$^{2}$/r=0.00157N; if his speed reaches the first cosmic speed ( 7.9 km per second ), then calculate: F=mv$^{2}$/r=980N, just overcome the gravity of the earth. A car with a certain velocity running in a straight highway ( actually with some slight curves ) can't fly up. However, if it encounters an arched bridge, it is possible for the car to fly. That is the consequence of the arch bridge has changed its movement radius, and the repulsion is increased. A aircraft's taking off and flying are not only because of the fluid - air, but also because of the change of its motion radius ( forming virtual arched bridge in the air ), which increases the repulsion.   

Supersymmetry approach to delocalization transition in the network model of weak field quantum Hall effect and related models

Recently, a two-channel random network model was proposed for quantum Hall effect in a weak (non-quantizing) magnetic field. For the anisotropic variant of this network, we analytically average over the randomness using the supersymmetry method, and map the network onto a model of interacting $u(1,1|2)$ superspins. We analyze the resulting superspin model by relating it to a nonlinear sigma-model. We argue that the phase diagram of the superspin model does not change qualitatively if we identify $u(1,1|2)$ superspins with ordinary $su(2)$ spin-1/2's. This allows us to develop a deeper insight and extend the phase diagram into the regions inaccessible to a sigma-model approach. We further extend our approach to a related two-channel network models and discuss their physical implications.   

Giant Electro-Mechanical Energy Conversion in [011] cut Relaxor Ferroelectric Single Crystals

Giant electro-mechanical energy conversion is demonstrated under a ferroelectric/ferroelectric phase transformation in [011] cut and poled lead titanate-based relaxor perovskite morphotropic single crystals. It is found that under mechanical pre-stress, a relatively small oscillatory stress drives the material reversibly between rhombohedral and orthorhombic phases with a remarkably high polarization and strain jumps induced at zero bias electric field and room temperature. The measured electrical output per cycle is more than an order of magnitude larger than that reported for linear piezoelectric materials. Ideal thermodynamic cycles are presented for this electro-mechanical energy conversion followed by a presentation and discussion of the experimental data.   

Direct and Indirect Two-color Coherent Control in Bulk Silicon

Using an empirical pseudopotential model for electron states and an adiabatic bond charge model for phonon states, we investigate the two-color direct and indirect coherent current injection with an incident optical field composed of a fundamental frequency and its second harmonic, and calculate the response tensors of the electron (hole) charge and spin currents. We show the current control for three different polarization scenarios: For co-circularly polarized beams, the direction of the charge current and the polarization direction of the spin current can be controlled by a relative-phase parameter; for the co-linearly and cross-linearly polarized beams, the current amplitude can be controlled by that parameter. For the indirect gap injection, the spectral dependence of the maximum swarm velocity shows that the direction of charge current reverses under an increase in photon energy.   

C-Axis Properties of DyNi$_{2}$B$_{2}$C System

We have measured the electrical resistivity along c-axis $\rho _{c}$(T, H) of the DyNi$_{2}$B$_{2}$C single crystal with the magnetic fields perpendicular to the c-axis and the magnetization isotherms M(H) of the DyNi$_{2}$B$_{2}$C single crystal with magnetic fields perpendicular and parallel to the c-axis. We confirmed that Neel temperature T$_{N}$ is 10.3K from the $\rho _{c}$(T) result which is consistent with that from previous $\rho _{ab}$(T) result. In addition, the constructed critical fields H$_{c2}$(T) curve and magnetic transitions diagram of DyNi$_{2}$B$_{2}$C from $\rho _{c}$(T) magnetic fields perpendicular to c-axis is similar to that of $\rho _{ab}$(T) result, which is thought to arise that 3 D magnetic structure of DyNi$_{2}$B$_{2}$C.   

Topological insulators on a Mobius Strip

We study the two dimensional Chern insulator and spin Hall insulator on a non-orientable Riemann surface, the Mobius strip, where the usual bandstructure topological invariant is not defined. We show that while the flow pattern of edge currents can detect the twist of the Mobius strip in the case of Chern insulator, it can not do so in spin Hall insulator [1]. \\[4pt] [1] Lang-Tao Huang and Dung-Hai Lee, Phys. Rev. B \textbf{84}, 193106 (2011)   

Extension of the Non-Hermitian Quantum Walk Model into a 2D Hexagonal Lattice

We generalize the 1D non-Hermitian quantum walk model of M. S. Rudner, and L. S. Levitov [1] to three different cases: 1D model with next nearest hopping, 2D model with and without next nearest hopping on hexagonal lattice respectively. We find that the quantization of the average decay length is always robust to next nearest hopping with real hopping parameter in 1D, but in the case of 2D, this quantization is only present in some special circumstances. We also find the 2D model is compatible to the 1D one by setting one of the hopping terms to zero. As to the Haldane model, quantization of this type exits only in $0$ or $\pi$ magnetic flux phases. \\[4pt] [1] M. S. Rudner and L. S. Levitov, Phys. Rev. Lett. {\bf102}, 065703 (2009).   

The Pirouette effect in turbulent flows

The dynamics of the velocity gradient tensor plays a crucial role for our understanding of turbulent flows. I will discuss recent numerical and experimental results concerning the ``perceived velocity gradient tensor,'' constructed with the help of 4 points (a tetrad). This tensor depends on the overall size of the tetrad, thus permitting to study the properties of the flow as a function of scale. I will in particular consider the statistical properties, as well as the dynamical properties of the velocity gradient tensor, in relation to the dynamics of the shape of the tetrad. These properties shed some new light on the intriguing preferential alignment between vorticity and the eigenvector, corresponding to the intermediate eigenvalue of the strain.   

Spintronic Oscillator based on feed-back mechanism

Nano-scale rf oscillators based on the magnetic tunnel junctions is an active area of research. These oscillators are based on the spin-transfer torque effect, in which a dc current drives the magnetization into precessional motion. Here we present a novel design of a spintronic oscillator which is not based on spin-transfer torque effect. This new oscillator is comprised of a magnetic tunnel junction whose top and bottom contacts are connected to a bias-T. A dc current is passed through the low frequency port of the bias-T and the high frequency port is connected to a ``feed-back'' wire which runs below the MTJ. Any fluctuation in the magnetization direction of the free layer of MTJ, drives ac current through the feed-back wire, which in turn exerts ac magnetic field on the free layer. The feedback wire is oriented such that the ac magnetic field amplifies the magnetization fluctuations for positive value of dc current. For negative value of dc current, the feedback loop suppresses the fluctuations. We find that if the positive dc current passing through the MTJ is more than a critical value, continuous precessing states of the magnetization are possible. Oscillators with better quality factors are possible using the feedback scheme.   

Stability and collapse of a dipolar condensate in a 1D optical lattice

The stability properties of a dipolar condensate are strongly affected by the external confining potential, as a consequence of the long range and anisotropic character of the dipole-dipole interaction. In contrast to a contact interacting gas, the presence of a 1D lattice induces a crossover from a dipolar destabilized to a dipolar stabilized regime for increasing lattice depth. In the deep lattice regime, a dipolar condensate can be stabilized at large negative scattering length in the interaction-dominated regime. As a consequence of the dipolar potential shape, the condensate collapse in the deep lattice regime features a {\em local} character (as opposed to the global character of the usual contact interacting gas): the condensate develops a surface density modulation resulting in several isolated local collapses. Finally, the stabilizing effect of the external potential plays an important role during the time-of-flight expansion. A dipolar condensate can be stable as long as it is trapped but can immediately collapse as soon as the external potential is removed. This makes the mapping of the time of flight expansion onto the momentum distribution highly non trivial.   

Spin wave excitation by spin-transfer torque in a magnetic insulator

We study the excitation of spin waves in magnetic insulators by the current-induced spin-transfer torque. We predict preferential excitation of surface spin waves induced by an easy-axis surface anisotropy. The critical excitation current for the surface spin wave is inversely proportional to the penetration depth and surface anisotropy. Compared to the bulk modes, the critical current and excitation power of such surface spin wave are greatly reduced and enhanced, respectively.   

Calculating the density of states in disordered systems using free probability

We approximate the density of states in disordered systems by decomposing the Hamiltonian into two random matrices and constructing their free convolution. The error in this approximation is determined using asymptotic moment expansions. Each moment can be decomposed into contributions from specific joint moments of the random matrices; each of which has a combinatorial interpretation as the weighted sum of returning trajectories. We show how the error, like the free convolution itself, can be calculated without explicit diagonalization of the Hamiltonian. We apply our theory to Hamiltonians for one-dimensional tight binding models with Gaussian and semicircular site disorder. We find that the particular choice of decomposition crucially determines the accuracy of the resultant density of states. From a partitioning of the Hamiltonian into diagonal and off-diagonal components, free convolution produces an approximate density of states which is correct to the eighth moment. This allows us to explain the accuracy of mean field theories such as the coherent potential approximation, as well as the results of isotropic entanglement theory.   

Transport Coefficients of a Normal Fermi Gas at Unitarity

We studied the transport properties of a two component Fermi gas in the unitary limit. Transport coefficients of the Fermi gas are calculated in the extreme low-temperature limit. To calculate the transport coefficients we need the scattering amplitudes. The scattering amplitudes are calculated from the Landau parameters. These parameters are obtained from the local version of the induced interaction model for computing Landau parameters [1]. The leading order finite temperature corrections to the transport coefficients are also calculated [2]. The calculated temperature dependent spin diffusivity is compared with the experimental measurement. A minimum of the spin diffusivity with a value of order h/m is observed at some finite temperature below the Fermi temperature.   

Thermally-induced resonance in folding-unfolding transition of a stretched RNA

The biopolymers are often situated in constrained, thermally fluctuating environment in the cell. To understand how they respond to a minute but temporally varying signal, we study folding-unfolding dynamics of a stretched RNA under a small oscillatory force. We find via numerical simulations that the small oscillation enhances the folding (unfolding) dynamics, even at a high (low) stretching condition where the folding (unfolding) is improbable, leading to a minimum mean transition time at an optimal frequency, a phenomenon dubbed as Resonant Activation (RA). In addition, the folding-unfolding transition can be maximally synchronous to the oscillation at another optimal frequency, characteristics of the Stochastic Resonance (SR). These noise-assisted resonance phenomena, RA and SR, which can also be largely modulated by the polymer parameters, provide a glimpse of how a biopolymer self-organizes in response to the environmental fluctuation in the cell.   

Effect of inter-layer single-electron hopping and pair-hopping on superconductivity in multi-layered cuprates

The multi-layered cuprates, which remain the highest-Tc material to date, still harbor unsolved problems. Specifically, the reason why Tc increases as we go from the single-layer system to trilayer has not been microscopically understood. Here we have studied the superconductivity in the n-layered Hg-series cuprate HgBa$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{2+2n+\delta }$ by solving the Eliashberg equation for a multi-orbital Hubbard model with the random phase approximation (RPA) as well as with the fluctuation exchange approximation (FLEX). The hopping parameters are obtained by downfolding from first principles calculations based on the density functional theory (DFT) for the Hg-series cuprates with n=1, 2, 3. The result indicates that the enhancement of Tc with n is not explained in terms of the band structure alone. We then consider the possible ingredients in multi-layered cuprates that may affect Tc for different numbers of CuO$_{2}$ planes, specifically, the inter-layer single-electron hopping and inter-layer Cooper-pair hopping originating from the inter-layer Coulomb interaction, which we vary to probe the superconducting nature.   

Superfluid correlations of ladder-type Hubbard model with trapping potential

In this study, we investigate possibility of superfluidity derived from repulsive interactions in optical lattice systems using the DMRG method. We calculate superfluid correlations in 2-leg ladder-type Hubbard models with varying the repulsive parameter $U$ and the trapping one $V$, and then clarify dependences of the superfluid correlations on the parameters. The calculation results show that there are two hotspots, whose superfluid correlations are remarkably high, in the space spanned by $U$ and $V $for suitable fillings. In this presentation, we present the results, compare them with those in more than 2-leg Hubbard models, and reveal the best situation in which the superfluidity correlation develops.   

Critical Lamellar Thickness of Polymer Crystal Growth Studied by Dynamic Monte Carlo Simulations

We set up a parallel-oriented slit to block lamellar crystal growth of chain molecules, and found a critical spacing of the slit gate, which reflected the critical lamellar thickness (L$_{min})$ for crystal growth. The values of L$_{min}$ measured at various temperatures provided useful information about the role of lamellar crystal thickness in the kinetics of polymer crystal growth. More interesting, we found that the excess lamellar thicknesses decreased with the increase of temperatures, in contradictory to the prediction of Lauritzen-Hoffman theory.   

Strain-induced transition from intramolecular to intermolecular crystal nucleation

We performed dynamic Monte Carlo simulations of strain-induced crystallization of bulk polymer chains. We observed that for small crystallites grown at relatively high temperatures, the probabilities of adjacent chain-folding jump down at a critical strain. The result implies that polymer chains prefer to choose intramolecular crystal nucleation at small strains, and the intermolecular crystal nucleation becomes dominant only when the strain is higher than the critical value. In addition, the critical strains appear as almost constant to temperatures.   

Single Crystalline TiO$_{2}$ Nanorod grown on Transparent Conducting Substrates for Dye-Sensitized Solar Cells

Single crystalline TiO$_{2}$ nanorods have been synthesized by pulse laser deposition technique on indium tin oxide glass substrates. During the deposition, oxygen pressures were kept under 100, 200 and 500mTorr, respectively. Scanning Electron Microscopy images show vertical nanorod arrays with the same diameters of 200-300 nm were obtained under different oxygen pressures, but with various density of the arrays. The TiO$_{2}$ nanorods growth under 200 mTorr oxygen pressures were pure highly crystalline anatase according to X-ray diffraction measurement. The highly crystalline and vertical TiO$_{2}$ nanorod arrays contributed to the achievement of the high conversion efficiency of lightto-electricity. High conversion efficiency was obtained with the vertically aligned TiO$_{2}$ single crystalline nanorod cell.   

Transitions between multiple attractors in a granular mixing experiment

We report on a granular experiment that produces multiple quasi-periodic patterns in a rotating flat container filled with a bidisperse mixture (0.3 mm and 0.9 mm diameter) of spherical grains. The cell depth is fixed to 0.5 cm (of the order of 10 bead layers), the height is 8 cm. Cell widths are either 17 cm or 50 cm. After an initial transient period, the system develops a lateral band texture of size-segregated grains. These bands show a pronounced spatio-temporally periodic drift in axial direction, sometimes with reflection at the cell ends. In the course of long durations of the experiment, the system switches between different states that appear to be marginally stable oscillatory solutions over many periods. Ageing of the particles as well as external influences on the experiment can be excluded as explanations. In the long term limit, the system does not tend towards a stationary state, neither to a complete segregation, nor to stable bands or uniformly mixed states. The results complement and extend previous observations in cylindrical geometries, and represent a challenge for modeling and theoretical description.   

Ultra high resolution neutron scattering: Neutron Resonance Spin-Echo and Larmor Diffraction

The TRISP spectrometer at the FRM II neutron source near Munich, Germany, is a unique world-leading neutron scattering instrument which employs the Neutron Resonance Spin-Echo technique (NRSE). Linewidths of dispersive excitations with energy transfers up to 50 meV can be measured with an energy resolution in the $\mu$eV range without the restrictive flux limitations that normally apply to high resolution neutron triple-axis spectrometers. Pioneering studies on the electron-phonon interaction in elemental superconductors\footnote{P. Aynajian et al., Science {\bf 319} 1509 (2008)} and the lifetimes of magnetic excitations in archetypal magnetic systems will be reviewed.\footnote{S. Bayrakci et al., Science {\bf 312} 1928 (2006)} The instrument can also be used as a Larmor diffractometer, enabling d-spacings to be measured with a resolution of $\frac{\Delta d}{d} \sim 10^{-6}$, i.e. more than one order of magnitude more sensitive than conventional diffraction techniques.\footnote{C. Pfleiderer et al., Science {\bf 316} 1871 (2007)} Ongoing and future NRSE and Larmor diffraction projects will be outlined, especially in regard to prospective studies which will take full advantage of the new low temperature and high pressure sample environment capabilities now available at TRISP.   

Quantum Diffraction Without de Broglie Waves

It is well established that composite particles, from neutrons to atoms to large molecules, sometimes behave as de Broglie waves, with wavelength $\lambda =h$/$p$, where $p$ is the total momentum of the given composite particle. The primary evidence of these de Broglie waves is the observation of diffraction patterns from diffracting objects such as crystal lattices or one or more orifices. Indeed, it is generally believed that such de Broglie waves are universal quantum aspects of all matter. It is argued here (see also [1]) that only primary quantum fields such as electrons are truly de Broglie waves. In contrast, composite particles are small confined quantum waves that follow essentially classical particle trajectories, without extended wavelike behavior given by the de Broglie relation. For example, a neutron is properly a particle on the fm scale, not an extended wave on the nm scale. The momentum change of such a quasiclassical particle during a collision is constrained by quantum transition rules of the crystal lattice or other extended object. In particular, for elastic scattering from a crystal lattice with reciprocal lattice vectors \textbf{G}$_{i}$, the allowable momentum changes are \textbf{$\Delta $p} = $h$\textbf{G}$_{i}$/2$\pi $, which reproduces the standard crystal diffraction pattern. This corresponds to emission of a degenerate phonon with zero energy. This provides a logically consistent picture of the microworld, avoiding most of the paradoxes associated with the orthodox interpretation of quantum mechanics. [1] A.M. Kadin, ``Waves, Particles, and Quantized Transitions: A New Realistic Model of the Microworld'', \underline {http://arxiv.org/abs/1107.5794} (2011).