Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J24: Frank J. Padden Award Symposium |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Azar Alizadeh, General Electric Research Center Room: Colorado Convention Center 201 |
Tuesday, March 6, 2007 11:15AM - 11:27AM |
J24.00001: How ideal are the ideal-like polymers David Shirvanyants, Sergey Panyukov, Michael Rubinstein The previously unknown long range correlations in the conformations of linear polymers in a $\theta$-solvent were found using analytical calculations and molecular dynamics simulations. Long range power law decay of the bond vector correlation function $\langle\cos\phi\rangle\sim s^{-3/2}$ dominate the standard exponential decay $\langle\cos\phi\rangle = e^{-s/l_p}$, where $\phi$ is the angle between the two bonds, $s$ is their separation along the chain and $l_p$ is the persistence length. These long-range correlations lead to significant deviations of polymer size from ideal with mean square end-to-end distance $\langle R^2 \rangle - b^2N \sim \sqrt N$, where $N$ is the number of Kuhn segments of size $b$. This new phenomena is explained by a fine interplay of polymer connectivity and the non-zero range of monomer interactions. Moreover, it is not specific for dilute $\theta$-solutions and exists in semidilute solutions and melts of polymers. Our results show good agreement with the experimental data on Flory characteristic ratio. [Preview Abstract] |
Tuesday, March 6, 2007 11:27AM - 11:39AM |
J24.00002: Self-Assembly of Block Copolymers in a Nematic Liquid Crystal Solvent Neal Scruggs, Rafael Verduzco, Julia Kornfield Solutions of side-group liquid crystal polymers (SGLCPs) in a liquid crystal (LC) solvent are governed by rich thermodynamics resulting from the competition between the solvent's orientational order and the polymer's conformational entropy. Solutions of SGLCP-random coil block copolymers in LC solvent have an additional layer of complexity deriving from lyophilic/lyophobic interactions between the blocks and the solvent. Dissolving a triblock copolymer yields an LC gel where synergistic coupling between polymer and solvent results in novel properties; the orientational order of the nematic LC solvent imparts electo-optic and mechano-optic properties that are forbidden by symmetry in isotropic gels, and the polymer network provides memory via long-time relaxation processes that do not exist in the bulk LC. The gels are thermoreversible because the random coil block becomes soluble above the solvent's isotropic transition temperature. The exceptional sensitivity of these block copolymers' self-assembled structures to temperature and polymer architecture is demonstrated by small-angle neutron scattering and rheometry. NMR and neutron scattering give a detailed understanding of the coupling between the solvent order and SGLCP conformation. [Preview Abstract] |
Tuesday, March 6, 2007 11:39AM - 11:51AM |
J24.00003: Threading Synthetic Polyelectrolytes through Protein Pores Ryan Murphy, Murugappan Muthukumar We have measured the ionic current signatures of sodium poly(styrene sulfonate) as its single molecules translocate through an alpha-hemolysin pore embedded into a bilayer in a salty aqueous medium under an externally applied electric field. As in the previous experiments involving DNA and RNA, the pore current, which is a measure of the ionic conductivity of the low molar mass electrolyte ions, is significantly reduced when the polymer molecule translocates through the pore. By studying thousands of single molecule events, we have constructed distribution functions for the extent of the reduced current and for the translocation time. By investigating over two orders of magnitude in the molecular weight of the polymer, the average translocation time is found to be proportional to the molecular weight and inversely proportional to the applied voltage. Our experiments open up many opportunities to systematically explore the fundamental physical principles behind translocation of single macromolecules, by resorting to the wide variety of synthetically available polymers. [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:03PM |
J24.00004: Order and Disorder in Polydisperse Block Copolymer Melts Nathaniel Lynd, Marc Hillmyer Utilizing creative strategies for the synthesis of model controlled-polydispersity poly(ethylene-\textit{alt}-propylene)-$b$-poly(\textsc{d,l}-lactide)(PEP-PLA) and polystyrene-$b$-polyisoprene(PS-PI) block copolymers, the effects of increased breadth in the molecular weight distribution on block copolymer self-assembly were investigated. Small-angle x-ray scattering and rheological measurements were carried out to characterize the morphological details of these self-assembled materials as a function of their polydispersity, interaction strengths, and compositions. A number of surprising consequences of increased breadth in the molecular weight distribution emerged; the domain spacing of the ordered structures increased, changes in morphology occurred, and the degree of segregation at the order-disorder transitions changed as well, particularly for asymmetric block copolymers. The change in the degree of segregation at the order-disorder transition as the polydispersity was increased was found to be dependent on the block copolymer composition, e.g., for PEP-PLA and PS-PI at asymmetric compositions, when the polydispersity was increased in the minority component, the degree of segregation at the order-disorder transition decreased, whereas when the polydispersity was increased in the majority component, the degree of segregation at the order-disorder transition increased. [Preview Abstract] |
Tuesday, March 6, 2007 12:03PM - 12:15PM |
J24.00005: Morphological Evolution of Poly (caprolactone) Dendrites during Isobaric Relaxation of Metastable Monolayers at the Air/Water Interface Bingbing Li, Alan Esker Isobaric crystallization at constant surface pressures of 11, 10.5, 10.3, 10, 9.5, and 8.5 mN/m were performed for a poly (caprolactone) (PCL)/poly (t-butyl acrylate) (PtBA) blend with a PtBA mole fraction of 0.14 to study the effect of surface pressure on the morphological evolution of PCL dendrites at the air/water interface. At 11 mN/m, corresponding to a higher degree of undercooling, morphological studies indicate that the side-branches in the two (100) sectors grow faster than those in the four (110) sectors, possibly because molecular diffusion effects arising from a limited material reservoir is more spatially constrained in the (110) sectors. During isobaric crystallization at 10.3 and 10 mN/m, four-arm dendrites observed show side-branches in the four (110) growth faces are better developed. Furthermore, isobaric crystallization at 9.5 mN/m causes a seaweedlike crystal morphology showing less branched and more compact structures with larger tip radii. Finally, PCL crystals grown at 8.5 mN/m show compact structures without side-branches. [Preview Abstract] |
Tuesday, March 6, 2007 12:15PM - 12:27PM |
J24.00006: How Polymers Diffuse in Molecularly-Thin Films Liang Hong, Steve Granick We explore the fundamental question, how polymer diffusion in molecularly-thin films differs from that in isotropic melts comprised of the same polymer. To explore this, a new surface forces apparatus was developed to enable, for the first time to the best of our knowledge, spectroscopic measurement in addition to force measurements of the traditional kind. Here we describe experiments using fluorescence correlation spectroscopy (FCS) and fluorescence recovery after photobleaching (FRAP). Melts of polydimethylsiloxane (PDMS) were confined between atomically-smooth mica surfaces at carefully controlled separations. Translational diffusion of the polymer was measured as a function of film thickness and molecular weight. We show that adsorption slows surface diffusion by over three orders of magnitude and that this effect propagates to thickness roughly 3 times the unperturbed radius of gyration (R$_{G})$. Confinement between two opposed surfaces further reduces the diffusion coefficient (D) by at most a factor of 5. Spatially-resolved measurements elucidate how D furthermore depends on the local pressure that squeezes the thin film, as well as on molecular weight. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J24.00007: Phononics and elastic moduli in polymeric and biological nanostructures Ryan Hartschuh, Johnathan Neiswinger, Huiming Xiong, Alexander Kisliuk, Alexei Sokolov, Stephen Wargacki, Richard Vaia Mechanical stability of nanostructured materials is of paramount importance for any nanotechnology. However, measuring mechanical properties at such small scales and understanding their dependence on confinement effects remain a great challenge. We use Brillouin light scattering to study the vibrational spectra and mechanical moduli of polymeric and biological nanostructures. We demonstrate that elastic properties neither change nor appear anisotropic in polymeric lines as narrow as 80 nm. As biological nanomachines, viruses may be useful for various applications ranging from gene therapy and drug delivery to photonic crystals. Analysis of phononic spectra of viral structures indicates strong mechanical coupling between viruses, helping to explain the stability of structures formed by viruses. Spectral analysis further indicates that DNA cores of viruses, not their protein shell, dominate elastic properties of viruses, in contrast to traditional model assumptions. This new finding will impact not only nano-patterning, but also biomedical applications involving viruses and possible treatments of viral infections. [Preview Abstract] |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J24.00008: Surface Dynamics of Glassy Polymer Films and Its Effect on Glass Transition Temperature Zahra Fakhraai, James A. Forrest The surface dynamics of thin polymer films is believed to be different from that of bulk and is cited to be the source of Tg anomalies in thin films. In this study ellipsometry is used to measure the cooling rate dependence of Tg in thin polystyrene films. It is shown that as the temperature decreases below bulk Tg, the relaxation time behavior changes from Vogel-Fulcher to Arrhenius, with an activation energy that decreases linearly with film thickness, indicating that the relaxation time of the surface (limit of zero thickness) is also Arrhenius. To measure the relaxation time of the surface directly, a novel technique is used to produce nanometer size holes with well defined shapes and driving forces on the film surface. AFM is used to monitor the depth of the holes as a function of time at different temperatures. The relaxation times are obtained from the exponential decay of the depths. It is shown that at the low temperature limit the behaviour is Arrhenius with an activation energy similar to the one predicted from ellipsometry measurements. By combining these results the Tg of the surface can be estimated. These results can explain various contradictions in the literature and can provide a framework for a new theory to explain Tg reductions in thin polymer films. [Preview Abstract] |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J24.00009: Thermodynamic and Kinetic Control of Charged Triblock Copolymer Assembly into Complex Nanostructures Honggang Cui, Darrin Pochan, Zhiyun Chen, Karen Wooley Self-assembly of poly (acrylic acid)-block-poly (methyl acrylate)-block-polystyrene triblock copolymers produces various ordered nano-domains in THF/water solution through the interaction with organic counterions. These assembled structures include classic micelles (spheres, cylinders and vesicles), and non-classic micelles (disks, toroids, branched micelles and segmented micelles). Each micelle structure is stable and reproducible at different assembly conditions depending on not only solution components (thermodynamics) but also mixing procedure and consequent self-assembly pathway (kinetics). The key factors that determine the thermodynamic interactions that help define the assembled structures and the kinetic assembly process include THF/water ratio, PS block length, the type and amount of organic counterions, and the mixing pathway. The complex phase behavior and controlled morphology production have been studied via in-situ cryogenic transmission electron microscopy in combination with scattering techniques (small angle neutron scattering and light scattering). Delicate control of the interplay of thermodynamics with slow chain kinetics of block copolymers in solution offers a new strategy to create unique, functional nanostructures. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J24.00010: Structured Interfaces of Surface Wrinkles for Adhesion, Optics and Sensors Edwin Chan, Alfred Crosby From the adhesion of the gecko to the optics of the dragonfly's eye, nature provides numerous examples that utilize optimized microstructures to control interfacial properties. Inspired by the natural world, many research groups have adopted similar approaches to generate devices that are based primarily on top-down strategies -- i.e. lithography. An attractive alternative for creating surface structures is surface wrinkling. Surface wrinkling has been observed by many researchers over the past several decades and is associated with the onset of an elastic instability. The wavelength and critical stress of formation are determined by a combination of geometry and materials properties. Here, we present a new approach to generating stable surface wrinkles based on swelling of an elastomer with a photocrosslinkable monomer formulation. We explore dimensional, orientational and spatial control of the wrinkled structures by creating polymer surfaces with defined regions of contrasting elastic moduli. Specifically, we present an experimental phase diagram of various wrinkle structures, which highlights the discovery of two morphologies that have not been previously observed. We demonstrate how these unique structures can be used for enhanced control of adhesion in soft polymers and the simple fabrication of microlens arrays and compound lens structures. [Preview Abstract] |
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