Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session V25: Focus Session: Interfaces and Adhesion I |
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Sponsoring Units: DPOLY Chair: Karl Freed, University of Chicago Room: Morial Convention Center 217 |
Thursday, March 13, 2008 11:15AM - 11:51AM |
V25.00001: A model for glass transitions in polymer thin films. Invited Speaker: Polymeric materials formulated as thin films can exhibit glass transition temperatures which are significantly shifted relative to bulk values. Depending on whether the film is supported (on a substrate) or freely-standing the temperature shift can go in different directions relative to the bulk. For all films the magnitude of the shift depends on film thickness. For supported films the shift appears to depend on substrate-polymer interactions, while for free-standing films there is a striking dependence on molecular weight. Experimental data published over the last five years have included some elegant and intriguing results which provide a significant challenge for those wishing to understand these phenomena. In this talk a model that predicts glass transitions in both free-standing and supported films will be presented and tested against extant data. Ideas for future experiments will also be discussed. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V25.00002: Creating defect free structures by directed photochemical reaction in a ternary phase separating system Pratyush Dayal, Olga Kuksenok, Anna Balazs We present a technique to create long range ordered structure in photosensitive reaction-diffusion systems. Our approach utilizes a reversible photochemical reaction between species A and B in a ternary immiscible ABC blend to yield a defect-free arrangement of phase separated ABC domains. The process essentially consists of two steps. First, the sample is irradiated by virtue of masks creating an illumination pattern which allows the migration of AB components to the covered regions at the expense of the C component. As a result the C component is pinned onto the irradiated regions which in turn leads to pinning of AB domains thereby creating a long range order in the system. Second, the masks are completely removed and whole sample is irradiated uniformly. In this case, the system evolves into a distinctly different ordered structure. The ordering of the system, either before or after removal of the mask, can be locked in by quenching the system at appropriate time. Simulation studies reveal that the ordering can be controlled by changing the initial concentration of the blend as well as by varying size and arrangement of the holes in the mask. The defect free morphology has been demonstrated for rectangular, hexagonal and parallel arrangement of the masks. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V25.00003: Entropic Effects in the Phase Behavior of Athermal Nanoparticle/Homopolymer Thin Film Mixtures Luciana Meli, Abraham Arceo, Peter Green The phase behavior of an athermal nanoparticle/polymer mixture, composed of polystyrene-coated gold nanoparticles embedded in polystyrene thin film hosts, was examined. It is shown that the spatial distribution of nanoparticles is readily tailored through control of: the grafting density of the brush, the length of both grafted and free chains, and the relative size of the nanoparticles in comparison to the size of the polymer melt chains. The distribution of nanoparticles within the polymer host, including a surface-induced phase separation, may be understood as a balance between the conformational entropy of the polymer chains, which is compromised when the chains have to stretch around the nanoparticles and penetrate their brush, and the translational entropy of the nanoparticles, which favors their homogeneous distribution. This is the first report that systematically studies the entropic effects that lead to surface phase transitions in polymer/nanoparticle thin film mixtures. Our results may also prove helpful in understanding nanofiller dispersion in analogous bulk mixtures. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V25.00004: Mechanism of Interfacial Instability in Thin Polymer Film in Controlled Solvent Atmosphere Parvaneh Mokarian-Tabari, Jonathan. R. Howse, Sasha Y. Heriot, Mark Geoghegan, Richard A.L. Jones Thin films of immiscible polymers made by spin coating have potential for many practical applications like field-effect transistors, LEDs and photovoltaic devices. We have developed a technique based on small angle light scattering and reflectivity to study the process of phase separation in spin cast films \textit{in situ} during formation. Previous experiments\footnote{ \textit{Heriot, S.Y. and R.A.L. Jones, Nature Materials, 2005. }\textbf{\textit{4}}\textit{(10): p. 782-786 }\par } proposed formation of a transient wetting layer which is followed by interfacial instability and leading to lateral phase separation. In our recent work the origin of this instability has been studied. Experiments have been designed to test the Marangoni instability by spin coating the PS/PMMA film in a controlled toluene vapour atmosphere. A fast evaporation rate leads to laterally phase separated structure whereas slow evaporation lowers the solvent gradient inside the film and leads to a self stratified structure. By comparing the data to a model a better understanding of film evolution has been established. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V25.00005: Spanning Trees and the Dynamics of Compact Polymers Armin Rahmani, Andrea Velenich, Claudio Chamon We introduce a lattice model for a compact loop polymer confined to a two-dimensional box. By a mapping to spanning trees on a square lattice we calculate the partition function and the energy of the system as a function of temperature, bending rigidity and elasticity of the polymer. We study the dynamics of the system using a kinetically constrained model whose elementary moves consist of polymer fingering or, in the language of the spanning trees, local bond flips taking place at the leaves of the tree. We study, through Monte-Carlo simulations, the time dependence of the energy and the number of leaves in the system when quenched from infinite temperature to various finite temperatures. We find that for temperatures above a critical value, these observables monotonically decay to their equilibrium values whereas, for lower temperatures, a broad non-equilibrium plateau emerges. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V25.00006: Uper Limit of Superheating in Polymer Crystals Revealed from Linear Heating Covering Seven Orders of Magnitude in Heating Rate Christoph Schick, Alexander Minakov, Andreas Wurm We report about superheating of polymer crystals on linear heating covering the scanning rate range from 0.02 to 1,000,000 K/s. Results obtained by super-fast scanning calorimetry using a thin film sensor [1] are combined with results from DSC. On slow heating semicrystalline polymers tend to recrystallize (reorganize) significantly fast. From previous measurements the onset of melting of isothermally crystallized samples can be attributed to the rising flank of the first melting peak, which is often called the annealing peak, rather than to the peak maximum. The later depends on the counterbalance of melting and recrystallization. Melting kinetics is described by the power law of superheating: $\sim $ ($T$ -- $T_{cr})^{\alpha }$ with \textit{$\alpha $} $<$ 0.2, which does not correspond to the heat transfer but rather to a nucleation process [2]. At high heating rates superheating saturates. The power law behavior and the saturation of superheating will be discussed. 1. A.A. Minakov, C. Schick, Rev. Sci. Instrum. 78 (2007) 073902. 2. A.A. Minakov, A. Wurm, C. Schick, Europ. Phys. J. E, 23 (2007) 43. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V25.00007: Molecular simulation of crystal nucleation of an n-alkane Peng Yi, Gregory Rutledge We report the results of molecular simulations to study crystal nucleation of n-octane from the melt. A realistic united atom force field was employed for n-octane. The melting behavior was first determined by ramping temperature in a set of Monte Carlo simulations. The adiabatic nucleation trajectory was then sampled using the umbrella sampling technique with a set of proposed global and local order parameters, and analyzed for selection of best order parameter. The transition state ensemble has been verified by molecular dynamics simulation. The structure of critical nuclei in the nucleation process is analyzed and the effect of intermediate phases discussed. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V25.00008: Entropically Driven Layering Near a Substrate: A Fluids DFT Study Erin McGarrity, Amalie Frischknecht, Michael Mackay We employ a fluids density functional theory to study the phase behavior of athermal polymer/nanoparticle blends near a hard substrate. These blends exhibit two types of first order, entropically driven layering transitions. In the first type of transition, the nanoparticles order to form a layer which is a fixed distance from the surface. The structure and location of this layer depends on nanoparticle radius. In the second type of transition, which occurs at melt-like densities, the nanoparticles and polymers form laminar structures which resemble colloidal crystals. We examine the effects of packing density, chain length and nanoparticle radius on the system and show that the transitions are first order. In addition we show that the crystalline phase is nucleated by the presence of the surface. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V25.00009: Properties of Ferroelectric Polyvinylidene fluoride-co-trifluoroethylene Nanorods Jodie Lutkenhaus, Thomas Russell Ferroelectric and piezoelectric nanorods of polyvinylidene fluoride (PVdF) are potential candidates for sensing mechanical stimuli on the nano-scale. Left untreated PVdF tends to crystallize in a non-ferroelectric ($\alpha )$ phase. PVdF-co-trifluoroethylene (PVdF-TrFE), on the other hand, readily crystallizes into the ferroelectric ($\beta )$ phase; however, the structure and properties can be significantly influenced by confinement. Here, the creation of PVdF-TrFE nanorods (dia. = 15 to 200 nm) using anodized aluminum oxide membranes was investigated. The crystallization behavior and the Curie transition (ferro- to paraelectric phase) were studied. Changes in structure and phase were investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The effects of electrical poling, which increases dipole-orientation within the ferroelectric phase, are discussed. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V25.00010: Thermodynamics and Kinetics of Crystallization of Flexible Molecules Bernhard Wunderlich The crystallization of a single atom (as in a metal) is a one- step transfer across the liquid-crystal interface. A flexible chain molecule of n segments needs considerably more steps. A simple description gives it (n - 2)$^3$ possible conformations in the liquid, i.e., for pentcontane C50H102 111,000, but only one conformation in the crystal, making the crystallization a multi-step process. At what length are there too many steps to allow the crystallization to be reversible? How can one describe phase separation for flexible molecules containing segments of different chemical nature? How do random segments of different nature and precisely spaced segments of different nature influence the phase separation on crystallization? How is reversible crystallization influenced by chain folding and crystal perfection in the solid state? Some of these old questions can now be answered by temperature-modulated differential scanning calorimetry of precisely made molecules of different length and copolymerized structure. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V25.00011: UV-convergent One-loop Theory of Binary Homopolymer Blends Jian Qin, Frank Bates, David Morse We analyze the effects of long wavelength composition fluctuations in binary homopolymer blends. We use a generalization of Wang's theory [1], in which all dependence upon short-wavelength structure is absorbed into a renormalization of an effective $\chi$ parameter $\chi_e$ and of statistical segment lengths. The theory allows us to calculate the collective correlation function $S(k)$, single chain correlation functions, and the free energy density in homogeneous mixtures. The same formalism can be used to study diblock copolymer melts. The value of $\chi_e N$ at the critical point of a binary blend exceeds that predicted by Flory-Huggins theory by an amount proportional to $1/\sqrt{N}$, though the width of the critical region is proportional to $1/N$. For strongly asymmetric blends, however, the binodal value of $\chi_e N$ is suppressed. The dimensions of individual chains decrease slightly with increasing $\chi$ or with decreasing $N$, even when $\chi_e=0$. [1] Z.-G. Wang, {\it J. Chem. Phys.}, 2002, 117:481. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V25.00012: Effect of intensity gradient profiles on crystal growth subject to holographic free radical photopolymerization Thein Kyu, Pankaj Rathi, Soojeoung Park A theoretical model has been developed to describe a unique phenomenon of photopolymerization-induced crystallization subjected to holographic intensity gradient profiles. A hypothetical phase diagram of a crystalline polymer solution (or blend) is constructed to guide the dynamics of directional crystallization. Calculations of holographic photopolymerization induced crystallization were carried out slightly above the melting temperature of the bends under the conditions of sinusoidal as well as sharp (square) interface. In the case of periodically varying interface, the dynamic calculations revealed that the emerged crystals (spherulites) have out-grown the patterned regions, which is consistent with the crystal growth behavior of the polyethylene oxide/diacrylate system. However, in the case of a sharp holographic the directional growth occurs along the stripes, which is confined within the stratified layers. [Preview Abstract] |
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