Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L1: John H. Dillon Medal Symposium in Honor of Venkat Ganesan |
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Sponsoring Units: DPOLY Chair: Glenn Fredrickson, University of California, Santa Barbara Room: Spirit of Pittsburgh Ballrom A |
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L1.00001: Dillion Medal Prize Lecture Invited Speaker: Many aspects of polymer research have undergone a paradigm shift in the past decade, with an increased emphasis on technological applications which propose the use of materials and devices created by controlling matter from the atomic scales to the bulk commodity level. This talk will focus on multicomponent polymeric materials (block copolymers, rod-coil polymers and mixtures like polymer blends and polymer nanocomposites), which have played a central role in enabling this paradigm shift in the context of polymeric materials. In this talk, I will discuss our recent researches on developing simulation tools that can predict the structure, morphology and flow behavior of such multicomponent polymers. In contrast to conventional (``particle-based'') Monte Carlo and Molecular dynamics approaches, our methods work at a coarse-grained description of the system to predict the thermodynamics and dynamics of such multicomponent polymers. This talk will focus on an outline of the simulation strategies and present some results concerning both the equilibrium and dynamical properties of such materials. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L1.00002: Chain Stretching and Order-Disorder Transitions in Block Copolymer Monolayers and Multilayers Edward J. Kramer, Vindhya Mishra, Gila E. Stein, Karen E. Sohn, Sumi Hur, Glenn H. Fredrickson, Eric W. Cochran Both monolayers of block copolymer cylinders and spheres undergo order to disorder transitions (ODT) at temperatures well below those of the bulk. Monolayers of PS-b-P2VP cylinders undergo a ``nematic'' to ``isotropic'' transition at temperatures about 20 K below the bulk ODT while monolayers of PS-b-P2VP with P2VP spheres undergo a 2D crystal to hexatic transition at least 10 K below the bulk ODT. Bilayers of each structure disorder at temperatures well above that of the monolayers. While one is tempted to attribute all of the difference to the fact that ordered monolayers are quasi 2 dimensional while bilayers are not, an alternative explanation exists. In the cylinder monolayer the corona PS chains must stretch to fill a nearly square cross-section domain rather than a hexagonal one in the bulk, while the corona PS chains in a sphere monolayer must stretch to fill a hexagonal prism rather than an octahedron in the bulk. The more non-uniform stretching of the chains in the monolayer should increase its free energy and decrease its order-disorder temperature. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L1.00003: Free Energy Estimation in Field-Theoretic Simulations Glenn Fredrickson, Erin Lennon, Kirill Katsov A new technique is presented for computing absolute and relative free energies of polymeric fluids in the context of field-theoretic simulations. Complex Langevin sampling is combined with a thermodynamic integration scheme to provide access to free energies of homogeneous and inhomogeneous polymer phases. The scheme utilizes a harmonic crystal reference state whose free energy can be computed analytically. The method is demonstrated in the context of the order-disorder transition of diblock copolymer melts. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L1.00004: Directed Crystallization in polymer solutions Murugappan Muthukumar Theoretical considerations of amyloid fibrillization in protein solutions and polymer-mediated crystallization of nanoparticles will be presented. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L1.00005: The O$^{52}$ Network by Molecular Design: CECD Tetrablock Terpolymers Frank S. Bates, Michael Bluemle, Guillaume Fleury, Timothy Lodge Varying the length of poly(dimethylsiloxane) in poly(cyclohexylethylene-$b$-ethylene-$b$-cyclohexylethylene-$b$-dimethylsiloxane) (CECD) tetrablock terpolymers between 0 and 20{\%} produces the sequence of ordered phases: cylindrical-to-network-to-cylindrical. Small-angle X-ray scattering and transmission electron microscopy demonstrate \textit{Pnna }space group symmetry and a unique network morphology stabilized by the asymmetric molecular architecture and block interactions. These results establish a new design principle for the generation of triply periodic and multiply continuous nanostructured soft material. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L1.00006: Suppression of Segmental Relaxation as the Origin of Strain Hardening in Polymer Glasses Kenneth Schweizer, Kang Chen A nanometer scale dynamical theory is proposed for the post-yield large amplitude strain hardening phenomenon in polymer glasses. The physical picture is that external deformation induces anisotropic chain conformations which modifies interchain packing, resulting in density fluctuation suppression and intensification of localizing dynamical constraints and activation barriers. The strain amplitude dependence of the resulting stresses are well described by classic rubber elasticity form. However, the hardening stress is of interchain origin and arises primarily from prolongation of segmental relaxation, not single strand entropic elasticity. Theoretical predictions for the magnitude, temperature and deformation rate dependence of the hardening modulus are consistent with experiments and simulations. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L1.00007: Disappearance of high frequency modes in polymer dilute solution viscoelasticity Ronald Larson, Semant Jain We address the problem of the ``missing modes'' in the high frequency rheology of dilute polymer solutions. According to the Rouse-Zimm theory, the slow viscoelastic response of dilute polymers is dominated by the collective motion of the chain, as described by a bead-spring model. However, one expects this description to break down at high frequencies at which chain motion on scales too small to be represented by beads and springs should be evident; this motion should be controlled by rotations of individual backbone bonds of the polymer. The viscoelastic response produced by these ``local modes'' is observable in polymer melts; however, for dilute polymer solutions, the ``local modes'' are absent from viscoelastic spectra, as shown by Schrag and coworkers (Peterson, et al., J. Polym. Sci. B, 39:2860 (2001)). Here we address this problem by directly simulating single polymer chains using Brownian dynamics simulations, with realistic bending and torsional potentials. We show using these simulations that the ``missing modes'' result from barriers to bond rotation that make the chain ``dynamically rigid'' at high frequencies. As a result, the ``dynamical Kuhn length'' of the chain exceeds the static one, and the chain at high frequencies is not able to explore local conformations as fast as would be needed for their relaxation to contribute to the mechanical relaxation spectrum. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L1.00008: Polymer Conductivity through Particle Connectivity Yueh-Lin Loo To promote solution processability of conductive polymers, polymer acids, instead of small-molecule acids, are frequently used as dopants. Generally, the conductive polymer is synthesized in the presence of the polymer acid; sub-micron size particles that are electrostatically stabilized result during polymerization. We discovered that the molecular characteristics of the polymer acid have great implications on the structure of these conductive polymer particles. Templating the synthesis of the conductive polymer with a higher molecular weight polymer acid results in larger particles, and templating with a polymer acid having a larger molecular weight distribution results in a large size distribution in the particles. Because conduction in such conductive polymers is governed by how these particles pack, we show that the macroscopic conductivity of these films is dictated by a single parameter, i.e., the particle density, that is reducible from the various molecular characteristics of the polymer acid we explored. In the specific case of polyaniline that is doped with poly(2-acrylamido-2-methyl-1-propane sulfonic acid), the particles are structurally and chemically inhomogeneous. The conductive portions of the polymer preferentially segregate to the particle surface. Conduction in these materials are therefore mediated by the particle surface and conductivity thus scales superlinearly with particle surface area per unit film volume. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L1.00009: Modeling the Self-Assembly of Nanoparticle Amphiphiles Sanat Kumar, Athanassios Panagiotopoulos We demonstrate that spherical nanoparticles, uniformly grafted with macromolecules, robustly self-assemble into a range of anisotropic superstructures when they are dispersed in the corresponding homopolymer matrix. Theory and simulations both suggest that this self-assembly process reflects a balance between the energy gain when particle cores approach and the entropy of distorting the grafted polymers. The effectively directional nature of the particle interactions is thus a many-body \textit{emergent property}. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L1.00010: Confinement Effects on Polymer Dynamics in Nanocomposites Ramanan Krishnamoorti, Tirtha Chatterjee, Mansour Abdulbaki, Madhusudan Tyagi The dynamics of polymers in systems with dispersed nanoparticles is studied using inelastic and quasi-elastic neutron scattering. In this study, the role of confinement between nanoparticles and the role of nanoparticle topology are examined by considering dispersions with spherical C60 buckeyballs, rod-like single walled carbon nanotubes and plate-like graphene. The polymers examined here include bisphenol A epicholorohydrin and bisphenol F epicholorohydrin. Significant changes in the dynamics of the polymer are observed and these will be examined in the context of mode coupling theories. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L1.00011: Long-time dynamics of chains in polymer nanocomposites Peter Green In polymer nanocomposites (PNCs), the presence of the nanoparticles has a marked effect on the dynamics and the T$_{g}$. In one limit, the chains become strongly attached to the particles, and two glass transitions, and bimodal dynamics, may be observed. In the other, where the chain/particle interactions are weak, the chain friction factor, z(T) can undergo significant changes, manifested in the translational diffusion and viscosity. In the polymethyl methacrylate (PMMA)/C$_{60}$ system, the dynamics slow down, accompanied by an increase in the glass transition. At the same time, the temperature dependence of the relaxations remains the same as pure PMMA. In polystyrene (PS)/Au-thiol capped PS ligands, the dynamics and the glass transition could be induced to increase or decrease, through manipulation of molecular parameters in the system. In this presentation, we propose a mechanism to describe translational diffusion and T$_{g}$ in PNC systems in which the polymer chain/nanoparticle interactions are weak. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L1.00012: Self-Assembly of Conjugated Rod-Coil Block Copolymers for Photovoltaic Applications R. A. Segalman, B.D. Olsen, Y. Tao, B. McCulloch The phase behavior of conjugated rod-coil block copolymers is significantly different from that of traditional block copolymers due to the interplay between liquid crystalline interactions of the rod blocks and microphase separation of the rods and coils. A universal phase diagram for rod-coil diblock copolymers depends on the strengths of the rod aligning interactions and the rod-coil repulsive interactions as well as the geometrical ratio of rod volume to coil and aspect ratios. In this talk, the experimental phase diagram of a weakly segregated model block copolymer will be compared to that predicted by self-consistent field theory. Conjugated rod-coil block copolymers with electron donating and accepting blocks are promising for photovoltaic applications. The self-assembly of poly(thiophene-b-acrylate perylene diimide) block copolymers as well as block copolymer-nanocrystal composites result in photovoltaic active layers with controllable degrees of order. We demonstrate that short range order on the nanoscale is beneficial to device performance. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L1.00013: Confinement Effects on Glassy-State Polymer Behavior in Thin Films, Nanocomposites, Tethered Nanoparticles, and Nanostructured Systems John Torkelson, Perla Rittigstein, Soyoung Kim, Rodney Priestley, Connie Roth, Manish Mundra Confinement of polymers at the nanoscale and even the microscale can lead to significant deviations in glass transition temperature, physical aging rate, and alpha-relaxation dynamics from bulk polymer behavior. Here we illustrate how model experiments involving several techniques applied to simple, thin polymer films help us to understand and predict qualitatively or semi-quantitatively the glassy-state response of more complex, confined systems, including nanocomposites, tethered nanospheres, nanostructured homopolymer films, and nanostructured systems consisting of more than one polymer component. We shall illustrate how the glass transition temperature can be altered by as much as 60 K and how physical aging can be nearly totally suppressed via confinement. The discovery of new confinement effects and implications for new applications of confined polymeric systems will be discussed. [Preview Abstract] |
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