Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H3: Polymer Physics Prize |
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Sponsoring Units: DPOLY Chair: Mark Ediger, University of Wisconsin-Madison Room: Morial Convention Center RO2 - RO3 |
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H3.00001: Polymer Prize Talk: Segmental Dynamics in Polymers : From Cold Melts to Aging and Stressed Glasses Invited Speaker: Kenneth Schweizer Polymers are excellent glass formers. In the cold molten state they exhibit chemically-specific and strongly non-Arrenhius segmental relaxation which sets the time scale for the generic chain scale dynamics. In the amorphous solid or plastic state the temperature dependence of the alpha relaxation time changes, physical aging emerges, and a rich mechanical response occurs characterized by the dynamic yielding, strain softening and strain hardening processes. We have developed a statistical mechanical theory of activated segmental relaxation in cold melts by combining and extending methods of mode coupling, dynamic density functional and activated hopping theories. The approach is built on the concept of a confining nonequilibrium free energy which quantifies local dynamical constraints and the barrier hopping process. The localizing consequences of interchain caging forces are quantified by the amplitude of nanometer scale density fluctuations (compressibility) and backbone stiffness. Predictions for the kinetic glass and dynamic crossover temperatures, dynamic fragility, and thermal dependence of the segmental relaxation time are consistent with experiments. The theory has been generalized to treat alpha relaxation, physical aging, and nonlinear mechanical properties in the glass. The structural component of density fluctuations become (partially) frozen resulting in a crossover to Arrenhius relaxation. Physical aging is modeled based on a kinetic equation for collective density fluctuations. At intermediate time scales the relaxation time (shear modulus) grows as a power law (logarithmic) function of aging time with a temperature dependent exponent. Applied stress weakens dynamical constraints thereby accelerating relaxation and softening the elastic modulus. A constitutive equation has been constructed from which the temperature dependent dynamic yielding and mechanical response under constant strain rate, constant stress (creep), and other modes of deformation can be calculated. This work was done in collaboration with Drs. Kang Chen and Erica J. Saltzman. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H3.00002: Application of Integral Equation Theory to Polymers in the Condensed State Invited Speaker: John G. Curro Large scale computer simulations involving many chains can be used to model the physical properties and structure of amorphous polymers, however, such simulations are computationally intensive. An alternative is to employ the approximate, but computationally less demanding, Polymer Reference Interaction Site Model or PRISM theory. PRISM theory is an extension to polymers of liquid state, integral equation theories originally developed for atomic and small molecule liquids. From a computational standpoint, PRISM theory can be viewed as a self-consistent formalism to map the statistical mechanics of the difficult many-chain system to a much simpler, single chain problem. Depending on the model, the statistics of the single chain can be solved analytically or through a single chain Monte Carlo simulation. Solution of PRISM theory yields the intermolecular and intramolecular radial distribution functions characterizing the packing of the polymer chains in the condensed liquid. These distribution functions can then be employed to compute the thermodynamic properties of the polymer system. Various applications of PRISM theory to polymer liquids, blends and copolymers will be discussed and comparisons will be made with exact MD simulations and scattering experiments. Finally, a more accurate approach will be discussed that involves mapping the many chain system to an equivalent two-chain problem. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:48AM |
H3.00003: Complex Fluid Microstructure, Rheology and Glass Transitions:~ Effect of Continuous Phase Molecular Weight Invited Speaker: Charles Zukoski The mechanical properties of suspensions depend dramatically on the suspension microstructure. Microstructure in turn depends on the nature of particle interaction potentials.~ For those systems that are thermally activated there will be an equilibrium microstructure and thus equilibrium transport properties.~ One of the model systems used to understand the links between interactions, microstructure and transport properties is that of hard spheres suspended in a Newtonian continuous phase.~ This model system can be studied experimentally and direct comparisons made with model predictions.~ With the increase in particle volume fraction, if the particles cannot crystallize, the suspension forms a glass where long range self diffusion is essentially eliminated.~ The approach to the glass transition has been studied experimentally and agreement with models is strong.~ In this talk we discuss what changes as the continuous phase takes on a granularity where the continuous phase molecules have substantial degrees of freedom. In particular, we investigate the mechanics and microstructure of hard sphere suspensions in polymer melts.~ The particles are composed of silica while we use polyethelene glycols of different weights ranging from small degrees of polymerization through the entanglement molecular weight.~ These studies are motivated by a desire to understand properties of polymer nanocomposites where the role of particle/polymer segment interactions is poorly understood but the state of particle dispersion is key to composite properties.~ In this talk I explore equilibrium and nonequilibrium phases of this system and compare with extant theoretical approaches. ~ [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:24AM |
H3.00004: Dynamics of fluids in complex environments Invited Speaker: Arun Yethiraj |
Tuesday, March 11, 2008 10:24AM - 11:00AM |
H3.00005: The Theta Point Of Long Flexible Polymer Chains: When Does It Exist? Invited Speaker: K. Binder The standard description of the conformation of a long flexible polymer coil in dilute solution implies a swollen state under good solvent conditions, while deterioration of solvent quality (by decrease of temperature) causes a (gradual) chain collapse below the Theta point. At the Theta point, the chain follows Gaussian statistics, apart from logarithmic corrections. Monte Carlo simulations of the bond fluctuation model will be discussed that provide evidence for a second scenario, where the chain experiences a first order transition from the swollen state to a dense solid phase, provided the range of effective attractive interactions is sufficiently short. This scenario then implies that in solution at finite concentration no vapor-liquid-like phase separation occurs. The analogy between this prediction and the behavior of some colloidal dispersions is discussed. [Preview Abstract] |
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