Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session R21: Polymer Glasses in Confinement and DeformationInvited
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Sponsoring Units: DPOLY Chair: Connie Roth, Emory University Room: 281-282 |
Thursday, March 16, 2017 8:00AM - 8:36AM |
R21.00001: Shear elasticity and shear relaxation in glass-forming polymer melts and films Invited Speaker: Jorg Baschnagel The shear modulus $G$ can be thought of as an order parameter distinguishing the liquid ($G = 0$) from the glass (solid, $G > 0$). Here we present results from molecular dynamics simulations for the temperature ($T$) dependence of $G$. Our simulations examine a coarse-grained polymer model for bulk polymer melts and free-standing films of various thicknesses. For the bulk we apply two methods to calculate $G(T)$: a method based on the fluctuations of the wave-vector dependent strain and the ``stress-fluctuation formalism'' which determines $G$ from the fluctuations of the shear stress (in different thermodynamic ensembles). We discuss both methods, show that they give consistent results, and also compare the resulting $G$ with estimates of the nonergodicity parameter from the shear-stress auto-correlation function and the monomer mean-square displacement. The analysis is then extended to free-standing films. We find that the presence of the free interfaces weakens the shear rigidity of the polymer glass relative to the bulk. We discuss the dependence of this effect on film thickness and on the distance to the free interface and compare our results to similar findings in the literature. [Preview Abstract] |
Thursday, March 16, 2017 8:36AM - 9:12AM |
R21.00002: Tg and Structural Recovery of Single Ultrathin Films Invited Speaker: Sindee Simon The behavior of materials confined at the nanoscale has been of considerable interest over the past two decades. Here, the focus is on recent results for single polystyrene ultrathin films studied with ultrafast scanning chip calorimetry. The T$_{\mathrm{g}}$ depression of a 20 nm-thick high-molecular-weight polystyrene film is found to be a function of cooling rate, decreasing with increasing cooling rate; whereas, at high enough cooling rates (e.g., 1000 K/s), T$_{\mathrm{g}}$ is the same as the bulk within the error of the measurements. Structural recovery is also performed with chip calorimetry as a function of aging time and temperature, and the evolution of the fictive temperature is followed. The advantages of the Flash DSC include sufficient sensitivity to measure enthalpy recovery for a single 20 nm-thick film, as well as extension of the measurements to aging temperatures as high as 15 K above nominal T$_{\mathrm{g}}$ and to aging times as short as 0.01 s. The aging behavior and relaxation time-temperature map for single ultrathin films are compared to those for bulk material. Comparison to behavior in other geometries will also be discussed. [Preview Abstract] |
Thursday, March 16, 2017 9:12AM - 9:48AM |
R21.00003: Confinement and Interfacial Effects on the Dynamics of Polymer Nanocomposites and Ultra-Thin Films Invited Speaker: Francis Starr Changes in the dynamics of confined polymer materials relative to the bulk polymer involve a complex convolution of effects, including the form and scale of confinement, as well as the interfacial interaction strength and surface roughness, leading to a seemingly intractable degree of complexity in describing these changes. In this talk, we review the results of molecular dynamics simulations of confinement effects on polymer dynamics in both ultra-thin films and nanocomposites. We show that all observed changes to the polymer dynamics can be understood in a unified way. In particular, we quantitatively describe the change in dynamics based on how the collective motion is perturbed. These changes are ultimately parameterized in terms of the high-temperature activation free energy, leading to the almost paradoxical finding that changes in glass formation are controlled by the changes to activation barriers in the high temperature limit. We also consider the peculiar case of strongly interacting interfaces, where experiments often report small or no changes in the glass transition temperature, $T_g$. We find that when interfacial interactions exceed the polymer-polymer interactions, a distinct relaxation that is slower than the main $\alpha$-relaxation emerges, arising from an adsorbed ``bound'' polymer layer near the interface. This bound layer ``cloaks'' the interfacial interactions, so that the dynamics of the matrix polymer is largely unaffected. Consequently, $T_g$ defined from the temperature dependence of the routinely measured thermodynamics or the polymer matrix relaxation can nearly independent of interfacial interaction strength. [Preview Abstract] |
Thursday, March 16, 2017 9:48AM - 10:24AM |
R21.00004: Deformation and Recovery of Polymer Glasses: Insights from Molecular Simulations Invited Speaker: Joerg Rottler This presentation will explore the atomistic level processes in quiescent and actively deformed polymer glasses with molecular simulations. Generic polymer models capture the macroscopic phenomenology of thermal creep or deformation at constant strain rate remarkably well. Monomer trajectories obtained from molecular dynamics simulations allow us to track the evolution of relaxation time distributions under different deformation protocols and thermal histories, and to relate spatially heterogeneous relaxation dynamics with local structure. Molecular simulations successfully reproduce experimentally observed phenomena such as accelerated segmental motion and narrowing of the relaxation time spectrum, and suggest a common framework in which to understand them. Polymer glasses are also ideal materials to study the interplay between physical aging (structural recovery) and erasure of memory (rejuvenation) due to mechanical driving, which is important in other structural glasses (amorphous metals or dense colloidal suspensions) as well. We conclude by testing the predictions of recently proposed theories for the behavior of the relaxation time during plastic flow against simulations, and thus identify the relevant deformation variables that control mechanical rejuvenation. [Preview Abstract] |
Thursday, March 16, 2017 10:24AM - 11:00AM |
R21.00005: What We Know and Don't Know About the Thermo-mechanical Behavior of Glassy Polymers Invited Speaker: James Caruthers The thermomechanical behavior of glassy polymers has been the subject of vigorous research for decades, but a fundamental understanding remains one of the outstanding challenges in condensed matter physics. First, the rich phenomenology of the thermo-mechanical behavior will be reviewed with emphasis on some of the more non-traditional experiments like multiple-step thermo-deformation histories and deformation induced changes in the enthalpic responses -- experimental data that provides both insight and challenges to a more complete understanding of the glassy state. Second, we will show how a recently developed Stochastic Constitutive Model (SCM) that acknowledges dynamic heterogeneity is beginning to unify the rich experimental phenomenology of glassy thermo-mechanical behavior into a consistent description. Finally, key unanswered questions will be raised -- questions that suggest directions for future experimental studies as well as outstanding theoretical challenges. [Preview Abstract] |
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