Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Z11: Invited Session: Nonlinear Mechanics of Glassy Polymers |
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Sponsoring Units: DPOLY Chair: Robert Hoy, University of South Florida Room: 310 |
Friday, March 22, 2013 11:15AM - 11:51AM |
Z11.00001: Rate- and Temperature-Dependent Softening in Polymer Glasses Invited Speaker: Leon Govaert It is well established that physical aging in polymer glasses leads to an increase in density, elastic modulus, yield stress and also strain softening. The latter, sometimes referred to as ``mechanical rejuvenation,'' is the phenomenon where the post-yield stress level initially decreases with further deformation until strain hardening sets in. In all constitutive models for glasses proposed until now, the rate and temperature-dependence of the yield stress is regarded to remain unchanged during strain softening. In the present study, it is demonstrated that a large number of polymer glasses (PMMA, PLLA, PS, PVC) display a pronounced change in kinetics (strain-rate dependence) after yield. The phenomenon finds its origin in the fact that, in specific ranges of temperature and strain rate, two different molecular mechanisms may contribute to the yield stress. Due to strain softening the post-yield response is only controlled by one of the two, resulting in a strain-rate and temperature dependence of the yield drop. The universality of the phenomenon is discussed in connection to the alleged influence of secondary transitions on the impact response of polymer glasses. A modification to the traditional models is proposed that enables an accurate description of the mechanical response of solid polymers in the transition range. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:27PM |
Z11.00002: How deformation enhances mobility in a polymer glass Invited Speaker: Daniel Lacks Recent experiments show that deformation of a polymer glass can lead to orders-of-magnitude enhancement in the atomic level dynamics. To determine why this change in dynamics occurs, we carry out molecular dynamics simulations and energy landscape analyses. The simulations address the coarse-grained polystyrene model of Kremer and co-workers, and the dynamics, as quantified by the van Hove function, are examined as the glass undergoes shear deformation. In agreement with experiment, the simulations find that deformation enhances the atomic mobility. The enhanced mobility is shown to arise from two mechanisms: First, active deformation continually reduces barriers for hopping events, and the importance of this mechanism is modulated by the rate of thermally activated transitions between adjacent energy minima. Second, deformation moves the system to higher-energy regions of the energy landscape, characterized by lower barriers. Both mechanisms enhance the dynamics during deformation, and the second mechanism is also relevant after deformation has ceased. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 1:03PM |
Z11.00003: Can intrachain contributions dominate the stress response of polymer glasses under large deformation? Invited Speaker: Shi-Qing Wang Polymer glasses are a structural hybrid in their mechanical responses to large deformation. The primary structure due to the short-range inter-segmental van der Waals bonds yields at small strains. In presence of chain connectivity, brittle failure may be avoided if the chain networking is adequately dense. We show in this presentation how the interplay between the primary structure and chain network dictates deformation, yielding, strain softening, strain localization and ``strain hardening'' during continuous uniaxial extension at room temperature of a variety of polymer glasses from the brittle (e.g., PS, PMMA) to the ductile (e.g., PC). In particular, our results identify straining of the chain network as the dominant contribution to the mechanical stress in the post-yield regimes. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:39PM |
Z11.00004: A Simple Model for Yielding and Strain Hardening in Glassy Polymers Invited Speaker: Ron Larson Strain hardening has long been an observed feature of polymer glasses in extension; explanations to date have often been phenomenological. Ediger and coworkers (Lee et al. \textit{Science} 323, 231, 2009) have shown in experiments on PMMA glasses that, in addition to strain hardening, polymeric glasses show a remarkable non-monotonicity in the segmental relaxation time both in loading and unloading of stress. Here, we develop a simple constitutive equation that combines recent theories for yielding in simple glasses (Brader et al. PNAS, 106, 15186, 2009) to represent local segmental modes in the polymer, with a dumbbell model for the slow polymer relaxation modes. For a polymer glass under uniaxial loading, the model predicts that the liquefaction of the segmental modes permits strain hardening of the polymer modes to emerge, and once this emerges, it slows the deformation of the material under constant load enough to partially re-vitrify the segmental modes even though the sample remains under stress. In this way, the observed non-monotonicity in the segmental relaxation modes is produced. We show the extension of the work to simple shearing flows, and make (as yet) untested predictions about segmental relaxation rates in shear flows. We also show how to extend the model to include Rouse chain dynamics in place of the over-simplified dumbbell. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 2:15PM |
Z11.00005: Impact-Induced Glass Transition in Elastomeric Coatings Invited Speaker: C.M. Roland When an elastomer layer is applied to the front surface of steel, the resistance to penetration by hard projectiles increases significantly. It is not obvious why a soft polymer should affect this property of metals, and most rubbers do not. However, we have found that a few are very effective; the requirement is that the polymer undergo a viscoelastic phase transition upon impact. This means that the frequency of its segmental dynamics correspond to the impact frequency. The latter is estimated as the ratio of the projectile velocity to the coating thickness, and is on the order of 10$^{5}$ s$^{-1}$ for the experiments herein. Our data and a non-linear dynamics finite-element analysis offer support for this resonance condition as a primary mechanism underlying the penetration-resistance of elastomer-coated metal substrates. The impact-induced phase transition causes large energy absorption, decreasing the kinetic energy of the impacting projectile. However, this energy absorption only accounts for about half the enhanced stopping power of the elastomer/steel bilayer. An additional mechanism is lateral spreading of the impact force, resulting from the transient hardening of the elastomeric during its transition to the glassy state -- the modulus of the rubber increases 1000-fold over a time period of microseconds. The penetration-resistance is a very nonlinear function of the coating thickness. Moreover, tests on various metals show that hardness is the principal substrate parameter controlling the contribution of the coating. [Preview Abstract] |
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