For many practical purposes, the mechanical yielding (i.e. irreversible deformation) of glassy polymers is a key feature. Typically, the rate of irreversible structural re-arrangement depends exponentially on the applied stress, according to Eyring’s approach [1], which has been employed rather successfully in advanced, but phenomenological, macro-scale models (e.g. [2, 3]). The goal of this paper is to underpin Eyring-based macroscopic models for yielding in glassy polymers, by relating them to the molecular details of the underlying polymer structure. To that end, two routes are followed. The first route consists in examining the molecular origin of the so-called activation volume, which enters the relation between the rate of structural re-arrangement and the applied stress, and is commonly considered a phenomenological quantity. For a series of glassy polymers, we have determined experimentally the activation volumes. In order to correlate the corresponding values with the molecular structure, we have performed molecular simulations for the same polymers. While we found no apparent correlation with the typical length scales of a polymer chain (e.g., persistence length), our results indicate that the activation volume correlates reasonably with the network density of the entangled chains. The second route goes beyond the first in that it targets a molecular description of the actual yielding kinetics in a glassy polymer, which is inaccessible to classical molecular dynamics. The key concept is the potential energy landscape (PEL), that assigns an energy value to every molecular arrangement of the glassy polymer chains based on an atomistic force field. Based on the PEL, a network is constructed that connects local minima (meta-stable states) via transitions over saddle points (infrequent events). For the specific case of atactic polystyrene, we demonstrate how the network is constructed, how it can be used to determine the linear-elastic behavior [4] and, with the help of the Dynamical Integration of a Markovian Web method [5], how dynamics on experimentally relevant time scales can be addressed. [1] A. Krausz, H. Eyring, Deformation Kinetics, Wiley-Interscience, London, 1975. [2] M.C. Boyce, et al., Mech. Mater. 7, 15 (1988). [3] T.A. Tervoort, et al., J. Rheol. 40, 779 (1996). [4] N. Lempesis, et al., Mol. Phys. 111, 3430 (2013). [5] G.C. Boulougouris and D.N. Theodorou, J. Chem Phys. 127, 084903 (2007).

Molecular aspects of yielding of glassy polymers

249th National Meeting (2015)

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