The Source of Strain Hardening in Glassy Polymers Investigated by Molecular Dynamics and Brownian Dynamics Simulations

Using both fine-grained Molecular Dynamics (MD) simulations and coarse-grained simulations we show how strain hardening in polymeric glasses under uniaxial extension arises from highly stretched strands that form as the polymer chains deform subaffinely on increasing length scales as strain increases. We find that although the HBD model ignores entanglements, it accurately predicts how the MD chain configurations evolve during deformation. Both models shows similar strain hardening modulus G_R that is much larger than the melt plateau modulus G_N because chain segments become highly stretched at modest Hencky strain (<1 ). Both models also capture the increase in strain hardening with increasing chain length that saturates in the long chain limit. We improve upon HBD’s ability to accurately capture stress-strain curves at small strains through yielding and strain softening by extending the theory to multiple segmental relaxation modes, whose strain-dependent relaxation times are obtained from small-molecule probe relaxation experiments by Ediger and coworkers [Bending, B. & Ediger, M. D. J. Polym. Sci. B 2016, 54, 1957-1967].