Simulation of polymer glasses: From segmental dynamics to bulk mechanics

Molecular dynamics (MD) simulations of the low-molecular-weight glass-former, isopropylbenzene (iPB), atactic-polystyrene (PS) and bisphenol A polycarbonate (PC) reveal the polymer-specific effects of the glassy segmental orientational dynamics. We also simulate the mechanical responce of these two typical amorphous polymers, brittle PS and tough PC, under uniaxial extension. The Young moduli, yield stresses, strain-softening and strain-hardening phenomena of the simulated polymers are numerically close to the experimental data. The local mobility of the segments in the deformation direction is increased drastically (but differently) for both polymers beyond the yield point. In the non-deformed case three relaxational processes have been found for all simulated glass-formers: high-frequency transient ballistic process, β process corresponding to the motions within the cage, and final cage relaxation. For all three glass-formers the slowing down of the cage relaxation (α relaxation) follows mode-coupling theory above Tg, with non-universal exponents, and continues below Tg as an activated relaxational process. For iPB, PS and PC at high temperature the α process is merged with the β process. The β process can be described by an activation law both above and below Tg.