Strain softening and hardening of amorphous polymers: Atomistic simulation of bulk mechanics and local dynamics

Molecular-dynamics ( MD) simulations have been performed for two amorphous polymers with extremely different mechanical properties, atactic polystyrene ( PS) and bisphenol A polycarbonate ( PC), in the isotropic state and under load. The glass transition temperatures, Young moduli, yield stresses and strain-hardening moduli are calculated and compared to the experimental data. Both chemistry-specific and mode-coupling aspects of the segmental mobility in the isotropic case and under the uniaxial deformation have been identified. The mobility of the PS segments in the deformation direction is increased drastically beyond the yield point. A weaker increase is observed for PC.