Simulation of mechanical properties of oriented glassy polystyrene

Mechanical properties of processed polymers depend sensitively on their microstructure. In order to understand how different processing conditions affect the mechanical properties of polymers, one needs a means to describe the process-induced microstructure. Because the characteristic relaxation times of processed polymer chains often span several orders of magnitude, it is commonly the case that partial relaxation of the chains is frozen into the final product. We report results of molecular simulations by the Semi-Grand Canonical Monte Carlo (SGMC) method to study the orientation-dependent elasticity of glassy polystyrene as a function of both the system-average degree of orientation and the degree of relaxation of chain ends at a constant average orientation, in accord with the tube model of Doi and Edwards. Our simulations reproduce quantitatively the experimentally observed trends in the tensile modulus E₁₁ as a function both of the system-average orientation and of the inhomogeneity of the orientation along the chain due to rapid relaxation of chain ends. The results show that the partial relaxation of the polymer chains is sufficient to explain the observed variation of mechanical properties for samples that differ in processing history, yet have the same observed birefringence.