Computer simulation study of bulk atactic polystyrene in the vicinity of the glass transition

Molecular dynamics (MD) simulations of bulk atactic polystyrene have been performed in a temperature range from 100 K to 650 K at atmospheric pressure. Local translational mobility has been investigated by measuring the mean square translational displacements of monomers. The long-time asymptotic slope of these dependencies is 0.54 at T>Tg, showing Rouse behavior. Cross-over from motion in the cage to Rouse like dynamics has been studied at T>Tg with a characteristic crossover time follows a power law behavior as a function of T, as predicted by mode-coupling theory (MCT). Local orientational mobility has been studied via the orientational autocorrelation functions, ACFs, (Legendre polynomials of the first and second, order) of both the main-chain and side-group bonds. The relaxation times of the orientational -relaxation follow the same power law (2.9) as the characteristic translational diffusion time. Below T>Tg both types of dynamics are described by the same activated law. The ACFs time-distribution functions reveal the existence of activated local rearrangements already above T>Tg