A STUDY OF COOPERATIVE PHENYL RING FLIP MOTIONS IN GLASSY POLYSTYRENEBY MOLECULAR SIMULATIONS

Phenyl ring flip motion in polystyrene involves a cooperative movement of the phenyl ring and the chain backbone. In previous work, we descr ibed this cooperative rotation in single chains of polystyrene by defi ning the conformational transition in terms of a multidimensional reac tion path and calculating torsion angle and energy profiles along this path. In this paper, we have extended our approach to study phenyl ri ng rotation in model structures of atactic polystyrene glasses. Reacti on paths, energy profiles, free energy barriers, and rate constants we re calculated for the rotation of 10 different phenyl rings in the sam e local backbone conformation. The results show that local intrachain steric interactions, that dominate rotation in single chains, do so in the polymer glass as well. In addition, the free energy barriers cove r a wide range of values corresponding to a distribution of characteri stic times for ring rotation that spans 21 decades. This distribution is also bimodal. The results are compared with observations from NMR e xperiments and previous molecular mechanics calculations.