BROWNIAN DYNAMICS SIMULATIONS OF LOCAL MOTIONS IN POLYISOPRENE

Brownian dynamics computer simulations have been performed on polyisoprene chains containing 67 repeat units. The simulated trajectories were used to generate three orientational autocorrelation functions analogous to those sensed by C-13 NMR T1 experiments. Agreement between the simulation and dilute-solution NMR experiments is encouraging. Differences in the dynamics of various C-H vectors in the polymer backbone are quantitatively reproduced by the simulation. The simulations indicate the presence of substantial librational motions on a time scale of a few picoseconds. Correlated pairs of transitions occur infrequently. The role of cooperativity in conformational transitions is examined, and a new method of examining cooperativity is presented. For polyisoprene, distortions in atomic positions accompanying a conformational transition are localized to about one repeat unit.