MOLECULAR-DYNAMICS COMPUTER-SIMULATION OF POLYISOPRENE LOCAL DYNAMICSIN DILUTE TOLUENE SOLUTION

Fully atomistic molecular dynamics computer simulations have been perf ormed on a polyisoprene chain of 35 repeat units in a solution of 207 toluene molecules. Simulated orientation correlation functions of poly mer C-H vectors compare favorably with C-13 NMR experiments. In partic ular, the ordering of the different correlation times is as expected, and their absolute values agree with experiment within a factor of 2. Also in good agreement with experiment, backbone C-H correlation funct ions were found to be nearly exponential after a very fast small-ampli tude decay. The simulation trajectories have been analyzed to examine the changes in atomic positions and torsional coordinates which accomp any conformational transitions. Adjustments in these degrees of freedo m are found to be localized to 1-2 repeat units in polyisoprene, in go od agreement with the results of previous Brownian dynamics simulation s. Cooperative pairs of conformational transitions account for less th an half of all transitions if coupling up to second neighbors is consi dered. About two-thirds of all transitions occur in cooperative pairs if coupling up to ninth neighbors is considered.