Computer simulations of polyisoprene local dynamics in vacuum, solution, and the melt: Are conformational transitions always important?

Molecular dynamics computer simulations have been performed on an isolated polyisoprene chain at 298 K. Simulations of polyisoprene in the melt (413 K) and in solution (298 K) have been previously reported. The local dynamics of polyisoprene chains in these three environments are compared. While conformational transition rates are similar in each environment, orientation correlation times for C-H bond vectors vary by a factor of 20. These simulation results are inconsistent with the common assumption that conformational transitions are largely responsible for the decay of the orientation correlation functions for C-H vectors. Substantial C-H vector relaxation results from coupled small amplitude motions of groups of adjacent torsions which do not involve conformational transitions. This mechanism plays a major role in C-H vector reorientation in the dilute solution simulations and also has a substantial influence in the melt simulations.