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 isol ated polyisoprene chain at 298 K. Simulations of polyisoprene in the m elt (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 e nvironment, orientation correlation times for C-H bond vectors vary by a factor of 20. These simulation results are inconsistent with the co mmon assumption that conformational transitions are largely responsibl e for the decay of the orientation correlation functions for C-H vecto rs. Substantial C-H vector relaxation results from coupled small ampli tude motions of groups of adjacent torsions which do not involve confo rmational transitions. This mechanism plays a major role in C-H vector reorientation in the dilute solution simulations and also has a subst antial influence in the melt simulations.