COOPERATIVITY OF LOCAL CONFORMATIONAL DYNAMICS IN SIMULATIONS OF POLYISOPRENE AND POLYETHYLENE

Brownian dynamics computer simulations of polyethylene and cis-polyisoprene are analyzed to determine how various degrees of freedom cooperate in localizing conformational transitions. Changes in atomic positions, bond angles, and torsional angles are considered. For both polyethylene and polyisoprene the distortions in atomic positions accompanying a conformational transition are localized to about four carbons along the backbone. Second-neighbor torsional coupling is found to play an important role in localizing conformational transitions for polyethylene but not for polyisoprene. The motion which occurs in connection with a conformational transition in polyethylene is almost independent of whether a second conformational transition occurs cooperatively with the first. Several aspects of the polyethylene results are consistent with earlier calculations using multidimensional Kramers' theory.