Polyisoprene local dynamics in solution: Comparison between molecular dynamics simulations and high order diffusion theory

The local dynamics of a cis-1,4-polyisoprene of ten monomer units in dilute toluene solution is approached by deterministic molecular dynamics (MD) si mulations and by a high-order mode-coupling diffusion theory. The first- an d second-rank orientation autocorrelation functions for virtual bonds conne cting the repeat units are calculated from the theory and compared to the s ame quantities calculated directly from the MD trajectory. The equilibrium statistical averages needed in the theory are calculated from a time averag e along the same MD trajectory. The correlation functions from the simulati on are found to be almost coincident when significant contributions to high orders in the mode-coupling expansion are considered. Even the first-order diffusion theory represents a good approximation to the MD result. Alterna tively, when the statistics are obtained by substituting the trajectory in the explicit solvent with a suitable trajectory in the vacuum, the local dy namics is still well-approximated by the diffusion theory. The study addres ses the relationship between correlation functions of different rank, which has been shown to be universal for chains undergoing a first-order Gaussia n random process. (C) 2001 American Institute of Physics.