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) simulations and by a high-order mode-coupling diffusion theory. The first- and second-rank orientation autocorrelation functions for virtual bonds connecting the repeat units are calculated from the theory and compared to the same quantities calculated directly from the MD trajectory. The equilibrium statistical averages needed in the theory are calculated from a time average along the same MD trajectory. The correlation functions from the simulation 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. Alternatively, when the statistics are obtained by substituting the trajectory in the explicit solvent with a suitable trajectory in the vacuum, the local dynamics is still well-approximated by the diffusion theory. The study addresses the relationship between correlation functions of different rank, which has been shown to be universal for chains undergoing a first-order Gaussian random process. (C) 2001 American Institute of Physics.