DYNAMICS OF POLYMER ISOTOPE MIXTURES - MOLECULAR-DYNAMICS SIMULATION AND ROUSE MODEL ANALYSIS

We report results of a molecular dynamics simulation of an ''isotope'' mixture of polymer chains, which are represented by a standard bead-s pring model, and whose two species differ only by their monomer masses . Detailed analysis of the Rouse modes shows that for sufficiently sho rt (non-entangled) chains this system can be well described by the Rou se model. Each species is described by its individual monomeric fricti on coefficient, whose dependence on both mass ratio as well as mixing ratio is studied. The main effect of mixing is an acceleration of the slower chains and a slowdown of the faster ones, while both species re main dynamically different. Some microscopic insight into the mechanis m is obtained by studying the short-time behavior of the monomeric vel ocity autocorrelation function. Studies in the slightly entangled regi me (chain length up to N = 150, where the typical entanglement chain l ength is N-e approximate to 35) seem to further corroborate the hypoth esis that the ''tube diameter'' of the reptation model is a quantity w hich results mainly from the static configurations, i.e., is an equili brium thermal average. The usefulness of recently suggested analysis m ethods in this regime is briefly discussed. (C) 1997 American Institut e of Physics. [S0021-9606(97)52141-6].