Dynamics of Entangled, Polydispersed Polymer Melts

While essentially all theoretical and computational studies of entangled polymer melts have focused on monodispersed samples, the best polymer synthesis routes result in some dispersity, albeit narrow, in the distribution of molecular weights (M_w/M_n ~ 1.01-1.04). The effects of dispersity on chain mobility and the stress relaxation have been studied using molecular dynamics simulations of entangled, disperse coarse-grained polyethylene melts. Polymer melts with M_w/M_n = 1.0 to 1.16 for times of ~ 500 μs, which is on the order of the terminal stress relaxation time at T=500 K, were studied. The chain lengths were set to follow a Schultz-Zimm distribution for the same average M_w = 36 kg/mol. We find that the entanglement time and tube diameter are unaffected. The average diffusion constant however, increases with dispersity, as the contribution of the shorter chains moving faster outweighs the longer chains moving slower. The stress autocorrelation function was fit to the theoretical expression proposed by Likhtman and McLeish. The resulting plateau modulus, terminal time and viscosity all decrease with increasing dispersity, corresponding to an apparent increase in the entanglement molecular weight.