Viscoelastic Response of Dispersed Entangled Polymer Melts

Polymer synthesis routes result in macromolecules with molecular weight dispersity D-M that depends on the polymerization mechanism. The lowest dispersity polymers are those made by anionic and atom-transfer radical polymerization, which exhibit narrow distributions D-M = M-w/M-n similar to 1.02-1.04. Even for small dispersity, the chain length can vary by a factor of two from the average. The impact of chain length dispersity on the viscoelastic response remains an open question. Here, the effects of dispersity on stress relaxation and shear viscosity of entangled polyethylene melts are studied using molecular dynamics simulations. Melts with chain length dispersity, which follow a Schulz-Zimm (SZ) distribution with D-M = 1.0-1.16, are studied for times up to 800 mu s, longer than the terminal time. These systems are compared to those with binary and ternary distributions. The stress relaxation functions are extracted from the Green-Kubo relation and from stress relaxation following a uniaxial extension. At short and intermediate time scales, both the mean squared displacement and the stress relaxation function G(t) are independent of D-M. At longer times, the terminal relaxation time decreases with increasing D-M. In this time range, the faster motion of the shorter chains results in constraint release for the longer chains.