Chain Length Dispersity Effects on Mobility of Entangled Polymers

TL;DR

This study investigates how slight dispersity in chain lengths affects the mobility of entangled polyethylene melts, revealing that dispersity influences the movement of shorter chains but not the overall entanglement characteristics.

Contribution

It provides the first detailed molecular dynamics analysis of dispersity effects on entangled polymer melt mobility, focusing on realistic narrow dispersity ranges.

Findings

Dispersity does not affect entanglement time or tube diameter.

Shorter chains move faster, enabling constraint release.

Dispersity influences chain mobility distribution, especially for shorter chains.

Abstract

While nearly all theoretical and computational studies of entangled polymer melts have focused on uniform samples, polymer synthesis routes always result in some dispersity, albeit narrow, of distribution of molecular weights (D_M=M_w/M_n ~ 1.02-1.04). Here the effects of dispersity on chain mobility are studied for entangled, disperse melts using a coarse-grained model for polyethylene. Polymer melts with chain lengths set to follow a Schulz-Zimm distribution for the same average M_w = 36 kg/mol with D_M = 1.0 to 1.16, were studied for times of $600-800$ $\mu$s using molecular dynamics simulations. This time frame is longer than the time required to reach the diffusive regime. We find that dispersity in this range does not affect the entanglement time or tube diameter. However, while there is negligible difference in the average mobility of chains for the uniform distribution D_M=1.0 and D_M = 1.02, the shortest chains move significantly faster than the longest ones offering a constraint release pathway for the melts for larger D_M.