Dynamics of Polymer Chains in Disperse Melts: Insights from Coarse-Grained Molecular Dynamics Simulations.

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Synthetic polymers have a distribution of chain lengths which can be characterized by dispersity, Đ. Their macroscopic properties are influenced by chain mobility in the melt, and controlling Đ can significantly impact these properties. In this work, we present a detailed study of the static and dynamic behavior of fully flexible polymer chains that follow the Schulz-Zimm molecular weight distribution up to Đ = 2.0 using coarse-grained molecular dynamics simulations. We analyze the behavior of test chains with molecular weights that are equal to, above, or below the molecular weight (Mw) of the melt. Static analysis shows that the conformation of these test chains remains unaffected by the heterogeneity of the surrounding chains. To study the dynamics, we computed the mean-squared displacement of test chains in melts of the same Mw and different dispersities. The mobility of test chains with N > Mw steadily increases with dispersity, due to the shorter chains contributing to early onset of disentanglement of the long chains. However, the dynamics of test chains of length N < Mw is nonmonotonic with respect to dispersity; this behavior arises from a trade-off between the increased mobility of shorter chains and the corresponding slowdown caused by the presence of longer chains. We examine the dynamic structure factor and find a weakening of tube confinement, with the effects becoming less pronounced with increasing dispersity and Mw. These findings provide insights into the rich dynamic heterogeneity of disperse polymer melts.