Modeling chain relaxation in moderately entangled polyisoprene melts

Chain relaxation in melts of moderately entangled macromolecules presents the first signs of topological constraints that are commonly manifested in the rheology of long commercial- grade polymers. In this study, we employ multiscale molecular dynamics simulations to examine the relaxation characteristics of model polyisoprene(PI) melts through the transition from the Rouse regime to entangled systems. We probed the impact of molecular weight and stereochemistry, and we evaluated the accuracy of theory in modeling the end-to-end relaxation. Our results support that trans-PI dynamics is distinct, with entanglements forming at a lower degree of polymerization than other compositions due to differences in the characteristic ratio and packing of this polymer. For all moderately entangled systems, modeling the end- to-end relaxation with the original Likhtman-McLeish theory with contour-length fluctuations presents deviations from recorded data both in the early-time and terminal regime. We propose numerical modifications that significantly improve the model for all systems studied without introducing additional parameters. Our approach allows accurate predictions for long macromolecules based on data from detailed simulations with moderately entangled polyisoprene systems.