Effect of Dispersity on the Behavior of Entangled Polymer Melts under Extensional Flow: Insights from Nonequilibrium Molecular Dynamics Simulations

This study employs nonequilibrium molecular dynamics (NEMD) simulations to investigate the elongational-flow behavior of model polymer melts with varying dispersity. We examine how chain-length heterogeneity influences the nonlinear response of polymer melts and establish connections among macroscopic stress, chain-conformation evolution, and entanglement dynamics. At low strain rates, strain hardening becomes stronger as dispersity increases. However, at high strain rates, this trend reverses: the initial strain hardening weakens with increasing dispersity, with monodisperse melts exhibiting a more pronounced steady-state stress, while disperse melts continue to build stress at large strains. To probe the molecular origins of these behaviors, we follow the flow response of chains of two lengths (N = 360 and 500) embedded in melts of different dispersities and compare them with the monodisperse counterparts up to a Hencky strain ε ≥ 6. Chain conformations reveal significant stretching under flow, while the progressive loss of entanglements reflects tube elongation and thinning. Dispersity modulates both stretching and disentanglement with pronounced effects for longer and tightly entangled test chains. We interpret these results using two theoretical frameworks: the Rolie-Double-Poly model and an entropic elasticity model that explicitly incorporates chain-specific entanglement evolution. The entropic model provides accurate predictions across all strain rates and dispersities studied. Overall, our findings demonstrate that dispersity plays a central role in dictating the far-from-equilibrium response of moderately entangled polymer melts, with the direction and magnitude of strain hardening determined by the interplay between deformation-driven chain stretching and the rate at which entanglements are lost under flow.