Influence of Dispersity on the Relaxation of Entangled Polymers from Molecular Dynamics Simulations

Abstract: Nonequilibrium molecular dynamics simulations are used to study the deformation behavior of disperse polymer melts by tracking test chains of length N = Mw, the weight average molecular weight, in melts of varying dispersity. At high strain rates, stress decreases with increasing dispersity up to the critical stretch ratio. After flow cessation, relaxation is anisotropic: the parallel component of the radius of gyration decreases monotonically with dispersity, but the perpendicular component exhibits non-monotonic behavior, with fast relaxation at low to intermediate stretching ratios, and slower relaxation at higher stretching ratios as dispersity increases. Single-chain structure factor shows a similar trend in the perpendicular direction, with fast relaxation at intermediate length scales in disperse systems, while the parallel component remains unaffected by dispersity. These findings reveal the complex, dispersity-dependent anisotropic relaxation mechanisms in moderately entangled polymer melts far from equilibrium.