Molecular-scale simulation of cross-flow migration in polymer melts

Abstract

The first ever molecular-scale simulation of cross-flow migration effects in dense polymer melts is presented; simulations for both unentangled and entangled chains are presented. At quiescence a small depletion next to the wall for the segmental densities of longer chains is present, a corresponding excess exists about one-half a radii of gyration away from the wall, and uniform values are observed further from the wall. In shear flow the melts exhibit similar behavior as the quiescent case; a constant shear rate across the gap does not induce chain length based migration. In contradistinction, parabolic flow (where gradients in shear rate are present) causes profound migration for both unentangled and entangled melts. Mapping onto polyethylene and calculating stress shows the system is far below the stress required to break chains. Accordingly, our findings are consistent with flow induced migration mechanisms predominating over competing chain degradation mechanisms thus resolving a 40 year old controversy.

Viscoelastic Response of Dispersed Entangled Polymer Melts

“…Therefore most numerical simulations have largely focused on monodispersed ( Đ M = 1) systems. Dispersed systems have mostly been modeled by blends of two chain lengths, − with only a few studies of polymer melts with a distribution of chain lengths. ,− Rorrer and Drogan − studied entangled melts using a lattice dynamic Monte Carlo method with three unique chain lengths to represent different degrees of dispersity. Recently, we directly probed the mobility of dispersed entangled polymer melts with distribution as narrow as experimentally attainable for long entangled polymers .…”

Effect of Chain Length Dispersity on the Mobility of Entangled Polymers

“…However, due to computational limitations, only the longer of the two chain lengths was well above the entanglement molecular weight M e . There have been few studies of polymer melts with a distribution of chains lengths, though mostly for short, unentangled polymers [19,23,[30][31][32][33][34][35][36][37]. Rorrer et al [19,[34][35][36] mapped a distribution of chain lengths on a small number of chain lengths and showed that for the same weight-averaged molecular weight, increasing the dispersity in chain lengths gives a lower Rouse time and introduces a broadening of the transition to reptation of the chains.…”

“…There have been few studies of polymer melts with a distribution of chains lengths, though mostly for short, unentangled polymers [19,23,[30][31][32][33][34][35][36][37]. Rorrer et al [19,[34][35][36] mapped a distribution of chain lengths on a small number of chain lengths and showed that for the same weight-averaged molecular weight, increasing the dispersity in chain lengths gives a lower Rouse time and introduces a broadening of the transition to reptation of the chains. Li et al [37] have shown that even very large dispersity has little effect on the polymer glass transition.…”

Die Drool and Polymer Degradation

“…This theory has been supported experimentally; gel permeation chromatography [11] and static laser light scattering [12] have demonstrated that die drool has a markedly different composition than the bulk extrudate. Theoretically, models have supported these experimental findings [13]. The die drool exhibits a larger fraction of lower molecular weight polymer.…”