Relating Chain Conformations to Extensional Stress In Entangled Polymer Melts

TL;DR

This study uses coarse-grained simulations to explore how entangled polymer chains deform under extensional stress, revealing the relationship between molecular structure, entropy, and viscosity in non-equilibrium conditions.

Contribution

It provides a detailed simulation-based analysis linking chain conformation changes to stress and viscosity in entangled polymer melts under extensional flow, extending understanding beyond equilibrium.

Findings

Simulations match experimental viscosity trends across flow conditions.

Elongation and thinning of the confining tube increase with Wi_R.

Stress correlates with decreasing chain entropy at the entanglement length.

Abstract

Nonlinear extensional flows are common in polymer processing but remain challenging theoretically because dramatic stretching of chains deforms the entanglement network far from equilibrium. Here, we present coarse-grained simulations of extensional flows in entangled polymer melts for Rouse-Weissenberg numbers $Wi_R=0.06$-$52$ and Hencky strains $\epsilon\geq6$. Simulations reproduce experimental trends in extensional viscosity with time, rate and molecular weight. Studies of molecular structure reveal an elongation and thinning of the confining tube with increasing $Wi_R$. The rising stress is quantitatively consistent with the decreasing entropy of chains at the equilibrium entanglement length. Molecular weight dependent trends in viscosity are related to a crossover from the Newtonian limit to a high rate limit that scales differently with chain length.