Modeling the Relaxation of Polymer Glasses under Shear and Elongational Loads

TL;DR

This paper presents a minimal model explaining the complex relaxation and flow behaviors of glassy polymers under shear and elongational loads, highlighting the interplay of flow-induced melting, stress buildup, and segmental re-vitrification.

Contribution

It introduces a unified model capturing the physics of polymer relaxation under different loading conditions, including new insights into unloading behavior and transient shear banding.

Findings

Segmental relaxation time dips during flow acceleration

Flow-induced melting leads to stress buildup and re-vitrification

Transient shear banding observed during startup of steady shear

Abstract

Glassy polymers show strain hardening: at constant extensional load, their flow first accelerates, then arrests. Recent experiments under such loading have found this to be accompanied by a striking dip in the segmental relaxation time. This can be explained by a minimal nonfactorable model combining flow-induced melting of a glass with the buildup of stress carried by strained polymers. Within this model, liquefaction of segmental motion permits strong flow that creates polymer-borne stress, slowing the deformation enough for the segmental (or solvent) modes to then re-vitrify. Here we present new results for the corresponding behavior under step-stress shear loading, to which very similar physics applies. To explain the unloading behavior in the extensional case requires introduction of a crinkle factor describing a rapid loss of segmental ordering. We discuss in more detail here the physics of this, which we argue involves non-entropic contributions to the polymer stress, and which might lead to some important differences between shear and elongation. We also discuss some fundamental and possibly testable issues concerning the physical meaning of entropic elasticity in vitrified polymers. Finally we present new results for the startup of steady shear flow, addressing the possible role of transient shear banding.