Strain Hardening of Polymer Glasses: Entanglements, Energetics, and Plasticity

TL;DR

This study uses simulations to investigate the microscopic origins of strain hardening in polymer glasses, revealing that energy increases from chain tautness and bond dynamics, rather than entropy, dominate the hardening process.

Contribution

It challenges entropic network models by showing that energy and bond dynamics are the primary factors in strain hardening of polymer glasses.

Findings

Stress is too large to be entropic and decreases with temperature.

Energy increases as chains are pulled taut, driving hardening.

Plasticity rate correlates with thermal stress contributions.

Abstract

Simulations are used to examine the microscopic origins of strain hardening in polymer glasses. While stress-strain curves for a wide range of temperature can be fit to the functional form predicted by entropic network models, many other results are fundamentally inconsistent with the physical picture underlying these models. Stresses are too large to be entropic and have the wrong trend with temperature. The most dramatic hardening at large strains reflects increases in energy as chains are pulled taut between entanglements rather than a change in entropy. A weak entropic stress is only observed in shape recovery of deformed samples when heated above the glass transition. While short chains do not form an entangled network, they exhibit partial shape recovery, orientation, and strain hardening. Stresses for all chain lengths collapse when plotted against a microscopic measure of chain stretching rather than the macroscopic stretch. The thermal contribution to the stress is directly proportional to the rate of plasticity as measured by breaking and reforming of interchain bonds. These observations suggest that the correct microscopic theory of strain hardening should be based on glassy state physics rather than rubber elasticity.