Growth, microstructure, and failure of crazes in glassy polymers

TL;DR

This study uses molecular dynamics simulations to explore the formation, structure, and failure mechanisms of crazes in glassy polymers, revealing molecular-level details and challenging existing continuum models.

Contribution

It provides new insights into craze nucleation, widening, and failure at the molecular level, highlighting the role of entanglement and force distributions in glassy polymers.

Findings

Craze extension ratio depends on entanglement length.

Failure mode shifts from disentanglement to scission at N/N_e~10.

Force distribution in crazes exhibits an exponential tail.

Abstract

We report on an extensive study of craze formation in glassy polymers. Molecular dynamics simulations of a coarse-grained bead-spring model were employed to investigate the molecular level processes during craze nucleation, widening, and breakdown for a wide range of temperature, polymer chain length $N$, entanglement length $N_e$ and strength of adhesive interactions between polymer chains. Craze widening proceeds via a fibril-drawing process at constant drawing stress. The extension ratio is determined by the entanglement length, and the characteristic length of stretched chain segments in the polymer craze is $N_e/3$. In the craze, tension is mostly carried by the covalent backbone bonds, and the force distribution develops an exponential tail at large tensile forces. The failure mode of crazes changes from disentanglement to scission for $N/N_e\sim 10$, and breakdown through scission is governed by large stress fluctuations. The simulations also reveal inconsistencies with previous theoretical models of craze widening that were based on continuum level hydrodynamics.