Molecular simulations of crazes in glassy polymers under cyclic loading

TL;DR

This study uses molecular dynamics simulations to explore the cyclic crazing behavior of glassy polymers, revealing how craze fibrils respond to loading and unloading, with implications for understanding their mechanical properties.

Contribution

It provides new insights into the microscopic mechanisms of craze formation, orientation, and stress response in glassy polymers under cyclic loading.

Findings

Hysteresis in stress response is quasi stationary after initial cycle.

Craze fibril behavior during unloading resembles string-like folding.

Craze stiffness is two orders of magnitude lower than bulk stiffness.

Abstract

We study with molecular dynamics simulations of a generic bead-spring model the cyclic crazing behaviour of glassy polymers. The aim is to elucidate the mechanical response of sole fibrillated craze matter as well as its interaction with bulk material. The macroscopic stress response exhibits a hysteresis, which is quasi stationary after the first cycle and largely independent of deformation rate and temperature. It results from a complex interplay between constraints imposed by the entanglement network, pore space and pore space closure. Once the craze fibrils are oriented, stretching of the covalent backbone bonds leads to a rapid stress increase. In the initial stages of unloading, a loss in entanglement contact yields a quick stress relaxation in the backbone. During unloading, the craze fibrils undergo a rigid body (i.e.\ stress-free) folding motion due to the surrounding pore space, so that the structural behaviour of craze fibrils during unloading is most accurately described as string-like. The reloading response depends significantly on the degree of pore space closure and the enforced intermolecular interaction during unloading. It ranges from a linear stress increase to a re-cavitation with a re-drawing response. Compared to the bulk stiffness, the craze stiffness is two orders of magnitude lower and as a result, the macro response of coexisting craze and bulk matter is governed by the sole fibrillated craze matter.