Molecular simulations of crazes in glassy polymers under cyclic loading

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.