The effect of molecular properties on the mechanical behaviour of PMMA—II. Environmental stress cracking in methanol

Abstract

The fracture process of low molecular weight PMMA in the presence of methanol is a crack predominant mechanism (Regime II). The activation energy of the cracking process calculated by Zhurkov's rate theory suggested that the failure mechanism is governed by a process where the slippage of chain molecules over each other results in the secondary bond breakage. Analysis based on a linear elastic fracture mechanics approach demonstrated the dependence of the crack opening displacement and the craze length on temperature and the stress intensity factor (SIF). Agreement between values of the diffusion coefficient at zero applied stress derived from the cracking data and those from methanol-equilibration of bulk PMMA indicated the role of diffusional behaviour of methanol in the failure mechanism. Increasing craze length with temperature or the SIF can be related to the enhancement of the diffusion coefficient at elevated temperature or increased SIF. The Williams-Marshall theory demonstrated quantitatively more rapid decay of the crazing stress as larger amounts of methanol are absorbed at the crack tip.