Growth kinetics of solvent crazes in glassy polymers

Abstract

The kinetics of craze growth from sharp cracks in polystyrene (PS) and poly(methyl methacrylate) (PMMA) in contact with liquid methanol were measured with time‐lapse photography as a function of the stress intensity factor K_I. At high K_I the craze length in both systems increases as √t if the sides of the craze are protected from methanol and as t if they are not, where t is the elapsed time after loading. If such a side‐protected craze is dried under load and then methanol is reintroduced to the crack tip, the methanol front advances with the same kinetics as the original craze growth. This experiment Proves that solvent crazing velocities are limited by the hydrodynamic transport of solvent through the porous craze structure under a capillary pressure driving force (which can be as high as 100 atm). An improved model of fluid flow through the craze is developed and shown to predict craze growth kinetics in good agreement with those observed. The hydraulic permeability of methanol crazes in PS was found to be independent of craze length at small craze length and to be independent of K_I except at very low K_I. Although increases in molecular weight in the range M_w = 200,000 to M_w = 670,000 do not markedly affect the crazing kinetics, they greatly increase the time to fracture of the craze.