Plasticization effects on environmental craze microstructure

Abstract

Environmental crazes were grown in thin films of polystyrene (PS) using the homologous series of alcohols, CH3OH, C2H5OH, n-C3H9OH and n-C4H9OH. The films were bonded to copper grids, strained to below the minimum crazing strain in air and exposed to the vapour of the various alcohols. The craze microstructure, as measured quantitatively by transmission electron microscopy, varies significantly along the series. The craze fibril volume-fraction, Vf, decreases monotonically from 0.25 for methanol, which depresses the glass transition temperature, Tg, of PS to 91° C, to 0.09 for n-butanol, which depresses Tg of PS to 71° C. All these slowly-growing vapour crazes thicken by drawing more fibrillar material from the craze surfaces rather than by fibril creep. The large decrease in Vf along the series of alcohols cannot be due to a change in the chain-entanglement molecular weight, as a result of swelling by the alcohols, but must result rather from an easier slippage of molecular entanglements in the drawing glassy fibrils. The large decrease in Vf from methanol to butanol crazes must also enhance the nucleation of cracks within these crazes, as evidenced by the ten-fold decrease in the environmental fatigue life of PS along the series from methanol to butanol.