Effects of orientation on the penetration, crazing, and dissolution of polystyrene by n-hexane

Abstract

Extruded, drawn, and quenched samples of polystyrene were subsequently immersed in liquid n-hexane at temperatures between 35° and 55°C for various time intervals. Samples were removed from the immersion bath, quenched and fractured. Subsequent microscopic examination of the cross-sections revealed a distinct boundary between a crazed outer shell and an essentially unpenetrated central core. The time dependence of the depth of penetration of the advancing craze front was measured at various temperatures for several draw ratios. The initial rate of penetration increased monotonically with draw ratio (orientation) and the advance of the penetrant front was completely controlled by diffusion for drawn samples at 55°C. More complex kinetics, involving relaxations at the moving boundary, describe the penetration at lower temperatures; a slight systematic variation in the relative contribution of diffusion and relaxation was observed with increasing draw ratio. An activation energy of 23.7 k cal/g-mole characterized the temperature dependence of the initial penetration rate, independent of sample orientation. Gravimetric swelling experiments were confounded by sample dissolution in the case of the oriented samples. Intriguing swelling patterns, including discernable differences between the pronounced edge effects in the draw and tranverse directions, were apparent. Conversely, diffusion transverse to the orientation direction was accelerated by the orientation resulting in an increasing component of relaxation control in the penetration experiments and increased rates of dissolution in the oriented samples.