The effect of cross-linking on crazing in polyethersulphone

Cross-links have been introduced into thin films of PES (polyethersulphone)/1 wt% sulphur by heating them in air at 350 °C. The effect of this is to suppress crazing in favour of shear deformation in high-temperature regimes where disentanglement crazing dominates for uncross-linked films of the same composition. We argue that light cross-linking (one or two cross-links per chain) is sufficient to give rise to a finite gel fraction in the films which, because it effectively forms an infinite network, cannot disentangle. Thus for crazing to occur, chains which form part of the gel fraction must always break rather than disentangle. This has the effect of raising the crazing stress relative to the yield stress in the weakly temperaturedependent regime of crazing at high temperature, where disentanglement is normally considered sufficiently rapid for entanglement loss not to contribute to the crazing stress. Hence as the gel fraction is increased by increasing the heat-treatment time, crazing is suppressed at the highest temperatures with respect to shear deformation, leading to a second transition, this time from crazing back to shear.