Improved thermodynamic approach for predicting the ESC behavior of polycarbonate in binary liquid mixtures

This article examines the environmental stress cracking (ESC) behavior of L exan polycarbonate in a range of mixed environments. The main aim of the wo rk was to establish whether thermodynamic correlations, which can predict E SC to some extent with single environments, hold when dealing with mixed en vironments. The binary solvent mixtures used were produced from combination s of water, ethanol, benzyl alcohol, trimethylhexanol, and ethylhexanol. Sa mples of polycarbonate were subject to three-point bend tests while immerse d in environments of various composition. From these, the critical strain ( epsilon(crit)) required to produce crazing was determined in each case. The use of the Hildebrand solubility parameter (delta) to predict ESC behavior was examined. A simple law of mixtures would suggest that given a mixture of two environments with delta(1) and delta(2) on either side of delta for the polymer there should exist a composition at which the solubility parame ter of the polymer and mixture should be equal, giving severe ESC or even s olvation. Plots of epsilon(crit) versus delta show that this is not necessa rily the case. Finally, an enthalpic parameter, based on the Flory-Huggins theory and utilizing partial solubility parameters, was applied and shown t o give a reasonable correlation with the critical strains obtained. (C) 199 9 John Wiley & Sons, Inc.