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

Abstract

This article examines the environmental stress cracking (ESC) behavior of Lexan polycarbonate in a range of mixed environments. The main aim of the work was to establish whether thermodynamic correlations, which can predict ESC to some extent with single environments, hold when dealing with mixed environments. The binary solvent mixtures used were produced from combinations of water, ethanol, benzyl alcohol, trimethylhexanol, and ethylhexanol. Samples of polycarbonate were subject to three-point bend tests while immersed in environments of various composition. From these, the critical strain (εcrit) required to produce crazing was determined in each case. The use of the Hildebrand solubility parameter (δ) to predict ESC behavior was examined. A simple law of mixtures would suggest that given a mixture of two environments with δ1 and δ2 on either side of δ for the polymer there should exist a composition at which the solubility parameter of the polymer and mixture should be equal, giving severe ESC or even solvation. Plots of εcrit versus δ show that this is not necessarily the case. Finally, an enthalpic parameter, based on the Flory–Huggins theory and utilizing partial solubility parameters, was applied and shown to give a reasonable correlation with the critical strains obtained. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 2155–2161, 1999