Glass Transition Temperature as a Function of Conversion in Thermosetting Polymers

A simple equation to model the dependence of the glass transition temperature (T_g) on the extent of reaction in highly cross-linked thermosetting polymers has been derived. The model assumes that the increase in T_g is caused by (a) decrease in chain-end concentration, (b) formation of effective cross-links, and (c) further decrease in the configurationl entropy due to departure from Gaussian behavior at high cross-link densities. Data of T_g as a function of conversion have been obtained for a multifunctional epoxynovolac system. This system undergoes a very sharp increase in T_g at high conversions, which can be adequately fit by the model developed here. It has also been found that the change in T_g at low conversions in the multifunctional system can be predicted from data obtained with a model monofunctional system. The equation derived here is compared to several equations published in the literature.