Glass transition temperature as a function of conversion in thermosetting polymers

A simple equation to model the dependence of the glass transition temperature (T sub g ) on the extent of reaction in highly cross-linked thermosetting polymers has been derived. The model assumes that the increase in T sub g is caused by (a) decrease in chain-end concentration, (b) formation of effective cross-links, and (c) further decrease in the configurational entropy due to departure from Gaussian behavior at high cross-link densities. Data of T sub g as a function of conversion have been obtained for a multifunctional epoxy-novolac system. This system undergoes a very sharp increase in T sub g at high conversions, which can be adequately fit by the model developed here. It has also been found that the change in T sub 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. Graphs. 65 ref.--AA