Molecular modeling of epoxide-amine systems: Topological cure conversion limit and its influence on material properties

The cure conversion is a key determinant of the characteristics of many thermosetting polymers. Their curing process can be monitored using experimental tools such as FTIR and DSC; however, these techniques can only provide a qualitative measure of the extent of curing reaction rather than a quantitative determination of cure conversion. In fact, the actual cure conversion in epoxy systems is not known. Many molecular simulation works assume that the cure conversion limit approaches 100% although it is unrealistic. In this work, a parametric study based on model size and a radial distribution function (RDF) of relevant functional groups reveals that a topological cure conversion limit may exist and is estimated to be about 70% for a typical epoxide-amine system based on the chosen force field and simulation conditions. Beyond this, atoms within the model may form isolated clusters and the molecular model fails to converge to an equilibrated topology with structural homogeneity. It is found that the predicted dependencies of thermal and mechanical properties on cure conversion are in concurrence with accepted norms and only show deviation when the cure conversion exceeds the limit.