THE EFFECT OF STOICHIOMETRY AND THERMAL HISTORY DURING CURE ON STRUCTURE AND PROPERTIES OF EPOXY NETWORKS

An investigation was carried out into the effect of amine/epoxy stoich iometry and thermal history during cure on physical and mechanical pro perties of epoxy networks. The formulation studied consisted of a digl ycidyl ether of bisphenol A epoxy resin and 4,4'-diaminodiphenyl sulfo ne curing agent. The experimental matrix was based upon three amine/ep oxy ratios and seven different thermal histories during cure. Techniqu es used included dynamic mechanical and fracture analysis, and Fourier transform infra-red (FTi.r.) spectroscopy. The highest glass transiti on temperature (T(g)) was observed in the stoichiometric formulation a nd the lowest in the epoxy-rich mixture. For a given stoichiometry, th e value of T(g.infinity) was not a function of thermal history during cure except, interestingly, in the case when the initial temperature w as 180-degrees-C. The highest rubbery state modulus and the lowest ave rage molecular weight between crosslinks were also found in the stoich iometric formulation. Our findings were rationalized in terms of the v arying degrees of crosslinking in different networks. The opposite tre nd was observed in the glassy state at 20-degrees-C, where the lowest flexular modulus belonged to the stoichiometric formulation. An explan ation for those results was offered in terms of the free volume concep t. FTi.r. analysis established clearly the existence of residual epoxy groups in all formulations, even after post-cure. Etherification reac tion between epoxy and hydroxyl groups takes place during post-cure, b ut a complete conversion of epoxy groups cannot be attained owing to t he topological constraints within the three-dimensional network in the later stages of cure. This finding is of particular significance in m echanistic kinetic models based upon the absolute value of epoxy conce ntration at all stages of cure.