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 stoichiometry and thermal history during cure on physical and mechanical properties of epoxy networks. The formulation studied consisted of a diglycidyl ether of bisphenol A epoxy resin and 4,4′-diaminodiphenyl sulfone curing agent. The experimental matrix was based upon three amine/epoxy ratios and seven different thermal histories during cure. Techniques used included dynamic mechanical and fracture analysis, and Fourier transform infra-red (FTi.r.) spectroscopy. The highest glass transition temperature (Tg) was observed in the stoichiometric formulation and the lowest in the epoxy-rich mixture. For a given stoichiometry, the value of Tg, infinity was not a function of thermal history during cure except, interestingly, in the case when the initial temperature was 180°C. The highest rubbery state modulus and the lowest average molecular weight between crosslinks were also found in the stoichiometric formulation. Our findings were rationalized in terms of the varying degrees of crosslinking in different networks. The opposite trend was observed in the glassy state at 20°C, where the lowest flexular modulus belonged to the stoichiometric formulation. An explanation for those results was offered in terms of the free volume concept. FTi.r. analysis established clearly the existence of residual epoxy groups in all formulations, even after post-cure. Etherification reaction between epoxy and hydroxyl groups takes place during post-cure, but a complete conversion of epoxy groups cannot be attained owing to the topological constraints within the three-dimensional network in the later stages of cure. This finding is of particular significance in mechanistic kinetic models based upon the absolute value of epoxy concentration at all stages of cure.