The effect of thermal history and strain rate on the mechanical properties of diethylenetriamine‐cured bisphenol‐A‐diglycidyl ether epoxies

Abstract

The tensile mechanical properties of diethylenetriamine (DETA)‐cured bisphenol‐A‐diglycidyl ether (DGEBA) epoxies prepared from 9, 11 and 13 phr DETA are reported as a function of thermal history, strain‐rate and test temperature. These epoxies exhibit macroscopic yield stresses and >10% ultimate elongations. The mechanical properties of these epoxies exhibit a free‐volume dependence as a function of thermal history. Annealing below T_g causes an increase in the macroscopic yield stress and a decrease in the ultimate elongation, whereas quenching from above T_g lowers the yield stress and increases the elongation. These mechanical property modifications are shown to be reversible with reversible thermal–anneal cycles. The activation volumes associated with Eyring's theory for stress‐activated viscous flow for the DGEBA‐DETA epoxies are within the range of values (9–12 nm³) reported for noncrosslinked polymers. These observations suggest that the DGEBA‐DETA epoxies are not as highly crosslinked as would be expected from normal addition reactions of epoxide groups with primary and secondary amines. The formation of lower crosslink density networks is discussed in terms of potential chemical reactions.