Off-stoichiometry effect on the physical properties of epoxy resins

Amines generally tend to segregate at the interface of the epoxy resins cured with amines. To gain better insight into the aggregation states and physical properties at the adhesive interface, we examined the cross-linking structure and physical properties of the cured epoxy resins with an off-stoichiometric ratio of epoxy and amine. As the excess amine increased, the amine not only remained as unreacted monomers or low-molecular-weight isolated chains within the cross-linking structure but also formed dangling chain ends, increasing heterogeneity in the cured epoxy resin. As a result, the cross-linking density and mass density decreased, along with a reduction in the glass transition temperature. On the other hand, Young’s modulus increased with the excess amount of amine. Wide-angle X-ray scattering experiments, in conjunction with molecular dynamics simulations, revealed that the excess amine suppressed the widening of the distance between phenyl groups during the curing reaction, suggesting that this could act as steric resistance during deformation.