Effect of a Heterogeneous Network on the Fracture Behavior of Epoxy Resins

Epoxy resins, which possess a three-dimensional network obtained by a curing reaction between epoxy and amine compounds, are in a glassy state at room temperature. They are therefore generally brittle and exhibit rather poor resistance to crack propagation, which is one of the greatest drawbacks for their practical application. To solve this problem, a better understanding of their fracture behavior, which is related to long-term durability, is strongly desired. We herein report on how the extent of network heterogeneity affects the fracture behavior of epoxy resins. Three types of epoxy resins were obtained under different curing conditions, which generated different extents of heterogeneity. The fracture behavior of the three epoxy resins was examined by in situ scanning electron microscopy in conjunction with microbeam X-ray scattering experiments. When the epoxy resin in which a notch was pre-introduced was stretched under a small strain, the internal network structure was deformed, and the extent was more significant with increasing heterogeneity. As further stretching was applied, a crack propagated from the notch. This occurred more easily with increasing heterogeneity. That is, the toughness of the epoxy resins decreased with increasing network heterogeneity. The knowledge obtained here will be useful for understanding and controlling the toughness of epoxy resins.