One-Time Network Rearrangement for Homogenization of Epoxy Resin Structures

Epoxy resins, which form highly cross-linked and permanent network structures by curing reactions between epoxy and amine compounds, are typically in a glassy state at room temperature. They are thus inherently brittle, a property that severely limits their broader practical application. To address this issue, the structural heterogeneity of the cured network, which is closely related to fracture behavior, must be precisely controlled, even after curing. Herein, we propose an epoxy resin system featuring a one-time rearrangeable network designed to reduce heterogeneity after curing. Disulfide bonds were introduced into the epoxy-amine network to enable disulfide exchange reactions, which promote network homogenization. Following this rearrangement, the resulting less-heterogeneous network was subsequently fixed via a chemical reaction between the disulfide bonds and terminal carbon-carbon double bonds on dangling chains. The resin was synthesized by curing a mixture of a bifunctional epoxy compound, a monofunctional epoxy compound containing a terminal double bond, and a bifunctional amine compound bearing disulfide linkages. Thermal annealing was used as a stimulus to activate both the disulfide exchange reactions and the subsequent fixing reaction. Upon annealing, the resin exhibited an increased glass transition temperature and a reduction in dynamic heterogeneity. Compared to a reference epoxy resin lacking disulfide bonds, the annealed resin showed a narrower distribution of relaxation times and higher fracture energy. These findings offer a valuable insight into the design of thermosetting resins, demonstrating that network heterogeneity can be reduced even after curing, while maintaining thermal and dimensional stability.