Preparation of self-healing and degradable high-performance epoxy thermosets containing disulfide bonds and hindered urea bonds

Compared with a single dynamic covalent network, the introduction of dual dynamic bonds can significantly improve the comprehensive properties of materials. In this study, we introduced dynamic hindered urea bonds and low-bond-energy dynamic disulfide bonds into the epoxy dynamic network and synthesized epoxy polymers based on dual dynamic covalent bonds. Among them, the existence of curing agent containing disulfide bond can improve the rigidity of the crosslinked network and greatly improve the tensile strength of epoxy resin. However, epoxy resin cured exclusively with disulfide-based agents exhibit elevated glass transition temperature, and the exchange reaction of disulfide bond is frozen. It usually takes a very high temperature (above 180 °C) to repair the damage by disulfide bond rearrangement. While the flexible urea bond curing agent can improve the movement ability of the chain segment of epoxy resin from the molecular structure, thus reduce the glass transition temperature, and greatly reduce the processing difficulty of epoxy resin while toughening. The experimental results show that the dual dynamic network demonstrates accelerated stress relaxation behavior at high temperatures, which can be completely relaxed within 1 min at 130 °C, the activation energy is only 58.19 kJ/mol, and the resin scratch can be completely repaired by heating at 130 °C for 5 min. What’s more, the mechanical properties of the dynamic network have been significantly improved due to the existence of disulfide cross-linking network with high strength and high cross-linking density, with the tensile strength reaching 92 MPa and the elongation at break reaching 11.9 %. In addition, in the mixed solution of N, N-dimethylformamide/DL-dithiothreitol (DMF/DTT), the resin can be completely degraded to a transparent solution. The construction of dual dynamic crosslinked networks successfully addresses the long-standing dilemma between dynamic performance and mechanical properties in conventional dynamic bond resins, while simultaneously overcoming the recycling and degradation challenges inherent to thermosetting epoxy resins, endowing epoxy dynamic network with more excellent comprehensive performance. The research results can also provide an effective solution for the waste treatment of epoxy resin/carbon fiber composites and has good application potential.