Interpenetrating Dual Epoxy Networks with Reversible Covalent Bonds: Shape-Constant Self-Healing Electrically Insulating Materials

Abstract

Novel dual interpenetrating epoxy networks (IPNs) with highly promising self-healing and electroinsulating properties were synthesized and systematically optimized. These systems display healing of microcracks and electrical tree damage. Their structure consists of a permanent epoxy-amine subnetwork (EP) interpenetrated with a reversible epoxy network (DA), the latter being cross-linked via Diels–Alder chemistry, through the reaction of maleimide and furan groups. The DA subnetwork provides excellent thermally triggered self-healing, while the permanent EP network ensures shape stability throughout the healing process. Although damage to the permanent EP subnetwork cannot be reversed, the mechanical and tensile properties of disrupted specimens are almost completely recovered after thermal healing. Both subnetworks incorporate flexible linear poly(propylene oxide) (PPO) segments of different molecular weights (230, 400, and 2000 g/mol). Increasing the PPO chain length significantly reduces the glass transition temperature and the rubbery modulus, while also slightly decreasing the dissociation temperature of the reversible DA subnetwork. Synthesis and healing conditions were carefully designed to suppress undesired internetwork cross-linking side reactions, thereby preserving the reversibility of the DA bonds over repeated healing cycles. The effectiveness of this strategy was confirmed by cyclic thermal analyses and tensile tests conducted on promising healed specimens. Dielectric analysis was performed to assess the suitability of the developed IPNs for use in electrical insulation applications. The highest self-healing efficiency was achieved for the IPN containing the longest PPO chains, whereas the best dielectric insulating performance was observed for the shortest PPO chains. The IPN containing PPO chains with a molecular weight of 400 g/mol provided the best balance between self-healing capability and electroinsulating performance, making it the most promising candidate for self-healing electrical insulation applications.