Hybrid Chemomechanical Yielding of Covalent Adaptable Networks for Toughening Ionic Semi-Crystalline Epoxy

Covalent adaptable network (CAN) in thermoset epoxy shows good recyclability due to its bond exchange reactions (BERs), which result in low mechanical strength and toughness. Many experimental studies on using the semi-crystalline structures and ions with the CAN have shown to improve the mechanical strength and toughness of these thermoset epoxies. However, their toughening mechanisms and coupling effects have not been well understood. Herein, we propose an extended Maxwell model to explore toughening mechanisms and coupling effects of the semi-crystallization process and formation of ion phases on CANs, which enables the thermoset epoxy with hybrid chemomechanical yielding. A constitutive stress-strain relationship is formulated to characterize the mechanical toughening and hybrid chemomechanical yielding behavior, where the first stage of yielding is originated from the effect of ions on amorphous phases, and the second stage of yielding is due to the coupling effect of semi-crystallization and formation of ions. The proposed models have been verified using the experimental results, and provide a fundamental approach to understanding the toughening mechanisms and coupling effects of semi-crystallization and ions in thermoset epoxy.