Revisiting the Origins of the Fracture Energy of Tough Double-Network Hydrogels with Quantitative Mechanochemical Characterization of the Damage Zone

The high fracture energy of tough soft materials can be attributed to the large energy dissipation zone around the crack tip. Hence, quantitative characterization of energy dissipation is the key to soft matter fracture mechanics. In this study, we quantified the energy dissipation in the damage zone of a double-network (DN) hydrogel using a mechanochemical technique based on mechanoradical polymerization combined with confocal fluorescence microscopy. We found that, in addition to energy dissipation in a relatively narrow yield region, the dissipation in the wide preyielding region and the intrinsic fracture energy also has a large contribution to the fracture energy. Moreover, the fracture energy of a prestretched sample, in which the dissipative capacity is nearly depleted, suggests that the intrinsic fracture energy is higher than the fracture energy of the second network. These findings modify the previous understanding that the fracture energy of DN gels is dominated by the energy of the yielding zone formation.