Quantitative characterization of the energy dissipative zone around the crack tip is the central issue in fracture mechanics of soft materials. In this research, we present a mechanochemical technique to visualize the bond scission of the first network in the damage zone of tough double-network hydrogels. The mechanoradicals generated by polymer chain scission are employed to initiate polymerization of a thermoresponsive polymer, which is visualized by a fluorophore. This technique records the spatial distribution of internal fracturing from the fractured surface to the bulk, which provides the spatial profiles of stress, strain, and energy dissipation around the crack-tip. The characterized results suggest that, in addition to the dissipation in relatively narrow yielded zone which is mostly focused in the previous works, the dissipation in wide pre-yielding zone and the intrinsic fracture energy have also significant contribution to the fracture energy of a DN gel.
Crack tip field of a double-network gel (SI)