Crack Tip Field of a Double-Network Gel: Visualization of Covalent Bond Scission through Mechanoradical Polymerization

Quantitative characterization of the energy dissipation zone around a crack tip is the focal point in the fracture mechanics of soft materials. In this report, we present a mechanochemical technique for the visualization and quantification of the degree of polymer strand scission in the damage zone of tough double-network hydrogels. This technique uses mechanoradicals generated by covalent bond scission to initiate radical polymerization, which records the internal fracturing around the crack tip during crack opening or propagation. We adopted the mechanoradical polymerization of N-isopropylacrylamide, which forms a thermoresponsive polymer and whose distribution was visualized using an environment-responsive fluorescent probe. Two- and three-dimensional damage distributions were captured using a laser scanning confocal microscope. This technique also allowed for the quantitative estimation of the spatial distribution of stress, strain, and energy dissipation around the crack tip. The advantages and limitations of this technique are also discussed.