Bulk Energy Dissipation Mechanism for the Fracture of Tough and Self-Healing Hydrogels

Recently, many tough and self-healing hydrogels have been developed based on physical bonds as reversible sacrificial bonds. As breaking and re-forming of physical bonds are time-dependent, these hydrogels are viscoelastic and the deformation rate and temperature pronouncedly influence their fracture behavior. Using a polyampholyte hydrogel as a model system, we observed that the time–temperature superposition principle is obeyed not only for the small strain rheology but also for the large strain hysteresis energy dissipation and the fracture energy below a certain temperature. The three processes possess the same shift factors that obey the equation of Williams, Landel, and Ferry (WLF) time–temperature equivalence. The fracture energy Γ scales with the crack velocity Vc over a wide velocity range as Γ ∼ Vcα (α = 0.21). The exponent α of the power law is well-related to the exponent κ of the relaxation modulus G(t) ∼ t–κ (κ = 0.26), obeying the prediction α = κ/(1 + κ) from classic viscoelasticity theory. These results show that the fracture energy of the polyampholyte gel is dominated by the bulk viscoelastic energy dissipated around the crack tip. This investigation gives an insight into designing tough and self-healing hydrogels and predicting their fracture behaviors from their dynamic mechanical spectrum.