Dynamic fracture and catastrophic crack branching in highly entangled hydrogels.

Despite extensive research on advanced soft materials, how fracture resistance evolves under dynamic loading and how cracks propagate remain incompletely understood. Here we show that dynamic fracture in long-chain, highly entangled polymer hydrogel networks with near-perfect elasticity is governed by localized viscous dissipation at the crack tip. Using highly entangled gels as a model system, we systematically vary network and loading parameters to evaluate the effects of loading rate, network topology, and solvent viscosity on fracture behavior. We find that, due to viscoelastic dissipation, the dynamic fracture toughness can be significantly lower than the quasi-static fracture toughness. Cracks can propagate at velocities approaching the shear-wave speed and undergo pronounced, unconventional branching. We develop a shear-lag model to describe the dissipation mechanism. The model predicts that the fracture energy scales inversely with a Weissenberg-like number ([Formula: see text]) that combines strain rate, chain length, mesh size, and solvent viscosity, and it recovers the Lake-Thomas limit at low values of [Formula: see text]. These results reveal how localized viscous dissipation governs fracture in highly elastic, entangled hydrogels and provide design principles for improving failure resistance in soft materials and devices operating under dynamic loading.