A local damage model for anomalous high toughness of double-network gels

We present a phenomenological model for anomalously high fracture energy of double-network (DN) gels, which consist of a substantially cross-linked polyelectrolyte gel (first network) and of a quite poorly cross-linked neutral polymer (second network) penetrating into the first network (Gong J. P., Katsuyama Y., Kurokawa T. and Osada Y., Adv. Mater., 15 (2003) 1155). The model assumes that the material locally softens around crack tip due to damage of the first network, and then the crack extends within the softened zone. An order estimation indicates that energy dissipation by the softening greatly exceeds the "bare fracture energy" of the softened material, and that the effective fracture energy can reach the order of 100 J/m2. This is consistent with the experimental value ∼400 J/m2.