Effect of Predamage on the Fracture Energy of Double-Network Hydrogels.

Double-network (DN) hydrogels are tough soft materials, and the high fracture resistance can be attributed to the formation of a large damage zone (internal fracture of the brittle first network) around the crack tip. In this work, we studied the effect of predamage in the brittle network on the fracture energy Γ c of DN hydrogels. The prestretch of the first network was induced by prestretching the DN gels to prestretch ratio λ pre . Depending on the λ pre in relative to the yielding stretch ratio λ y , above which the brittle first network starts to break into discontinuous fragments inside DN gels, two regimes were observed: Γ c decreases monotonically with λ pre in the regime of λ pre < λ y , mainly due to the decreasing contribution from the bulk internal damage, while Γ c increases with λ pre in the regime of λ pre > λ y . The latter can be understood by the release of the hidden length of the stretchable network strands by the rupture of the brittle network, whereby the broken fragments of the brittle network could serve as sliding cross-links to further delocalize the stress-concentration near the crack tip and prevent chain scissions.