Fracture of the Physically Cross-Linked First Network in Hybrid Double Network Hydrogels

Fundamental understanding of the fracture process and toughening mechanisms of double network (DN) hydrogels is critical for rational design of the next generation of tough DN gels with desirable mechanical properties. However, current knowledge of DN gels from synthesis methods to toughening mechanisms mainly comes from chemically cross-linked DN gels. Little is known about hybrid physically chemically cross-linked DN gels. Herein, we synthesize tough DN hydrogels by combining two types of cross-linked polymer networks: a physically cross-linked first network of agar and a covalently cross-linked second network of polyacrylamide (PAM). The resulting Agar/PAM DN gels achieved high toughness of 500-1000 J/m(2). More importantly, we reveal several differences and similarities between hybrid Agar/PAM DN gels and chemically linked PAMPS/PAM DN gels. First, different from the nearly velocity-independent mechanical properties in chemically linked DN gels, hybrid Agar/PAM DN gels show velocity-dependent fracture behaviors and toughness. Second, successive cyclic loading-unloading tests indicate the continuous fracture of the first agar network, instead of a phase transition from continuous to discontinuous in chemically linked DN gels. Third, Agar/PAM DN gels exhibit different yielding and necking behaviors from chemically linked DN gels, including much lower yielding stress/strain, no stable necking platform, and simultaneous necking. We thus propose a chain pulling-out model to interpret the continuous fracture process of the first agar network and associated energy dissipation mechanism for hybrid Agar/PAM DN gels. This work strives to provide a better fundamental understanding of structure-property relationship of DN gels, which help to develop new DN gels with desirable properties.