Unique crack propagation of double network hydrogels under high stretch

Double network (DN) gels, consisting of two contrasting interpenetrated polymer networks, exhibit large resistances to crack initiation and propagation. For practical applications, crack resistances of materials in highly stretched states are important. In this study, we investigated the crack growth behaviors of DN gels in stretched states by inducing crack seeds into these gels under various degrees of tension. This examination enables the analysis of crack propagation for a wide range of bulk energy release rate G. The power-law elationship between G and crack growth velocity v was investigated for DN gels with different tensile behaviors. We found that for brittle and unnecking DN gels, the velocities changed from slow mode (quasi-stationary fracture) to fast mode (dynamic fracture) with an increase in G, similar to that of a single network gel; in contrast, for necking DN gels having higher crack resistances than those of the unnecking DN gels, only fast modes were observed once the G is above a threshold. Real-time birefringence observation reveals a large damage zone around the crack tip at G slightly lower than the threshold, while the damage zone is hardly observed at G higher than the threshold. The results indicate that, for the necking DN gels, crack initiation has a large energy barrier owing to the formation of the damage zone; once this barrier is overcome, the excess energy release accelerates the crack propagation and therefore the gels exhibit dynamic fracture.