Large strain hysteresis and Mullins effect of tough double-network hydrogels
Systematic loading and unloading experiments, in uniaxial tension and uniaxial compression, have been performed on a double-network hydrogel exhibiting a very high toughness. We observed a significant hysteresis during the first loading cycle that increased strongly with the applied maximum deformation. A large hysteresis was not observed during a second loading cycle, implying that the initial hysteresis can be attributed to the fracture of covalent bonds in the primary network. We report this type of dissipative mechanism for polymer gels for the first time. Assuming that the entire energy dissipated during the hysteresis cycle can be attributed to the fracture of network strands by a Lake-Thomas mechanism, our results suggest that the fracture and unloading of only 1% of the bonds within the network leads to a decrease of up to 80% of the number of strands. These results also demonstrate the very large degree of heterogeneity within the hydrogel network. If such a dissipative mechanism is active at the crack tip, it will most likely greatly increase the energy necessary to propagate a macroscopic crack, elucidating the origin of the toughness in these interesting materials.