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,…