Effect of the Second Network on First Network Rupture and the Origin of Energy Dissipation in Double Network Hydrogels

The detection of mechanoradicals generated by homolytic bond scission enables the quantitative determination of the polymer network rupture during deformation in hydrogels. In this study, we investigate, in double network (DN) hydrogels, how the stretchable second network affects the stress distribution and rupture of the first network and how it contributes to energy dissipation by correlating bond scission with mechanical hysteresis. By increasing the second network density while keeping the first network constant, the tensile behavior of DN gels transitions from typical DN characteristics, exhibiting yielding with necking and strain-hardening, to an oversupported state, showing yielding and strain-hardening without necking, accompanied by reduced bond scission in the first network. Notably, the first network bond scission occurs extensively beyond the yield point and saturates at large strains, irrespective of necking behavior. Furthermore, the energy dissipated per bond cleavage remains constant in the necking regime but increases significantly in the strain-hardening regime and rises with higher second network density, indicating dissipation in the second network through internetwork entanglements. These trends are consistent across DN gels with both weak and strong cross-linkers in the first network, with the weak cross-linker leading to greater bond scission. This study not only deepens the understanding of the molecular mechanism underlying DN materials but also assists in designing mechanoresponsive functional materials based on DN mechanochemistry.