Tensile Behaviors of Double Network Hydrogels with Varied First Network Topological and Chemical Structures

In this work, we systematically investigated how the topological and chemical structures of the first network affect the tensile behaviors of double network hydrogels. By varying the monomer type, concentration, and cross-linking density of the first network while keeping the second network’s structure constant, we identified four distinct categories in tensile behavior. These categories arise from changes in the topological structure of the first network, which alters the mechanical balance of the two networks, independent of the first network’s chemical composition. Universal scaling relationships were observed for the strand density of the first network, including yield stress, yield stretch ratio, plateau stress, stretch ratio at strain-hardening, degree of strain-hardening, and the fracture stress. These findings suggest that ductile DN gels are defect-insensitive, with both internal and final fractures determined by their average structures. We also estimated critical number density ratios of the two network strands required for different mechanical responses. This insight allows us to predict the mechanical behavior of DN materials based on these structural averages. The established quantitative link between the topological structure of the networks and the mechanical properties of DN gels opens possibilities for customized applications.