Hydrogels, characterized by their 3D cross-linked hydrophilic polymer networks, are capable of retaining a significant amount of water without dissolution. However, the high water content often results in low mechanical strength and toughness, posing challenges for practical applications such as soft robotics, tissue engineering, and drug delivery. Double-network hydrogels (DNHs), featuring two interpenetrating networks with contrasting characteristics, effectively address this issue. During deformation, the tightly cross-linked brittle network breaks first, while the loosely cross-linked ductile network remains intact. The internal fracture of the first network dissipates a large amount of energy before the second network carries the load, significantly enhancing the toughness of the material.

In this study, DNHs comprising poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) as the rigid first network and polyacrylamide (PAAm) as the ductile second network were prepared. The synthesis was adapted from Gong's previous methodology with a reduced crosslinking time, and the resulting hydrogels were subsequently evaluated through extensive mechanical testing.

Under cyclic uniaxial tension, three characteristic regions of tough DNH can be observed, encompassing the pre-necking, necking, and hardening phases. The hysteresis loops observed in all three regions reflect stress softening and energy dissipation. Multi-stage relaxation tests within a small-strain range further demonstrate the progressive fracture of the first network, as stress relaxation during the strain-holding period becomes more pronounced with increasing strain levels, while remaining minimal during unloading and subsequent reloading cycles.

The multiple tension test regimes can be complemented by confocal laser scanning microscopy, X-ray scattering, and digital image correlation to enable a more comprehensive investigation of internal fractures and to provide detailed insights into the localized microstructural damage within the material.