Molecular engineering of backbone rotation in an energy-dissipative hydrogel for combining ultra-high stiffness and toughness

Hydrogels hold great promise for various applications, from soft robotics to electrolytes in energy storage devices. However, their mechanical strength, stiffness, and toughness are inherently limited, and due to their mutually exclusive nature, it is rare to find reports on the enhancement of both stiffness and toughness properties simultaneously. This study introduces a novel strategy termed "Hofmeister effect induced Arrested chain Rotation and energy Dissipation" (HARD), which synergistically combines ultra-high stiffness and toughness in hydrogels. As a representative example, a dual-cross-linked hydrogel demonstrated an increase in stiffness by ∼7000 fold to 326 MPa and toughness by ∼200 fold to 25.5 MJ m^-3, compared to the corresponding chemically cross-linked hydrogel. It is further elucidated that the Hofmeister effect immobilizes the polymer segmental motion by restricting backbone rotation, utilizing the hydrophobic pendant methyl groups, while the secondary cross-links function as energy-dissipating elements. The synergistic stress transfer from the primary network to the secondary cross-links effectively integrates these typically opposing mechanical traits. Additionally, we applied the HARD strategy to a double-network hydrogel system, demonstrating its versatility and broad applicability. The dynamic and highly tunable mechanical enhancement makes this strategy a powerful tool for advancing hydrogel design across various applications, as demonstrated by case studies on shape recovery and anti-freezing properties.