Physics

ABSTRACT Hydrogels are increasingly explored as structural candidates for impact‐mitigation and protection systems because of their biocompatibility, softness, and design versatility. However, conventional hydrogels typically suffer from inadequate mechanical properties and environmental instability, which severely hamper their practical applications. Here we report the use of quaternary ammonium‐regulated networks to overcome these limitations, without the need for cumbersome postprocessing or high energy consumption. The approach enables controlled structural organization in polymers containing regularly spaced polar groups (e.g., polyvinyl alcohol, poly(acrylic acid), and polyacrylamide), transforming large, densely packed rigid domains into dynamic, adaptable supramolecular architectures. This arises from strong quaternary‐ammonium–polymer associations that restrict local chain mobility and promote ordered segmental packing. Consequently, by tuning polymer composition, the hydrogels have widely tunable mechanics from brittle to ductile behavior, with ultimate stresses of 32.9–101.9 MPa, strains of 29%–1670%, and toughness values of 22.7–434.1 MJ m − 3 . They also show exceptional impact‐resistant performance (426.7 MPa), combined with high energy dissipation (96.02%) and puncture resistance (1.3 J), which compares favorably to those of other tough hydrogels and even natural materials. The presented strategy is generalizable to other polymers, and could expand the applicability of structural hydrogels to more mechanically demanding conditions.

We study auxetic lattice structures with curved bi‐material ligaments using the finite element method. The overall Poisson's ratio and coefficient of thermal expansion of the lattices can be simultaneously tuned to be negative by adjusting their microstructural geometries and constituent material parameters. The Young's modulus of ligaments plays an important role in controlling the effective Poisson's ratio and coefficient of thermal expansion. The size and Young's modulus of the joints that connect ligaments strongly affect the effective mechanical properties. When Young's modulus of the joint is given, a larger joint size gives rise to stronger auxeticity, but less negative thermal expansion. By tuning the coefficient of thermal expansion of each constituent, the overall coefficient of thermal expansion may be negative, zero, or positive. The bicamaterial auxetic structures studied here may be designed to possess desired properties for real‐world applications when their microstructural properties are appropriately tuned.