Design of tough hydrogels has made great progress in the past two decades. However, the synthetic tough gels are usually much softer than some biotissues (e.g., skins with modulus up to 100 MPa). Here we report a new class of ultrastiff and tough supramolecular hydrogels facilely prepared by copolymerization of methacrylic acid and methacrylamide. The gels with water content of approximately 50–70 wt % possessed remarkable mechanical properties, with Young’s modulus of 2.3–217.3 MPa, tensile breaking stress of 1.2–8.3 MPa, breaking strain of 200–620%, and tearing fracture energy of 2.9–23.5 kJ/m2, superior to most existing hydrogels, especially in terms of modulus. Typical yielding and crazing were observed in the gel under tensile loading, indicating the forced elastic deformation of these hydrogels in a glassy state, as confirmed by dynamic mechanical analysis. The ultrahigh stiffness was attributed to the dense cross-linking and reduced segmental mobility caused by the robust intra- and interchain hydrogen bonds. Because of the dynamic nature of noncovalent bonds, these supramolecular gels also showed rate-dependent mechanical performances along with good shape memory and recyclability. This strategy should be applicable for other systems toward robust mechanical properties, versatile functionalities, and promising applications of hydrogel materials as structural elements.

Hydrogels are usually recognized as soft and weak materials, the poor mechanical properties of which greatly limit their applications as structural elements. Designing of hydrogels with high strength and high modulus has both fundamental and practical significances. Herein we report a series of tough, stiff, and transparent hydrogels facilely prepared by copolymerization of 1-vinylimidazole and methacrylic acid in dimethyl sulfoxide followed by solvent exchange to water. The equilibrated hydrogels with water content of 50–60 wt % possessed excellent mechanical properties, with tensile breaking stress, breaking strain, Young’s modulus, and tearing fracture energy of 1.3–5.4 MPa, 40–330%, 20–170 MPa, and 600–4500 J/m2, respectively. These tough hydrogels were also stable over a wide pH range (2 ≤ pH ≤ 10), resulting from the formation of dense and robust hydrogen bonds between imidazole and carboxylic acid groups. Moreover, the water content and mechanical properties of one gel can be adjusted over a wide range by controlling the dissociation and re-formation of hydrogen bonds during the solvent exchange and heating process; the treated hydrogel with specific characters was stable in water at room temperature. This is because the density of hydrogen bonds can be modulated at high temperature yet immediately fixed at room temperature due to the high stiffness and glassy state of the hydrogel. This strategy to prepare tough and stiff hydrogels should be applicable to other systems as structural materials with promising applications in diverse fields.

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Loss factor tan δ determines the viscoelasticity of a material. Higher or lower loss factor tanδ (>1 or <1) suggests a viscous or elastic material. Most polymer pressure‐sensitive adhesives (PSAs) possess a limited operational temperature range (near room temperature), above which the PSAs trend to be more viscous (un‐crosslinked) or more elastic (crosslinked), and below which PSAs become more elastic. These properties are unfavorable for PSA operation. Herein, an underwater PSA possessing short hydrophobic side chains and weak hydrogen bond interactions are described. Proper modulus and stable loss factor close to 1 contributes to an efficient adhesion underwater over a temperature range of 0–100 °C. Moreover, by introducing Teflon particles, the adhesion can be operated under silicon oil from room temperature to 150 °C due to the formation of a drainage surface structure and its temperature insensitivity.