Weak Coordination-Induced Dynamic Crosslinked, Adjustable Strength, Reversible Adhesion Supramolecular Hydrogel

Natural soft materials can generate a reversible transition between soft and tough states triggered by environmental stimuli. Inspired by this, various synthetic hydrogels with dynamic reversible crosslinks are developed. In this study, a weak coordination-induced dynamic crosslinked hydrogel is successfully synthesized. High concentrations of Ca2+ ions are coordinated with acrylamide, which then catalyze ammonium persulfate decomposition for rapid polymerization to a polyacrylamide (PAAm)/Ca hydrogel at room temperature. The preassembled coordination provides sufficient crosslinks, accompanied by enhanced hydrogen bonding during dehydration, which together endows the gel with high mechanical properties. This weak coordinated crosslinked structure is affected by humidity, leading to a wide range of mechanical properties of the gel, with the tensile strength ranging from 44.2 kPa to 5.6 MPa, elastic modulus from 17.3 kPa to 2.2 MPa, and toughness from 312.3 kJ/m3 to 7.8 MJ/m3. Additionally, Ca2+ ions preferentially coordinate with amides in the hydration process, and the gel exhibits an abnormal hydration phenomenon under low humidity. The gel with high polymer content has a higher water loss ratio and volume shrinkage. On this basis, we design an asymmetric bilayer structure gel and realize the curling and stretching of leaves and petals. Interestingly, the bilayer gel exhibits reversible adhesion properties and the tearing energy is up to 3985.9 J/m2 under low humidity. This principle may provide ideas and inspiration for producing hydrogels with adjustable mechanical strength and develop a route to construct versatile stimulus-responsive materials.