Ferric Salt-MediatedTough Zwitterionic Hydrogel

Polyzwitterionic gels possess a high density of ionic groups and present great potential in ionotronics. However, zwitterionic chains are easy to be highly hydrated, leading to weak interchain interactions and a poor mechanical performance. Although metal ions like Fe³⁺ show great effectiveness in toughening hydrogels, they usually weaken the zwitterionic gel inversely due to the anti-polyelectrolyte effect. Herein, a special co-ion effect of Fe(ClO₄)₃ is found, which is adopted to toughen the zwitterionic hydrogel. In contrast to the weak effect of Fe(NO₃)₃, FeBr₃, and FeCl₃ salts on gel toughness, Fe(ClO₄)₃ can toughen the imidazolium-based polyzwitterion (pVBIPS) effectively. The Fe(ClO₄)₃-treated pVBIPS gel exhibits a high ultimate tensile stress of 1.34 MPa, a Young’s modulus of 1.2 MPa, and tensile work of 2.29 MJ/m³, surpassing many existing pure zwitterionic hydrogels. It is found that the ClO₄^– ions can effectively bind to the imidazolium moiety of pVBIPS and promote the formation of SO₃^––Fe³⁺ interactions; the trivalent Fe³⁺ ions can then bridge multiple sulfonate groups to impart the strength. This work should offer a new strategy for developing tough zwitterionic hydrogels.