Ferric Salt-Mediated Tough 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.