Synergistically Toughening Non-Neutral Polyampholyte Hydrogels by Ionic and Coordination Bonds at Low Metal-Ion Contents

Polyampholyte (PA) hydrogels, composed of charged hydrophilic networks with both positive and negative groups, have attracted great attention due to the unique structure and excellent antifouling properties. Yet, the superhydrophilicity usually makes non-neutral PA (n-PA) gels highly swollen and mechanically very weak in aqueous environments, severely limiting their applications. Herein metal-coordination bonds are designed to introduce to synergistically toughen n-PA hydrogels with ionic bonds via a secondary equilibrium strategy. In the design, as-prepared n-PA gels are dialyzed in metal-ion solutions and deionized water in sequence to achieve the tough gels. Through this strategy, the weak n-PA gels can be significantly toughened by the synergy of ionic and metal-coordination bonds. A systematic study indicates that both the molar ratio of oppositely charged monomers and the metal-ion concentration affect the mechanical enhancements clearly. The universality of the proposed strategy is further proved by selecting different gel systems and multivalent metal ions. Notably, low metal-ion concentrations (≤0.1 m) of dialysis solutions can enable synergistic toughening. Theoretical models are also adopted to disclose the toughening mechanism. This work not only expands the understanding on the fabrication of strong and tough PA hydrogels but also provides some insights for PA gels in electrolyte solutions.