Toughening Weak Polyampholyte Hydrogels with Weak Chain Entanglements via a Secondary Equilibrium Approach

Polyampholyte (PA) hydrogels are randomly copolymerized from anionic and cationic monomers, showing good mechanical properties owing to the existence of numerous ionic bonds in the networks. However, relatively tough PA gels can be synthesized successfully only at high monomer concentrations (C_M), where relatively strong chain entanglements exist to stabilize the primary supramolecular networks. This study aims to toughen weak PA gels with relatively weak primary topological entanglements (at relatively low C_M) via a secondary equilibrium approach. According to this approach, an as-prepared PA gel is first dialyzed in a FeCl₃ solution to reach a swelling equilibrium and then dialyzed in sufficient deionized water to remove excess free ions to achieve a new equilibrium, resulting in the modified PA gels. It is proved that the modified PA gels are eventually constructed by both ionic and metal coordination bonds, which could synergistically enhance the chain interactions and enable the network toughening. Systematic studies indicate that both C_M and FeCl₃ concentration (CFeCl3) influence the enhancement effectiveness of the modified PA gels, although all the gels could be dramatically enhanced. The mechanical properties of the modified PA gel could be optimized at C_M = 2.0 M and CFeCl3 = 0.3 M, where the Young's modulus, tensile fracture strength, and work of tension are improved by 1800%, 600%, and 820%, respectively, comparing to these of the original PA gel. By selecting a different PA gel system and diverse metal ions (i.e., Al³⁺, Mg²⁺, Ca²⁺), we further prove that the proposed approach is generally appliable. A theoretical model is used to understand the toughening mechanism. This work well extends the simple yet general approach for the toughening of weak PA gels with relatively weak chain entanglements.