Mechanical Performance of Polyampholyte Hydrogels Influenced by Ionic Bond Strength under Isochoric Conditions

Hydrogels composed of hierarchical structures are notable for their exceptional strength and toughness. Understanding the toughening mechanisms associated with these hierarchical structures is essential for the application and design of tough soft materials. Using polyampholyte hydrogels as a model system, this study introduces a novel approach that combines electrolyte-induced ionic bond dissociation with PEG-induced osmotic stress to systematically investigate how ionic bond strength and phase separation influence mechanical properties under isochoric conditions. We reveal that increasing electrolyte concentrations induces a structural transition from strong bicontinuous phase-separated networks to a homogeneous structure and eventually to weak bicontinuous phase-separated networks, accompanied by characteristic relaxation times that initially decrease and subsequently increase. This transition leads to abnormal nonmonotonic changes in mechanical properties. We further elucidate that the nonmonotonic behavior in fracture stress and work of extension to fracture, along with self-recovery dynamics, is governed by the phase-separated structural transition, while variations in fracture energy are primarily related to the dynamics of ionic bonds. This work provides valuable insights into the design of tough soft materials through the modulation of the strengths of physical associations.