Dehydration-rehydration-driven reinforcement and toughening of hydrogels based on weakly associated polyion pairs

Constructing polyion complex (PIC) networks with dense electrostatic interactions is a well-established strategy to enhance the strength and toughness of hydrogels. However, for polyion combinations with weak association energy, the construction of robust hydrogel networks is typically challenging. In this work, a PIC hydrogel integrating electrostatic and hydrogen bonding interactions was fabricated by polymerizing acrylic acid (AA) and acrylamide (AM) in a partially deprotonated polyallylamine hydrochloride (DPAH) solution. Subsequent dehydration-rehydration treatment enabled densification of the network structure. It is revealed that the dehydration-rehydration process not only reinforces the ionic cross-linked network and significantly elevates the network's flow activation energy but also enhances the hydrogel's hydrophobicity via enhanced ion-pairing interactions. Conversely, hydrogen bonding interactions from the introduced amide groups disrupt the originally dense intermolecular electrostatic interactions and accelerate the relaxation kinetics of the polymeric network. After dehydration-rehydration treatment, the tensile strength of the hydrogel was enhanced by approximately 32-fold, reaching ∼10 MPa, and the fracture toughness was elevated from about 0.05 to ∼11.61 MJ m^-3, demonstrating ideal toughening and strengthening effects. Owing to its superior anti-swelling property derived from the enhanced hydrophobicity, the hydrogel can be well applied as flexible sensors for real-time monitoring of underwater motion, exhibiting robust signal transmission stability. This work presents a facile and novel strategy to diversify high-performance PIC hydrogel systems, especially those with inherently weak association energy, thus providing a new perspective for the design of hydrogel platforms for underwater applications.