Locking of poly (sodium acrylate) hydrogels for underwater ultrastretchability but overwater non-flexibility

Abstract

Although water is an essential component of hydrogels, developing hydrogels showing distinct deformability depending on the surroundings remains interesting and challenge. Herein, a structural remolding strategy of hydrophilic poly (sodium acrylate) is proposed to develop anti-swelling hydrogels, which interestingly show a significant and reversible mechanical switch of underwater ultra-stretchability and overwater non-flexibility. The super anti-swelling hydrogels are derived based on super-swelling poly (sodium acrylate) hydrogels by simply immersing into 0.2~0.8 mol L-1 CaCl2 solutions to form strong Ca2+-carboxyl coordination. The strong Ca2+-carboxyl coordination in hydrogels serves as locks, which bundles neighboring poly (sodium acrylate) chains to resist water intrusion into polymer network for structural maintenance at underwater conditions, as well as constructs loose porous structures exposing inside water to surroundings. The obtained hydrogels with coordination locked networks show long-term stability, self-healing, and ultra-high stretchability of ~30 with equilibrium-water-content ~70 wt% at versatile underwater conditions, and achieve rapid dehydration and loss of flexibility within 30 mins at overwater conditions. Owing to the unique switchable characteristics, the hydrogels demonstrate multiple encrypted functionalities applied only for underwater conditions. The work here deepens understanding of poly (sodium acrylate) hydrogels, and paves a new way on design and remolding of hydrogel topology tailoring for underwater-only functions.