CO₂-Responsive Superabsorbent Hydrogels Capable of >90% Dewatering When Immersed in Water

Superabsorbent polymer (SAP) hydrogels function by absorbing and retaining water. Facile removal of the absorbed water would make it easier to recycle used SAP hydrogels. However, the hydrophilic interior of traditional SAP hydrogels inhibits the desorption of the absorbed fluid. Herein, we report the synthesis and properties of CO2-responsive SAP hydrogels capable of switching from being relatively hydrophobic to hydrophilic in the presence of CO2, and vice versa when the CO2 is removed, accompanied by the simultaneous absorption and expulsion of large volumes of water respectively, mimicking a CO2-directed sponge. The hydrogels studied are synthesized by the free-radical copolymerization of either N-[3-(dimethylamino)propyl]methacrylamide (DMAPMAm) or 2-N-morpholinoethyl methacrylate (MEMA) with N,N′-dimethylacrylamide (DMAAm), which acts as both a monomer and a self-cross-linker. In the presence of CO2, both p(DMAAm-co-DMAPMAm) and p(DMAAm-co-MEMA) gels were able to achieve a maximum swelling ratio (SR) of ∼800, demonstrating that they are superabsorbent. These gels release more than 70% of the absorbed water if they are immersed in noncarbonated water. Repeated swelling/deswelling of the hydrogels (with recovery of the original swelling ratio) in this manner over four cycles demonstrates the reusability of these materials and their potential use in a variety of applications.