Mechanically Strong Superabsorbent Terpolymer Hydrogels Based on AMPS via Hydrogen-Bonding Interactions

Abstract

Polymers based on 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) attract significant attention due to their large water absorption capacity when swollen in water. Poly(AMPS) (PAMPS) hydrogels are usually synthesized via free-radical cross-linking copolymerization of AMPS and a chemical cross-linker in aqueous solutions. Owing to the covalently cross-linked network structure of PAMPS hydrogels preventing dissipation of the crack energy, they exhibit poor mechanical properties. Herein, we demonstrate that the terpolymerization of AMPS, methacrylic acid (MAAc), and N,N-dimethylacrylamide (DMAA) in an aqueous solution under UV light without a chemical cross-linker produces mechanically strong hydrogen-bonded hydrogels that are durable in water. The terpolymer hydrogels formed at a MAAc/DMAA molar ratio of 4:1 exhibit a high Young’s modulus (26 ± 2 MPa) and toughness (31 ± 5 MJ·m-3) and are able to absorb 2035 ± 255 times their mass in water without dissolving. The water content at the gel preparation, denoted by w, significantly affects the microstructure of terpolymer hydrogels. Decreasing the water content w at gelation increases the length of the primary chains forming the three-dimensional (3D) network and hence the number of interchain H-bonds due to the proximity effect. An optically transparent-to-opaque transition accompanied with a strong-to-weak transition in the mechanical properties was detected with increasing w due to the transformation of the uniform network into a colloidal network composed of phase-separated and highly hydrogen-bonded AMPS-poor aggregates interconnected by AMPS-rich terpolymer chains.