Swelling or deswelling? Composition-dependent CO₂-responsive swelling behaviors of poly(acrylamide-co-2-dimethylaminoethyl methacrylate) hydrogels

Use of CO₂ to realize both swelling and deswelling in a same type of copolymer is a challenging topic. Herein, we synthesized poly(acrylamide-co-2-dimethylaminoethyl methacrylate) (P(AM-co-DMAEMA)) copolymer hydrogels by easy solution copolymerization that showed both swelling and deswelling when exposed to CO₂. Interestingly, the copolymer hydrogels were found to show a convincing CO₂-induced “composition-dependent” swelling behaviors. The as-prepared hydrogels with 4.1 to 23.3 mol% DMAEMA showed a decreased swelling ratio when CO₂ was purged. In contrast, further raising DMAEMA molar ratio from 23.3 to 67.8%, the copolymer hydrogels showed CO₂-induced increased swelling. Insights from NMR and XPS results revealed different characteristics of poly(N,N-dimethylaminoethyl methacrylate in the hydrogel samples with varying monomer compositions, which were considered to be responsible for the composition-dependent swelling behavior. By discovering a simple method to synthesize a copolymer hydrogel which showed CO₂-induced composition-dependent swelling behaviors, this study is conducive to developing CO₂-tunable smart materials.