CO₂-Triggered Hydrogelation: Architectural Insights from Random and Graft Polyelectrolytes

The sol–gel transition of CO2-responsive polyelectrolytes is driven by electrostatic interactions between anionic groups and protonated cationic moieties, yet the critical roles of ionic stoichiometry and spatial architecture in mediating gelation behavior remain poorly understood. To address this knowledge gap, two distinct copolymer series with varying configurations and anion/CO2-responsive monomer ratios were synthesized: P(AA-co-DPM) random copolymers via free radical copolymerization of acrylamide, acrylic acid, and dimethylaminopropyl methacrylamide (DMAPMAm), while PAA-g-PDPM graft copolymers through the covalent attachment of poly(DMAPMAm) side chains to poly(AM-co-AA) backbones. CO2-induced protonation led to contrasting phase behaviors contingent on ionic balance. For P(AA-co-DPM) aqueous solution, viscosity decreased when nDMAPMAm ≤ 0.83nNaAA, increased when nDMAPMAm ≥ 1.13 nNaAA, and resulted in precipitation when nDMAPMAm = 0.83nNaAA. Conversely, the PAA-g-PDPM aqueous solution exhibited continuous viscosity enhancement up to nDMAPMAm ≤ 0.65nNaAA, followed by dehydration at higher cationic ratios. Notably, the graft architecture demonstrated superior gelation under CO2 exposure, forming robust hydrogels with frequency-independent modulus (0.01–0.1 rad·s–1) and minimal energy dissipation (tan δ < 0.1). In contrast, random copolymers yielded fragile networks displaying frequency-dependent moduli and elevated tan δ (>0.1). Rheology-small angle neutron scattering analysis revealed that graft copolymers underwent CO2-induced chain collapse from swollen to compact conformations, creating physical cross-links, whereas random chains maintained Gaussian statistics regardless of protonation state. These findings highlight that extended cationic side chains in graft architectures enhance intermolecular entanglement and directional electrostatic interactions, offering a strategic approach for designing CO2-responsive hydrogels with tailored mechanical properties through molecular architecture engineering.