Smart Crowding on pH-Induced Elasticity of Weakly Anionic poly(N-Isopropylacrylamide)-Based Semi-Interpenetrating Polymer Networks via Integration of Methacrylic Acid and Linear Polyacrylamide Chains

Weakly anionic semi-interpenetrating polymer networks (semi-IPNs), comprised of copolymer poly(N-isopropylacrylamide-co-methacrylic acid) P(NIPA-MA) and linear poly(acrylamide) (LPA) chains as macromolecular crowding agent, are designed to evaluate pH-induced swelling and elasticity. Uniaxial compression testing after swelling in various pH-conditions is used to analyze the compressive elasticity as a function of swelling pH and LPA-content. The swelling of P(NIPA-MA)/LPA semi-IPNs is strongly pH-dependent due to MA units incorporated into the copolymer network which already exhibits temperature-sensitivity by presence of PNIPA counterpart. Since the behavior of semi-IPNs is a combination of PMA, LPA, and PNIPA moieties, the sensitivity of swelling to external pH can be modified with increasing swelling temperature. At high pH conditions, LPA-doped semi-IPNs show elasticity representing soft and loosely cross-linked structure. Elastic modulus is higher in acidic pH condition due to the less swelling tendency, while in basic pH, the modulus decreases significantly in coordination with swelling. Oscillatory swelling reveals how fast semi-IPNs can respond to environmental pH change (2.1–10.7). By describing adsorption potential of semi-IPNs for cationic methylene blue uptake by pseudo-first-order and Freundlich model, the designed poly(NIPA-MA)/LPA semi-IPNs emerge as promising smart materials in applications requiring rapid response to changes in temperature and pH via diffusional properties.