Tunable thermoresponsive starch-based superabsorbent polymers for temperature-triggered water desorption in alkaline...

Corn starch is an abundant natural biological macromolecule with hydroxyl rich chains, making it a renewable backbone for constructing functional superabsorbent polymers. In this study, a thermoresponsive starch-based superabsorbent polymer (TR-SAP) was synthesized by grafting acrylic acid and N-isopropylacrylamide (NIPAM) onto corn starch. Its structural characteristics, composition-dependent absorption behavior, volume phase transition temperature (VPTT) regulation, temperature-induced water desorption, and functional response in alkaline ionic cementitious environments were investigated. The results show that acrylic acid and NIPAM were grafted onto the starch backbone, disrupting the crystalline regions of native starch and forming an amorphous crosslinked macromolecular network. The TR-SAPs with different compositions exhibited high monomer conversions, while the grafting ratio initially increased and then decreased with increasing NIPAM content, reaching the highest value for TR-SAP-65 at a NIPAM content of 65%. The most pronounced thermoresponsive behavior was observed at NIPAM contents of 65-80%. When the ambient temperature exceeded the VPTT, polymer-water hydrogen bonding weakened, whereas hydrophobic association among the isopropyl groups became dominant, causing the swollen three dimensional network to contract and desorb the absorbed water, with up to 68.8% of the stored water being desorbed. After 10 swelling-deswelling cycles, TR-SAP-65 recovered 93.8% of its initial water absorption at low temperature, while its cumulative water-desorption ratio reached 56.8% upon reaching 60 °C. When the VPTT of TR-SAP was matched to the ambient temperature, temperature-triggered water desorption was achieved, thereby regulating hydration heat evolution and hydration-product formation during cement hydration.