Construction and property characterization of PNIPAAm-grafted chitosan/sodium alginate composite Thermo-responsive hydrogel films

A series of thermo-responsive PNIPAAm-CS/SA composite hydrogel films were synthesized via radical polymerization and ionic cross-linking. FTIR, XPS, and NMR confirmed the successful covalent grafting of PNIPAAm onto the chitosan (CS) backbone. XRD and SSNMR analyses revealed that the grafting disrupted the CS crystalline regions, inducing an amorphous structural transition. The PNIPAAm-CS copolymer solutions exhibited LCST values in the range of 30–32 °C. Notably, compared with the corresponding precursor systems, the hydrated films exhibited an apparent thermo-responsive transition onset approximately 1 °C higher in temperature-dependent optical measurements, suggesting that film-network formation influenced the thermal response behavior of the PNIPAAm-containing system. Grafting modification significantly improved optical and mechanical properties: PNIPAAm-CS/SA-4 achieved a peak transmittance of 82.81% (vs. 50.78% for pure CS/SA), while PNIPAAm-CS/SA-2 reached an optimal balance with 22.6 MPa tensile strength and ∼ 100% elongation. Furthermore, antibacterial evaluation of the PNIPAAm-CS copolymers showed that, although the inhibitory activity decreased with increasing PNIPAAm incorporation, measurable antibacterial effects against E. coli and S. aureus were retained. Together with the preliminary package-integrated visual response demonstration, these results suggest that the hydrated films may serve as candidate temperature-responsive indicator materials for further development in intelligent packaging.