Agarose/PNIPAAm semi-interpenetrating network hydrogels with enhanced mechanical and optical properties for thermoregulating smart windows

We report a novel semi-interpenetrating network (s-IPN) hydrogel fabricated using a simple diffusion method that incorporates poly(<i>N</i>-isopropylacrylamide) (PNIPAAm) into agarose matrices. The agarose serves as a structural framework while PNIPAAm provides thermoresponsive capability, creating a straightforward, stable, and thermally responsive material for practical applications. This approach notably reduces volume shrinkage from 80-90% (typical of pure PNIPAAm) to approximately 12%, corresponding to only 4% linear thermal contraction, while preserving complete thermoresponsive functionality. The optimized composition (2% agarose/8% PNIPAAm) exhibits approximately 90% visible light transmittance at room temperature while becoming opaque above its lower critical solution temperature (LCST) of 32.1 °C. Thermogravimetric analysis and FTIR spectroscopy reveal enhanced thermal stability and molecular interactions between the agarose and PNIPAAm networks through hydrogen bonding. The properties of the PNIPAAm-agarose s-IPN hydrogel can be systematically controlled by simply adjusting the concentration of each polymer, enabling customization of the smart hydrogel properties. When incorporated into a glass-polymer-glass sandwich structure, these s-IPN hydrogels function effectively as smart window materials, providing autonomous temperature regulation by modulating solar transmittance in response to temperature changes.