Toughness and Thermoresponsive Hydrogel for Sandwich Smart Window with Adaptive Solar Modulation and Energy Saving.

Thermochromic hydrogels with self-regulating solar transmittance are gaining increasing attention due to their significant potential in the fields of smart windows and energy conservation. Smart windows incorporating viscosity-tough hydrogels as an interlayer exhibit enhanced advantages in resisting external forces. In this study, a tough and thermoresponsive composite hydrogel was developed by incorporating poly(N-isopropylacrylamide) nanoparticles (PNIPAM NPs) and W-doped VO₂ into a polyacrylamide-agar (PAM-Agar) double network hydrogel. Upon solar irradiation, thermochromism of PNIPAM NPs could regulate the visible light transmittance of the composite hydrogel and the photothermal effect of W-VO₂ contributes to the optical regulation and NIR shielding. The smart window, with the composite hydrogel as an interlayer, demonstrates excellent optical modulation capabilities, with a luminous transmittance (T_um(20 °C)) of 86.81%, high light modulation (ΔT_um = 78.89%), a high solar modulation (T_sol) of 83.59%, and a lower critical solution temperature (LCST) of 32.6 °C. The composite hydrogel's superior toughness (0.215 MJ/m³) also enhances the impact resistance of the smart window glass. Additionally, the adhesion between the hydrogel and the glass, with a maximum peeling force of up to 151 N/m (attributed to interactions between the amide groups and the silicon hydroxyl groups), was confirmed through a falling ball experiment. Moreover, the hydrogel exhibits a certain degree of thermal insulation, further promoting its utility in energy-saving applications. In conclusion, this study highlights the significant potential of such composite hydrogels in the development of smart windows for energy-efficient buildings.