Thermal and solvent responsive hydrogels for active–passive dual-control smart windows

Hydrogel smart windows with light modulation capability have received widespread attention because they can meet the requirements of indoor lighting at daytime, privacy protection at night and energy saving. However, it remains challenging to achieve these multiple goals simultaneously and enable active actuation to meet the temporary needs of users. In this study, a thermal and solvent responsive hydrogel smart window was developed by introducing hydroxypropyl cellulose (HPC) and sodium dodecyl sulfate (SDS) micelles into a polyacrylamide (PAAm) cross-linked network. By adding different concentrations of NaCl solution, the hydrogels underwent upper critical solution temperature (UCST)- and lower critical solution temperature (LCST)-type phase transitions in the ranges of 1 − 15 °C and 25 − 80 °C, respectively, which enabled the hydrogel to achieve a reversible transition between opacity and transparency with temperature change. The smart window exhibited an excellent solar modulation ability (T_lum = 91.39%, ΔT_sol,_15−5℃ = 68.67%, ΔT_sol,_15−40℃ = 62.50%) and effective IR shielding. Furthermore, actively actuated light modulation of hydrogel smart windows was achieved by spraying ethanol on the hydrogel surface. Ethanol stimulation and volatilization induced the reconfiguration and dissociation of hydrogen bonds between HPC and PAAm molecular chains, resulting in reversible transparency transitions. We anticipate that this facile strategy for preparing thermal and solvent responsive hydrogels could serve as a promising alternative for designing phase change hydrogels for a variety of applications.