Dual-function PEG engineering enables room temperature-switchable hydrogel smart windows with ultrahigh solar modulation

Thermochromic hydroxypropyl cellulose (HPC) hydrogel-based smart windows have attracted considerable attention owing to their significant potential to reduce the building energy consumption. Lowering their low critical transition temperature (LCST, >40 °C) to room temperature (∼25 °C) is the primary goal for practical application, but this is always accompanied by a compromise in solar modulation efficiency (Δ T sol ). To address this issue, we proposed a ‘two birds with one stone’ approach: introducing polyethylene glycol (PEG) into hydrogels. By adjusting the content and molecular weight of PEG, LCST could be flexibly controlled from 45.5 to 24 °C by tailoring the hydrogen bonding strength between HPC and water. Meanwhile, the introduction of PEG formed a denser double network structure. This induced more pronounced aggregation upon phase transition, thereby completely blocking near-infrared light and resulting in enhanced NIR modulation efficiency (Δ T NIR ) and Δ T sol . The HPC/PEG composite hydrogel smart window with PEG2000 content of 30 wt% showed optimal overall performance with luminous transmittance ( T lum ) of 92.9 %, Δ T NIR of 73.7 %, Δ T sol of 84.2 % and a low LCST of 26.7 °C. The proof-of-concept experiment demonstrated that the smart window reduced the indoor temperature by 12 °C compared to the blank glass, indicating good energy-saving potential.