Energy-efficient smart windows based on Na₂SiO₃/PNIPAM-HPC composite hydrogels with tunable phase transition

Abstract Building window glass is one of the main interfaces for solar radiation transmission and energy exchange in buildings. Therefore, the development of intelligent window glass materials that can dynamically adjust the light transmittance and thermal insulation performance is an effective way to reduce building energy consumption and improve indoor thermal comfort. However, the existing thermochromic smart windows are still difficult to achieve collaborative optimization between high visible light transmittance, strong solar modulation capability, suitable phase transition temperature and long-term structural stability. In order to solve this problem, a thermal response hydrogel layer was introduced between two glass substrates by sandwich assembly, and a Na2SiO3/ poly-N-isopropylacrylamide (PNIPAM)-hydroxypropyl cellulose (HPC) composite hydrogel smart window was constructed. Among them, the PNIPAM-HPC composite hydrogel has both the temperature response characteristics of PNIPAM and the network enhancement effect of HPC: PNIPAM undergoes a reversible hydrophilic /hydrophobic transition near the low critical solution temperature ( LCST ) to achieve transparent and scattering state switching ; Na2SiO3 reduces the LCST by regulating the polymer-water interaction, so that the material triggers shading at a temperature closer to the thermal comfort of the human body ; HPC increases network density and inhibits high temperature volume shrinkage, thereby enhancing structural stability. The optimized device exhibits excellent overall performance, with a visible light transmittance (Tlum of 84.1 % and a solar modulation efficiency (ΔTsol) of 79.2 %. Under the action of Na2SiO3, the LCST decreases to 27.9°C, which can achieve earlier thermal response shading. At the same time, under the synergistic effect of HPC, the volume shrinkage rate is only 5 %, which significantly improves the high temperature stability. This study provides an effective design idea for the collaborative optimization of hydrogel smart window glass between optical performance, response temperature and structural stability, and shows the application potential in the field of energy-saving buildings.