Physically Cross-Linked Hydrogel Designed for Thermochromic Smart Windows: Balance between Thermal Stability and Processability

Abstract

Thermosensitive polymer solutions and hydrogels show great potential for thermochromic windows. However, current research often overlooks two critical factors: thermal stability under heating and processability, which are typically contradictory. This study develops a physically cross-linked hydrogel specifically for thermochromic smart windows, aiming to achieve an optimal balance between these two factors. The hydrogel is made from low-cost, environmentally friendly materials, using commercial block polyether (L62) combined with a poly(vinyl alcohol)-borosilicate network. By optimizing PVA and borax concentrations at 2 and 0.3%, the hydrogel demonstrates good thermal stability, showing no significant changes after repeated heating to 60 °C for 9 h. Additionally, it exhibits excellent processability, allowing it to flow and fill gaps between glass slides when heated. The transition temperature for transmittance and solar modulation can be easily adjusted by varying the concentration of L62. The resulting hydrogel achieves tunable transition temperature, high luminous transmittance (71.2%), solar modulation efficiency (72.2%), and excellent durability. Compared to air-sandwiched windows, these thermochromic smart windows exhibit a significant cooling effect, with temperature responsiveness contributing up to 50%, depending on hydrogel thickness. This work addresses key issues that will advance the industrial application of hydrogel-derived thermochromic smart windows.