Thermo-Responsive Poly(N-isopropylacrylamide)/Hydroxypropylmethyl Cellulose Hydrogel with High Luminous Transmittance and Solar Modulation for Smart Windows

Thermochromic smart windows are considered to be promising energy-saving devices for reducing energy consumption in buildings. The ideal materials for thermochromic smart windows should have high transmittance, high solar modulation, low phase-transition temperature, and excellent high-temperature thermal stability, which are difficult to achieve simultaneously. This work reports a simple one-step low-temperature polymerization method to prepare a thermo-responsive poly(N-isopropylacrylamide)/hydroxypropylmethyl cellulose (PNIPAM/HPMC) hydrogel achieving the above performances simultaneously. The low-temperature polymerization environment endowed the hydrogel with a high luminous transmittance (T_lum) of 90.82%. HPMC as a functional material effectively enhanced the mechanical properties and thermal stability of the hydrogel. Meanwhile, the PNIPAM/HPMC hydrogel showed a low phase-transition temperature (∼32 °C) and high solar modulation (ΔT_sol = 81.52%), which proved that it is an ideal material for thermochromic smart windows. Moreover, a PNIPAM/HPMC smart window exhibited high light transmittance (T_380-760 = 86.27%), excellent light modulation (ΔT₃₆₅ = 74.27%, ΔT_380-760 = 86.17%, and ΔT₉₄₀ = 63.93%), good indoor temperature regulation ability and stability, which indicated that it was an attractive candidate for application in reducing energy consumption in buildings. This work also provides an option and direction for modifying PNIPAM-based thermochromic smart windows.