A long-term stable thermochromic smart window of PNIPAM-based hydrogels with excellent water retention and visible light modulation

Poly(N-isopropylacrylamide) (PNIPAM)-based hydrogel thermochromic smart windows exhibit advantages in near-room-temperature phase transition and strong solar modulation. However, their poor water retention performance limits the phase transition cycle stability. Here, PNIPAM microgels as thermochromic components, gelatin, poly(acrylamide-acrylic cacid) (PAM-AA) and AlCl3 were combined to form a PNIPAM/gelatin/PAM-AA/Al3+ hydrogel by copolymerization and cross-linking. This composite hydrogel was then sandwiched between two pieces of glass to form a smart window. The multiple interpenetrating polymer networks can improve the mechanical properties of the hydrogel, and the hydrophilic groups of PAM-AA and gelatin can enhance the water retention capacity of the hydrogel, ensuring the phase transition cycle stability of the smart window. The elongation at break, Young's modulus and toughness of the hydrogel reach up to 460%, 65 kPa and 0.52 MJ/m3, respectively. The smart window exhibits excellent visible light transmittance (Tlum=85.6%) and visible light modulation (ΔTlum=51.8%). During 150 phase transition cycles, the visible light modulation capability of the smart window remained highly stable. In comparison with an air sandwich window, the smart window equipped on a simulated chamber can achieve a maximum temperature reduction of ~ 7 °C in a 7-day outdoor experiment.