Physical crosslinked hydrogel-derived smart windows: anti-freezing and fast thermal responsive performance

Abstract

Thermochromic hydrogels are versatile smart materials that have many applications, including in smart windows, sensing, camouflage, <i>etc.</i> The previous reports of hydrogel smart windows have been based on covalent crosslinking, requiring multistep processing, and complicated preparation. Moreover, most research studies focused on enhancing the luminous transmittance (<i>T</i>_lum) and modulating ability (Δ<i>T</i>_sol), while the structural integrity and antifreezing ability, which are essential in practical applications, have been compromised and rarely investigated. Herein, we develop a new physical (noncovalent crosslinked) hydrogel-derived smart window by introducing an <i>in situ</i> free radical polymerization (FRP) of <i>N</i>-isopropylacrylamide (NIPAM) in a glycerol-water (GW) binary solvent system. The noncovalent crosslinked PNIPAM GW solutions are facilely synthesized, giving outstanding freezing tolerance (∼-18 °C), a comparably high <i>T</i>_lum of 90%, and Δ<i>T</i>_sol of 60.8%, together with added advantages of fast response time (∼10 s) and good structural integrity before and after phase transition. This work could provide a new strategy to design and fabricate heat stimulated smart hydrogels not limited to energy saving smart windows.