High Solar Modulation and Fast-Response Thermoresponsive Poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) Hydrogel for Energy-Efficient Smart Windows

Thermochromic smart windows represent an efficient technology for building energy conservation. The ideal thermochromic smart window should possess high optical transmittance, strong solar modulation ability, rapid thermal response, and appropriate lower critical solution temperature. In this study, 2-hydroxyethyl methacrylate (HEMA) was introduced into the polymer chains of poly(N-isopropylacrylamide) to synthesize composite hydrogels, which were subsequently applied to smart window systems. The resulting composite hydrogels exhibited good optical properties, with a solar transmittance (Tsol) of up to 93.89% before phase transition and a solar modulation ability (ΔTsol) of 88.38%. The incorporation of HEMA reduced the response time of the hydrogel from 59 to 39.3 s, representing an acceleration of approximately 33.4%. Meanwhile, the formation of intramolecular hydrogen bonds from HEMA enhanced the stability of the polymeric network, increasing the Young’s modulus from 6 × 10–4 to 1.05 × 10–3 Pa, approximately a 1.75-fold improvement. After 100 heating–cooling cycles, the composite hydrogels maintained stable optical performance. These enhancements clearly demonstrate the great potential of this hydrogel for applications in smart window technologies.