Synergistic upper critical solution temperature–lower critical solution temperature microgel–hydrogel composites for bidirectional thermoresponsive smart windows with superior optical modulation and dimensional stability

Smart windows are crucial for reducing building energy consumption, yet conventional thermochromic hydrogels, especially poly(N-isopropylacrylamide) (PNIPAM), suffer from unidirectional response, volumetric instability, and slow switching kinetics. Herein, we report an additive-free, purely polymeric smart window composed of carboxyl-functionalized P(NIPAM-co-MAA) microgels embedded in a polyacrylamide (PAM) matrix via in situ polymerization. This synergistic upper critical solution temperature–lower critical solution temperature (UCST–LCST) composite enables reversible tristate optical switching with ultrahigh integrated luminous transmittance (96.97% at 24 °C) and outstanding solar transmission modulation (ΔT_sol,low = 60.44%; ΔT_sol,high = 78.42%). The transition temperatures are tunable between 11 and 39 °C for broad climatic adaptability. Moreover, the composite exhibits exceptional dimensional stability, rapid response, and mechanical robustness. Outdoor simulations demonstrate a temperature reduction of up to 14.4 °C, and greenhouse tests show enhanced plant growth. This work provides a robust platform for energy-efficient buildings and smart agricultural systems.