Semi-crystalline Methacrylate Hydrogels for Multistage Solar and Thermal-Radiative Regulation.

Passive smart windows require materials that combine high daylight transparency, strong solar-heat shielding, thermal buffering, mechanical robustness, and cycling stability without external energy input. Here, we report a semi-crystalline hydrogel that couples stearyl methacrylate (SMA) side-chain crystallization/melting with oligo(ethylene glycol) methyl ether methacrylate (OEGMA) lower critical solution temperature phase separation through 1,6-hexanediol diacrylate (HDDA) crosslinking. This dual-phase program generates three adaptive states: low-temperature latent-heat release with a temperature-dependent mid-infrared response, moderate-temperature transparency, and high-temperature broadband solar shielding. The optimized SMA60-OEGMA40-0.5HDDA hydrogel exhibits 92.25% luminous transmittance, 84.14% solar modulation efficiency, 75.11% near-infrared modulation, 1.02 MPa fracture stress, and 207.07% elongation. In model-window tests, it lowers the maximum indoor temperature by 12 °C relative to a commercial low-emissivity film and yields a 39% lower relative cooling-load index. By synergistically integrating phase-change thermal-energy storage and thermally induced optical regulation within a single material, this work provides a material-design strategy for mitigating conventional trade-offs among transparent daylighting, solar-heat shielding, and thermal management across different temperature ranges and demonstrates potential for passive thermal-management systems in energy-efficient building windows.