Cost‐Effective Paraffin Particles‐Based Thermochromic Composites for Smart Windows and Greenhouses

This study presents low‐cost, high‐performing, and long‐lasting scattering‐based thermochromic composites for light transmittance and thermal management, which are based on poly(vinyl alcohol) films embedding paraffin micro/nanoparticles. The thermoresponsive behavior relies on matching (transparent state)/mismatching (opaque state) of the refractive index of the polymer matrix with that of the micro/nanoparticles in their solid/liquid state. The fine control of the paraffin particle composition and size, of the crystalline phase at the micro/nanoscale, and of the preparation process, permits accomplishing coatings manifesting high luminous transmittance in the nonactivated state (% T lum = 77.3%), strong light modulation (Δ T solar = 58.6%, Δ T NIR = 45.0%), and thermal self‐regulation capabilities. These features, combined with their excellent photostability and water resistance, position the composites among the best performing dye‐free thermotropic coatings reported so far. Besides, the introduction of graphene oxide as a photothermal agent in the polymer turns it photoresponsive. Noticeably, these coatings are prepared through a straightforward, low‐energy, sustainable, and scalable blade technique, allowing precise control over the material properties onto various substrates, including flexible and adhesive poly(ethylene terephthalate), and glass. This allows obtaining different prototypes demonstrating their adaptability and potential for real‐world applications, such as energy‐saving smart windows, greenhouses, and adhesive films, applied to real train windows.