A Transparent Semicrystalline Polymer Used in Thermally Responsive Smart Window Application

Abstract Thermally responsive smart windows based on phase‐changing polymers usually face challenges such as low transmittance at room temperature and limited solar modulation when demanded. A clear poly(SA) or CPSA composed of poly(stearyl acrylate) (poly(SA)) and ethoxylated trimethylolpropane triacrylate is introduced. High optical transparency and high crystallinity at ambient temperature are achieved by ultraviolet curing at a high temperature to suppress the crystallite domain size to the nanometer range. Adding poly(hydroxyethyl methacrylate) (poly(HEMA)) in the formula produces a micrometer‐size phase‐separated terpolymer system where the CPSA phase retains its crystallinity. The refractive index of the poly(HEMA) phase is tuned to match that of the CPSA phase, and the terpolymer film is transparent at 20 °C with visible transmittance up to 91.4%. Above the transition temperature of 42 to 46 °C, CPSA crystal melting leads to light scattering at the phase boundaries, creating an opaque appearance. The transmittance modulation is 81.4%, 87.7%, and 76.5% for solar (300–2500 nm), visible (380–780 nm), and near‐infrared (780–2500 nm) spectrum, respectively. Furthermore, a styrofoam chamber covered with the terpolymer smart window can effectively reduce the chamber interior temperature increase by 8.5 °C compared to the reference glass window.