Mie-optimized PMMA particles for fully polymer-based radiative cooling coatings with high reflectance and hydrophobicity.

Passive radiative cooling materials achieve cooling without energy consumption by reflecting sunlight and emitting thermal radiation to the cold outer space (3 K). This study synthesizes poly(methyl methacrylate) (PMMA) particles with Mie scattering properties through emulsifier-free emulsion and dispersion polymerization to control particle size, enhancing their scattering and reflectivity. A high-emissivity polydimethylsiloxane (PDMS) matrix is used to fabricate a composite material, enabling complementary radiative effects between PMMA and PDMS within the atmospheric transparency window, thus enhancing the cooling efficiency. The impact of PMMA particle size on reflectance was investigated, and the performance of varying PMMA-PDMS ratios in radiative cooling was assessed. Experimental results indicate that PMMA particles synthesized in-house, particularly PMMA-1, exhibit optimal radiative properties, with a reflectance of 93.7% and emissivity of 93.2%. The composite AS coating with an optimized PMMA : PDMS ratio of 7 : 3 exhibited a solar reflectance of 96.9% and an emissivity of 94.0% within the atmospheric window wavelength range. Outdoor testing showed that this AS coating achieves an average temperature reduction of 3.4 °C and a maximum of 8.6 °C below ambient temperature, outperforming commercial coatings. Additionally, water contact angle measurements indicated hydrophobicity (111.4°), suggesting self-cleaning potential. These findings reveal that the AS coating exhibits promising cooling performance, self-cleaning capabilities, and cost-effectiveness, highlighting its potential for practical applications in radiative cooling.