Hot-press melt-assembly anisotropic porous structure with enhanced radiative cooling

Passive daytime radiative cooling (PDRC) is a promising and sustainable technology, achieving subambient cooling of objects without electrical consumption. Current PDRC designs realize efficient cooling performance, but their complicated and expensive fabrication approaches remain a challenge. Herein, we present a simple, versatile, and effective morphology control method based on the hot-press melt-assembly approach for fabricating tunable anisotropic porous polymethyl methacrylate (PMMA) films with high-efficiency daytime radiative cooling capability. The abundant and disordered micro/nanopores in the PMMA films caused by localized melting and bonding of particles reinforce scattering of the incident sunlight. Apart from the high refractive index contrast between PMMA and air within the porous structure, the dispersed PMMA particles exhibit an apparent micro/nanoplatelet morphology after hot pressing, enhancing the backscattering of sunlight within a porous PMMA film and contributing to the high-performance subambient radiative cooling. Therefore, the porous PMMA film features an ultrahigh solar reflectance of 0.95 and strong mid-infrared thermal emittance of 0.93, allowing for a peak subambient cooling temperature reduction of 9.9 °C under a solar irradiance of 700 W/m2 with an average radiative cooling power of 98 W/m2 during the daytime. Furthermore, the hot-press melt-assembly method is a cost-effective and high-efficient process for fabricating the desirable micro/nanoporous structure, providing a practical and effective pathway toward a large-scale production for excellent passive radiative cooling.