Biodegradable and Hierarchically Designed Polymer Film for Sustainable Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) with high solar reflectivity and mid-infrared emissivity supplies a sustainable strategy for global cooling. Especially, polymer-based PDRC materials have attracted much attention because of their advantages of easy-processing, low cost, and outstanding optical properties. However, environmental pollution resulting from polymers has been one of the most serious issues, as most polymers cannot be naturally degraded when discarded and easily lead to severe environmental pollution. Here, we demonstrate a hierarchically designed PDRC film based on eco-friendly polylactic acid polymer (PLA) originated from natural corns, which shows excellent optical performance with rapidly natural degradation properties. Specifically, the intrinsic C═O (1825–1725 cm –1 ), C–O–C (1260–1110 cm –1 ), −COO (870–820 cm –1 ) bondings of PLA enable high mid-infrared emissivity, and the scalable processing on the PLA with controlled diameter endows a high sunlight reflectivity. Consequently, we achieved an average subambient cooling temperature of 5.7 °C under 800 W/m 2 of solar irradiance. In addition, the designed PDRC–PLA film exhibited the most rapid degradation rate in soil with over 92% weight loss after 45 days of natural degradation, which was 12 times higher than that of other polymers commonly used for radiative cooling. This hierarchically designed PDRC–PLA film with excellent biodegradable properties opens an alternative pathway toward sustainable radiative cooling.