Selection of polymers with functional groups for daytime radiative cooling

The infrared ‘fingerprint’ of polymers that many functional groups/bonds such as C–F and C–O emit overlaps with the atmospheric transmittance window and is responsible for the radiative cooling property of polymers. In this work, we aim to select common polymers applicable to daytime radiative cooling by studying the emission/absorption peaks of functional groups in the atmospheric transmittance window. We demonstrate week-long subambient radiative cooling of water with two common polymers—polyvinylidene fluoride (PVDF) and poly(methyl methacrylate) (PMMA) films—by simply placing them on a silver mirror layer. Although both films have the same thickness of 50 μm, the PVDF film has lower solar absorptivity, higher atmospheric window emissivity, and slightly higher selectivity. Consequently, its daytime and nighttime cooling performances are both better than those of the PMMA film, with peak sun-hour subambient cooling degrees being 6 °C vs. 4 °C and peak nighttime subambient cooling degrees being 9 °C vs. 8 °C. If used in large-scale cooling systems, such a nuance in cooling performance of polymers could lead to a significant economic difference over the long term.