Selection of polymers with functional groups for day-time radiative cooling

Abstract 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 are responsible for the radiative cooling property of polymers. In this work, we aim to select common polymers applicable to day-time radiative cooling by studying the emission/absorption peaks of functional groups in the atmospheric transmittance window. We demonstrate week-long sub-ambient radiative cooling of water with two common polymers – PVDF and PMMA films by simply placing them on a silver mirror layer. While both films having 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 that of the PMMA film: with peak sun-hour sub-ambient cooling degrees being 6 o C vs. 4 o C, and peak nighttime sub-ambient cooling degrees being 9 o C vs. 8 o C. If utilized in large scale cooling systems, such a nuance in cooling performance of polymers could lead to a significant economic difference over the long term.