Hierarchically\nMicro- and Nanostructured Polymer via\nCrystallinity Alteration for Sustainable Environmental Cooling

Cooling environments are a pervasive need in our society,\nwith\nconventional air conditioners being the most popular approach. However,\nair conditioners rely heavily on electricity and Freon, a chemical\nthat depletes ozone and contributes to greenhouse gas effects. To\naddress this issue, passive daytime radiative coolers (PDRCs) have\nbeen proposed to achieve cooling by simultaneously reflecting sunlight\nand allowing internal heat to escape without electricity. Despite\ntheir potential, most high-performance PDRCs are composed of thick\npolymer films, which increases material costs during PDRC preparation\nand limits thermal transport. In this work, we introduced an economical\nand scalable solvent evaporation-based method to prepare a relatively\nthin hierarchically micro- and nanostructured poly(vinylidene fluoride-trifluoroethylene)\nvia crystallinity alteration. Particularly, we find that the key to\ngenerating nanosized pores is to remove the water residual within\nthe film without sample annealing, which significantly enhances the\nscattering efficiency across the solar spectrum. With our design,\nwe demonstrate effective cooling of the outdoor environment, achieving\na cooling temperature of Δ2.5 °C, with a film thickness\nof only 215 μm. Furthermore, our model suggested that applying\nthis material could lead to annual energy savings of up to ∼39%\nin warmer climates across the country and up to 715 GJ nationwide.\nDeveloping effective PDRCs with reduced material thickness, such as\nthe one discussed here, is imperative for implementing sustainable\ncooling solutions and reducing our carbon footprint.