Solar heat reflective coating consisting of hierarchically assembled polystyrene nanoparticles

A polystyrene (PS) thin film usually displays a very low light reflectivity (< 0.1) because of its low refractive index (n) and frequency-unresponsive n values. This study yet observes a significant boost of light reflectivity to above 0.7 over the visible and near infrared range when the microstructure of PS film is incubated by the drying-induced phase inversion mechanism. Specifically, a liquid layer of PS in dimethylformamide (DMF) cast on a flat substrate is subjected to drying in air at ambient humidity (ca. 65% RH), through which the condensation of moisture and the evaporation of DMF prompt a gradual phase inversion because water and DMF are miscible and DMF vaporizes slowly. PS chains undergo steady contractions accompanying the drying of the cast liquid film, leaving behind a matrix consisting of grains with sizes of ca. 50 nm, of which each is the assembly of smaller nanoparticles. Furthermore, the infrared spectroscopy offers an assessment for the packing density of PS chains in these nanoparticles. In addition, the film possesses a topographic contour composed of adjoining spherulites in micron sizes according to AFM characterizations. In contrast, two control samples fabricated by using typical solvents for PS, toluene and chloroform, invariably present a solar reflectivity of 0.1. In addition to strong surface solar reflectivity, the PS film formed through the phase inversion holds a surface temperature 3.7 °C below that on a TiO2 coating under direct sunlight.