Preparation and properties of cellulose nanocrystal based structurally colored radiative cooling composite films

Radiative cooling lowers its own temperature by emitting heat into outer space and is a green, low-carbon and sustainable cooling strategy. Most radiative cooling materials have monotonous color appearance, mostly white or transparent, and the addition of traditional colorants will cause the materials to absorb heat and reduce the radiative cooling performance. Cellulose nanocrystal/polyethylene glycol (CNC/PEG) composite radiative cooling films with tunable structural colors were prepared by self-assembly method. And the composite films were combined with cellulose acetate (CA) films with porous structure to obtain structure-colored radiation-cooled bilayer composite films. The results show that the cellulose nanocrystalline composite films have bright structural colors with obvious birefringence phenomenon. With the increase of PEG content, the pitch of the composite film structure increases, and the color changes from blue-green to red. the CNC/PEG structural color composite film has the highest reflectance up to 68.5% in the visible light band, and the emissivity in the atmospheric window band is up to 93%, with an ambient cooling effect of about 3.4 ℃. the CNC/PEG-CA bilayer composite film has the highest reflectance in the visible light band up to 91.8%, and the emissivity in the atmospheric window band is up to 32.2%. Compared with the composite film, the bilayer composite film has better cooling performance, with a temperature difference of about 14.3 ℃ compared with the ambient temperature.In outdoor tests, compared with the ambient temperature, the composite film can achieve a cooling effect of about 2 ℃, and the bilayer composite film can achieve a cooling effect of about 6 ℃.