Self-assembling hierarchical flexible cellulose films assisted by electrostatic field for passive daytime radiative cooling

Natural cellulose materials possess hierarchical microstructures and high crystallinity, which gives the possibility to develop high-performance cellulose structural materials. However, the hierarchical microstructures of natural cellulose materials will be disappeared in regenerated cellulose materials owing to the dissolution process. Achieving hierarchical microstructures and high crystallinity as natural cellulose materials in flexible regenerated cellulose materials is an arduous challenge. Herein, a simple electrostatic-field-assisted self-assembly strategy is proposed to greatly boost the crystallization of regenerated cellulose materials and obtain hierarchical flexible fibrous cellulose film. Large areas of cellulose crystallites with the lattice spacing close to monoclinic cellulose II are presented, and crystallinity as high as 71.0 % is achieved for the self-assembled fibrous cellulose film. Moreover, the self-assembled cellulose film has a bioinspired fibrous morphology with anisotropic hierarchical microstructures, and therefore can be used as flexible passive daytime radiative cooling materials with subambient cooling up to 11.3 °C in hot summer. It is the first time to realize high crystallinity and hierarchical microstructures in regenerated cellulose materials by the self-assembly of cellulose macromolecules at the molecular level. This strategy provides new perspectives for the development of advanced flexible cellulose materials with hierarchical microstructures and high crystallization from natural cellulose resources.