Ice-Assisted Assembly of Liquid Crystalline Cellulose Nanocrystals for Preparing Anisotropic Aerogels with Ordered Structures

Imparting aerogels with structural anisotropy endows these porous materials with direction-dependent properties and promising novel applicability in material science. However, facile fabrication of such anisotropic aerogels under mild conditions still remains a major challenge. Here, we report on the fabrication of anisotropic aerogels from liquid crystalline ordered cellulose nanocrystals (CNCs) by a directional freeze casting process. The resulting CNCs aerogels were millimeter sized with lamellar macrostructural features and optical anisotropy, exhibiting hierarchical structures perpendicular to the direction of freezing. We show that the birefringence of CNCs aerogels arose from the alignment of CNC liquid crystals, rather than from the shape of rod-like nanoparticles. Moreover, the versatility of the directional freezing approach was expanded to a CNC-silica system, resulting in anisotropic silica aerogels with an ordered mesopore-imprinted structure template by the liquid crystalline ordered CNCs. The highly directional alignment, optical anisotropy, hierarchical porosity, and large internal surface area of the CNC-based aerogels exhibit considerable potential for future fire-resistant, direction-dependent mechanical and electrical insulator applications.