Ultralight and highly flexible aerogels with long cellulose I nanofibers

Ultralight and highly flexible aerogels with long cellulose I nanofibers (CNFs) were produced via purification and defibrillation of wood fibers into CNF hydrogels, followed by freeze-drying. The aerogels had a web-like entangled structure, low density, and high water uptake capability. With the increase of CNF content in hydrogels from 0.1 to 1.5%, the bulk density of the aerogels increased from 1.3 x 10(-3) to 17.0 x 10(-3) g cm(-3), whereas the water uptake ratio (WUR) decreased from 155 to 54. The microstructure of the aerogels can be transformed from open 3D porous nanofibrillar network to 2D sheet-like skeletons by adjusting the CNF content of the hydrogels. A possible assembly mechanism was proposed based on this transformation. When the transparent supernatant fraction, which has similar to 0.018% solid content obtained via centrifugation of the hydrogels, was subjected to freeze-drying, ultra-low density aerogels (0.2 x 10(-3) g cm(-3)) consisting of nanofibers with lengths above 1 mm and a width range of 40-180 nm, were successfully produced due to the self-assembled of the tiny CNFs and their bundles along the longitudinal direction.