Ultralight and highly flexible aerogels with long cellulose I nanofibers

Ultralight and highly flexible aerogels with long cellulose I nanofibers (CNFs) were produced viapurification 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 × 10−3 to 17.0 × 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 ∼0.018% solid content obtained viacentrifugation of the hydrogels, was subjected to freeze-drying, ultra-low density aerogels (0.2 × 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.