Super water absorbing and shape memory nanocellulose aerogels from TEMPO-oxidized cellulose nanofibrils via cyclic freezing-thawing

Mechanically robust cellulose nanofibril (CNF) aerogels with ultralow density (8 mg cm(-3)), superior porosity (99.5%), super water absorbency (104 g g(-1) water/dried mass), high crystallinity (68.5%) as well as exceptional wet resilience and water activated shape recovery were facilely fabricated for the first time by ice-crystal templated self-assembly of TEMPO oxidized CNFs via cyclic freezing-thawing. With ultrathin widths (1-2 nm), high aspect ratios (several hundreds) and numerous surface polar hydroxyls and carboxyls, TEMPO oxidized CNFs behaved similar to aqueous soluble polymers to form strong freestanding hydrogels by repetitive freezing (-20 degrees C, 15 h) and thawing (room temperature, 9 h). The spaces occupied by the several hundred microns wide ice crystals were well preserved upon freeze-drying, deriving macroporous CNF aerogels with over 99% porosity of interconnected pores. The freezing induced self-assembling of CNFs was observed at a low concentration of 0.05%, whilst more ordered macroporous honeycomb structures were observed at and above 0.2%. Exchanging water in the CNF hydrogel with tert-butanol generated hierarchical CNF aerogels containing several hundred microns sized macroscopic as well as mesoscopic pores ranging from 2 to 90 nm with further improved specific surface area (117.8 m(2) g(-1)), pore volume (1.19 cm(3) g(-1)) and water absorption (116 g g(-1)). All CNF aerogels demonstrated super water absorbency, fast water-activated shape recovery in 4 s and reusability for at least 20 times.