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 °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 m2 g−1), pore volume (1.19 cm3 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.