Mechanically tough, transparent, low dielectric, and mesoporous nanocellulose cryogels formed via osmotically tuned swelling of dense nanocellulose assembly

Abstract Cellulose nanofibers (CNFs) possess excellent physical properties and are utilized as building blocks of functional materials. Highly porous CNF structures (typically > 97%) with large specific surface areas are prepared by supercritical drying or freeze-drying of CNF dispersions or hydrogels. However, their mechanical strength is limited by the extremely low CNF network density. Increasing the network density is challenging because the required high concentration CNF dispersions are highly viscous, making it difficult to mold into regular shapes. In this study, mechanically strong and tough porous CNF structures were prepared through the expansion of dense CNF networks via osmotic swelling, followed by industrially feasible freeze-drying assisted by a tert -butyl alcohol/water mixture. This process enabled tunable porosities in the range of 47–61%. The resulting CNF structures were composed of layered CNF networks with interlayer mesopores and exhibited high specific surface areas of 137–209 m 2 g −1 , which were relatively higher than those of typically reported freeze-dried CNF structures from aqueous CNF dispersion or hydrogel (~10 to 100 m 2 g −1 ). They were optically transparent and showed high strength (~ 100 to 115 MPa) and high toughness (~ 6 to 6.5 MJ m −3 ), which was approximately twice the toughness of the dense CNF network. Furthermore, porosity tuning reduced the refractive index from 1.53 in the dense CNF network to 1.29 and the dielectric constant from 4.90 to 2.57, highlighting the potential of these porous CNF structures for functional applications.