Water-resistant, flexible and transparent cellulose nanofibril film for exceptional oxygen-barrier performance under high humidity

Water sensitivity and brittleness have limited significantly the practical application of cellulose nanofibril (CNF) films. Herein, butane tetracarboxylic acid (BTCA) was integrated into CNF film to construct advanced material. Structurally, the multiple carboxylic acid groups in BTCA were able to form robust ionic bonding with quaternized CNF to improve the water stability. The optimal film, prepared by cross-linking with 0.1 mol L^-1 of BTCA solution at pH 8.0, exhibited dramatically reduced water uptake, two orders of magnitude lower than that of the pristine film. After conditioned at 50% RH and 25 °C, the film showed a roughly doubled strain-at-break with only a negligible decline in tensile strength. Under wet conditions, this film demonstrated an ultrahigh strain-at-break of 26.5%, almost comparable to a stiff hard rubber (20%-50%), while the pristine film became too swollen and fragile to handle. Notably, the film retained its optical transparency, exhibiting optimal transmittance of 87.0% at 600 nm. Moreover, it displayed an exceptionally low oxygen permeability of 1.5 mL μm m^-2 day^-1 kPa^-1 even at 90% RH, outperforming other recently reported nanocellulose-based films (3-60 mL μm m^-2 day^-1 kPa^-1). The multifunctional CNF film is a promising alternative to petroleum-based materials for visible packaging, flexible electronics and biomedical materials.