Ternary Synergistic Strengthening and Toughening of Bio-Inspired TEMPO-Oxidized Cellulose Nanofibers/Borax/Polyvinyl Alcohol Composite Film with High Transparency

Utilizing and exploring degradable and renewable resources are of importance for environmental protection, and one efficient strategy is to use green cellulose-based materials as alternatives of eco-unfriendly polymers. However, compared to other polymeric and inorganic fibers, the cellulose-based materials show weak mechanical strength, which limits their applications in some extreme conditions. To obtain TEMPO-oxidized cellulose nanofiber (TOCN) films with high strength and toughness simultaneously, we prepared the ternary TOCN/borax/polyvinyl alcohol (PVA) composite films through a vacuum-assisted filtration method in this work. By controlling the additional contents of borax and PVA in the composites, the stress, elongation, and toughness of the TOCN/borax/PVA composites can be up to 226.53 MPa, 5.98%, and 9.84 MJ·m–3, which are 1.89, 2.51, and 6.39 times of a pure TOCN film, respectively. The outstanding mechanical behaviors of the TOCN/borax/PVA composites are attributed to the synergic strengthening effect of hydrogen and covalent bonding as well as the intrinsic and extrinsic toughening mechanisms. In addition, the TOCN/borax/PVA composites also maintain a high visible-light transparency of 83.36% at 800 nm, which has potential applications in solar cells, transparency electrodes, and flexible electric devices as substrates.