Unique Stress Whitening and High-Toughness Double-Cross-Linked Cellulose Films

Polysaccharide-based materials, which have the advantages of abundant reserves and excellent biocompatibility and biodegradability, have attracted growing interest due to public awareness of sustainable development. Herein, we demonstrate the formation of high-strength and high-toughness double-cross-linked (DC) cellulose films. For the first time, stress whitening of DC cellulose films is reported, which has never been observed in cellulose-based films or other polysaccharide-based materials. The epichlorohydrin-to-anhydroglucose unit of cellulose (ECH-to-AGU) molar ratio, ethanol concentration, and relative humidity are critical parameters that influence the microstructure and stress whitening of DC cellulose films. Moreover, the incorporation of chemically and physically cross-linked heterogeneous structures, strong hydrogen bonding, and irreversible chemical covalent interactions among cellulose chains endows DC cellulose films with excellent mechanical properties and superior toughness. The drawing orientation can produce extremely high-strength and high-toughness DC cellulose films with tensile strength, Young's modulus, and work of fracture values of 234 MPa, 9.3 GPa, and 28.2 MJ.m(-3), respectively. The developed DC cellulose films also exhibited excellent thermomechanical properties, moderate thermal stability, and extremely low oxygen permeability and should contribute to potential applications in food and drug packaging, battery separators, and biodegradable flexible electronics.