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.