Strong, Ductile, Transparent, Water-Resistant Cellulose Nanofibril Composite Films via UV-Induced Inter-Cross-Linked Networks

Brittleness and high water sensitivity have so far hindered the utilization of cellulose nanofibril (CNF) films in many applications. In this study, strong, ductile, transparent, and water-resistant CNF-based composite films were prepared by incorporating UV-curable glycerol dimethacrylate (GDMA) and poly(ethylene oxide) (PEO) into CNFs. Fourier transform infrared spectroscopy and other test results confirmed the cross-linking between PEO chains, inter-cross-linking between PEO and GDMA, as well as self-polymerization of GDMA after UV irradiation. These reactions had a synergistic effect on the mechanical properties and water resistance of the films, leading to outstanding mechanical properties of the films in both wet and dry states. Specifically, an optimal formulation, CNF/15%GDMA/50%PEO, showed a tensile strength, modulus, and toughness at the wet state of 42.1 MPa, 0.3 GPa, and 3.7 MJ/m3, about 6.6, 4.3, and 11.1 times those of the neat CNF film, respectively. This ternary composite film also exhibited a higher transparency than the neat CNF film with an optical transmittance of ∼84.0% at 550 nm wavelength. The film could be pressed into a double-curved structure without a fracture with moisture plasticization, demonstrating its high ductility and formability. These properties indicate that the new CNF-based composite films can be promising alternatives to petroleum-based materials for packaging, flexible electronics, and many other applications that require material strength, toughness, and water resistance at the same time.