Ductile All-Cellulose Nanocomposite Films Fabricated\nfrom Core–Shell Structured Cellulose Nanofibrils

Abstract

Cellulosic materials have many desirable\nproperties such as high\nmechanical strength and low oxygen permeability and will be an important\ncomponent in a sustainable biomaterial-based society, but unfortunately\nthey often lack the ductility and formability offered by petroleum-based\nmaterials. This paper describes the fabrication and characterization\nof nanocomposite films made of core–shell modified cellulose\nnanofibrils (CNFs) surrounded by a shell of ductile dialcohol cellulose,\ncreated by heterogeneous periodate oxidation followed by borohydride\nreduction of the native cellulose in the external parts of the individual\nfibrils. The oxidation with periodate selectively produces dialdehyde\ncellulose, and the process does not increase the charge density of\nthe material. Yet the modified cellulose fibers could easily be homogenized\nto CNFs. Prior to film fabrication, the CNF was shown by atomic force\nmicroscopy to be 0.5–2 μm long and 4–10 nm wide.\nThe films were fabricated by filtration, and besides uniaxial tensile\ntesting at different relative humidities, they were characterized\nby scanning electron microscopy and oxygen permeability. The strength-at-break\nat 23 °C and 50% RH was 175 MPa, and the films could, before\nrupture, be strained, mainly by plastic deformation, to about 15%\nand 37% at 50% RH and 90% RH, respectively. This moisture plasticization\nwas further utilized to form a demonstrator consisting of a double-curved\nstructure with a nominal strain of 24% over the curvature. At a relative\nhumidity of 80%, the films still acted as a good oxygen barrier, having\nan oxygen permeability of 5.5 mL·μL/(m²·24\nh·kPa). These properties indicate that this new material has\na potential for use as a barrier in complex-shaped structures and\nhence ultimately reduce the need for petroleum-based plastics.