The Effect of Ionic Liquid Treatment on the Morphology and Mechanical Properties of Cellulose Fibers

Cellulose is a major component of biomass and its dissolution is a critical step for its chemical modification and transformation into different products, such as man-made fibers, and films.
Compared to traditional organic solvents, ionic liquids are considered more environmentally friendly, making them an attractive alternative for more sustainable processing technologies. However, little information is available on the dissolution process, especially from a mechanics-point-of-view. In this work, we show that it is possible with different treatment times to dissolve the primary cell wall of flax fibers to gain information about the secondary cell wall, which is crucial for the fibers' mechanical behavior. We study the morphology by atomic force microscopy and employ wide-angle x-ray scattering to determine the microfibril angle of the fibers. Since the treatment with ionic liquids strongly influences the mechanical properties caused by the partial dissolution process, we perform single fiber tensile testing for the untreated and treated fibers and also apply Brillouin spectroscopy, a non-contact technique to study elastic material properties, to gain information on the normal and shear stiffness of the untreated and treated flax fibers. As expected, there is a decrease in mechanical strength, and the combination of tensile testing and Brillouin spectroscopy yields interesting information about the material behavior at two different frequency scales.