Molecular orientation in cellulose fibers and composites

Doctoral thesis, 2020

The scope of this thesis is to quantitatively investigate the molecular orientation distribution of regenerated cellulose fibers and composites. The molecular orientation is known to affect macroscopic properties such as tensile strength of the fiber. In addition, the quality of a carbon fiber is, to a great extent, determined by the molecular orientation of the precursor. A plethora of techniques are paramount for materials characterization and a handful of these are suited for determination of molecular orientation. Since different methods have various experimental limitations, methodological awareness is crucial in the strive for quantitative data and in particular when cellulose fibers and other polymers are chemically modified, or a part of a composite. This work concerns three methods in order to investigate molecular orientation: rotor synchronized magic angle spinning solid-state nuclear magnetic resonance spectroscopy (ROSMAS), polarized Raman spectroscopy, and X-ray scattering. The latter is already a proven method for analyzing molecular orientation and was therefore used as a reference for the two first methods, which have never previously been applied on cellulose fibers. ROSMAS was used to investigate the chemical shift anisotropy, which relates to molecular orientation, on a bundle of Lyocell fibers. Polarized Raman spectroscopy was used to analyze the molecular orientation distribution from the Raman vibrational tensor on a single fiber. A new method was developed for polarized Raman spectroscopy by assuming a wrapped Lorentzian orientation distribution function, as measured from X-ray scattering patterns. The results from both ROSMAS and polarized Raman spectroscopy were in agreement with X-ray scattering on a highly oriented cellulose fiber bundle and on a single regenerated cellulose fiber, respectively, indicating that these methods are quantitative. The ROSMAS and X-ray methodologies were applied to a stretched fiber consisting of a regenerated cellulose-lignin composite intended as a carbon fiber precursor. Finally, ROSMAS was also used for determination of the complete chemical shift anisotropy in the molecular reference frame on regio-regular poly(3-hexylthiophene) fibers, in addition to elucidation of backbone and side chain orientation.

In the grand perspective, resources have to be used efficiently to minimize environmental impact. Therefore, this work explores man-made environmentally benign cellulose alternatives to cotton and other polymers. These processes refine cellulose from plant life, typically trees, which can grow without pesticide on non-arable land.