Investigating cellulose structure using solid-state NMR spectroscopy
Cellulose is the most abundant polymer in nature and it is an important constituent in most flowers, trees, and even some animals. Cellulose has an established use in many important products, such as textile fibers, paper and paperboard, and recently new applications have received increased attention. The crystalline parts of the cellulose wood fiber can be extracted and this material can be used for applications such as forming thin films or as stabilizers in emulsions. New and more environmentally friendly processes to dissolve cellulose and produce textile fibers are also of interest. However, both native and regenerated cellulose build up complicated supramolecular structures that are, to some extent, still not completely resolved. In order to be able to utilize the cellulose material fully, additional knowledge of the material is needed.
In the work covered in this thesis, solid-state NMR was used to gain information of the supramolecular structure of cellulose. Measurements were conducted on both native and regenerated cellulose, exposed to a range of different treatments. Using the 1D solid-state NMR spectrum, the effects on the material after a certain treatment could be followed and elucidated.
During the production of regenerated cellulose, the structure of the material was altered by changing the properties of either the solvent or the coagulant, changing the initial cellulose concentration, or by post-treatment, e.g. drying and re-wetting. This latter process induces an irreversible pore collapse, called hornification. This phenomenon was seen for both native and regenerated cellulose and it was concluded from the studies that co-crystallization contributes to hornification. Using 2D solid-state NMR correlation spectroscopy, a full chemical shift assignment of the 1D NMR spectrum of regenerated cellulose was obtained. This assignment made it possible to follow treatments of the material from the full solid-state NMR spectrum. Using the chemical shift assignment, a global spectral deconvolution could be applied on the 1D spectrum of regenerated cellulose, making it possible to, with increased accuracy, determine the crystallinity of the material.