Wood Hemicelluloses - Fundamental Insights on Biological and Technical Properties
Sammanfattning: Hemicelluloses are a group of heterogeneous polysaccharides representing around 30 % of wood where the dominating types are xylans, glucomannans and xyloglucans. Hemicelluloses complex molecular structure makes it difficult to understand the relationship between structure and properties entirely, and their biological role is not yet fully verified. Additionally, hemicelluloses are sensitive to chemical processing and are not utilized to their full potentials for production of value-added products such as materials, additives to food and pharmaceutical products, etc. Increased knowledge regarding their functions is important for the development of both processes and products. The aim with this work has therefore been to increase the fundamental understanding about how the structure and properties of wood hemicelluloses are correlated, and properties such as flexibility, interaction with cellulose, solubility, resistance to chemical-, thermal-, and enzymatic degradation have been explored.Molecular dynamics (MD) simulations were used to, in detail, study the structures found in wood hemicelluloses. The flexibility was evaluated by comparing the impact of backbone sugars on the conformational space and also the impact of side groups was considered. Based on the conformational space of backbone glycosidic linkages the flexibility order of hemicelluloses in an aqueous environment was determined to be: xylan > glucomannan > xyloglucan. Additionally, the impact of xylan structure on cellulose interaction was evaluated by MD methods.Hemicelluloses were extracted from birch and spruce, and were used to fabricate different composite hydrogels with bacterial cellulose. These materials were studied with regards to mechanical properties, and it was shown that galactoglucomannans mainly contributed to an increased modulus in compression, whereas the most significant effect from xylan was increased strain under uniaxial tensile testing. Besides, other polysaccharides of similar structure as galactoglucomannans were modified and used as pure, well defined, models. Acetyl groups are naturally occurring decorations of wood hemicelluloses and can also be chemically introduced. Here, mannans with different degrees of acetylation were prepared and the influence of structure on solubility in water and the organic solvent DMSO were evaluated. Furthermore, the structure and water solubility influenced the interaction with cellulose. Acetylation also showed to increase the thermal and biological stability of mannans.With chemical pulping processes in mind, the degradability of spruce galactoglucomannans in alkaline solution were studied with regards to the structure, and the content of more or less stable structural regions were proposed.
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