Defence of doctoral thesis: Saina Kishani – On the Solubility of Wood Hemicelluloses in Water and its Influence on the Adsorption at Cellulose/Water Interfaces

Abstract

Hemicelluloses are a group of natural polysaccharides and one of the main components of wood. The availability and biodegradability of these polymers have created a great interest in developing new bio-based materials or composites from these materials for different end-use purposes. To develop new materials from these polysaccharides with better functionalities, it is essential to understand the fundamental properties of these compounds. The solubility of hemicelluloses is one of these crucial characteristics since the material has to be dissolved in an appropriate solvent if these biopolymers are to be used in, for example, dense and strong films once the solvents are removed. The interaction of these polysaccharides with different solid surfaces is also significantly influenced by their solubility at the surface/water interface and an understanding of this interaction is essential for describing composite formation, since the polysaccharides are most frequently used together with reinforcing materials such as anisotropic cellulose fibers and fibrils.

In the work described in this thesis, a novel methodology has been developed for characterizing in detail the solubility of the extracted and model mannans, arabinoxylans and xyloglucan polysaccharides. Different chemical structural analyses, chromatography, light scattering and microscopy techniques have been applied to achieve an accurate understanding of the solubility of the polysaccharides in aqueous media. A careful study has been performed to isolate and purify softwood polysaccharides, followed by the preparation of model samples to investigate the influence of processing, structural substitutions and molar mass on the solubility. Association and the phase separation of hemicelluloses have been identified in aqueous media despite their clear and transparent appearance to the naked eye.

Natural hemicelluloses are used in combination with cellulose as composite materials both to introduce different functionalities and to utilize the great mechanical properties of cellulose fibrils/fibers. Accordingly, there was a great need to study the influence of the solubility on the interaction and adsorption of these polysaccharides at the cellulose/water interface. The adsorption at the cellulose/water interface was indeed affected by the physicochemical structures and solubility of the polysaccharides, and it has been shown that an increasing molar mass and an increasing polymer concentration lead to formation of associated structures and a phase separation at cellulose model surfaces.