Polysaccharide thin film studies - Adsorption of oxidized xylan on cellulose

Licentiate thesis, 2021

Wood xylans are renewable natural materials that have potential for future use in bio-based products. However, their high price and low production capacity restrict their industrial use and current applications. This research aimed to explore the potential of wood xylans for film applications. Most xylans extracted from wood do not dissolve in water, which is a common solvent in many industries, such as the pulp and paper industry. Furthermore, bio-based materials are expected to be processed in a green approach, where water is a preferable solvent, and has benefit in terms of low-cost, and being an environmentally benign solvent. The poor water interaction of xylans hinders dissolution and film production, but can be overcome by chemical modification of xylan to alter its properties. Regioselective oxidation—a well-known modification in polysaccharides—can increase the flexibility and water interaction of xylans by opening their monosaccharide ring and equipping them with aldehyde groups. The research employed quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR) to study the adsorption of macromolecules on film surfaces. The analytical combination was first established for studying protein adsorption on cellulose film with different morphology, and structure, and then oxidized xylan adsorption on nanocellulose films, which was the focus of this research. The influence of the molecular weight of oxidized xylan on adsorption in aqueous conditions, and its effect on film swelling were explored. The behavior of oxidized xylan in solution was observed using dynamic light scattering, showing that high molecular weight oxidized xylan formed larger aggregates when the concentration increased compared to low molecular weight oxidized xylan. The QCM-D and SPR investigations revealed that a high molecular weight grade adsorbed preferentially on a neutral cellulose surface, while both grades were adsorbed independently of molecular weight on a negatively charged surface. The impact of oxidized xylan on the swelling of cellulose films, regardless of molecular weight, was due to oxidized xylan bringing more water to the non-charged films, while the opposite was true of negatively charged films. Also, applying the combination of QCM-D and SPR techniques to oxidized xylan adsorption on cellulose, water content and adsorbed dry mass were obtained. This study provides a stepping-stone toward the future use of bio-based materials obtained from forest sources.