Surface modification of cellulose-based fibres for use in advanced materials

Abstract: The awareness of our need for a sustainable society has encouraged the search for renewable, high quality materials that can replace oil-based products. This, in combination with increased competition in the forest industry, has stimulated a lot of research into different types of wood-based materials where cellulose-rich fibres are combined with different types of polymers. There is hence a large need to develop efficient fibre modification techniques by which the fibres can be tailored to obtain specific properties. Furthermore, by modifying only the surface of fibers a significant change in properties can be achieved although only a relatively small amount of the total fibre material is modified. The potential impact of a surface modification increases tremendously when nano-sized fibres are used due to the larger interfacial area between the fibres and their surroundings. Interest in smaller building blocks in the nanometer range has continued to grow rapidly in recent years due to both the availability and preparation/synthesis of smaller building blocks and to the discovery of the high performance of these types of nanocomposites. In this thesis, three different types of surface modifications are presented as new tools to design the properties of new novel cellulose-based materials. In the first work, thermoresponsive nanocomposites have been assembled from specially designed thermoresponsive polymers and nanofibrillated cellulose. The polymers have one thermoresponsive block and one cationically charged block which can thus attach the polymer to an oppositely charged fibre/fibril surface. Multilayers were assembled with these polymers and the nanofibrillated cellulose utilizing the layer-by-layer technique, resulting in thin films with thermoresponsive behavior which for example could be used for controlled drug-release applications. In the second work, an amphiphilic block copolymer with one high molecular weight hydrophobic polystyrene block and one cationic block was synthesized for use as a compatibilizer between fibres and hydrophobic polymer matrices in composites. These polymers self-assemble into micelles in water with the hydrophobic part in the core of the micelle and the cationic part in the shell. Due to the cationic charges, these micelles adsorb to oppositely charged surfaces where the hydrophobic parts can be liberated on the surface by a heat treatment, leading to a new, less hydrophilic, surface with a contact angle close to that of pure polystyrene. Atomic force microscopy was used to measure the adhesive properties of a polymer-treated surface using a polystyrene probe at different temperatures and contact times. The adhesion increased with increasing contact time for the treated surfaces, probably due to entanglement between the polystyrene blocks at the treated surface and the probe. The relative increase in adhesion with contact time was higher at the lower temperature whereas the absolute value of the adhesion was higher at the higher temperature, most probably due to a larger molecular contact area. This odification technique could be utilized to make fibrebased composite materials with better mechanical properties. In the third work, click chemistry was used to covalently attach dendrons to cellulose surfaces and further to modify them with mannose groups to obtain specific interactions with  Concanavalin A. The protein interactions were studied at different protein concentrations with a quartz crystal microbalance. The multivalent dendronized surface showed a 10-fold increase in sensitivity to the protein compared to a monovalent reference surface. This could be used to design more sensitive cellulose-based biosensors in the future.