On the Mechanism of Assembly of Xylans onto Cellulose Surfaces

Doctoral thesis, 2003

Cellulose is the most abundant biopolymer on earth and the major component in cell walls of wood and terrestrial plants. Commercial applications include paper, plastics, hygiene and pharmaceutical products. The composition and morphology of cellulose surfaces affect the product properties. The development of environmentally attractive methods for surface modification is thus important. Hemicelluloses are suitable candidates for cellulose modification as they are present in large amounts in wood and plants and interact strongly with cellulose, both during cell wall biosynthesis and pulping. The major hemicelluloses in hardwoods are xylans, which are characterized by a .BETA.-(1->4)-D-Xylp backbone and the presence of acidic substituents. This work has focused on preparation of cellulosic fibers with tailored surface morphologies by controlled assembly of xylans onto the surfaces. The assembly of xylan onto cellulose was shown to be related to the accessible surface area of the cellulose substrates and the assembly process could be controlled by variation of experimental conditions such as temperature, treatment time, and solution pH. Microscopy analysis showed that the xylans assemble on the cellulose surfaces as small particles, ranging in sizes from about 50 nm to a few micrometers. The formation of xylan surface structures was correlated to the aggregation of xylans in aqueous solution, as shown by light scattering studies. Assembly of xylans onto cellulose surfaces under the conditions used in this work is proposed to take place by an adsorption process involving diffusion to, and interaction with, cellulose surfaces of xylan aggregate structures formed in the solution. Assembly of xylan and formation of surface structures on lignocellulosic fibers was shown to significantly improve the wettability and liquid absorption rate of the fibers by creating hydrophilic surfaces with topographies that promote liquid spreading. This research work provides a deeper understanding of the behavior of xylans in solution and their interactions with cellulose surfaces. Potential applications include incorporation of xylan assembly into existing kraft pulping processes to improve pulp properties such as wetting and fiber strength. Hopefully, the knowledge gained in this work can contribute to development of new materials based on polysaccharides.