Interactions between Wood Polymers in Wood Cell Walls and Cellulose/Hemicellulose Biocomposites

Doctoral thesis, 2011

A wood fibre is a complex multi component biocomposite that is hierarchically organised. The arrangement of a wood fibre on the ultrastructural level is highly controlled by the interactions between the main structural polymers, i.e. cellulose, various hemicelluloses and lignin, as well as in some parts also pectin and protein. This further determines the mechanical and physical properties of the wood material, and consequently its targeted applications. Despite considerable research in this field during a number of years, the current knowledge on the interactions between the wood polymers is still incomplete and needs improvement. Also, mimicking of natural structures is an inspiration when preparing new materials from renewable resources, hence a comprehensive understanding of the polymer interactions is required. The first part of this study was to improve the understanding on the molecular interactions within the primary cell wall of spruce wood fibres, and its importance for the energy demand for the refining process during the thermomechanical pulp (TMP) manufacturing. Dynamic FTIR (Fourier transform infrared) spectroscopy in combination with dynamic 2D (two-dimensional) FTIR spectroscopy was used to examine how the lignin, protein, pectin, xyloglucan and cellulose interact in an enriched primary cell wall material. Measurements indicated that strong interactions exist between lignin, protein and pectin, as well as between cellulose, xyloglucan and pectin in this particular layer. Further, by applying a low degree of sulphonation pre-treatment to spruce wood chips, it was shown that the desired ultrastructural changes in the sulphonated primary cell wall material were reached. This selective reaction caused a weakening of the interactions between lignin;pectin, lignin;protein and pectin;protein,as well as an increased softening and swelling of the material, which were the reasons for the noted energy savings when refining such low sulphonated spruce wood chips. The second part of this study was to improve the current knowledge on the molecular interaction within the secondary cell wall of a spruce wood fibre, as well as its ultrastructure by studying the orientation of the cellulose, glucomannan, xylan and lignin. Imaging FTIR microscopy was a technique used for studying, which indicated a parallel orientation of polysaccharides with respect to each other and the fibre axis and a partial parallel orientation of lignin. It was suggested that the interactions between the cellulose and glucomannan and the xylan and lignin are dominant but also that interactions between the two hemicelluloses, i.e. glucomannan and xylan, exist. The third part of this study was to prepare biomimetic biocomposite films, based on cellulose reinforced hemicelluloses. The interactions between these polysaccharides were studied through the prior examined mechanical properties of the biocomposite films using dynamic mechanical analysis (DMA). Here, the understanding of the ultrastructural organisation of the primary and secondary cell walls has been taken as a basis. Intramolecular hydrogen bonding might be created between the cellulose molecules and the unsubstituted areas of the backbone of the hemicellulose molecules, where the lower degree of substitution of xylan results in a more frequent occurrence of intermolecular hydrogen bonding. This study contributes to an increased understanding between interactions of wood polymers in the primary and secondary cell walls of a softwood tracheid and may serve as a guide for the new generation of biomimetic biocomposites.