Structural Organisation of the Wood Polymers in the Wood Fibre Structure

The organization of the major polymers in the wood fiber has a large impact on the properties of the structure. Numerous studies have been devoted to the cellulose microfibril arrangement, providing the longitudinal strength of the fiber, while less is known regarding the structural organization of other components, such as hemicelluloses and lignin. For the hemicelluloses, as being part of the cellulose aggregation process, indications of a strong coupling to the cellulose structure have been shown. For lignin, being laid down in a later stage, no clear picture has emerged. Here the orientation of lignin vis-à-vis the cellulose orientation was examined for a number of different fiber structures. It was shown that the lignin in the middle lamella region seems to be nonoriented, thus more resembling an isotropic material, while the lignin in the secondary wall is to some extent oriented. The orientation of this lignin is less pronounced than the orientation of cellulose but has a preferential alignment in the direction of the fiber axis. The reason for this alignment could be related to structural restrictions of this lignin, deposited in the spaces remaining after the initial forming of the structured cellulose/hemicellulose fibrillar structure.