Cellulose microfibril angles in a spruce branch and mechanical implications

The nanostructure of the wood cell wall and, in particular the tilt angle o f the cellulose fibrils versus the longitudinal cell axis (microfibril angl e, MFA), are known to play a key role in determining the mechanical propert ies of wood. A variation of microfibril angles during growth may therefore be regarded as a means to adapt to different loading situations. In the pre sent study, a branch of Norway spruce (Picea abies) was used as a model sys tem. The change of microfibril angles with increasing age and size of the b ranch and therefore increasing gravitational load was systematically invest igated. Small angle X-ray scattering (SAXS) was applied to obtain a map of MFA all over the branch as a function of the distance from the trunk within each annual ring. It was found that in compression wood the MFA decreased continuously from the trunk towards the tip in all annual rings. In opposit e wood, however, the course of microfibril angles was found to change consi derably with the age of the branch: in the outer annual rings, very small m icrofibril angles occurred in the middle part of the branch. The results ar e discussed in view of the mechanical implications of different microfibril angles. (C) 2001 Kluwer Academic Publishers.