Cellulose microfibril angles in a spruce branch and mechanical implications

The nanostructure of the wood cell wall and, in particular the tilt angle of the cellulose fibrils versus the longitudinal cell axis (microfibril angle, MFA), are known to play a key role in determining the mechanical properties of wood. A variation of microfibril angles during growth may therefore be regarded as a means to adapt to different loading situations. In the present study, a branch of Norway spruce (Picea abies) was used as a model system. The change of microfibril angles with increasing age and size of the branch and therefore increasing gravitational load was systematically investigated. 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 opposite wood, however, the course of microfibril angles was found to change considerably with the age of the branch: in the outer annual rings, very small microfibril angles occurred in the middle part of the branch. The results are discussed in view of the mechanical implications of different microfibril angles.