The function of complex biological materials such as wood or bones is related to their hierarchical structure. The local variation of the nanostructure can be non-destructively investigated by microbeam scanning diffractometry [1, 2]. Experiments performed on ID13 combine reciprocal space resolution on the unit cell level with the resolution of a light microscope provided by micrometre-sized scanning-steps. This method has been applied to the study of the orientation of cellulose fibrils in a single wood cell wall [3]. On the nanometric scale, the wood cell wall is a fibre composite consisting of an amorphous matrix and crystalline cellulose microfibrils. In the dominating S2 layer, the cellulose microfibrils are tilted with respect to the longitudinal cell (MFA: microfibril angle) and are helically wound around the cell wall (Figure 89). While standard X-ray diffraction techniques are capable of determining an average tilt angle, information on the helical superstructure has been principally derived from optical techniques. In view of the importance of this superstructure for the mechanical properties of wood it was of interest to verify whether scanning X-ray diffractometry also provides this information.

Experiments were performed on a cross section (thickness 10 µm) of Picea abies (Norwegian spruce) tracheid cells, with the X-ray beam parallel to the cell walls. The result of a complete scan of a 52 x 42 µm2 area is shown in Figure 90. Individual diffraction patterns recorded by a CCD detector are limited to the range of the 110/200 reflections. The outline of the wood cells can be obtained immediately. As shown in Figure 90, the orientation of the microfibrils relative to the X-ray beam modifies the diffraction pattern in a characteristic way. An arrow here symbolises the orientation. The patterns can be perfectly simulated by assuming a sharply defined MFA-angle of about 23° [3]. The overall projection of the fibril orientation is shown schematically in Figure 91. The arrows form a flow pattern around the wood cell and there is a clear line of cell walls belonging to neighbouring cells. The flow pattern is an exact representation of the projection of the helical superstructure (Figure 89) on a plane and proves the presence of a right-handed Z helix in spruce wood.

References
[1] P. Fratzl et al., J. Appl. Cryst., 30, 765-769 (1997).
[2] C. Riekel et al., Macromolecules, 30(4), 1033-1037 (1997).
[3] H. Lichtenegger et al., J. Appl. Cryst., 32, 1127-1133 (1999).

Authors
H. Lichtenegger (a), M. Müller (b), O. Paris (a), C. Riekel (b), P. Fratzl (a).

(a) Erich Schmid Institute of Materials Science, Austrian Academy of Sciences and Metal Physics Institute, University of Leoben (Austria)
(b) ESRF