In the mid-1990s, with the arrival of third generation synchrotron radiation sources, it became possible to focus highly brilliant X-rays down to spot sizes of a few micrometers and still provide a sufficient X-ray flux to obtain diffraction patterns from weakly scattering materials within a few seconds. This makes it possible to investigate structural parameters such as cellulose microfibril orientation and crystal size within single cellulose fibers and single wood cell walls directly.<br/>The X-ray diffraction of poplar tension wood fibers is largely dominated by the highly oriented gelatinous (G) layer. By investigating single tension wood cells fiber diffraction diagrams with an unprecedented (for native samples) resolution of 0.11 nm are measured. Lines of diffuse scattering in the fiber diffraction pattern are probably arising from defects (possibly stacking faults) in the crystalline microfibrils. Moreover, a direct comparison of crystal parameters and sizes in the G and the S2 layer is possible.<br/>Many experiments on wood profit from the high spatial resolution of micro diffraction. The nanostructure of the soft wood cell wall exhibits a characteristic layered structure, introducing another structural parameter, the microfibril (helix) angle (MFA). With a “coarse” resolution of 2 µm, only the thickest secondary layer S2 significantly contributes to the scattering signal. Looking at cross sections, the helical arrangement of cellulose microfibrils is directly imaged. Extended high-resolution scans over annual rings reveal details of the seasonal variation of the MFA and its connection to mechanical and acoustic properties of different wood species. The micro diffraction analysis of developing xylem tissue provides an elegant and direct access to the sub-micron thick S1 layer properties even with a beam larger than the cell wall layers.<br/>Nowadays, with X-ray nano beams a spatial resolution of 100-200 nm is achieved so that the individual cell wall layers are directly separated in cross sections. Recent examples of such results will be shown. Indications for a local breakdown of the ideal cellulose fiber texture in the cell wall are found. The homogeneity of the MFA inside a cell wall layer and the size of possible helicoidal transition layers are estimated.

Investigating individual wood cell wall layers with X-ray micro and nano beams

255th National Meeting (2018)

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