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 microfibril orientation and crystal size of single cellulose fibers directly.  The first demonstration of the capabilities of X-ray microbeams is a comparison of cellulose microfibril orientation in single flax fibers and fiber bundles (in this case using small-angle X-ray scattering with a 2 �m size beam). The study shows that orientation estimates by conventional scattering techniques overestimate the misalignment with respect to the fiber axis by several degrees. The results were later complemented by fiber diffraction experiments, which reveal lines of diffuse scattering in the two-dimensional fiber diffraction pattern, probably arising from defects within the crystalline microfibrils. By investigating single tension wood fibers with their characteristic large and highly oriented cellulose microfibrils fiber diffraction diagrams with the unprecedented (for native samples) resolution of 0.11 nm can be measured. All these structural properties are relevant for the unique mechanical properties of the biocomposite material cellulose. X-ray diffraction can be combined in situwith mechanical tests and is thus an ideal tool to learn more about the yield mechanisms of cellulose fiber. X-ray microbeams additionally provide spatial resolution below the fiber dimensions, making the effect of inhomogeneities in native fibers accessible.  Many other experiments profit from the high spatial resolution of microdiffraction. Very recent examples of such results will be shown. Indications for a local breakdown of the ideal fiber texture are found with nanodiffraction.

Fifteen years of microbeam X-ray diffraction of single cellulose fibers and wood cell walls

245th National Meeting (2013)

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