On axis fibre microdiffraction

Microfocus X-ray diffraction is one of very few techniques capable of studying single fibres without the need for destructive sample preparation. Using the microbeams available at the ID13 beamline, variations across single fibre diameters can be assessed. However, a significant limitation of the 'standard' fibre diffraction geometry is a complete lack of information concerning the fibre's skin-core structure along the beam path. Any in-depth analysis requires a modelling approach which inherently biases the results according to the model's definition.

As an alternative to modelling, ID13 has recently demonstrated that microdiffraction along the fibre axis is also feasible. Using a two-dimensional raster scan over the fibre cross-section, an accurate picture of the fibre's skin-core morphology can be reconstructed. This contains much more microstructural information than can be obtained using the normal fibre diffraction geometry. This includes detail of local texture, domain orientation, and void distribution (for on-axis SAXS).

The technique requires very short fibre segments, prepared with minimal damage to the fibre's free ends. Such sections can now be produced using a laser cutter which is available at the ESRF.

The images on the right show typical on-axis diffraction patterns collected from different positions on a single fibre cross-section. The left hand image is from a disordered region whilst the right hand image is from a highly-oriented region. Parameters such as reflection azimuthal width can be used as a qualitative indicator of local crystalline domain orientation.

In a recent demonstration of the general method, crystallographic texture and rotational disorder was investigated within a single poly(p-phenylene benzobisoxazole) fibre. PBO is a high performance fibre which exhibits remarkable mechanical properties. The results show that the fibre exhibits a radial microstructure which persists over the entire fibre cross-section. A greater degree of domain misorientation is apparent in the fibre skin and core regions, which are seperated by an intermediate region exhibiting a more ordered structure. Further details of this technique, along with more information on PBO skin-core morphology, are published in the journal Macromolecules.

Probing the Internal Structure of High-Performance Fibers by On-Axis Scanning Diffractometry
Davies, R. J.; Burghammer, M.; Riekel, C.
Macromolecules 40(14), 5038-5046 (2007)