Improvement of alignment in diffuse fibre diffraction patterns

Molecular alignment of crystalline polymers leads to 'fibre type' X-ray diffraction patterns which are a rich source of structural information. For aligned glassy polymers, however, it is only possible to obtain a very diffuse fibre pattern. This paper describes a technique developed to improve the alignment of such patterns by azimuthal sharpening. The technique is based on consideration of the contribution to the diffraction pattern due to scattering from a fibre entity oriented at an angle to the specimen's extension axis. The distribution of these entities about the specimen axis is determined from the azimuthal profile of a diffuse diffraction halo which has concentrated on the equator of the pattern. Integral equations are developed which describe the appearance of a smeared reflexion on the diffraction pattern in terms of the entity distribution. Solution of these equations by an iterative procedure enables the diffraction pattern to be azimuthally sharpened. The result is a diffuse version of a typical fibre pattern, which is useful in the prediction of chain conformation and packing as well as levels of disorder present.