Direct determination of polymer crystal structures from fibre and powder X-ray data

Powder and fibre X-ray diffraction intensities, respectively from two synthetic polymers, polyethylene and poly(epsilon-caprolactone), were used to evaluate the prospects for structure analysis via direct methods. For the former, centrosymmetric, example, an entirely correct phase set was found when the origin-defining reflections were extended by the Sayre equation. Prediction of structure factor magnitudes by this equation also successfully deconvoluted clusters of overlapping reflections into their component values. In the analysis of the latter, non-centrosymmetric, problem with the Sayre equation, a very accurate phase set could also be derived, particularly when the initial model was improved by Fourier refinement (final mean error: 19 degrees). Again, direct prediction of structure factor magnitudes was helpful for the separation of overlapping reflections during the course of the analysis. Copyright (C) 1996 Elsevier Science Ltd.