Molecular dynamics in perfluorohyphen;nhyphen;eicosane. II. Components of disorder

摘要

The defect structure in the rhombohedral modificationRof perfluorohyphen;nhyphen;eicosane has been analyzed in a temperaturehyphen;dependent xhyphen;ray and neutron scattering study. The analysis was based on an evaluation of Bragghyphen;reflection intensities and of the diffuse scattering. Different components of disorder were detected and characterized. At the transition from the low temperature phase to modificationR, rotation of the molecules about their long axes sets in. As shown by the azimuthal distribution of the diffuse scattering, rotations occur cooperatively with correlations between neighbors. The Debyendash;Waller factors measured for the layer reflections are indicative for a moderate roughening of the interfaces due to a restricted longitudinal diffusive motion. Molecules are perturbed by the formation of helix reversal defects. Evaluation of the diffuse scattering yields the associated finite azimuthal correlation length. There is also evidence for a continuous slow untwisting of the helix. With increasing temperature, it approaches the allhyphen;transform.