The defect structure in the rhombohedral modification R of perfluoro-n-eicosane has been analyzed in a temperature-dependent x-ray and neutron scattering study. The analysis was based on an evaluation of Bragg-reflection intensities and of the diffuse scattering. Different components of disorder were detected and characterized. At the transition from the low temperature phase to modification R, 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 Debye–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 all-trans form.
In an X-ray reflectivity study of thin films of several liquid crystal-side group polymers, boundary layers with a smectic structure were detected in the isotropic phase. They possess a remarkable stability and exist also at larger distances from the clearing point. Reflectivity curves were evaluated in detail by application of the Born approximation on a convenient structure model of the film. A stepwise procedure for the data evaluation is suggested, which enables a separate determination for different groups of model parameters.
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Pretransitional surface ordering phenomena, as they occur in the isotropic phase of liquid crystals upon approaching the clearing point, were studied by variable angle ellipsometry and X‐ray reflectivity measurements. Ellipsometric data obtained for two phenylcyclohexanes indicate a logarithmic divergence of the thickness of a nematic boundary layer with homeotropic orientation, which corresponds to a complete wetting behaviour. Data can be represented by a model based on a Landau‐de Gennes approach. An investigation of a LC‐side group polyacrylate showed a boundary layer with planar orientation. The thickness, which increases on approaching the clearing point, is clearly larger than in the case of the low molar mass samples. In X‐ray reflectivity studies on a film of a LC‐side group polymer boundary layers with smectic order were detected. They possess a remarkable stability and exist also at larger distances from the clearing point. A stepwise procedure in the data analysis enables a detailed, complete characterization.
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