Nematic and smectic surface order in the isotropic phase of low molar mass and polymeric liquid crystals

Abstract

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.