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 homoeotropic 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.