G. Strobl

Nematic wetting at the free surface of 4‐cyano‐4′‐n‐alkyl‐biphenyls

The free surface of a homologous series of low molecular weight liquid crystals 4‐cyano‐4′‐n‐alkyl‐biphenyls (nCB, n=5,6,7,8) is studied by reflection ellipsometry. On approaching the bulk isotropic–nematic transition temperature TNI from the isotropic phase, the growth of a nematic layer at the surface is observed. The director orientation in the layer is identified as homeotropic. The ellipsometric data are analyzed in terms of a perturbation calculation. It is shown that for 7CB and 8CB very close to TNI, where the layer has a thickness d≥50 A, both d and the order parameter S0 at the surface can be determined. In all other cases only one parameter, which corresponds to the nematic coverage, can be extracted from the data. Results are discussed in terms of a wetting process. Partial wetting by the nematic phase is observed for n=6,7,8. On approaching TNI from lower temperatures where the bulk is in the nematic phase a small increase of excess surface order is measured which is interpreted as a homeotro...

Pretransitional nematic surface order in the isotropic phase of phenylcyclohexanes

Pretransitional order at the free surface of the isotropic phases of two phenylcyclohexances (PCH5 and PCH7) was studied by reflection ellipsometry under variation of the angle of incidence. Data analysis shows a logarithmic divergence of the boundary layer thickness when the temperature approaches the isotropic‐nematic transition point, indicating complete wetting behavior. The data are well represented by fits to a model based on a Landau–de Gennes approach, yielding order parameter profiles of the layer.

The effect of molecular association and tube dilation on the rotational viscosity and rotational diffusion in nematic liquid crystals

A combination of director reorientation experiments with dielectric relaxation spectroscopy was used to study the relation between the rotational viscosity γ1 and the rotational diffusion constants in nematic compounds with different association tendency. Increased values for γ1 were found for compounds showing strong association. This is explained by increased rotational friction of the associates due to the necessary translational motion of the molecules within these groups. For nonassociating compounds, γ1 was found to be described quantitatively by Marrucci’s theory, when, in addition, the tube dilation effect described by Doi for the rotational diffusion in an orientationally ordered environment is taken into account.

Dynamics of Rotational Motion in Liquid Crystalline Systems

Liquid crystals combine the anisotropic optical properties, as they are typical for birefringent crystals, with the viscous behaviour of liquids [1, 2]. Application of these materials is based on this specific pattern of properties. Well known, and of great technological importance is the use of nematic liquid crystals in modern displays. Here electrical fields are applied to induce local changes in the orientation of the optic axis. Switching times should be as short as possible and usually are in the order of 10–100 ms. The decisive parameter is the rotational viscosity γ, which describes the ratio between the torque acting on the director (i.e. the optic axis) and its angular velocity. For displays low values of γl are desirable. In a different range of applications nematogenic materials are used to prepare optical components with specific permanent birefringence profiles. Principally this can be achieved by setting up in the nematic phase a specific director field and then fixing it by a quench into the glassy state. This procedure is now gaining special importance in attempts to produce components with nonlinear optical properties. Liquid crystalline polymers, composed of mesogenic groups which are laterally attached to a flexible backbone chain, constitute a class of materials which is convenient for this purpose [3, 4]. These “LC-side group polymers” usually show a glass transition at temperatures above room temperature, so that optical structures prepared in the nematic phase can be fixed by a quenching and then remain stable at ambient temperature.