Determination of the Degree of Orientation in Thin Films of Nematic Liquid Crystals from Infrared Dichroic Measurements in a Homogeneous Electric Field

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Abstract
Two methods are described for the determination of the degree of orientation in thin films of nematic liquid crystals. Both methods are based on the measurement of infrared absorption intensities in the presence of a homogeneous d.c. electric field. In the first method the infrared dichroic ratio is determined by applying the electric field perpendicular to the direction of propagation of the polarized radiation. In the second method the electric field is applied parallel to the direction of propagation and does not require polarized radiation. An expression is derived for the dependence of the dichroic ratio on the magnitude of the applied field using a simple model of a domain acting as a rigid dipole. These methods were applied to the liquid crystal p-methoxybenzylidene-p'-cyanoaniline. The degree of orientation was determined over the temperature range of the liquid crystal. The maximum degree of orientation was found to be 0.33 at 106°C.