Topical interest in photochromic systems is due in part to their application in silver‐free photography, in data display technology, and possibly even in solar energy storage. The new system (1)⇌(2) is readily accessible and open to wide structural variation.
Water‐sensitive catalysts work in water! Metallocene catalysts that normally have to be handled under the strict exclusion of air and moisture, for example [TiCp*(OMe)3] (1, Cp*=pentamethylcyclopentadienyl), are successfully employed in the aqueous emulsion polymerization of styrene to syndiotactic polystyrene. The catalyst is encapsulated inside the monomer droplets and thus protected against attack by water.
Das aktuelle Interesse an photochromen Systemen beruht u. a. auf ihrer Anwendungsmöglichkeit in der silberfreien Photographie, in der Daten‐Display‐Technik und vielleicht sogar als Sonnenenergiespeicher. Das neue System (1)⇌(2) ist gut zugänglich und strukturell sehr variabel.
A simple conoscopic optical method of determining liquid crystal low tilt angles with sufficient accuracy for many practica,l purposes is described. The similarity between this method and the crystal rotation one is shown. The limits of applicability of the conoscopic optical method are also given.Eine einfache optische Methode zur Bestimmung kleiner Tiltwinkel flussiger Kristalle niit einer fur vide praktische Fiille itusreichenden Genauigkeit wird beschrieben. Diese Methode wird mit der Kristall-Rotations-Methode verglichen. Die Grenzen der Anwendbarkeit der konoskopischen Methode werden aufgezeigt.
IndroductionIt is well known that the performimce of Liquid Crystal Display Devices strongly depends on the tilt angle -the angle between the optical axis of a nematic liquid crystal and the boundary surface (BAUR et al. 1975; GOODMAN et al.). In order to optimize device performance, it is important to have an accurate technique to measure tilt angles.The methods for measuring the tilt angle, which have been proposed recently, are possible to be devided into two categories according to whether or not an external field is used. At the capacitive (MEYERHOFER; KAHN; SUZUKI et al.) and the magnetic null methods (SCIIEFFER, NEHRING ; TORIYAMA, ISHIBASHI) an external field is applied. The purpose of this paper is to describe a simple conoscopic optical method of determining liquid crystal low tilt angles with sufficient accuracy, that can be applied over the range of practical interest, and experimental data obtained with it.
TheoryAs pointed out by SCHEFFER, NEHRWG and FAHRENSCHON, SCHIEKEL an useful geometry for observation of angular dependence of transmitted light intensity T ( y ) is the rotation of the liquid crystal cell between parallel or crossed polarizers about an axis which is perpendicular to the principle plane of the cell. This plane is defined by the surface normal and the liquid crystal director n. We consider the case of uniformly aligned liquid crystal cell with low tilt angle placed between crossed polarizers and
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