Phase Types, Structures and Chemistry of Liquid Crystals
Liquid crystals are well known today to a broad community. Liquid crystal displays in electronic watches and calculators are wide spread. However, much less is commonly known about the function of these displays or the scientific background of the liquid
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Phase Types, Structures and Chemistry of Liquid Crystals
D. Demus
1.1
Historical
Liquid crystals are well known today to a broad community. Liquid crystal displays in electronic watches and calculators are wide spread. However, much less is commonly known about the function of these displays or the scientific background ofthe liquid crystalline state which may be considered as a special physical condition. The liquid crystalline state was detected more than 100 years ago. In 1888, Friedrich Reinitzer, an Austrian botanist and chemist at the University of Graz, synthesized several esters of cholesterol, a natural product occurring in plants and animals. Reinitzer found in these esters the phenomenon of "double melting," i.e., at a certain temperature the compound changes from the crystalline solid phase to an opaque liquid which transforms at a defined higher temperature to an optically clear liquid. These phase changes were reproducible with increasing and decreasing temperature in several compounds. Looking to the literature before Reinitzer, some indication can be found that several scientists indeed dealt with liquid crystals, but did not notice the unique phenomena and therefore did not become aware of this new state. Reinitzer himself was not able to explain the curious phenomenon of "double melting" and the existence of the opaque liquid. Therefore, he sent samples of his compounds to Otto Lehmann, who was professor of physics (successor of Heinrich Hertz) at the Technical High School of Karlsruhe. Lehmann, at that time, was the leading crystallographer in Germany, and quickly found the optical anisotropy of the opaque liquid phases of Reinitzer's cholesterol esters. Despite the fact that the chemical constitution of cholesterol was elucidated much later, Lehmann intuitively argued that the optical anisotropy of these liquids would be due to elongated molecules which are oriented parallel with the long axes. Today, it is clear that, in principle, this explanation was valid. Lehmann created the designation "fluid crystals" and "liquid crystals" ("flieBende Kristalle," "fliissige Kristalle"). Compounds which exhibit the liquid crystalline state in a certain temperature interval are called "thermotropic" liquid crystalline. Lehmann and others also experimented with mixtures, especially mixtures of certain organic salts and water, and detected a variety of liquid crystals which is existent only in mixtures of this kind and which are called "lyotropic" liquid crystals. Over the next decade about 15 liquid crystalline compounds became known; in all these cases the detection of the liquid crystalline phases occurred by chance since no connection between the molecular shape and the liquid crystalline state was known. Soon after 1900, Daniel VorUinder, who was professor of chemistry at the University of Halle, started his systematic synthetic work in order to find connections 1
H. Stegemeyer et al. (eds.), Liquid Crystals © Springer-Verlag Berlin Heidelberg 1994
between the molecular structure of chemical compo
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