Structure, properties, and some applications of liquid crystals*

Liquid crystals may be classified in two major categories, namely, thermotropic and lyotropic. Thermotropic liquid crystals are commonly prepared by heating organic compounds, whereas lyotropic liquid crystals are made by mixing two or more components. Thermotropic liquid crystals are mainly of interest in this paper. They have been classified as nematic (threadlike) and smectic (soapy). Most commercial applications have focused on thermotropic liquid crystals; however, lyotropic liquid crystals are important in the detergent industry and in living systems. Thermotropic and lyotropic liquid crystals show polymorphism. The properties and structure of the different polymorphic forms are discussed. The two most-active areas of applications of liquid crystals are nondestructive testing and display media; both are considered. Liquid crystals have been used as solvents in structure determination (nmr) in chromatography and in study of chemical kinetics.