Remarks about the Phase Identification of Mesomorphic States

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Abstract
Experimental and conceptual problems associated with the identification of thermotropic mesophases are reviewed and discussed. Theories linking the observed optical properties of mesophases with their molecular building principle have considerable shortcomings. Therefore the identification of mesophases is mainly based on empirical rules pertaining to the optical behavior of characteristic textures. Certain ambiguities of a strictly optical approach may only challenge the ingenuity of the investigator, while other difficulties can prevent the determination of mesomesophase and liquid-mesophase transition temperatures. Since phase transitions are defined by variations of more general thermodynamic quantities, thermal analysis is suggested as a remedy. The requirements on temperature scanning thermal analysis necessary to insure detection of all mesophases of a given sample are specified. The significance and the determination of sample impurities by themal analysis are discussed. Comments are made on the correlation between characteristics textures and mesophases. The relevancy of uniaxial optical properties to phase identification and the homeotropic conditions of cholestreic mesophases are considered briefly.