Characterization of Mesophases

Abstract

Most crystals transform directly into the liquid phase, so that the long-range translational order and the long-range orientational order of the molecules are destroyed simultaneously. However, if the constituent molecules have pronounced anisotropy of shape, the disappearance in one, two or three dimensions of the long-range translational periodicity in the crystal may precede the collapse of the long-range orientational order. Such compounds do not show a single transition from solid to liquid but rather a cascade of transitions involving new phases; the mechanical properties and the symmetry properties of these phases are intermediate between those of a liquid and those of a crystal. These intermediate states of matter were first termed ‘liquid crystals’ by Lehmann (1889). They were later termed ‘Les etats mesomorphes’ by Friedel (1922) to avoid the ambiguities and controversies inherent in a term such as liquid crystals’ or ‘crystalline liquids’ and to indicate explicitly their character as quite distinct states intermediate between the perfectly ordered periodic structure of solid crystals and the perfectly disordered structure of the amorphous state. Nowadays the terms liquid crystal (LC), mesomorphic or (mesomorphous) state and mesophase are employed almost synonymously. Substances that under suitable conditions form mesophases are referred to as ‘mesogens’.