Mechanical and Thermophysical Properties of Polymer Liquid Crystals

may never overcome the effects of hysteresis and stress (see Chapters 6 and 12). The first sentence of the reference work, Handbook of Liquid Crystals, reads: The terms liquid crystals, crystalline liquid, mesophase, and mesomorphous state are used synonymously to describe a state of aggregation that exhibits a molecular order in a size range similar to that of a crystal but acts more or less as a viscous liquid: [2] In other words, molecules within a liquid crystalline phase possess some orientational order and lack positional order; furthermore, the shape of a liquid crystalline sample is determined by the vessel in which it is contained rather than by the orientational order of its aggregated molecules. The authors recognized the limitations and imprecision of this definition but, like others preceding them, could not devise a simple and generally applicable one that is better. Regardless, the terms 'liquid crystal' and 'mesophase' should not be used interchangeably. As mentioned above, all liquid crystals are mesophases, but all mesophases are not liquid crystals. Recent studies, employing elaborate and sophisticated analytical techniques, have permitted finer distinctions between classical crystals and mesophases. At the same time, they have made definitions like that from the Handbook of Liquid Crystals somewhat obsolete for reasons other than terminology. One part of the problem arises from the use of a combination of bulk properties (like flow) and microscopic properties (like molecular ordering) within the same definition.