Introduction to Thermotropic Liquid Crystal Phases
The basic features that distinguish the various liquid crystalline phases from the other condensed phases will be given, and the type of chemical compounds will be discussed. The various types of thermotropic mesophases formed from rod-like molecules will be described in some detail and the order parameters that characterize them will be defined. Apart from the nematic phase, that is characterized by orientational order and translational symmetry of the molecular positions, various types of smectic phase can be distinguished. Their common feature is the existence of a one-dimensional density wave. Additional symmetry properties distinguish the different types of smectic phase. Thermotropic mesophases with disc-like molecules can also lead to a nematic phase. Reduced positional order now gives so-called columnar phases. The analogies and differences with the phases of rod-like molecules will be discussed. Both symmetry elements can be combined in lath-like molecules, leading to possible transitions between the two types of phase. In the first Chapter of this section a short overview of the various types of liquid crystals that can be observed will be given, including a discussion of the types of molecules from which they can be formed. In addition the order parameters of the most relevant phases will be defined. Emphasis will be on the smectic phases, anticipating what I suppose might be relevant to other lectures in this school. I will restrict myself to thermotropic liquid crystals (for which temperature is the important variable for the phase behaviour), and will not consider lyotropic liquid crystals (for which the variation of concentration in a solution is a dominant aspect). In spite of their increasing importance liquid crystalline polymers will also not be included.
KeywordsLiquid Crystal Nematic Phase Orientational Order Smectic Phase Positional Order
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