5 - Smectic liquid crystals
Published online by Cambridge University Press: 08 January 2010
Summary
Classification of the smectic phases
Present classification of smectic liquid crystals is based largely on the optical and miscibility studies of Sackmann and Demus. The miscibility criterion relies on the postulate that two liquid crystalline modifications which are continuously miscible (without crossing a transition line) in the isobaric temperature–concentration diagram have the same symmetry and therefore can be designated by the same symbol. It is not clear whether this criterion is valid regardless of the differences in the molecular shapes and dimensions of the two components, but empirically Sackmann and Demus have found that in no case does a phase of a given symbol mix continuously with a phase of another symbol. The method is simple and has been used for the identification of a number of new phases, but, of course, it does not throw light on the precise nature of the molecular order in these phases. Systematic X-ray investigations have been carried out during the last decade at several laboratories, and particularly with the availability of synchrotron X-ray sources, considerable progress has been made in elucidating the structures.
The notation of Sackmann and Demus is according to the order of the discovery of the different phases and bears no relation to the molecular packing. The broad structural features of these phases are summarized in table 5.1.1. A more detailed description of these structures may be found in the excellent reviews by Pershan and by Leadbetter.
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- Liquid Crystals , pp. 300 - 387Publisher: Cambridge University PressPrint publication year: 1992
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