Structure of smectic phases

Once the helical structure of the Sc phase is unwound, ferroelectricity is displayed (see Chapter 6 for the details). In recent years, many experimental studies have revealed that some liquid crystal compounds show new types of smectic phases with complex tilt and dipole order, such as the anti-ferroelectric smectic C phase, Sca phase, and the ferrielectric smectic C phase, Sc7 phase. For instance, in the Sca phase, the spontaneous polarization Ps is opposite for successive layers. It was found experimentally that the chiral So phase is in fact similar to the anti-ferroelectric Sca phase. 

The texture of polymeric smectic. As discussed in Chapter 1 there are many subcategories of smectic phases. The identification of a smectic phase by POM becomes more difficult with less certainty when the order of molecular packing in the phase is increased. Other techniques such as WAXS are often used together with POM for conclusive identification of smectics (Gray and Goodby, 1984). Because of the complexity in the molecular structure of liquid crystalline polymers there are fewer types of polymeric smectics. The polymeric smectics most often observed are the less ordered types A and C. 

Now that we have discussed all of the structures of the phases that play a part in the formation of twist grain boundary phases, let us now consider the events that can occur at a nematic to smectic phase transition. At a normal chiral... 

The first evidence that more than one form of smectic A exists came from an observation of Sigaud et of a phase transition that was detected by calorimetry, but could not be observed optically. X-ray studies revealed that this was a transition between two forms of the A phase the higher temperature phase was characterized by a pair of refiexions corresponding to a layer spacing and the lower temperature one by two pairs of reflexions corresponding to fi / and 21 respectively. The former type of smectic A is called the monolayer (A ) phase and the latter the bilayer (Aj) phase. A third type which has a layer spacing intermediate between / and 21, has been identified and is called the partially bilayer (A ) phase. The structures of these phases are represented schematically in fig. 5.6.1. 

Fig. 5.6.10. The structure of smectic A or the twist grain boundary phase.

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