Textures of polymeric liquid crystals

Nevertheless in polymeric liquid crystals the same types of orientational defects and thus the same types of textures as present in the low mass counterparts have been observed. The textures often formed by polymers are the threaded texture, the schlieren texture and the focal conic texture of smectics. As is for low mass liquid crystals, the texture is a consequence of defects (disclinations and dislocations, refer to Chapter 1) present in the liquid crystal and is characteristic of a specific type of the phase. The texture examination has become a very useful tool in the determination of the type and nature of the polymeric liquid crystals. 

In a nematic phase, the strength of disclination may have values that are half-numbered or whole-numbered. However because the disclination energy is proportional to s2 (Nehring and Saupe, 1972), it is rare to observe singularities with s 1. In studies of a series of nematic polymers with two-dimensional mesogenic units Zhou and coworkers (1993) have been 

Disclination in smectic phases may also show up in the form of schlieren textures. It was believed that only the whole-numbered singularities are present in these phases. However more recent work has shown that certain smectic Ca phases can also give schlieren textures with half-numbered singularities (Watanabe et al, 1989 1992 Niori et al., 1995). Other methods such as X-ray scattering may be needed for an unambiguous characterization of the 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. 

The often observed textures for Sa are the focal-conic fan texture and the homeotropic texture. When the molecular orientation is homeotropic, the optic axis is perpendicular to the film surface so that the preparation appears black (pseudoisotropic) on a crossed polarizing microscope. As with the homeotropic nematics, the homeotropic Sa phase can be differentiated from the true isotropic phase by conoscopic observations. Further more, if the cover slip of the homeotropic Sa preparation is slightly moved, the orientation is disturbed resulting in oily streaks in form of bright bands. The homeotropic Sa phase may thus be distinguished from the homeotropic nematic phase. 

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