Defects 335-7 focal conic textures

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. 

The formation of focal-conic textures is characteristic of smectic phases (Gray and Goodby, 1984). These textures are the result of smectic layers arranged in Dupin cyclides (Friedel, 1922). The common defects included in these arrangements have the form of ellipses and hyperbolae in certain confocal relationship. Dependent on the direction of observation relative to the defects, the dark lines shown in Figure 4.20 may be observed in focal-conic domains. 

Figure 4.21 shows a focal-conic fan texture of a smectic A phase in which is observable some defects represented by Figure 4.20(b). While Sa forms the fan-like focal-conic textures, the focal-conics formed by the Sc phase are often broken and less distinct. In addition, as stated above, Sa is uniaxial but Sc is biaxial. Sa may take a homeotropic alignment with the axis normal to the sample plane and extinct on POM with crossed polars. The two smectic phases are therefore distinguishable with POM. Nevertheless, it is desirable to include WAXS studies in order to identify the two with assurance (Section 4.3). 

Because of the layered structure, defects in the cholesteric can be likened in many respects to those in smectic A. Both of them exhibit focal conic textures and both allow for the existence of screw and edge dislocations. To discuss these similarities we employ a coarse-grained approximation in which the cholesteric distortions are considered to be small and to vary slowly over a pitch. In this approximation the free energy of distortion may be expressed in terms of layer displacement u parallel to the twist axis ... 

The polarised tight is now affected by the anisotropic, birefringent nature of the phase stmcture and a bright texture appears that is representative of the defects within the stmcture. Ideally, both the focal conic texture and the homeotropic texture should be... 

The texture transitions have been observed in smectics A many years ago [104], however, they are especially pronounced in materials with large dielectric anisotropy. A high-quality planar texture goes to a quasi-homeo-tropic optically transparent texture via intermediate structural defects (80CB, Ae = 8.2, d = 50 /xm, and voltage 30 V) [105]. Other substances were studied in [106]. A homeotropic texture can also be obtained by applying the field to the less-ordered focal-conic texture if Ae > 0 [107] (erasing a defect structure produced either by the thermal action of a laser or by electrohydrodynamic instability [108]). 

Friedel did however understand the layered nature of smectics, firstly through the stepped edges possessed by smectic droplets with a free surface, and secondly through his detailed studies of the optical microscopic textures of thin films of smectic phases. He understood the optical discontinuities, i.e., the defects, of the smectic focal-conic texture and saw the relationship of the black lines delineating ellipses of different eccentricities and their associated hyperbolae in terms of focal-conic domains which may be divided into a series of parallel, curved surfaces known as Dupin cy-clides. He also understood that the optically extinct homeotropic textures of smectics of the type he studied gave positive uniaxial interference figures consistent with systems of layers lying flat to the surface. His microscopic studies demonstrated the immense value of the optical microscope as a precise scientific instrument in studies of all types of liquid crystal phases. 

Under the polarized optical microscope, the liquid crystal films show colorful patterns, i.e., the optical textures. Each liquid crystal phase shows its typical texture which provides the means to identify the phase of the liquid crystals. The typical textures are the Schlieren, threadlike, homeotropic, homogeneous, marble, finger-print, focal-conic, Dupin cyclide, fan-shape, sanded, mosaic, and so on. They are the integrations of many typical defects mentioned above. Demus Richter (1978) were the first to review... 

POM), the Schlieren and homeotropic defect textures of the nematic phase, and the focal conic and homeotropic defect textures of the smectic A phase could be observed (Fig. 4.3). In addition, the parallel and perpendicular dielectric permittivities of mixtures of the GNPs dispersed in host nematic LCs were determined by using one-cell method , which was reported by Clark et al. [47 9]. 

Fig. 4.3 a, b Homeotropic and schlieren textures of the nematic phase of 8CB containing 5 wt% 1 coated GNPs. c Sheared nematic texture near to the edge of the cover slip, d Focal-conic defect texture of the smectic A phase. Magnification xlOO [46], Copyright from Wiley-VCH 2011... 

Textures of SmA samples with focal conic defects are shown in Figure 6.10. 

More recently it has been proved that it is possible to prepare cholesteric homopolymers.Krigbaum et al. observed fan-like texture for polyester synthesized from 4,4 -dihydroxy-a-methylstilbene and (+)-3-methyladipic acid. These authors suggested the formation of a strongly twisted cholesteric. Indeed, in low molecular weight cholesterics with high twist, the defects and textures resemble those of smectics, especially smectic A. Such cholesteric substances may yield non-planar textures. They may appear in fan-shaped, focal conic or polygonal textures. In the case of... 

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