Focal-conic fan texture

They observed that the sample with a polyion backbone formed a smectic A phase with a focal conic fan texture and a perpendicular structure. On the other hand, the material based on the neutral backbone formed a nematic phase with a schlieren texture. Once again, the presence of charges in the polymer severely influenced the polymorphism of the compounds. The authors showed the potential of these LC systems in the fields of photomemory, optical storage, and light drive display, especially because these amphiphilic polymers yield excellent azodense LC thin films [96]. 

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. 

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). 

Figure 4.21. Focal-conic fan texture of a smectic A phase.

Sc Sa, Sa->Ch, and Ch->I respectively. The smectic D phase is observed in the hexyl series but not in the decyl analogs. This observation suggests that the increased flexibility provided by the longer alkyl group destabilizes the higher temperature smectic phases, while stabilizing the smectic C. Optical micrographs of the PDBPB polymer exhibit a fine broken focal conic fan texture for Sq phase, and focal conic fan textures for the Sa phase and cholesteric texture. 

Figure 5.16b shows a broken fan texture, which results from cooling down the sample from the focal conic fan texture of the lamellar L phase. The continuity of the fans suggests that the lamellar structure persists at the phase transition. However, the breaking of the fans again indicates the formation of a tilted structure. 

Plate 2 Focal conic fan texture of the smectic A liquid crystal phase. 

The elhpses and hyperbolae of the focal-conic fan texture appear as black lines in the texture because these are the areas where the sharp changes in the direction of the optic axis are foimd. The area of a sudden change in the direction of the optic axis is isotropic and so this region appears black when viewed using optical polarising microscopy. 

The crystal B phase possesses an even more ordered arrangement of molecules (see Chapter 3) in which the correlation of the hexagonal nets of molecules extends three-dimensionally over large distances, and hence this phase is a disordered crystal phase rather than a liquid crystal phase. Accordingly, this phase does not produce a genuine focal-conic fan texture because curved layers are not possible. However, what appears to be a focal-conic fan texture does form on cooling from the phase. Instead of the fans... 

Broken focal conic fan texture and schlieren textme of the smectic C liquid crystal phase. 

Fig. 2. Photomicrograph of a focal-conic fan texture of P6BA/80Sz appearing from an isotropic phase at 177 on cooling viewed between crossed polarizers. 

Nierengarten et al. reported liquid crystalline pillar[5]- and pillar[6]ar-enes. They synthesized pillar[5]arene 6.32 and pillar[6]arene 6.33 with 10 and 12 mesogenic cyanobiphenyl units, respectively (Figure 6.20). Both compounds showed smectic A phase (focal-conic fan texture and homotropic areas) over a broader temperature range when compared with the model compound. This can be explained as the macrocyclic effect Inter-molecular 71-71 interactions between neighboring pillar[n]arene cores stabilized the smectic A phase. 

Polarized photomicrographs of smectic A samples show so-called focal-conic fan textures (Fig. 6.29). Similar but not identical structures are also found in smectic C phases. The origin of these structures is the preference for splay distortion as opposed to the unfavourable twist and bend distortions in these smectics. 

Figure 13 (a) Photograph of the focal conic fan texture of smectic A liquid crystal... 

The appearance of focal conic structures depends on the direction of viewing. When the epitaxial interactions between the mesophase and the substrate surface, cause the axes of the focal conic units to lie vertical, the polygonal texture appears (Figs. 1.23 and 1.24). On the other hand, samples with focal conic units lying obliquely to the viewing direction, give the characteristic fan texture (Fig. 1.25). [A note of caution for decades, when faced with an undecipherable optical texture with a jumbled mass of domains, workers in the liquid crystal field have often taken the easy option and simply called it a focal conic fan texture to disguise the fact that they had no idea what it was. They have usually been correct, but not always, and mesophases of very different structures have been seriously misidentified on more than one occasion.]... 

Fig. 1.22 The geometry of focal conic units, (a) the simplest geometry, with a circular cross-section and a straight, central discilination line, (b) the more general geometry, with an ellipse and hyperbola, (c) a slanting section of a focal conic unit, showing the origin of the characteristic focal conic fan texture...

Smectic C modifications exhibit two microscopic textures the Schlieren texture and the focal-conic fan texture. As previously discussed, the Schlieren texture of the smectic C phase can be distinguished from that of the nematic phase by the fact that it exhibits only singularities with S = 1. Compared to the corresponding texture in smectic A, the broken focal-conic fan texture of a smectic C is less regular and disturbed by additional disclinations. If the C phase is formed on cooling a smectic A phase, then the Schlieren texture will be obtained from a homeotropic A texture and the broken fan texture will be obtained from the simple focal-conic texture of the preceding A phase. 

Transitions with the participation of liquid crystals may show characteristic phenomena. If a nematic modification turns to a smectic A or smectic C phase, transient stripes in the form of a myelinic texture (also called chevron texture or striated texture) are often visible. Another example is the paramorphotic arced focal-conic fan texture of the smectic... 

Fig. 2.26. Paramorphotic arced focal-conic fan texture observed upon cooling from the smectic A state (cf Fig. 2.22) for the polyester prepared from di-n-propyl-p-terphenyl-4,4"-carboxylate and HO—CH(CH3)—CH(CH3)—OH.

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