Broken focal-conic texture

The same authors increased the complexity of their systems by introducing in a polyester chain both ionic and chiral chain segments. The series containing both the isosorbide chiral units and the ionic moieties yielded chiral smectic C (SmC ) and chiral smectic B (SmB ) liquid-crystalline phases, exhibiting broken focal-conic texture and schlieren texture. Not surprisingly, the analogous polymer without the chiral units exhibited only the nonchiral SmC mesophase. On the other hand, in this case, the effect of ionic units on the phase behavior was negligible [91]. 

Fig. 9. Broken focal conic texture for a homopolyester prepared from di-n-propyl p-terphenyl-, i+ carhoxylate and HOfCH -CH -O-), H. (Crossed polarizers, 2 0 C, x 200) From ref. 20-... 

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). 

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. 

Smectic phases show a number of characteristic textures including (1) the focal conic and fan texture, characteristic of smectic A, and often formed from the coalescence of batoimets (2) the mosaic texture observed when the smectic B phase is formed and (3) the broken focal conic structure resulting from smectic C phases that can also show the Schheren patten described earho-. Examples of focal conic textnres arc shown in Figure 11.13. 

By studying paramorphotic patterns and the way that they appear in focal-conics, phase identification can be accomplished and information on mesophase structure can be obtained. However, the problems of phase identification are greatly cased when the focal-conic texture is accompanied by a homeotropically oriented texture. For example, the smectic A phase can exhibit the unbroken focal-conic and optically extinct homeotropic textures together, whereas the smectic C phase exhibits broken focal-conic and schlieren textures, and the E phase exhibits banded focal-conic and mosaic textures. Thus, the... 

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

The texture change or memory effect is observed in cholesteric materials with negative dielectric anisotropy [71]. The liquid crystal layer is homogeneously oriented by boundary forces to form the planar texture which is completely transparent if the band of selective light reflection is outside the visible spectrum. The substrates are covered with conducting films that are in contact with the liquid crystal. When a d.c. or low frequency field is applied, the sample is transformed to the so-called focal conic texture. In this texture, the liquid crystal is broken up into small domains which are randomly oriented and have diameters of a few microns. Since these domains are optically anisotropic, they act as scattering centers for visible light. Therefore the focal conic texture exhibits a milky white appearance. 

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. 

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. 

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