Schlieren nematic texture

The optical polarizing microscopy analysis of these copolymers showed in all cases schlieren nematic textures, in contrast to the corresponding polyethers which present smectic type textures. All these copolyethers are soluble in aromatic and halogenated solvents. 

Disclinations in tire nematic phase produce tire characteristic Schlieren texture, observed under tire microscope using crossed polars for samples between glass plates when tire director takes nonunifonn orientations parallel to tire plates. In thicker films of nematics, textures of dark flexible filaments are observed, whetlier in polarized light or not. This texture, in fact, gave rise to tire tenn nematic (from tire Greek for tliread ) [40]. The director fields... 

When an isotropic texture appears on cooling from an isotropic melt, it is formed from the coalescence of droplets that separate from the liquid. This mechanism is indicative of a nematic phase, and one of three possible characteristic textures may result, depending on the way the droplets form larger domains. These are the Schlieren, the threaded, or the marbled nematic textures. An example of Schlieren texture is shown in Figure II. 12. 

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... 

Lyotropic nematic phases were first reported by Lawson and Flautt (62) for mixtures of Cg and Cio alkyl sulfates, together with their corresponding alcohols in water. They are somewhat less common than the mesophases discussed so far. When they do form, they occur at the boundary between an isotropic micellar phase (LQ and the hexagonal phase (Li/HQ, or between Li and the lamellar phase (Li/L ). As their name implies, they have a similar micellar order to that of the molecules in a thermotropic nematic phase. This long-range micellar orientational and translational order is lower than in the other lyotropic phases described above. Like the thermotropic phases, they are of low viscosity and can be aligned in a magnetic field. It is possible to identify nematic phases optically because of their characteristic schlieren optical texture. 

For (lb), having the long flexible spacer of six methylene units a normal nematic phase is observed displaying the typical vi/ell known nematic textures under the polarizing microscope. Marbled textures as well as Schlieren textures can be seen, and they exhibit the same types of defects as ordinary l.-lc s. Conoscopic observations clearly prove the optically uniaxial positive character. The absolute value of the birefringence An as well as the temperature dependence of An correspond to chemically similar l.-lc s ... 

Mostly thermotropic polyesters show a nematic phase with a typical appearance that is called a marbled texture, a Schlieren texture, a threaded texture, or, a nematic droplets depending on the thickness and temperature of the polyester. Only in a few cases, they exhibit low-order smectic phases [100], which are identified through the observations of fan-shaped and broken fan textures in their LC phases. No report of higher-order smectic phases has been observed in this class of pol)nners. It is worth mentioning that nematic texture can simply be frozen into a glassy state in the case where there is no interference from the crystallization process. An intriguing property of an LC polyester is its biphase structure over... 

FIGURE 2.5 A typical planar-aligned nematic texture imaged with polarized light microscopy showing the characteristic "Schlieren texture."... 

Fig. 5. Nematic schlieren texture observed between crossed polarizers. Courtesy of G. H. Brown, Liquid Crystal Institute, Kent State University.

Note 3 Schlieren textures observed in nematic samples with planar alignment show defect centers with two or four emerging brushes. Schlieren textures in nematic samples with tilted alignments show centers with four brushes centers with two brushes are caused by defect walls. 

Similarly with low-molecular nematics is manifested in that the nematic polymers may form equally well schlieren texture, typical for low-molecular nematics (Fig. 18a) (polymers B.3.3-B.3.4, Table 9). The enthalpy of transition from LC state to isotropic melt is also close to that for low-molecular nematics. At the same time, there also exist definite structural differences. X-ray patterns of the same polymers, even in unoriented state, display certain elements of structural ordering in the arrangement of side branches (a weak diffuse halo at small angles), which could indicate a sibotactic nematic type of ordering. These differences are most distinct for oriented polymer films. As an example Fig. 18b, c, present X-ray patterns of unoriented and oriented samples of one and the same nematic polymer 121 l24. In fact two sharp small angle... 

After DMAP-catalyzed esterification of 42a with tri-benzoate esters, liquid crystals 55, 56 (Scheme 32) were obtained, which both displayed narrow nematic phases as identified by their typical Schlieren texture. The melting and clearing points are high and, in the case of 55, accompanied by decomposition. Compounds with shorter arms were also synthesized but found to be crystalline. 

However, only 65a exhibits thermotropic nematic behavior if amorphous thin films are heated to 155 °C, at which a Schlieren texture develops. The material decomposes above... 

Above 110 °C, this arrangement becomes mobile, and a smectic C liquid-crystalline phase is entered. Samples cooled down from the isotropic melt (140 °C) show Schlieren and banded textures when viewed under crossed polarizers (Figure 8). These textures look similar to nematic Schlieren textures, but from the X-ray diffraction data it is clear that 12c forms a homeotropically oriented smectic C phase. In a nematic phase, the small-angle diffraction peak would be absent, and a broad scattering feature, a nematic streak , would be observed. Polymer 12c was the first example of a PPE derivative for which three states of matter, i.e. crystalline, thermotropic liquid crystalline, and a highly viscous isotropic liquid, were accessible [46]. 

None of the ferrocene derivatives 13 showed liquid crystal properties on heating. They all melted into an isotropic melt. When cooled from the isotropic liquid, the first members of the series [n = 1 — 6) exhibited a monotropic nematic phase. A representative example of a nematic schlieren texture is shown in Fig. 9-10. 

Fig. 9-10. Representative thermal polarized optical micrograph of the nematic schlieren texture displayed by 13 ( = 5) on cooling from the isotropic melt to 159 C.

Fig. 4.2 Schlieren texture of the nematic phase of 1 (TfjN" salt) (Reproduced from Ref. [6] with kind permission of The American Chemical Society)...

One of the few examples of ionic nematic (monotropic phase) was reported by Cardinaels and coworkers [6]. The Tf N imidazoUum salt 1, symmetrically substituted with two mesogenic cores of alkoxycyanobiphenyl, showed a nematic phase between 47 C and 31°C. Typical schlieren textures are shown in Fig. 4.2. 

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]. 

A second noteworthy series is that of the dirhodium tetraalkanaotes (Figure 80 above) which have been extensively studied as thermotropic mesogens (see 5. 2. above). However, in solution in hexadecane, nematic phases are formed as evidenced by the schlieren textures observed [188],... 

The polycarbonates 62a-f and 63a-f were prepared by transesterification of the corresponding diols with the biscarbonate 55 in the molten state. At least after annealing all members of both series were semicrystalline showing a melting endotherm in the DSC heating traces. Unfortunately, DSC curves of 62a-f were not published. An unidentified birefringent mesophase was mentioned for 62a-e, whereas a nematic melt was reported for 62f. However, the published texture of this melt is not a typical nematic Schlieren texture, but looks like a suspension of solid particles in an isotropic melt. For all members of series 63a-f a nematic... 

Figure 13.11 (a) Thread-like domains in a nematic liquid crystal of thickness 100 4m viewed under crossed polarisers. (b) Schlieren texture in a nematic film of thickness 10 J,m (reproduced with... 

A schlieren texture is often observed for polymeric nematics. This texture is characterized by point singularities from which two (s = 1/2) or four dark (s = 1) brushes radiate. A representative photomicrograph of the texture is given in Figure 4.13. In this nematic a schlieren texture is observable for both the singularities of two and four brushes. 

Figure 4.13. Photomicrograph of the schlieren texture of a polymeric nematic phase.

---