Schlieren textures

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... 

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

Figure 9. Top Two tetraurea calixarene monomers connected by a rigid spacer at their bottom rim display diverging hydrogen-bonding sites ideally suited for polymerization. The polymer (53) bears capsules of ca. 1.6 nm x 2.2 nm dimensions like beads on a string. Bottom Photomicrographs of typical Schlieren textures of 53 in chloroform (top row, left) and p-difluorobenzene (top row, middle) as viewed...

Note 1 Diselinations are responsible for some optical textures seen with a polarizing microscope, such as the schlieren texture formed by disclination lines in nearly vertical orientations, whose projections are seen as dark points with two or four emerging dark stripes or brushes (see Definition 4.9.2). 

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. 

Note 4 A thin sample of a smectic C phase with the layers parallel to the sample surfaces gives schlieren textures with centers that have four brushes. However, a smectic C phase formed by odd-membered liquid-crystal dimers (see Definition 2.11.2.9, Note 5) has schlieren textures with two or four brushes. 

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.

These different contrast mechanisms can all be used to reveal the scale of liquid crystalline polymer microstructures. In specimens that exhibit a mosaic texture, and in those that contain predominantly planar defects, domain size is easily defined in terms of areas that uniformly show extinction between crossed polars. However, if the defects are predominantly linear, as in specimens that exhibit schlieren textures, such simple characterization of microstructural scale is no longer possible. Here it is more convenient to look at the length of disclination line per unit volume, which is equivalent to the number of lines intersecting unit area, and analogous to the dislocation density as defined for crystalline solids. Good contrast is essential in order to obtain an accurate count. Technologically, microstructural scale is of growing interest because of its relevance to processability, mechanical properties and optical transparency. 

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. 

Fig. 4.3 Schlieren textures of SmC phase of the symmetric nonyl derivative 1 (bromide salt) at 90°C with only four-brush defects (Reproduced from Ref [31] with kind permission of Elsevier)...

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]. 

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... 

All other members of this series were cholesteric displaying colourful Schlieren textures. The capability to form GJ-textures was completely lost, when the molar fraction of isosorbide was increased to 20% (or higher). In other... 

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... 

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. 

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.

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. 

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