Dark brushes

Note 2 The radial droplet texture shows four dark brushes located in the regions where the director is in the polarization plane of either the polarizer or the analyser. 

In a previous paper (1), phase segregation by spinodal decomposition in mixtures of polyethylene terephthalate and polyhydroxybenzoic acid copolymer (PET-PHB) and polycarbonate (PC) has been investigated. It was shown that thermally induced phase segregation takes place above the Tg of PC and exhibits a lower critical solution temperature (LCST). However, the phase separated domains do not grow until the temperature exceeds 255°C. Some disclinations developed within the liquid crystal rich regions. Even in the pure PET-PHB component, four dark brushes with negative sense of disclinations form around 240°C, indicating the presence of nematic liquid crystals. Paci and coworkers (2) claimed that a smectic-nematic transition exists near 270°C in this liquid crystalline copolyester. 

For example. Fig. 2 shows a SchHeren texture, which is typical of nematic distortions of the director field (visible as dark branches usually named brushes). These distortions are induced by the perpendicular anchoring on the microscope slide of topological defects, called disclination lines, which are numbered from 1 to 7 in Fig. 2. These disclination lines can be classified into two groups according to their local topology as the dark brushes seen around these defects can rotate either clockwise or counterclockwise as the crossed polarizers are rotated simultaneously while the sample is kept fixed. This is due to the two different possible defect topologies illustrated in Fig. 2 for cases 1 and 2 [1]. 

Fig. 3.5.1. Director orientation (indicated by arrows) along a polar line making an angle . Incident light that is linearly polarized at angle or 7il2 will be extinguished by a crossed analyser and will give rise to a dark brush.

X axis, it is seen from fig. 3.5.1 that the polarization will be unchanged at all points on the polar line a and hence will not be transmitted by the analyser. This will result in a black brush at an angle a. A similar situation will arise when changes by n/2. The angle between two successive dark brushes is therefore Aa = = n/2s. Thus the number of dark brushes... 

From the nature of the director patterns it is clear that dark brushes of the schlieren type will not be seen under the polarizing microscope for light propagating normal to the film (see 4.1.1). Twist disclinations may therefore be expected to be less conspicuous than wedge disclinations, and few observations have been reported of their existence in ordinary nematics. They do, however, reveal themselves under favourable circumstances in twisted nematics, often as loops separating regions of different twist. ... 

The SchhCToi textures show large dark brush patterns, corresponding to the extinction zones where the mesograis arc ahgned perpendicular to the glass slide. Also noticeable are the points where two or four of these brushes meet if the texture shows points where only two brushes meet, this is an unambiguous indication of a nematic phase. It should be noted that if a Schheren texture shows only points where four brushes meet, then this represents a smectic C texture, or its chiral modification. 

The threaded texture changes to the Schlieren texture when the lines are all perpendicular to the sample boundaries. This texture observed between crossed polarizers displays dark brushes (also called black... 

Fig. 8.13 Two disclinations fixed by their end at the two glasses limiting a layer of a nematic liquid crystal. They interact with each other by the elastic force proportional to 1/Pi2 (a). The structure of the director field n(r) near the two disclinations of positive and negative strength and four dark brushes corresponding to the j = 1 disclinations (b)...

Nematic phases typically show a schlieren texture between crossed polarizers if the director is oriented perpendicular to the viewing direction. One feature of the schlieren texture is the occurrence of topological point defects. At these point defects either two or four dark brushes meet. The corresponding defects are denoted as 1/2 or 1, respectively. Further characteristic textures of the nematic phase are the thread-like texture, which exhibits n disclinations parallel to the substrate, and the marble texture, in which areas of differing uniform director orientations occur. 

Many liquid crystalline polymers exhibit Schlieren textures, which display dark brushes. These correspond to extinction positions of the mesophase. At certain points, two or more dark brushes meet at points called disclinations see Figure 7.11 (27). The disclination strength is calculated from the number of dark brushes meeting at one point ... 

In the case of 2,3,4-trialkoxycinnamic acids (compound 3) [18,23], the conoscopic observations were made on much thinner samples ( 20 pm). These acids, sandwiched between two glass plates, the inside surfaces of which were coated with transparent tin oxide, were aligned homeotropically using an electric field. The interference pattern obtained under these conditions is shown in Fig. 7. The position of the dark brushes is independent of the temperature. Rotation of the sample around 90° makes the brushes coalesce and reappear in the perpendicular direction, as shown. 

Schlieren texture A planar liquid crystal birefringence texture characterized by dark brushes. ... 

Fig. 11 illustrates these characters. The plane which contains the ellipse is always parallel to the layers belonging to the homeotropic region. By changing the focusing of the microscope, it is possible to follow a line singularity, which lies in the vertical plane containing the major axis of the ellipse and passes through the focus of the ellipse. This line is one branch of hyperbola. Between crossed nicols, the focus of the ellipse is the center of four dark brushes (fig.l 1). These observations lead to the conclusion that these domains are FCD s of the first species. 

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