Texture, planar

To interprete these X-ray spacings, let us first examine the structure of model compounds PMAA-n and PMMAA-n. The macromolecules of these polymers have a chemical structure similar to that of Pc5iMAA-n with the difference that they do not contain cholesterol groups in the side chains. 

On the basis of the obtained X-ray data, the structure of cholesterol-containing pol3rmers of the PChMAA-n series may be presented as follows. 

First of all, it is evident that the structU3 e of PChMA and PChMAA-2 differs from that of PChMAA-n where n 5. This difference msmifests itself both in the values of dj and the dependence of small-angle X-ray spacings d2, 4 

Another reason why it is difficult to discuss the structural model of cholesterol-containing pol pers is the fact that so far no adequately substantiated structural model of low-molecular cholesteric liquid crystals exists, apart from the already mentioned packing pattern proposed in Ref. (26), It seems that for a full understanding of the mesophase structure of cholesterol esters, one should, first of all, carefully study their crystalline structure. Such a study, described in Refs, (27, 28), has not yet lead to a complete description of molecular packing in cholesterol esters. 

Although the structure of cholesterol-containing polsrmers is not yet completely understood, there is no doubt that the liquid crystalline properties exhibited by these polymers are due to a particular oirder in the arrangement of cholesterol groups. This is corroborated by the results of studying the structure and optical properties of model compounds not containing cholesterol. The above-mentioned PMAA-n and PMMAA-n are optically isotropic in all three physical states vitrified, elastic and viscous. 

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Another technique widely used to measure the cholesteric pitch is based on the Bragg scattering of monochromatic light (obtained by a helium-neon laser) from fingerprint or planar textures of the cholesteric32 Its angular dependence is strictly related to the cholesteric pitch. 

Note 3 The angular distribution of the director around a defect in a nematic planar texture, in the X-Y projection, can be expressed in terms of the polar angle 0 ... 

A (110) planar texture formed (67) with no epitaxial relationship to the substrate. 

Another and more characteristic texture observable of the cholesteric phase is the Grandjean planar texture with a series of regularly spaced bands separated by sharp lines. This was observed by F. Grandjean (1921) when a wedge-shaped split between two sheets of fresh mica (or clean glass slides) was used as the cell for examination under crossed polars. Under such conditions the cholesteric phase formed on cooling a homeotropic planar texture with visible bands. 

A sheet of cholesteric liquid crystals is sandwiched between two glass plates separated by a gap of tens microns. The cholesteric liquid crystals on two glass plates are homogeneously aligned to form the planar texture. The cell displays a bright color. The color varies according to the view angle and temperature. This is an important characteristic of cholesteric liquid crystals—selective reflection. 

We next examine the effect of a magnetic field acting along the helical axis of a cholesteric film having a planar texture. If > 0 and boundary constraints are absent, there is a possibility of a 90° rotation of the helical axis because > Xi- on the other hand, boundary effects are... 

Figure 4. Refractive indices of a lyotropic cellulosic system (3C1-CTC/ EMMAc, concentration c=0.8 g/ml) as a function of temperature, measured on samples exhibiting planar textures with an Abbe refractometer. n ordinary refractive index Oe- extraordinary refractive index 0 n refractive index in the isotropic phase . The birefringence is defined by An y,=ng-n and shown in the insert (b) defines...

The optical properties of chiral nematic phases are closely related to their supermolecular Structures, as stated by the considerations of de Vries. In particular, the planar textures exhibit beautiful colors correlated to the pitch P of the helicoidal structures by Eq. (1), if the selective reflection wavelength lies in the visible range, and many examples are shown in Fig. 2. 

Thus, all monomers of the ChMAA-n series fonn a monotropic liquid crystalline phase of the cholesteric type, whose temperature interval of existence depends on the rate of cooling. The liquid crystalline phase is unstable and is transformed to crystal phase so soon that X-ray examination of the mesophase structure becomes difficult. Nevertheless, polarization-optical studies have made it possible to draw certain conclusions as to the nature of the liquid crystalline phase of monomers. Cooling of isotropic melts of monomers results in a confocal texture which turns to a planar one when a mechanical field is superimposed on the sample, for example, by shifting a cover glass in the cell of the polarizing microscope (Figure 4). The observed planar texture exhibits the property of selective light reflection, which is typical of low-molecular cholesteric liquid crystals. 

Figure 4. Optical microphotograph of the planar texture ChMAA-11 (crossed polarizers)...

Fig. 15 Schematic representation of the different types of supramolecular arrangements of discotics on surfaces with a edge-on orientation of the molecules, where the columnar axis is oriented parallel to the substrate (planar texture), and b face-on arrangement of the discotics leading to a homeotropic texture. Reproduced from [125]. (2005) Wiley-VCH...

Fig. 7 Typical planar texture obtained at 260°C by the addition of an optically active compoimd to the polyazomethine III ...

Cholesteric phases are quite complex and are easily deformed by even small forces, causing a broad range of observable textures. Among the observed textures, the planar textures, known as oily streaks, are among the most commonly observed textures [1,12],... 

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