Homeotropic texture

Fig. 27a and b. Scheme of thermorecording using the film of the homeotropic-oriented nematic LC polymer (a) and thermorecording of letters on the film of polymer XII at 105,5° (film thickness 40 pm) (b) I — initial homeotropic texture II — texture with the laser thermo-recorded letters... 

The mesophase exhibiting a homeotropic texture can still shew stir opalescence, which is a method of identification particularly suited for thermotropic polymers. This somewhat crude method of characterizing a liquid crystalline material is performed by shearing a thin film of the mesophase and looking for momentary appearances of turbidity in the otherwise transparent melt. No microscope is need to observe stir opalescence, but simple shearing of the homeotropic melt between crossed-polars can also reveal the mesophase. 

The often observed textures for Sa are the focal-conic fan texture and the homeotropic texture. When the molecular orientation is homeotropic, the optic axis is perpendicular to the film surface so that the preparation appears black (pseudoisotropic) on a crossed polarizing microscope. As with the homeotropic nematics, the homeotropic Sa phase can be differentiated from the true isotropic phase by conoscopic observations. Further more, if the cover slip of the homeotropic Sa preparation is slightly moved, the orientation is disturbed resulting in oily streaks in form of bright bands. The homeotropic Sa phase may thus be distinguished from the homeotropic nematic phase. 

The Sc phase exhibits four textures including the pseudo-homeotropic texture, the petal texture, the focal-conic texture and the lined texture (Gray and Goodby, 1984). The last occurs when the layer planes are not parallel to the surface. The lined appearance may be similar to what have been discussed for the cholesteric phase, but can also occur as being superimposed on the focal-conics. Examples of such lined and focal-conic... 

Figure 22. Development of homeotropic texture from copolyester 11 at 330°C (crossed polarizers). Annealing time (a) t=4 min, (b) t=8 min and (c) t=28 min (from [75]).

Homeotropic texture (transparent state) (light-scattering state)... 

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...

Thin layers of these mesophases exhibit other nematic characteristics such as intense movement within the melt and scintillation effects due to a directly observable Brownian motion. Homeotropic textur gca ged gy a spontaneous orientation of the sample is also found . This texture occurs when long axes of the meso-... 

Fig. 22. Conoscopic interference pattern given by homeotropic texture of nematic phase of copolyester prepared from a 50/25/25 (mole %) mixture of terephthalic acid, pyrocate-chol and methylhydroquinone ...

In these local regions the liquid crystal passes to an isotropic melt and the homeotropic orientation is destroyed. Instead of a transparent monodomain of homeotropic texture, a polydomain texture that scatters light is formed on cooling. In this way, information can be recorded on the transparent film. This information may be... 

Figure 2 Side eievation of a specimen of a smectic A phase that exhibits a homeotropic texture. The moiecuies, shown as rods, are arranged in more disorganized iayers than those shown. (Data from Gray and Goodby, 1984.)...

A homeotropic texture can also be obtained for smectic phases by creating a freely suspended film of... 

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... 

Mosaics can be observed for a number of phases, for example, smectics F and hexatic B, and crystals B, E, G, J, K, and H, all exhibit mosaics of one form or another (see Figure 10). It is very difficult to identify these phases from their mosaic textures as they all look very similar. Two exceptions are worthy of note the E phase, because it is biaxial, has a very characteristic mosaic pattern where the domains tend to overlap to give a ghost-like appearance and the B phases, where the mosaic texture is often accompanied by a homeotropic texture that remains optically extinct on rotation of the microscope stage. Figure 11 shows a variety of mosaic textures where the molecules are either tilted or perpendicular with respect to the layer planes. 

Fig. 12.22 Operation of BiNem bistable device. Two stable states are the uniform (n = 0) and 71-twisted ( = 1). Anchoring is strong at the top plate and weak at the bottom one. A strong field pulse E breaks anchoring and creates a transient quasi-homeotropic texture. If the rear edge of the pulse is short, the backflow develops and the pulse writes a signal in the form of the n-twisted texture. To erase the signal, a strong pulse with a step-like rear edge creates the same transient state, which relaxes to the uniform stable texture due solely to the elastic force...

The first scientists to investigate liquid crystals soon noticed that sample orientation was strongly influenced by the glass slides between which they sandwiched their nematic phases. Simply rubbing the slide in one direction with drawing paper, molecules are anchored to the surface, parallel to the direction of rubbing. We thus obtain a nematic monocrystal. Optically, the texture is uniform. Suitable surface treatments allow perpendicular anchoring of the molecules and a so-called homeotropic texture. 

FIGURE 12 Mechanisms for inducing transitions between scattering (focal conic) and clear (homeotropic) textures in a smectic A device. The material is assumed to have positive dielectric anisotropy. 

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