Conoscopic pattern

The deformation of the director can be probed by optical transmission, electric capacitance, conoscopic pattern, etc. The experimental set-up... 

Interference figure n. The conoscopic pattern of extinction positions of a crystal superimposed on the pattern of interference colors corresponding to the full cone of directions by which the crystal is illuminated, each direction showing its own interference color. 

All PCH and BPpolymers with liquid crystalline groups in the side chain exhibited fanshaped textures and uniaxial conoscopic patterns in a polarizing optical microscope that are characteristic of the smectic A phase. Typical fan-shaped textures of the polymers prepared with Fe-based and Mo based catalysts are shown in Figure 9-3. 

Figure 3 Conoscopic pattern taken with crossed polars. The sample is a uniaxial crystal with its optic axis parallel to the axis of the microscope. The dark cross shows the polarization directions for the polarizer and the analyzer. The rings are at retardations of 1,2, and 3 wavelengths.

The conoscopic interference patterns are also quite useful to identify uniaxial and biaxial nematics. For a uniaxial specimen, the conoscopic image consists of a dark cross... 

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 ...

More recently, the existence of transverse band structures during shear has been discounted. Ernst and Navard [56] used SALS of HPC -f water and found no patterns indicative of band structures, even at a shear rate when was negative. A confirming negative result was found by Picken ei al. [13], who used conoscopic SALS and found no evidence of meridional reflections during steady shear. 

One of the most important and useful techniques for confirming the existence of an Nb phase is conoscopy. From a good interference pattern, it is possible to establish whether or not a system is biaxial. The evidence put forward using conoscopic observations for the compounds discussed in the previous section is given below. 

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. 

The wide angle X-ray diffraction method was employed in the case of a sanidic aromatic polyamide [17] which exhibited a nematic phase. The characteristic feature of the X-ray pattern was the observation of two amorphous halos, which indicated the absence of rotational symmetry about the long chain axis. Based on this observation as well as the conoscopic interference pattern, it was concluded that the polyamides exhibit the Nb phase. 

Conoscopic images under white light illumination, (a and b) Uniaxial material an interference pattern consisting of two intersecting black bars (termed isogyres) forming Maltese cross-Kke pattern and circular distributions of interference colors (c) biaxial material. 

---