Conoscopic observation

While in orthoscopic studies the parallel incident lights are desirable, in conoscopic examinations high-aperture condenser and objective are used so that a wide-angle cone of light is allowed to pass through the sample. The cone is composed of bundles of parallel rays along different directions. 

Biaxial liquid crystals result in much different interference figures dependent on how the specimen is cut relative to the two axes. For example, if it is cut perpendicular to one of the axes, the figure-8-like fringes of equal retardation may be observed. Thus the uniaxial and biaxial liquid crystals may be differentiated. Since biaxial liquid crystals are very rare in polymers, no further discussion is given. 

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Fig. 15 Conoscopic observation of the alignment of azobenzene mesogens as a function of change in incident angle. From A to F before irradiation, after irradiation at incident angles of...

In studies of the optical properties of the liquid crystal phase the microscope may be arranged for orthoscopic observations between crossed polars, or it may be arranged for conoscopic observations. These two arrangements are shown in Figure 4.4. 

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. 

With the aid of conoscopic observations, Finkelmann has also proved the positive uniaxial character of comb-like polymers ... 

For (lb), having the long flexible spacer of six methylene units a normal nematic phase is observed displaying the typical vi/ell known nematic textures under the polarizing microscope. Marbled textures as well as Schlieren textures can be seen, and they exhibit the same types of defects as ordinary l.-lc s. Conoscopic observations clearly prove the optically uniaxial positive character. The absolute value of the birefringence An as well as the temperature dependence of An correspond to chemically similar l.-lc s ... 

Bertrand lens n. An auxiliary, low-power lens which maybe inserted in the bodytube of the microscope between the eyepiece and the objective for observing the back focal plane of the objective useful for conoscopic observations in crystal optics and for checking microscope illumination quality. 

Conoscopic observation n. The study of the back focal plane of the objective by removing the eyepiece, by inserting a Bertrand lens, by examining the image at the eye-point above the eyepiece with a magnifier or by using a phase telescope is called conoscopic because the observations are associated with the cone of light furnished by the condenser and viewed by the objective (cf., orthoscopic). 

Orthoscopic observation n. The normal way of viewing an object microscopically (cf, conoscopic observation). With Kohler illumination the field diaphragm and the ocular front focal plane as well as the specimen will be in focus. 

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. 

Figure 10. Conoscopic observation of induced alignment change of PMeOAzO/DON103/ODS cell by slantwise exposure to UV (a) and visible (b) lights. Exposure was made from the left of the microscope. Pictures were taken every 2s for (a) and 0.5s for (b) as the order shown above the pictures.

Leube et al. achieved a homeotropic alignment by using an appropriate surface treatment and confirmed it by conoscopy. Furthermore, conoscopic observations, namely a Maltese cross that did not change upon rotation, proved that a macroscopic alignment of the minor directors was not present. Afterwards, they obtained... 

Finally, polarized IR spectroscopy is another sensitive method for the study of phase symmetry in liquid crystals. It was recently applied to prove phase biaxiality in organo-siloxane tetrapodes [37] - a supermolecular system forming quasi-flat platelets. Planar homogenously and homeotropically oriented samples were studied in order to derive all relevant order parameters from the three components of the IR absorbance. The presence of both a uniaxial and a biaxial nematic phase was detected, and again, optical textures and conoscopic observations supported these findings. 

Fig. 10. (a) Conoscopic observation of the clear stored texture of Fig. 9, showing homeotropic alignment of mesogenic groups, and (b) positive uniaxial behaviour as evidenced by the insertion of a A/4 plate. 

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