Optical conoscopy

Direct experimental determinations of the sign of a2/a in LCPs by optical conoscopy or similar direct methods are rare, mainly because the optical observations must be made... 

FIGURE 3.6 Typical optical microscopy images of homeotropically aligned columnar phase of discotic liquid crystals with ((a), inset conoscopy image) and without (b) crossed polarizers, and X-ray diffraction patterns of homeotropic layers (c). For details see Reference 39. 

Many of the more specialised techniques for the optical examination of specimens have not been covered above, for reasons of space. These include conoscopy [35], infrared microscopy [46], fluorescence microscopy [47], and, of particular interest in connection with polymers, Raman microscopy [2]. 

It is possible to use both the optical and electron microscopes for scattering/diffraction studies. The observation of the scattering pattern in an optical microscope is referred to as conoscopy. It is also possible by closing the field iris and the aperture iris to obtain a small-angle light scattering pattern (Chapter 7). Electron diffraction, which is carried out in an electron microscope, is a classical method. The scattering methods are presented in Chapter 12. 

Conoscopy provides an extremely sensitive method with which to determine the degree of biaxiality. By the early 1990 s, conoscopic measurements had already indicated the presence of phase biaxiality in a nematic side-on liquid crystalline side-chain polymer [9]. However, the method s sensitivity is also its weak point because surface effects may induce optical biaxiality in an actual uniaxial system. For this reason, deuterium NMR was used to confirm phase biaxiality in a liquid crystalline polymer system similar to the one investigated with conoscopy by Leube [11-13]. Due to the fairly high viscosity of the polymeric samples, the tilt experiment, employed by Yu and Saupe to show phase biaxiality in a lyotropic liquid crystal [4], was used. The results obtained in this way are in good agreement with observations of optical textures in a biaxial cholesteric copolymer [16], where phase biaxiality disturbs the smooth optical periodicity of the cholesteric phase structure. 

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