Conoscopic method

Fig. 4.30. Conoscopic method with the help of a microscope and with the naked eye...

A thin film of the LC-elastomer with m=3 (x=0) has been stretched approximately 20 % at room temperature (KT ) in the LC-state and then observed in the polarizing microscope using the conoscopic method. 

The n and nj values as functions of ten jerature were determined with the aid of a Leitz-Jelley-microrefractometer, using a sodium lamp as light source. The birefringence was also determined by a conoscopic method using a polarizing microscope. The temperature in both experiments was adjusted by a Mettler hot stage FP-52 with the control unit FP-5. 

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. 

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. 

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. 

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