Uniaxial positive birefringence

Fig. 5.101. (a) Index ellipsoid and refractive indices no and Hq for two directions of the electric vector of the wave in a plane perpendicular to the wave propagation k. (b) Dependence of Ho and He on the angle 0 between the wave vector k and the optical axis of a uniaxial positive birefringent crystal... 

Such polymers adopt, when affected by a mechanical field, an optically uniaxial homeotropic structure polymers B.1.2, B.1.7, B.1.8 (Table 8) have positive birefringence polymers B.1.1, B.1.8. (Table 9) have negative birefringence, which has not been reported to our knowledge, for low-molecular nematic liquid crystals. Although the authors do not comment on the cause for the observed phenomenon, the fact in itself is sufficiently uncommon. 

Figure 2.7 The refractive index ellipsoid of a uniaxial liquid crystal phase with the optical axis parallel to ihe x-axis. The refractive index, no, of the ordinary ray is independent of the direction of propagation. The refractive index, ng, of the extraordinary ray is larger than n if the liquid crystalline phase is of positive birefringence. ...

Birefringence The difference between the refractive index for the extraordinary and ordinary rays (he — no) in a uniaxial or biaxial crystal. If a crystal, such as quartz, is positively birefringent, no is less than ns, and the velocity of the ordinary-ray is greater than that of the extraordinary ray. The reverse is true for a negatively birefringent crystal. 

Optical investigations of smetic liquids indicated a behaviour of uniaxial or biaxial crystals depending on the special type of material. They are usually positive birefringent, which means that with transmitted light the ordinary beam has a lower refractive index. The nematic liquid is optically positive. 

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 ... 

The uniaxial crystal is called positively birefringent if and negatively... 

An interesting feature of polymer nematics was discovered in studying the orientation of some polymethacrylate polymers and cross-linked LC elastomers based on polysiloxanes [57]. Most nematic polymers form an optically positive, uniaxial, homeotropic structure under the effect of a mechanical Held such polymers have a positive birefringence (An > 0), like most low-molecular-weight liquid crystals. 

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