Static dielectric constants, nematics

The response of the nematic layer to a low frequency electric field depends crucially on the sign of the anisotropy of the static dielectric constants, — ef where and ef denote the dielectric con-... 

Figure 9 presents the static dielectric constants, e n and e j, as a function of pressure at two temperatures within the nematic phase of 5CB and 6CB. The average permittivity values, e = (e n + 2e j )/3, are practically independent of pressure and are the same at both temperatures. 

A second effect of the same sample shown in Fig. 3 is also interesting. By cooling the isotropic liquid deviations of the static dielectric constant q from the straight line by approaching the is/N transition were measured. Such behavior was at first detected by Bradshaw and Raynes [24], confirmed by Thoen and Menu [25] and explained as an antiparallel correlation of the strong dipoles in the nematic-like clusters formed in the isotropic phase. For less polar molecules the driving force for the antiparallel orientation can also be produced by a combination of the steric and dipolar forces as shown by Kresse and Kremer [26] and demonstrated in Fig. 3. 

Maier and Meier [35] worked out these ideas more quantitatively by applying Onsager s theory of static polarization for nematic liquid crystals. The Onsager theory relates the dielectric constant to molecular properties, namely the molecular polarizability a and the permanent electrical dipole moment /i. In the case of nematic liquid crystals, however, the polarizability has to be treated as a tensor with principal values an and a and an angle ]8 has to be introduced to describe the... 

Figure 4. The expected static dielectric behaviour of a nematic liquid crystal with A o>0 according to Eq. (1). e 02=high frequency dielectric constant with respect to the low frequency absorption (/r, A )...

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