Static Dielectric Constants of Nematic Samples

Firstly the nematic phase and the theoretical description of its dielectric behavior will be discussed briefly. In the isotropic state anisotropic molecules are statistically dis- 

For such a comparison one has to consider the step of the density at the phase transition. All the other relations which connect molecular parameters such as the molecular dipole moment /i, the polarizability a and the angle between the molecular long axis and fj, with each other have a general problem the calculation of the internal field and its anisotropy. Therefore, all the equations given in Vol. 1, Chap. VII.2 are necessary and useful but one has to take always into account the limitations of the models. Nevertheless, the Onsager theory [27] (basis for the Maier-Meier model [28]) and Kirkwood-Frbhlich model [29] have been 

From the beginning of the 20th century to 1970 the dielectric behavior of liquid crystals was more of academic interest. Especially dielectric anisotropy at low frequencies, A o = o- j o was one of many qualities for the characterization of liquid crystals. Results of 18 samples measured by Specht, Vaupel, Sulze, Errera, Jezewski, Schulwas-Sorokina, Maier and the respective coworkers are summarized in the tables by Landold-Bdrnstein [30]. Based on these data Maier and Meier could give a connection between the dielectric constants o, e iq and molecular parameters [28]. In this model the dielectric anisotropy 

Ny = number of molecules per unit volume F, h = constants of the internal field [27] 

Aa = anisotropy of polarizability, Aa 0 P = angle between the molecular long axis and the dipole moment /t k = Boltzmann constant 

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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. 

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