Dielectric relaxation in the uniaxial phase

The character of DR spectra in liquid crystals refiects both the symmetry of these phases and the dynamics of molecules under the influence of the intermolecular ordering potential. In order to explain the observed DR spectra in liquid crystals, a number of different theories based on the use of a general expansion of the orientational probability distribution 

It has been shown by Kozak et that a qualitative explana- 

Let us assume that the rod-like liquid-crystalline molecule possesses a permanent dipole moment u(mi, u ). In order to estimate DR spectra in the two principal geometries, i.e. for the probing electric field either parallel or perpendicular to the director, one has to know the correlation functions of projections of u on the Z- and X-axes of the laboratory frame (cf. Section 4.2)  

Firstly, we note that respective contributions depend on the degree of orientational order. For example, in the case of perfect orientational order (S=l) we should not be able to see any contribution to M (0)M (0 from and to Mi(0)wi(t) from Wj. On the other hand, in 

Secondly, a decay of the contribution to M (0)M (t) from ux is given by Aoo(t), which is the autocorrelation function of Dqo(0) (cf. eqns (4.40) and (4.25)). Since Dqo(0) depends only on the P angle (cf. Table 4.2), U] contributes to W (0)M (t) solely via reorientation of the molecule about the short molecular axis (see Fig. 4.7). Moreover, because of the symmetry of the phase, it must be identified with the end-over-end reorientation of the long axis. 

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