Dielectric Relaxation in Nematic Phases

As shown in Fig. 1 the two main relaxation mechanisms (reorientation around the short and long molecular axis) are influenced in different way by the phase transitions. Thereby the reorientation around the short molecular axis characterized by T or (T = (27i/r ) changes stepwise at this transition. From this step the change in the degree of order can be calculated [23], but one has to use the same mechanism in the isotropic state as reference. The last condition is not always fulfilled because of a superposition of different mechanisms 

If the last case is excluded one can try to make an universal model. Starting point is the idea that in general, the relaxation times Til give a straight line in an Arrhenius plot  

(2) can be also used for the clearing temperature T(N/I). By subtracting the same equation at the clearing temperature one can obtain 

A second result is interesting from theoretical point of view. The question is are there different relaxation times of mole- 

For comparison Cole-Cole plots of the basic mixture with D are given in Fig. 19. In contrast to Fig. 18, the added compound D reorients faster than the mixture. By plotting the experimentally obtained relaxation frequencies versus the reciprocal temperature, the following activation energies could be calculated for the solute [150,151]. 

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Theories of dielectric relaxation in the nematic phase which considered particular models of the molecular dynamics have assumed the existence... 

J. Chrusciel, H. Kresse, S. Urban, Megahertz [192] dielectric relaxation process in the nematic and smectic phases of two thiol esters (9S5 and... 

Nwabunma D, Kyu T (2(X)1) Phase behavior, photopolymerization and morphology development in mixtures of eutectic nematic liquid crystal and photocurable monomer. Polymer 42 801-806 Ohta S, hiasawa S, Yamaguchi Y (2012) Size control of phase-separated liquid crystal droplets in a polymer matrix based on the phase diagram. J Polym Sci Part B Polym Phys 50 863-869 Parab SS, Malik MK, Deshmukh RR (2012) Dielectric relaxation and electro-optical switching behavior of nematic liquid crystal dispersed in poly (methyl methacrylate). J NonCryst Solids 358 2713-2722... 

The orientational dynamics of liquid crystals in the nematic phase has been widely investigated using NMR and dielectric relaxation spectroscopy. Here, we briefly discuss the applications of the two techniques and present the experimental data concerning a few liquid crystals. 

In FIGURES 4 and 5 the available experimental data for the dielectric relaxation times are collected for 7CB and 8CB, respectively. The general behaviour is very similar in the two cases. In the isotropic phase a single relaxation time is measured, corresponding to the rotational (fynamics of the molecules about their short axes. However, the relaxation becomes bimodal for tempo-atures lower than Tni. In the nematic phase, dielectric data sets have been collected with the electric field parallel (8 ) and perpendicular (ij ) to the nematic director. In the parallel geometry a single relaxation process is observed, characterised by a relaxation time which increases rapidly at low temperatures. A second relaxation process appears in the perpendicular geometry its characteristic time is only... 

Fig. 4.6. Argand plots of dielectric relaxation spectra of the nematic phase of some typical liquid crystals in two principal geometries of measurement. Numbers refer to frequency (in MHz) of the probing electric field. (From Ref. 23).

Extensive DR studies of rod-like polymers in solution covering the isotropic, biphasic and nematic states of solution have been carried out only for two different PAICs in toluene a 1/1 copolymer of n-butyl- and n-nonyl isocyanate (PBNIC) and homopoly(n-hexyl isocyanate) (PHIC). The relaxation process was studied as a function of both concentration and temperature. Only one relaxation process, with a broad distribution of relaxation times, was observed in the isotropic and nematic phases up at relatively low frequencies (10" --10 Hz) (see Fig. 4.15). The most important results are summarized in Fig. 4.16. All three characteristic dielectric parameters—the dielectric increment A8, the maximum of the loss factor 8, and the logarithm of the mean relaxation rate /c—undergo significant changes across the isotropic-biphasic-nematic concentration range. 

Fig. 4.23. Summary representation of the temperature dependence of the relaxation rate for five different dielectric relaxation processes observed in side chain polyacrylate liquid crystals. Tg denotes the glassy transition, and I, N, and S stand for the isotropic, nematic and smectic phases, respectively. The broken line represents results of NMR investigations (see text).

In equations (5)-(8), i is the molecule s moment of Inertia, v the flow velocity, K is the appropriate elastic constant, e the dielectric anisotropy, 8 is the angle between the optical field and the nematic liquid crystal director axis y the viscosity coefficient, the tensorial order parameter (for isotropic phase), the optical electric field, T the nematic-isotropic phase transition temperature, S the order parameter (for liquid-crystal phase), the thermal conductivity, a the absorption constant, pj the density, C the specific heat, B the bulk modulus, v, the velocity of sound, y the electrostrictive coefficient. Table 1 summarizes these optical nonlinearities, their magnitudes and typical relaxation time constants. Also included in Table 1 is the extraordinary large optical nonlinearity we recently observed in excited dye-molecules doped liquid... 

Consider first an anisometric molecule with the longitudinal p, and transversal p, permanent dipole moments in an isotropic phase. There are two relaxation modes mode 1, rotations of p, around the long axis, and mode 2, reorientation of p,. Figure 10-1. The mode 1 has a smaller relaxation time, Tj < Tj, because of the smaller moments of inertia involved. When this isotropic fluid is cooled down into the NEC phase, the dynamics is affected by the appearance of the nematic potential associated with the orientational order along the director n. The mode 1 remains almost the same as in the isotropic phase, and contributes to both the parallel and perpendicular components of dielectric polarization (determined with respect to n). Mode 2 is associated with small changes of the angle between p, and n it contributes to the parallel component of dielectric polarization. Mode 3 is associated with conical rotations of p, around the director (as the axis of the cone) it is effective when the applied electric field is perpendicular to n and contributes... 

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