Dielectric Relaxation in a Deeply Supercooled Liquid Crystals Confined in Random Porous Media
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27 Mat. Res. Soc. Symp. Proc. Vol. 543 © 1999 Materials Research Society
EXPERIMENT We used porous silica glasses with randomly oriented and interconnected pores with mean pore size of 100 A and volume fraction 27% as matrices. These matrices were solid plates with dimensions 25 mm x 25 mm x 1 mm. The porous silica glasses have negligible electrical conductivity, and their dielectric permittivity is practically independent of frequency and temperature. We impregnated these porous glasses with 5Cb and 8CB at temperatures corresponding to isotropic phase. The bulk 5CB has a nematic phase in the temperature range of 22.5-35'C. 8CB has a smectic phase in the temperature range of 21.1-33.5'C in addition to the nematic range of 33.5-40.8°C. Measurements of the real (e') and the imaginary (6") parts of the complex dielectric permittivity in the frequency range 10-' Hz to 1.5 GHz were performed using two sets of devices. In the range from 10- Hz to 3 MHz we used the Schlumberger Technologies 1260 Impedance/Gain-Phase Analyzer in combination with Novocontrol Broad Band Dielectric Converter and an active sample cell (BDC-S). The sample was mounted between two gold plated parallel plates and placed in the shielded cell. For measurements in the frequency range 1 MHz-1.5 GHz we used Hewlett-Packard 4291A rf Impedance Analyzer. The temperature stabilization was better than 0.01GC. For the quantitative analysis of the dielectric spectra the Havriliak-Negami function [14] has been used. For the case of more than one relaxation process, taking into account the contribution of the dc conductivity to the imaginary part of dielectric permittivity, the Havriliak-Negami function is given by 6*
=: 60
+
~
AEj
a-
j [1 + (i2 irfrj)l -oayj,82iri0of '
where c.. is the high-frequency limit of the permittivity, Acj the dielectric strength, r-, the mean relaxation time, and j the number of the relaxation process. The exponents a3 and 83 describe the symmetric and asymmetric distribution of relaxation times. The term i a/27rfofn accounts for the contribution of conductivity a, with n as fitting parameter. RESULTS AND DISCUSSION It is known that bulk 5CB and 8CB are non-glass formers and do not to have a glass transition. In the liquid crystalline phases of bulk 5CB and 8CB there are two dielectrically active relaxation processes of molecular origin [15-19]. For a geometry in which the electric field E is parallel to the director n i.e. Eln, the Debye type process due to the restricted rotation of the molecules about their short axis exists. The characteristic frequency of this process is - 5 MHz and the temperature dependence of the corresponding relaxation times obeys empirical Arrhenius equation. For the geometry in which the electric field E is perpendicular to the director n, Emn the most prominent relaxation process with characteristic frequency about 70 MHz was observed, which has been attributed to the tumbling of the molecules about their molecular short axis [9]. No dielectrically active collective modes are present in these LCs.
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