Nanocomposite Material, Liquid Crystal-Aerosil Particles: Dielectric and Photon Correlation Spectroscopy Investigations
- PDF / 437,618 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 48 Downloads / 213 Views
polarization modes, and conductivity can be investigated in different systems. Photon correlation spectroscopy applied to nematic liquid crystal provides information on the dynamics of director fluctuations determined by elastic and viscous properties of LC. EXPERIMENT We used liquid crystalline 5CB filled with Aerosil particles, both hydrophilic (A200) as well as hydrophobic (R974), as materials. The volume concentration of the filling particles was 2.3 % for both samples. The hydrophilic particles are made of silica with OH-groups (1.4 - 2 groups per nm 2) on the surface. In the hydrophobic Aerosil particles, about 70 % of the surface hydrophilic groups are replaced by hydrophobic groups reducing the interaction with the liquid crystal molecules and between particles. Investigations have shown that the agglomeration of 2-3 volume percent of Aerosil particles in a nematic phase forms a three-dimensional network dividing the liquid crystal into LC domains with a linear size of approximately 250 nm [2] with random distribution of director orientation of each domain. The bulk 5CB has a nematic phase in the temperature range of 22.5-35°C. Measurements of the real (c) and the imaginary (C") parts of the complex dielectric permittivity in the frequency range 10-3 Hz to 1.5 GHz were performed using two sets of devices. In the range from 10-3 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). For measurements in the frequency range from 1 MHz to 1.5 GHz we used Hewlett-Packard 4291A rf Impedance Analyzer. Photon correlation measurements were performed using a A = 0.6328pim He-Ne laser and the ALV-5000/Fast Digital Multiple Tau Correlator (real time) operating over delay times from 12.5 ns up to 103 s with the Thorn EMI 9130/100B03 photomultiplier and the ALV preamplifier. The temperature stabilization in both experiments was better than 0.01°C. DIELECTRIC RELAXATION In the nematic phase of bulk 5CB there are two dielectrically active relaxation processes of molecular origin [11-15]. For a geometry in which the electric field E is parallel to the director n i.e. Ejjn, 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 dependencies of the corresponding relaxation times obey empirical Arrhenius equation. For the geometry in which the electric field E is perpendicular to the director n, i.e. ELn, the most prominent relaxation process with a characteristic frequency about 70 MHz has been attributed to the tumbling of the molecules [16,17]. No dielectrically active collective modes are present in 5CB. For the quantitative analysis of the dielectric spectra the Havriliak-Negami function [18] 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 Havril
Data Loading...
