Slow Dynamics and Glass-Like Behavior of Liquid Crystals Dispersed in Nanoporous Media
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discovered and studied in these materials. Although there has been great success in the investigations of the physical properties of confined fluids [1-4] and liquid crystals [5-10], there are still open questions in the understanding of the influence of confinement on the dynamical behavior of both ordinary and anisotropic liquids. Confinement of fluids to porous media leads to changes in the phase and glass transition temperature and molecular mobility, and to a substantial broadening of the distribution of relaxation times [4]. In this paper we present some evidence of glass like behavior in confined and filled liquid crystals. These systems are anisotropic (at least at short scales) and heterogeneous materials characterized by a very developed interface. We performed dynamic light scattering and dielectric measurements in nematic LC confined in porous matrices. We have investigated pentylcyanobiphenyl (5CB) confined in porous glasses with interconnected randomly oriented pores, and in Anopore membranes with parallel cylindrical pores. The experiments show significant changes in physical properties of confined LC and suggest that there is some evidence for glass-like dynamical behavior, although bulk liquid crystal does not have glassy properties in both anisotropic and isotropic phases. EXPERIMENT We performed photon correlation measurements using a 6328 A 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 depolarized component of scattered light were investigated. All dynamic light 273 Mat. Res. Soc. Symp. Proc. Vol. 455 01997 Materials Research Society
scattering data we discuss below were obtained at the scattering angle e=30'. Measurements of the real (e') and the imaginary (e") parts of the complex dielectric permittivity in the frequency range 0.1 Hz - 3 MHz were carried out at different temperatures using a computer controlled Schlumberger Technologies 1260 Impedance/Gain-Phase Analyzer. For measurements in the frequency range 1MHz - 1.5GHz we used HP 4291A RF Impedance Analyzer with a calibrated HP 16453A Dielectric Material Test Fixture. We used matrices with randomly oriented, interconnected pores (porous glasses with average pore sizes of 10 nm and 100 nm) and parallel cylindrical pores (Anopore membranes with pore diameters of 20 nm and 200 nm). These matrices were impregnated with 5CB. The phase transition temperatures of bulk 5CB are TCN= 2 9 5 K and TNI=308.27 K. We also used filled nematic liquid crystals [11]. This material is a suspension of small silica particles, which are about 10-17 nm in diameter and the concentration of these particles in LC was 2.8 % of the volume. These filled nematics were prepared by Dr. M. Kreuzer. PHOTON CORRELATION SPECTROSCOPY OF CONFINED AND FILLED NEMATIC LIQUID CRYSTALS
First of all from static light scattering experiments we obtained that the nematic isotropic phase transition in 1000 A random pores, 20
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