Collective and Molecular Dielectric Relaxation in Confined Liquid Crystals: Effect of Different Layer Thicknesses and Bo
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Collective and Molecular Dielectric Relaxation in Confined Liquid Crystals: Effect of Different Layer Thicknesses and Boundary Conditions Fouad M. Aliev and Manuel Rivera Bengoechea Department of Physics, University of Puerto Rico, San Juan, PR 00931-2343, USA ABSTRACT Broadband dielectric spectroscopy was used to study the influence of boundary conditions and layer thickness of liquid crystal (LC) confined to cylindrical pores on low frequency and high frequency relaxation processes. Low frequency measurements provided information on the relaxation of surface polarization that arose at LC – pore wall interface. The dynamics of molecular reorientations were investigated in high frequency experiments. In samples with axial orientation of molecules, the dielectric mode due to reorientation of molecules around their short axis was investigated. The homeotropic alignment of molecules facilitated the investigation of the librational mode. The behavior of this mode was different from the behavior observed in investigations of relaxation due to reorientation of molecules around their short axis. Broadening of the dielectric spectra was observed in confined LC. The broadening increases with decreasing liquid crystal layer thickness. INTRODUCTION Dielectric spectroscopy applied to bulk nematic and smectic-A LCs studies relaxation processes of molecular origin, which are relatively fast. Since the bulk phase of these LCs has no polar ordering there are no dielectrically active processes that would be associated with order parameter relaxation and therefore slow relaxation dynamics of order parameter is not involved. Alkylcyanobiphenyls, such as 5CB and 8CB are liquid crystals, which have been investigated in detail in the past and therefore their dielectric bulk properties are well understood [1-5]. Their molecules have a large dipole moment ≈ 5D oriented along the molecular long axis, which simplifies the understanding of the dielectric behavior. In the nematic phase in a geometry in which the electric field E is parallel to the director n, the real ε' and imaginary ε" parts of the dielectric permittivity have a dispersion region with a characteristic frequency at about 5 MHz. For the geometry in which the electric field E is perpendicular to the director n, the characteristic frequency of the most prominent relaxation process is about 70 MHz due to librational motion of the molecules was observed. However information on the dynamics of the librational mode is scarce even for bulk liquid crystals. Dielectric spectroscopy can be applied [6-9] to investigate multiple aspects of the influence of confinement on the dynamic properties of liquid crystals. In this paper we report the influence on dielectric relaxation of different boundary conditions (planar-axial and homeotropic-radial) and LC surface layer thickness as confined in cylindrical pores matrices. We used dielectric spectroscopy to investigate the relaxation properties of the librational mode of 8CB confined in 200 nm cylindrical pores of Anopore membranes tre
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