Dielectric relaxation in bulk and cylindrically confined octylcyanobiphenyl (8CB)

Abstract

The dielectric relaxation spectrum over the frequency range 10 2 to 1.8610 9 Hz of 4-octyl-49-cyanobiphenyl, 8CB, in bulk and confined to 200 nm diameter cylindrical pores is reported. We used matrices with parallel cylindrical pores, obtaining different alignments of the molecular director depending on the treatment. Results show that there are two relaxations in the isotropic phase and in the mesophases for parallel alignment and three for perpendicular alignment. The molecular origin of theses modes and the effect of the confinement on their dynamics are discussed. To compare properly the results for bulk and confined 8CB, a rescaling of the experimental data is proposed.

Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

“…The relaxation time of the fast process practically coincides with that of bulk 7CB [59]. It is typical of the rotational dynamics around the short molecular axis (δ-relaxation), the end-over-end tumbling [64,68,82,83]. The T dependences of the extracted fit parameters are depicted in Figs.…”

Eutectic liquid crystal mixture E7 in nanoporous alumina. Effects of confinement on the thermal and concentration fluctuations

“…Theoretical models which employ orientational probability distribution functions and take into account only the point symmetry of the nematic phase predict the existence of four dielectrically active relaxation domains. 37,38 These are associated with molecules aligned either parallel or perpendicular to the probing eld performing end-over-end rotation about the short molecular axis (slow) and precession/nutation about the nematic director or rotation about the long molecular axis (fast). Experimental studies on liquid crystalline Table 1 Constituent molecules of E7 along with their gas phase dipole moment, mass, clearing and glass temperature.…”

“…Here, Ds, is the difference in surface tension between the nematic and isotropic phases, DH is the enthalpy associated with the transition and r the density of the liquid crystal. From eqn (6) Representative experimental values of DT are equal to 5.5 K and 6.8 K in native 400 nm and 25 nm pores, respectively. We may conclude that the depression of T N/I in E7 contains contributions from both elastic distortions and surface tension effects.…”

“…However, as a signicant portion of LCs in technological applications are in contact with hard interfaces, 1,2 it is necessary to investigate the effects of geometric connement on the thermodynamics and dynamics of LCs. Apart from size effects, LCs in conned geometries [3][4][5][6] are susceptible to surface interactions such as anchoring and density modulations. 7,8 Therefore, the director eld conguration and, macroscopically, the material's optical and dielectric properties, are directly related to the size of the system as well as the competition of surface effects with the intrinsic elastic forces.…”

Dielectric investigations in the nematic and hexatic smectic B phases of 4-hexyl-4′-[2-(4-isothiocyanatophenyl)ethyl]-1,1′-biphenyl

“…Measurements at static field give information about the electric structure of molecules and, in some cases, the dipole-dipole correlation between the molecules. Measurement of dynamic dielectric permittivity, "*( f ) ¼ " 0 ( f ) À i" 00 ( f ), enables one to determine the dielectric relaxation time, which is an important parameter describing the influence of neighbours on molecular rotation [6,7]. The dielectric properties of nematic liquid crystals change drastically at the nematicisotropic transition temperature.…”