Dielectric Relaxation in Nematic Liquid Crystals

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Abstract
In homogeneously ordered liquid crystals of the nematic type, two principal dielectric constants can be measured: ϵ1 parallel and ϵ2 perpendicular to the symmetry axis of the liquid crystal. The two dielectric constants may show a very different relaxation behavior. With alkoxy derivatives of azobenzene, only the normal Debye dispersion region at microwave frequencies has been observed. With alkoxy derivatives of azoxybenzene, ϵ2 shows again only the normal Debye dispersion region at microwave frequencies, but ϵ1 has an additional dispersion at very unusually low frequencies in the radio region. This dispersion is due to the permanent electric dipole momment of the azoxy group, more exactly to its component parallel to the long molecular axis. The reorientation of this component in direction 1 is hindered by a potential barrier due to intermolecular forces. The corresponding relaxation time is increased by a factor g. Starting from a modification of the Debye theory of dielectric relaxation, this retardation factor may be used to calculate the height of the potential barrier. Numerical values are in satisfactory agreement with other values, calculated from the observed long range order.