Dielectric relaxation in liquid crystalline dimers

Abstract

Dielectric measurements are reported for liquid crystalline dimers consisting of two mesogenic groups joined through ether links by a flexible alkyl chain. Results are presented for the odd-symmetric dimer, α,ω-bis[(4-cyanobiphenyl)-4′-yloxy]undecane (BCB.O11), and the odd-asymmetric dimer, α-[(4-cyanobiphenyl)-4′-yloxy]-ω-(4-decylanilinebenzylidene-4′-oxy) nonane (CB.O9O.10). The real and imaginary parts of the electric permittivities of aligned samples were measured as functions of frequency over the range 102–109 Hz at temperatures throughout the nematic phase. Measurements were fitted as a function of frequency to the Havriliak–Negami function, and yielded relaxation times and dielectric strengths for the relaxations at each of the temperatures studied. The static dielectric anisotropy for both materials was positive. For BCB.O11, the parallel and perpendicular components of the permittivity exhibited one low frequency and one high frequency relaxation, while the parallel component of the permittivity for CB.O9O.10 showed three relaxations, which were well-separated in frequency. A generic model of liquid crystalline dimers based on mixtures of linear and bent conformers with the tetrahedral angle has been proposed [A. Ferrarini et al., Chem. Phys. Lett. 214, 409 (1993)]. This model is used to provide a qualitative interpretation of the dielectric measurements reported. To obtain a satisfactory explanation of the results, it is necessary to include additional conformers having either a hairpin shape or a bent shape with an angle of 180° minus the tetrahedral angle. The temperature dependence measured for the strengths of the dielectric relaxations is explained in terms of changes of the order parameter and changes in the relative probabilities of different shaped conformers.