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.

Molecular dynamics of a binary mixture of twist-bend nematic liquid crystal dimers studied by dielectric spectroscopy

“…has a monoexponential decay, and so only a single low-317 frequency absorption, which, as has been shown for CB7CB 318 and CB9CB [6,11], can be detected at frequencies correspond-319 ing to those of the intermediate frequency relaxation observed 320 here and whose strength depends on the conformational 321 distribution. On the other hand, for nonsymmetric dimers, 322 relaxation rates would be different, and, depending on the 323 shape and dipole moments of each unit, a variety of frequency 324 relaxation profiles is predicted.…”

“…Liquid crystal dimers consisting of two promesogenic 24 groups joined by a flexible methylene chain containing an 25 odd number of carbon atoms have been the focus of intense 26 research activity over the last few years [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. It was found 27 that these compounds, of which 1 ,7 -bis(4-cyanobiphenyl- 28 4 -yl) heptane, CB7CB, is the most studied, show a first 29 order nematic-nematic phase transition.…”

“…3 there is only one mode, in both nematic phases the dielectric 278 permittivity shows three relaxations, as in other nonsymmetric 279 dimers [11,28,29] and in particular as in FFO9OCB [30]. (inverse process).…”

“…Although the precise 351 temperature dependence is intricate, the usual trend [11,28,29] 352 is that the strength of the lower-frequency mode (m 1L ) 353 increases while that of m 1H decreases when the temperature is 354 reduced as found for FFO9OCB [30]. Despite the similarity of …”

Dielectric properties of liquid crystalline dimer mixtures exhibiting the nematic and twist-bend nematic phases

“…It is an excellent technique to probe conformational distribution and changes in liquid crystalline structures. Variation in conformations and their population distribution gives an insight into the molecular shape of the liquid crystal, which is an important feature in determining phase behaviour and physical properties [15,[27][28][29][30][31][32][33]. This is based on the principle that a change in the molecular conformation would change the mean-square dipole moment, which provides a direct link to the molecular shape.…”