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Dielectric relaxation in liquid crystalline dimers

Dielectric relaxation in liquid crystalline dimers
Dielectric relaxation in liquid crystalline dimers
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, alpha,omega -bis[(4-cyanobiphenyl)-4'-yloxy]undecane (BCB.O11), and the odd-asymmetric dimer, alpha-[(4-cyanobiphenyl)-4'-yloxy]-omega-(4-decylanilinebenzylidene-4'-o xy) 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 10(2)-10(9) 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 , 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 degrees 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.
transitional properties, nematogenic molecules, order parameters, flexible core, dipole-moment, behavior, phase
0021-9606
8681-8691
Dunmur, D. A.
04540c17-effa-49e6-9a78-b661af481a27
Luckhurst, G. R.
7807d1c6-54a1-414f-9abe-22de4d9c30f7
De la Fuente, M. R.
e498c2b8-27c6-409f-9cd6-657b1551b068
Diez, S.
fecc40b4-833d-4127-83d4-cd3539f78142
Jubindo, M. A. P.
a20c9729-4275-4905-894e-9e54e4ab5879
Dunmur, D. A.
04540c17-effa-49e6-9a78-b661af481a27
Luckhurst, G. R.
7807d1c6-54a1-414f-9abe-22de4d9c30f7
De la Fuente, M. R.
e498c2b8-27c6-409f-9cd6-657b1551b068
Diez, S.
fecc40b4-833d-4127-83d4-cd3539f78142
Jubindo, M. A. P.
a20c9729-4275-4905-894e-9e54e4ab5879

Dunmur, D. A., Luckhurst, G. R., De la Fuente, M. R., Diez, S. and Jubindo, M. A. P. (2001) Dielectric relaxation in liquid crystalline dimers. The Journal of Chemical Physics, 115 (18), 8681-8691. (doi:10.1063/1.1409365).

Record type: Article

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, alpha,omega -bis[(4-cyanobiphenyl)-4'-yloxy]undecane (BCB.O11), and the odd-asymmetric dimer, alpha-[(4-cyanobiphenyl)-4'-yloxy]-omega-(4-decylanilinebenzylidene-4'-o xy) 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 10(2)-10(9) 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 , 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 degrees 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.

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More information

Published date: 1 November 2001
Keywords: transitional properties, nematogenic molecules, order parameters, flexible core, dipole-moment, behavior, phase

Identifiers

Local EPrints ID: 19469
URI: http://eprints.soton.ac.uk/id/eprint/19469
ISSN: 0021-9606
PURE UUID: 46278c70-87db-4aec-97ac-781a8e5f9ca2

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Date deposited: 14 Feb 2006
Last modified: 15 Jul 2019 19:27

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