Sergio Diez

Dielectric studies of a laterally-linked siloxane ester dimer

This paper reports measurements of the dielectric response over the frequency range 102 to 109 Hz of a liquid crystal dimer consisting of two ester mesogens laterally linked by an alkoxy chain containing a siloxane group. The synthesis and phase behaviour of the siloxane dimer are also reported. Results show that there are two relaxations in the isotropic phase and four in the nematic phase of the material. The possible molecular origins for these modes are given. It is found that there is a coupling between internal and external modes which gives rise to a cooperative mode as the temperature in the nematic phase is lowered towards a glass transition.

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, 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...

Dielectric studies of liquid crystals: the influence of molecular shape

One of the first applications of the molecular field theory developed by Maier and Saupe (Maier W.; Saupe, A. Z. Naturforsch. 1958, 13a, 564–566) was to the static dielectric properties of a nematic liquid crystal by Maier and Meier (Maier, W.; Meier, G. Z. Naturforsch. 1961, 16a, 262). Then, in a paper published in 1966 with G. Meier, (Meier, G.; Saupe, A. Mol. Cryst. 1966, 1, 515) Saupe developed a simple theory to explain the low-frequency dielectric relaxation observed in the nematic phase around 1 MHz, but absent in the isotropic phase, and indeed absent in all other isotropic molecular fluids. This paper reviews this work of Saupe and the subsequent development of the theory by Martin, Meier and Saupe (Martin, A.J.; Meier, G.; Saupe, A. Symp. Faraday Soc. 1971 , 5, 119), and then by others such as Nordio, Rigatti and Segre (Nordio, P.L.; Rigatti, G.; Segre, U. Mol. Phys. 1973, 25, 129). These theories described the effect of a nematic potential on the dipole relaxation times for a molecule, and provided a framework within which to analyse experimental measurements of dielectric relaxation in liquid crystals. The purpose of this review is to examine the application of simple ideas from the theories of Saupe and others to a variety of experimental results for primarily nematic liquid crystals. A particular theme is developed which categorises liquid crystal-forming molecules (mesogens) in terms of their molecular shape. Experimental results of dielectric relaxation for examples of different shaped mesogens are then explained in terms of the theory. Recent work has extended the types of mesogen to include bent-core or V-shaped molecules and flexible-core mesogens, the latter of which, liquid crystal dimers are examples. New features have appeared in the experimental results for these materials, which have stimulated the development of a new theory of dielectric relaxation in flexible dimeric liquid crystals (Stocchero, M.; Ferrarini, A.; Moro, G.J.; Dunmur, D.A.; Luckhurst, G.R. J. Chem. Phys. 2004, 121, 8079). These results are reviewed.

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

The dielectric relaxation spectrum over the frequency range 102 to 1.8×109 Hz of 4‐octyl‐4′‐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 re‐scaling of the experimental data is proposed.