Mise au point. Constante dielectrique et relaxation dielectrique dans des cristaux liquides nematiques et smectiques en relation avec l'orientation moleculaire. Comportement dielectrique des phases reentrantes. Relaxation dielectrique dans des cristaux liquides polymeres

Dielectric Behavior of Liquid Crystals

Broadband dielectric spectroscopy (up to 109 Hz) is employed to study the molecular dynamics of the liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) in the free bulk phase and confined in cylindrical channels of Anopore membranes having a diameter of 0.2 μm and length of about 60 μm. The bulk samples of 5CB orient almost homeotropically between the untreated metal electrodes of the measurement set-up, and two relaxation processes are observed: the slower δ-relaxation is assigned to hindered rotation (180° flips) of the molecules around their molecular short axis, and a faster second process is attributed to the tumbling of the molecules about this axis. In the confined 5CB samples, the membrane pores align the nematic director axially or radially depending upon their surface preparation. Planar (axial) alignment is always found in untreated membranes, whereas radial alignment was achieved by treatment with decanoic acid. Consequently the director field is fixed perpendicular or parallel to the el...

Dielectric properties of the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl in porous membranes

Dielectric spectroscopy measurements have been performed on a bent-core nematic liquid crystal and on its binary mixtures with a calamitic nematic. We have detected more dispersions in the bent-core compound than in the calamitic one, including one at an unusually low frequency of a few kilohertz. The dispersions detected in the mixtures have been identified and the spectra have been split into contributions of the constituents. In order to connect the dielectric increment with the molecular dipole moment we have applied a sophisticated conformational calculation not performed before for a large, flexible mesogen molecule with numerous polar groups.

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Dielectric properties of mixtures of a bent-core and a calamitic liquid crystal

Optically isotropic films of an amorphous copolymer containing azobenzene moieties (pDR1M-co-MMA) were irradiated by interfering two circularly contrarotating polarized laser beams and permanent ho...

Molecular Orientations in Azopolymer Holographic Diffraction Gratings as Studied by Raman Confocal Microspectroscopy

It is shown how the broad and asymmetric dielectric loss spectra obtained for a nematogenic side-chain siloxane polymer can be fitted by the superposition of two Fuoss-Kirkwood curves. The degree of director alignment and the second rank orientational order parameters of the sample are estimated by relating the intensities of the two curves to the intensities of the loss processes as given by the generalization of the Nordio-Rigatti-Segre theory to the case of arbitrary director alignment. It appears that the nematic phase of the material studied has a high degree of planar director alignment and that the order parameters are much smaller than those observed for low molar mass nematogens.

The determination of the order parameters and director alignment in the nematic phase of a liquid crystalline side-chain polymer by the resolution of dielectric loss spectra

The dielectric spectrum of n-heptyl-cyanobiphenyl(7CB) and n-heptoxy-cyanobiphenyl(7OCB) are reported over a frequency range up to 18 GHz and over a temperature range well into the isotropic phase. Two relaxation processes are observed in the isotropic phase of both compounds, as well as in the nematic phase (when measurements are made parallel to the nematic director). Possible mechanisms for the two relaxation processes are discussed.

Dielectric Relaxation in the Nematic and Isotropic Phases of n-heptyl- and n- heptoxy-cyanobiphenyl

The dielectric properties of 8CB in the smectic and nematic phase over the frequency range up to 200 MHz are reported. Two relaxation processes are observed in samples where the director is aligned parallel to the measuring electric field and a broad relaxation process is observed in the perpendicular alignment. The relaxation times do not change significantly at the transition between the smectic and nematic phase, but the activation energy for dipole reorientation appears lower in the smectic phase than in the nematic phase. The results are interpreted to yield the dipole components responsible for the various relaxation processes and also the order parameters.

Dielectric relaxation and order parameters in the nematic and smectic phases of 4-n-octyl-4-cyanobiphenyl (8CB)

The non-linear Raman spectrum of 4-cyano-4′-heptylbiphenyl is reported. Measurements of depolarization ratios of the vibrational bands at 2224 cm−1 (CN stretching), 1606 cm−1 (aromatic ring stretching) and 1185 cm−1 (CH aromatic in-plane deformation) are used to evaluate the order parameters (〈P2〉 and 〈P4〉) using a ‘line model’ for the molecules. The results are internally consistent and also show the same general features (including some discrepancies in 〈P4〉) reported by other workers. Nevertheless, the results demonstrate that the technique is capable of wide application to the study of the anisotropic properties of liquid crystals and provides a valuable probe into the study of the biaxial nature of these sytems.

Non-linear Raman spectroscopy of liquid crystals: Polarization measurements and relaxation processes in 4-cyano-4′-heptylbiphenyl (7CB)

Dielectric absorption measurements are reported for MBBA in both the parallel and perpendicular alignments. Two absorptions are observed in each alignment. The intensity of the absorptions agree with those calculated using the Maier and Meier mean field theory. In the perpendicular alignment the high frequency absorption is attributed to the internal rotation of the methoxy-group.

Dielectric Relaxation in MBBA

Dielectric studies on MBBA and its solutions doped with 4-cyanobiphenyl (CB) are reported at reduced temperatures of 0.996 over the frequency range of 0 to 200 MHz. The longitudinal and transverse dipole components are evaluated as well as the order parameter. The results are self consistent only after an additional relaxation process is presumed at frequencies greater than 200 MHz. This may arise from the internal rotation of the terminal methoxy group in MBBA.

Alun H. Price

Papers

Dielectric Relaxation in the Nematic and Isotropic Phases of n-heptyl- and n- heptoxy-cyanobiphenyl

Dielectric relaxation and order parameters in the nematic and smectic phases of 4-n-octyl-4-cyanobiphenyl (8CB)

Non-linear Raman spectroscopy of liquid crystals: Polarization measurements and relaxation processes in 4-cyano-4′-heptylbiphenyl (7CB)

Dielectric Relaxation in MBBA

Dielectric Relaxation and Order Parameters in Mixtures of 4-cyanobiphenyl and MBBA