Dielectric Relaxation in Low Temperature Smectic Phases of 4-n-Alkyloxy-benzylidene-amino-4′-cyanobiphenyls

The dielectric behaviour of five homologous 4-n-alkyloxy-benzylidene-amino-4′-cyanobiphenyls was investigated. Especially, we were interested in the dielectric relaxation frequencies of the smectic low temperature phases S_B and S_E as well as the dielectric behaviour in the solid state. We have found in all liquid crystalline phases a low frequency relaxation region explained by a hindered rotation of the molecules around their short axes. By supercooling of the hexyl and heptyl derivatives a new liquid crystalline phase can be obtained. A dielectric relaxation in the solid phase could not be found.     

β Dielectric relaxation mode in side-chain liquid crystalline polymer film

In this paper, side-chain liquid crystalline polymer (SLCP) film was studied by dielectric relaxation spectroscopy (DRS). Dielectric properties were measured in the frequency range from 1 kHz to 10 MHz at some fixed temperatures. The experimental data were fit non-Debye type Cole-Cole dielectric relaxation function. A single dielectric relaxation mode was observed in polymer film. It is observed that relaxation characteristics show Arrhenius type behavior. Activation energy value obtained from the relaxation data was found to be 0.1 eV. The relaxation process is interpreted in terms of side-chain dipole group motion (β relaxation).     

Modification of the smectic B phase by the aid of dielectric relaxation

The analysis of the dielectric relaxation data in the liquid crystalline S_B phase allow to classify the substances into two groups: S_B phases which are similar to the low or high temperature modifications. The experimental results are correlated to the recently discussed „heaxtic”︁ and „solid like”︁ S_B modifications.     

MOLECULAR AND COLLECTIVE PROCESSES IN THE ANTIFERROELECTRIC PHASE OF A CHLORINATED COMPOUND

Abstract

New liquid crystalline substance, (S) 4-(1-methylheptyloxycarbonyl)-3-chlorophenyl 4′-(4-butanoiloxybut-1-oxy)biphenyl-4-carboxylate, exhibiting ferroelectric and antiferroelectric phases have been studied by means of frequency domain dielectric spectroscopy. DSC calorimetric and electrooptic measurements were done to study its phase transitions and electrooptic behaviour. In the antiferroelectric SmC_A* phase one of the dielectric relaxation modes exhibits a non-Arrhenius behaviour. The plots of critical frequencies and dielectric increments vs. temperature show an anomalous behaviour of the relaxation processes in the neighbourhood of the ferroelectric — antiferroelectric phase transition. Reversal current method reveals one current peak in the ferroelectric phase and two in the antiferroelectric one. Temperature dependencies of tilt angle, spontaneous polarization, switching time and transmittance are also presented and discussed in scope of the mean-field theory.     

TSDC as a tool to study slow molecular mobility in condensed complex systems

The usefulness of the dielectric technique of thermally stimulated depolarization currents (TSDC) to characterize the slow molecular mobility in complex condensed systems is emphasized. The experimental results obtained by TSDC with the glass-forming liquid salicylsalicylic acid were used to illustrate different points. The TSDC signatures of the α- and β-relaxations are presented and commented, and the physical significance of the information provided by TSDC is discussed. The need for complementary use of the two dielectric techniques of thermally stimulated depolarization currents and dielectric relaxation spectroscopy (DRS) is stressed.     

On the Dielectric Behavior of Unaligned Samples of 4-n-Octyl-4′-cyanobiphenyl (8CB)

Data on the dielectric properties of the liquid crystal 4-octyl-4′-cyanobiphenyl (8CB) have been determined onto unaligned samples, for a large scale of frequency, as function of temperature. The components of the complex dielectric permittivity as well as the corresponding dielectric parameters were evaluated using Havriliak-Negami (HN) functions. The dielectric characteristics were discussed and were compared with the literature, a good agreement being observed. The method allows to obtain for the first time the dielectric strength for a large range of the temperature and gives the two functions describing the relaxation processes in the isotropic range.     

Ultrasonic Investigations in the Liquid Crystal p-Cyanophenyl-trans-4-Propylocyclohexane Carboxylate

The variation of ultrasonic velocity with temperature in the liquid crystal p-cyanophenyl-trans-4-propylcyclohexane carboxylate at four different fixed frequencies is studied. From the observed temperature variation of the ultrasonic velocities, relaxation parameters like adiabatic relaxation time, velocity at infinite frequency in the isotropic as well as in the nematic liquid crystalline phases are determined. The results are discussed.     

Dielectric spectroscopy of low-loss sugar lyophiles: II. Relaxation mechanisms in freeze-dried lactose and lactose monohydrate

This article presents an analysis of the dielectric relaxation mechanisms of the disaccharide lactose, in both the freeze-dried state and the crystalline monohydrate. Complex dielectric permittivity spectra for both forms of lactose were measured over wide ranges of frequency (0.1 Hz to 1 MHz) and temperature (−120 °C to +100 °C). The crystalline monohydrate displays a sharp peak in the temperature domain which was ascribed to the percolation of migrating protons over the surfaces of the crystals. The freeze-dried material displayed three relaxation processes. The first (γ) and second (β) processes were observed at low temperature and present as very broad relaxations with low dielectric strength. The relaxation times for both processes showed Arrhenius temperature-dependencies, with activation energies of 52 kJ/mol and 72 kJ/mol, respectively. The mechanisms of dielectric relaxation were ascribed to the motion of the pendant hydroxymethyl group and the relative motion of the pair of saccharide rings, respectively. Analysis and discussion of the third relaxation process is beyond of the scope of this article.     

Real time dielectric monitoring of glass transition in n-vinyl pyrrolidone polymerization

Real time dielectric spectroscopy was used for monitoring the dynamics during liquid glass transition in radical bulk polymerization of n-vinyl pyrrolidone. Two different relaxation mechanisms were identified. Initially segmental α-relaxation was dominant. Subsequently, contribution from slower motions became more significant and ε′(t_r, ω) underwent maxima in all frequency bands up to 10 MHz. Peak amplitude increased and the occurring time was delayed with decreasing frequency. The slow region dynamics were similar to the dynamics of Chamberlin’s domain-like model of glass transition. The glassy phase is formed by a segmental relaxation and a relaxation due to glassy regions. The characteristic relaxation times of slow and segmental motions were well described by a modified Vogel–Fulcher–Tammann relation. The high frequency behavior, associated with the segmental motions, is well described by Havriliak–Negami and Kohlrausch–Williams–Watts formulas, which did not provide satisfactory fittings for the dynamics of slow regions. Both real and imaginary parts of the dielectric data were fully described by a complex dielectric relaxation function, the parameters of which are found by the eigen-coordinates (EC) method.     

Dielectric Spectroscopy of Ferronematics Based on 6CHBT Liquid Crystal

In our study, the dielectric behaviour of the rod-like liquid crystal (6CHBT) doped with magnetic nanoparticles of spherical shape was investigated by means of dielectric spectroscopy in the frequency range from 20 Hz to 2 MHz. A low frequency dielectric dispersion in the nematic and isotropic phases of the pure liquid crystal (LC) has been assigned to the space charge polarization. After doping the host LC with the magnetic nano particles, a nearly Debye-like relaxation process was observed with the temperature dependence obeying the Arrhenius law. Considering a possible electric double layer formation on the particle surfaces, the detected relaxation process in the doped LC can be associated with the electric double layer polarization. The experimental results point out that in the measured frequency range the space charge and interfacial effects constitute the main dielectric response. Any anchoring effects were not observed and are therefore expected to appear in higher frequencies.     

A nematic two frequency adressing mixture with extremely low isotropy frequencies

New liquid crystalline substances with four benzene rings have been synthesized. A nematic two frequency mixture containing some of these compounds was produced and dielectrically as well as electrooptically investigated. With dielectric measurements a very low frequent and distributed relaxation mechanism at room temperature could be estimated (f_R = 0.0565 kHz, h = 0.25 at 294.2 K). Although new substances have been used the switching behaviour in a TN cell showed disturbances as on an earlier developed two frequency mixture.     

Structure and thermal fluctuations in thin films of smectic liquid crystals

Thermal fluctuations of the smectic layers in freely suspended thin liquid crystalline films have been studied by the methods of X-ray diffraction and reflectometry. The fluctuation dynamics is studied by the methods of photon correlation spectroscopy with the use of a source of coherent synchrotron radiation. In freely suspended smectic films, the typical relaxation times are of the order of several microseconds. In thin liquid crystalline films, the simultaneous damped and oscillatory behavior was observed for the layer undulations.     

Molecular orientation and dielectric anisotropy properties of 4-cyano-4′-n-heptylbiphenyl-TiO2 liquid crystal composite

The molecular orientation and the dielectric anisotropy of the nematic liquid crystal (LC) 4-cyano-4′-n-heptylbiphenyl (7CB) and of TiO₂-doped 7CB have been investigated. The dielectric properties of the LCs exhibit a relaxation peak that shifts to lower frequencies with increasing voltages. The relaxation frequencies of 7CB and 7CB/TiO₂ liquid crystals were calculated and found to decrease as the bias voltage increases. This is attributed to molecular reorientation. The dielectric anisotropy of the LCs changes from the positive type to negative type and the static electric permittivity and dielectric anisotropy values were found to be lower for the 7CB/TiO₂ system.     

Optical and static dielectric study of a terminally fluorinated liquid crystalline compound

Optical and static dielectric studies on a terminally fluorinated liquid crystalline compound have been carried out. Measurements of temperature variation of refractive indices of the compound are done by using thin prism method. A four-parameter model is validated by fitting the experimentally measured values of refractive indices, birefringence and average refractive indices of the compound with the theoretical ones. Refractive index and density data are utilized for determination of orientational order parameter. The temperature variation of dielectric permittivities of the compound are measured by a LCR meter.     

Digital estimators of relaxation spectra

Determination of the distribution of relaxation times (DRT) from a wide variety of the time- and the frequency-domain material functions, such as polarization current and charge, real and imaginary parts of complex dielectric permittivity and complex dielectric modulus, the appropriate mechanical and magnetic counterparts is generalized as a filtering problem on a logarithmic time or frequency scale. Algorithms of the appropriate digital DRT estimators are derived. A novel regularization strategy is proposed based on choosing sampling rate for the input data, which ensures acceptably low random error of the recovered spectra. Optimum frequency ranges and sampling rates are found for determination of the relaxation spectrum from the real part of complex permittivity and complex modulus. A multi-filter DRT recovery strategy is suggested by a bank of filters with different smoothing abilities.     

Dielectric Relaxation in Liquid Crystals

We present recent progress in the study of the liquid crystals by dielectric relaxation. First, we discuss the possible mechanism controlling the rotation of the molecules around a short axis: rotational diffusion or free volume effect. In the case of phase transition between Sm A phases, dielectric relaxation can bring information about the short range order and also the long range order when a dipole ordered structure is established.     

Erratum

The meaning of the rotational friction and diffusion tensor in Liquid crystalline phases characterized by anisotropic viscosities is discussed, by generalizing the hydrodynamic Stokes-Einstein relationships. Corrections due to the viscosity anisotropy effects to the rotational correlation times, relevant for the interpretation of magnetic and dielectric relaxation experiments, have been computed by solving the appropriate Fokker-Planck equation.     

The dielectric properties of as-received and gamma irradiated fused silica

Audio frequency capacitance and conductance measurements have been carried out on 15 kinds of fused silica over the temperature range 5.5–380 K. The results are used to arrive at corresponding values for the complex dielectric constant. It is shown that the relative strengths of the three relaxations observed in as-received fused silica are characteristic of the type of fused silica. In addition, it is found that one of the relaxations can be destroyed by gamma rays and that the destruction is accompanied by the creation of a lower activation energy relaxation. A corresponding growth of optical absorption bands in the visible portion of the spectrum is observed. The gamma-ray sensitive relaxation is attributed to an aluminum-alkali center and the gamma-ray induced relaxation is attributed either to an aluminum-oxygen-hole center or to a “non-local” alkali ion. Finally, accurate values for the real part of the dielectric constant at various temperatures are presented.     

Molecular mobility in piezoelectric hybrid nanocomposites with 0–3 connectivity: Volume fraction influence

Thermo Stimulated Current and Dynamic Dielectric Spectroscopy studies were carried out on hybrid ferroelectric Polyamide 11/Barium Titanate to investigate dielectric relaxation modes. The correlated results obtained by both methods allow us to describe precisely the molecular mobility of this 0–3 nanocomposite; in this work we will focus on the influence of the 700 nm nanoparticles volume fraction. The dielectric spectroscopy shows that the molecular mobility associated with the liquid dynamic is not influenced by the volume fraction. The evolution of low frequency relaxation, observed by depolarization currents techniques, have been attributed to the decrease of Cooperative Rearranging Region size and the increase of intra/inter macromolecular interactions in the soft amorphous phase.