Dielectric Behavior of Nematic Liquid Crystals under DC Electric Field

Dielectric behavior of nematic liquid crystals of p-methoxybenzylidene-p-n-butylaniline and p-ethoxybenzylidene-p-n-butylaniline have been investigated under DC electric field in relation to their optical behavior. Both dielectric permittivity and transmitted light intensity decrease at a threshold voltage of external DC electric field, and the latter reaches a constant low value at higher field, while dielectric permittivity increases via minimum to approach a constant value at higher field strength of about 2 × 10⁴ V/cm. Such a change of dielectric permittivity under DC electric field is characteristic of nematic and supercooled states, and exhibits large dependence on both temperature and sample thickness. This dielectrical observation is discussed in terms of dielectric anisotropy of the nematic liquids oriented to the field, and the disturbance of molecular orientation due to the dynamic scattering at higher DC electric field.     

Dielectric and Electrooptic Investigations in the Nematic Phase of 5-n-Pentyl-2-[4-n-nonyloxyphenyl]-1,3-dioxane

The dielectric constant and the dielectric loss of 5-n-pentyl-2-[4-n-nonyloxyphenyl]-1,3-dioxane were investigated in the region of 0.1 to 10 MHz. An isotropy temperature was detected, at which the static dielectric anisotropy Δε₀₁ = 0. Using both the Freedericksz and the DAP effect, we measured the frequency dependence of the threshold voltage U₀ and compared the results with those of the dielectric measurements. In the case of DAP effect we could find two different types of dispersions curves of U₀.     

Influence of a.c. Electric Field on Infrared Absorption Spectra of Liquid Crystals and Determination of Orientational Order Parameter by Infrared Dichroism. II. Molecules with Weak Negative Dielectric Anisotropy

The influence of a.c. electric field (10 kHz) on the IR spectra of the nematic phase of six p-alkoxybenzylidene-p,n-butylanilines with weak negative dielectric anisotropy was studied and the temperature dependence of the orientational order parameter and threshold voltage have been investigated. The values of band elastic constants K₃₃ were calculated for MBBA and EBBA.     

Diamagnetic Anisotropy Measurements of Nematic Liquid Crystals

Electro-optic effect of liquid crystals in the presence of a magnetic field was investigated. Two independent techniques: magnetic field dependent threshold voltage and magnetic field-assisted relaxation time, were used to evaluate the diamagnetic anisotropy for three nematic liquid crystals: BDH-E7, RO-TN-103 and ZLI-1132. Correlation between the diamagnetic anisotropy and the molecular structures was obtained. This information is particularly important for synthesizing liquid crystal components with desired diamagnetic anisotropy.     

Some Novel Nematic Liquid Crystals of Negative Dielectric Anisotropy

A number of ether and ester derivatives of 1-(trans-4-n-alkylcyclohexyl)-2-(4-hydroxyphenyl)ethane bearing lateral substituents on the phenyl ring are reported. Several of the novel compounds exhibit enantiotropic nematic phases at room temperature. All of the liquid crystals described are of negative dielectric anisotropy (-0.5 -3.5). The effect of lateral substituents on the liquid crystal transition temperatures of these ethanes has been studied.     

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.     

Magnetic Field Dependence of Laser Light Scattering through CBOOA in Nematic Phase

In the nematic phase of homeotropically aligned liquid crystals which have the smectic A phase at lower temperature, interference rings have been observed above some threshold magnetic field H_c which nearly coincides with the ocurrence of Freedericksz transition, only for the light polarized parallel to the direction of magnetic field. This interference ring disappears with increasing magnetic field above second threshold H_s . The origin of these interference rings is explained tentatively by a periodic deformation of bulk directors.

The bend elastic constant of CBOOA estimated from the measurements of the interference ring as a function of temperature, diverges with the critical exponent (0.5 ± 0.02) near the smectic A-nematic transition temperature and agrees well with a recent suggestion of McMillan based on the mean field theory.     

Dipole-Dipole Correlation in Nematic Liquid Crystals

The theory of dipole-dipole correlation in nematic liquid crystals is presented and it is shown that different models for the internal field lead to different values for dipole correlation factors. Results of calculations are presented for three liquid crystals and commented upon. Using the calculated correlation factors and a simplified dielectric theory, predictions are made of the temperature derivatives of the mean permittivity and permittivity anisotropy and compared with experimental values.     

On the Ability of Homogeneously Aligned Nematic Mesophases with a Positive Dielectric Anisotropy to Exhibit Williams Domains as a Threshold Effect

The ability of homogeneously aligned nematics with a positive dielectric anisotropy to exhibit Williams Domain as a threshold effect is numerically investigated. A simplified two-dimensional model which has, already, given satisfactory results in the negative dielectric anisotropy case is used. The frequency dependence of the threshold fields obtained for materials with small dielectric anisotropies is in satisfactory qualitative agreement with the known experiments. The frequency dependence of the threshold spatial wavenumbers is calculated. In accordance with earlier work, a positive dielectric anisotropy nematic which normally does not exhibit Williams Domains, does so under the action of a stabilizing magnetic field. This effect is confirmed and the frequency dependence of the threshold fields as well as the assoiated threshold spatial wavenumbers, for various values of the stabilizing field, are calculated. In addition, the model predicts the existence of a class of positive dielectric anisotropy nematics which do not form Williams Domains whatever the applied magnetic field is. This suggests a distinction between three classes of positive dielectric anisotropy nematics.     

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.     

Dependence of the Electrically Controlled Birefringence on Applied Frequency in a Nematic Liquid Crystal

Detailed studies on the dependence of the ECB effect on the applied frequency have been made in a nematic liquid crystal MBBA. It is found experimentally that, when the applied voltage is just above the threshold voltage, there are strong dependences of both the delay time and the mean deformation angle of the molecular director subjected to a constant voltage setup on the applied frequency. Those applied frequency dependences of the ECB effect can be interpreted in terms of the dielectric alignment in an electrically conducting nematic liquid crystal. We have measured the capacitance of the deformed liquid crystal cell varying the applied frequency. The experimental results are compared with the Gruler and Cheung theoretical prediction, and good agreements are obtained between them when the applied voltage is just above the threshold voltage.     

Dielectric Studies of Three Liquid Crystals HBT,E8 and PCH-1132

The dielectric behavior of a Schiff base N-(P-Hexyloxybenzylidene)-P-toluidine (HBT) and two technologically important mixtures, E₈ and PCH-1132, are reported in the radio frequency region. The value of the mean dielectric constant increases by ～5% on going from the nematic to the isotropic phase in the case of E₈ and PCH-1132. This may perhaps be due to antiferroelectric arrangement of the molecules in the nematic phase. These mesogens show unusual dielectric behavior and exhibit positive anisotropy at lower frequencies and negative dielectric anisotropy above a critical frequency termed as isotropic frequency. The isotropic frequency is found to be temperature dependent. All these nematogens exhibit single relaxation in the radio frequency region which is due to hindered rotation of the molecules under nematic potential. The relaxation frequencies are 3.98 MHz (at 61.4°C), 2.27 MHz (at 34.3°C), 1.87 MHz (at 33°C) in case of HBT, E₈ and PCH-1132 respectively. The activation energy is found to be 16.7 KCal/mole, 14.6 KCal/mole and 16.1 KCal/mole for HBT, Ea and PCH-1132 respectively. In smectic B phase of HBT, the dipolar orientations are found to be somewhat “locked” similar to that in solids. The inversion of the sign of dielectric anisotropy in S_B phase of HBT is also discussed.     

Nematic and Cholesteric PDLC Elaborated under Shear Stress

Abstract

The properties of nematic and cholesteric Polymer Dispersed Liquid Crystal (PDLC) with ellipsoidal droplets are reported. The shape of the droplets are modified by a shear stress during the preparation. Different values of the shear stress were used which permit an ellipticity ratio modification by a factor 10. The electro-optical properties are measured for different types of samples and compared with theoretical previsions: transmission or reflection under voltage, threshold voltage, response times. We also describe the preparation of the sample, the experimental apparatus used for the shear stress, and the expected theoretical quantities like response times. When the liquid crystals are nematics, we obtain an increase of the threshold voltages, as predicted. The relaxation time decreases with increasing deformation but the diminution is not as great as predicted. When the liquid crystals are cholesterics, the high deformations induce polygonal fields, which strongly modify the electro-optical properties of the micro-composite. The threshold voltages and response time at applied voltage decrease whereas the relaxation time is increasing.     

Determination of Elastic Constant K 33 for Some Nematic Liquid Crystals with Strong Positive Dielectric Anisotropy by Means of IR Absorption Spectroscopy

The influence of a.c. electric field (2000 Hz) on the half-width of some bands from IR spectrum of three liquid crystals with strong positive dielectric anisotropy is investigated and U _c and K ₃₃ values are calculated.     

In-Plane Switching of Chiral Nematic Liquid Crystals with Different Dielectric Anisotropies

The electro-optic response in planar-aligned chiral nematic liquid crystals in an electric field oriented perpendicular to the helical structure axis is studied as a function of the magnitude and sign of the dielectric anisotropy of liquid crystal in order to understand the mechanism of this electro-optic effect. The main contribution to the electro-optic response is shown to be due to the interaction of the electric field with the dielectric anisotropy. The flexoelectric effect also contributes to the electro-optic response, but its magnitude is much smaller.     

Truxene Derivatives

Eight hexa-n-alkanoyloxy truxenes are presented. These new disc-like liquid crystals exhibit both columnar and N _D nematic phases. The N _D nematic phase is the first low temperature one. All these compounds show an inverted nematic-columnar sequence: the fluid N _D nematic phase is observed while heating after the crystalline phase below one or two viscous columnar phases and shows a phenomenon quite similar to the reentrant nematic behaviour.     

Application of Infrared ATR Spectroscopy to Liquid Crystals. III

Infrared absorption measurements of nematic N-(p-ethoxybenzylidene)-p'-cyanoaniline (EBCA) under a DC electric field were performed by means of our previously reported ATR method. DC voltages up to 15V were applied to the nematic liquid crystal 15 μm thick between a silicon ATR prism (probing electrode) and an In₂O₃-coated glass electrode (opposite electrode). By using this method the orientational behavior of molecules in boundary layers about 1 μm thick at the Si electrode was investigated.

The threshold voltage for the dielectric orientation varied from 2 to 6 V depending upon the boundary conditions at the opposite electrode, and the maximum degree of order of homeotropic orientation was found to be 0.6–0.7. A storage effect, in which a homeotropic texture induced by the electric field remains unaltered even if the field is removed or reversed, was observed.     

Domain structure of a planar layer of a nematic liquid crystal in oscillating Couette flow

The spatially periodic structure arising in a planar layer of a nematic liquid crystal in oscillating Couette flow is described theoretically. The analysis of this effect is based on the hydrodynamic equations for nematic liquid crystals, from which a self-consistent system of equations is singled out for perturbations of hydrodynamic variables. It is shown that the structure and threshold parameters of the effect depend on the oscillation frequency and the layer thickness through the scaling combination ωh ². The dependence of the configuration of distortions on the viscosity α₃ is analyzed.     

Liquid crystalline swallow-tailed compounds (II) dielectric relaxation

Dielectric investigations in the kHz-range on swallow-tailed liquid crystals show a strong decrease of the dielectric constant as well as of the dielectric loss parallel to the nematic director with decreasing temperature. In the perpendicular direction no strong anomaly has been observed. By consideration of X-ray data of the S_A phase the dielectric results can be interpreted by the dipole-dipole interaction which is forced by the anisotropy of repulsive forces of the respective molecules.