Field and Frequency Effects in a Nematic Mixture of Negative and Positive Dielectric Anisotropy

The molecular alignment in the nematic liquid crystal mixture of p-ethoxy benzylidene p′-n-butyl aniline (EBBA) and p-ethoxy benzylidene p′-cyano aniline (EBCA) has been investigated by measuring the dielectric constant at 1 MHz. Detailed investigation has been carried out for a composition of 2.74%, of EBCA in EBBA for different electric and magnetic field strengths and at different frequencies of the applied electric field. Both parallel and perpendicular electric and magnetic field configurations have been employed. Temperature and frequency dependence of the dielectric anisotropy have been studied.

Two interesting features have been observed. I) In the crossed field configuration complete molecular alignment could be observed by suitably changing the electric field strength. The range of electric field strength required to change the molecular ordering, is frequency dependent. This range shows a minimum in the neighbourhood of 2 kHz. 2) It has also been observed that in the parallel field configuration minimum dielectric constant change is observed apparently independent of frequency and electric field strength. Detailed results of the field and frequency effects and of optical studies in this mixture are reported.     

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.     

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.     

Anisotropic Surface Order in the Isotropic Phase of Nematic Liquid Crystals

A spinning-drop technique was used to study the thermal variation of surface tension of two nematic liquid crystals at the interface with an isotropic fluid. A sharp increase in interfacial tension was observed in the vicinity of the nematic-isotropic transition. The interfacial tension-temperature characteristics, unlike monotonically decreasing dependence found in most isotropic fluids, showed regions of positive slope, both in the isotropic and anisotropic phases. A positive slope in the isotropic region of the liquid crystal is interesting since it implies that excess order has developed at the surface while the buk interior remains isotropic.     

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.     

Development of the Nematic Liquid Crystal Compounds Derived from Phenyl Acetylene Group with Large Birefringence

Abstract

Novel liquid crystal compounds with extraordinary large birefringence have been studided from the viewpoint of optical anisotropy. A large birefrinence was obtained in the compounds compared with that in 5CB, while the orientational order parameters determined by FT-IR measurement are almost the same. From the calculated results of molecular polarizability, it is concluded that the large optical anisotropy is mainly caused by the electric anisotropy.     

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.     

Refractive Indices and Dielectric Studies of Three Phenylcyclohexane Liquid Crystals in the Nematic Phase

The refractive indices n_e, n_o and densities and the dielectric primittivities ε_∥ and ε_⊥ of three P-cyano-P-alkylcyclohexanes (PHC-3, PCH-5, PCH-7) have been measured at different temperatures. The order parameters calculated by using Neugebauer and Vuks' methods were comparable. A comparison of order parameters of the PCH with the corresponding alkylcyanobiphenyls at reduced temperatures τ (= T-T_NI/T_NI) shows that the replacement of a phenyl ring with a cyclohexyl ring decreases the order parameter appreciably. The experimental values of dielectric permittivities ε_⊥ in each of the PCHs are found to be less than half of the ε_⊥-value calculated from Maier and Meier equations using the polarizability and the order parameter obtained from refactive index measurements. This was explained as due to short range antiparallel ordering of strongly polar molecules in the nematic phase, as proposed by Chandrasekhar and Madhusudan, which was not taken into consideration in Maier and Meiers' theory     

A Positive Contrast Guest-Host Display Using a Liquid Crystal of Negative Dielectric Anisotropy

Recently we described a novel guest-host cell which exhibits a positive contrast (dark digits on a clear background) employing a mixture of a nematic liquid crystal with negative dielectric anisotropy, a chiral component, and a dichroic dye. In this cell, the cell thickness and the alignment layers are chosen such that in the absence of a field, the mixture shows a uniform homeotropic structure. When a voltage greater than the threshold value is applied, a helicoïdal structure is adopted, which absorbs the incident illumination.

In this work the expression for the threshold voltage is derived and compared with experimental results. Experimental results are also presented which indicate, for a given cell thickness, the influence of the pitch of the mixture on the contrast and the response time.     

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.     

Anisotropy of Ultrasonic Attenuation in Nematic Liquid Crystals

Anisotropic ultrasonic attenuation in nematic liquid crystals is accounted for by an intramolecular mechanism. Anisotropy calculated for n-(p-methoxybenzylidene)-p-butylaniline (MBBA), on the basis of the proposed model, is in good agreement with experimental results.     

Deformation of Nematic Liquid Crystals in Electric and Magnetic Fields as seen by Proton Nuclear Magnetic Resonance

We have calculated the deformation of the director field within a nematic liquid-crystal cell when both extenal electric and magnetic fields are present. It can be proved experimentally that the line shape of proton nuclear magnetic resonance is strongly affected by the orientational distribution of the director.

The analysis of the NMR-line shape of LC-sandwich cells in this manner can yield various parameters relating to the liquid crystalline material.     

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.     

Intensity and Anisotropy of the Dynamic Ligh Scattering in Nematic Liquid Crystals

The dependence on voltage of the correlation length of turbulence in the dynamic scattering mode in an ac excited nematic liquid crystal has been measured and discussed. The influence of a high frequency stabilizing field has been shown. The total anisotropy of the turbulent medium has been demonstrated, using measurements on a wedge-shaped sample.     

Nematic Behavior of 5OCB in a Dielectric Medium at Phase Transition Temperature ‐A Statistical Analysis

A statistical analysis has been carried out to determine the configurationally preference of a pair of 5OCB (4′‐n‐pentyloxy‐4‐biphenylcarbonitrile) molecules with respect to translatory and orientational motions. The CNDO/2 method has been employed to evaluate the net atomic charge and atomic dipole components at each atomic centre of the molecule. Modified Rayleigh‐Schrodinger perturbation theory along with multicentered‐multipole expansion method has been employed to evaluate long‐range intermolecular interactions, while a ‘6‐exp’ potential function has been assumed for short‐range interactions. The total interaction energy values obtained through these computations were used to calculate the probability of each configuration in vacuum and in a dielectric medium at phase transition temperature using Maxwell‐Boltzmann formula. It has been observed that the probabilities are redistributed in dielectric medium and there is considerable rise in the probabilities of interactions, although the order of preference remains the same. On the basis of stacking, in‐plane and terminal interaction energy calculations, all possible geometrical arrangements of the molecular pairs have been considered. The results are discussed in the light of experimental as well as other theoretical observations.     

Dielectric Anisotropy and the Order Parameter of HXBBA

Abstract

Measurements of dielectric anisotropy (Δ?), refractive indices (n _e , n _o), birefringence (Δn) and density (ρ) have been made in the nematic and smectic phases of N(-p-hexyloxybenzylidene)-p-butylaniline (HXBBA). The results indicate that the various transitions are of the first order type except smectic B-smectic G, which may be a second order transition. The order parameter S has been determined using the isotropic internal field model (Vuks approach) and the anisotropic internal field model (Neugebauer's approach) and both the values agree fairly well. The dielectric anisotropy (Δ?) increases strongly in the smectic phases whle S increases only slowly. It is interpreted by an increase of dipole-dipole correlations.     

Electric Field-Induced Phase Transitions in Electro-Optic Cell with Dielectrically Negative Chiral Nematic Liquid Crystal

Behaviour of a chiral nematic mixture with a negative dielectric anisotropy was studied between two electrodes with homeotropic anchoring in the presence of an a.c. electric field. By changing the sample thickness d and field strength E at constant cholesteric pitch p, the phase diagram in the parameter plane (d/p, E) was determined. Usefullness of this diagram for realization of positive contrast dichroic LC displays with nematic-cholesteric phase change is discussed.     

Surface and Magnetic Orientation in a Type II Nematic Lyomesophase

Abstract

A type II nematic lyomesophase formed by a quaternary system (K decanoate/water/decanol/KCI) has been studied by X-ray diffraction (XD) and optical microscopy (OM) at room temperature. Samples in capillaries of various thicknesses, materials and geometries have been studied under the influence of wall effects, residual magnetic orientation and orientation in presence of magnetic fields. Results evidence that surface orientation extend typically up to 0.5 mm. The planar diffracting units have a characteristic distance in the director direction of 36 Å. The analysis of the compromise between surface and magnetic orientation by XD allowed to determine a critical field of 200 G for a 0.5 mm thickness; the elastic constant is estimated as 10^?-⁷ dynes. Analised by OM, the sample is axially positive, but shows also a weak biaxiality.     

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.     

Electro-Optical Properties of Electric Field-Induced-Polymer Stabilization at Blue Phase of Chiral Nematic Liquid Crystal

Polymer stabilization of the blue phase induced a wide temperature range of more than 60 K, but the high driving voltage and the large switching hysteresis compared to the low molecular weight blue phase of the chiral nematic liquid crystal. In this study, we newly proposed electric field-induced-polymer stabilization of blue phase (EFIPSBP) which the electric field was applied to the blue phase during polymer stabilization and investigated the driving voltage and the switching hysteresis using various voltage-applied EFIPSBP cells. We confirmed that the driving voltage and the switching hysteresis of the EFIPSBP cell were decreased compared to those of conventional polymer stabilized BP at 8OCB chiral nematic liquid crystal mixture. This could be suggested as a potential method for improving the driving voltage and the switching hysteresis of conventional polymer stabilized BP.