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.     

PHYSICAL PROPERTIES OF SOME NOVEL NEMATIC BIMESOGENS FOR THE FLEXOELECTRIC EFFECT

Abstract

In a chiral nematic liquid crystal, the flexoelectric effect consists of a fast and linear coupling with an applied electric field. One difficulty to overcome is the unwinding of the helix that occurs at higher fields due to dielectric coupling. The use of bimesogens, which possess very low molecular dielectric anisotropy can improve flexoelectric characteristics. New bimesogen compounds have recently been synthesised that exhibit switching angles of 45° for applied fields of about 9 V.μm^?1. In this paper, results from dielectric, electro-optic and dynamic light scattering measurements are reported for the new bimesogenic mixture. The dielectric anisotropy Δε changes sign with temperature and its values range between -0.2 and 0.3 for the temperature range studied. For Δε weakly positive, no electric field Freedericksz transition could be induced but Williams domains are observed instead. The large decrease in the bend elastic constant to viscosity coefficient ratio is attributed to a large increase in the bend viscosity coefficient.     

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.     

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.     

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.     

Electric Field Effects on Sensitized Fluorescence Emission From Nematic Liquid Crystal Cells

Remarkable electric field effects were observed with sensitized fluorescence intensities of a perylene derivative (BPDC), when rectangular AC waves (up to 10 V_0-p ) were applied to nematic liquid crystal cells containing a sensitizer (C307 or AHDA) and an acceptor (BPDC) as the guest molecules. Measurements of the fluorescence lifetime of AHDA indicated that the rates of energy transfer from AHDA to BPDC were independent of the electric fields. The electric field effects completely changed the sign when the intensity variation detected at vertical direction was compared with that at the horizontal direction with respect to the liquid crystal cell. The observed electric field effects on the sensitized fluorescence were consequently suggested to come from the orientation control of transition dipole moment of BPDC through the molecular alignment of the liquid crystal. The suggestion was confirmed by the measurement of dichroic absorption of the donor-acceptor systems.     

Study of Liquid Crystals in Flow: I. Conventional Viscometry and Density Measurements

Using the technique of conventional viscometry (Haake-Hoeffler Falling Ball and Ostwald-Poiseuille viscometers), we have measured the temperature dependence of the apparent viscosity of three liquid crystals: methoxybenzylidene butylaniline (MBBA), hexyloxybenzylidene amino benzonitrile (HBAB) and cyanobenzylidene octyloxyaniline (CBOOA). The measurements with the falling-ball show a small discontinuity in the apparent viscosity of (HBAB) at ～ 91°C, where the flow properties of this compound have been found to change from one flow regime in which the “classical” molecular alignment with the flow can be observed (T > 91°C) to another regime in which this alignment cannot be observed (T < 91°C). However, in spite of this small discontinuity, the apparent falling-ball viscosity which we measured for (HBAB) is still very close to the Miesowicz viscosity, η_B, obtained by constraining the director n to remain parallel to the velocity vector during the measurements. Measurements performed with the Ostwald-Poiseuille viscometer show: (i) that the apparent viscosity and the viscous heat of the nematic and smectic-A mesophases depend upon the radius of the capillary; (ii) that the apparent viscosity, in both the flow-alignment and the non-flow-alignment regimes, is very close to η_B and (iii) that there is no convincing evidence of a discontinuity in the apparent viscosity of (HBAB) at 91°C.

Our measurements of the temperature dependence of the density of the mesophases show that the isobaric coefficients of thermal expansion for (HBAB) and (CBOOA) are different in the flow alignment and in the non-flow-alignment regimes. This is consistent with the hypothesis that there is a greater degree of molecular association in the non-flow-alignment regime.     

Piezoelektrische und elastische Eigenschaften von Triglycinselenatkristallen im Bereich des Phasenüberganges

The piezoelectric constants d₂₁, d₂₃, d₂₅ and the elastic constants S₁₁, S₁₅, S₃₃ and S₃₅ of triglycine selenate are measured by the resonance method in the region of phase transformation. The elastic and piezoelectric constants are shown to have sharp maxima in the Curie point. The influence of a constant electric field on the temperature dependence of the piezoelectric constant d₂₃ and of the elastic constant S₃₃ is studied as well as the dependence of the mentioned constants on the intensity of a constant electric field at temperatures below and above the Curie temperature. A considerable increase of the elastic constant S₃₃ is found in weak electric fields of the ferroelectric phase. The electrostrictional coefficients Q₂₁, Q₂₃ and Q₂₅ are determined by means of the piezoelectric constants, spontane polarisation and dielectric receptivity. The found experimental dependences are explained on the base of the character of the domain structure of this crystal.     

The Dielectric and Optical Properties of the Homologous Series of Cyano-Alkyl-Biphenyl Liquid Crystals

The electric permittivities, refractive indices and densities of the homologous series of alkyl-cyano-biphenyls (C_nH_2n+1.φ.φ.CN) have been measured as a function of temperature in the nematic phases for n = 5 to 9, and in the smectic phases for n = 8 and 9. The results have been analysed in terms of the Maier and Meier theory to yield values for the molecular dipole moments and polarizabilities. Kerr effect measurements on dilute solutions of 44′-n-pentyl-cyano-biphenyl at different temperatures are reported, and in conjunction with similar dielectric and optical measurements have provided results for the free molecule dipole moment and polarizabilites; values for the higher homologues are predicted using a bond additivity model. Bordewijk's theory is applied to the permittivity measurements, and using appropriate molecular parameters, dipole-dipole correlation factors are calculated.     

Some special dielectric characteristics of TlGaSe2 layered crystals

TlGaSe₂ layered crystals were characterized with dielectric spectroscopy measurements. We have studied the memory effect within soliton regime, time relaxation in annealing temperature, and the effect of different cooling rate on conductivity and dielectric constant. The effects connected with the annealing time and cooling rate processes may be explained the theory of DDW. The measurements were carried out in a narrow temperature region (95‐135 K). The temperature dependence of dielectric constant measurements show that the crystal reveals a weak dielectric anomaly at 101 K. A well‐defined ferroelectric behaviour of first order transition was observed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure,Stability and Response to the Electric Field of Antiferroelectric Smectic Materials

Abstract

The structure and switching of five antiferroelectric mesogens are studied by the constant temperature-pressure molecular dynamics simulation. The conformations of the end chains in the smectic phase are compared with those at an isolated single molecule estimated by the method of molecular orbitals, where the bending angle of the chiral chain as well as the angle of the achiral one is shown to be enhanced. The tilt angles obtained here are in well agreement with the experimental ones. The response of the anticlinic alignment to the transverse electric field is also tested.     

Phase Transition of 1,16-Hexadecanediol

It was found that 1,16-hexadecanediol exhibited a solid-solid phase transition. X-ray diffraction experiments and dielectric constant measurements were carried out in order to investigate the structure of the high-temperature phase. A model of molecular arrangement in the high-temperature phase was presented. The high-dielectric constant values were observed in the high-temperature phase. It is considered that the high-temperature phase of 1,16-hexadecanediol is a rotator phase in analogy with n-higher alcohols.     

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.     

Crystal growth and physical properties of monoclinic L-arginine hydrochloride monohydrate,C6H14O2N4HCl · H2O,and L-arginine hydrobromide monohydrate,C6H14O2N4HBr · H2O

Single crystals of optical quality of L-arginine HCl · H₂O and L-arginine HBr · H₂O with dimensions up to 80 × 60 × 30 mm have been grown from aqueous solutions by controlled evaporation at about 310 K. The isotypic crystals exhibit a quite similar behaviour with respect to morphological, pyroelectric, dielectric, optical, thermal expansion, electrostrictive, electrooptic, elastic, thermoelastic and non-linear optical properties. A distinct anisotropy of the longitudinal elastic stiffness is observed with a maximum along the direction of the alignment of the arginine ions. Nearly in the same direction a minimum of thermal expansion, dielectric constant and index of refraction is found. The pyroelectric, electrostrictive, electrooptic and non-linear-optical effects are of only mediocre magnitude except the electrostrictive constant d₂₁₁ which surmounts the longitudinal electrostrictive constant d₁₁₁ of alpha-quartz by a factor three.     

Diaqua tetrabenzo (b,f,j,n,) {1,5,9,13} tetraazacyclohexadecine copper(II) and nickel(II) bisanthraflavates: crystal and molecular structure

Charge transfer molecular solids [diaqua tetrabenzo (b,f,j,n,) {1,5,9,13) tetraazacyclohexadecine] Ni(II) and Cu(II) bisanthraflavates were synthesized and characterized by IR, Mass Spectrometry, TGA, and X-ray diffraction on single crystals. Unit cell dimensions (Å) a = 12.291(1), c = 14.574(1) Å and a = 12.434(1), c = 14.066(1) Å for Cu(II) and Ni(II) derivatives, respectively, space group P4₂/n in both cases. The obtained compounds show expected chemical and structural similarities. Tetraazamacrocycles are surrounded individually by a zig-zag, ribbon-like motif extending in the crystal by head-to-tail hydrogen bonding of monosubstituted anthraflavates. Electric conductivities were measured by the four points method as pellets from the variation of electric current through the sample as a function of temperature, for fixed voltages in the ohmic regime. Preliminary results indicate low conductivity values at room temperature but semiconductivity behaviour in the temperature range –150 to 170°C.     

The Dielectric Anomaly Near the Transition from the Smectic A* to Smectic C* Phase and Visco-Elastic Properties of Ferroelectric Liquid Crystals

A pyro-electric technique is developed which allows the measurement of the dielectric response near the A*-C* phase transition in ferroelectric liquid crystals. The temperature dependence of the elastic modulus K _θ(T) corresponding to the molecular tilt in smectic layers is calculated from the experimental data. Direct pyro-electric measurements of the relaxation time for the spontaneous polarization P _s and the data on K _θ(T) allow us to calculate the temperature behaviour of the twist-viscosity γ₁(T) for the smectic C* phase. The curves γ₁(T) are compared for the smectic C* and the nematic phase of the same compound and the dependence of the twist viscosity on the molecular tilt angle in the C* phase is investigated. The results of the dielectic measurements are discussed in terms of the mean-field approximation.     

Dielectric studies on binary mixtures containing 4-trans-4′ -n-hexyl-cyclohexyl-isothiocyanato-benzene

We have studied binary mixtures with an induced smectic phase of 4-trans-4′ -n-hexylcycohexyl isothiocyanatobenzene (6CHBT) with 4-n-ethyl-4′-pentylazoxybenzene (EPAB) by means of dielectric, microscopic and refractometric measurements. The effective values of components p ⟂ and p∥ of the dipole moment per one molecule of the mixture have been calculated from values of ϵ ⟂ and ϵ ∥. As a result of our studies, we have established that the heterocomplexes formed in the mixture are so weak that they do not cause any essential changes in the distribution of molecule charges. The observed deviations from additivity of the dielectric constant of the mixture are caused solely by a non-additive behavior of the mixture density.     

Dielectric Study on the Alignment or Orientation of N(4-n-heptyloxy benzylidene)4´-n-butylaniline and the Anomalous Dielectric Behavior in the Smectic G Phase

The present article reports on the alignment or orientation of a particular compound N(4-n-heptyloxy benzylidene)4´-n-butylaniline through dielectric study. The effect of using two different alignment layers (for homogenous alignment) and doping silver nanoparticles (0.25% by weight) on the behavior in dielectric property with varying temperature and biasing electric field was studied in terms of liquid crystal alignment or orientation using the mechanism proposed by earlier workers. The study includes the anomalous dielectric behavior in SmG phase by presenting a technique for the analysis of the absorption process, which exists only in SmG phase at fixed frequency while varying temperature, is also presented.     

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.     

Dielectric Studies of the Ordering Due to an External DC Electric Field in Nematic 4,4′-di-n-hexyloxyazoxybenzene

Studies of dielectric permittivity changes under the action of a DC electric field on nematic 4,4′-di-n-hexyloxyazoxybenzene (DHAB) have been performed. Observations of time dependent processes occurring during the application of a voltage were made for samples of 0.5–2 mm thickness as a function of the applied voltage. The dielectric permittivity changes which are related to changes in the ordering of the samples were interpreted in dielectric as well as in electrohydrodynamic terms. Three voltage regions were distinguished which are characterised by ordering due to dielectric orientation, hydrodynamic laminar flow and turbulent flow.