Dielectric,viscous and elastic properties of nematogenic 1-(4- trans -propylcyclohexyl)-2-(4-cyanophenyl)ethane

This paper presents the results of measurements of the static and dynamic electric permittivity, the shear viscosity of a freely flowing sample, and the splay and bend elastic constants for 1-(4-trans -propylcyclohexyl)-2-(4-cyanophenyl)ethane (C3H7-CyHx-CH2CH2-Ph-C=N) (3CCPE). The static permittivity of the isotropic phase shows pre-nematic critical behaviour which is discussed in the frame of the fluid-like model of Mukherjee. From the temperature dependence of the dielectric relaxation time (corresponding to the molecular rotation around the short axis) and the shear viscosity, the strength of the nematic potential and the effective length of the 3CCPE molecule (in the isotropic phase), were estimated. The splay and bend elastic constants were determined from the voltage dependence of the capacitance of a planar nematic cell.     

Measurement of the orientational elastic constants and the twist viscosity of nematic side chain polymers

Abstract

The temperature dependence of the three elastic constants k _ii (i = 1, 2, 3) and the twist viscosity γ₁ of two nematic side chain polyacrylates and one comparable low molecular weight compound have been measured by means of the Freedericksz effect. The change from the low to the high molecular liquid crystal causes a change of the ratio k ₃₃/k ₁₁ from greater to less than unity, but the order of magnitude of the elastic constants remains the same. In contrast, the twist viscosity of the polymeric liquid crystal is about 1000 times greater in magnitude than that of a comparable low molecular weight liquid crystal. The activation energy for the viscosity of the polymer differs by a factor 3–4 from that of the low molecular weight liquid crystal. The elastic constants as well as the twist viscosity show a quadratic dependence on the order parameter S over a wide temperature range.     

Dielectric, viscous and elastic properties of nematogenic 1-(4- trans -propylcyclohexyl)-2-(4-cyanophenyl)ethane

This paper presents the results of measurements of the static and dynamic electric permittivity, the shear viscosity of a freely flowing sample, and the splay and bend elastic constants for 1-(4- trans -propylcyclohexyl)-2-(4-cyanophenyl)ethane (C 3 H 7 -CyHx-CH 2 CH 2 -Ph-C=N) (3CCPE). The static permittivity of the isotropic phase shows pre-nematic critical behaviour which is discussed in the frame of the fluid-like model of Mukherjee. From the temperature dependence of the dielectric relaxation time (corresponding to the molecular rotation around the short axis) and the shear viscosity, the strength of the nematic potential and the effective length of the 3CCPE molecule (in the isotropic phase), were estimated. The splay and bend elastic constants were determined from the voltage dependence of the capacitance of a planar nematic cell.     

Determination of the elastic constants of nematic liquid crystals by means of the affine transformation model

In this paper the affine connection approach will be used to calculate the elastic constants of nematic liquid crystals. Following this approach, which was originally conceived to compute the nematic viscosity coefficients, an expression for the elastic constants, without adjustable free parameters, will be formulated in terms of a temperature dependent metric, whose non‐isotropic part is proportional to the tensorial order parameter of the nematic phase. The dependence of the elastic constants on the scalar order parameter, in the geometry of the nematic molecules, and in the anisotropic part of the molecular interaction, will be determined.     

Determination of the elastic constants of nematic liquid crystals by means of the affine transformation model

In this paper the affine connection approach will be used to calculate the elastic constants of nematic liquid crystals. Following this approach, which was originally conceived to compute the nematic viscosity coefficients, an expression for the elastic constants, without adjustable free parameters, will be formulated in terms of a temperature dependent metric, whose non-isotropic part is proportional to the tensorial order parameter of the nematic phase. The dependence of the elastic constants on the scalar order parameter, in the geometry of the nematic molecules, and in the anisotropic part of the molecular interaction, will be determined.     

Elastic,dielectric and optical constants of 4'-pentyl-4-cyanobiphenyl

4'-n-Pentyl-4-cyanobiphenyl (5CB) is a room temperature nematic liquid crystal with a high positive dielectric anisotropy and a high chemical stability. Many experimental results concerning the elastic and dielectric constants of 5CB are available in the literature, although there is often no satisfactory agreement between the experimental data obtained by different groups, especially as far as the dielectric constants are concerned. Furthermore, no detailed investigation of the temperature dependence of the elastic and dielectric constants close to the nematic-isotropic transition temperature T _NI has yet been reported. In this paper, we report the measurement of the elastic and dielectric constants of 5CB, and the temperature behaviour close to T _NI has been investigated in detail. The experiment consists in the measurement of the director deformation induced by an electric field using simultaneously both a dielectric and an optical method. The simultaneous use of these two methods provides an indirect check on the reliability of the measurements. Special attention has been devoted to control possible sources of uncertainty. In particular, the effects of finite anchoring energy and of finite pretilt angle have been considered. The temperature dependence of the anisotropy of the refractive indices is also obtained in the experiment.     

Investigation of dielectric and electro-optical properties of nematic liquid crystal with the suspension of biowaste-based porous carbon nanoparticles

ABSTRACT

The effect of biowaste porous carbon nanoparticles (PCNPs) on the dielectric and electro-optical properties of nematic liquid crystal (LC) mixture (1823A) of 4-(4-alkyl-cyclohexyl) benzene isothiocyanates and 4-(4-alkyl-cyclohexyl) biphenyl isothiocyanates has been studied. The dielectric permittivity of nematic LC has been increased with the dispersion of carbon NPs. The dielectric anisotropy has been calculated and found to be decreased with the dispersion of PCNPs into the pure nematic LC. The response time and birefringence have been also observed with the variation of temperature, frequency as well as the concentrations of carbon NPs. After the dispersion of PCNPs achieved better birefringence and faster response in the dispersed system, which is the significant application in display devices. Threshold voltage splay elastic coefficient and rotational viscosity have been calculated for both pure and NPs dispersed nematic system. Its value is increased with the dispersion of NPs. Additionally, photoluminescence and figure of merit have investigated as a comparative study of nematic matrix as well the dispersed system. The experimental results have been found to have good agreement with the theoretical data of nematic LC. An effort has been made to explain these experimental results on the basis of interaction between nematic molecules and carbon NPs.     

Elastic,dielectric and optical constants of the nematic mixture E49

The Merck nematic mixture E49 exhibits a large nematic interval (0–100 °C) and a large dielectric anisotropy. Both of these features make E49 interesting for applications and basic physics. Unfortunately, no systematic measurements of the material constants of this mixture and their temperature dependence have been reported in the literature. In this paper we report experimental measurements of the splay and bend elastic constants (K ₁₁ and K ₃₃) of the ordinary and the extraordinary refractive indices (n _ort and n _par) at the wavelength λ?=?632.8 nm and of the two elastic constants parallel and orthogonal to the director (ε_par and ε_ort) at the frequency ν?=?5?kHz. The temperature dependence of all of these parameters is found in the temperature range 25–99 °C. The measurements of the elastic constants are performed using both a dielectric and an optical method simultaneously on the same nematic sample. The results obtained using the two methods are in a satisfactory agreement between them within the estimated experimental uncertainty. The ordinary and the extraordinary indices are measured using the prism method.     

Measurement of splay elastic constant and rotational viscosity of an induced nematic binary system

We investigated the temperature dependence of the splay elastic constant (K₁₁) and rotational viscosity (γ₁) of a binary liquid crystal system comprising 5-trans-n-butyl-2-(4-isothiocyanatophenyl)-1,3-dioxane (4DBT) and 4-cyano-4′-n-undecyloxy-biphenyl (11OCB), exhibiting induced nematic phase. Both the splay elastic constant and rotational viscosity increased following lowering of the temperature. In the vicinity of smectic A-nematic (SmA-N) transition, both the relaxation time and rotational viscosity exhibited a strong pretransitional behaviour. The critical exponent (ν) presented here led to valuable qualitative information about the pre-transitional behaviour of the rotational viscosity data near the SmA-N phase transition. The extracted ν values ranging between 0.336 and 0.352 are in fair agreement with those predicted in the de Gennes model as compared to that by the mean-field model.     

Effect of a rigid,non-polar solute on the dielectric anisotropy,the splay and bend elastic constants,and on the rotational viscosity coefficient of 4-4′-n-heptylcyanobiphenyl

The effect of mixing a rigid, non-polar, non-mesogenic solute, biphenyl (C₆H₅-C₆H₅), in the nematic solvent 7CB (4,4′-n-heptylcyanobiphenyl) is investigated. The solute is found to reduce the nematic order and a two-phase region appears. We report measurements of the transition temperatures, dielectric anisotropy, and splay and bend elastic constants, as well as the rotational viscosity coefficient by the method of electric field-induced Fréedericksz transition for biphenyl concentrations up to 8.0%.     

NMR relaxation of rigid molecules in the nematic phase of a discotic liquid crystal

Abstract

Frequency and temperature dependent NMR relaxation measurements were performed on deuteriated benzene, pyrene and triphenylene dissolved in the nematic phase of a discotic liquid crystal. The results show a strong frequency dependence of the spectral densities. Based on the symmetries of the system and the usual model for director fluctuations this frequency dependence should be equal for J ₁ and J ₂. From fitting the commonly used model of rotational diffusion and director fluctuations to the data we see that this is not the case for benzene and triphenylene, even though the fits themselves are satisfactory. Values for the elastic constants, effective viscosity and translational diffusion in similar discotic liquid crystals do not account quantitatively for the frequency dependence of benzene. For both pyrene and triphenylene quantitative comparison was impossible due to lack of translational diffusion data. We also find that the so-called cut-off wave-length is of the order of the dimensions of the liquid crystal molecules, just as in ordinary nematics.     

Fluoro-substitution in hydrogen bonding liquid crystal benzoic acid: dielectric,electro-optic and optical proprieties and inducing polar nematic phase

ABSTRACT

Dielectric and electro-optic properties have been studied in order to understand the effect of the fluoro-substitution on hydrogen bonded liquid crystal proprieties. The fluoro-substitution alters the characteristics of pure LC structure and changes its physical parameters. In the present paper, variation in physical parameters like dielectric permittivity, response time, elastic constant, rotational viscosity etc. has been explained based on many factors. In addition, the fluorine substitution has lead to the appearance of the polar nematic phase.     

Effect of functionalised silver nanoparticle on the elastic constants and ionic transport of a nematic liquid crystal

ABSTRACT

Addition of nanomaterial into pure nematic liquid crystals (NLCs) leads to improvement in the various physical properties of the liquid crystal (LC) host. Doping of nanomaterials affects the local molecular arrangement of the LC molecules. Here, we present the results of our investigation on the effect of functionalised silver nanoparticles (f-AgNPs) on the physical properties of the rod-shaped NLC, 4-trans-pentyl-cyclohexyl cyanobenzene (5PCH). The dielectric constant, threshold voltage, elastic constants, birefringence and conductivity measurements were performed on pure 5PCH and its f-AgNPs doped nanocomposites as a function of temperature in planar cell. The magnitude of dielectric anisotropy, elastic constants and birefringence in nanocomposites were enhanced with increasing concentration of f-AgNPs indicating enhancement of order parameter in the nematic medium. Threshold voltage decreases with increasing concentration of f-AgNPs. Both parallel and perpendicular components of conductivity decrease with increasing concentration of f-AgNPs due to the absorption of ion by the doped f-AgNPs. This observed decrease in conductivity in nanocomposites is further confirmed by calculating the ion transportation number and time of flight. The ion transport number i.e ionic contribution present in the LC cell was found to be 0.966 in pure 5PCH, whereas 0.830 in 0.5 wt% of f-AgNPs nanocomposite of 5PCH.     

Phase transition and elastic constants of 4-n-octyl-4′-cyanobiphenyl (8CB) obtained using the light-scattering anisotropy of turbidity

Abstract

A light-scattering technique was used to study the anisotropy of turbidity and the three elastic constants K ₁, K ₂ and K ₃ of 8CB as a function of temperature and sample thickness. The turbidity was measured in the nematic and schematic A phases at sample thicknesses l of 0.02, 0.04, 0.1 and 0.2 cm. The effect of the smectic-like (cybotactic nematic) order was observed near the smectic A-nematic phase transition. Owing to the surface-enhanced cybotactic order, evaluation of the elastic constants and order parameter was possible only from the turbidity data at l = 0.2 cm. From the divergence of both K ₂ and K ₃ near T_{S A N} we estimated an average critical exponent value v of 0.65, suggesting that S_A-N in 8CB is a second-order phase transition. The magnetic-field quenching of director fluctuations showed observed effects on the order of magnitude of the temperature dependence of the turbidities, elastic constants and order parameter.     

Elastic, dielectric and optical constants of 4'-pentyl-4-cyanobiphenyl

4'-n-Pentyl-4-cyanobiphenyl (5CB) is a room temperature nematic liquid crystal with a high positive dielectric anisotropy and a high chemical stability. Many experimental results concerning the elastic and dielectric constants of 5CB are available in the literature, although there is often no satisfactory agreement between the experimental data obtained by different groups, especially as far as the dielectric constants are concerned. Furthermore, no detailed investigation of the temperature dependence of the elastic and dielectric constants close to the nematic-isotropic transition temperature T_NI has yet been reported. In this paper, we report the measurement of the elastic and dielectric constants of 5CB, and the temperature behaviour close to T_NI has been investigated in detail. The experiment consists in the measurement of the director deformation induced by an electric field using simultaneously both a dielectric and an optical method. The simultaneous use of these two methods provides an indirect check on the reliability of the measurements. Special attention has been devoted to control possible sources of uncertainty. In particular, the effects of finite anchoring energy and of finite pretilt angle have been considered. The temperature dependence of the anisotropy of the refractive indices is also obtained in the experiment.     

High pressure studies of the static permittivity tensor components in the nematic phase of 6CB

The dielectric permittivity tensor components, ε_II and ε_⊥, in the nematic phase of 6CB (4‐n‐hexyl‐4′‐cyanobiphenyl) were measured in the pressure range 0.1–130 MPa and the temperature range 12–58°C. The dielectric anisotropy, Δε(p, V, T) = ε_II ‐ ε_⊥, was analysed in isothermal, isobaric and isochoric conditions taking into account the pVT data and the well known Maier and Meier equation. On that basis the nematic order parameter S(p, V, T) was determined. This was used to calculate the parameter Γ relating the interaction potential with the volume (density). Its value Γ = 4.1 agrees very well with other estimates.     

Accurate measurement of the twist elastic constant of liquid crystal by using capacitance method

In this study, the twist elastic constant (k₂₂) of liquid crystals (LCs) was accurately measured using capacitance method. The constant can be obtained on the basis of accurate measurement of other LC parameters, such as parallel and vertical dielectric constants (ε_// and ε_⊥), splay and bend elastic constants (k₁₁ and k₃₃), and rotational viscosity coefficient (γ₁). First, by using dual-cell capacitance method and an LC cell capacitance model to measure ε_// and ε_⊥, k₁₁ and k₃₃ can be obtained from the threshold voltage determined from the voltage–capacitance curve of the parallel-aligned nematic LC layer and the comparison between the experimental and theoretical results based on the Frank elastic theory, respectively. In addition, γ₁ can be obtained from the measurement of the dynamic response in the parallel-aligned nematic cell. Finally, k₂₂ can be accurately determined using the threshold voltage of the twisted nematic LC cell. By adopting the above method, the measured k₂₂ for LC E7 was 6.7 × 10^?12 N. The proposed method is more rigorous and yields a more accurate measurement result than the other methods reported in the literature.     

Liquid-crystalline reference compounds

Abstract

A polar (PCH-5) and a non-polar (152) compound are introduced as liquid-crystalline bench-mark compounds. The materials chosen are affordable, stable, easy to handle and possess nematic mesophases between 30–54·9 and 24–103·4°C. All relevant macroscopic physical properties, such as refractive indices n _o n _e, dielectric permittivities ∥_?, elastic constants K ₁₁, K ₂₂ and K ₃₃, and bulk and rotational viscosities are given as a function of temperature, wavelength and frequency, when applicable. The reference compounds are available from E. Merck, Darmstadt.     

Electroplating of Steel in the Presence of 5,6-Dihydropyrimidine-2-(1H)-thione, 2-Methylthiopyrimidine-4-(1H)-one, 2-Thiopyrimidine-4-(1H)-ones,and 2,4-Pyrimidine(1H,3H)-dione Derivatives as Organic Additives

The electrode processes on steel in acidified CuSO₄ were finding to depend on the organic additives as well as their concentrations. They also depend on the type of the anode and temperature. The activation energy proves that the reaction is diffusion controlled. The percentage of inhibition for all solutions ranged from 6.9% to 44.7% and depended on the type of the cathode and the concentration of the additive. The order of inhibition efficiency of steel cathode is: compound 4 > compound 5 > compound 3 > compound 2 > compound 6 > compound 1. The overall mass transfer correlation proves that the electroplating reaction is natural convection, which is in accordance with our previous studies.     

Thermodynamic study of complex formation process of free base meso-tetraphenylporphyrins with dimethyl and dibutyltin(IV) dichloride: a new algorithm for a single thermometric titration

A novel, fast and easy single sample measurement has been developed based upon temperature dependence of equilibrium constant in order to determine the enthalpy and entropy changes of a complexation reaction using spectrophotometric temperature titration. The method can be used in determination of the formation constant and thermodynamic parameters of the solutions that there are difficulties in their titration where volatile compounds are studying. Knowledge of component spectra is not required for the analysis. The formation constants of the interactions of ß-di and tri-brominated meso-tetraphenylporphyrins, and meso-tetrakis(4-methylphenyl) and (4-methoxyphenyl) porphyrins with Me₂SnCl₂ and Bu₂SnCl₂, have been determined in range of 0–25 °C utilizing van’t Hoff relation, mass balance and equilibrium constant equations by an iterative least squares method with ΔH ⁰ as adjustable parameter. The outputs of analysis are the equilibrium constants, ligand and adduct spectral profiles, their concentrations as a function of temperature, the adjusted values of the standard enthalpy ΔH ⁰, and entropy ΔS ⁰ changes. The order of formation constants of the resulting 1:1 complexes decreased with increasing number of bromide substituents and increased with adding methyl and methoxy groups, and vary as H₂T(4-CH₃O)PP > H₂T(4-CH₃)PP > H₂TPP > H₂TPPBr₂ > H₂TPPBr₃ and Me₂SnCl₂ > Bu₂SnCl₂.