Nuclear magnetic relaxation of nematic liquid crystals,N-(4-methoxybenzylidene)-4′-n-butylaniline and N-(2-hydroxy-4-methoxybenzylidene)-4′-n-butylaniline

Nuclear magnetic resonance linewidth and spin-lattice relaxation time T₁ were measured for 4-methoxybenzylidene-4′-n-butylaniline (MBBA) and N-(2-hydroxy-4-methoxybenzylidene)-4′-n-butylaniline )OHMBBA) in their liquid, nematic, undercooled nematic, and glassy nematic liquid crystal states as well as in the stable crystalline states. In the case of OHMBBA, the motional narrowing occurs in three steps corresponding to excitation of end-group reorientations, self-diffusion, and molecular tumbling. T₁ of OHMBBA in the nematic and undercooled nematic state has two minima at 264 K (24 ± 4 msec) and at 140 K (22.0 ± 0.5 msec); the motions responsible for the minima are the self-diffusion and the end-group reorientations with the activation energies 29 ± 1 J mole⁻¹ and 5.8 ± 0.2 kJ mole⁻¹ respectively. Analogous results were obtained for MBBA. Some measurements were also made for p-azoxyanisole for the sake of comparison. The effect of external magnetic field on the formation of the glassy state was examined.     

Changes of thermo-morphologic and thermotropic properties of liquid crystals at direct and reverse nematic mesophase ↔ isotropic liquid phase transitions: surface-induced effect

In this work, the effect of thin films on the thermo-morphologic and thermotropic properties of the phase transitions between the nematic mesophase and isotropic liquid has been investigated. Investigations have been carried out for both the heating and cooling processes. The temperature and linear widths of the biphasic regions of the direct and reverse phase transitions in nematic liquid crystals versus thickness of the thin films have been calculated with a high accuracy. The shift of the nematic mesophase–isotropic liquid and the isotropic liquid–nematic mesophase phase transition temperatures to higher temperatures and the enlargement of the temperature and linear widths of the biphasic regions as the effect of surfaces have been found.     

Mössbauer spectra of the Γ8 quadruplet in the presence of spin-lattice relaxation

The influence of crystal field fluctuations on Mössbauer hyperfine spectra is analyzed. These fluctuations lead to violation of cubic symmetry of the environment, to a time-dependent splitting of a quadruplet and, as a consequence, to a temporal modulation of the hyperfine interaction. It is found that fluctuations of E-type do not lead to a complete averaging of the hyperfine interaction in the fast relaxation limit, unlike the case of e.g. magnetic relaxation and any other type of crystal field fluctuations. Moreover, for ions with J=7/2 in the fast relaxation limit the relaxation spectra of ₈ are identical to the spectra of the ₈ doublet.     

Lowering of the Magnetic Fréedericksz transition threshold in nematic liquid crystals doped with ferromagnetic nanoparticles

We show that magnetic colloids could increase the coupling of a liquid crystal to a week magnetic field. Magnetic Fréedericksz transition in a planarly oriented nematic liquid crystal doped with spherical ferromagnetic nanoparticles is considered theoretically. It is shown that even spherical nanoparticles could decrease a threshold of this transition because they strongly increase the magnetic field in the medium. This phenomenon indicates the possibility to use the considered system in magneto-optical displays.     

Observation of cancellation and second light-induced Fréedericksz transition in nematic liquid crystals

We describe the observation of a new phenomenon: the cancellation of molecular reorientation that has been achieved in nematic liquid crystals acted on by two competing (pulsed) light beams. The effect is unstable: Starting from the cancellation condition, we have observed patterns related to the presence of a nonlinear phase shift in both beams when their intensity was increased simultaneously. We characterize the behavior of the induced phase shift as a function of the impinging power.     

Calculating the dielectric anisotropy of nematic liquid crystals: a reinvestigation of the Maier——Meier theory

This paper investigates the average dielectric permittivity (\overline ε ) in the Maier--Meier theory for calculating the dielectric anisotropy (Δε) of nematic liquid crystals. For the reason that \overline ε of nematics has the same expression as the dielectric permittivity of the isotropic state, the Onsager equation for isotropic dielectric was used to calculate it. The computed \overline ε shows reasonable agreement with the results of the numerical methods used in the literature. Molecular parameters, such as the polarizability and its anisotropy, the dipole moment and its angle with the molecular long axis, were taken from semi-empirical quantum chemistry (MOCPAC/AM1) modeling. The calculated values of Δε according to the Maier--Meier equation are in good agreement with the experimental results for the investigated compounds having different core structures and polar substituents.     

Temperature dependence of ratio between dielectric anisotropy and order parameter in fluorinated nematic liquid crystals

Temperature dependence of ratio between dielectric anisotropy and order parameter of fluorinated nematic liquid crystal is investigated by using a semi-empirical molecular orbital package that can accurately calculate an angle between molecular dipole moment and long axis. We optimize the molecular conformations with three semi-empirical Hamiltonians AM1, PM3 and PM5, and then make a comparison between computational results and experimental measurements. It is shown that the results obtained from AM1 method are in good agreement with the measurements. The present study offers an applicable method to predict the dielectric properties of liquid crystal material.     

Dielectric properties of 4′-n-alkyl-4-cyanobiphenyls in their nematic phases

The principal dielectric constants of the pentyl to octyl derivatives of 4′-n-alkyl-4-cyanobiphenyl have been measured as functions of temperature in their nematic and isotropic phases. All the compounds exhibit a strong positive dielectric anisotropy due to the presence of a large dipole moment along the major molecular axis. The principal dielectric constantsε _‖ andε _⊥ as well as the anisotropy Δε decrease with increasing alkyl chain length. The experimental value of (ε_‖+2ε_⊥) decreases with decreasing temperature, and is throughout less than the extrapolated isotropic value, in conformity with the model of antiferroelectric short range order proposed by Madhusudana and Chandrasekhar.     

NMR study of the Rα structures of 2-chloro- and 2-bromothiophene partially oriented by nematic liquid crystals

The ¹H NMR spectra with the ¹³C satellites and the '3C spectra of 2-chloro- and 2-bromothiophene in nematic Merck ZLI 1167 and ZLI 1132 liquid crystal solutions were recorded and analyzed. The 'H spectra and three of the four ¹³C-satellite spectra appeared to be deceptively simple and prevented the determinations of certain individual coupling constants. As a consequence, the problems became under-determined. However, the use of two liquid crystal solvents and the assumption of unchanged structures in the solutions made it possible to determine the complete r_αr_α structures, although the linear dependence of some dipolar coupling constants led to more poorly determined groups of equations than was expected. The CH and CC distances, bond angles, and orientation matrix elements are reported for both 2-chloro- and 2-bromothiophene.     

Theoretical estimation of mesogenic characteristics of 4-methyl (2′-hydroxy,4′-n-hexadecyloxy) azobenzene – a nematic liquid crystal

The compound 4-methyl (2′-hydroxy,4′-n-hexadecyloxy) azobenzene was synthesized by Prajapati and co-workers (Mol. Cryst. Liq. Cryst. 369 (2001), pp. 37–46). Subsequent experiments (D. Pal, [PhD thesis], University of Lucknow, Lucknow, India, 2007) confirmed that the compound exists in nematic phase for a small range of temperature (72°C–80°C). In the present work, optimization of molecular geometry has been carried out by employing the Gaussian 03 suit of programs without any constraint using density functional B3LYP along with 6-31G** basis set and checked for imaginary frequencies. A detailed investigation on intermolecular interaction energy at various interacting configurations has been carried out. In order to study the mesogenic characteristics of the molecule, an attempt has been made to estimate the variation of order parameter with respect to the change in temperature as well as degrees of freedom. These studies will be helpful to understanding the mesogenic character of any molecule prior to synthesis and promises future application in molecular engineering.     

Influence of nematic and smectic orders on polarizability of molecules of ethyl-p-(4-ethoxybenzylideneamino-)α-methyl cinnamate liquid crystals

Experimental values of the polarizability tensor γ components of molecules of ethyl-p-(4-ethoxybenzylideneamino-)α-methyl cinnamate liquid crystals in the nematic and smectic A phases have been obtained. Quadratic dependences of the longitudinal, γ _l , and transverse, γ _t , components, the mean value $\bar \gamma $ , and anisotropy Δγ in both phases have been established as functions of the orientational order parameter S of molecules in a maximally wide range of S. The nematic-smectic A phase transition with a continuous variation of S manifests itself as kinks in the linear dependences of $\bar \gamma $ , Δγ, and γ _l on S ² and does not influence the dependence γ _t (S ²). The observed dependences have been derived in the framework of the theory of the nematic-smectic A phase transition. The factors responsible for different influences of the orientational and positional orderings of molecules on the polarizability parameters have been determined.     

Distribution function of relaxation times for a 4-<Emphasis Type="Italic">n</Emphasis>-pentyl-4′-cyanobiphenyl liquid crystal

The distribution function of relaxation times is reconstructed from the dielectric loss spectrum measured over a wide range of frequencies for a 4-n-pentyl-4′-cyanobiphenyl (5CB) liquid crystal. It is demonstrated that the distribution function for the isotropic and nematic phases is asymmetric in shape. Comparison shows that the reconstructed distribution function is in qualitative agreement with similar functions that are analytically derived from the Cole-Davidson and Havriliak-Negami empirical equations. The specific features observed in the behavior of the distribution function with a variation in the angle between the direction of polarization of a microwave electric field and the director of the liquid-crystal molecules are analyzed. A complex dependence of the permittivity for the liquid crystal in the range of ultrahigh frequencies is explained in terms of additional relaxation mechanisms associated with different motions of molecular fragments of alkyl chains.     

Preparation and characterization of vinyltriethoxysilane based inorganic–organic gel glasses dispersed 4′-n-pentyl-4-cyanobiphenyl nematic liquid crystal

Vinyltriethoxysilane based inorganic–organic glasses dispersed with 4′-n-pentyl-4-cyanobiphenyl nematic liquid crystal are designed to fabricate a new type of gel glass dispersed with liquid crystal. Scanning electron microscopy shows that the nematic liquid crystal phase, occupying the interconnected cavities, is continuously embedded in the inorganic–organic matrixes. The effect of the nematic liquid crystal weight concentration on the morphology is investigated. The differential scanning calorimetry shows that the nematic-isotropic transition temperature depends on the nematic liquid crystal weight concentration. The Raman spectra show that there are no obvious Raman frequency shifts, compared with pure nematic liquid crystal. The in situ diffuse reflectance FTIR spectra show that the integral intensity of bands of hydroxyls has not varied.     

Laser light scattering technique for non-invasive in situ simultaneous measurements on elastic constants and viscosity coefficients of nematic liquid crystals

Theelasticconstantsandtheviscositycoefficientsarethekeyparametersforthephysicalpropertiesofnematicliquidcrystals(LCs)[1,2],fortheyarecorrespondingtothethresholdvoltageandtherespondingtimeofliquidcrystaldisplaydevices[3,4].Sofarnotechniquethatcansimultaneouslymeasurethetwoparametersinsituhasbeenreported.TheelasticconstantsandviscositycoefficientsofnematicLCsatthepresentaremeasuredusingdifferentmethodsindependently.TheviscositycoefficientsisgenerallymeasuredwithOstwaldviscosimeter,andtheelasti…     

Nonlinear dielectric response of antiferroelectric liquid crystals in the smectic Cα * phase

Nonlinear dielectric response of antiferroelectric liquid crystals has been studied in the smectic C_α* (SmC_α*) phase. The linear dielectric spectrum shows a single relaxation of Debye type and its relaxation frequency is as high as one hundred kHz. The profile of the third-order nonlinear dielectric spectrum varies in complex fashion with temperature. In the vicinity of the SmA-SmC_α* phase transition, experimentally obtained nonlinear spectra are well described by those calculated with phenomenological theory of Landau type. The soft mode of the SmC_α* phase shows critical slowing-down near the transition temperature. In the lower-temperature region of the SmC_α* phase, the contribution from the soft mode of the SmC_α* phase reduces and the other relaxation mode with Debye-type spectrum appears at several hundred Hz in the nonlinear spectrum. The appearance of this low-frequency mode suggests that the cooperative fluctuation of directors over long range exists in the SmC_α* phase. Received 19 April 2002 and Received in final form 23 July 2002 RID="a" ID="a"e-mail: kimura@exp.t.u-tokyo.ac.jp     

Kinetics of the isotropic–nematic phase transition in melted multi-component liquid crystal mixtures upon cooling

The kinetics of the phase separation and the nematic phase growth in two melted commercial multi-component liquid crystal mixtures upon cooling was studied using polarising optical microscopy and IR spectrometry. The droplets of the nematic phase revealed in the optical images across the phase transition were segmented and treated statistically. In the resulting size histograms of mixture B, two overlapping statistical ensembles related to two co-existing nematic phases were recognised; these phases were shown to be different in their chemical structures. In mixture A, any separation within the nematic phase was not found. The statistical ensembles of the nematic droplets were successfully described using the principles of irreversible thermodynamics. Analysis of the mean droplet diameter as a function of time allowed recognition of two regimes of the nematic phase evolution: (1) nucleation and rapid nucleus growth and (2) nucleus coalescence. Both the regimes were quantitatively described with the universal law for the cluster growth.