A simple spin echo measurement of the anisotropy of the self-diffusion coefficient in a smectic A and B type liquid crystal

The temperature dependence of the self diffusion coefficients D_ along D₆ perpendicular to the molecular director of p-dodecanoyl-benzylidene-p -aminoazobenzene has been investigated in both the smectic A and B regime.     

Wedge disclinations as models of curvature walls in smectic A liquid crystals

The displacement of layers caused by a wedge disclination and the interaction energy of a wedge disclination pair in a smectic A liquid crystal is determined. Wedge disclinations are used to model and to discuss curvature walls (e.g. chevron structures of twin walls) observed experimentally in smectic A liquid crystals.Dedicated to my teacher Frantiek Kroupa on the occasion of his 65th birthday.     

13C PGSE NMR experiment with heteronuclear dipolar decoupling to measure diffusion in liquid crystals and solids

A new PGSE NMR experiment, designed to measure molecular diffusion coefficients in systems with nonvanishing static dipolar coupling, is described. The fast static dipolar dephasing of the single-quantum (13)C coherences is removed by multiple-pulse heteronuclear decoupling. The resulting slow dephasing of the (13)C coherences allows for inserting appropriate gradient pulses into the pulse sequence. The presence of the large magnetic field gradient reduces the efficiency of the decoupling sequences which is compensated for by introducing a scheme of sequential slice selection across the sample. The method is demonstrated by (19)F-decoupled (13)C PGSE NMR experiments in a lyotropic nematic and lamellar liquid crystal.     

Discontinuity walls terminated by twist disclinations in smectic a liquid crystals

A discontinuity wall in smectic A liquid crystals was discussed in M. Kléman:Points, Lines and Walls in Liquid Crystals, Magnetic Systems and Various Ordered Media (J. Wiley, Chichester, 1983). Possible solutions describing a singularity — twist disclination which terminates the discontinuity wall in an infinite model smectic A liquid crystal are discussed. One of solutions proposed can be used to describe the so-called zigzag defect line in chiral smectic C liquid crystals on which opposite oriented chevrons are terminated. This work was supported by Grant No.: 202/96/1687 of the Grant Agency of the Czech Republic.     

Effects of anisotropy,order parameter,and viscosity on translational self-diffusion in nematic liquid crystals

A theory developed for the translational diffusion in nematic liquids crystals shows a dependence on the viscosities, order parameter, and molecular structure. Theoretical results for self-diffusion in p-azoxyanisole at 125°C are D^{∥ = 4.3 × 10?6, D⊥ = 3.1 × 10?6 c2/s}, andD^∥/D^⊥ = 1.4.     

Dielectric permittivity in the smectic a phase of liquid crystals

We propose a functional dependence of the dielectric constant with the smectic order parameter of liquid crystals in their smectic A phase. The proposal is verified by comparing with the published values of heptylazoxybenzene.     

On a mechanism of nucleation in crystals with combined anisotropy

The stable states of magnetic inhomogeneities of the “static”-soliton type in a (111) plate of a garnet ferrite with combined anisotropy are investigated theoretically, and the conditions for their occurrence are established. A model of static solitons is considered, and its fundamental properties are determined by solving the corresponding variational problem numerically. It is shown that these inhomogeneities originate at crystal defects and can significantly affect the kinetics of a phase transition of the spin-reorientational type.     

A model of crack based on dislocations in smectic A liquid crystals

A plastic crack model for smectic A liquid crystals under longitudinal shear is suggested. The solution of the screw dislocation in smectic A is the key to the correct result that we obtained by overcoming a longstanding puzzle. We further use the dislocation pile-up principle and the singular integral equation method to construct the solution of the crack in the phase. From the solution, we can determine the size of the plastic zone at the crack tip and the crack tip opening （tearing） displacement, which are the parameters relevant to the local stability/instability of materials. Our results may be useful for developing soft-matter mechanics.     

A study of polarization in smectic liquid crystals via statistical-physics methods

The spontaneous polarization processes in synclinic and anticlinic smectic liquid crystals are discussed in the context of molecular statistical theory. The effects of interaction between molecules, external electric fields, and bending of molecules via polarization are elucidated as well.     

Reactivity in smectic liquid crystals. Rearrangement of MSE induced by the smectic B mesophase of ZLI 1544

Summary The rearrangement of MSE has been carried out in the smecticB phase of ZLI 1544 (4-(trans-4-n-pentyclyclohexyl)-4′-trans-4-n-propylcyclohexyl-biphenyl) in order to probe the occurrence of liquid-crystalline-solvent effects. The different mesophases of ZLI 1544 have been previously studied by the X-ray diffraction technique in order to characterize the structure of the two unknown smectic mesophases.

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Riassunto Si è ottenuto il riordinamento del MSE nella fase smetticaB dello ZLI 1544 (4-(trans-4-n-pentilcicloesil)-4′-trans-4-n-propilcicloesil-bifenile) per evidenziare gli effetti di un solvente liquido cristallino. Le differenti mesofasi dello ZLI 1544 sono state prima studiate con tecniche di diffrazione di raggi X per caratterizzare la struttura delle due mesofasi smettiche sconosciute.

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Резюме Осуществляется перестройка MSE в смектическойB фазе ZLI 1544 (4-(транс-4-n-пентилциклогексил)-4′-транс-4-n-пропилциклоге-ксил-бифенил), чтобы исследовать влияние растворяющих эффектов жидкого кристалла. Предварительно изучались различные мезофазы ZLI 1544 с помощью методов рентгеновской дифракции, чтобы охарактеризовать структуру двух неизвестных смектических мезофаз.

     

Dichroism of absorption by impurity molecules with internal rotation in nematic liquid crystals

Expressions are obtained for the components of the permittivity tensor in the optical region for a nematic liquid crystal with impurity molecules having the internal rotation conformational degree of freedom. The dependence of the intensity and dichroism of the impurity absorption bands in isotropic and nematic phases on the parameters of conformational, orientation, and mixed conformation-orientation orders of molecules is found. The influence of the mutual correlation of conformational and orientation degrees of freedom of molecules on these parameters is studied. The dependence of the oscillator strengths of molecular transitions on the phase state of a medium and the orientation order of the impurity subsystem is analyzed. The self-consistent nature of molecular, structural, and spectral changes upon the nematic-isotropic liquid phase transition is demonstrated.     

Quenching of self-diffusion by impurities in plastic crystals as a tentative explanation of experimental discrepancies

A model has been developed which can explain observed discrepancies in the determination by different techniques of the activation energy for self-diffusion in plastic crystals. In this model, impurities play an essential role. The basic hypothesis is that an impurity stabilizes the crystal lattice by reducing the self-diffusion around it, to a range of several molecular distances. This hypothesis allows a semi-quantitative fit of published experimental N.M.R. data on perfluorocyclohexane, and the parameters of the model are found to be consistent with previous work on cyclohexane. Moreover, as observed experimentally, it leads to the conclusion that activation energies deduced from N.M.R. linewidth measurements are in general smaller than those deduced from radiotracer data. It is concluded that for infinitely pure samples, all techniques should lead to convergent results for self-diffusion activation energies.     

On one mechanism of magnetization reversal in crystals with combined anisotropy

The effect of a magnetic field on the stability of null domain walls is considered in terms of a variational model. The walls are localized near defects in a (001)-oriented plate. The critical fields at which the inhomogeneities exist are found, and their role in magnetization processes taking place in the crystals under study is considered.     

Uniform rotation of smectic layers of a ferroelectric liquid crystal in an asymmetric electric field

An asymmetric ac electric field with amplitudeE=10 V/μm gives rise to a matched rotation of the normal to the smectic layers, while a sinusoidal fieldE=1 V/μm is used to study the rotation in the method of modulation total internal reflection ellipsometry, which makes it possible to probe the region of a ferroelectric liquid crystal (FLC) next to the electrode (h≈0.7 μm). It is shown that the angle of rotation of the normal to the smectic layers near the surface of the electrode varies reversibly as a function of the polarity and number of electric pulses applied, just as in the interior region. The characteristic dynamic properties of thin layers of nematic liquid crystals, such as an anomalously long relaxation time and a high-frequency relaxation process, are observed in thin FLC layers.     

An EPR study of the orientational transition in smectic liquid crystals in a magnetic field

Spin probe EPR spectroscopy is used to detect an orientational transition in the smectic mesophase in 4-n-octyl-4'-cyanobiphenyl (8CB) liquid crystal under the action of an intense magnetic field (5.87 T).     

Theory of spherulitic domains in cholesteric liquid crystals with positive dielectric anisotropy

New localized axisymmetric nonsingular solutions minimizing the Frank functional have been found for a cholesteric layer with homeotropic anchoring. They have a continuous distribution of the director field and are characterized by a convex shape. These solutions describe the so-called spherulitic domains which have been observed in a large-pitch cholesteric near the unwinding transition.     

Localized modes in optics of photonic liquid crystals with local anisotropy of absorption

The localized optical modes in spiral photonic liquid crystals are theoretically studied for the certainty at the example of chiral liquid crystals (CLCs) for the case of CLC with an anisotropic local absorption. The model adopted here (absence of dielectric interfaces in the structures under investigation) makes it possible to get rid of mixing of polarizations on the surfaces of the CLC layer and of the defect structure and to reduce the corresponding equations to only the equations for light with polarization diffracting in the CLC. The dispersion equations determining connection of the edge mode (EM) and defect mode (DM) frequencies with the CLC layer parameters (anisotropy of local absorption, CLC order parameter) and other parameters of the DMS are obtained. Analytic expressions for the transmission and reflection coefficients of CLC layer and DMS for the case of CLC with an anisotropic local absorption are presented and analyzed. It is shown that the CLC layers with locally anisotropic absorption reduce the EM and DM lifetimes (and increase the lasing threshold) in the way different from the case of CLC with an isotropic local absorption. Due to the Borrmann effect revealing of which is different at the opposite stop-band edges in the case of CLC layers with an anisotropic local absorption the EM life-times for the EM frequencies at the opposite stop-bands edges may be significantly different. The options of experimental observations of the theoretically revealed phenomena are briefly discussed.     

The hydrogen chemical shielding anisotropies of the methyl halides as measured in smectic liquid crystal solvents

Hydrogen chemical shielding anisotropies for the methyl halides are measured using the smectic phase liquid crystals 4-n-pentyloxybenzylidene-4′-p-aminoacetophenone (PBAA), and 4-n-hexyloxybenzylidene-4′-p-aminoacetophenone (HBAA). The method enables one to obtained shielding anisotropies from measurements in the smectic phase only and at a single temperature, and essentially eliminates most effects attributed the changes of phase and temperature. A trend is observed for the shielding anisotropies of the methyl halides, with the anisotropy of methyl fluoride being somewhat large and negative, that of methyl chloride and of methyl bromide being small and positive, and that of methyl iodide being somewhat large and positive. The use of 50 mol % (PBAA)-50 mol % (HBAA) mixture enables one to perform measurements in the smectic phase from approximately room temperature to 100°C.