D. Dardas

Non-linear electrooptic effect in antiferroelectric liquid crystal

Electrooptic phenomena caused by weak electric fields, much lower than those needed for the helix unwinding, in helical smectic liquid crystals were studied in thin planar samples. The investigations were performed in chiral liquid crystal 4-(1-methyl-heptyloxycarbonyl) phenyl 4′-(3-butanoyloxy propyl-1-oxy) biphenyl-4-carboxylate which exhibits antiferro-electric properties. We have found that electric field applied to a helical smectic liquid crystal caused two effects. First, the helix was deformed and the position of effective optic axis changed by an angle proportional to the field strength. The second effect, quadratic in field, causes the change in the shape of the indicatrix. As a consequence, the relative changes in the light intensity caused by external electric field consist of two components. The first component represents the modulation with the fundamental frequency and the second one with the doubled frequency (second harmonic of the electrooptic effect). The ab- solute values of the first- and second-order electrooptic coefficients have been determined and their temperature dependence discussed.

Measurements of absolute values of electrooptic coefficients in a ferroelectric liquid crystal

Weak external electric field E causes two effects in chiral smectic liquid crystals: linear change of the optic axis direction and modification of the shape of the indicatrix. The relative changes in the light intensity consist of two components. The first component represents the modulation with the fundamental frequency f and the second one, with the doubled frequency 2f (second harmonic). We carried out measurements of the electrooptic modulation at both the first and second harmonics frequency. The applied calibration procedure allowed for expressing the experimental results as angular quantities independent of experimental conditions. Therefore we were able to determine the absolute values of the coefficients describing both effects of the electric field on the optical properties of the sample. We studied the temperature dependence of the first and second order coefficients in the ferroelectric smectic C* and paraelectric smectic A phases. The experiments delivered an information on the structure of tilted smectic liquid crystals and its dependence on the electric field.

Liquid-Crystalline Polymorphism of Symmetrical Azobananas: bis(4-(4-alkylphenyl)azophenyl) 2-nitroisophtalates

In this paper we present a series of novel compounds, bis(4-(4-alkylphenyl)azophenyl) 2-nitroisophtalates, which exhibit nematic and banana-type liquid-crystalline phases. The alkyl chain length varies from 1 to 18 carbons. The first ten members of this series exhibit nematic phase. The last eleven compounds exhibit banana-type liquid crystalline phases. The propyl and pentyl derivatives have extra second type of banana mesophase.

Director distribution and surface anchoring potential in Grandjean-Cano wedge

Theoretical and experimental studies of the director distribution of a chiral nematic liquid crystal placed in a wedge-shaped cell with weak surface anchoring were performed. We measured local value of pitch as function of position inside each Grandjean-Cano zone by registering selective reflection spectra from narrow stripes cut from each Cano zone. We also measured the rotation of the polarisation plane as function of thickness of the wedge sample. These data allowed determination of the angle between the local director and the easy direction on the surface. The obtained director distributions in individual Cano zones were compared with the results of theory and thus used for restoration of the actual shape of the surface anchoring potential.

Determination of twist elastic constant in antiferroelectric liquid crystals

An electro-optic method for determining the twist elastic coupling between smectic layers in antiferroelectric liquid crystals with a helical superstructure has been introduced. This method is based on a calibration procedure which enables a modulation of light intensity under an alternating applied electric field to relate to a respective modulation caused by mechanical oscillations of a sample. The elastic constant of antiferroelectric liquid-crystalline materials has been obtained by applying the method to a liquid crystal which displays a direct transition from antiferroelectric C*a phase to the smectic A phase.

Field-induced dynamics of ferroelectric liquid crystals with elastic interfacial confinement

Dynamic properties of surface stabilized ferroelectric liquid crystals driven by an alternating external electric field and confined inside measuring cells by air are studied. Using the electro-optic response method, it has been possible to register response spectra of small parts of liquid crystals near surfaces of their contact with air. This has allowed the direct determination of the dependence of changes of such partial electro-optic response spectra on the distance of illuminated liquid-crystal areas from the contact surfaces. The interfacial dynamic processes are found to enhance considerably the electro-optic response over relatively large distances from contact surfaces, within a wide range of field frequencies. Experimental data obtained from the used approach to determine the fragmentary electro-optic response indicate that the approach can be very effective for studying various interfacial phenomena in thin liquid-crystal systems.

Determination of the bulk rotational viscosity coefficient in a chiral smectic C* liquid crystal

The method of determination of the bulk values of the rotational viscosity coefficient of the smectic c-director in chiral smectic liquid crystals with a helical structure is described. The measurements were performed using optical detection in the small deformation limit. In contrast to the usual methods, the initial deformation of the helix (caused by strong surface interactions) was avoided by using homeotropic aligned thick samples. The coefficient was measured in 4-methylbutyloxy phenyl-4-octyloxy-benzoate (C8) in the presence of laminar flow. The critical temperature dependence of the measured coefficient was found. The relation between the measured coefficient and the smectic C order parameter is shown.

High permittivity of chiral tilted smectic phases in a binary mixture

Characterization of structure and properties of the bicomponent mixture of partially fluorinated chiral three ring ester with an chiral alkyl compound by applied different methods such as the differential scanning calorimetry, the electro-optic and the dielectric spectroscopy, and the helical pitch measurement is presented. In particular, we investigate the influence of the texture changes in order to determine the phase transitions in the mixture under investigation.

Comparison of dielectric and optical responses of chevron ferroelectric liquid crystals

An analysis of consistency of dielectric and optical response methods is carried out for surface stabilized ferroelectric liquid crystals (SSFLC) with chevron geometry. The consistency is found both theoretically and experimentally for weak external electric fields of intermediate frequencies, for which the response of SSFLC is dominated by collective relaxation processes due to azimuthal reorientation of molecules arranging chevron layers. The methods are experimentally shown to lack consistency within very low, relatively low, and high field-frequency ranges. The disagreement appearing at relatively low frequencies is argued to be a consequence of different recording by dielectric and optical techniques the dynamics of zig-zag walls, forming defects in chevron structure.

Characterization of Some Fluorinated Mesogens for Application in Liquid Crystal Displays

The long-range orientational order of some fluoro-substituted liquid crystals with different molecular structure was determined on the basis of the optical birefringence measurements. The high values of the second-rank order parameter in the whole mesophase range for the liquid crystals studied were found. The liquid crystals with the negative dielectric anisotropy were mixed with an arachidic acid at various molar fractions. The behavior of the mixtures in monolayers formed by using Langmuir-Blodgett technique was investigated.