W. Kuczyński

Dielectric Study of Ferroelectric Properties in Chiral Smectic C

Abstract Measurements of electric permittivity, and polarization are reported for chiral liquid crystal DOBAMBC. Dielectric hysteresis was found. Temperature-variations of spontaneous polarization and relaxation time are studied. The dielectric properties of chiral smectic C and those of crystalline ferroelectrics are compared.

Determination of Orientational Order Parameter in Various Liquid-Crystalline Phases

Many methods for the determination of the order parameter in nematic liquid crystals are known. However, most of them are not applicable to cholesteric and smectic phases. Therefore, experimental data on the order parameter in cholesteric and smectic phases are hardly available. To determine the orientational order parameter S we exploited the de Gennes observation, that any anisotropic physical quantity may be a measure of orientational ordering in nematic mesophase. The order parameter can be calculated directly from a macroscopic quantity measured in the experiment. From this idea we developed a simple method for the determination of the order parameter based on birefringence measurements, which can be applied for nematic, cholesteric, and smectic phases. A precise and simple method for measurement of the optical birefringence is described. An approximate empirical formula is applied for normalization of the optical birefringence similar to that used by Haller for polarizabilities. The results are in excellent agreement with the literature data. We proved that in the case of optical anisotropy the order parameter is equal within ±1% to the one evaluated from polarizability anisotropy. Thus, the discussion concerning the local field can be omitted. Using the described method we determined the orientational order parameter in various mesophases.

INVESTIGATIONS OF THE STRUCTURE OF A CHOLESTERIC PHASE WITH A TEMPERATURE INDUCED HELIX INVERSION AND OF THE SUCCEEDING SC-ASTERISK PHASE IN THIN LIQUID-CRYSTAL CELLS

Abstract Investigations of 4-[(S, S)-2, 3 epoxyhexyloxy]-phenyl-4-(decyloxy)-benzoate by polarizing microscopy, the Cano-Grandjean method, optical rotation dispersion and UV-VIS spectroscopy reveal a cholesteric phase with temperature induced reversal of the helical twist. Switching time experiments in the Sc* phase show that the intrinsic helix can be unwound reversibly and irreveribly by application of electric fields of different strengths.

Optical Study of a Chiral Smectic C Under Shear

Abstract We study the distortion of a chiral smectic C, under a shear applied parallel to the plane of the layers, using an original technique based on the modification of the conoscopic image. Both the amplitude of the distortion and the phase lag between the alternating applied shear and the deformation are found to vary like the flow induced transverse polarization (described in Ref. 7). The behaviour is understood in terms of a viscoelastic-nematic-like model and of a simplified optical description which gives account of the displacement without deformation of the conoscopic image.

Twist Grain Boundary Phases in Binary Mixtures

Abstract Two twist grain boundary phases (TGB-A and TGB-C*) have been observed in binary mixtures exhibiting a NAC multicritical point. The observed TGB phases are of very high thermal stability - for some concentrations the temperature range of their existence was as large as 40 K. Optical properties and textures has been investigated. Cano lines in the whole region of the TGB-A phase were observed. The pitch of the helix along with the rotation of polarization plane of light has been measured. A distinct influence of surface interactions on the stability of the twisted smectic A phase has been found.

Bicomponent Systems with Induced or Enhanced Antirerroelectric SmCA* Phase

Abstract A new method of the formulation of liquid crystalline antiferroelectric (CA*) materials from smectic C* and smectic A components is demonstrated. The phase diagrams of the induced CA* systems and the temperature dependence of their smectic layer spacings and switching behavior are shown.

The helical structure of highly ordered smectic phases

Abstract Optical studies of smectic phases have been performed in homogeneously oriented samples of chiral 4-(2′-methylbutyl) phenyl-4′-n-octylbiphenyl-4-carboxylate (CE8). The helix structure has been found in smectic phases C, I and J, but not in the smectic G phase. Two chiral phases have been found between SI* and SG phases. Up to now one of them has not been observed. The pitch of the helix has been measured in all of the twisted smectic phases, including the SJ* phase. The existence of the helix in this phase suggests that the correlations between smectic layers are not very strong.

Orientational Order of Difluorinated Liquid Crystals : A Comparative 13C-NMR, Optical, and Dielectric Study in Nematic and Smectic B Phases

Structural and orientational order properties of 3Cy2CyBF2 and of 5CyCy2BF2 have been investigated by means of (13)C-NMR, optical, and dielectric spectroscopy methods. In the case of NMR, order parameters have been independently obtained from the analysis of either (13)C-(19)F dipolar couplings or (13)C chemical shift anisotropies, both measured from (13)C-{(1)H} NMR static spectra. The assignment of the (13)C resonances has been carried out thanks to the comparison with solution state spectra and DFT calculations, and the relevant geometrical parameters and (13)C chemical shift tensors needed to derive orientational order parameters have been calculated by DFT methods. In the analysis of (13)C-(19)F dipolar couplings, empirical corrections for vibrations and anisotropic scalar couplings have been included. Dielectric measurements have been performed over a broad frequency range for two orientations of the nematic director with respect to the measuring field. At low frequencies (static case) a positive dielectric anisotropy has been determined, which has enabled the calculation of the order parameters according to a well-tested procedure. At high frequencies the dielectric anisotropy changes its sign, a property which can be useful in designing a dual addressing display. The nematic order parameter determined from optical, dielectric, and NMR methods have been compared: their trends with temperature are very similar, apart from some slight shifts, and were analyzed by Haller and Chirtoc models. The differences among the results obtained by the four methods have been discussed in detail, also with reference to the assumptions and approximations used in each case, and to the results recently reported for similar fluorinated nematogens. The presence of a non-negligible order biaxiality has been related to the presence of a CH2CH2 bridging group, linking one cyclohexylic unit with either the other cyclohexyl or the phenyl ring.

A hard-disc system, an experimental model

Abstract An experimental model of a hard-disc system (the two-dimensional analogue of a hard-sphere system) is described. The melting transition is studied qualitatively by a photographic technique. The half-width of the singlet distribution function is measured and compared with results obtained by the method of molecular dynamics. Images of the pair distribution function are presented.

Dielectric Investigations of Induced Twist Grain Boundary Phases in the Binary Mixtures of Cholesteryl Benzoate and Di-Heptyloxyazoxybenzene

The binary mixtures of Di-heptyloxyazoxybenzene (HOAB) and Cholesteryl Benzoate (ChB) show the longest ever temperature range of TGBA and TGBC* phases for ChB concentration 25 and 18 mole percent respectively. Comprehensive dielectric studies of these two binary mixtures have been carried out in the frequency range of 1 Hz to 10 MHz for different condition of molecular anchoring. Both mixtures show negative dielectric anisotropy (Δ ϵ′ = ϵ‖′−ϵ⊥′ < 0) in all the mesophases. One mode of dielectric relaxation showing Arrhenius behavior has been detected in N*, TGBA and TGBC* phases for the homeotropic aligned sample for both mixtures. This mode corresponds to rotation of molecules about their short axes. Planar oriented molecules of TGBC* and SmC* phase show a mode of relaxation presumably due to same reason as stated above. Various electrical parameter viz. dielectric anisotropy, relaxation frequency, dielectric strength, activation energy have been determined for TGB phases.