On a Periodically Grooved Nematic Cell

The mean permittivity behaviour of a periodically grooved nematic liquid crystal cell has been theoretically investigated as a function of the cell geometry in the strong anchoring hypothesis, with the aim of predicting the degree of homeotropic alignment. The capacitance measurements, performed on two MBBA cells of different thicknesses, are in good agreement with our calculations.     

Nematic Liquid Crystals with a Trifluoromethyl Group

Abstract

We have synthesised a new class of compounds incorporating a trifluoromethyl group in the terminal alkoxy or alkenyloxy chain. Compounds containing several aromatic rings were synthesised with a view to producing compounds of high birefringence. This also included the synthesis of compounds containing a carbon-carbon triple bond. Compounds with three 1,4-disubstituted rings were synthesised in order to produce materials with a high nematic clearing point and as low a melting point as possible. Laterally fluoro-substituted compounds were prepared to generate a high positive value of the dielectric anisotropy, a low melting point and no smectic mesophases. Substances containing a cyclohexyl ring, which tend to induce a lower viscosity and compounds incorporating a bicyclo[2.2.2]octane ring were also synthesised in order to produce a high nematic clearing point and to influence the elastic constants. The (E)-trifluorobut-2-enyloxy-compounds were prepared in attempts to produce compounds with appropriate elastic constants and a high birefringence.     

Aggregation of Anthraquinone Dye Molecules in a Nematic Liquid Crystal

We report an experimental evidence that molecules of anthraquinone dye aggregate in an anisotropic solvent, a nematic liquid crystal (NLC). We observe strong nonlinear dependencies of the dielectric permittivity and anisotropy of light absorption on the dye concentration c. Moreover, the data suggest that the aggregates are linear, but, in contrast to the isotropic case, their formation in a NLC undergoes two different concentration regimes with a well pronounced crossover. These effects are explained by the interplay between the aggregates’ lengths and their orientational ordering due to the interaction with the nematic director.     

Viscometry of Nematic Liquid Crystals

Viscometric experiments have played a significant part in establishing continuum theory for nematic liquid crystals. Such theory supports the hypothesis that observed non-Newtonian behaviour stems from competition between the aligning influences of flow and solid surfaces, and as a consequence predicts rather unusual scaling for the apparent viscosity. This paper first describes such scaling and its subsequent experimental confirmation emphasising the full implications of the latter. The relevance of the theory thus established, there follows a simple analysis of alignment in shear flow which leads to conditions on material coefficients necessary to ensure consistency with observations. Our discussion turns next to an account of solutions exhibiting non-Newtonian behaviour, and also mention of a recent stability analysis which attempts to discriminate between the different solutions that are possible. The final section considers oscillatory shear flow and presents an analysis of an instability which can occur in certain nematics.     

Criteria of Color Contrast in a Nematic Liquid Crystal Display

Color contrast in a nematic liquid crystal display is defined experimentally. Liquid crystal cell shows a positive dielectric anisotropy and is operated in a tunable birefringence mode. Munsell color chips used as background color. Colorimetric data are shown in the 1976 CIE (L*, a*, b*) color scale. First, a predict equation of color shift induced by background color is derived and shows a good agreement with the result of experiment. Second, criterial of color contrast is established by taking into account the color shift. The values of criteria nearly coincide with that of psychophysical experiment by single-stimulur method, and the validity of criteria is confirmed.     

Infrared Linear Dichroism of Nematic Solutions I. Orientation of Guest Molecules in Nematic Hosts

The orientational order and the biaxiality of some mono-substituted benzenes in namatic liquid crystals have been determined from infrared linear dichroism measurements. The experimental results are in good agreement with the theoretical predictions of the mean field theory.     

Effects of Nematic Compounds with High Clearing Points on the Multiplexing Performance of Twisted Nematic Displays

The addition of nematic compounds with high clearing points (N_h) to multiplexable nematic liquid crystals decreases the temperature dependence of the threshold voltage of twisted nematic displays. This effect depends on the dielectric anisotropy of the nematic liquid crystals containing N_h, as well as on the increase in T _c. In the case of 4-n-butylphenyl 4'-(4”-butyibenzoyloxy) benzoate, the low threshold voltage was exceptionally compatible with a small temperature dependence of threshold voltage, and this result was attributed to the large temperature dependence of the dielectric anisotropy in comparison with that of nematic liquid crystals containing other N_h's.     

Dynamic Scattering and Material Flow in a Nematic Liquid Crystal

Results are presented which show how defects in a nematic liquid crystal can be used to investigate material flow when external electric fields are applied. These results include measurements of the size of flow cells, which were created between the electrodes by an electric field. The electrodes, which were made of transparent conductive coated glass, were placed in a vertical position so that flow cells could be observed from the top while dynamic scattering was observed when viewing normal to the electrodes.     

Pressure Effects of Nematic Liquid Crystals

There exists a variety of important new phenomena and plenty of experimental data on the pressure effects of liquid crystals. Yet, no systematic phenomenological or microscopic theory is available. In this paper, the ordinary Landau-deGennes theory is generalized so that the free energy becomes G = G _o(P, T) + a/2 [T - T*(P)]S ² - B(P)/3 S ³ + C(P)/4 S ⁴, T*(P) = T _o + bP - eP ². All the known pressure experiments of nematic PAA (where B and C are independent of P) are explained in one stroke and all parameters are determined. New results including reentrant I phase (I represents isotropic phase), and the independence of T _c—T* and T*—T _c on P, etc., are predicted. Simple methods of experimental confirmation are proposed. Similar discussions on the cases of MBBA, EBBA, etc. are also given. Our theory differs from that of Lin-Keyes-Daniels but agrees better with experiments. Results related to G _O(P, T) will be reported in a separate paper.     

Surface and Magnetic Orientation in a Type II Nematic Lyomesophase

Abstract

A type II nematic lyomesophase formed by a quaternary system (K decanoate/water/decanol/KCI) has been studied by X-ray diffraction (XD) and optical microscopy (OM) at room temperature. Samples in capillaries of various thicknesses, materials and geometries have been studied under the influence of wall effects, residual magnetic orientation and orientation in presence of magnetic fields. Results evidence that surface orientation extend typically up to 0.5 mm. The planar diffracting units have a characteristic distance in the director direction of 36 Å. The analysis of the compromise between surface and magnetic orientation by XD allowed to determine a critical field of 200 G for a 0.5 mm thickness; the elastic constant is estimated as 10^?-⁷ dynes. Analised by OM, the sample is axially positive, but shows also a weak biaxiality.     

Measurements of dc Kerr Constants for a Homologous Series of Nematic Compounds

The dc Kerr constants of seven p,p′-n-alkoxyazoxybenzene homologous compounds have been measured as functions of temperature in the isotropic phase. The observed pretransitional behavior near the isotropic-nematic transition is consistent with the phenomenological model of Landau-de Gennes with the Onsager's loal-field correction included in the dielectric tensor. The dc polarizability anisotropy and the average orientation of the permanent dipole with respect to the long molecular axis are deduced from the experimental results. Their variations with increase of methylene groups in the alkyl chains are discussed.     

Study of the Smectic a Nematic Transition in Octyl and Nonyl Cyanobiphenyl

A new DSC method is used to study a smectic A to nematic transition. The measurement of the height of the DSC transition peak gives the order of the transition. The smectic A to nematic transition of octyl cyanobiphenyl was found to be second order while the same transition was found to be first order for nonyl cyanobiphenyl.     

Relaxation of Distortions in a Nematic Thin Film with Weak Anchoring Condition

Dynamical properties of a nematic thin film are investigated under the weak anchoring condition on the boundaries. By solving the equations for small distortions, their asymptotic behaviors are found. Numerical solutions of nonlinear equations for the director field are obtained both in case of small and large distortions around the lower and upper threshold fields. The asymptotic behaviors of these solutions are compared with the analytical results. It is concluded that the dynamics around the upper critical field is governed by the boundaries and strongly space dependent.     

Light Scattering from Nematic Liquid Crystals in a Nonuniform Velocity Field

Fluctuations about a nonequilibrium steady state of a nematic liquid crystal subject to small, linear shear, are studied by means of a generalized Langevin method. We find a symmetric broadening of the director power spectrum and estimate the numerical size of the effect for a typical material.     

Structure of the Nematic Phase of Pbaba

X-ray diffraction investigations of the nematic phase (191°C - 276°C) of p-n-propyloxybenzylidene-p-amino benzoic acid have shown that this phase is a skewed cybotactic nematic phase. The diffraction patterns, obtained with aligned samples, reveal an additional surprising structural feature, - the presence of one dimensional molecular correlation in a direction parallel to the nematic director. At 192°C, it is found that the molecular unit, thus correlated, is about 37 Å in length (dimer length) and the correlation range extends up to about seven such units. This l-d correlation persists up to the N-I transition.     

Optics of Twisted Nematic Liquid Crystal Layers

Calculations have been made of the optical properties of various liquid crystal displays (LCDs) based on twisted nematic liquid crystal layers. It is shown that the steepness of the electro-optical characteristics of TN LCDs can be reduced and consequently the gray-scale capability can be improved by decreasing the twist angle. The twist reduction permits the realization of displays which, due to their outstanding gray-scale capability and low inherent coloration, are particularly suited for active matrix TV-applications. In addition to calculations made on displays based on weakly twisted layers, a comparison is made of the optical properties of displays with a high multiplex capability, which utilize highly twisted nematic liquid crystal layers. It is shown that the brightness of the OMI LCD can be improved without deterioration of the weak inherent coloration. This renders OMI LCDs particularly well suited for applications as time-multiplexed, full colour, high information content panels.     

Modulated Microdomain Switching of Nematic Liquid Crystals

Abstract

Modulated microdomain switching of nematic liquid crystals have been investigated. This display mode used for light switching of micron-sized domains regions using a complicated electric field with special mesh electrode structures. Under He-Ne laser irradiation, contrast ratio was 22:1 for homeotropic alignment cell and transmittance change was no dependent of incident polarization condition. Under white light illumination, the contrast ratio was 4.3:1.     

Laser Induced Reorientation of Nematic Liquid Crystals

A significant increase of the divergence and appearance of a fringe system has been observed by illuminating of MBBA (p-n-methoxybenzilidene-p-butylaniline) and OCB (octyl-cyano-biphenyl) nematic liquid crystalline samples with a collimated beam of an argon ion laser. The dependence of this effect on laser power, beam polarization, and angle of incidence has been studied in homeopolar and planar sandwich-like cells of 50-150 μm thickness, in the nematic phase. At a homeopolar cell of MBBA (150 μm thickness), at normal incidence a threshold laser power of 45 m W was found. The phenomena can be explained as deformation of the orientation by the Fredericks effect due to light fields. The observations are in good agreement with the theoretical predictions including an estimation of the laser power threshold.     

Landau Theory of Biaxial Nematic Liquid Crystals

The orientational order of molecules in a liquid crystal is traditionally described by spatial or temporal averages of functions that are quadratic in the direction cosines of molecular axes with respect to laboratory fixed axes. The free energy describing the nematic phase must be independent of the choice of coordinate systems and therefore rotationally invariant with respect to both laboratory and molecular frames. A Landau theory for biaxial and uniaxial nematics based on six fundamental invariants is developed which is properly rotationally invariant. Four possible nematic phases, two uniaxial and two biaxial, result. NMR experiments should be able to identify which of these actually occur in specific materials.