Liquid Crystal Molecular Orientation Research Articles

Molecular orientation in liquid crystals on a photochromic polyion complex LB film by FTIR spectroscopy

The molecular orientation of liquid crystals mixed with stearic acid-d 35 in Langmuir–Blodgett (LB) films on the polyion complex with azobenzene-containing fatty acid and polycation was examined using Fourier transform infrared (FTIR) reflection-absorption (RA) and UV–Vis spectra. It was found from UV–Vis measurements that the polyion complex in these LB films shows reversible photoisomerization when irradiated by UV or visible light. Further, changes in the RA spectra revealed that the orientation of molecules in liquid crystal mixed with stearic acid-d 35 is controlled by the trans–cis photoisomerization of the polyion complex at 45 °C, but their orientation is unaffected at 25 °C. This result can be explained by the difference in the state of the liquid crystal at these different temperatures.

Liquid Crystal Display Devices with Slit-Patterned Electrode Structures

We present a novel liquid crystal (LC) display device based on the molecular orientation effect referred to as the electrically induced hybrid twisted nematic (EHTN) LC molecular orientation in homogeneously aligned LC cells with slit-patterned electrode structures. With a simplified EHTN orientation model, we investigate the viewing-angle and contrast ratio characteristics of this display mode by a computer simulation using Berreman's 4×4 matrix calculus. A very high contrast ratio property is exhibited due to very low transmission levels in the “off" state. Moreover, the electrooptical characteristics of the EHTN LC display mode are studied by measuring the transmittances at various viewing angles. A wide viewing-angle and high contrast ratio property is demonstrated experimentally.

Molecular Orientations and Optical Transmission Properties of Liquid Crystal Cells with Slit-Patterned Electrodes

The molecular orientations in slit-patterned electrode liquid crystal (LC) cells with homogeneous initial alignment parallel to the slit pattern are investigated in detail using a 3-dimensional polarization observation method. In addition to conventional observations of interference fringes, we study the molecular orientations from cross sections of the LC cells using photoelasticity experimental techniques and a UV-curable LC material. A model of LC molecular orientations referred to as the electrically induced hybrid twisted nematic (EHTN) molecular orientation, is proposed. Moreover, optical transmission properties of such LC cells with different structural parameters are measured as a function of the applied voltage. The possibility of application of these kinds of LC cells to LC display devices is discussed.

Surface Morphology of the Rubbed Polyimide and Polystyrene Films and Their Liquid Crystal Aligning Capability

Abstract Atomic force microscope (AFM) observation has been made on rubbed polymer films for liquid crystal (LC) molecular orientation such as those of alkylbranched and branchless polyimide (PI) together with polystyrene (PS); other characterizations such as Fourier analysis and roughness evaluation have also been made. It has been investigated how these results of the characterization relate to the LC aligning capabilities such as microscopic texture and pretilt angle. It is shown that in a PI film with alkylbranches there exist grooves owing to strong rubbing, while the branchless does not reveal grooves at the AFM resolution. However, both of them are shown to be capable of aligning nematic (N) LCs very well. Regarding PS film, the grooves run parallel to the rubbing direction but LC alignment is perpendicular to that of rubbing, in accordance with the previous report (D.-S. Seo et al., Jpn. J. Appl. Phys. 31, 2165C 1992)).

Local Channeled Spectrum Analysis as a Measure of the Local Birefringence and Molecular Orientation in Liquid Crystals

Abstract A new experimental technique is described for the measurement of local birefringence and molecular orientation in liquid crystals. This technique consists in analyzing the local channeled spectra obtained by optical polarising microscopy.

A new experimental technique for measuring the local birefringence and molecular orientation in liquid crystals

Abstract A new experimental technique is described for the measurement of local birefringence and molecular orientation in liquid crystals. This technique consists in analyzing the local channeled spectra obtained by optical polarising microscopy.

Relaxation and gradient methods for molecular orientation in liquid crystals

Relaxation and gradient methods have been developed during the past several years for the computation of molecular orientation in liquid crystals. These methods have contributed to the classification of stable defect structures and the description of some of the transitions that occur under the influence of electric and magnetic fields. The bulk energy of the liquid crystal depends on the orientation of the optic axis, and this gives the variational problem a nonconvex constraints. Further, minimum energy configurations can be discontinous and can have singularities. In this paper, new relaxation and gradient methods are proposed to handle this nonconvex constraint. The results of numerical experiments and error analysis are presented.

Surface memory effects in liquid crystals: Influence of surface composition.

Certain polymer-coated solid surfaces prepared to produce an isotropic (random) planar liquid-crystal boundary condition can be rendered strongly anisotropic by exposure to orientationally ordered liquid-crystal phases. This effect reveals a plastic coupling of liquid crystal molecular orientation to hydrophobic surfaces which is absent for hydrophyilic surfaces.

Grain Boundaries Obsevation of Polycrystalline Sillcon Using Nematic Liquid Crystals with Positive Dielectric Anisotropy

Abstract A new method to analyze electrically active grain boundaries of polycrystalline silicon by using a field effect of nematic liquid crystal with positive dielectric anisotropy is described. In addition, threshold voltage of a liquid crystal planar cell with interdigited comb metal electrdes is measured as a function of frequency and temperature. From the results, the fringing electric field effect on the liquid crystal molecular orientation is clarified. This method is advantageous in the wide frequency range (0.1–105 Hz), fast response time and low threshild voltage by comparison with the method using nematic liquid crystal with negative dielectric anisotropy.

Liquid Crystal Orientation on the Surface of Obliquely-Evaporated Silicon Monoxide with Homeotropic Surface Treatment

We have investigated the liquid crystal molecular orientation on a surface coated by oblique evaporation of SiO followed by treatment with homeotropic surface coupling agents. This is found to be a useful technique for obtaining the liquid crystal orientation with a small pretilt from homeotropic alignment. The orientation mechanism on surfaces with and without homeotropic treatment is also investigated. The results show that the relationships among evaporation conditions, surface topography and liquid crystal orientation can be explained well by using the columnar structure model of obliquely-evaporated films proposed by Goodman-van de Waterbeemd.

Surface-induced molecular orientation of liquid crystals by carboxylatochromium complexes

Tetrachloro–μ-hydroxo–μ-carboxylatodichromium (III) complexes have been used to induce homeotropic alignment of nematic, smectic, and large-pitch cholesteric liquid crystals. A dynamic scattering degradation test has shown that this orientation is remarkably stable.

Elastic continuum theory cutoffs and order in nematics and solids

Thermal disorder in liquid crystal molecular orientation and thermal displacement of atoms in monatomic crystals can be described with analogous elastic continuum theories having short wavelength cutoffs. Half the short wavelength cutoff that gives the observed displacement is remarkably close to 1.8 times the cube root of the atomic volume in monatomic crystals. Half the short wavelength cutoff that gives the observed order parameter in PAA is also about 1.8 times the cube root of the molecular volume. Some data give a number about twice as large for MBBA. This large cutoff may be due to a large error in measured order parameter, or it may suggest that MBBA molecules tend to form groups in the nematic phase.

Molecular Orientation of Liquid Crystals by Cr-Complexes

Molecular Orientation of Liquid Crystals by Cr-Complexes

The orientation of liquid crystals by temperature gradients

It has been known for some time that the molecular orientation of liquid crystals can be affected by the presence of a temperature gradient.Stewart reported in 1936 that, whenp-azoxyanisol was placed in a vertical temperature gradient, the orientation adopted was vertical when the higher temperature was at the bottom and horizontal when the higher temperature was at the top. The continuum theory for nematic liquid crystals, proposed byLeslie, is used to explain this phenomenon. An examination of the stability of the static orientation adopted by the director in the presence of a vertical temperature gradient shows that the stable orientations are either vertical or horizontal, as found experimentally.

The boundary value problem of asymmetric elasticity theory in quasi-classical approximation: PMM vol.36, n≗2, 1972, pp.282–290

Considered is the static boundary value problem of the asymmetric theory of elasticity for media in which the moment effects [couple stresses] supply a small contribution to the elastic energy. The elastic coefficients in the equilibrium equations, having the dimensions of a squared length, are taken to be small in comparison with the squares of the characteristic dimensions of the body. One obtains the solution of the equilibrium equations, containing small parameters in the leading derivatives. By an approximation method one constructs the solution for the field of displacements and rotations in the form of the sum of their classical limits and moment terms having the form of boundary layer functions. Boundary conditions of kinematic type are considered and a scheme is developed in order to satisfy them by the method of successive approximations. Most of the media whose viscoelastic behavior is described within the limits of the asymmetric continuum mechanics (liquid) crystals, ferromagnetics, in a series of cases dislocation media and suspensions) are characterized by a small contribution of the moment terms in the general energetic balance of the deformation and flow processes. However, without taking into account the interaction of the moments one cannot give and interpretation to an entire series of delicate singularities of their viscoelastic behavior (the effects of the elastic distortion in the field of directions of the axes of molecular orientation in liquid crystals, the formation of spin waves in ferromagnetics, the effects of hardening in the plastic deformation, peculiarities of blood flow, etc.) In connection with this there aiises the problem of analyzing the simplifications which can be introduced in the asymmetric mechanics by the investigation of media with weak moments. Below we consider elastic Isotropie media which are characterized by additional coefficients of rotational elasticity λ and moment elasticity η, τ, θ [1]. The energetic contribution of the moment terms to the elastic potential is determined by the ratio between these coefficients and tine moduli λ, μ of the classical elasticity. If these ratios are small (in some sense which will be specified later), then we will say that the medium has weak moments [couple stresses]. For such media we investigate boundary value problem of the asymmetric elasticity theory.