Planar Liquid Crystal Research Articles

The principle of non-mechanical transport of a liquid crystal in thin capillaries

The non-mechanical principle of transport of a liquid crystal (LC) encapsulated in a narrow cavity between two coaxially arranged cylinders is introduced based on the interaction of the temperature and director field gradients. The temperature gradient is created due to a heat flow from the inner cylinder surface, whereas the temperature on the outer cylinder surface is maintained constant. The director field gradient is caused by the deformation of the planar-oriented LC cavity upon exposure to a double electrostatic layer, which naturally appears at the LC phase-solid interface. The size of the gap between the bounding surfaces, cylinder curvatures, and thermal conditions are determined, which allow initiation of the LC phase flow in the horizontal direction.

Polarization Properties and Structure Changes of Plane-Oriented Nematic LCs with Provitamin D3 Chiral Dopant

At the first time a combined method of UV-absorption spectroscopy and Stokes-polarimetry were used for investigations of cholesteric liquid crystals induced by UV-photosensitive steroid provitamin D3 molecules. It is determined that trans-isomer tachysterol is the left-handed chiral dopant in contrary to initial provitamin D3. The measurements of detailed distributions of light polarization ellipticity after passing through planar liquid crystal cells have shown that ellipticity changes are in close agreement with the variations of calculated equilibrium cholesteric pitch. It has been shown that UV-induced changes of cholesteric structure and alignment depend both on the liquid crystal thickness and initial concentration of provitamin D3 dopant.

Polarization-resolved conoscopic patterns of planar oriented liquid crystal cells

We theoretically and experimentally investigate the angular structure of polarization of light transmitted through planar nematic and cholesteric liquid crystal (LC) cells by analyzing the polarization state as a function of the incidence angles and the polarization characteristics of the incident wave (the ellipticity and the azimuth of polarization). The geometry of polarization-resolved conoscopic (angular) patterns emerging after the planar nematic and cholesteric LC cells is described in terms of the polarization singularities such as C-points (points of circular polarization) and L-lines (lines of linear polarization). The characteristic feature of polarization-resolved conoscopic patterns under consideration is that the conditions of C-point formation are very sensitive to polarization parameters of the incident wave. Full text: PDF

Study of Liquid Crystal Alignment Formed Using Slit Coater

Liquid-crystal (LC)-coated substrates with a planar LC alignment can be fabricated by UV irradiation during LC coating using a slit coater. The alignment transition from a planar alignment to a vertical alignment was caused by UV irradiation after LC coating when a certain UV-curable additive was doped with the nematic LC. These results are applicable to the development of a novel fabrication technology for printable LC devices.

Liquid Crystal-Carbon Nanotubes Composites with the Induced Chirality: The Way Towards Enhancement of Electro-Optic Memory

It is found that the effect of electro-optic memory earlier revealed in the liquid crystal (LC) – carbon nanotube (CNT) suspensions can be substantially enhanced by doping the LC with small amount of chiral agent. The induced chirality weakens homeotropic anchoring set by the aligning layers helping CNT network to maintain state of planar LC alignment realized in the electric field.

Structure of polarization-resolved conoscopic patterns of planar oriented liquid crystal cells

The geometry of distributions of the polarization of light in conoscopic patterns of planar oriented nematic and cholesteric liquid crystal (LC) cells is described in terms of the polarization singularities including C-points (points of circular polarization) and L lines (lines of linear polarization). Conditions for the formation of polarization singularities (C-points) in an ensemble of conoscopic patterns parametrized by the polarization azimuth and ellipticity of the incident light wave have been studied. A characteristic feature of these conditions is selectivity with respect to the polarization parameters of the incident light wave. The polarization azimuth and ellipticity are determining parameters for nematic and cholesteric LC cells, respectively.

BaTiO 3 ferroelectric nanoparticles dispersed in 5CB nematic liquid crystal: Synthesis and electro-optical characterization

We report in this paper studies on a mixture of nematic liquid crystal (5CB) and BaTiO3 ferroelectric nanoparticles (NPs). Apart a drastic change in the electro-optical (EO) properties of this mixture, which is reported as a conclusion, we focused our attention onto the characteristics of those NPs. The initial powder obtained by solid way is first analyzed: this material is crystallized in the ferroelectric phase as shown by x-ray diffraction and Raman spectroscopy. A high speed second milling was performed to obtain NPs: they have been characterized by high resolution transmission electron microscopy (size and structural characterizations) and by dynamic light scattering. Then these NPs were blended with a nematic liquid crystal (5CB). Raman spectroscopy measurements were used to measure and compare the scalar order parameter of both samples. The EO properties were measured in planar liquid crystal cell configuration by using classical EO setup. We amazingly found a significant decrease in the Freedericksz threshold, compared to the pure material. This is to be compared to the Glushchenko’s measurement, who observed no variation, for a similar mixture in concentration, but for smaller NPs. Switching times were measured using the same setup and we noticed a decrease in the “τon” for the 5CB+BTO mixture compared to the pure 5CB one.

Dielectric spectroscopy of liquid crystal doped with Fe3O[formula omitted] nanoparticles

The influence of Fe3O4 nanoparticles on dielectric properties of planar oriented liquid crystal were studied at the frequency range 10−1–106 Hz and in temperature interval of 292–345 K. It was shown that the dielectric spectrum of doped liquid crystal can be divided into two areas. Parameters obtained from the dielectric spectrum for frequencies lower than 10 Hz characterize near electrode processes, and parameters obtained for frequencies more than 10 Hz characterize the bulk properties of the sample. The relaxation time and near-electrode area thickness have been estimated. It was experimentally shown that magnetic nanoparticles change the near-electrode area parameters.

Liquid crystal alignment properties of polystyrene derivatives containing fluorinated side groups

The liquid crystal (LC) alignment properties of a series of polystyrene derivatives containing various fluorinated side chain terminal moieties, such as 4-fluorophenoxymethyl, 3,4,5-trifluorophenoxymethyl, pentafluorophenoxymethyl, and 4-trifluoromethoxyphenoxymethyl groups, were investigated. These polymers could induce homeotropic and/or homogeneous planar LC alignment on their surfaces by changing the LC molecules and rubbing conditions. For example, when a positive (MJ001929 from Merck) and negative dielectric anisotropic LC mixture (MLC-7026-000 from Merck) were used, homeotropic LC alignment was observed in the LC cells made from unrubbed films of pentafluorophenoxymethyl- and 4-trifluoromethoxyphenoxymethyl-substituted polystyrenes with a smaller surface energy, whereas random planar LC alignment was observed in those made from the other polymers with a larger surface energy. On the other hand, homeotropic LC alignment behavior was observed in all the LC cells made from unrubbed films of all the polystyrene derivatives when 5CB was used as the LC material.

Influence of additional electric field on the birefringence of a nematic liquid crystal

Influence of the longitudinal voltage applied along the semiconductor substrate of a planar-oriented liquid crystal cell on the birefringence of a nematic liquid crystal (NLC) was investigated. It is shown that for a fixed magnitude of the transverse field the application of a longitudinal voltage creates an additional possibility to control the electro-optically induced phase incursion of the radiation passing through NLC. Dependence of the phase incursion on the control voltages is obtained. The results of simulation of the longitudinal voltage effect on the NLC reorientation are described. The obtained results can be used in development and designing of NLC phase modulators.

Polyimide blend alignment layers for control of liquid crystal pretilt angle through baking

Polyimide films were used for liquid crystal (LC) alignment layers to control LC pretilt angles over the full range (8°–90°). The pretilt angles could be controlled using polyimide films prepared from polyamic acid for vertical LC alignment and using polyimide blend films prepared from two types of polyamic acids, one for vertical LC alignment and the other for planar LC alignment, by changing the baking times ranging from 40 to 180min at 230°C. The polyimide blend film could control the pretilt angle better than the polyimide prepared from just one polymer component. The LC alignment behavior was well correlated with the wettability of the polyimide films due to the fragmentation of the long alkyl side group on the polyimide surfaces by the baking process.

Nanotopology of Polyimide Films Obliquely Treated by Plasma Beam and Liquid Crystal Alignment

The correlation between nanorelief of polyimide films obliquely treated by the beam of accelerated Ar plasma and liquid crystal (LC) alignment on these films is investigated. It is shown that the plasma beam treatment results in the change of films' roughness in a nanometer scale. In this case, the values of roughness in the directions parallel and perpendicular to the plasma beam projection on the films are different and dependent on the exposure dose. Clear correlation between the anisotropy of surface roughness and the direction of LC alignment is observed. This implies that topology is important, possibly decisive, factor of planar LC alignment on the surfaces obliquely treated by plasma beam.

The effect of phenoxymethyl side groups on the liquid crystal alignment behavior of polystyrene derivatives

We synthesized a series of polystyrene derivatives containing various side chain terminal moieties, such as phenoxymethyl, 4-methoxyphenoxymethyl, 4-fluorophenoxymethyl, 4-methylphenoxymethyl, and 4-trifluoromethoxyphenoxymethyl groups, using polymer analogous reactions, in order to investigate the effect of the side group on their liquid crystal (LC) alignment behaviors. The polymers containing 4-fluorophenoxymethyl, 4-methylphenoxymethyl, or 4-trifluoromethoxyphenoxymethyl side groups had lower surface energy values and the LC cells fabricated using the unrubbed films of these polymers showed homeotropic LC alignment behavior. The LC cells fabricated using the rubbed films of the polymers containing phenoxymethyl or 4-fluorophenoxymethyl groups showed homogeneous planar LC alignment behavior in which the LCs were aligned perpendicular to the rubbing direction. This homogeneous planar and perpendicular alignment behavior was ascribed to the favorable anisotropic interactions between the LC molecules and the side groups preferentially oriented perpendicular to the rubbing direction.

Sound propagation in 5CB liquid crystals homogeneously confined in a planar cell

The Brillouin spectrum of 4?‐n‐pentyl‐4‐cyano‐biphenyl (5CB) liquid crystals homogeneously confined in a planar liquid crystal (LC) cell was measured using a 6‐pass tandem Fabry‐Perot interferometer. By adopting a special right‐angle scattering geometry, the sound velocity of 5CB was estimated from the Brillouin shift without knowing the refractive index. The sound velocity of the longitudinal wave propagating along the direction of the directors aligned parallel to the glass plates of the LC cell was 1784±7 m/s at 300 K. The attenuation coefficient α was estimated to be approximately 1.9 × 106 m‐1, which is about twice as large as that of the longitudinal sound wave propagating along the direction perpendicular to the directors. The present method may be very useful in the evaluation of the elastic properties of the materials used in display devices, whose refractive indices are not known.

4‐Alkylphenoxymethyl‐Substituted Polystyrenes for Liquid Crystal Alignment Layers

AbstractWe synthesized a phenoxymethyl‐substituted polystyrene (PHP) and a series of 4‐alkylphenoxymethyl‐substituted polystyrenes, where the alkyl group is (CH2)nH (n = 1, 2, 4, 6, and 8), using polymer analogous reactions, in order to study the effect of the phenoxymethyl and 4‐alkylphenoxymethyl side groups on the liquid crystal (LC) alignment properties. The LC cell fabricated with the rubbed PHP film exhibited homogeneous planar and perpendicular LC alignment with respect to the rubbing direction. On the contrary, the LC cells made from the 4‐alkylphenoxymethyl‐substituted polystyrenes showed homeotropic LC alignment behaviors even at a very high rubbing density of 250, regardless of the length of the alkyl groups. Previously, n‐alkylsulfonylmethyl‐substituted polystyrenes with alkyl groups having more than 8 carbons (n > 8) showed homeotropic LC alignment behavior. Therefore, the additional phenoxy group in the side chain was found to improve the homeotropic LC aligning ability of the polystyrene derivatives.magnified image

Theory of surface-potential-mediated photorefractivelike effects in liquid crystals

We make a phenomenological model of optical two-beam interaction in a model planar liquid crystal cell. The liquid crystal is subject to homeotropic anchoring at the cell walls, is surrounded by thin photosensitive layers, and is subject to a variable potential across the cell. These systems are often known as liquid crystal photorefractive systems. The interference between the two obliquely incident beams causes a time-independent periodic modulation in electric field intensity in the direction transverse to the cell normal. Our model includes this field phenomenologically by supposing an effect on the electric potential at the cell walls. The transverse periodic surface potential causes spatially periodic departures from a pure homeotropic texture. The texture modulation acts as a grating for the incident light. The incident light is both directly transmitted and also subject to diffraction. The lowest order diffracted beams correspond to energy exchange between the beams. We find that the degree of energy exchange can be strongly sensitive to the mean angle of incidence, the angle between the beams, and the imposed potential across the cell. We use the model to speculate about what factors optimize nonlinear optical interaction in liquid crystalline photorefractive systems.

Structural transitions of encapsidated polyelectrolytes

Conformations and structural transitions of polyelectrolytes strictly confined onto a spherical 2D surface have been investigated by scaling descriptions based on physical arguments concerning polyelectrolyte adsorption onto planar surface and liquid crystals as well as by Monte Carlo simulations using a bead-spring model with short-range and electrostatic repulsions. In case of the electrostatic screened regime, a disordered-ordered (spiral) transition at increasing persistence length of the chain was found. It was predicted that the transition occurred when the persistence length is comparable with the mean spacing between adjacent strands of the ordered chain. The presence of a non-screened electrostatic repulsion led to a more complex behavior with i) a re-entrant order-disorder transition and ii) a tennis ball texture as an additional smectic/nematic structure. The various competing structures given by the theory were recovered by the Monte Carlo simulations, which also indicated that the tennis ball texture was favored over the spiral structure by the long-range interactions for semi-flexible chains.

Inside Front Cover: Unprecedented Dual Alignment Mode and Freedericksz Transition in Planar Nematic Liquid Crystal Cells Doped with Gold Nanoclusters (Adv. Funct. Mater. 2/2008)

AbstractOn p. 212, Torsten Hegmann and co‐workers describe nematic liquid crystals (N‐LCs) confined in planar liquid crystal cells after doping with small quantities of gold nanoclusters. These give rise to a dual alignment mode and electro‐optic response (Freedericksz transition). By fine‐tuning of experimental conditions, N‐LCs doped with gold nanoclusters can be electrically reoriented and aligned either like N‐LCs with a positive dielectric anisotropy (used in twisted nematic displays) in a planar cell or alternatively as N‐LCs with a negative dielectric anisotropy (used in large LCD TVs based on the vertical alignment mode).We demonstrate that alkylthiol‐capped gold nanoclusters doped into nematic liquid crystals (N‐LCs) with positive dielectric anisotropy give rise to an unprecedented dual alignment mode and electro‐optical response, which has a potential impact on current liquid crystal (LC) display technologies and N‐LC optical‐biosensor design. By fine‐tuning experimental conditions (temperature, electric field, and alignment), N‐LCs doped with gold nanoclusters can be aligned and electrically reoriented either like N‐LCs with a positive dielectric anisotropy in a planar cell or, alternatively, as N‐LCs with a negative dielectric anisotropy in a homeotropic cell, both at lower threshold voltages than the pure N‐LC.

Liquid Crystal Alignment Effects on SiNx Thin Film Layers Treated by Ion-Beam Irradiation

Nematic liquid crystal (NLC) alignment effects on SiNx thin film layers treated by ion-beam irradiation for three types of N ratio were successfully studied for the first time. The SiNx thin film was deposited by plasma-enhanced chemical vapor deposition using three types of N ratio. To characterize the film, atomic force microscopy was performed. Good LC aligning capabilities on the SiNx thin film treated by ion-beam irradiation for all N ratios can be achieved. The low pretilt angle for an NLC on the SiNx thin film treated by ion-beam irradiation was observed and could be adopted in planar alignment liquid crystal display applications as in-plane switching and fringe-field switching modes.

Controlled thin-film quasi-waveguide dye-laser in a liquid-crystal matrix

Laser generation in a thin oriented liquid crystal-dye layer is investigated experimentally. By inserting an electro-tunable planar-oriented liquid crystal cell into the resonator, tuning of the generation wavelength was realized. Studies were performed in the quasi-waveguide generation regime.