Pretilt Angle Research Articles

Effects of Pretilt Angle on Electro-Optical Properties of Optically Compensated Bend Liquid Crystal Devices With a Mixed Polyimide Alignment Layer

The effects of pretilt angle on electro-optical properties of optically compensated bend liquid crystal cells with a mixed polyimide alignment layer are investigated. By adjusting the mixed polyimide concentration, several kinds of alignment layers are produced, and a wide range of pretilt angles (2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> -66 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> ) are achieved. Differing from the conventional addresses, the maximum-light-transmittance voltage, obtained from the transmittance-voltage curve of the cell undergoing the reverse measurement, is employed, making a ~ 14% enhancement in the output transmittance. Compared to the 0-wt% cell, the 0.6-wt% cell with 16 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> pretilt angle achieves ~ 38% and ~ 50% improvement in the rising-time and falling-time response, respectively.

Póincare Sphere Analysis of the Pretilt Angle Effect on the Viewing Angle of a Single-Domain FFS Liquid-Crystal Mode

We demonstrated the pretilt angle effect on the viewing angle properties of a single-domain fringe-field switching (FFS) liquid crystal (LC) mode. By performing the Poincare sphere analysis, we investigated, in detail, the origin of the viewing angle asymmetry that exists in the single-domain FFS LC mode both in the field-on and field-off states. Using this analysis, we confirmed that the pretilt angle reduces the viewing angle symmetry in the single-domain FFS LC mode. Finally, we examined the effect of a zero pretilt angle on the viewing angle symmetry by evaluating real single-domain FFS LC cells.

Polystyrene Blend Alignment Layers for High Pretilt Angle Control

ABSTRACTWe prepared blend alignment layer based on 4-ethylphenoxymethyl-substituted polystyrene (P4EP) and polystyrene containing 7-hydroxycoumarin (P7COU) for liquid crystal (LC) alignment. Stable and uniform vertical LC cells having pretilt angles adjustable from 90° to 81.2° were obtained from these polystyrene films having molar contents of P4EP and P7COU in the ranges of 100–70 and 0–30 mol%, respectively. The LC alignment behavior was well correlated with the wettability of the polymer films. These polystyrene blends having good solubility in many organic solvents and their films having low process (curing) temperature can be candidates for LC alignment layer in the flexible display. This study contributes to the latest efforts to develop new alignment layers for pretilt angle control.

Tunable Liquid Crystal Lens Based on Pretilt Angle Gradient Alignment

ABSTRACTThe new method of LC lenses fabrication based on innovative alignment material for pretilt angle gradient generation is developed. The alignment layer provides LC lens pretilt angle gradient via two-step treatment: uniform rubbing and gradient nonpolarized UV exposure. LC lens has uniform cell-gap, 2–10 V tunable voltage range. The fabrication method is scalable for lens arrays for two-dimensional/three-dimensional autostereoscopic and light-field displays.

Liquid Crystal Alignment Behaviors on Comb-Like Fluorinated Polystyrene Films

ABSTRACTWe synthesized a series of fluorinated polystyrene (PCMSF#, # = 50, 80, and 100), where # is the molar content of 1H,1H,2H,2H-perfluorodecanethiolate moiety, using polymer analogous reactions to investigate their liquid crystal (LC) alignment properties. In general, the LC cell fabricated using the polymer film having a higher molar content of fluorinated side group showed vertical LC alignment behavior with a pretilt angle of about 90°. The vertical alignment behavior was well correlated with the surface energy value and fluorine atomic percent of these polymer films. For example, vertical alignment was observed when the surface energy values of the polymer were smaller than about 8.77 mJ/m2 generated by the nonpolar and long fluorocarbon groups. Good electro-optical characteristics, such as voltage holding ratio (VHR) and residual DC voltage (R-DC), and aligning stabilities at 250°C were observed for the LC cells fabricated using PCMSF100 as a LC alignment layer.

Improvement of image sticking on liquid crystal displays with polymer layers produced from mixed monomers

ABSTRACTAs a model of polymer-sustained-alignment liquid crystal display (PSA-LCD), the LC cells having the polymer layers produced from the homopolymers and copolymers of 4,4ʹ-dimethacryloyl-oxy-biphenyl (4,4ʹ-DMABiph) and 2,6-dimethacryloyl-oxy-naphthalene (2,6-DMANaph) were prepared and investigated image sticking phenomenon with evaluations of residual direct current voltage (VrDC) and difference in pre-tilt angle before and after application of alternate current (AC) voltage (Δtilt). The VrDC was effectively improved by adding a small amount of 2,6-DMANaph to 4,4ʹ-DMABiph because the concentrations of radicals and ions in the LC layer were decreased due to increase in the rate constant of the polymerisation with the addition of 2,6-DMANaph under UV light exposure. The Δtilt was proportionally increased with increasing the weight ratio of 2,6-DMANaph in the mixed monomers of 4,4ʹ-DMABiph and 2,6-DMANaph. We confirmed that the range of the weight ratio for 2,6-DMANaph in the mixed monomers of 4,4ʹ-DMABiph and 2,6-DMANaph from 10 to 25 wt% was useful for obtaining the small level of image sticking in the PSA-LCD.

Alignment of liquid crystal molecules on solution-derived zinc-tin-oxide films via ion beam irradiation

We present the characteristics of annealing temperature-dependent, zinc-tin-oxide (ZTO) films deposited by a solution process for application in liquid crystal displays (LCDs). ZTO surfaces supported homogeneously-aligned liquid crystal (LC) molecules based on an ion beam irradiation system. Uniform LC alignment and a precise pretilt angle were obtained at an annealing temperature greater than 300 °C. The oxidation of ZTO films was confirmed using field-emission-scanning electron microscopy and X-ray photoelectron spectroscopy. The electro-optical characteristics of electrically controlled birefringence (ECB) cells based on the ZTO films were superior to those based on polyimide. Especially, IB-irradiated ZTO films exhibited superior performance with respect to response time. This result indicates that this approach will allow for the fabrication of advanced LCDs with high performance.

Controlling liquid crystal pretilt angle with photocurable prepolymer and vertically aligned substrate.

We demonstrate a new approach to control the pretilt angle of liquid crystals (LCs) with photocurable prepolymer in a cell fabricated with vertically aligned substrates. During UV exposure, prepolymer approaches and is polymerized on the substrate surfaces because of the vertical phase separation induced by differences in the surface tensions of the employed materials. After polymerization, the polymer structure formed on the substrate alters its surface polarity and changes the pretilt angle of the LC cell. The LC pretilt angle can be controlled from 87.3° to 2.5° when the prepolymer concentration ranges from 0 wt% to 2.5 wt%.

Surface flattening of a polyimide layer in a liquid crystal cell fabricated by using a field-induced UV-alignment method

We found that the decrease in pretilt angle is related to the flattened surface in a homogeneously aligned LC cell fabricated with the field-induced UV-alignment method.

Electro-Optical Performance of a Zero Pre-Tilt Liquid Crystal Cell Fabricated by Using the Field-Induced UV-Alignment Method

The liquid crystal alignment method using a UV-curable polymer has been reported for the zero pre-tilt angle of liquid crystals (see the paper by Oh in Optics Express, vol. 23, p. 1044, 2015). In this paper, we report the electro-optical characteristics and parameters related to the image quality of a fringe-field switching cell fabricated using the field-induced UV-alignment method as functions of the monomer concentration and the UV irradiation time. The results show that the field-induced UV-alignment method has a high potential for practical display applications.

Photoalignment Behavior on Polystyrene Films Containing Coumarin Moieties

We synthesized a series of polystyrene derivatives containing coumarin side groups, poly(6-(4-vinylbenzyloxy)coumarin)(P6COU#), where # is the molar content of 6-hydroxycoumarin using polymer modification reaction, in order to study the effect of the 6-hydroxycoumarin side groups on the liquid crystal (LC) alignment properties. The LC alignment behavior of these polymers was investigated using photoalignment method. The LC cells made from photoirradiated P6COU# films exhibited homogeneous planar LC alignment with a pretilt angle of approximately 0°. We found that LC aligning ability of the LC cells made from photoirradiated P6COU# films was affected by the molar content of 6-hydroxycoumarin side groups. For example, the azimuthal anchoring energy of a P6COU80 compared to P6COU25 is about 1 × 10−5 J/m2 and 5 × 10−7 J/m2, respectively. The electro-optical characteristics of the LC cells fabricated with P6COU80 films such as voltage holding ratio and residual DC voltage, threshold voltage, driving voltage, and response time were as good as those fabricated from rubbed polyimide films, the most commonly used LC alignment layers.

Determination of the director pretilt angle at liquid-crystal layer boundaries by polarimetric and crystal rotation methods in the same cell

The pretilt angle of the director at the liquid crystal–glass interface is determined with the polarimetric and crystal rotation methods. The polarimetric method has been modified with the aim of increasing the measurement accuracy. The results obtained with the modified polarimetric method are verified with the use of reference plane-parallel iceland spar plates with known pretilt angles of the optical axes. It is shown that the pretilt angles found with the crystal rotation and modified polarimetric methods coincide within the measurement accuracy.

A Technique for determining the director pretilt angle in cells with hybrid or homeotropic alignment of a nematic liquid crystal

A new technique for determining the director pretilt angle in cells with hybrid or homeotropic alignment of a nematic liquid crystal has been developed. To use this technique, it is necessary to experimentally determine the transmission of an optical system in parallel polarizers and maximum transmission of a cell in crossed polarizers. The technique makes it possible to locally control the liquid crystal director alignment on an aligning surface.

Coaxially bifocal liquid crystal lens with switchable optical aperture

ABSTRACTAn electrically tunable liquid crystal (LC) lens with dual hole-patterned electrodes is demonstrated. When the LC lens is operated at low voltages, the dual hole-patterned electrodes with different diameters impart the lens with a coaxial bifocal characteristic. At high voltages, the proposed LC lens functions as a conventional lens with a single focal length but with a switchable optical aperture. The demonstrated LC lens is free of disclination lines because of the presence of voltage-assisted high pretilt angles created from the upmost hole-patterned electrode with small diameter.

Nickel Oxide Nanoparticles Doped Liquid Crystal System for Superior Electro-Optical Properties.

We examined the properties of nematic liquid crystal (N-LC) systems with dispersed nickel oxide nanoparticles (NPs). Uniform LC alignments with regular pretilt angles were achieved on rubbed polymer surface regardless of NiO nanoparticles concentration. We confirmed the electro-optical characteristics of twisted nematic (TN) cells containing NiO nanoparticles on rubbed polymer surface, which exhibited lower threshold voltages and faster response times with less capacitance hysteresis than pure LC cells. It is clear that the response time of TN cells on rubbed polymer surfaces decreases with increasing the NiO nanoparticles concentration. These results demonstrate the relationship between NP doping concentration and trapping of impurity ions, and were confirmed by a software simulation of electric flux and field density. NiO nanoparticles in the LC cells focused the electric field flux and strengthened the electric field. Further, NiO nanoparticles in LC medium trapped charged ionic impurities and suppressed the screen effect, leading to a stronger electric field and the van der Waals interactions between LC molecules and the alignment layers.

Polymer-Layer-Free Alignment for Fast Switching Nematic Liquid Crystals by Multifunctional Nanostructured Substrate.

A novel polymer-layer-free system for liquid-crystal alignment is demonstrated by various shaped indium tin oxide (ITO) patterns. Liquid crystals are aligned along the ITO line pattern and secondary sputtering lithography can change the shape of the ITO line pattern. Different shapes can control the direction and size of the pretilt angle. This effect eliminates defects and reduces the response time.

Enhancement of viewing angle properties of a single-domain fringe-field switching mode using zero pretilt alignment

We demonstrated the effect of the pretilt angle on the viewing angle properties of the single-domain fringe-field switching (FFS) mode, and proposed a method to enhance the viewing angle properties. Firstly, we investigated the origin of the asymmetric viewing angle problems in the field-on and off states by performing a field-induced liquid crystal (LC) reorientation analysis. Because of the asymmetric properties induced by the coupling between the tilting angle of the LC director and the polar component of the electric field, a zero pretilt angle is essential for the symmetric viewing angle properties. Secondly, to eliminate the pretilt angle, which is generally inevitable with the rubbing process, we applied polystyrene (PS) alignment layer which generates an easy axis perpendicular to the rubbing direction. Azimuthal anchoring energy and thermal stability of the PS layer were improved by employing UV-curable reactive mesogen (RM) within the LC cell. The RM-stabilized PS layer preserved the zero pretilt angle with enhanced thermal stability. Finally, by evaluating 2.4 in QVGA single-domain FFS LC cells, we confirmed the improved alignment ability of the RM-stabilized PS layer and the effect of the zero pretilt angle on electro-optical properties such as response time and viewing angle.

PA09 水平・垂直混合配向膜により付与されるプレチルト角の理論的検討(液晶物理・物性,ポスター発表,2015年日本液晶学会討論会)

PA09 水平・垂直混合配向膜により付与されるプレチルト角の理論的検討(液晶物理・物性,ポスター発表,2015年日本液晶学会討論会)

Anchoring energy enhancement and pretilt angle control of liquid crystal alignment on polymerized surfaces

We demonstrate enhanced surface anchoring energy and control of pretilt angle in a nematic liquid crystal cell with vertical alignment and polymerized surfaces (PS-VA). The polymerized surfaces are formed by ultraviolet (UV) irradiation-induced phase separation of a minute amount of a reactive monomer in the vertical-aligned nematic liquid crystal. By introducing a bias voltage during UV curing, surface-localized polymer protrusions with a dimension of 100nm and a field-induced pretilt angle are observed. Experimental evidences and theoretical analyses validate that PS-VA has increased surface anchoring strength by two folds and pretilt angle has been changed from 89° to 86° compared to those of a VA cell. The enabling PS-VA cell technique with excel electro-optical properties such as very good dark state, high optical contrast, and fast rise and decay times may lead to development of a wide range of applications.

Dielectric properties of liquid crystals for display and sensor applications

AbstractTemperature dependence of dielectric properties of nematic liquid crystals are investigated for two types of devices: wide temperature range liquid crystal (LC) displays and capacitor temperature sensor. Both real and imaginary components of the dielectric constant have been measured in wide frequency and temperature range including t &lt; 0 °C as well as versus the angle θ between the directions of both magnetic and electric field in the measuring scheme. Some physical parameters of the nematic LC (NLC) dielectric relaxation have been determined. Effective values of the LC dielectric permittivity with different values of the LC pretilt angles in different parts of a complex LC cell have been simulated.