Effect Of Pretilt Angle Research Articles

Novel properties of high-performance multi-component mixture for Vertically Aligned mode LCDs

ABSTRACT Vertically Aligned mode Liquid Crystal Displays (VALCDs) which have generated huge research interest due to their excellent contrast and wide viewing angle require materials with negative dielectric anisotropy. This work highlights the systematic development of a fifteen-component mixture from pure liquid crystalline compounds comprising laterally fluorinated bi-, tri- phenyl, tolane, bicyclohexane compounds and non-mesogenic compounds exhibiting negative dielectric anisotropy. The temperature dependence of birefringence (Δn), dielectric anisotropy (Δε), threshold voltage (Vth), bend elastic constant (K33), relaxation time (τ0) and rotational viscosity (γ1) as a function of temperature of this fifteen-component nematic mixture have been reported. This mixture exhibits a high Figure of Merit (FoM) which is desirable for reducing the device response time. Additionally, the orientational order parameter (OOP) values, activation energy (Ea), visco-elastic co-efficient (γ1/K33) and material parameters (A 0 ) have also been reported. The effect of pretilt angle in the alignment layers on the threshold voltage (Vth) and switching time (τ0) to ascertain the applicability of this material in Vertically Aligned mode Liquid Crystal Displays (VALCDs) have also been discussed.

Magnetic Freedericksz transition in a ferronematic liquid crystal doped with spindle magnetic particles

In this work we studied new composite ferronematic system based on the liquid crystal 4-n-hexyl-4′-cyanobiphenyl (6CB) doped with hematite spindle-like magnetic particles. We experimentally showed that magnetic moments of the elongated spindle particles are oriented perpendicularly to their main axes. It is a fundamental difference of our systems from ferronematics, which were synthesized before by many authors. For a research of magnetic and orientational properties of new composite materials, the ferronematic samples were prepared with different length of the particles and with the volume concentrations of solid impurity ϕ1=10−3, ϕ2=10−4 and ϕ3=10−5. The structural Freedericksz transition in ferronematic samples were observed by capacitance measurements in a capacitor made of ITO-coated glass electrodes. Experimental results showed a decrease of the critical field in the ferronematic samples in comparison with the pure 6CB, and an increase of the threshold shift with a growth of both the volume fraction of solid impurity and the length of particles. The response of capacitance to low external magnetic field - far below the magnetic Freedericksz transition threshold of prepared samples was also observed experimentally. For a theoretical description of this magnetically induced behavior of composite systems, the continual model of ferronematics with bistable coupling between colloidal particles and the nematic matrix was used. This theory permits the equilibrium tilted orientation of the particles with respect to the nematic director. We showed that it creates a so-called “pretilt angle effect” which explains the behavior of the ferronematic systems in a low field regime as well as decrease of the threshold field and the dependence of this field on the volume fraction of solid impurity and the length of the particles.

Effects of pretilt angle and anchoring energy on alignment of uniformly lying helix mode

ABSTRACTWe investigated the effects of pretilt angle and anchoring energy on the formation of a uniformly lying helix (ULH) texture of cholesteric liquid crystals (CLCs). Pretilt angle was controlled by the thickness of a vertical alignment layer coated onto a planar alignment layer. In the given pretilt angle, the anchoring energy was enhanced by introducing reactive mesogen to the vertical alignment material. To characterise quantitatively the formation of the ULH texture we introduced reflectance, governed by areas of the ULH region and the planar-aligned CLC region. We found that the ULH texture was more widely formed under the condition of higher pretilt angle and weaker anchoring energy. Also, a more uniform alignment of the ULH texture was achieved with the higher pretilt angle even under the same anchoring energy condition.

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.

P‐170L: Late‐News Poster: Viewing Angle Enhancement Technology with the Polymer Stabilized In‐Plane Switching Mode Liquid Crystal Display

The polymer stabilized effect of the in‐plane switching(IPS) liquid crystal(LC) mode has been investigated. The polymer stabilized IPS LC mode exhibits low pretilt angle properties caused by additional new layer formation between bulk LC and alignment layer. Theoretically, low pretilt angle formation in the IPS LC mode is very helpful to improve viewing angle property in black state. but, until know, it is almost impossible to realize low pretilt angle IPS LC mode with usual rubbing process. With this low pretilt angle effect, the polymer stabilized IPS technology was confirmed to have good viewing angle properties in black state.

Recent advances in polymer network liquid crystal spatial light modulators

ABSTRACTPolymer network liquid crystal (PNLC) spatial light modulators are attractive for display and photonic applications because they can achieve submillisecond response time while keeping a large phase change. However, their on‐state scattering caused by the grain boundary of LC multidomains hinders their applications. In this article, we review recent progress on the development of scattering‐free PNLCs extending from short‐wavelength infrared to visible region by reducing the domain sizes to ∼200 nm through low temperature curing process. To reduce operation voltage, both transmissive and reflective modes, LC material properties (birefringence and dielectric anisotropy), polymer composition and concentration, and pretilt angle effect are analyzed. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 183–192

Dielectric and visco-elastic properties of laterally fluorinated liquid crystal single compounds and their mixture

The physical properties of three laterally fluorinated liquid crystalline compounds with negative dielectric anisotropy have been studied from static dielectric permittivity, optical birefringence, bend elastic constant, relaxation time and rotational viscosity measurements. Such negative dielectric anisotropy materials find use as components of mixtures for application in vertically aligned mode liquid crystal displays. Moreover, the physical properties of one phenyl cyclohexane compound with positive dielectric anisotropy have also been studied. A five-component mixture comprising these four mesogens and a non-mesogenic component has been formulated and its physical properties have been thoroughly investigated. An attempt has been made to strike a balance between the optical birefringence of the mixture to adjust the cell gap and the dielectric anisotropy and threshold voltage (Vth ) to ensure low driving voltages. The pretilt angle effect on the threshold voltage and the relaxation time has also been studied. At T = 20°C, the response time decreases to 22% and 41% for the mixture for 2° and 5° pretilt as compared to zero pretilt. On the other hand, at the same temperature the Vth values are decreased by 5% and 9%, respectively.

Influence of pretilt angle on disclination lines of liquid crystal lens

This article investigates the effect of pretilt angle on disclination lines of liquid crystal (LC) lenses. When the pretilt angle of LCs is higher than 7°, the disclination lines are reduced and are moved to the boundary of the LC lens. The disclination lines at the boundary do not influence the focused beam profile of the LC lens. As the pretilt angle of LCs further increases, the disclination lines at the boundary become almost invisible. However, the interference rings become asymmetrical. The response time of an LC lens with a pretilt angle higher than 7° is ∼60% of the conventionally homogeneous LC lens. This value is a result of the decrease in the rotation angle of the LCs and the reduced disclination lines.

Polymer Stabilized VA Mode Liquid Crystal Display

We used a polymer network to improve the response time of a vertical aligned (VA) mode liquid crystal display (LCD). The polymer network was anisotropic and was oriented in the same direction as the liquid crystal in the dark state. We studied the effect of pretilt angle and polymer concentration and measured the transmittance as well as scattering. With the polymer network, the turn-off time of VA-LCD significantly improved while the contrast ratio remained high.

Study of a liquid crystal structure with improved electro-optical characteristics

This paper introduces the vertically aligned twisted nematic in-plane switching (VPS) structure, and compares the electro-optical (EO) characteristics of VPS liquid crystal displays (LCDs) and in-plane switching (IPS) LCDs. Various VPS-LCD pretilt angles were induced, and ion beam (IB) irradiation was used to enhance the EO properties. An x-ray photoelectron spectroscopic analysis of the IB-irradiated polymer surface confirmed that the intensity of C=O bonding as a function of exposure time traced the pretilt angles. The pretilt angle effect on optical properties was first analyzed in the experimental results. While multidomain vertical alignment LCDs and IPS-LCDs only resolve one difficulty, the VPS-LCDs described here overcome several viewing angle and transmittance curve problems.

Stability of Bistable Chiral-Tilted Homeotropic Nematic Liquid Crystal Displays

We investigated the effects of pretilt angle and d/ p on the stability of bistable chiral-tilted homeotropic nematic (BHN) liquid crystal (LC) devices by experiments and simulation. The stability and experimental phenomena of BHN LC devices can be explained explicitly by calculating the free energies of the devices. Among the three BHN LC device varieties, (0,2π), (-π/2,3π/2), and (π/2,5π/2), the former two varieties are truly bistable devices in the high pretilt angle regime. However, the π/2 and 5π/2 states of the (π/2,5π/2) BHN LC device will always relax to the intermediate 3π/2 twisted state. We also analyze the energy barriers between the two stable states of (0,2π) and (-π/2,3π/2) BHN LC devices.

Pretilt Angle Effects on Liquid Crystal Response Time

Pretilt angle effect on liquid crystal dynamics is analyzed theoretically. Analytical expressions are derived to describe liquid crystal response time under nonzero pretilt angle conditions. The theoretical analysis is confirmed experimentally using vertically aligned liquid crystal cells. This finding quantitatively correlates pretilt angles with liquid crystal response time. This study improves the understanding of the liquid crystal dynamic process which is helpful for optimizing liquid crystal response time.

An O-Plate Compensated In-Plane Switching Liquid Crystal Display

Pretilt angle effect on the dark-state light leakage and viewing angle of in-plane switching (IPS) liquid crystal display is investigated. As the liquid crystal (LC) pretilt angle increases, the light leakage becomes more severe and the off-axis light leakage from the out-of-plane LC directors cannot be neglected. Introducing an oblique compensation plate (O-plate) reduces the off-axis light leakages from the crossed linear polarizers and the anisotropic LC media simultaneously which, in turn, improves the viewing angle of the IPS LCD.

Simulation study of an e‐beam addressed liquid crystal display for projection

We have carried out a simulation study on an e‐beam addressed liquid crystal projection display in which the liquid crystal is switched by the electric field of the charge, produced by an electron beam, on the surface of the display. We calculated the electric field produced by the surface charge, the liquid crystal director configuration and the profile of the transmitted light. We studied the factors affecting the resolution of the display and the effect of pretilt angle on the performance of the display. The e‐beam addressed liquid crystal projection display potentially has the advantages of high resolution and high brightness.

Dynamic simulation of Pi-cell liquid crystal displays

The dynamics of π-cell liquid crystal displays was studied by detailed computer simulation. The time evolution of the director configuration and the velocity of flow reveals the mechanism of the fast response of π-cells. The effect of pretilt angle on the dynamics of π-cells was also studied.

Effect of pretilt angle on the polar anchoring energy in NLCs on weakly rubbed polyimide surfaces

The polar anchoring strength and pretilt angle generation in nematic liquid crystals (NLCs), on three kinds of rubbed polyimide (PI) surfaces, were investigated. The pretilt angle generated in 4-n-pentyl-4'-cyanobiphenyl (5CB) is large compared with ZLI-4792 (a fluorinated mixture type NLC) for all rubbed PI surfaces. The high pretilt angle in 5CB is attributed to a much larger birefringence and much larger perpendicular component of permittivity. The polar anchoring energy of ZLI-4792 is high compared with 5CB on all rubbed PI surfaces for a weak rubbing strength. It is suggested that the high anchoring energy of ZLI-4792 may be attributed to the low NLC pretilt angle.

Effect of pretilt angle on the polar anchoring energy in NLCs on weakly rubbed polyimide surfaces

Effect of pretilt angle on the polar anchoring energy in NLCs on weakly rubbed polyimide surfaces

Effect of pretilt angle on the polar anchoring energy in NLCs on weakly rubbed polyimide surfaces

The polar anchoring strength and pretilt angle generation in nematic liquid crystals (NLCs), on three kinds of rubbed polyimide (PI) surfaces, were investigated. The pretilt angle generated in 4-n-pentyl-4'-cyanobiphenyl (5CB) is large compared with ZLI-4792 (a fluorinated mixture type NLC) for all rubbed PI surfaces. The high pretilt angle in 5CB is attributed to a much larger birefringence and much larger perpendicular component of permittivity. The polar anchoring energy of ZLI-4792 is high compared with 5CB on all rubbed PI surfaces for a weak rubbing strength. It is suggested that the high anchoring energy of ZLI-4792 may be attributed to the low NLC pretilt angle.

The effects of pre-tilt angle on the stability of C1 and C2 in rubbed polymer aligned FLC cells

Abstract Observations of a boundary layer structure near the surfaces of FLC devices, where the director reorients to the nematic alignment direction, are inconsistent with previous models of the chevron structure, where the the surface director lies at the intersection of the smetic cone with a horizontal (or tilted) plane. Two models of the surface alignment are presented: (i) strong surface anchoring where the smectic layers must bend and (ii) fixed layering where the position of the director at the surface is determined by finite surface is determined by finite surface anchorin. The corresponding director profiles and energies are claculted using models for a range of pre-tilt angles for C1 and C2 states.