Surface Pretilt Angle Research Articles

Liquid-Crystal-on-Silicon 소자에서 액정의 프리틸트각에 따른 Disclination Line 발생

We have studied how surface pretilt angle affects generation of disclination line in liquid-crystal-on silicon cells for 45<TEX>$^{\circ}$</TEX>-twisted nematic (TN) and vertical alignment (VA) modes with pixel size of 15<TEX>$\mu\textrm{m}$</TEX>. Our studies show that when the pretilt angle is increased from 0<TEX>$^{\circ}$</TEX>to 3<TEX>$^{\circ}$</TEX> in the 45<TEX>$^{\circ}$</TEX>-TN cell, the disclination line at left side of on-pixel becomes weak and is well suppressed with <TEX>$\theta$</TEX><TEX>$\_$</TEX>p/=3<TEX>$^{\circ}$</TEX> although the pixel size Is decreased. In the VA cell, when the pretilt angle is decreased from 89<TEX>$^{\circ}$</TEX> to 86<TEX>$^{\circ}$</TEX>, the disclination line at right side of the on-pixel is suppressed well and even for a smaller pixel size, it does not exist when <TEX>$\theta$</TEX><TEX>$\_$</TEX>p/=86<TEX>$^{\circ}$</TEX>. The results inform that the pretilt angle strongly affects the image quality of microdisplays.

Monte Carlo simulation of twisted nematic and supertwisted nematic liquid crystal cellsPresented at the 19th International Liquid Crystal Conference, 30 June–5 July 2002, Edinburgh, UK.

The orientational properties of liquid crystals in twisted nematic (TN) and supertwisted nematic (STN) cells were studied by Monte Carlo (MC) simulation depending on the strength of an external field. The liquid crystals sandwiched between planar surfaces were described by three-dimensional lattice systems of dimensions 10 × 10 × 13 up to 10 × 10 × 33 with pairwise interactions given by the chiral Lebwohl–Lasher potential between objects taken as directors representing a cluster of neighbouring molecules in a cubic unit cell. The electro-distortional curves showing the midlayer tilt angle as function of the applied voltage have been investigated depending on a set of cell and material parameters including e.g. the cell twist angle, the surface pretilt angle, and the thickness to pitch ratio. At very low temperature, in spite of the small system sizes studied a very good agreement has been obtained for MC simulation results and continuum theoretical predictions, both with respect to the dependency of the obtained threshold voltages necessary for the field-induced unwinding transition on cell twist angle and thickness to pitch ratio even for non-zero surface pretilt angles. This enabled the calculation of the elastic constants of the chiral Lebwohl–Lasher model. The applied method, the analysis of director arrangements in liquid crystal cells by means of Monte Carlo simulations starting from pairwise interactions, will be helpful as an alternative route to a continuum theoretical approach in designing advanced liquid crystal displays.

Surface free energy and pretilt angle on rubbed polyimide surfaces

Surface free energy and pretilt angle on rubbed polyimide surfaces

Surface Pretilt Effects of Bistable Twisted Nematic Liquid Crystal Display

Electrooptical properties of a bistable twisted nematic liquid crystal display are studied as a factor of surface pretilt angle. It is shown experimentally that the selection voltage decreased and the maximum transmittance increased as the surface pretilt angle is increased. These effects can be explained by the backflow effect and the stabilities of 0° and 360° twisted states upon changing the surface pretilt angle.

Surface Alignment Effect and Pretilt Angle Generation in NLC with Oblique Non-Polarized UV Light Irradiation on Polymer Surface

In this study, we have investigated the generation of pretilt angle in nematic liquid crystal (NLC) in the cell with oblique non-polarized ultraviolet (UV) light irradiation on two kinds of the polyimide (PI) surfaces. It was found that the monodomain alignment in NLC is obtained having a PI surface without side chain. We successfully observed that the generated pretilt angle of the NLC is about 3° with an angle of incidence of 70° on the PI surface without side chain. It is considered that the pretilt angle generation in NLC is attributed to the interaction between the LC molecules and the polymer surfaces due to photo-depolymerization reaction. Also, the uniform alignment of NLC is attributed to anisotropic dispersion force due to photo-depolymerization of polymer on PI surfaces.

A Combined Method Based on Rubbing and UV-Irradiation for Preparing Stable Alignment-Layers with High Pretilt Angles

A novel two-step method combining rubbing and UV-irradiation based on ladderlike polysilsesquioxanes (LPS) containing cinnamate and azobenzene side chains was developed to obtain alignment layers which features a high surface pretilt angle and good stability. At the first step, a low pretilt angle of 1–3° was generated by rubbing and at the second step, an increased pretilt angle of about 11° was obtained through further UV-irradiation. The polarized optical micrograph and the conoscopic picture prove that good LC alignment state with a stable high pretilt angle can be obtained by in situ photo-crosslinking of the cinnamate side-chains through proper UV-irradiation. The high pretilt angle arises from the photo-induced tautomerization of azobenzene side chains and that is confirmed by AFM study. In particular, the annealing test near the clearing point indicates the high stability of the high pretilt state, which heralds their potential application in LCD such as producing a patterned alignment layer for improving view angle.

Surface free energy and pretilt angle on rubbed polyimide surfaces

To investigate surface free energy of rubbed polyimide surfaces as a function of the rubbing strength, we measured the contact angle of distilled water and methylene iodide on the polyimide surfaces. For this purpose, a main-chain-type and two side-chain-type polyimides were used. The dispersion force (γd) and the polar force (γp) of the rubbed polyimide surfaces was measured. The rubbing induced polarity on the polyimide surfaces. Also, polar forces contributed to form a pretilt angle of liquid crystals on the rubbed polyimide surfaces. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 267–271, 1999

Influence of the inhomogeneous surface pretilt on zig-zag defects

The influence of the inhomogeneous surface pretilt (slope) on chevron structures and on zig-zag defects in a SmC liquid-crystal cell is studied. The experimentally observed effect of the confining substrate with controlled variation of the surface slope on the formation of zig-zag defects is qualitatively explained with the Landau–Ginzburg-type free-energy expanded in terms of the nematic director field and the smectic complex order parameter. Transitions in the C1 and C2 chevron structures are studied as functions of the surface pretilt angle θt. The threshold conditions are calculated. The width of the straight element of the domain wall (running parallel to the smectic layers) and the costs for its formation are estimated.

Effects of dielectric biaxiallity and surface pretilt on τ-V characteristics of SSFLCs

The surface pretilt angle in the surface-stabilized state and the dielectric biaxiallity of two kinds of ferroelectric liquid crystalline mixtures are evaluated based on the molecular orientational model. Using these values, the switching characteristics are calculated based on the elastic continuum theory of SmC* liquid crystals. These results are compared with the experimental ones, and the effects of the dielectric biaxiallity and the surface pretilt angle on τ—V characteristics are discussed.

The Effect of Material Parameters on the Hysteresis of Bistable STN Displays

We have investigated the hysteretic electro-optic behavior of super-twisted nematic (STN) displays with the twist angies larger than 270°. A pair of Lagrange's equations based on the continuum theory were solved numerically. The Gibbs energy and elastic energy of the nematic liquid crystal medium under the influence of external electric field were calculated. The director configurations at an applied field may be significantly different depending on the initial conditions. The elastic energy of the liquid crystal medium is used to describe its bistability. The total Gibbs energy, which represents the stability of the system, is discussed with each bistable state. The effects of cell twist angle, elastic constants, thickness to pitch ratio, dielectric parameter, and surface pretilt angle are presented.

Liquid Crystalline Polymer Stabilized FLCDs with Conventional Rubbed Polyimide Films or with Photo Alignment Films of Poly(vinyl Cinnamate)

Polymer stabilized SSFLCDs have been successfully fabricated. A liquid crystalline polymer network in a device cell is formed by the photo-cure of UV photo-currable liquid crystalline monomers that are doped in an FLC, where both the rubbed polyimide (PI) and non-rubbed but linearly polarized UV irradiated poly(vinyl cinnamate) (PVCi) films are utilized as surface FLC molecular alignment. The actual monomers adopted is acrylate with a mesogenic side chain. The PVCi is one of the photo alignment polymer. The fabrication process of these SSFLC cells is as follows; first the photo-cure is done on a monomers and initiators doped LC medium at the temperature where this material takes SmA phase and then the cell is cooled down to room temperature, at which the medium takes SmC* phase. The layer structure of the liquid crystalline polymer stabilized (LC-PS) FLC cells has been investigated by X-ray diffraction measurement. As the experimental results, it is found that the formed polymer network suppresses the formation of a chevron layer structure, resulting in the formation of a quasi-bookshelf layer structure; the only C1-uniform state is spontaneously formed without giving a high surface pretilt angle; and a fairly good memory state with an excellent contrast ratio and a low threshold voltage are obtained for a low concentration of doped monomers (2 ∼ 3 wt%) in particular with photo-alignment layers of PVCi; and furthermore, in a cell with a higher concentration of doped monomers (4 wt%) an excellent electroclinic like behavior (called the quasi-electroclinic effect) is obtained.

UV modification of surface pretilt of alignment layers for multidomain liquid crystal displays

Two types of ultraviolet (UV) light induced modification of the surface pretilt of alignment layers are reported. The UV modification allows the liquid crystal (LC) surface pretilt angle of a polyimide film to be selectively altered in a small area. Two device structures for fabricating two-domain liquid crystal displays based on this selective alteration of the LC surface pretilt angle of polyimides are proposed. Results from 3D simulation show that each pixel splits into two domains for the proposed structures. Experimentally, two-domain twisted nematic (TN) test panels and thin-film-transistor (TFT) addressed two-domain TN panels were fabricated using this method. The UV modification described in this letter does not require conventional photoresist technology as do other methods of fabricating two domain liquid crystal displays (LCDs). This process is simple as it requires only one polyimide coating and one rubbing step for each substrate.

Spatially Resolved Study of the Fréedericksz Transition in Thin Nematic Cells

Abstract We present a novel method for spatially resolved measurements of the amplitudes of thermal fluctuations and surface pretilt angles in thin cells filled with nematic liquid crystals (LC). We use the idea that the dynamical response of the director field during a switching process critically depends on the initial conditions of the director field which vary over the cell plane. The applied switching voltage acts as an amplifier which augments initial director deformations by orders of magnitude. By means of a CCD line scan camera combined with a polarizing microscope, we study the time dependence of the optical transmission curves. Their slopes give information on viscoelastic parameters whereas their delay can be related directly to the initial director angle.

PTFE Drawn Films as Alignment Agents for Liquid Crystals

Abstract It has been reported that thin films of Poly-tetra-fluoro-ethylene (PTFE) deposited in a suitable manner will induce alignment in a liquid crystal1. These films are deposited in a one step dry process which offers advantages over polyimide or SiO2 alignment layer application methods. Investigations of the effect of application conditions (pressure, temperature, deposition velocity) of this type of alignment layer on the quality of alignment in liquid crystals have been carried out. The surface pretilt angle induced in the liquid crystal by such an alignment layer has been measured and the PTFE films induce a low surface pretilt angle of the order of 1°. Some results of attempts to control this pretilt value by varying the conditions under which the alignment layer is deposited are presented. TN devices utilising PTFE alignment layers have been constructed and operating characteristics shown to be comparable to those for devices utilising commercially available polyimide alignment layers. There is...

Study of molecular orientational states of ferroelectric liquid crystals in a surface stabilized geometry

Abstract The molecular orientational states of homogeneously aligned, helix unwinding, chiral smectic C liquid crystals placed in a thin cell (surface-stabilized ferroelectric liquid crystals [SSFLC]) were studied. They were classified by the optical viewing conditions and the relationship between the directions of the chevron layer structure and the surface pretilt. The molecular orientational models of the states were considered and illustrated with regard to the experimental results. The models of molecular orientation give us a total understanding of the orientational states which appear in SSFLCs with parallel rubbing. Furthermore, the effect of the surface pretilt angle on the orientational and optical properties of SSFLCs is discussed.

Frame Sequential Color Thin Film Transistor/Liquid Crystal Displays: Display Characteristics and Grayscale Consideration

We report a theoretical study on the optical characteristics of a frame sequential color thin film transistor/liquid crystal display (TFT/LCD) operating with the TN LCD. Important liquid crystal (LC) and display parameters (cell gap, LC surface pretilt angle, applied voltage, LC viscosity, frame rate, and operation sequence) are considered and optimized to improve the display performances. Both the modeling and experimental results for a TN LCD using an EM Merck ZLI-3651 LC material are presented. We also present detailed theoretical results for a TFT/LCD operating at frame rates of 30 Hz and 71 Hz. Sufficiently fast switching-on and switching-off dynamic responses can be obtained by raising the temperature to 40-60°C to reduce the LC rotational viscosity. A display operating at the 71 Hz frame rate, can obtain a good bi-level display, but it is difficult to obtain a good display with 512 colors.

Cell thickness and surface pretilt angle measurements of a planar liquid-crystal cell with obliquely incident light

Two simple and fast optical methods are presented for the cell thickness and the surface pretilt angle measurements of a planar-oriented liquid-crystal (LC) cell, in which the LC director always lies in a plane of a Cartesian coordinate system. The methods employ an obliquely incident light propagating in the same plane containing the liquid-crystal molecules. Simple analytic formulas for the angular-dependent optical transmission are obtained. The phase retardation between the ordinary and extraordinary waves is used to measure the cell thickness with the use of a compensator. Independent data can be obtained for the cell thickness measurement by varying the angle of incidence of the incident light. The phase retardation-extremum method is used to measure the pretilt angle. Approximate formulas relating the pretilt angle and the angle of incidence corresponding to the transmission minimum are also obtained for the pretilt angle measurement for vertically and parallel-oriented LC cells with a small pretilt angle. For the parallel cell, the approximate formula agrees with that obtained by Birecki and Kahn [The Physics and Chemistry of Liquid Crystal Devices, edited by G. J. Sprokel (Plenum, New York, 1980), p. 115]. The methods are applicable to a planar LC cell, including the planar parallel nematic LC, homeotropic nematic LC cell, obliquely oriented nematic LC cell, and the planar-oriented ferroelectric smectic LC cell.

Alignment of Nematic and Smectic Liquid Crystals

Abstract Some remarks on the alignment of nematics are given. The orientation of smectic A-liquid crystals at different surface pretilt angles and for variable layer thickness has been studied.

Alignment treatments for surface stabilised ferroelectric liquid crystals

Abstract The alignment characteristics for a range of Sc materials are reported. Different types of polymer have been used as the aligning agent and various processing techniques of these polymers have been studied. A comparison has been made between three polymers; PVA polyimide and Nylon 6. In each case the alignment was assessed with the rubbing directions orientated parallel, antiparallel and with only one surface rubbed. The results show that in some cases, there must be a surface pretilt of the director in SSFLCD. Different thermal treatments of polyimide can vary this surface pretilt angle, and the effects of this have been studied. In addition, a novel hybrid alignment is presented. One surface is coated with rubbed nylon 6 to produce a homogeneous alignment and the other surface is coated with a polysilane to produce homeotropic alignment. Good alignment can be obtained using this technique and the bistability is always asymmetrical.

Origin and characteristics of the optical properties of general twisted nematic liquid-crystal displays

The origin and characteristics of the field-off state optical properties of the general twisted nematic liquid-crystal displays (LCD) are discussed. The optical field propagating normally in a uniformly twisted nematic structure can be represented by two mutually orthogonal elliptically polarized normal modes, one with left elliptical polarization and the other with right elliptical polarization. The interference between the two normal modes explains precisely, for the first time, the working mechanism for the field-off state optical performance of all twisted nematic liquid-crystal displays. The sum of the intensities of the two modes is independent of the polarizer orientation and always constitutes one half of the total intensity. The interference between the two modes contributes the second half of the total intensity and gives rise to two separate terms, each of which can be controlled by the entrance polarizer. Each of their contributions to the transmitted intensity can be minimized or maximized by the entrance polarizer in a complementary way. In addition, the ordinary and the extraordinary waves are always excited and strongly coupled in the twisted nematic (TN) structures. We also show that the field amplitudes of the waves are, in general, nonzero. The ordinary wave is zero only at some particular spatial points when the entrance polarizer is oriented parallel to entrance LC director; the extraordinary wave is zero only at the same particular spatial points when the entrance polarizer is oriented normal to the entrance LC director. In all other spatial points, both waves are nonzero even if the entrance polarizer is oriented parallel or normal to the entrance LC director. Using the solution for the optical field, analytic expressions are obtained for the field-off state optical transmissions of the general twisted nematic liquid-crystal displays with arbitrary polarizer orientations, liquid-crystal indices of refraction, twist angle, surface pretilt angle, and cell thickness. The field-off state optical working mechanism and characteristics of various twisted nematic liquid-crystal displays, including the standard 90° twisted nematic, the lower-twisted nematic, the optical mode interference, the supertwisted nematic, and the supertwisted birefringence effects liquid-crystal displays, are discussed. All the previously reported analytic results for the TN LCDs can be shown to be special cases of our general transmission formula. However, the Gooch and Tarry approach contains a few major incorrect intermediate results, despite the correct final results. We also obtained a simple expression for the polarizer orientations, for which the field-off state transmission has an extreme value under parallel or crossed polarizers. In general, for every twist geometry, there are two possible optimized polarizer orientations for the field-off state with parallel (or crossed) polarizers configuration. In particular, for the 180° supertwisted devices, one polarizer condition describes the standard 180° supertwisted nematic display, and the second polarizer condition describes the black and white 180° optical mode interference display.