Surface Pretilt Angle Research Articles

Influence of the surface pretilt angle on spatially periodic deformations in nematic layers

ABSTRACTThe deformations induced by electric field in twisted or untwisted flexoelectric nematic layers can be homogeneous (i.e. one-dimensional) or spatially periodic (i.e. two-dimensional). The periodic deformations are undesirable from an applicative point of view since they destroy the homogeneous appearance of the area of an excited pixel of a display. They are particularly favoured when the nematic material possesses flexoelectric properties. In order to check whether the unwanted periodic deformations can be eliminated by means of suitable surface pretilt angle, the small deformations arising just above the corresponding threshold voltage were investigated numerically. The nematic materials exhibiting both weak and strong flexoelectricity were taken into account. The surface pretilt angles ranging from 0° to 30° were adopted. It was shown that the periodic patterns, arising in the case of planar surface alignment, disappear if sufficiently large surface pretilt angle is applied.

Improving profile of multi-electrode Fresnel-type liquid crystal lens utilizing local control of pretilt angle

A multi-electrode driving Fresnel liquid crystal lenticular lens (MeD-Fresnel type LC lens) for switchable 2D/3D liquid crystal displays has been studied. The demonstrated lens which has fine-patterned electrodes only on one substrate, minimizing lens error by mis-assembly between top and bottom substrate, and uses a discontinuous non-uniform electric field with local control of surface pretilt angle by polymer stabilization technique, exhibits excellent lens profile compared with conventional lens without local control of pretilt angle.

Phase separation of monomer in liquid crystal mixtures and surface morphology in polymer-stabilized vertical alignment liquid crystal displays

The polymer-stabilized vertically aligned (PS-VA) liquid crystal display (LCD) driving mode has high potential for manufacturing low power consuming displays due to the higher transmittance and fast response as compared with the existing patterned vertically aligned and multi-domain vertically aligned modes. In this paper we have investigated the reaction mechanisms of monomer–liquid crystal blends to form a surface pre-tilt angle of liquid crystal in vertical alignment LCD associated with a fishbone electrode structure. The observed sizes of polymer bumps formed on the substrates were found to be dependent on the exposed UV wavelength and were almost equal in both top and bottom substrates. When a large UV wavelength was used, the phase separation mechanism of monomer in PS-VA mode was found nearly isotropic rather than anisotropic in contrast to the previous studies.

P‐143: Polymer Stabilized In‐Plane Field Driven Vertical Alignment Liquid Crystal Device

AbstractWe propose surface polymer‐stabilized in‐plane field driven vertical alignment device using a liquid crystal with positive dielectric anisotropy. The device requires long distance between electrodes to achieve higher transmittance, however, such a design results in increase in operating voltage and slow response time. Forming surface pretilt angle with utilization of polymer stabilization technique, operating voltage is reduced and response time is greatly improved even with long electrode distance.

Transflective fringe-field switching liquid crystal display without any retarder

Conventional transflective liquid crystal displays (LCDs) with single cell-gap requires optical compensation films or patterned retarders to balance the optical path-length difference between transmissive and reflective regions. In this paper, novel single cell-gap transflective LCDs driven by fringe electric field without using the compensation film or in-cell retarder have been proposed. The liquid crystal director is aligned parallel to analyzer but makes an angle of 45° with respect to analyzer in reflective region. In addition, the surface pretilt angle in the reflective region is controlled by vertical field and polymerization of an UV curable reactive mesogen at the same time and thus, the effective cell retardation in the reflective region becomes smaller than that in transmissive region. Consequently, without using any compensation film or in-cell retarder, the single cell-gap and single-gamma transflective LCD with high performance is realized.

Effect of the skeleton structures in the side chain of polyimides on their film surface properties and pretilt angles of liquid crystal molecules

Polyimides (1c–4c, 1c'–4c') with side chains (1–4) containing four different skeletons were synthesised and characterised. Results showed that a similar high side chain abundance took place on the surface of the films, which was independent of the skeleton structures, while the pretilt angles of liquid crystal molecules were considerably affected by the skeleton structures. 1c, 1c' and 2c, 2c' achieved vertical alignment while 3c, 3c' and 4c, 4c' only obtained low pretilt angles. The main reason, based on surface testing results and molecular modelling, was the different interactions between liquid crystal molecules and the skeletons in the side chains.

Studies of Frictional Properties on TN mode Alignment Film Surfaces with AFM/FFM

The surface characteristics of hybrid type twisted nematic (TN) mode alignment film (AF) were studied with atomic force microscopy (AFM) and frictional force microscopy (FFM). As a result, the contrast of friction on FFM image had changed when the probe was scanned in rubbing directions. In the friction loop, the frictional asymmetry was detected differently according to the drying temperature conditions. The difference was explained by pretilt uniformity of side chain molecules arrangement on polyimide (PI) layer surface. The measured data of surface energy and pretilt angle after liquid crystal (LC) injection couldn't represent realistic layer frictional characteristics of AF surface.

A single-gap transflective liquid crystal driven by fringe and vertical electric fields

A single-gap transflective liquid-crystal display driven by a fringe electric field in the transmissive (T) region and a vertical electric field in the reflective (R) region was designed. In the device, a homogeneously aligned liquid crystal (LC) rotates almost in plane by a fringe field in the T-region whereas the LC tilts upwards by a vertical field in the R-region. A high surface pre-tilt angle of the LC in the R-region is achieved through polymerization of an UV curable reactive mesogen monomer at the surfaces and thus the effective cell retardation in the R-region becomes half of that in the T-region. Consequently, a transflective display driven by a vertical and a fringe electric field with a single cell gap and single gamma curves is realized.

Review Paper: Emerging vertical‐alignment liquid‐crystal technology associated with surface modification using UV‐curable monomer

Abstract— The recent development of polymer‐induced pretilt angle in multi‐domain vertical‐alignment liquid‐crystal (LC) structures is reviewed. To create a small but well‐defined pretilt angle, ∼0.1 wt.% of a photo‐curable monomer was mixed in an LC host and a bias voltage was applied to reorient the LC directors within each domain. The monomers are polymerized near the substrate surfaces by UV exposure. The formed polymer layers change the surface pretilt angle of the LC from 90° to about 89° with a defined azimuthal orientation. Consequently, within each domain the LC reorientation direction responding to the external field is well‐defined which leads to faster rise time and higher transmittance. This new technology overcomes the long standing problems of conventional MVA devices and is therefore expected to play a dominant role in the future.

Liquid-crystal variable-focus lenses with a spatially-distributed tilt angles

A pretilt angle controlling method by the density of rubbings using a tiny stylus is proposed. The control of the surface pretilt angle is achieved by rubbing a side-chain type polyimide film for a homeotropic alignment. Smooth liquid crystal (LC) director distribution in the bulk layer is successfully obtained even though the rough surface orientation. This approach is applied to LC cylindrical and rectangular lenses with a variable-focusing function. The distribution profile of the rubbing pitch (the reciprocal of the rubbing density) for small aberration is determined to be quadratic. The variable focusing function is successfully achieved in the LC rectangular lens, and the voltage dependence of the focal length is tried to be explained by the LC molecular reorientation behavior.

Electro-optic characteristics as a function of pretilt angle in the optically compensated splay liquid crystal cell

Optically compensated splay (OCS) mode, where the liquid crystals (LCs) are almost homeotropically aligned with rubbed surfaces at parallel directions and a mid-director orients parallel to the substrate, was known to show a wide-viewing-angle due to self-compensation effect. The device requires a setting voltage and phase transition time in order for LC to have a transition from almost vertical alignment to the splay state. The setting voltage, transition time, and response time of the device as a function of surface pretilt angle were studied. The results indicate that with decreasing surface pretilt angle in the OCS cell, both phase transition time and setting voltage decrease and response time becomes faster.

Splay-to-Bend Transition-Free Reactive Monomer Modified Pi-Cell

A reactive monomer modified Pi-cell (RMM-Pi-cell) comprising a layer of liquid crystal (LC) reactive monomer on one surface was prepared to control the surface pretilt angle. The simulation results suggested that a transition-free Pi-cell can be prepared by asymmetrical cell with one 8 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> pretilt angle and the other surface greater than 47 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> when the cell gap was smaller than 4 mum. The nematic reactive monomer layer has a molecular average tilt angle over 80 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> which allowed the LC molecules to be arranged in a favored bend state in the asymmetrical cell. The critical voltages cannot be found in all 3-mu m RMM-Pi-cells. The cell retardation data confirmed the initial bend orientation with zero voltage applied. Moreover, the light leakage of dark state was reduced. The contrast ratio of RMM-Pi-cell was improved by a factor of 11 compared with an original Pi-cell without using compensation film.

Surface Polymer-Stabilized Vertically Aligned Liquid Crystal Cells with Various Polymer Wall Structures

Vertically aligned liquid crystal (LC) surrounded by polymer wall, which is also known as locked-super homeotropic (LSH) mode has several advantages such as rubbing- and spacer-free, wide-viewing angle and stable LC dynamics against external pressure. However, the LSH mode shows slow response time due to the instantaneous collision between the LC molecules when a vertical electric field is applied, because the reorientation of chiral doped vertically aligned LC is only determined by the polymer wall. We found that the slow response time can be improved by surface polymer-stabilized technique where surface pretilt angle is defined on alignment layers. This technology can be used in any shape of polymer wall. (PACS 42.79.Kr, 85.60.-q).

Medical displays by using plasma‐beam‐alignment technology

Abstract— Novel anode layer plasma within minimum chamber space was developed for non‐contact alignment process. The plasma‐treated polyimide (PI) surface showed no particle contamination and no micro‐scratches. Surface morphology was investigated by using scanning electron microscope (SEM), an atomic force microscope (AFM), and X‐ray photoemission spectroscopy. The different oxygen‐to‐carbon ratio ([O]/[C] ratio) for XPS spectra indicated a composition change after plasma treatment. Surface pretilt angles were varied from 0 to 2.1° under different plasma exposure times. Finally, a prototype 20.8‐in. QXGA IPS‐mode gray‐scale medical liquid‐crystal display was successfully demonstrated with high contrast ratio, excellent uniformity, and wide viewing angle using this new plasma‐beam‐alignment technique.

Basic properties of cell fabricated by ion‐beam treatment for in‐plane switching LCD

We investigated the horizontal alignment properties such as surface morphology, pretilt angle, and polar and azimuthal anchoring energy of organic alignment material‐coated surface treated by ion beam. In this investigation the energy and incident angle of ion beam were changed. We also fabricated an in‐plane switching (IPS) cell by ion beam alignment. The results showed similar voltage‐transmittance characteristics to those of a rubbed cell and better dark state.

P-174: In-Plane Switching LC Cell Fabricated by Ion-Beam Treatment

We investigated the horizontal alignment characteristics such as surface morphology, pretilt angle, and polar and azimuthal anchoring energy by treating the thin film surfaces coated with organic alignment materials by ion beam. We changed the energy and incident angle of ion beam. We also fabricated an in-plane switching (IPS) cell by ion beam alignment. It shows similar voltage-transmittance characteristics to those of a rubbed cell and better dark state.

Transflective Liquid Crystal Display with Single Cell Gap and Single Gamma Curve

The vertical electric field-driven transflective nematic liquid crystal display with dual orientation has a problem that the voltage-dependent transmittance and reflectance curves do not match each other. Thus a dual driving circuit is required. This study shows that optimization of the surface pretilt angle in the transmittance solves this problem so that the transflective display with a single cell gap and single gamma curve for reflective and transmissive region is possible.

Four-domain twisted vertically aligned liquid crystal pixels using microrubbing

Microrubbing (μ-rubbing) technique is used to create four-domain twisted vertically aligned liquid crystal pixels for wide viewing angle displays. A metallic sphere of 1 mm diameter is used to locally rub a homeotropic polyimide surface to create a four-domain pixel. We use μ-rubbing to accurately control the surface pretilt angle of the liquid crystal from the normal, which directs the liquid crystal molecules to respective direction on applying a voltage. We report on twisted vertically aligned configurations with symmetric isointensity curves, high contrast, and attractive electro-optic performance. The optical simulations of the liquid crystal director orientation, viewing angle characteristics and switching behavior of the four-domain are also presented.

High pretilt four-domain twisted nematic liquid crystal display by microrubbing: Process, characterization, and optical simulation

A microrubbing (μ-rubbing) technique was utilized to create four-domain alignment in a liquid crystal display sample. A small metallic sphere under sufficient load was used to directly rub a homeotropic polyimide alignment layer to create a large surface pretilt angle. We demonstrate a 47-μm linewidth and a variable pretilt angle with respect to the substrate plane for different loads. The homeotropic polyimide surface was μ rubbed in such a way that neighboring alignment regions are rubbed in opposite directions. liquid crystal cells were constructed with two μ-rubbed substrates where the rubbing directions are orthogonal to each other. With this cell configuration we obtained four domains consisting of two left-handed and two right-handed twisted nematic subpixels. We report on the electro-optic performance properties, viewing angle characteristics, the relationship between pretilt angle versus load, and thermal aging study of the four-domain liquid crystal cell. The optical simulations of the liquid crystal director orientation, viewing angle characteristics, and the contrast ratio of the four domains are also presented. The experimental results are in agreement with the simulation results.

P-127: Multidomain Twisted Vertically Aligned Display by Microrubbing and its Simulations

A microrubbing (μ-rubbing) technique is used to create multidomain vertically aligned liquid crystal pixels for wide viewing angle liquid crystal display (LCD) applications. A metallic sphere of 1 mm diameter is used to rub a homeotropic polyimide surface to manipulate the local pretilt angle to create a four-domain pixel configuration. We use μ-rubbing to accurately control the surface pretilt angle of the liquid crystal from the normal, which directs the liquid crystal molecules to respective direction on applying a voltage. We report on the twisted vertically aligned (TVA) configurations showing symmetric isointensity curves, high contrast and attractive electro-optic performance parameters. The viewing angle and electro-optical characteristics are simulated and compared with the experimental results.