Pretilt Angle Research Articles

Alignment properties of liquid crystal on DLC film using ion beam

Uniform alignment of the liquid crystals is necessary for high quality liquid crystal displays (LCDs). In this study, the rubbing-free method was carried out for the modification of the DLC (Diamond Like Carbon) surface by Ar + ion beam bombardment. By ion beam conditions, such as the energy and fluence of the incident ions at incident angle of 45º, the unidirectional out-of-plane liquid crystal pre-tilt angle is changed. Also, we measured the surface microstructure changes by ion beam using XPS, Raman spectroscopy and AFM analysis. Crystal rotation method was used for measuring the pre-tilt angle of LC and we checked voltage–transmittance (V–T) to evaluate the electro-optical characteristics. Also, the changes of the structural property were analyzed with AFM analysis. LC alignment and electric-optical characteristics which were obtained through LCDs cell alignment by ion beam treatment technology were confirmed that it is identical and suitable to the rubbing method to apply industrial application.

Tunable lens by spatially varying liquid crystal pretilt angles

We report a method of obtaining controllable spatially varying liquid crystal pretilt angles using a stacked alignment layer. The stacked alignment layer consists of nano-domains of horizontal and vertical alignment materials. The pretilt angle is controlled by varying the domain ratio of the two layers. By using photoalignment material as the top layer, the pretilt angle can be controlled by varying the UV light dosage. A spatially variable UV light beam can be used to control the pretilt angle spatially. An electrically tunable-focus liquid crystal lens is obtained using this method.

Eliminating optical bounce of homeotropic liquid crystal cells with sputtered silicon dioxide alignment films by rubbing treatment

In this article, we propose a mechanical rubbing method to eliminate the optical bounce and decrease the response time of homeotropic liquid crystal (LC) cells with sputtered silicon dioxide alignment films. After gently rubbing the inorganic alignment film (sputtered silicon oxide, SiO 2), the decreasing polar anchoring energy results in slightly smaller pretilt angle which gives LC a toppling direction to shorten the transition time while LC cell is applied by an external electric field. The results of our work show that this rubbing remedy can suppress the optical bounce. This simple and efficient surface treatment could be applied to liquid crystal on silicon (LCOS) technique for high resolution microdisplays and portable electronic products.

Perfluoropolymer as Planar Alignment Layer for Liquid Crystal Mixtures

We report a perfluoropolymer (CYTOP) provides planar alignment to liquid crystal mixtures (e.g., ZLI-2293) that are used in display. We found that the anchoring energy of ZLI-2293 on CYTOP is almost eighteen times less and the pretilt angle is slightly larger than on AL-1254. It is expected that the low surface anchoring and high transmission in CYTOP cell will make CYTOP a suitable aligning agent for LC displays as well as many other LC devices.

Vertical Alignment of Liquid Crytals on Polydimethylsiloxane Thin Films

Studies on the use of polydimethylsiloxane to align liquid crystals have been carried out. The thin films prepared using the silicon-based elastomer, with a pretty low surface free energy, are found to be capable of vertically aligning liquid crystals. The pretilt angle of calamitic liquid crystals sandwiched between thin silicon elastomer films was virtually 90 degrees. Liquid crystal devices consisting of elastomer alignment layers can operate in the in-plane-switch mode. Full text: PDF

Van der Waals force contribution to the vertical alignment of liquid crystal on Al2O3 films using ion-beam method

This study demonstrates liquid crystal (LC) alignment on Al2O3 films using the ion-beam (IB) method as well as the optimization of the IB irradiation condition. Uniform LC alignment was achieved at an IB incident energy of 1800eV with an IB incident angle of 45°, while inferior LC alignments were observed in other tested conditions. The pretilt angles and transmittances of the LC cell were also shown as part of the same trend for the LC alignment states. This result was subject to van der Waals forces which were caused by topographical changes during the IB process.

An Optical Method for Pretilt and Profile Determination in LCOS VAN Displays

Pretilt angle and cell thickness are two extremely important parameters in predicting the behavior of vertically aligned negative nematic liquid crystal (LC) displays. The accurate estimation of pretilt angle and cell thickness is not a trivial task when these devices work in reflective mode, as in liquid crystal on silicon (LCOS) vertically aligned nematic (VAN) displays. Usual experimental setups are based on the proportionality between the retardation of the polarization components of the incident light and the product effective birefringence times thickness. However, any attempt to separate the two product variables is cancelled out by symmetry from reflection. This work shows a relatively simple method capable of separating both variables, allowing accurate, independent measurements of pretilt and thickness, as well as other configurations details, such as residual twist. A simulation model based on the properties of actual reflective displays has been developed. An experimental setup specifically designed for measuring LCOS VAN cells has been prepared. Initial comparisons between experimental measurements of intensity and theoretical results showed some discrepancies that could be explained assuming that the LC profile contains a residual twist from the manufacturing process. Including that twist in the model, an excellent agreement between theory and experiment has been achieved. Matching simulations and experimental results yield separate determinations of pretilt angle and thickness, and give good estimates for the residual twist angle.

The effect of surface alignment on analog control of director rotation in polarization stiffened SmC* devices

The effect of surface alignment on the achievement of analog (“V”-shaped) electric field control of director rotation in SmC* liquid crystal devices is investigated experimentally and through numerical modeling. Ferroelectric SmC* liquid crystals are intrinsically analog and thresholdless, i.e. the director can be rotated freely around the tilt cone. Whether or not a SmC* liquid crystal cell exhibits thresholdless switching depends strongly on the influence of the cell’s alignment layers, on the magnitude of the liquid crystal’s spontaneous polarization, and on whether smectic layers adopt a bookshelf or chevron configuration. To study the effect of the surface alignment layers, we have exploited a technique for the vertical (bookshelf) alignment of the smectic layers that does not depend on surface anisotropy. The alignment technique allows an experimental study of the influence of surfaces spanning a wide range of pretilt angles, azimuthal and zenithal anchoring energies. This technique is used to study the effect of surfaces on the threshold behavior of director rotation in SmC* materials under the influence of an electric field. The alignment technique also allowed us to use a high-PS liquid crystal material having an I-A-C phase sequence and reduced layer shrinkage thought to be well suited to thresholdless switching. We show that the alignment layer has a strong effect, and that excellent analog response can be achieved for the case of alignment layers which promote homeotropic director orientation. We further model and discuss the potential effect of a thin layer of nematic at the surface and the possibility of gliding of the easy axis during switching.

A Mean-Field Photoreaction Model for the Pretilt Generation of a Liquid Crystal on Photopolymer Layers upon Ultraviolet Exposure

We present a mean-field photoreaction model for the pretilt generation of a nematic liquid crystal (NLC) on the surfaces of photopolymers upon the exposure of ultraviolet (UV) light. The angular distribution function of photopolymer side chains, calculated in the photoreaction model, is used for determining the surface anchoring energy within the Rapini–Papoular approximation. The pretilt angle transition from the homeotropic alignment to the planar alignment of the NLC is demonstrated in two photopolymers with different alkyl chain lengths as a function of the UV exposure time. The main features of the experimental results agree well with theoretical predictions.

A Mean-Field Photoreaction Model for the Pretilt Generation of a Liquid Crystal on Photopolymer Layers upon Ultraviolet Exposure

We present a mean-field photoreaction model for the pretilt generation of a nematic liquid crystal (NLC) on the surfaces of photopolymers upon the exposure of ultraviolet (UV) light. The angular distribution function of photopolymer side chains, calculated in the photoreaction model, is used for determining the surface anchoring energy within the Rapini–Papoular approximation. The pretilt angle transition from the homeotropic alignment to the planar alignment of the NLC is demonstrated in two photopolymers with different alkyl chain lengths as a function of the UV exposure time. The main features of the experimental results agree well with theoretical predictions.

Effects of backbone conformation and surface texture of polyimide alignment film on the pretilt angle of liquid crystals

Polyimides (PIs) with different inclination angle of polymer backbones, together with polar hydroxyl group and/or nonpolar trifluoromethyl group at various sites of the backbone were synthesized and used as liquid crystal alignment layers. The molecular conformation, surface chemistry, surface energy, surface morphology, and pretilt angle of the PI film were investigated. The distributions of fluorinated group and hydroxyl group at different depths of the PI surfaces were analyzed by X-ray photoelectron spectroscopy. Effects of the conformation of the PI molecular backbone on the surface morphology of the rubbed PI layer, the pretilt angle and surface energy of the alignment film were studied. The PI which contains both nonpolar fluorinated groups sticking out of the surface and the polar hydroxyl groups on the surface exhibits high pretilt angle.

Vertically aligned liquid crystal molecules on a silicon nitride film treated by ion-beam irradiation

Vertical alignment (VA) of a liquid crystal display (LCD) device with a stable pretilt angle has been achieved on ion-bombarded silicon nitride (SiN x) surfaces. The surface energy variation of the SiN x surface with the bonding conversion achieved via ion beam bombardment makes it possible to control the liquid crystal (LC) pretilt angle. X-ray photoelectron spectroscopy revealed that the vertical orientation of the LC molecules occurs on the SiO 2 surface and that this orientation is directionally transformed upon ion bombardment of SiN x thin films. Moreover, the upgraded electro-optical characteristics of VA-LCDs showed potential for high-performance LCD applications on modified SiN x surfaces.

Stability in the memory state of the silica nanoparticle-doped hybrid aligned nematic device

We investigate the stability in the memory state of the silica nanoparticle doped hybrid aligned nematic (SN-HAN) cell. The mixed polyimide (PI)-coated planar substrate provides the SN-HAN cell with a stable memory state. The mixed PI comprises the homogeneous PI and small amounts of the homeotropic PI (H-PI). The tiny H-PI dopant decreases the surface energy, increases the roughness of the planar substrate, and increases the pretilt angle of the liquid crystals (LCs). When the pretilt angle is high, the relaxation torque that rewinds the LCs from the electrically addressed homeotropic state to the originally HAN state is too small to break the formed aggregated silica networks, which stabilize the LCs at the electrically addressed homeotropic state. Consequently, the memory state of the SN-HAN cell is stable when the pretilt angle of the LCs is high.

Development of Novel Polyimide Alignment Films for Liquid Crystal Display Televisions

We developed novel organic-solvent-soluble polyimide (PI) alignment films with a steroidal structure for liquid crystal display (LCD) televisions (TVs). The PIs can generate any desired pretilt angles between 3 - 90 °, which are needed to realize LCD-TVs such as twisted nematic, optically compensated bend, and vertical alignment LCD modes. The PIs also showed very stable pretilt angles, together with good electrical properties. The developed PI alignment films have been widely used for large screen size LCD-TVs with the display performance such as high contrast, fast response time, and wide viewing angle.

基于4-辛氧基联苯酚-3,5-二氨基苯甲酸酯的聚酰亚胺的合成及性能研究

The first series of polyimides with different ratio of side chain were copolymerized from 4-octyloxy-biphenyl-3,5-diaminobenzoate(C8),3,3′-dimethyl-4,4′-methyl-enedianiline(DMMDA) and 4,4-oxydiphthalic(ODPA) via one-step method.The effect of the diamine content of these PIs on their solubility,alignment,pretilt angle and thermal stability was investigated.Similarly,the second series of polyimides with different main chain were copolymerized from C8,DMMDA and different dianhydrides which include ODPA,pyromellitic dianhydride(PMDA),3,3′,4,4′-dibenzophenonetetracarboxylic dianhydride(BTDA) via one-step method.The effect of dianhydride structure on the solubility and thermal stability was explored.

Effects of alignment layer thickness on the pretilt angle of liquid crystals

Mixture solutions of vertical- and planar-aligning polyimide precursors were coated on bare glass. The concentrations of the solutions were varied to control the thicknesses of the films. The resulting blend films were baked to induce imidization and then rubbed. The thicknesses (t) of the blend film and of the pure vertical-alignment film affected their surface energies; the pretilt angle can be fully controlled in the range 5.5°≤Θ0≤87° by adjusting t. The surface energy of pure planar-alignment layers was independent of t.

Electrically switchable liquid crystal Fresnel lens using UV-modified alignment film

A simple method to make a switchable liquid crystal (LC) Fresnel lens with high diffraction efficiency and a low driving voltage was proposed based on the photo-induced surface modification of the vertical alignment layer. UV illumination alters the pretilt angle of alignment layers, a Fresnel zone-distribution hybrid alignment in the homeotropic LC cell can be straightforwardly achieved through UV exposure, yielding a concentric structure of the Fresnel phase LC lens. A remarkable diffraction efficiency of ~31.4%, close to the measured diffraction efficiency of the used Fresnel-zone-plate mask of 32%, was detected using a linearly polarized incident beam.

Correlation of pretilt angles and surface chemical structures of polyimide alignment films after direct fluorination

A series of polyimides (PIs) with side chains were fabricated from 6-(4-phenylphenoxy)hexyl-3,5-diaminobenzoate of various ratios with 4,4′-oxydianiline and 4,4′-oxydiphthalic anhydride. After PI alignment, films were directly fluorinated using a 10 vol% fluorine/nitrogen mix. The pretilt angle of liquid crystals on the PI films is much improved to more than 20°, and can be continuously controlled by changing the side-chain content of the PI films from 20 to 100%. The change of the pretilt angles is ascribed to the fluorinated microstructure of molecules at the surface rather than to the surface energy or surface morphology. Moreover, the improvement of rigidity and the steric repulsion of the fluorinated side chains result in an upright conformation of the side chains that leads to the high pretilt angle. Copyright © 2010 Society of Chemical Industry

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.

Continuous Pretilt Angle Controlled No-Bias-Bend Pi Cell via Blended Polyimide Liquid Crystal System

The controllable pre-tilt angle of liquid crystals was investigated using a blend of horizontal and vertical polyimide prepared using a rubbing method. Various pretilt angles ranging from 0° to 90° were achieved as a function of the vertical polyimide content. We observed uniform liquid crystal alignment on the rubbing-treated blended polyimide layer. A no-bias-bend (NBB) pi cell with an intermediate pretilt angle of 47.8° was manufactured. This cell had no initial bias voltage and a low threshold voltage, which indicates that it has low power consumption. In addition, the response time of the NBB pi cell was rapid.