Rubbed Alignment Layer Research Articles

Backflow-assisted time-resolved phase modulation in nematic liquid crystal Pi-Cells

By monitoring the time-dependent phase modulation behavior, we demonstrate the benefits of backflow on the phase modulation depth of nematic liquid crystal (LC) pi-cells in comparison to the phase modulation observed for other nematic LC device configurations. Specifically, results are presented for the time-resolved phase modulation of three different nematic LC device configurations under a range of electric field conditions: pi-cell (parallel-rubbed alignment layers), Fréedericksz cell (anti-parallel rubbed alignment layers), and a hybrid aligned nematic (HAN) device. The time-dependent behavior is obtained experimentally with a Michelson interferometer with a piezoelectric scanning mirror in the reference arm, which provides a continuous phase ramp in the reference signal. By recording the interference signal intensity, it is then possible to extract the change in optical phase as a function of time. It is shown that both the pi-cell and Fréedericksz cell exhibit a modulation that is greater than π radians for a double-pass configuration (2π radians for a four-pass configuration), with the lowest voltage amplitude corresponding to π phase modulation being observed for the pi-cell. Through the time-resolved response, it is shown that the presence of backflow hinders the dynamic phase response of the Fréedericksz device but does not negatively impact the behavior of the pi-cell. These results are found to be in good agreement with simulations of the phase modulation for the three nematic devices when backflow is taken into consideration.

Gradient pretilt angle alignment materials with different photosensitivity for tunable polarization‐independent self‐aligned liquid crystal lens

AbstractAlignment of benzaldehyde materials with different photosensitivity thresholds, capable of changing the pretilt angles of nematic liquid crystal (LC) from 90° to 0° in a controlled manner under UV‐B exposure, has been developed. Inhomogeneous exposure of uniform rubbed alignment layers allows the formation of a refractive index gradient inside the LC cell. The concept of tunable polarization‐independent self‐aligned LC lens based on gradient pretilt angle alignment materials with different photosensitivity is demonstrated. Self‐alignment of two polarization‐dependent sub‐lens is achieved due to a single exposure act of two alignment layers, which are located on the same piece of glass on both sides, forming one common optical axis for a polarization‐independent LC lens. The sub‐lens cells have uniform cell gap and are independently controlled using low‐voltage driving. Devices based on gradient benzaldehyde alignment materials can be used in many modern optical and photonic devices.

Syntheses and Properties of New Smectic Reactive Mesogens and Their Application in Guest-Host Polarizer

We designed and synthesized new smectic liquid crystalline monomers containing photopolymerizable acrylate groups and a mesogenic core unit composed of two benzene rings, one pyrimidine-based heteroaromatic ring and an acetylene linking group. The structures of monomers were characterized by 1H nuclear magnetic resonance spectroscopy, and their mesomorphic behaviors were investigated by differential scanning calorimetry and polarized optical microscopy. Both of monomers showed an enantiotropic phase transition behavior with exhibiting both nematic phase and highly ordered smectic phase. Both of monomers showed smectic phases during the cooling process in the temperature range of 144.2–94.0 °C and 154.0–85.8 °C, respectively. Neither of these compounds showed a homogeneous alignment even at various temperatures, and therefore, their 50:50 mixture was chosen for the preparation of guest-host mixture because of its lowest crystallization point, wide smectic phase temperature range and superior aligning property. Polymerizable “host-guest” mixture formulated from the host liquid crystalline monomer mixture, dichroic dye and additives was injected into the sandwiched glass substrates having rubbed alignment layers at the isotropic temperature. Subsequent in-situ photopolymerization by UV irradiation at room temperature successfully resulted in a thin film polarizer with good polarizing properties. The fabricated guest-host polarizer showed a dichroic ratio (DR) of 13.1 and a degree of polarization (DOP) of 95.5% with the thickness of 5 µm.

The effect of monomer and chiral dopant content on reverse-mode polymer stabilized cholesteric liquid crystal display

Liquid crystal (LC)/polymer composites have great application potential in many fields. LC/polymer composites can be classified into normal mode LC/polymer composites and reverse-mode LC/polymer composites (RLCPCs). As an important application, reverse-mode polymer stabilized cholesteric LC displays (RPSCLCDs) is widely studied. In this paper, a bifunctional monomer (BPEFDA) which has V-shaped polymerizable branched chains and a monofunctional monomer (OPPEA) has been introduced. The effects of total monomer content, ratio of different monomers and chiral dopant content on the electro-optical properties of RPSCLCDs without any surface treatment are studied. The sample with the best comprehensive performance is obtained in the experiment by optimizing the percentage of monomer and chiral dopant. Compared with the traditional RPSCLCDs with rubbed alignment layers, RPSCLCDs without any surface treatment have the simple fabrication processes, low saturation voltage, small hysteresis, and fast response speed.

GRADIENT ALIGNMENT MATERIALS WITH DIFFERENT PHOTOSENSITIVITY FOR TUNABLE POLARIZATION-INDEPENDENT LIQUID CRYSTAL LENS

The development of electrically tunable liquid crystal (LC) lenses is perspective and promising for a wide range of applications, for example, for imaging system, pico projectors, optical zoom systems, ophthalmology applications and other. Of particular note is the development of polarization-independent LC lenses, as eliminates polarizers from application devices that reduce the efficiency of light transmission through optical systems. Alignment benzaldehyde photosensitive materials, capable of changing the pretilt angles of nematic LC from 90 to 0 ºС in a controlled manner under UV exposure are developed. The anisotropy of the benzaldehyde alignment layers is generated by a two-stage treatment consisting of uniform rubbing with a cloth and subsequent non-polarized UV exposure. Inhomogeneous UV exposure of uniformly rubbed alignment layers allows formation of refractive index gradient inside the LC cell. The concept of tunable polarization-independent self-aligned LC lens based on gradient pretilt angle alignment materials with different photosensitivity is demonstrated. Self-alignment of two polarization-dependent sub-lens is achieved due to a single UV exposure act of two alignment layers, which are located on the same piece of glass on both sides, forming one common optical axis for a polarization-independent LC lens. The independence of the polarization of LC lenses is achieved by setting the azimuthal rubbing direction of the alignment layers of two polarizationdependent LC lenses perpendicular to each other. The sub-lens cells have uniform cell gap and are independently controlled using low-voltage driving. Devices based on gradient benzaldehyde alignment materials can be used in many modern optical and photonic devices.

Birefringence-dependent linearly-polarized emission in a liquid crystalline organic light emitting polymer.

We investigated the linearly polarized emission of uniformly aligned poly(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thia-diazol-4,8-diyl) (F8BT) with a liquid crystalline phase on a rubbed alignment layer. The polarization ratio, defined by the ratio of luminous intensities polarized parallel and perpendicular to the rubbed direction, gradually decreased with increasing thickness of the F8BT film. In the photoluminescence (PL) process, the polarized light is emitted throughout the whole F8BT film, while in the electroluminescence (EL) process, the polarized light is emitted at a certain region within the F8BT film. The thickness-dependent polarization ratios in both PL and EL processes were successfully described based on a simple model wherein the mean optical birefringence was expressed as a function of the thickness of the F8BT film.

Twisted-nematic-mode liquid crystal display with a DNA alignment layer

In this study, we investigated the use of biomaterials in optoelectronic applications. One of the most abundant biomaterials in nature is deoxyribonucleic acid (DNA). It was used in this study as an alignment layer to fabricate a twisted-nematic-mode (TN-mode) liquid crystal display (LCD). To create the LCD, a cheap DNA material extracted from salmon was used to coat an indium tin oxide glass substrate, and the DNA chains were aligned with a simple but effective mechanical shearing method at room temperature. Then small rod-type LC molecules were spread on top of the alignment layer, obliquely aligned with respect to the long axis of the DNA chains because of the polar and topographical interactions between the LC molecules and the DNA material. The electro-optical characteristics of the LCD device were examined to compare the properties of the DNA-based LCD with those of a TN-mode LCD fabricated with the conventional rubbed polymer alignment layer. We believe our method provides an easy and useful tool for the fabrication of biomaterial-based optoelectronic devices.

Anchoring strength of dual rubbed alignment layers in liquid crystal cells

Interaction between liquid crystals and alignment layers is of great scientific and technical importance, but is not fully understood. We studied the aligning effect of orthogonally dual rubbed alignment layers in liquid crystal cells. We experimentally observed that with proper numbers of rubbing in the two directions, the generated easy axis of the aligning direction lay between them. We measured the polar and azimuthal anchoring strengths of the alignment layers. We observed that the rubbing in the second direction significantly changed the azimuthal anchoring direction and strength but not the polar anchoring. We theoretically found that Rapini–Papoular formula with quadratic and quartic terms can predict the aligning behavior of the alignment layers. We successfully determined the quartic azimuthal anchoring coefficient. The results are of significance for the understanding of the aligning mechanism of alignment layers.

Effects of surface modification with reactive mesogen on the anchoring strength of liquid crystals

We studied the effects of polymer chain ordering in the alignment layer and resulting molecular interactions on the surface anchoring energy by introducing a reactive mesogen (RM) coating to the alignment layer. Directionally polymerized RMs on the rubbed alignment layer decrease the steric repulsion and increase the electronic interaction with liquid crystal molecules, and, as a result, the surface anchoring energy is enhanced in both the out-of plane and in-plane directions. We also demonstrated that this enhanced surface anchoring energy can be used to improve the response time characteristics of liquid crystal displays.

Symmetric-viewing liquid crystal display with alternating alignment layers in an inverse-twisted-nematic configuration

A symmetric-viewing inverse-twisted-nematic (ITN) liquid crystal display (LCD) with alternating alignment layers was developed using a stamping-assisted rubbing (SAR) technique. A patterned layer of a fluorinated acrylate polymer was transferred onto the first rubbed vertical-alignment layer prepared on a substrate by stamping. The fluorinated acrylate polymer provided a protective layer covering the first rubbed alignment layer during the second rubbing process, which promoted the vertical alignment of the LC molecules. The LC cell in the ITN geometry with two orthogonally rubbed alignment layers showed symmetric-viewing characteristics with fourfold symmetry. The SAR technique was shown to be a mask-free alignment method of producing multidomains for symmetric-viewing LCDs.

LCD 제조용 러빙포 물성에 따른 러빙된 폴리이미드 배향막의 특성 및 러빙효과 분석

러빙공정에서는 러빙 시 공정변수와 배향막의 특성 그리고 러빙포의 물성이 중요한 변수로 작용한다. 이러한 변수들은 러빙밀도(rubbing density)와 러빙력(rubbing force)에 의해서 결정되는 러빙된 정도(크기)를 통하여 배향막의 배향성에 영향을 미치게 된다. 본 연구에서는 러빙 변수 중 러빙포의 물성이 러빙밀도(길이)와 러빙력에 미치는 영향에 대하여 알아보기 위하여 다른 물성(파일 밀도, 강직도)을 가지는 러빙포를 이용하여 러빙효과(정도)를 분석하였다. 러빙포의 파일 밀도가 커지고 강직도가 강할수록 러빙된 배향막의 이방성과 표면조도가 커졌으나 러빙효과의 형태는 서로 달랐다. 러빙포의 파일밀도는 러빙에 관여하는 파일의 수와 밀접한 관련이 되어 있어 러빙하는 밀도(길이) 영향을 미치지만 강직도는 러빙밀도(길이)보다는 러빙력과 보다 밀접하게 관련성이 있었다.

P‐125: Wide‐viewing Twisted‐Nematic Liquid Crystal Display Using a Stamping‐assisted Rubbing Process

AbstractWe developed a wide‐viewing twisted‐nematic (TN) liquid crystal display (LCD) with two bidirectional alignment layers using a stamping‐assisted rubbing process. The bidirectionally rubbed alignment layer through a stamping and peel‐off process provides the multi‐domain alignment of the LC molecules. The patterned fluorinated acrylate polymer, used as a protective layer, was produced to cover the rubbed alignment layer during the second rubbing process for the bidirectional LC alignment. Our multi‐domain TN‐LCD shows wide‐viewing characteristics that are symmetric with respect to the rubbing directions.

Stamping‐assisted fabrication technique of the bidirectional alignment layer for wide‐viewing twisted‐nematic liquid crystal displays

A stamping‐assisted rubbing technique for generating bidirectional alignment in the fabrication of wide‐viewing twistednematic (TN) liquid crystal displays (LCDs) was developed. A patterned layer of a fluorinated acrylate polymer was transferred onto the first rubbed alignment layer prepared on a substrate by stamping. The fluorinated acrylate polymer provides a protective layer that covers the first alignment layer during the second rubbing process to facilitate the bidirectional alignment of the LC molecules. The LC cell in the twisted geometry with two bidirectional‐alignment layers showed stable electro‐optic properties and wide‐viewing characteristics. The stamping‐assisted rubbing technique serves as a mask‐free alignment method of producing multidomains for wide‐viewing LCDs.

Anisotropic optical constants of in-plane oriented polyfluorene thin films on rubbed substrate

We present a photometric method to determine the anisotropic optical constants of several aligned polyfluorene films. These polymers exhibit liquid crystal characteristic under heat treatment and polymer chains are preferentially in-plane oriented on a rubbed alignment layer. A self-consistent dispersion formula of Forouhi–Bloomer model is introduced to fit the measured polarized reflectance and transmittance curves by a global optimization algorithm. The very good agreements between the experimental and theoretical spectra allow us to shed light on the parallel and perpendicular components of optical constant. On this basis, light-emitting devices are fabricated using the anisotropic active films. The measured polarized electroluminescence spectra confirm the optical birefringence.

Fabrication of Single Substrate Flexible Twisted Nematic LC Cell Using a Photo-Polymer Cover Film

A single substrate flexible display was fabricated by laminating a thin cover film of UV curable photo-polymer on the plastic bottom substrate. The micro-walls of photoresist on the bottom substrate are tightly bound with the cover film to provide the solid mechanical support for preserving the molecular alignment of liquid crystals (LCs). The ability of aligning LCs is induced by rubbed alignment layer on the bottom substrate and micro-grooves formed on the photo-polymer film. We demonstrated a twisted nematic LC mode on the plastic substrate with in-plane switching electrode. The fabrication process and electro-optical characteristic of the device are described.

FFS 모드에서 표면 엥커링에너지에 따른 전기광학특성의 연구

Liquid crystal (LC) alignment using a photoalignment method by irradiation of polarized ultraviolet (UV) on an alignment layer has been investigated. Photoalignment method exhibits weaker anchoring energy than rubbing method so that we have studied electro-optic characteristics of fringe-field switching (FFS) mode with alignment layers using the photo and rubbing alignment methods. The cell using photo alignment layer shows lower threshold and operation voltage than those using rubbed alignment layer. Also, the former method shows higher transmittance than that of the latter.

Strongly Polarized and Efficient Blue Organic Light‐Emitting Diodes Using Monodisperse Glassy Nematic Oligo(fluorene)s

Monodisperse oligo(fluorene)s have been successfully employed for the construction of polarized blue organic light‐emitting diodes. Uniaxial alignment is accomplished by spin‐casting oligo(fluorene)s onto a rubbed alignment layer with subsequent thermal annealing. The resultant films yield the best set of performance data ever reported in terms of emission dichroic ratio, luminance yield, and chromaticity (see Figure).

Morphological and Transistor Studies of Organic Molecular Semiconductors with Anisotropic Electrical Characteristics

Oriented thin films of organic semiconducting small molecules were prepared by crystallization on rubbed alignment layers. Polarized absorption spectra showed that the long axis of the conjugated backbones was highly oriented along the rubbing direction and parallel to the substrates. Transmission electron microscopy and diffraction confirmed that the molecules and in many cases the resulting crystals are aligned. Using the above aligned films as semiconducting layers, we fabricated field-effect transistors having anisotropic mobilities with ratios greater than 15. Several common organic semiconductors have been investigated, and the results indicate that this growth method is generally successful for achieving macroscopic alignment of these semiconducting molecules (and frequently their crystals, as well).

3-2b ラビング処理条件による液晶配向膜の膜構造変化とプレチルト角の関係

3-2b ラビング処理条件による液晶配向膜の膜構造変化とプレチルト角の関係

Electrooptical Properties of Phase Separated Composite Organic Films of Ferroelectric Liquid Crystals

AbstractA new structure of a light modulating element based on ferroelectric liquid crystal (FLC) is fabricated. It is prepared by the photopolymerization induced phase separation of an initially isotropic solution of FLC and a prepolymer. The element consists of two successive layers parallel to the cell substrates. One layer is the isotropic polymer and the other is the FLC macroscopically aligned by a rubbed alignment layer. A study of electrooptical properties of these FLC elements reveals a continuous gray scale, contrast ratio 100:1 along the cell normal, line addressing times of microseconds in the active mode. Because of their internal structure, they are insensitive to mechanical stress.