Pretilt Angle Research Articles

Tunable pretilt angle based on gelator-doped planar liquid crystal cells

ABSTRACT This study demonstrates a simple approach for controlling the pretilt angles of liquid crystals (LCs) using gelator-doped cells fabricated with homogeneously aligned substrates. The competition between the gelator-induced spontaneous vertical alignment and planar alignment layer coated on the substrates results in various pretilt angles of LCs. The LC pretilt angle can thus be controlled from a homeotropic to planar state by adjusting the anchoring energy of the planar alignment layer and gelator concentration used for doping. A no-bias pi cell with multistable property is demonstrated by utilizing the proposed method.

Магнитоориентационный отклик ферронематика с наклонным сцеплением директора и намагниченности

The article considers orientational transitions induced by a magnetic field in a highly dispersed suspension of ferromagnetic particles in a nematic liquid crystal – ferronematic. We research the case when, in the absence of an external magnetic field between the directions of the liquid crystal director and magnetization of the impurity particles, a constant pretilt angle is maintained, and the coupling of the liquid crystal molecules to the surface of the ferroparticles is assumed to be soft and planar. An example of such an impurity is ferromagnetic carbon nanotubes magnetized at a certain angle to their long axes. On the basis of the continuum theory, the equilibrium values of the orientation angles of the liquid crystal director and magnetization are calculated for different values of the pretilt angle. The results of calculations are compared with the previously considered case of planar coupling between the director and magnetization. Analytical expressions are obtained for determining the orientational and magnetic structure of the ferronematic in the case of weak magnetic fields. The magnetization of the ferronematic is studied for different values of the pretilt angle.

Enhanced Electro-optical Properties of Low Viscous Nematic Liquid Crystal Doped with Mixed Phase Anatase/Rutile TiO<sub>2</sub> Nanoparticles for Display Applications

Organic–inorganic composite based on liquid crystalline and TiO2 nanoparticles were obtained and investigated taking into account the crystallographic form of TiO2 i.e., anatase/rutile mixed phase. TiO2 is an important class of material having various dielectric and electro-optical properties. The existent research presents the electro-optical properties of nematic liquid crystal (NLC) E204 and TiO2 nanocomposites over an extensive range of frequencies. Various important display parameters such as response time, threshold voltage, pretilt angle and activation energy of pristine as well as TiO2 doped composites systems were measured and analyzed. In comparison with the pure, TiO2-doped composite systems has approximately 50% faster response time, owing to its remarkable decline in the relaxation time and activation energy of the LCs. The alteration in the optical intensity of the NLC composites as a function of the concentration of TiO2 nanoparticles (NPs) was also examined. It was established that the optical intensity in nano-nematic composites was decreasing with the concentration of TiO2 NPs. Also, it was observed that an escalation in the TiO2 NPs concentration in NLC composites indicates to an escalation in the birefringence. Probable mechanisms of the interactivity between NLC molecules and TiO2 NPs have been discussed. The current work shows that the TiO2 NPs doping has an encouraging application in the display devices, including other electro-optical as well as photonic applications.

Soft imprint lithography for liquid crystal alignment using a wrinkled UVO-treated PDMS transferring method

Soft imprint lithography for liquid crystal (LC) alignment using a poly(dimethylsiloxane) (PDMS) wrinkled structure formed by UV–ozone (UVO) treatment was investigated in this study. Oxidation of the PDMS surface formed a silica-like outer layer that caused elastic moduli mismatch with the inner PDMS layer, leading to the formation of a wrinkled structure. This wrinkled PDMS was used as a stamp for LC alignment via soft imprint lithography. The wrinkled structure was transferred to the polyimide alignment layer and then LC cells were fabricated using the wrinkled substrates. The pretilt angle and electro-optical properties of the LC cells fabricated in this manner had performance similar to that of LC cells fabricated using the conventional rubbing method.

Application of Bombyx mori Silk Fibroin Films for Liquid-Crystal Devices.

Biocompatible and biodegradable silk fibroin films show promise as an eco-friendly biomaterial with excellent mechanical, thermal, and optical transparency properties. In contrast, polyimide (PI) films adopted in the liquid-crystal display (LCD) industry for aligning LC molecules are synthesized using toxic chemicals, which are nonrecyclable and nonbiodegradable. In this work, Bombyx mori silk fibroin films are fabricated from the aqueous solution and applied as alignment films for LCDs. The thermal properties of the prepared regenerated silk fibroin materials under different heat treatment temperatures are investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The crystallinity of different heat-treated silk fibroin materials is determined by infrared spectroscopy. The silk fibroin film treated at a higher temperature exhibits better thermal stability due to the higher crystallinity of the β-form structure. The LCDs using silk fibroin alignment films show a low pretilt angle of 0.5° and an anchoring energy of ∼10-3 J/m2 similar to those of the conventional polyimide films.

Study on the effect of small molecule photosensitizer and photoinitiator on alignment behavior of photosensitive polyimide

ABSTRACT In order to improve the photosensitivity of photosensitive polyimide (PSPI) and greatly reduce the exposure time of non-polarised ultraviolet light (NPUVL). A novel PSPI (ODPA-PI-CA) with flexible main chain is successfully synthesised via esterification of PI containing hydroxyl groups with cinnamoyl chloride (CL). A photoinitiator, 2, 4, 6-trimethylbenzoyldiphenyl phosphine oxide (TPO) and a photosensitiser, benzophenone (BP) are doped into ODPA-PI-CA film to prepare ODPA-PI-CA+BP and ODPA-PI-CA+TPO films. When the irradiation time is the same, LC alignment ability of the ODPA-PI-CA+BP films is higher than that of ODPA-PI-CA+TPO films. The ODPA-PI-CA+BP can induce uniform alignment of liquid crystal (LC) after NPUVL exposure for only 5 min. In addition, the possible mechanism of alignment of LC induced by ODPA-PI-CA+BP films is discussed in detail. Furthermore, the pretilt angle of the LC cells is 0.5°.

Rational Analysis of Self-Alignment Force of Reactive Additive Monomer with Multiple Methods

Analysis self-alignment force (SAF) of reactive self-assambly additive monomer is highly valuable in self-alignment vertical alignment mode(SAVA), because this force directly influences performance of optoelectronics device. In this work, SAF has been studied by multiple methods, such as atomic force microscopy, mueller matrix imaging polarimeter. Reactive additive monomer and LC molecular were immobilized on the surface of an AFM probe and ITO, respectively. Due to the complexity of the additive monomer-hydroxy, additive monomer-LC moluclar interaction, parallel experiments were designed to discriminate specific interactions. The difference of pretilt angle between TFT side and CF side also reveals the difference of SAF at various structure cell.The additive with strong SAF showed well alignment state after forming into optoelectronice device.

Shear Induced TiO2 Nano Structure Using Brush-Coating for Liquid Crystal Alignment

We have developed a very useful and cost-effective liquid crystal (LC) alignment layer of brush-coated TiO2 that is solution-processable for twisted nematic (TN) LC cells. TiO2 was prepared via the sol-gel method. The TiO2 solution was brush-coated on the substrate, followed by an annealing process. During the brush-coating process, a retracting force is generated on the deposited TiO solutions along the coating direction. The annealing process hardens the TiO2 and generates shearing stress arising from the retracting force along the brush-coating direction. The shearing stress created highly oriented nano/microstructure and uniformly aligned LCs with a stable pretilt angle of 0.6°. TN mode LC cells based on brush-coated TiO2 exhibited a performance of 12.5 ms of response and a threshold voltage of 1.8 V. Our brush-coated TiO2 incorporates two steps of the film deposition and alignment process into one step.

Preparation and performance analysis of novel liquid crystal alignment films based on macromolecular photosensitiser and photosensitive polyimide

ABSTRACT A photosensitive polyimide (BF-CA) and a macromolecular photosensitiser (BB-CA) are successfully synthesised. The BF&BB-CA blend films based on BF-CA and BB-CA can induce uniform alignment of liquid crystal (LC) after the non-polarised ultraviolet light (NPUVL) exposure for 15 min. After exposure for the same time, LC alignment ability of the BF&BB-CA blend films is higher than that of BF-CA films and BB-CA films. The alignment performance of LC and pretilt angles induced by the BF&BB-CA films after treatment at 120°C for 4 h are nearly unchanged, indicating that the prepared films have excellent thermal stability. Furthermore, the pretilt angle of the LC cells is 0.5°, which can be used in In-Plane Switching mode LCD.

Origin of oblique optical axis of electrically tunable focusing lenses arising from initial anisotropic molecular tilts under a symmetric electric field. I

The optical axes of electrically tunable liquid crystal (LC) lenses are usually tilted, and the corresponding asymmetric wavefront aberrations hinder the lenses from being diffraction-limited. Looking back at the literature studies since 1979, researchers used different approaches to compensate the tilting of the optical axis for achieving a perfect lens, or they utilized the intrinsically asymmetric wavefront aberration as a physical-planar free-form optics. However, the physics behind axis tilting has not been discussed yet, and the origin of the oblique optical axes of LC lenses in terms of anisotropic properties of molecules requires an investigation. In this paper, we study the origin of the asymmetrical tilting of optical axes of LC lenses. We found that the initial anisotropic molecular tilts (the so-called pretilt angle) result in the dielectric torque difference even under a rotationally symmetric electric field. Moreover, it is discovered that the rotational symmetry of the wavefront can be broken by generating uneven tilt angles of the LC molecules even though the electric potential is rotationally symmetric. Numerical results are presented and discussed to illustrate the mechanism.

A Study on Alignment Process of Ion Beam Irradiation to Improve LCD Productivity in Electric Installation Application

In this study, we fabricated TN-LCD using DuoPIGatron ion beam with obliquely irradiated polyimide film and TN-LCD using conventional rubbing method, and compared the electro-optic characteristics. In the TN-LCD using the rubbing method, a pretilt angle of 0.05° was obtained, and it was confirmed that an excellent pretilt angle of 1.13° also occurred in the TN-LCD irradiated with an ion beam. In addition, voltage-transmittance characteristic, which is a characteristic for realizing gray scale, was measured using the TN-LCD manufactured as described above. And, the response time characteristic was measured as a basic feature to compose a screen without motion blur. As a result, both the TN-LCD using the rubbing method and the TN-LCD manufactured by ion beam irradiation showed good voltage-transmittance characteristics in the driving voltage range of 1.5–3.5 V. In particular, as a result of measuring the response time characteristics, the light-on time was measured at 5.11 ms and the light-off time was measured at 10.98 ms. This is a very good result at the level of TN-LCD without chiral dopant, which can be evaluated as comparable with the result of the production level.

Liquid crystal-photopolymer composite films for label-free single-substrate protein quantitation and immunoassay.

Conventional liquid crystal (LC)-based biosensing at the LC-glass interface requires the assembly of an LC cell formed by two glass substrates with an LC film sandwiched in between. As most biochemical and clinical assays are performed on a single solid substrate, the feasibility of a single-substrate biodetection platform based on a thin film of LC-photopolymer composite was explored in this study. The LC mixture, consisting of nematic LC, E7 or AY40-006, doped with a small amount (≤ 5 wt%) of a photocurable prepolymer was spin-coated on a glass substrate modified with dimethyloctadecyl[3-trimethoxysilyl)propyl] ammonium chloride (DMOAP), a vertical alignment reagent, followed by irradiation with ultraviolet (UV) light. During the photopolymerization process, the accumulated and polymerized NOA65 at the LC-glass interface weakened the anchoring strength of DMOAP, resulting in a decrease in the pretilt angle of LC and allowing the LC molecules to be more easily disturbed in the presence of biomolecules, compared with vertically aligned LC in the absence of polymerized NOA65. Incorporating NOA65 in the LC film therefore provides a means for signal amplification. When an LC-photopolymer composite film consisting of AY40-006 and 4-wt% NOA65 was exposed to UV at 15 mW/cm2 for 30 s and utilized as the biosensing mesogen, the limits of detection were 1.6 × 10-12 g/ml for the direct detection of bovine serum albumin (BSA) and 2.1 × 10-8 g/ml for the immunoassay of the cancer biomarker CA125, significantly lower than those detected with AY40-006 alone or AY40-006/NOA65 mixture without UV irradiation. The results from this study offer a compelling implication on the biomedical application of LC-photopolymer composites in label-free and single-substrate biodetection.

P‐158: Generation of Pretilt Angle and an Alignment Layer Utilizing Binary Mixture in PI‐less Vertical Alignment Mode

We demonstrate new approach of polyimide (PI)‐less polymer stabilized vertical alignment (PS‐VA) mode in which binary mixture of single kind of reactive mesogen and LC induces a VA with temperature treatment. The induced VA is fixed at first UV exposure and then a pretilt angle is generated with second UV exposure. The proposed device with two‐step UV exposure exhibits excellent electro‐optics such as fast response time and enhanced on‐state transmittance. This approach fixes a vertical alignment physically unlike chemical bonding in conventional PI‐less VA mode, giving rise to high reliability.

P‐153: High Performance Photo‐aligned Surface of Vertical Alignment Liquid Crystal Displays for Mass Production

Photoalignment technology has been proven useful in the patterned alignment display industry. For large area display, most of the available in‐plane switching (IPS) mode TVs or panels use photoalignment technology. Not only it allows free pattern alignment direction to the liquid crystal compared to the conventional rubbed polyimide, but also it provides high yield and stability. However, the cost of the photoalignment technology could be very high. The materials often require polarized deep UV irradiation with a finite dosage to generate pretilt angles. Due to the improper control of rubbing on the alignment layer, the low yield rate problem still exists in the passive LCD world, especially for vertical photoalignment LCDs. It would be ineffective to improve the yield rate problem with this technology. Furthermore, the required materials are not common in the market. In view of the situation, a Vertical Alignment (VA) surface by non‐polarized LED light irradiation with a large wavelength is proposed and demonstrated in this paper. The cost of the LED exposure equipment is relatively more affordable. In a series of factory pilot runs on the proposed alignment characteristics, it shows comparable result to conventional polyimides in terms of performance. It achieves a stable and uniform pretilt angle, with high anchoring energy and fast response time. To conclude, the proposed robust alignment layer could benefit the industry with a better performance at low cost.

66‐3: The Way to Improve Black Circle Mura in Curved Display by Polyimide‐less Technology

In the fabrication of curved diplay, there always exist black circle mura, which is induced by the curvature. To improve this phenomenon, polyimide‐less technology was introduced. Polyimide‐less technology is a technology in which an additive can replace polyimide film to align LC molecules. In this technology, the additive added in liquid crystal (LC) host can not only align the LC molecules, but also have different pretilt angle generation ratio compared with PI film. In this paper, we fabricated curved panels using CF substrate with PI and TFT substrate without PI.The additive in PI‐less liquid crystals could anchor the TFT substrate and align the LC molecules on the TFT side. After UV process, the CF substrate had smaller pretilt angle. And the black circle mura appeared in the curved panel could be solved completely. Then we also studied the alignment, optical and image sticking performance, there existed no difference between the curved panels using PI‐less technology and the curved ones with normal PI layers.

P‐156: Distinguished Poster: Curved Vertical‐alignment Liquid Crystal Display with Uniform brightness: from Pixel Design to Verification

Curved liquid crystal displays (LCD) suffers from an issue of uneven brightness among the vertical‐alignment (VA) LCD panels based on thin glass substrates. In this work, we investigared its origin through optical simulation and successfully realized uniform brilliance for 27‐inch curved LCD panels. The optical simulation revealed that the dark areas on the bending sides of the curved LCD panel originated from the reduced azimuth angle of the liquid crystals (LC) next to the ITO trunk. This issue could be resolved by decreasing the pretilt angle of the LC on the color filter (CF) side. Through developing pretilt angle tuning (PAT) technologies, we experimentally verified that the uniform brilliance could be achieved for 27‐inch curved LCD panels with 900‐mm curvature on thin glass substrates.

P‐136: A Low Voltage, Switchable, Bistable‐Twist‐Nematic Display

The most common electronic paper technology is E‐ink, which enables many novel applications such as e‐readers, watches and electronic shelf labels. However, due to the TFT backplane driving and complex production processes, the cost of this technology is high, which limits its popularity. Recently, a low‐cost π‐bistable twist nematic (π‐BTN) liquid crystal display with a high pretilt angle is proposed. It has the same structure and production method as an ordinary liquid crystal display. Passive driving enables the manufacture of high‐resolution bistable displays without the use of thin‐film transistors. Compared with the current bistable twist nematic display technology, the high pretilt angles structure helps to achieve a low‐voltage and fast‐respond bistable display. This wide viewing angle, high contrast and TFT‐free display is a promising candidate for flexible displays.

P‐157: Rational Analysis of Self‐Alignment Force of Reactive Additive Monomer with Multiple Methods

Analysis self‐alignment force (SAF) of reactive self‐assembly additive monomer is highly valuable in self‐alignment vertical alignment mode(SAVA), because this force directly influence performance of optoelectronics device. In this work, SAF has been studied by multiple methods, such as atomic force microscopy, mueller matrix imaging polarimeter. Reactive additive monomer and LC molecular were immobilized on the surface of an AFM probe and ITO, respectively. Due to the complexity of the additive monomer‐hydroxy, additive monomer‐LC moluclar interaction, parallel experiments were designed to discriminate specific interactions. The difference of pretilt angle between TFT side and CF side also reveals the difference of SAF at various structure cell. The additive with strong SAF showed well alignment state after forming into optoelectronic device.

Preparation of polyimide alignment films with high photosensitivity and low solid content

ABSTRACT In order to reduce the cost, a polyimide (BF-CA) with high photosensitivity and low solid content was prepared by one-step method using 2,2-bis (3-amino-4-hydroxyphenyl) propane (BAP), 4,4ʹ- (hexafluoroisopropyl) phthalic anhydride (6FDA) as monomer and cinnamoyl chloride as photosensitive group. The BF-CA films exhibit good alignment performance after non-polarised ultraviolet light (NPUVL) irradiation for 20 minutes. Meanwhile, the effect of solid content on the alignment performance is mainly discussed, because reducing solid content helps to reduce costs. In addition, the BF-CA shows good thermal stability. The pretilt angle of the liquid crystal cells is 0.5°, which meet the requirements of practical application in In-Plane Switching liquid crystal display.

Polymer-sustained-alignment liquid crystal cell produced from the monomer containing benzophenone-moiety and its image-sticking property

ABSTRACT As a model of polymer-sustained-alignment liquid crystal display (PSA-LCD), the LC cells having the polymer layers produced from mixed monomers of 4,4ʹ-dimethacryloyl-oxy-biphenyl (4,4ʹ-DMABiph) and 4,4ʹ-dimethacryloyl-oxy-benzophenone (4,4ʹ-DMABenz) were fabricated and investigated image-sticking phenomenon with evaluations of residual direct current voltage (V rDC) and difference in pretilt angle before and after application of alternate current voltage (∆tilt). A rate constant of the polymerisation became large with an addition of a small amount of 4,4ʹ-DMABenz into 4,4ʹ-DMABiph. The V rDC was effectively improved with the addition of a small amount of 4,4ʹ-DMABenz because the concentrations of radical and ion in the LC layer were decreased due to enlargement for the rate constant of the polymerisation. The Δtilt was small enough only below 14% of 4,4ʹ-DMABenz in the mixed monomers of 4,4ʹ-DMABiph and 4,4ʹ-DMABenz. We confirmed that the PSA-LC cell produced from the mixed monomers of 4,4ʹ-DMABiph and 4,4ʹ-DMABenz with the weight ratio of 4,4ʹ-DMABenz between 9% and 14% was useful for obtaining the small level of image sticking.