Continuous viewing angle controllable patterned vertical alignment liquid crystal display

A continuous viewing angle switchable patterned vertical alignment liquid crystal display (PVA-LCD) with circle polarisers is proposed. In wide viewing angle (WVA) mode, the viewing cone with contrast ratio larger than 1000:1 is extended to 50° and the contrast ratio larger than 100:1 is extended to 80°. Meanwhile, the viewing cone with contrast ratio more than 10:1 is limited to 40° in all direction for narrow viewing angle (NVA) mode. The viewing angle of the display can be controlled by applying a bias voltage on the common electrode. With the increasing bias voltage, the viewing angle cone with contrast ratio more than 10:1 can be changed from 40° to 20°. The proposed PVA-LCD has high transmittance (~93%) under the circle polarisers because the dead zone in the normal PVA-LCD with linear polarisers is eliminated.     

Effects of rubbing angle on maximum transmittance of in-plane switching liquid crystal display

Measurements of the maximum transmittance of an in-plane switching liquid crystal display showed that it increases as rubbing angle is increased from 10 to 20°. This dependence was analysed in terms of the local variation of electric field intensity between electrodes, which in turn makes liquid crystal at various positions between the electrodes rotate to different angles. The local variation of electric field becomes prominent, especially in the case that the distance between the electrodes is much larger than the cell gap or electrode width.     

A transflective and viewing angle controllable blue-phase liquid crystal display

A polymer-stabilised blue-phase liquid crystal display (PSBP-LCD) with double-side in-plane switching (DS-IPS) electrode structure is proposed. This structure shows the transflective characteristics because the bottom electrodes are made by aluminium material. For transflective displays, it exhibits a well-matched voltage-dependent transmission and reflection curves through designing the width and gap of the top electrodes. When a bias voltage is applied on the top electrodes, it exhibits as a good viewing angle controllable display.     

Response mechanism of a vertical alignment mode driven by fringe-field switching

The response mechanism of a vertical alignment mode, driven by a fringe field, is investigated in detail using small-angle approximation. The flow effects can be ignored when using theoretical analysis. The period of the liquid crystal (LC) deformation in the transversal direction, instead of the lognitudinal direction, shows the cell gap effect on the response time in the LC layer's thickness. The authors' analytical results indicate that a liquid crystal display (LCD) mode with a small transversal period could provide a new method that gives a fast response.     

Surface polymer-stabilised in-plane field driven vertical alignment liquid crystal device

In-plane field driven vertical alignment device using a liquid crystal with positive dielectric anisotropy has been studied. In the device, the distance between inter-digital electrodes needs to be increased to achieve higher transmittance; however, such a design results in an increase in operating voltage and slower response time. In this work, we use polymer stabilisation technique, which generates surface tilt angle other than 90^o, to improve upon these drawbacks. As a result, the proposed device shows lower operating voltage and faster response time while keeping transmittance at the same level, compared to those prior to polymer stabilisation.     

Mixed-field-switching liquid crystal mode using in-plane and fringe fields self-adjusted by bottom floating electrode for transmittance enhancement

We demonstrate a liquid crystal (LC) mode switched by mixed electric fields of in-plane and fringe fields, which are self-adjusted by adopting a bottom floating electrode for enhanced electro-optical properties. In our LC mode structure, conventional in-plane switching (IPS) electrodes are formed as pixel electrodes and common electrodes on an insulating layer and floating electrodes that are patterned per the sub-pixels. When the areas of the pixel and common electrodes are identical, the voltage of the bottom floating electrode is spontaneously determined to be half the value of the pixel voltage, which ideally generates symmetric fringe fields with both pixel and common electrodes. Due to the in-plane fields additionally generated between the pixel and common electrodes, the proposed LC structure operates by mixed-field switching (MFS), which shows higher transmittance than fringe-field switching (FFS) and IPS LC modes. Transmittance of the conventional FFS and IPS LC modes is highly sensitive to the in-plane electrode’s width (w) and spacing (l) condition, but the proposed MFS LC mode shows good transmittance without degradation with large variations of the in-plane electrode’s spacing-to-width ratio (l/w).     

Preparation and characterization of photo-decomposable ester-containing semi-alicyclic polyimide alignment layers for potential applications in in-plane switching thin-film transistor liquid crystal display devices

Poly(amic acid)s (PAAs) alignment agents have been prepared from the alicyclic dianhydrides, including 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA, I), 1,3-dimethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride (DMCBDA, II), and ester-linked aromatic diamines, including 4-aminophenyl-4′-aminobenzoate (APAB, a) and bis(4-aminophenyl)terephthalate (BPTP, b), respectively. The derived PAAs, including PAA-I_a (CBDA-APAB), PAA-I_b (CBDA-BPTP), and PAA-II_a (DMCBDA-APAB) exhibited the preferable molecular weights, while the PAA-II_b (DMCBDA-BPTP) showed the low one due to the low polymerization reactivity both for the DMCBDA dianhydride and the BPTP diamine. All the PAA solutions except PAA-II_b were adopted as the alignment components to fabricate liquid crystal (LC) minicells with a mode of in-plane switching (IPS). The polyimides (PI) alignment layers derived from the thermal dehydration reaction of the PAA precursors at 230°C for 30 min showed good alignments effects to the LC molecules, which exhibited the pretilt angles (θ_p) from 0.09 to 0.15° after being exposed by the linear polarized ultraviolet light sources with the wavelength of 254 nm. In addition, the PI alignment layers afforded good optoelectronic features to the minicells, including the voltage holding ratio values over 97% at room temperature, and the residual direct circuit voltages lower than 1.0 V. The anchoring energy results indicated that the PI-II_a (DMCBDA-APAB) alignment layer showed the highest interaction with the LC molecules, and thus exhibited the highest threshold voltage (V_th) in the voltage-transmittance (V-T) measurements for the minicells.     

Fast switching and luminance-controlled fringe-field switching liquid crystal device for vehicle display

ABSTRACT

Liquid crystal displays (LCDs) for vehicle displays should exhibit a fast response time in wide temperature range and wide-viewing angle in horizontal and downward directions without grey-scale inversion but limited brightness in the upward direction because the display images can be reproduced in the front window glass of a vehicle, affecting driver’s front visibility. Currently, fringe-field switching (FFS) liquid crystal device is widely commercialised for high resolution and wide-viewing-angle LCD; however, it needs to improve response times and limit the display brightness in the upward direction. As a solution, we propose a homogeneously aligned liquid crystal device in which liquid crystal director does tilt as well as twist deformation in a confined area by both vertical- and fringe-electric fields, exhibiting about two times faster decay response time than that of conventional FFS mode with suppressed luminance in the upward direction. The proposed liquid crystal device can be applied to LCDs for vehicle displays.     

Improvement of asymmetric viewing angle properties in single-domain fringe-field-switching liquid crystal mode by using parallel-rubbed alignment surfaces

In this paper, we discuss the viewing angle properties of single-domain fringe-field-switching (FFS) liquid crystal (LC) mode aligned by using parallel-rubbed polyimide surfaces. Due to the reduced initial tilting angle distribution in the bulk LC layer under parallel-rubbed surface alignment conditions, the problems of greyscale inversion and off-axis colour shift in the dark state and luminance asymmetry distribution in the low grey level, observed in the conventional single-domain FFS LC mode, can be improved effectively. The viewing angle properties of the proposed structure were analysed by using the Póincare sphere and fringe-field-induced LC distribution.     

Synthesis of novel soluble rubbing-resistant polyimides used as liquid crystal vertical alignment layers

A novel functional diamine N12, containing triphenylamine moiety, biphenyl, tert-butyl substituents and long alkyl chain, N,N-bis(4-aminophenyl)-p-(3,5-di-tert-butyl-4-dodecyloxy phenyl) aniline (N12), was synthesised and characterised. A series of polyimides (PIs) were prepared based on 2,2?-bis(trifluoromethyl)benzidine, 4,4?-oxydiphthalicanhydride and different contents of N12 via a conventional two-step procedure that included a ring-opening polyaddition to gain polyamic acids, followed by chemical cyclodehydration. The chemical structures of the intermediates, diamines and PIs were characterised by Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy. All of the PIs were amorphous and exhibited good solubility in both polar aprotic solvents and some low boiling point solvents. PIs containing different content of N12 could induce highly uniform vertical alignment of liquid crystals (LCs). More importantly, PIs derived from N12 showed a good rubbing resistance. Besides, all PI films showed high transmittance in the wavelength range 400–700 nm and good thermal stabilities. Such PIs could be suitable candidates for alignment layers used in the manufacture of high-performance vertical alignment mode LC displays.     

Nematic tribenzosilatranes

The linkage of benzylsilatrane groups to laterally functionalized mesogens allows the investigation of molecular systems which contain propeller shaped mesogens and exhibit nematic phase behaviour.     

Investigation of alignment direction in wide view film and rubbing angle of twisted nematic liquid crystal display mode

Wide view (WV) film is an important material in the polariser. It is a hybrid-aligned, discotic liquid crystal (DLC) on an alignment layer on the tri-acetyl cellulose film, which is used as compensation film in twisted nematic (TN) mode liquid crystal display (LCD). The relation between the alignment direction of the DLC in the WV film and the rubbing direction of the polyimide on the glass substrate in the TN mode LCD was investigated. The results indicated that the contrast ratio (CR) of the TN mode LCD can be increased by adjusting this angle . When an 88°?rubbing angle was used in this work, the CR of the TN mode LCD could reach 1000:1 and the horizontal and vertical viewing angles were 170 and 160°, respectively.     

Liquid crystals comprising hydrogen-bonded organic acids I. Mixtures of non-mesogenic acids

Stable nematic phases are reported for binary mixtures of p-alkylbenzoic acids (nBA; where n = 1,2,3 denotes methyl, ethyl and propyl), p-alkoxybenzoic acids (nOBA, n = 1,2), and p-fluorobenzoic acid, which separately do not exhibit liquid crystalline phases. The mesophase stability increases in equimolar mixtures of acids having incomensurate lengths; the larger the difference in the tail substituents in the mixed organic acids, the broader the nematic phase. The trends in excess nematic range exhibited by the H-bonded supramesogens in binary mixtures parallels the behaviour of covalent mesogens. This is indicative of an influence of heterodimers comprising H-bonded pairs of the two different acids in equimolar mixtures of the non-mesogenic acids. It would appear that dimerized organic acids constitute a viable molecular-structural 'bread-board' for rapid screening for potential mesogen constituents.     

Optimisation of alignment materials for minimising residual retardation in in-plane switching liquid crystal display

The light leakage in a black state of in-plane switching (IPS) liquid crystal display (LCD) associated with rubbing process has been investigated. The mechanical rubbing process with a cloth caused orientation disorders in the liquid crystal directors and these partial orientation disorders result in residual retardations of the IPS LCD, causing the light leakages at the black state. In this study, we theoretically estimated how the light leakage is associated with the rubbing uniformity using 2 × 2 Jones matrix equation and also experimentally confirmed how it is associated with structural properties of the alignment layer. The light leakage was clearly reduced in the alignment layer with reduced crystallinity and flexibility.     

Photoracemization broadening of selective reflection and polarization band of glassy chiral-nematic films

Glass-forming liquid crystals consisting of a cyclohexane central core with (S)-1-phenyl-ethylamine and (4-cyanophenyl)naphthalene pendants were synthesized as hosts for racemizable (R)-dinaphtho[2,1-d:1',2'-f][1,3]dioxepin, a chiral dopant. Chiral-nematic films 14, 22, and 35 mum thick were prepared for thermal and photoinduced racemization at temperatures from 95 to 130°C, i.e. in the mesomorphic temperature range, over a period of hours to days. Spatially modulated photoracemization was accomplished with an insignificant contribution from the thermal process at temperatures around 100°C over a period of up to 3 h. With an absorbance per unit thickness of 6.2 mum^-1 at 334 nm, the photochemical process was essentially confined to the irradiated surface, thereby setting up counter-diffusion of the two enantiomers through the film, and hence the pitch gradient as visualized by atomic force microscopy. The significantly widened selective reflection band was interpreted with the Good-Karali theory extended for a gradient-pitch film. Furthermore, the bandwidth was found to increase with decreasing racemization temperature or with increasing film thickness, further validating the presence of a pitch gradient as a result of controlled photoracemization.     

Ferroelectric liquid crystals from banana-shaped achiral molecules with vinyl end groups

Banana-shaped achiral compounds, the 1,3-phenylene bis[4-(alkenyloxyphenyliminomethyl)benzoate]s, were synthesized with varying length of the alkenyl group; their ferroelectric properties are described. The smectic mesophases, including a switchable chiral smectic C (SmC*) phase, were characterized by differential scanning calorimetry, polarizing optical microscopy and the triangular wave method. The presence of vinyl groups at the ends of the linear side-wings in the banana-shaped achiral molecules, containing a Schiff's base mesogen, induced a decrease in melting temperature and formation of the switchable SmC* phase in the melt. The compound having the octenyloxy group exhibited a spontaneous polarization of 120 nC cm^?2 on reversal of an applied electric field.     

Photo-induced alignment behaviour of polymerisable liquid crystals on bisazide in a polymer matrix

Photo-reactive bisazide in a polymer matrix containing acryloyl groups on the side chain was investigated as a photoalignment layer for polymerizable liquid crystals (PLC). We found the thin film of bisazide (2,6-bis(4-azidobenzylidene)-4-methyl-1-cyclohexanone) in a polymer matrix, irradiated by linearly polarised ultraviolet light (LPUVL), was able to homogeneously align PLC. The LPUVL irradiation dose changed the orientation direction of the PLC on the thin film of bisazide in the polymer matrix. In addition, the direction of the slow axis for the retardation of the photoalignment layer changed from parallel to perpendicular to the LPUVL electric field with the irradiation dose. From these results, it was suggested that the PLC was likely to be aligned along the slow axis of the retardation of the photoalignment layer. We concluded that the key mechanism that changed the direction of the slow axis in a plane was the photoreaction of azide–acrylate at low irradiation dose and that of bis(benzylidene)cyclohexanone at high irradiation dose. Although the photoalignment as a result of a simple photo cross-linking was previously little known except for photo-dimerisation, we revealed that the photoaddition of azide–acrylate is able to achieve the photoalignment.     

Development of non-reactive fluorine-rich biphenyl molecules and their incorporation into a PDLC system

Novel highly fluorinated molecules containing biphenyl moieties were synthesized via N,N ′-dicyclohexylcarbodiimide esterification reactions. Chemical structures were confirmed by FTIR, NMR and elemental analysis. These non-reactive fluorinated molecules were added in increasing concentrations to a pentaacrylate-based polymer dispersed liquid crystal formulation and thin films were made by homogeneous illumination with a white light source. Refractive index and dynamic scattering measurements, in addition to direct polymerization attempts, indicate that the fluorinated compounds are non-reactive in the photopolymerization process. Electro-optic measurements suggest that the fluorinated molecules preferentially occupy the interface between the crosslinked polymer matrix and the liquid crystal domains. As a consequence, threshold voltages are lowered and relaxation times increased.     

Investigation on newly designed low resistivity polyimide-type alignment layer for reducing DC image sticking of in-plane switching liquid crystal display

Soluble polyimide-type alignment layer is widely used in in-plane switching (IPS) liquid crystal display (LCD) because of its excellent reliability owing to high imidisation ratio during long-term driving, high voltage-holding ratio and low ion density. Nevertheless, it exhibits slow direct current (DC) discharging property due to its high resistivity, causing significant DC image-sticking problem. In this study, we doped inorganic salt to control the resistivity of soluble polyimide-type alignment layer and found that this approach reduced DC image sticking greatly without any loss of reliability property in IPS-LCD.     

A simple method to measure the twist elastic constant of a nematic liquid crystal

We demonstrate a simple method for measuring the twist elastic constant (K₂₂) of a nematic liquid crystal (LC). By adding some chiral dopant to an LC host, the LC directors rotate 180° in a homogeneous cell, which is known as 180° super-twisted nematic (STN) cell. By preparing two such STN cells with different chiral concentrations and measuring their Fréedericksz threshold voltages, we can obtain the K₂₂ and helical twisting power simultaneously. In the whole process, there is no need to measure the pitch length. Our obtained K₂₂ values agree well with those reported by using other methods.