High contrast ratio of a vertically aligned liquid crystal cell using photocrosslinking alignment

A photo-induced alignment layer for LCDs has been successfully fabricated using the polarized UV photoreaction of a photo-crosslinkable polymer with the incident UV light at some angle with respect to the cell normal. The surface alignment and electro-optic properties were investigated for various UV exposure times. The homeotropic alignment layer showed a discrete anisotropic dichroic ratio, its surface morphology became smoother as the UV exposure time increased, and the LCs were arranged in a perpendicular direction to the PUVL direction with a proper pretilt angle. The cell showed no defects under cross-polarized microscopy and the contrast ratio was as high as 550:1 in transmittance. The contrast ratio pattern was found to be very similar to that of dichroic ratio as a function of UV exposure time and depended upon the frequency change to some degree. The response time was also investigated.     

Photo-dimerization of a chalcone-based side chain polymer for the alignment of ferroelectric liquid crystals

The alignment and optical properties of ferroelectric liquid crystal cells, having alignment films of a chalcone-based side chain polymer treated by linearly polarized UV irradiation were investigated. The long absorption band of the UV/Vis spectra gradually decreased and the FTIR spectra shifted as the irradiation times increased, indicating that cyclo-addition and isomerization reactions of the chalcone-based side chains occurred. UV dichroism demonstrated anisotropic changes in the alignment films, with a maximum at low exposure energy (0.5 J cm^-2). Liquid crystal molecules were aligned perpendicular to the polarization direction of the linearly polarized UV radiation. The azimuthal anchoring energy of liquid crystal E7 on a chalcone-based side chain polymer surface increased with exposure energy. Well aligned defect-free cells and high contrast ratio were achieved with irradiation of longer than 5 min; the geometric conditions for a stable C2 structure may be satisfied at low temperature with slowly cooling.     

Photo-alignment of liquid crystals using a crosslinked discotic film

A new photo-alignment layer using a crosslinked discotic compound was demonstrated. This discotic compound exhibits excellent solubility in common organic solvents, the possibility of low temperature processing and good thermal stability. A linearly polarized long wavelength ultraviolet (LPUV) light (λ = 350 nm) was used to initiate the crosslinking process and induce liquid crystal alignment on the discotic film surface. The induced LC directors were found parallel to the electric field direction of the LPUV light. A 1.2° pre-tilt angle was achieved using a single exposure at a 30° oblique angle.     

Electro-optical characteristics of photo-aligned TN-LCD on PM4Ch surface

In this study, a photo-alignment material PM4Ch, poly(4-methacryloyloxychalcone), was synthesized. The electro-optical characteristics of a photo-aligned twisted nematic liquid crystal display (TN-LCD), with linearly polarized UV exposure at normal direction on a PM4Ch surface, were investigated. The NLC alignment was uniform. Excellent voltage-transmittance characteristics were obtained; a fast response time was also achieved. Reduction of d.c. voltage decreases with increasing UV exposure time.     

Electro-optical characteristics of photo-aligned TN-LCD on PM4Ch surface

In this study, a photo-alignment material PM4Ch, poly(4-methacryloyloxychalcone), was synthesized. The electro-optical characteristics of a photo-aligned twisted nematic liquid crystal display (TN-LCD), with linearly polarized UV exposure at normal direction on a PM4Ch surface, were investigated. The NLC alignment was uniform. Excellent voltage-transmittance characteristics were obtained; a fast response time was also achieved. Reduction of d.c. voltage decreases with increasing UV exposure time.     

Command surfaces 12 [1]. Factors affecting in-plane photoregulation of liquid crystal alignment by surface azobenzenes on a silica substrate

In-plane alignment of nematic liquid crystals was regulated by polarized-light-irradiation of a cell assembled with a silica plate, the surface of which was modified by attaching 4-hexyl-4'-hexyloxyazobenzene at its o-position through surface silylation. The photoisomerizability of the chemisorbed azo-chromophore was affected by their surface density and the nature of photoinactive co-modifiers. The efficiency of the photoregulation of liquid crystal alignment was optimized by two-dimensional dilution of the chromophore with ethyltriethoxysilane (ETS) or 3-aminopropyltriethoxysilane (ATS). As a result, favourable procedure was to modify a silica surface with a crude azo-silylating reagent contaminated by ATS. The rate of the photoinduced reorientation of liquid crystals was followed by monitoring the alteration of the alignment direction of a dichroic dye dissolving in a mesophasic layer upon exposure to linearly polarized light. Exposure energy for the in-plane reorientation of a liquid crystal was about 100mJcm^-2 of 445 nm light at elevated temperatures close to T_NI. The effect of positional isomerism on the photoregulation was determined with the use of 4-cyano-4'-hexyloxyazobenzene as a commander molecule. It was confirmed that the photoalignment efficiency was markedly enhanced by attaching the chromophore at the ortho- or meta-position. The exposure energy for the reorientation of a liquid crystal was reduced by linking the p-cyanoazobenzene at the meta-position so that the reorientation was complete with an exposure energy of 20mJcm^-2.     

Effect of chemical structures of polyimides on photosensitivity of liquid crystal alignment using a polarized UV exposure

Unidirectional liquid crystal (LC) alignment produced by polarized UV exposure was examined using polyimides (PIs) synthesized using different diamines. The dichroic ratio of the resulting LC cells suggests that the UV photosensitivity is primarily controlled by the chemical structures of the PIs used. The UV absorption and fluorescence spectra of the PI films, and molecular conformations of the diamines, indicate that the photosensitivity is controlled by the UV absorption efficiencies and molecular conformations of the PIs.     

Photo-induced liquid crystal alignment on polyimide containing fluorinated groups

Polyimides were prepared from pyromellitic dianhydride, 4,4'-bis[2-(4-aminophenyl)hexafluoroprop-2-yl]diphenyl ether and 4,4'-diamodiphenyl ether (PMDA-BDAF-ODA) and used for liquid crystal alignment using linearly polarized UV exposure. The alignment properties of a LC on the polyimide films were found to depend on the fluorine content in the PMDA-BDAF-ODA alignment layer and on the UV exposure time. Pretilt angles were obtained in the range 0° to 90° dependent upon the fluorine content in the polyimide film and the UV exposure time. These effects seem to be closely related to the surface energy of the photo-alignment layer.     

Large area liquid crystal monodomain field-effect transistors

Butyl, hexyl, and decyl derivatives of the liquid-crystalline organic semiconductor 5,5' '-bis(5-alkyl-2-thienylethynyl)-2,2':5',2' '-terthiophene were synthesized and studied with respect to their structural, optical, and electrical properties. By means of an optimized thermal annealing scheme the hexyl and decyl compounds could be processed into self-assembled monodomain films of up to 150 mm in diameter. These were investigated with X-ray diffractometry, which revealed a clearly single-crystalline monoclinic morphology with lamellae parallel to the substrate. Within the lamellae the molecules were found to arrange with a tilt of about 50 degrees with the rubbing direction of the polyimide alignment layer. The resulting, close side-to-side packing was confirmed by measurements of the UV/vis absorption, which showed a dichroic ratio of 19 and indicated H-aggregation. AFM analyses revealed self-affinity in the surface roughness of the monodomain. The compounds showed bipolar charge transport in TOF measurements, with hole mobilities reaching up to 0.02 cm(2)/Vs and maximum electron mobilities around 0.002 cm(2)/Vs. The hexyl derivative was processed into large-area monodomain top-gate field-effect transistors, which were stable for months and showed anisotropic hole mobilities of up to 0.02 cm(2)/Vs. Compared to multidomain bottom-gate transistors the monodomain formation allowed for a mobility increase by 1 order of magnitude.     

Zigzag defect-free alignment of surface stabilized ferroelectric liquid crystal cells with a polyimide irradiated by polarized UV light

Zigzag defect-free surface stabilized ferroelectric liquid crystal (SSFLC) cells were prepared using a photodegradable polyimide (PI) having a cyclobutane ring in the backbone. The PI layers were irradiated by polarized ultraviolet light (PUVL) at normal incidence to the surface, and characterized by UV and FTIR spectroscopy. The anisotropy originates from preferential cleavage of PI chains oriented parallel to the polarization direction of the irradiating PUVL. After the polarized UV light irradiation, the PI surface was much flatter than that after rubbing, but it induced a similar order parameter of dye-doped nematic LC molecules to that for a rubbed cell. Alignment of both the FLC molecules and the layer structure is important in SSFLC. After 40 min irradiation, the FLC molecules were well aligned homogeneously, and the FLC cells showed a uniform texture without zigzag defects which also indicates a well aligned layer structure. Zigzag defect-free alignment may result from the flatter surface, the much smaller and more constant pretilt angles, and the bigger cone angle than those achieved by rubbing.     

Zigzag defect-free alignment of surface stabilized ferroelectric liquid crystal cells with a polyimide irradiated by polarized UV light

Zigzag defect-free surface stabilized ferroelectric liquid crystal (SSFLC) cells were prepared using a photodegradable polyimide (PI) having a cyclobutane ring in the backbone. The PI layers were irradiated by polarized ultraviolet light (PUVL) at normal incidence to the surface, and characterized by UV and FTIR spectroscopy. The anisotropy originates from preferential cleavage of PI chains oriented parallel to the polarization direction of the irradiating PUVL. After the polarized UV light irradiation, the PI surface was much flatter than that after rubbing, but it induced a similar order parameter of dye-doped nematic LC molecules to that for a rubbed cell. Alignment of both the FLC molecules and the layer structure is important in SSFLC. After 40 min irradiation, the FLC molecules were well aligned homogeneously, and the FLC cells showed a uniform texture without zigzag defects which also indicates a well aligned layer structure. Zigzag defect-free alignment may result from the flatter surface, the much smaller and more constant pretilt angles, and the bigger cone angle than those achieved by rubbing.     

Response time mechanism for a photo-aligned vertical-alignment liquid crystal display on a homeotropic alignment layer

The electro-optical characteristics of the photo-aligned vertical-alignment liquid crystal display (VA-LCD) with a non-polarized UV exposure of 45° on homeotropic polyimide (PI) surface was investigated. The domain size of the photo-aligned VA-LCD increases proportionately with the UV exposure time. The LC alignment of the photo-aligned VA-LCD is attributed to photo-dissociation of the polymer by UV exposure on the homeotropic PI surface. Good voltage-transmittance characteristics of the photo-aligned VA-LCD without negative compensation film was measured. The response time of the photo-aligned VA-LCD was slow compared with a rubbing-aligned VA-LCD; this is considered to be due to the alignment of LC molecules.     

Response time mechanism for a photo-aligned vertical-alignment liquid crystal display on a homeotropic alignment layer

The electro-optical characteristics of the photo-aligned vertical-alignment liquid crystal display (VA-LCD) with a non-polarized UV exposure of 45° on homeotropic polyimide (PI) surface was investigated. The domain size of the photo-aligned VA-LCD increases proportionately with the UV exposure time. The LC alignment of the photo-aligned VA-LCD is attributed to photo-dissociation of the polymer by UV exposure on the homeotropic PI surface. Good voltage-transmittance characteristics of the photo-aligned VA-LCD without negative compensation film was measured. The response time of the photo-aligned VA-LCD was slow compared with a rubbing-aligned VA-LCD; this is considered to be due to the alignment of LC molecules.     

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.     

Alignment mechanism of a nematic liquid crystal on a pre-rubbed polyimide film with laser-induced periodic surface structure

A periodic surface structure was prepared on a pre-rubbed polyimide (PI) film surface with a pulsed UV laser polarized perpendicular to the rubbing direction. The experimental results demonstrate that the rubbing-induced molecular anisotropic orientation was relaxed by the pulsed laser irradiation, and the laser induced molecular orientation was perpendicular to the line of the laser-induced periodic structure. The dichroism of the anisotropy of molecular orientation increased with the increase of laser energy. Since the direction of the laser-induced molecular anisotropy was perpendicular to the surface groove direction of the pre-rubbed PI surface, the effects of surface microgroove and anisotropic molecular orientation of the PI chain on liquid crystal (LC) alignment can be distinguished from each other. LC alignment was investigated by evaluating the anchoring energy of the PI surface, which was calculated according to Berreman's theory using the twist angle of the LC in the cells. The experimental results demonstrate that the exact alignment direction of the LC molecules is determined by the relative strength of both factors.     

Determination of the molecular orientation of very thin films on solid substrates: surface liquid crystal layers and rubbed polyimide films

The molecular orientation of very thin films on solid substrates can be determined quantitatively by measuring the polarized infrared (IR) absorption spectra of samples as a function of angle of incidence. The quantitative molecular orientation is derived by fitting the incident angle dependence and the dichroic ratio with theoretical calculations. We applied this method to a technologically important system: liquid crystal (LC)/rubbed polyimide film. To understand the alignment mechanism of LC molecules in contact with rubbed polyimide films, we have quantitatively determined the molecular orientation of rubbed polyimide films and a surface LC layer in contact with a rubbed polyimide film. In this paper two relations are discussed: (1) correlation between the inclination angle of polyimide backbone structures in rubbed films and the pretilt angle of bulk LC in contact with them, and (2) relation among the molecular orientation of a rubbed polyimide film and those of surface and bulk LC layers in contact with it.     

Photoinduced liquid crystal alignment by layer-by-layer ultrathin films with dual photoreaction moieties

A novel layer-by-layer (LBL) film containing dual photoreaction groups, cinnamoyl and azobenzene, was prepared from poly(diallyldimethylammonium chloride) (PDDA) and a photosensitive polyanion, PCAzo, in aqueous solution via electrostatic attraction. The film was able to induce uniform alignment of liquid crystals (LCs) with good stability and 2.3° pretilt angle by oblique irradiation with linearly polarised ultraviolet light (LPUVL). UV absorption and FTIR spectroscopic results indicate that the photoreactions of the two photoreactive groups jointly participate in generating the anisotropy of the film. The dichroic ratio of the film was found to depend on the number of adsorbed layers. The thicker film has the larger dichroic ratio after the LPUVL irradiation. The reorientation behaviour of the LC molecules was found to be associated with the LBL film thickness. Experiment results revealed that the photo-crosslinking of the cinnamoyl groups was responsible for the stability of the anisotropic orientation, and the isomerisation of the azobenzene chromophores led primarily to the appropriate pretilt angle.     

Alignment mechanism of a nematic liquid crystal on a pre-rubbed polyimide film with laser-induced periodic surface structure

A periodic surface structure was prepared on a pre-rubbed polyimide (PI) film surface with a pulsed UV laser polarized perpendicular to the rubbing direction. The experimental results demonstrate that the rubbing-induced molecular anisotropic orientation was relaxed by the pulsed laser irradiation, and the laser induced molecular orientation was perpendicular to the line of the laser-induced periodic structure. The dichroism of the anisotropy of molecular orientation increased with the increase of laser energy. Since the direction of the laser-induced molecular anisotropy was perpendicular to the surface groove direction of the pre-rubbed PI surface, the effects of surface microgroove and anisotropic molecular orientation of the PI chain on liquid crystal (LC) alignment can be distinguished from each other. LC alignment was investigated by evaluating the anchoring energy of the PI surface, which was calculated according to Berreman's theory using the twist angle of the LC in the cells. The experimental results demonstrate that the exact alignment direction of the LC molecules is determined by the relative strength of both factors.     

Alignment of liquid crystals induced by a photopolymerized self‐assembled film

An alignment film derived from a photopolymerized self‐assembled film may be used to orient nematic liquid crystals after irradiating the film with linearly polarized UV (LPUV). A photosensitive cationic amphiphile was first synthesized containing two double bonds and which could be polymerized by UV. A layer‐by‐layer self‐assembled multilayer film was next prepared in an aqueous solution of the cationic amphiphile and poly(sodium 4‐styrenesulphonate); the UV‐Vis spectra showed that each layer of the LBL multilayer film was uniform. When the film was irradiated by LPUV, the photosensitive double bonds underwent [2+2] cycloaddition along the vector direction of LPUV. The polarized UV‐Vis absorption spectra also provided evidence that the film was anisotropic, i.e. the photopolymerization was along a certain direction. The anisotropic film was used as an alignment layer for nematic liquid crystals, and observations under a polarizing microscope indicated that the alignment of the liquid crystals was good, as expected, and that the orientation direction of the liquid crystals was always perpendicular to the electric vector of the irradiating LPUV.     

Large-scale periodic pattern in homeotropic liquid crystals induced by electric field

In-plane periodic modulation of the director field of an initially homeotropic liquid crystal layer with negative dielectric anisotropy is studied in external electric field with time varying amplitude envelope. Periodically turning a low frequency electric field on and off the cell, an in-plane periodic modulation of the director develops with a length scale that is orders of magnitude larger than the cell gap and can be as large as couple of centimetres. Doping the liquid crystal layer with low concentration of dichroic dye allows easy mapping of the director. We show that the periodic pattern is a periodic arrangement of +1 and ?1 point defects, so called ‘umbilics’. We argue that the in-plane direction of the director is governed by the local flow that accompanies the turn on and off of the electric field. The finite size and the shape of the cell, incompressibility constraint and the nature of the surface alignment material, all together with the flow, are ultimately responsible for the development of the observed periodic director modulation.