The optical response of liquid crystal cells to a low frequency driving voltage

This study investigates the optical response of liquid crystal cells to a low frequency square wave voltage of 0.1 Hz. It is found that there are three physical phenomena that dominate the overall properties of the device. The first is the discharging effect whereby the effective voltage over the liquid crystal layer decreases as a function of time; this occurs due to mobile ions being present within the liquid crystal material. The second is the charging-up of the cell where the effective voltage increases with time; this is attributed to charge separation taking place within the polyimide layer upon application of the d.c. voltage component. The third effect is cell asymmetry whereby the effective voltage depends upon the polarity of the externally applied field; this is the result of a locked-in d.c. holding voltage being present within the cell layers. These three effects are analysed in some detail with the view of developing a liquid crystal cell capable of being driven with a low frequency square wave voltage. A model of a liquid crystal cell in which the liquid crystal material can dissolve impurity ions from the alignment layers and in which the ions can then become re-adsorbed into the polyimide layer is deduced.     

Rollable ferroelectric liquid crystal devices monostabilized with molecularly aligned polymer walls and networks

This paper describes a flexible monostabilized ferroelectric liquid crystal (FLC) device using molecularly aligned polymer walls and networks, which are sequentially formed with a two-step photopolymerization-induced phase separation. When ultraviolet (UV) light was irradiated through a photomask onto a heated nematic phase solution of FLC and monomer, monomer molecules that had flowed into the irradiated areas were photopolymerized, and polymer walls aligned along the rubbing direction of the polyimide alignment layers on plastic film substrates were formed. After uniform UV irradiation without a photomask, polymer networks aligned along the rubbing direction were formed in the FLC. The FLC molecules were monostabilized in the rubbing direction by the strong anchoring of the polymer networks. The device, which was sandwiched between crossed polarizers, exhibited a monostable electro-optic characteristic with a high contrast ratio of over 100:1 and a response time of less than 1 ms. The FLC device exhibited spatially uniform operation even when it was rolled up with a radius of curvature of 1.5 cm.     

Liquid crystal alignment on a ZrO2 thin film as a function of ion beam incident angle

Nematic liquid crystal alignment capabilities and electro-optical performance characteristics have first been embodied in a ZrO₂ layer using ion-beam irradiation. The study demonstrates that liquid crystal layers can be aligned homogeneously as a function of the incident angle of the ion beam device, which causes a uniform dense plasma. X-ray photoelectron spectroscopy indicated that full oxidation of the ZrO₂ thin-film surfaces was produced by ion irradiation, shifting the Zr 3d spectra to lower binding energies. In addition, the electro-optical performance characteristics of twisted nematic cells on a ZrO₂ thin-film layer after ion beam irradiation showed similar characteristics to those of a rubbed polyimide cell.     

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.     

Photo‐aligning of polyimide layers for liquid crystals

A series of soluble and highly transparent semi‐alicyclic polyimides (PIs) with designed flexible linkages have been synthesized derived from an alicyclic aromatic dianhydride (1,2,3,4‐cyclobutanetetracarboxylic dianhydride, CBDA) and various aromatic ether‐bridged diamines. The semi‐alicyclic PIs were evaluated as the photo‐alignment layers of liquid crystal (LC) molecules in liquid crystal display (LCD). Experimental results indicate that the photo‐alignment characteristics of LC molecules induced by the photo‐aligned PI layers and the electro‐optical (EO) properties of the LC cell devices are closely related with PI backbone structures. The retardation of the photo‐aligned PI layers is correlated with the ultraviolet (UV) absorption intensity of PI at 220 to approximately 330 nm. The higher UV absorption intensity PI has, the higher retardation and lower pre‐tilt angle the photo‐aligned PI layer exhibits. The defect‐free and photo‐aligned PI layer could result into the uniform LC texture, which is highly desired for in‐plane switching (IPS) mode LCD devices. In comparison, PI layer containing trifluoromethyl moiety shows poor photo‐aligning performance because of the strong electronic withdrawing effect of the fluorinated linkage.     

Advanced electro-optical smart window based on PSLC using a photoconductive TiOPc electrode

This study demonstrates an electro-optical controllable smart window based on a liquid crystal device. The device integrates a photoconductive electrode based on TiOPc films into a liquid crystal shutter based on polymer-stabilised homeotropic aligned nematic liquid crystals (PSLC) connecting in series. The PSLC light shutter is transparent in electric field-off state; it scatters light as it switches to multi-domain structures in electric field-on states. The TiOPc film has an optically controllable impedance that manipulates the electric field on the PSLC film. The window transits into multi-transmission states according to the intensities of exposure light with an extra applied bias voltage.     

有机偏振发光材料及其成膜技术研究进展

本文对近年来有机偏振发光及其发光材料和成膜技术的发展进行了综述。重点从材料化学结构与偏振发光的关系出发,系统地讨论了几类发光材料的性质、定向排列成膜的方法及其有机偏振发光器件的制作和应用。     

Momentum space imaging of a liquid crystal filled Fabry-Perot

Liquid crystal layer characteristics have for some time been determined by analysis of the optical guided modes supported within such layers. A novel technique for guided mode analysis, relying on scattering and re-radiation of light from a liquid crystal filled Fabry-Perot, is presented here and compared with a more standard technique, where the transmissivity of light through such a structure, as a function of the angle of incidence, is measured. The new technique which we label 'momentum space imaging' is found to hold some advantages over traditional methods, with data acquisition on the millisecond time-scale and full liquid crystal layer characterization being achievable with minor modification to existing theoretical multilayer modelling.     

FT-IR study of liquid crystal molecules on alignment layers

Alignment control at the molecular level is crucial for realizing high-performance LCDs. In particular, the structure of the interface between the liquid crystal and the alignment layer must be clarified. By utilizing RAS (reflection-absorption spectroscopy), a highly sensitive FT-IR spectroscopic technique, we have obtained the following information about the orientation of liquid crystal molecules; (1) the cyano group in a liquid crystal molecule behaves like an electron donor with respect to an SiO layer, but like an electron acceptor with respect to a polyimide layer; (2) with the use of polarized IR spectroscopy, it was discovered that the liquid crystal 8CB is aligned with a slant away from the aligning direction, regardless of the type of alignment layer. The angle between the molecular short axis within the plane of the core and the surface normal was found to be smaller on polyimide layers than on SiO layers.     

Orientation control and fixation of discotic liquid crystal

We established hybrid, horizontal, vertical, and vertical-twisted alignment of discotic liquid crystals on web-coating thin film. Newly developed materials are the photopolymerizable discotic liquid crystals, in order to fix desired novel alignment, the alignment promoters that deposit toward air surface during web coating, followed by drying of solvent, and then accomplish desired alignment of liquid crystals, the alignment layers that promote desired alignment of liquid crystals from the substrate side and the chiral agents, which have strong chiral powers in small amounts. Hybrid aligned discotic liquid crystal thin film has proven quite effective in expanding the viewing angle of thin film transistor liquid crystal displays.     

A simple transflective liquid crystal display with composite dielectric layer

ABSTRACT

A simple transflective liquid crystal display with a vertically aligned cell using a composite dielectric layer is demonstrated. The top substrate has a top planar common electrode, two transparent dielectric layers with different dielectric constants are coated on the bottom planar pixel electrode to generate linearly varying electric potential from the transmissive region (T region) to the reflective region (R region), while two bumpy reflectors are coated under the bottom substrate. In this device, with the composite dielectric layer, the common and pixel electrodes generate a strong electric potential in the T region and a relatively weak electric potential in the R region. Consequently, the T and R regions accumulate the same electro-optical characteristics. The simulation results show that the display exhibits reasonably low operating voltage, high optical efficiency and well-matched voltage-dependent transmittance (VT) and reflectance (VR) curves. Besides, the driving mode and the fabrication process of the transflective liquid crystal display are fairly simple and it is suitable for mobile applications.     

Liquid crystal photo-alignment layers made from aromatic bismaleimides

We propose a new type of liquid crystal photo-alignment layer made from aromatic bismaleimides. Aromatic bismaleimides are expected to react anisotropically by irradiation of polarized ultraviolet (UV) light. In the fabrication process of the new photo-alignment layers the curing process is much shorter than that needed for conventional polymeric types of photo-alignment layers. We have proved that the use of aromatic bismaleimides enables liquid crystal molecules to be aligned homogeneously by irradiation of polarized UV light. In particular N , N '- p -phenylenebismaleimide shows good photo-alignment characteristics and high UV sensitivity. In studying the relationship between molecular structures of aromatic bismaleimides and photo-alignment properties, we find that aromatic bismaleimides that have a linear molecular structure and resonance structure produce good photo-alignment layers.     

Nature-inspired light-harvesting liquid crystalline porphyrins for organic photovoltaics

A new class of nanoscale light-harvesting discotic liquid crystalline porphyrins, with the same basic structure of the best photoreceptor in nature (chlorophyll), was synthesized. These materials can be exceptionally aligned into a highly ordered architecture in which the columns formed by intermolecular π-π stacking are spontaneously perpendicular to the substrate. The homeotropic alignment, well confirmed by synchrotron X-ray diffraction, could not only provide the most efficient pathway for hole conduction along the columnar axis crossing the device thickness, but also offer the largest area to the incident light for optimized light harvesting. Their preliminary photocurrent generation and photovoltaic performances were also demonstrated. The results provide new and efficient pathways to the development of organic photovoltaics by using homeotropically aligned liquid crystal thin films.     

Nature‐inspired light‐harvesting liquid crystalline porphyrins for organic photovoltaics

A new class of nanoscale light‐harvesting discotic liquid crystalline porphyrins, with the same basic structure of the best photoreceptor in nature (chlorophyll), was synthesized. These materials can be exceptionally aligned into a highly ordered architecture in which the columns formed by intermolecular π–π stacking are spontaneously perpendicular to the substrate. The homeotropic alignment, well confirmed by synchrotron X‐ray diffraction, could not only provide the most efficient pathway for hole conduction along the columnar axis crossing the device thickness, but also offer the largest area to the incident light for optimized light harvesting. Their preliminary photocurrent generation and photovoltaic performances were also demonstrated. The results provide new and efficient pathways to the development of organic photovoltaics by using homeotropically aligned liquid crystal thin films.     

Liquid crystal photo-alignment layers made from aromatic bismaleimides

We propose a new type of liquid crystal photo-alignment layer made from aromatic bismaleimides. Aromatic bismaleimides are expected to react anisotropically by irradiation of polarized ultraviolet (UV) light. In the fabrication process of the new photo-alignment layers the curing process is much shorter than that needed for conventional polymeric types of photo-alignment layers. We have proved that the use of aromatic bismaleimides enables liquid crystal molecules to be aligned homogeneously by irradiation of polarized UV light. In particular N,N′-p-phenylenebismaleimide shows good photo-alignment characteristics and high UV sensitivity. In studying the relationship between molecular structures of aromatic bismaleimides and photo-alignment properties, we find that aromatic bismaleimides that have a linear molecular structure and resonance structure produce good photo-alignment layers.     

A mathematical model for blade coating of a nematic liquid crystal

The standard industrial process of blade-coating is now being used to produce new liquid crystal displays (LCDs) in which a liquid crystal and optical layers are coated onto a substrate. Motivated by this new LCD manufacturing process, we use the Ericksen-Leslie equations to develop a simple mathematical model for blade coating of a nematic liquid crystal. The direction and uniformity of the director are important factors for the performance of the displays, particularly when this alignment is 'frozen in' within optical layers. For this reason we investigate the flow and director within a liquid crystal film both after emerging from the region under a blade (the so-called 'drag-out' problem) and before entering the region under a blade (the so-called 'drag-in' problem). We restrict our attention to thin films and small director angles, and we study two particular cases in which either orientational elasticity effects or flow effects dominate the alignment of the liquid crystal. We find that there is a unique solution of the drag-out problem, whereas there may be multiple solutions of the drag-in problem. When orientational elasticity effects dominate we obtain a simple analytical solution for the director. When flow effects dominate we find that the director is uniform in the bulk of the liquid crystal, which exhibits thin orientational boundary layers near the substrate and the free surface, within which the director orientation changes rapidly from its prescribed boundary value to the flow alignment angle. These boundary layers may be potential locations for the nucleation of defects.     

Monitoring spatial distribution of ethanol in microfluidic channels by using a thin layer of cholesteric liquid crystal

Monitoring spatial distribution of chemicals in microfluidic devices by using traditional sensors is a challenging task. In this paper, we report utilization of a thin layer of cholesteric liquid crystal for monitoring ethanol inside microfluidic channels. This thin layer can be either a polymer dispersed cholesteric liquid crystal (PDCLC) layer or a free cholesteric liquid crystal (CLC) layer separated from the microfluidic device by using a thin film of PDMS. They both show visible colorimetric responses to 4% of ethanol solution inside the microfluidic channels. Moreover, the spatial distribution of ethanol inside the microfluidic channel can be reflected as a color map on the CLC sensing layers. By using this device, we successfully detected ethanol produced from fermentation taking place inside the microfluidic channel. These microfluidic channels with embedded PDCLC or embedded CLC offer a new sensing solution for monitoring volatile organic compounds in microfluidic devices.     

Microtubule particle dispersion in liquid crystal hosts

Microtubule particles and metal-coated microtubules were dispersed in various host liquid crystal mixtures. Dispersion effects were evaluated as a function of liquid crystal type, viscosity, dielectric anisotropy and surface interaction. Experimental results indicated that all the types of liquid crystals studied were aligned perpendicular to the microtubule surfaces, regardless of liquid crystal composition or various surface coatings used on the metal-coated microtubules. Low concentrations of the metal-coated microtubules in nematic liquid crystal hosts were aligned by flow or cell surface alignment conditions, and could be modulated by electric or magnetic fields. We observed better microtubules dispersion uniformity in high viscosity liquid crystal host mixtures and in liquid crystal-monomers than in isotropic fluids. Microtubules particles dispersed in ROTN-404 liquid crystal mixture had a much higher birefringence in the microwave region than dispersion in a paraffin oil.     

Planar and twisted lyotropic chromonic liquid crystal cells as optical compensators for twisted nematic displays

We have explored the use of lyotropic chromonic liquid crystals (LCLCs) as promising materials for optical compensators for liquid crystal displays. We used aqueous solutions of disodium cromoglycate in the range of concentrations that yield a chromonic N phase. The N phase is aligned by rubbed polyimide layers giving planar orientation. Two different types of LCLC-based elements are described: (a) a uniformly aligned planar N phase that forms an optically uniaxial plate with a negative birefringence; (b) twisted cells of LCLC obtained by doping the N phase with chiral amino acids. Both planar and twisted N* phase cells were used for compensation of the positive birefringence of twisted nematic (TN) displays; the TN devices were appreciably improved. We achieved achromatic dark state and contrast ratio up to 50:1 at all directions within a 40° cone of viewing angle.     

Mechanical model for anisotropic curved interfaces with applications to surfactant-laden liquid-liquid crystal interfaces

A mechanical model for anisotropic curved interfaces, applicable to thermodynamically closed surfactant-laden liquid-liquid crystal interfaces is developed. The model takes into account the mechanical effects due to surface bending and surface tilting (anchoring) and incorporates liquid crystal anisotropy into classical fluid membrane mechanics. In the absence of the aligned liquid crystal, the model converges to the fluid membrane mechanical model, and in the absence of surfactant, it converges to the nematic interface mechanical model. Use of the well-known Helfrich-Rapini-Papoular surface energies leads to the Laplace equation for anisotropic curved interfaces, whose material limits are the vesicle shape equation and the liquid crystal Herring equation. Applications of the model to shape selection in liquid drops embedded in aligned nematic liquid crystals illustrates the competition between anchoring and bending and shows how anisotropic surface tension distorts the droplet and how bending tends to restore the spherical shape. This theory presented in this article shows that the interaction of interfacial anchoring and bending creates new regimes in classical fluid membrane mechanics.