Vertical Alignment Liquid Crystal Research Articles

Synthesis of novel soluble polyimides containing triphenylamine groups for liquid crystal vertical alignment layers

A novel functional diamine containing triphenylamine and biphenyl moieties as well as a long alkyl chain, N,N-bis (4-aminophenyl)-4-(dodecyloxy-biphenyl)-4′-aminophenyl ether (B12), was successfully synthesized. A series of novel polyimides (PIs) with triphenylamine units in the polymer backbone were prepared from B12, 3,3′-dimethyl-4,4′-methylene-dianiline (DMMDA), and 4,4′-oxydiphthalic anhydride (ODPA) via a conventional two-step procedure that included a ring-opening polyaddition to give polyamic acids (PAAs) followed by chemical cyclodehydration. The chemical structures of the intermediates, diamine, and PIs were characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (1H NMR). All PIs were amorphous and readily soluble in many organic solvents, including tetrahydrofuran. PI films attained from a casting method showed high transmittance (above 90 %) in the wavelength range 400–700 nm and induced highly uniform vertical alignment of liquid crystals (LCs). PIs-B12 derived from B12, DMMDA, and ODPA exhibited much higher thermal stabilities than with the corresponding PIs-D12 from 4-dodecyloxy-biphenyl-3′,5′-diaminobenzoate (D12). The results demonstrated that the introduction of triphenylamine units into the polymer backbone improves the thermal stability of the PIs without deteriorating their solubility.

Achromatic wide-view circular polarizers for a high-transmittance vertically-aligned liquid crystal cell.

We propose an optical compensation scheme through which we can eliminate the off-axis light leakage in a vertically-aligned liquid crystal cell with circular polarizers. In this scheme, four uniaxial films with complementary dispersion characteristics are used to compensate one another, resulting in achromatic effective phase retardation for off-axis angles. By using the proposed optical compensation, a contrast ratio higher than 2000:1 can be realized over the entire 55° viewing cone in a multi-domain vertical-alignment liquid crystal cell with circular polarizers.

Optical properties of hybrid aligned nematic cells with different pretilt angles.

The phase retardation difference, ΔΦ, is calculated for hybrid liquid crystal (LC) cells as a function of LC pretilt angles, θ0(1), θ0(2), on the opposite substrates of the cell for the case of an arbitrary angle of light incidence in the range from 0 to 90°. An LC director configuration is suggested for its application in optical compensators. Design and fabrication methods of hybrid aligned nematic (HAN) cells with an arbitrary LC pretilt angle are described. The LC pretilt angle is measured in the HAN cells with a given planar or vertical LC alignment on one of the substrates.

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.

P‐129: Fast Switching of a Vertically‐Aligned Liquid Crystal Cell by Forming Polymer Networks at a Low Temperature

AbstractWe propose a method for fast switching of a vertical alignment (VA) liquid crystal (LC) cell by forming polymer networks at a low temperature. We found by experiment that the response time of a VA cell can be reduced by low‐temperature curing. The turn‐off time of the fabricated VA cell was reduced by 54% when the polymers were cured at ‐20°C.

P‐131: Surface Anchoring of Vertical Alignment Liquid Crystal Displays Enhanced by Surface Polymer Stabilization

AbstractA new method to enhance the surface anchoring of vertical alignment liquid crystal devices using surface polymer stabilization is introduced. Mixture made of a nematic liquid crystal and a minute amount of reactive monomer is polymerized to form sub‐micron sized polymer protrusions on the two substrates of the cell. Several kinds of monomers are tested.

Fast-Switching Vertical Alignment Liquid Crystal Displays Driven by Surface Polymer Stabilization

A new method of improving the response time of vertical alignment (VA) liquid crystal devices using surface polymer stabilization is introduced. A minute amount of a reactive monomer and photo-initiator is mixed with a nematic liquid crystal host and the mixture is polymerized under UV light to form sub-micron sized polymer protrusions on the two substrates of the cell with an applied bias voltage. Several kinds of reactive monomers with different molecular structures are tested, and a faster response time and a lower light leakage at dark state are achieved. This finding will be useful for display applications.

Evaluating and improving color washout of vertical aligned liquid crystal display

We demonstrated an optimized 8-domain vertical aligned (VA) liquid crystal display (LCD) by minimizing its color washout. The index G is adopted to analyze the degree of color washout through simulation. By using linearly and circularly polarized incident light, the optimized regime of the area and applied voltage ratios of sub-pixels is obtained. In the experiments with the circularly polarized light, the images are sensuously improved in a coupled capacity type LCD by applying the simulated applied voltage ratio of two sub-pixels.

Effects of three-dimensional polymer networks in vertical alignment liquid crystal display controlled by in-plane field.

Polymer network in vertical alignment liquid crystal cell driven by in-plane field (VA-IPS) is formed in three dimensions to achieve fast response time and to keep the liquid crystal alignment even when an external pressure is applied to the cell. The network formed by UV irradiation to vertically aligned liquid crystal cell with reactive mesogen does not disturb a dark state while exhibiting very fast decaying response time less than 2ms in all grey scales and almost zero pooling mura. The proposed device has a strong potential to be applicable to field sequential display owing to super-fast response time and flexible display owing to polymer network in bulk which supports a gap between two substrates.

Asymmetric Organic–Inorganic Hybrid Giant Molecule: Cyanobiphenyl Monosubstituted Polyhedral Oligomeric Silsesquioxane Nanoparticles for Vertical Alignment of Liquid Crystals

For liquid crystal (LC) alignment, polyhedral oligomeric silsesquioxanes (POSS) can be considered as one of the promising candidates for the formation of vertical alignment (VA) of LC. However, because of their poor compatibility and weak interaction with LC hosts, the pristine POSS are highly aggregate themselves in the LC media and create the macroscopic particles, resulting in severe light scatterings. To overcome this barrier, we proposed and successfully synthesized the cyanobiphenyl monosubstituted POSS giant molecule (abbreviated as POSS-CBP1), which showed an excellent dispersion in nematic (N) LC media and formed the perfect VA of LC without using conventional polymer-based VA layers. On the basis of the systematic experiments and careful analysis, we realized that the cyanobiphenyl moiety chemically attached to the pristine POSS with an alkyl chain can significantly improve the initial solubility and interaction with LC media but finely tune POSS-CBP1 to gradually diffuse onto the substrate of L...

Advanced Patterned Vertical Aligned Nematic Mode to Elevate Transmittance

This paper presents an advanced patterned vertical aligned nematic liquid crystal (APVA) mode as a way of improving the transmittance of the conventional patterned, vertically-aligned nematic liquid crystal (PVA) mode. The APVA was characterized by multi-electrodes, which play an important role in generating a horizontal electric field that prevents LC disclination in the vicinity of the middle region of electrodes between top and bottom slits in PVA mode. The proposed APVA mode focused on improving dramatically the transmittance with excellent image quality. In the simulation, the APVA mode improved the transmittance by more than 22%, compared to the conventional VA modes. Therefore, it is useful to upgrade the LCD display quality.

Fabrication of 10 nm-Scale Complex 3D Nanopatterns with Multiple Shapes and Components by Secondary Sputtering Phenomenon

We introduce an advanced ultrahigh-resolution (∼ 15 nm) patterning technique that enables the fabrication of various 3D high aspect ratio multicomponents/shaped nanostructures. This methodology utilizes the repetitive secondary sputtering phenomenon under etching plasma conditions and prepatterned fabrication control. The secondary sputtering phenomenon repetitively generates an angular distribution of target particles during ion-bombardment. This method, advanced repetitive secondary sputtering lithography, provides many strategies to fabricate complex continuous patterns and multilayer/material patterns with 10 nm-scale resolution. To demonstrate the versatility of this method, we show induced vertical alignment of liquid crystals (LCs) on indium-tin-oxide (ITO) grid patterns without any alignment layers. The ITO grid pattern fabricated in this method is found to have not only an alignment capability but also electrode properties without electrical or optical damage.

Design of an electrode structure for the elimination of the off-axis gamma shift in a multi-domain vertical-alignment liquid crystal cell

Various types of electrode structures were investigated to obtain the threshold voltage difference that allows for the elimination of the off-axis gamma shift in a multi-domain vertical-alignment liquid crystal cell. The threshold voltage difference can be obtained simply by changing the width of the electrodes or the distance between the electrodes in a two-terminal cell. It can also be obtained by applying a different bias voltage to a three-terminal (3T) cell. The advantages and disadvantages of three types of cell structures were compared. By optimizing the electrode structure of a 3T cell with the top electrode grounded, a 2.9 V threshold voltage difference was experimentally obtained.

Effect of cetyltrimethylammonium bromide coatings on indium tin oxide surfaces in reverse mode polymer dispersed liquid crystal films

The vertical alignment of liquid crystals (LCs) on cetyltrimethylammonium bromide-coated (CTAB-coated)indium tin oxide (ITO) glass is the consequence of specific interactions between the LCs, CTAB and ITO surface. The CTAB concentration influences the LC orientation, which changes the ITO surface energy. Changes in the surface energy of CTAB-coated ITO glass are investigated by the Owens–Wendt method. Both polar- and dispersive-component energies on the ITO surface decrease as the CTAB concentration increases, and reach saturation around 0.002% CTAB. The physico-chemical interaction between the LCs and the ITO surface was diminished by the presence of the CTAB layer, which induced the vertical alignment of LCs having positive or negative dielectric anisotropy. Vertical alignment is confirmed using crossed polarisers and the crystal rotation method. The electro-optical properties of fabricated reverse mode polymer dispersed liquid crystal films display transmittance of Ton: 29.7%, Toff: 85% and a response time of τon: 33 ms, τoff: 107 ms.

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 90o, 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.

Vertical Alignment Liquid Crystal Device With Reduced Off-Axis Gamma Shift

We propose a vertical alignment (VA) liquid crystal (LC) device with reduced off-axis gamma shift. We can form a multi-domain structure by simply designing the electrode structure . By applying a different bias voltage to each sub-pixel, we obtained a threshold voltage difference of 2.4 V, without any complex manufacturing process. We confirmed that the off-axis gamma shift can be significantly reduced by the proposed method. We believe that the proposed method can be easily applied to reduce the off-axis gamma shift in vertical alignment LC devices.

The effect of phthalimide side chains on the thermal stability and rubbing resistance of polyimide used as a liquid crystal vertical alignment layer

Two functional diamines, octyl-4-(3,5-diaminobenzamido)benzoate (O8, containing phenyl rings) and 3,5-diamino-N-(2-octyl-1,3-dioxoisoindolin-5-yl)benzamide (D8, containing a phthalimide ring), were designed and successfully synthesized. Two kinds of PIs were obtained by copolymerizing 3,3′-dimethyl-4,4′-methylenediamine (DMMDA), 4,4′-oxydiphthalic anhydride (ODPA), and D8 or O8. All of the PIs possessed excellent solubility. The PI films exhibited high transmittance, and were able to align liquid crystals (LCs) vertically. In contrast to the PIs generated from O8 (PI-O8), PIs created from D8 (PI-D8) were able to align LCs vertically even after rubbing, and also exhibited better thermal stability. The temperature at which 5 % of the weight of PI-D8 was lost (T 5) was higher than that for PI-O8. The results showed that the introduction of imide groups to the side chain improved the thermal stability and rubbing resistance of PIs without sacrificing their solubility and transmittance.

Vertical alignment of liquid crystals with zinc oxide nanorods

The alignment of liquid crystals (LCs) on zinc oxide (ZnO) nanorods grown vertically on an indium tin oxide (ITO) layer has been investigated as an alternative alignment layer for the vertical alignment of LCs. We found that the degree of vertical alignment strongly depends on the length and density of the vertically aligned ZnO nanorods on the ITO layer and also that a uniform vertical alignment using the proposed structure can be achieved. Finally, vertically aligned LC cells with ZnO nanorods were fabricated and their electro-optical properties were evaluated and compared with those of a conventional vertically aligned LC cell with a polymer alignment layer.

In-plane switching of vertically aligned negative liquid crystals for high transmittance and wide viewing angle

We propose a method for in-plane switching of vertically aligned negative liquid crystals (LCs) for high transmittance and wide viewing angle. By applying an in-plane electric field using a double-layered electrode structure, LC molecules can be rotated by the vertical as well as the in-plane components of the applied field over the entire region so that high transmittance can be achieved. The threshold voltage difference can be obtained simply by varying the electrode structure, which can reduce the off-axis gamma shift in a multidomain vertical alignment LC cell.

P.100: Multi‐domain Vertical Alignment of Liquid Crystals Through Control of the Anchoring Energy

AbstractWe propose a method for multi‐domain vertical alignment of liquid crystals without additional transistors. By irradiating UV light to the polyimide through a patterned photomask, we can obtain different threshold voltage in a VA cell. There is no light leakage in the initial dark state so that the contrast ratio is not reduced in the proposed structure. We expect that the proposed method can be used to realize an eight‐domain structure for reduction of the off‐axis gamma shift