Angle Of Nematic Liquid Crystal Research Articles

Nematic Structures under Conical Anchoring at Various Director Tilt Angles Specified by Polymethacrylate Compositions.

Dependence of the director tilt angle of nematic liquid crystal (LC) under conical anchoring from the two-component polymer mixture composition has been studied. We varied the ratio of poly(isobutyl methacrylate) (PiBMA), which specifies a conical anchoring for the nematic liquid crystal LN-396, and poly(methylmethacrylate) (PMMA) assigning a tangential alignment for the same nematic. An oblique incidence light technique to determine a tilt angle has been used. It has been shown that the tilt angle increases from to when PiBMA:PMMA ratio changes in the range 30:70 to 100:0. The specific optical textures viewed under the polarizing microscope and proper orientational structures have been considered for various compositions of the polymer films. An electric field action on the formed orientational structures has been investigated. The obtained results are promising for the application in various electro-optical LC devices with a conical anchoring in which the director tilt angle is a crucial parameter: a controlled diffraction gratings, an electrically operated achromatic rotators of linear light polarization, etc.

Gradient pretilt angle alignment materials with different photosensitivity for tunable polarization‐independent self‐aligned liquid crystal lens

AbstractAlignment of benzaldehyde materials with different photosensitivity thresholds, capable of changing the pretilt angles of nematic liquid crystal (LC) from 90° to 0° in a controlled manner under UV‐B exposure, has been developed. Inhomogeneous exposure of uniform rubbed alignment layers allows the formation of a refractive index gradient inside the LC cell. The concept of tunable polarization‐independent self‐aligned LC lens based on gradient pretilt angle alignment materials with different photosensitivity is demonstrated. Self‐alignment of two polarization‐dependent sub‐lens is achieved due to a single exposure act of two alignment layers, which are located on the same piece of glass on both sides, forming one common optical axis for a polarization‐independent LC lens. The sub‐lens cells have uniform cell gap and are independently controlled using low‐voltage driving. Devices based on gradient benzaldehyde alignment materials can be used in many modern optical and photonic devices.

GRADIENT ALIGNMENT MATERIALS WITH DIFFERENT PHOTOSENSITIVITY FOR TUNABLE POLARIZATION-INDEPENDENT LIQUID CRYSTAL LENS

The development of electrically tunable liquid crystal (LC) lenses is perspective and promising for a wide range of applications, for example, for imaging system, pico projectors, optical zoom systems, ophthalmology applications and other. Of particular note is the development of polarization-independent LC lenses, as eliminates polarizers from application devices that reduce the efficiency of light transmission through optical systems. Alignment benzaldehyde photosensitive materials, capable of changing the pretilt angles of nematic LC from 90 to 0 ºС in a controlled manner under UV exposure are developed. The anisotropy of the benzaldehyde alignment layers is generated by a two-stage treatment consisting of uniform rubbing with a cloth and subsequent non-polarized UV exposure. Inhomogeneous UV exposure of uniformly rubbed alignment layers allows formation of refractive index gradient inside the LC cell. The concept of tunable polarization-independent self-aligned LC lens based on gradient pretilt angle alignment materials with different photosensitivity is demonstrated. Self-alignment of two polarization-dependent sub-lens is achieved due to a single UV exposure act of two alignment layers, which are located on the same piece of glass on both sides, forming one common optical axis for a polarization-independent LC lens. The independence of the polarization of LC lenses is achieved by setting the azimuthal rubbing direction of the alignment layers of two polarizationdependent LC lenses perpendicular to each other. The sub-lens cells have uniform cell gap and are independently controlled using low-voltage driving. Devices based on gradient benzaldehyde alignment materials can be used in many modern optical and photonic devices.

Polarization-Dependence of Liquid Crystal Alignment on an Organic Insulation Thin Film with Ion Beam Irradiation Explained by Brewster's Law

We used Brewster's Law to examine the mechanism of liquid crystal (LC) alignment on an organic insulation layer when subjected to ion-beam irradiation. Brewster's Law implies that the maximum rate polarized ray on a slanted insulation layers on the substrate and it illustrates the dependence of polarization and the mechanical structure on the ion beam irradiation process. The pretilt angle of nematic LCs on the organic insulation surface was about 1.13° for an ion beam exposure of 45° for 1 minute at 1800 eV. This shows the dependence of LC alignment on the polarization ratio in a slanted organic insulation layer. We also discussed the electro-optical characteristic of twisted nematic (TN) LCD using ion beam irradiation on organic overcoat layer.

Correlations Between the Surface Energy and the Tilt Angle on the Homeotropic Polyimide Surface

We investigated the correlations between the surface energy and the tilt angle on homeotropic polyimide (PI) surfaces subjected to rubbing using ion beam (IB) irradiation. The tilt angle of nematic liquid crystal (NLC) decreased with increased rubbing times. Also, the tilt angle of NLC was measured to be homeotropically aligned and then rapidly changed to homogeneous alignment with increasing IB irradiation time. We assumed that the LC alignment depends strongly on the C–O bonds created from bonds on the PI surface broken with IB irradiation, which was confirmed through X-ray photoelectron spectroscopy measurements. In addition, the contact angle on the PI surface decreased with increasing IB irradiation time. Therefore, we conclude that the IB irradiation is attributed to the high surface energy and the compositional variation of the homeotropic PI, resulting in a transition of NLC orientation.

Effects of Alignment Layer on Pretilt Angle and Electrical Characteristics for The Photo-Aligned Twisted Nematic Cell on Polyimide Surfaces

The alignment layer effects on the pretilt angle generation and electrical characteristics of photoaligned twisted nematic (TN)-liquid crystal displays (LCDs) for a polyimide (PI) surface with side chains were studied. The generated pretilt angle of nematic liquid crystal (NLC) on the rubbed PI surface with one layer was almost the same as that on the PI surface with two layers. However, the generated pretilt angle of NLC on the photoinduced PI surface with two layers was larger than that on the PI surface with one layer generated by oblique UV exposure on the PI surface. The different mechanisms of pretilt generation in NLC in the rubbing- and photoalignment methods were examined. The pretilt angle of the NLC on the photoinduced PI surface is attributed to surface roughness and side chains due to photodissociation of the polymer with UV exposure on the PI surface. We observed the same characteristics of voltage–transmittance (V–T) and response time for one and two layers on the PI surface. We also observed the same voltage holding ratio (VHR) characteristics for one and two layers on the photoaligned TN-LCD. Finally, we suggest that the VHR of photoaligned TN-LCDs was higher than that of the rubbing-aligned TN-LCDs.

Relationship between Pretilt Angle of Nematic Liquid Crystal and Surface Structure of Alignment Layer

Fabrication method of alignment layer was investigated to verify which mechanism is dominant in generating the pretilt angle of LC. The surface energy and the geometrical surface structure were systematically controlled by combining the rubbing and Langmuir-Blodgett(LB) methods. Two layers of LB film were deposited on the top of rubbed polyimide films to remove the surface energy generated by rubbing. In a rubbed polyimide film, polymer chains array in a certain angle which increase with the rubbing strength. The rubbed film with two LB layer showed the contact angle to be the same as the LB film, while the pretilt angle of LB layer on rubbed film was reduced. Therefore it is concluded that the pretilt is more affected by the surface structure of the polyimide alignment layer than the surface energy.

Investigation of Liquid Crystal Alignment and Pretilt Angle Generation in the Cell with Linearly Polarized UV Light Irradiation on Polymer Surface

We have investigated the generation of pretilt angle in nematic liquid crystal (NLC) and electro-optical (EO) characteristics of photo-aligned TN-LCD with polarized ultraviolet (UV) light irradiated on polyimide (PI) surface. It was found that the LC alignment capability increases with increasing the UV light irradiation time. In case of the fact that the polarized UV light of oblique angle of 60° irradiated on PI surface rotated by 90° at 10 min. after being normally irradiated on PI surface at 30 min., the generated pretilt angle of NLC is about 1.5°. Also, we obtained that the pretilt angle in NLC strongly depends on oblique angle and UV light irradiation time. We considered that the pretilt angle of NLC is attributed to the interaction between the LC molecules and the polymer surface due to photo-depolymerization of polymer. Finally, we measured that the EO characteristics of photo-aligned TN-LCD is almost same in comparison to rubbing-aligned TN-LCD.

Preliminary communication Generation of high pretilt angle in a nematic liquid crystal with single oblique polarized UV light irradiation on polyimide surfaces

We propose and investigate obtaining high pretilt nematic alignment using a new photoalignment method: single oblique polarized ultraviolet (UV) light irradiation on polyimide (PI) surfaces. It was found that the monodomain alignment in a nematic liquid crystal (NLC) is obtained with PI surfaces having side chains. We successfully observed that the generated high pretilt angles of the NLC are about 6 with an angle of incidence of 5 80 on the PI surface. Also, the generated pretilt angle of NLC is about 6 with an UV light irradiation of 30 min for angle of incidence of 5, and decreases with increasing UV light irradiation time. High pretilt orientation of the NLC is successfully observed in cells with a single oblique UV light irradiation on PI surfaces.

Liquid Crystal Alignment Film with Mixture of Polyimide and Side Chain LC by Langmuir-Blodgett Technique

The mixed films of poly(amic acid) and a side-chain liquid crystal, 11-[(4-cyano-4′-biphenyl) oxy] undecanyl vinyl ether(CN-11-M) were investigated as a LC alignment film to control the pretilt angle of a nematic LC. The mixed films were fabricated by the Langmuir-Blodgett technique. A stable monolayer of poly (amic acid) alkylamine salt(PAAS) and CN-11-M was formed at the air-water interface. A Y-type film on 1TO glass was established at 25 mN/m and room temperature and was confirmed by FTIR spectroscopy. The number of layer was 4, and the ratio of PAAS and CN-11-M was varied. Surface tensions of these films were observed by measuring the contact angle of test liquid. Antiparallel cells were made to measure the pretilt angle of LC by crystal rotation method. Polar force in the mixed film increased with the amount of CN-11-M, but showed a maximum. The pretilt angles of nematic liquid crystal also showed the maximum at a fixed ratio of PAA and CN-11-M.

Preliminary Communication : Generation of high pretilt angles of nematic liquid crystal (5CB) on rubbed organic solvent soluble polyimide surfaces with helical backbone structure and trifluoromethyl moieties

High pretilt angles of nematic liquid crystals (NLC) have been successfully generated on rubbed organic solvent soluble polyimide (PI) surfaces with a helical backbone structure and trifluoromethyl moieties. We have found that the pretilt angle of in 4-n-penty-4-cyanobiphenyl (5CB) on rubbed PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings is larger compared to that on rubbed PI surfaces with trifluoromethyl moieties attached to the polymer backbone. The observed pretilt angle of 5CB on unidirectionally rubbed PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings is about 15 degrees in the wide rubbing region. From these results, it is expected that the microscopic surface structure of the polymer strongly contributes to the pretilt angle generation of the LCs at the surface.

Measurement of Tilt Angle of Nematic Liquid Crystal Near the Surface of Rubbed Polyimide by Means of the ATR Method

Abstract An attenuated total reflection (ATR) method has been used to measure the changes of the average tilt angle of nematic liquid crystals near the surface of alignment film under the application of an electric field. The respondence of molecular alignment, in the region of about 140nm from the liquid crystal/alignment film interface, upon the applied voltage, was obtained by this experiment. The results show that the average tilt angle near the interface increases monotonically with the applied field and tends to saturate to be 35° when an adequate field is applied up to 80 volts; this saturated value is small compared with the tilt angle at the mid-plane which is 90°. This behavior is well explained numerically in terms of elastic theory by assuming strong surface anchoring.

A simple method to determine the pretilt angle of nematic guest-host liquid crystals

The phase retardation-extremum method offers a fast and simple way to measure the pretilt angle of nematic liquid crystal (NLC) cells. Due to optical absorption of the dissolved dyes, such a method is not suitable to measure the pretilt angle of a guest-host (GH) NLC cell. We have obtained a simple and fast method to determine the pretilt angle of a GH NLC cell from the incident angle corresponding to the minimum transmission intensity of a polarized light passing through the GH cell. Using this simple method, we have measured the pretilt angles of GH NLC cells with different dye concentrations, and compared them with the pretilt angles of the corresponding host NLCs measured by the retardation-extremum method. Excellent agreement between the two methods was observed with a dye concentration of not more than 1%. Within our experimental errors, dye concentration of up to 5.8% (by weight) seems not to change the pretilt angle of the GH NLC from that of the corresponding host NLC.