Pretilt Angle Research Articles

Soluble polyimide films as alignment layers for bistable chiral-tilted homeotropic nematic liquid crystal display applications

Soluble polyimides (PI) with and without aliphatic side chains were synthesized by a one-pot synthesis method and acted as homogeneous or homeotropic liquid crystal alignment layers. Homeotropic PI was synthesized by grafting an alkyl side chain on the backbone of homogeneous PI. The vertical alignment properties can be tuned by adjusting the grafting density and the length of the side chains. In addition, various homogeneous and homeotropic PIs were blended by a different ratio to adjust the pretilt angle of the alignment film. The long-term alignment properties and thermal resistance of the alignment films were investigated. Combining the dual frequency liquid crystal with the alignment layers which have an intermediate pretilt angle of 75°, a bistable chiral-tilted homeotropic nematic liquid crystal display device is demonstrated.

Electro-optical Properties of Twisted Nematic Liquid Crystal Displays Fabricated with TIPS-pentacene Doping

This paper introduces 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) doped liquid crystal (LC) alignment properties on a rubbed polyimide (PI) layer as a function of the doping concentration of TIPS pentacene. Pretilt angles, photomicrographs, and electro-optical properties of TIPS pentacene doped LCs were comparable to those of pure LCs. However, TIPS pentacene in a LC medium supported twisted nematic-liquid crystal displays (TN-LCDs) to improve electro-optical properties. The threshold voltages observed in the TN cells decreased as the TIPS pentacene concentration increased. In addition, suitable response times were observed in TN cells.

Measurement of the liquid crystal pretilt angle in cells with homogeneous and inhomogeneous liquid crystal director configuration

Optical and electro-optical methods of liquid crystal (LC) director pretilt angle measurement are described for LC cells with homogeneous and inhomogeneous LC director distribution. The LC pretilt on both LC substrates can have the same or opposite direction. The phase retardation difference of both extraordinary and ordinary polarized rays passing through an LC cell with homogeneous and inhomogeneous LC director distribution has been calculated versus the LC pretilt angle θ(0) on the cell's substrates in the range 0≤θ(0)≤90°. The experimental procedure for phase retardation difference determination by measurement of the LC cell transmission between crossed polarizers for cells with LC tilted alignment is described. The method developed can also be used in optical compensator design.

Synthesis and characterisation of rubbing-resistant polyimides with naphthalimide side-chain for liquid-crystal alignment layers

A novel diamine with a side-chain containing naphthalimide ring and non-polar alkyl end group, N-octyl-4-(3,5-dinitrobenzoyl)-amido-1,8-naphthalimide (N8), was synthesised and its chemical structure was confirmed by FTIR (fourier transform infrared) spectroscopy and nuclear magnetic resonance spectroscopy (1H-NMR). Then, a new family of polyimides (PIs) containing naphthalimide unit in the side-chains has been successfully synthesised by reaction of 4,4′-oxydiphthalic anhydride (ODPA) and 3,3′-dimethyl-4,4′-methylenediamine (DMMDA) with the novel functional diamine N8. The obtained polymers showed excellent solubility in a broad range of solvents, including tetrahydrofuran. Thermal properties of polymers were good enough to permit the use of these PIs on liquid-crystal displays (LCDs) applications. Alignment films obtained by casting offered outstanding rubbing-resistant ability, meanwhile kept the pretilt angle high above 89°. The PI seems to be prospective materials for alignment layers in LCDs.

Increasing pretilt angle by grafting hexafluorobutyl acrylate into the surface of polyimide alignment films via electron beam irradiation

In this study, the polyimide (PI) alignment film whose surface contained fluorinated side chain was prepared by a new method. The fluorine compound, hexafluorobutyl acrylate (HFBA), was grafted into the surface of main-chained PI film as side chain via electron beam irradiation. The pretilt angle of liquid crystal (LC) was significantly improved from 2° to 20° or above. It was indicated by X-ray photoelectron spectroscopy spectra and attenuated total reflection Fourier transform infrared spectroscopy spectra that HFBA was successfully grafted into the thin surface layer of PI alignment films in the form of covalent bond. Scanning electron microscopy images showed that the irradiation process had no obvious influence on the surface morphology of the PI film. The surface energy of the PI film decreased from 49.85 to 31.50 mN/m after surface modification. Besides, the reason for the increase of pretilt angles was discussed.

Dielectric Permittivity and Viscoelastic Measurements οf Two Tricomponent Mixtures Consisting οf Laterally Fluorinated Terphenyl Derivatives

Two tricomponent room temperature nematic eutectic mixtures ABC and ABD with laterally uorinated constituents A and B were prepared as base mixtures for vertically aligned mode LCD's. The physical properties of these mixtures viz. birefringence, dielectric anisotropy, bend elastic constant, relaxation time and rotational viscosities were determined in order to compare with the optimum values required to achieve the target speci cations of VA mode materials. The dielectric anisotropy, ∆e, and optical birefringence, ∆n, of these mixtures were found to be in the range of ( 1.3 to 1.4) and (0.13 and 0.14), respectively, at around 20 ◦C. The gure of merit for the ABD mixture has been found to be higher than that of ABC mixture throughout the entire temperature range. The pretilt angle e ect in the physical parameters has also been studied. At T = 20 ◦C, the response time decreases to 25% and 35% for mixture ABC for 2◦ and 5◦ pretilt respectively in comparison to zero pretilt. On the other hand, at the same temperature for the ABD mixture these values are reduced by 16% and 35%, respectively.

Liquid crystal alignment properties of poly(styrenesulphonate)/alkyltrimethylammonium complexes

We synthesised a series of poly(4-styrenesulphonate)/alkyltrimethylammonium (PSS-#Cx, # = 12, 14 and 16; x = 80, 60, 40 and 20) complexes, where # is the number of carbon atoms in the alkyl groups in alkyltrimethylammonium bromide, and x is the molar content (%) of alkyltrimethylammonium moiety, using polymer analogous reactions to investigate their liquid crystal (LC) alignment properties. In general, the LC cell fabricated using the polymer film having a longer alkyl side group and a higher molar content of alkyl side group showed homeotropic LC alignment behaviour with a pretilt angle of about 90°. The homeotropic LC alignment behaviour was well correlated with the surface energy of the polymer films. Homeotropic LC alignment was observed when the surface energy values of the polymer were smaller than about 44.87 mJ/m2.

Evaluation technique of cell thickness, pretilt angle, and twist angle for guest–host liquid crystal displays

In this study, we demonstrated the evaluation of the device parameters, such as the cell thickness d, pretilt angles at the top and bottom substrates θ0 and θd, and twist angle φt for the guest-host (GH)-type electrically controlled birefringence (ECB) and twisted nematic (TN) modes, using the renormalized transmission ellipsometry (RTE). In the proposed technique, the extended Cauchy equation and the extinction coefficients for the ordinary and extraordinary rays based on the three Gaussian functions were employed as a description of the dielectric function. As a result, the numerically calculated phase difference Δ and angle of amplitude ratio Ψ determined using the proposed technique provided good agreement with the measured Δ and Ψ by introducing the extinction coefficient to the RTE. Furthermore, the device parameters for the GH-ECB and the GH-TN cells were obtained. It was confirmed that the extinction coefficient should be taken into consideration in an analysis of GH liquid crystal displays.

Liquid crystal alignment by polyhedral oligomeric silsesquioxane (POSS)–polyimide nanocomposites

Polyimide (PI) films are widely used in the liquid crystal display (LCD) industry to align liquid crystal (LC) molecules in a specific orientation with a pretilt angle θp on the PI alignment films. It was observed that physical dispersion of polyhedral oligomeric silsesquioxane (POSS) nanoparticles in commercial homogenous PIs decreases the surface energy of the PI alignment films and generates a controllable θp in the range 0°<θp<90°, which is not easily achieved by complicate PI synthesis. Characteristics of POSS–PI nanocomposites were studied to investigate the influence of POSS nanoparticles on PIs. Increased absorption in the infrared spectra and decreased decomposition temperature and glass transition temperature with POSS doped concentration in PI were observed due to the increase in free volume of POSS–PI nanocomposites. Such nanoscale hybridization suggests a novel approach to tune the properties of PIs through modification of molecular interaction. A fast response no-bias optically-compensated bend (OCB) LCD with a pretilt angle of 68° was also demonstrated in this work.

Surface localized polymer aligned liquid crystal lens

The surface localized polymer alignment (SLPA) method allows complete control of the polar pretilt angle as a function of position in liquid crystal devices. In this work, a liquid crystal (LC) cylindrical lens is fabricated by the SLPA method. The focal length of the LC lens is set by the polymerization conditions, and can be varied by a non-segmented electrode. The LC lens does not require a shaped substrate, or complicated electrode patterns, to achieve a desired parabolic phase profile. Therefore, both fabrication and driving process are relatively simple.

Transmitted Ellipsometry Method for Extracting Physical Parameters of TN/VA/Inverse-TN Liquid Crystal Cells

A transmitted ellipsometry method is proposed for extracting the ellipsometric parameters (Ψ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</i> , Ψ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ps</i> , Ψ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</i> , Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</i> , Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ps</i> and Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</i> ) of twisted nematic (TN), vertical alignment (VA), and inverse-TN (ITN) liquid crystal (LC) cells from the output Stokes parameters corresponding to five input polarized lights. The validity of the proposed approach is confirmed by comparing the experimental values of Ψ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</i> , Ψ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ps</i> , Ψ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</i> , Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</i> , Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ps</i> , and Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</i> with the simulated values obtained via a Genetic Algorithm curve-fitting approach. In experiments, the cell gap, pretilt angle, twist angle, and rubbing direction of TNLC and the cell gap, pretilt angle, and rubbing direction of VALC are extracted successfully. The sensitivity analysis results reveal that the ellipsometric parameters of the TNLC cell are sensitive to the cell gap, pretilt angle, twist angle and rubbing direction. However, those of the VALC and ITNLC cells are sensitive only to the cell gap, pretilt angle and rubbing direction. Overall, the results presented in this paper show that the proposed transmitted ellipsometry method provides a straightforward and accurate means of determining the ellipsometric and physical parameters of TN, VA, and inverse-TN LC cells.

Phase retardation vs pretilt angle in liquid crystal cells with homogeneous and inhomogeneous LC director configuration

Phase retardation of both extraordinary and ordinary polarized rays passing through a liquid crystal (LC) cell with homogeneous and inhomogeneous LC director distribution is calculated as a function of the LC pretilt angle θ₀ on the cell substrates in the range 0 ≤ θ₀ ≤ 90°. The LC pretilt on both substrates can have the same or opposite direction, thereby forming homogeneous, splay, or bend director configurations. At the same pretilt angle value, the largest phase retardation ΔΦ is observed in splay LC cells, whereas the smallest phase retardation is observed in bend cells. For the θ₀ values close to 0, 45°, and 90°, analytical approximations are derived, showing that phase retardation depends on LC birefringence variation.

Simple method to obtain accurate pretilt angles for thin twisted nematic liquid-crystal cells

Modern thin-film-transistor-driven twisted nematic (TN) products on the market require thin cell gaps below the value to satisfy the condition of the Gooch-Tarry first minimum in transmission for fast response times and wide viewing angles. For the first time to our knowledge, we have developed a simple experimental method based on a new configuration of out-of-plane cell rotation and analyzed our data by extended Jones matrix method to obtain accurate pretilt angles for such thin TN cells where the well-accepted configuration of the out-of-plane cell rotation method analyzed by Birecki and Kahn [The Physics and Chemistry of Liquid Crystal Devices (1980), p. 115] was insufficient to address.

侧链密度对聚酰亚胺取向膜性能的影响

4-dodecyloxy-phenyl-4′,4″-diaminotriphenylamine(DPDTA) containing triphenylamine were successfully synthesized.Polyimides(PIs) containing triphenylamine units in the main chain were synthesized with the co-condensation of DMMDA,DPDTA and aromatic dianhydride 4,4′-oxydiphthalic anhydride(ODPA) by thermal imidization.The structures of the PIs were confirmed by FT-IR.The alignment behavior of PIs as liquid crystal alignment layers was also examined with polarized optical micrograph(POM).The properties of the PIs were characterized by TGA,UV-Vis and DSC etc.It was found that side chain density of PIs had important influence on properties of liquid crystal alignment film.Before rubbing,PI containing 25% DPDTA could induce uniform vertical alignment and a pretilt angle of 89.7°.The liquid crystal was not aligned on the surface of PI film which contained 10% DPDTA without the rubbing treatment.After the rubbing process,the pretilt angle of liquid crystal was 2.4,when PI film contained 25% DPDTA and the pretilt angle of liquid crystal was 1.8°,when PI contained 10% DPDTA.The transmittances of the PIs in 500～800 nm wavelengths are more than 80%,and their glass transition temperatures are above 230 ℃.

A measurement method of cell gap for liquid crystal lens

There was a problem in current cell gap measurement of liquid crystal (LC) lens. A method was proposed by adopting interference of linearly polarized light in this paper. It was claimed that the incident light was monochromatic light with a uniform distribution of light intensity. The polarization axis of the analyzer was perpendicular or parallel to the polarization axis of polarizer. The difference between light and dark interference fringes was the biggest when the angle was equal to 45° between the rubbing direction of alignment layer in the LC lens and the polarization axis of polarizer. The cell gap of LC lens can be calculated by the transmittance of LC lens center and the max transmittance of its other locations. The tilt angle was equal to the pre-tilt angle of LC at the LC lens center. This method was simple and reliable. Its accuracy is relatively high due to the use of optical interference principle.

Surface reformation on solution-derived zinc oxide films for liquid crystal systems via ion-beam irradiation

We investigated solution-derived ZnO films as liquid crystal (LC) alignment layers via ion-beam (IB) irradiation. Solution processing was adopted to replace the sputtering method for the deposition of ZnO films as LC alignment layers. Uniform and homogeneous LC alignment was achieved, and a high-performance LC system was achieved using these substrates. X-ray photoelectron spectroscopy (XPS) analysis showed that surface reformation of ZnO films by annealing and IB irradiation affects the uniformity of the LC alignment and allows control of the pretilt angle. Superior electro-optical (EO) characteristics of twisted nematic-LC cells constructed with IB-irradiated ZnO films were observed, which indicates that the proposed solution-derived ZnO films have strong potential for use in the production of advanced LC displays.

Low temperature nanocomposite alignment films for optically compensated bend liquid crystal displays

Low temperature processes have been highly desired for the development of flexible displays. However, traditional polyimide alignment films require high-temperature hard baking for at least 1h to complete the cyclization polymerization. It is thus difficult to design liquid crystal displays with flexible substrates. In this paper, we proposed to use ultraviolet-curable liquid crystal polymer (LCP) in conjunction with polyhedral oligomeric silsequioxane (POSS) nano-particles (2–5nm) to develop a low temperature alignment technology with wide range of liquid crystal pre-tilt angles. The photo-polymerization process was relatively fast, and the polymer rubbing particle issues could be avoided. The results showed that, if 0.05wt.% POSS nano-particle was added in the 28-nm LCP nanocomposite film, the pretilt angle would be 42° for the optically compensated bend mode liquid crystal displays. The cell gap was about 4μm. The POSS nano-particles provided a homeotropic alignment force, while the LCP gave homogeneous alignment force in the horizontal direction. The force balance would result wide range pretilt angles, according to the nanocomposite compositions. The low temperature alignment technology will help the advancements in fast-response and flexible displays.

Controllable pretilt angle of liquid crystals with the formation of microgrooves

This work investigates the controllability of the pretilt angle of liquid crystals (LCs) in a cell with an initial vertical alignment through microgrooves induced by the holographic fast-writing method. By changing the writing time or intensity of the pumped beam, the microgrooves with various surface modulations can form and provide distinct planar anchoring strength for the LCs. The pretilt angle of the LCs can be controlled over a range from 18.3° to 89°. The writing time of the microgrooves takes less than 25 s, which is a two-fold reduction in the time taken in previous investigations using other photoaligned methods.

Spatial and orientational control of liquid crystal alignment using a surface localized polymer layer

We present an alignment method for the surface contacting liquid crystal (LC) director. This method allows complete control of the polar pretilt angle as a function of position in a liquid crystal device, and has the potential of controlling the azimuthal orientation of LC. Important considerations of this method are to form a thin layer of reactive monomers at the LC cell interior surface, and to control the deleterious effects of flow due to polymerization induced concentration gradients. To achieve these, the voltage and frequency of the applied electric field and the UV intensity during the polymerization process are significant.

Voltage holding properties of bend liquid crystal cells with treatments for bend alignment stabilisation

We investigated the voltage holding property of cells with additional fabrication processes for the formation of high and low pretilt angle regions within a dot for achieving a smooth splay-to-bend transition and of π-cells with a polymer network in a liquid crystal (LC) layer to stabilise a bend state. The pretilt angle could be adjusted by irradiation of deep ultraviolet (UV) light to a vertical alignment polyimide layer. The voltage holding ratio (VHR) was above 97% even with irradiation with deep UV light of 10 J/cm2. On the contrary, the VHR was decreased with increasing concentration of photoinitiator, which was added to LC mixture to form the polymer network. The VHR was 90% at the concentration of 0.3 wt%. The results indicate that the formation of the high and low pretilt angle regions within a dot is very useful from the point of reliability of the π-cell.