US20050260334A1

US20050260334A1 - Method of achieving high pretilt angles in a lilquid crystal cell

Info

Publication number: US20050260334A1
Application number: US10/853,348
Authority: US
Inventors: Hoi Kwok; Fion Yeung; Fengchao Xie; Ophelia Tsui; Ping Sheng
Original assignee: Hong Kong University of Science and Technology
Current assignee: HONG KONG UNIVERSITY OF SCIENCE AND TECHNOLOGY A CORP OF HONG KONG; Hong Kong University of Science and Technology
Priority date: 2004-05-24
Filing date: 2004-05-24
Publication date: 2005-11-24
Also published as: CN1985213A; TWI360003B; TW200538824A; US20050260426A1; WO2005116742A1; HK1102337A1; US7968158B2; CN100447637C

Abstract

A new method of obtaining high pretilt angles of predetermined values in a liquid crystal cell is disclosed. Appropriate amounts of compatible homogeneous and homeotropic alignment materials are mixed with special treatment methods to achieve the desired results.

Description

FIELD OF THE INVENTION

This invention relates to method of producing a high pretilt angle in a liquid crystal cell that can be controlled reliably and more specifically relates to a method enabling pretilt angles from 10 to 80 degrees to be achieved.

BACKGROUND OF THE INVENTION

A liquid crystal display consists, among many other things, of a liquid crystal layer. The alignment behavior of this liquid crystal layer determines most of the optical properties of the liquid crystal display. This alignment is determined by the alignment layers in contact with the liquid crystal layer itself. The most common form of this alignment layer is a thin film of polyimide deposited on the glass substrate. The preparation conditions of these top and bottom alignment layers provide an alignment direction as well as a pretilt angle for the liquid crystal molecules near the surface of the liquid crystal layer.
The pretilt angles achievable with common polyimides are predetermined and cannot be varied. In particular, it is very difficult to obtain large values of θ(0) from 10°-80°. There are needs to make liquid crystal cells with large pretilt angles in the region near 40-60 degrees. Many new types of liquid crystal displays including bistable displays can be made only if high pretilt angles are available. Additionally, the fast response liquid crystal displays can be better made with large pretilt angles.
Traditionally, the best way to obtain high pretilt angles controllably is by oblique evaporation of SiO_xin a vacuum. This is a well-known art and has been discussed in many open publications (see David Armitage, J Appl Phys, vol 51, p 2552, 1980). Another well-known art is the use of molecular Langmuir-Bloggett films. All of these methods are, however, impractical for mass production.
More recently a new method of ion beam alignment has been disclosed by Chaudhari et al. in U.S. Pat. No. 6,195,146. Many different pretilt angles can be made similar to SiO_xevaporation. Additionally, several inventions have been disclosed recently to address the same issue of large pretilt angles. Harada et al. in U.S. Pat. No. 5,744,203 disclose a new alignment material that can achieve high pretilt by simple rubbing. Brosig et al. in U.S. Pat. No. 5,172,255 teach a method of achieving high pretilt angle in a homogeneous polyimide by rubbing the same surface twice in opposite directions. It was said that by varying the rubbing strength of the second rub, various pretilt angles can be achieved. Resnikov et al. Kim et al. and Gibbons et al. in U.S. Pat. Nos. 6,633,355, 5,882,238 and 5,856,430 respectively disclose means of achieving high pretilt angle by the method of photo alignment. These methods are different in the particular material of the alignment layer used, and the difference in the light illumination geometry. It is claimed that the photon dosage will vary the pretilt angle achieved ultimately. The detailed physics of the photoalignment technique is still not clear.
Accordingly it is an object of the present invention to provide a novel method of achieving high pretilt angles in a liquid crystal cell. It is also an object of the present invention to provide a novel method of producing an alignment layer for a liquid crystal layer in a liquid crystal cell.

BRIEF SUMMARY OF THE INVENTION

This invention provides a novel method of obtaining controllable pretilt angles by the method of rubbing a specially prepared alignment layer. This method can achieve any predetermined pretilt angle from 10°-80°. It is also commensurate with traditional mass production technology and is based on mechanical rubbing. Basically, nano- and micro-domains are formed in the alignment layer. Such nano- and micro-domains contain different alignment properties. The interaction of the domains gives rise to large pretilt angles in a controllable manner.
Accordingly the present invention consists in providing a method of producing an alignment layer in a liquid crystal cell, comprising the steps of: a) forming a solution of a homogeneous (horizontal) alignment material; b) forming a solution of a homeotropic (vertical) alignment material; c) mixing the alignment materials in steps (a) and (b) to form a clear and slightly colored solution without precipitates; d) applying the mixture in step (c) on a substrate to form a solid film; e) curing the film in step (d), in one or multiple steps to form a hardened solid film; and f) treating the solid film in step (e) mechanically to obtain a uniform alignment direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the typical liquid crystal display.
FIG. 2 is a diagram showing the details of a liquid crystal cell.
FIG. 3 is a set of photographs showing examples of some nano- and micro-domains of the solid film in accordance with the present invention.
FIG. 4 is a graph showing the dependence of the pretilt angle on the composition of the alignment layer for one particular material combination in accordance with the present invention.
FIG. 5 is a graph showing the polar anchoring energy achievable with the present invention as a function of the composition of the alignment layer.

DETAILED DESCRIPTION OF THE INVENTION

A liquid crystal display is usually made of a liquid crystal cell 1 and two polarizers 2, 3. The simplest liquid crystal cell is composed of top and bottom glass plates 4, 5, transparent conductive electrodes 6, 7, alignment layers 8, 9 and the liquid crystal layer 10. The angles of the liquid crystal layer on the contact surfaces with the alignment layers are called the pretilt angles. As shown in FIG. 2, the pretilt angles on the top and bottom layers can be different. The transmission or reflectance of light by the liquid crystal cell is determined completely by knowing the polarizer angles α and γ, and the alignment condition of the liquid crystal layer 10. The liquid crystal layer is defined completely by the tilt angle θ(z) and twist angle φ(z) distributions of the liquid crystal director vector n as a function of its position in the one-dimensional case, and θ(x,y,z) and φ(x,y,z) in the three-dimensional case.
The electrodes 6, 7 and the alignment layers 8, 9 are used to control the alignment conditions of the liquid crystal layer 10. The electrodes provide the voltage to control the values of θ(z) and φ(z). The alignment layers and their treatment determine the values of θ(0) and φ(0). The values of θ(0) and φ(0), together with the elastic Eulers equation, determine the solution of θ(z) and φ(z). The physics of the alignment of the liquid crystal layer is well-known in the art and is well covered in the literature, such as given in the monograph “Electrooptic Effects in Liquid Crystal Materials” written by Blinov and Chigrinov, published by Springer in 1994. θ(0) and φ(0) are known as the easy axes for liquid crystal alignment. However, it should be noted that the actual alignment directions of the liquid crystal on the surfaces is also dependent on the anchoring energies of the alignment surfaces. The anchoring energy is a measure of how strong the anchoring condition is. If the anchoring energy is large, then it is difficult to deviate from this condition and the alignment angles are given by the easy axes directions. However for weak anchoring, the actual angles of the liquid crystals on the surface may deviate from θ(0) and φ(0). Again, well-known formulas are available to calculate the alignment of the liquid crystal molecules for all values of z given the anchoring energies.
Obviously, the values of θ(0) and φ(0) or the alignment of the liquid crystal molecules right near the alignment layer are important in designing the electro optical properties of the liquid crystal cell. The alignment of the liquid crystal molecules can be achieved by many means and is a well studied problem in liquid crystal physics and engineering. The predetermined alignment conditions are usually achieved, for example, by rubbing the alignment layers 8, 9. While the rubbing direction determines θ(0), the value of the pretilt angle θ(0) is determined mostly by the material of the alignment layers 8, 9. There are homogeneous alignment materials such as polyimides that can provide pretilt angles of 1°-8° for manufacturing twisted nematic (TN) and supertwisted nematic (STN) liquid crystal displays. There are also materials that can provide homeotropic alignment with pretilt angles of 85°-90° for manufacturing vertically aligned nematic (VAN) liquid crystal displays. These homogeneous and homeotropic materials are available commercially. Many inventions have been disclosed on different types of chemicals that can provide planar or vertical alignments. But it is noted that these alignment layers can only provide either planar or vertical alignments. It is impossible to obtain alignment polar angles that are in between. In particular, no known polyimide alignment materials are known that can give a pretilt angle of near 45°. These polyimide alignment materials have served the liquid crystal display industry well. Large quantities are used for making practical LCD.
Alignment layers are used routinely to obtain alignment of liquid crystal layers to make liquid crystal displays. There are many alignment materials reported in the literature for this purpose. These materials are mostly polymers that are stable against heat and light. Examples are polyimide (PI), polyvinyl alcohol (PVA), polyester and polyamic acid (PA). These materials are commonly spin coated or screen printed on the glass substrate 4 and 5. Prebaking and final baking steps are needed to harden and cure the polymeric materials. Some of these polymeric alignment agents can provide a homogeneous alignment condition with a pretilt angle of a few degrees. Some special alignment agents can provide a homeotropic or vertical alignment for the liquid crystals with a pretilt angle of near 90 degrees. Both types of polymers can be coated and cured on glass substrates for making alignment layers on the glass substrates 4 and 5, and are well-known in the art.
In accordance with the present invention, there is disclosed a method of forming nano- and micro-domains of a mixture of the vertical alignment polymers (V) and the horizontal homogeneous alignment polymers (H), and a way of using such micro-domains for LC alignment. Chemically, H and V type polymers are difficult to mix together. They always have different glass transition temperatures (T_g) and solubility in various solvents. Even if they are miscible in some common solvent, after the solvent is driven out in the solid film formation process, these polymers will solidify at different concentrations and times. Thus domains will form.
In accordance with the present invention, a polymer suitable for homeotropic alignment is first diluted with a suitable solvent such as methyl-2-pyrrolidone (NMP), dimethyl formamide (DMF), -butyrolactone (γBL), Butyl cellosolve (BC) or THF (tetrahydro furan). Examples of suitable homeotropic alignment materials are polymers that include polyimide (PI), polystyrene (PS), poly-methyl methacrylate (PMMA), polycarbonates (PC), polyamic acid (PAA) or polyvinyl alcohol (PVA). At the same time, a solution of an appropriately chosen homogeneous alignment material is also prepared using an appropriate solvent such as methyl-2-pyrrolidone (NMP), dimethyl formamide (DMF), -butyrolactone (γBL), Butyl cellosolve (BC) or THF (tetrahydro furan). This solvent may be different from or may be the same as that used for diluting the homeotropic alignment material. Examples of suitable homogeneous alignment materials include polymers such as polyimide (PI), polystyrene (PS), poly-methyl methacrylate (PMMA), polycarbonates (PC), polyamic acid (PAA) or polyvinyl alcohol (PVA).
The two solutions are then mixed together to obtain a clear solution. It is important at this stage that the two solutions be miscible to form an intimate mixture of the vertical and homogeneous alignment materials. The miscibility can be assured by noting that the mixture is free from precipitates and does not appear milky. It should be clear and slightly colored. This miscibility is not in general possible and can be achieved only by a judicious choice of the solvents. Moreover, some polyimide that contains a fluorine group may be more difficult to mix with other polyimides. Preferably, it should also be ensured that the solubility of the vertical alignment material and of the horizontal alignment material are different in the total solvent mixture. Also, the alignment materials are preferably mixed in an appropriate ratio such that the weight ratio of the homeotropic material is from 0.1 to 99 percent of the total weight of the final mixture excluding the weight of the solvent(s).
The total mixture is then spin coated or screen printed on a glass substrate to form a solid film. After prebaking and final baking to cure the mixture, a new alignment layer is produced. The curing method can for example be thermal curing or photo-curing. The hardened solid film is preferably of thickness between 10 nm and 300 nm. The alignment layer is then rubbed mechanically to produce the needed liquid crystal alignment layer. The mechanical rubbing can be achieved by, for example, mechanical rubbing by a piece of fabric in a fixed direction, or by irradiation by an ion beam in a vacuum in a fixed direction at a fixed incident angle.
Since the two alignment materials precipitate at different times in the solid film formation process, nano- and micro-domains will be formed. FIG. 3 shows examples of the film structure formed in accordance with the present invention. In these examples, the domains are of the size of a fraction of a micrometer. In many other cases, the domains can be as small as a few nanometers. The domain structure is usually of the form of islands of vertical alignment material in a background of the homogeneous alignment material. This structure is determined by the surface tension, surface energies, elasticity and other physical properties of the two materials as well as that of the common solvent. They are all useful for this embodiment.
Physically, the two types of domains will interact with the liquid crystal molecules. Their alignment forces will compete with each other resulting in an alignment which is intermediate between vertical and homogeneous. By varying the relative concentrations of the homeotropic and homogeneous alignment materials, pretilt angles of various values can be produced. FIG. 4 shows examples of such pretilt angles. That figure shows the measured pretilt angles of the liquid crystal layer as a function of the relative concentration of the homeotropic alignment material, JALS2021, which is available from Japan Synthetic Rubber Company. The homogeneous alignment agent is also from the same company with a model number of JALS9203. Many other materials will show the same trends. In fact the materials can be from different companies with different solvents. It has indeed been checked that combinations of other polyimides can produce similar results. For example, the homogeneous alignment agent from Nissan SE-610 and the homeotropic alignment agent JALS2021 can work together as well. The values of the pretilt angles obtained are slightly different though. However, the trend of increasing pretilt angle as the percentage of JALS2021 is increased is still the same.
The polar anchoring energy achievable with this new method is very good. FIG. 5 shows the measured anchoring energies as a function of the percentage of the vertical alignment agent. It can be seen that the value of the anchoring energy is of the order of 10⁻³J/cm². This is similar to values obtained with conventional methods.
Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. Therefore, the present invention is not limited by the specific disclosure herein.

Claims

1. A method of producing an alignment layer in a liquid crystal cell, comprising the steps of:

a) forming a solution of a homogeneous (horizontal) alignment material;

b) forming a solution of a homeotropic (vertical) alignment material;

c) mixing the alignment materials in steps (a) and (b) to form a clear and slightly colored solution without precipitates;

d) applying the mixture in step (c) on a substrate to form a solid film;

e) curing the film in step (e), in one or multiple steps to form a hardened solid film; and

f) treating the solid film in step (f) mechanically to obtain a uniform alignment direction.

2. The method of claim 1, wherein said homogeneous alignment material is a polymer such as polyimide (PI), polystyrene (PS), poly-methyl methacrylate (PMMA), polycarbonates (PC), polyamic acid (PAA) or polyvinyl alcohol (PVA).

3. The method of claim 1, wherein said homeotropic alignment material is a polymer such as polyimide (PI), polystyrene (PS), poly-methyl methacrylate (PMMA), polycarbonates (PC), polyamic acid (PAA) or polyvinyl alcohol (PVA).

4. The method of claim 1, wherein step (c) includes the step of mixing the alignment materials in steps (a) and (b) in an appropriate ratio such that the weight ratio of the homeotropic material is from 0.1 to 99 percent of the total weight of the final mixture excluding the weight of the solvent(s).

5. The method of claim 1, wherein step (c) includes the step of ensuring that the solubility of said homogeneous alignment material and the solubility of said homeotropic alignment material are different in the solvent mixture formed in step (c).

6. The method of claim 1, wherein the hardened solid film of step (e) is of thickness between 10 nm and 300 nm.

7. The method of claim 1, wherein the application of the mixture of said homogeneous and homeotropic alignment materials to said substrate is by the method of spin coating.

8. The method of claim 1, wherein the application of the mixture of said homogeneous and homeotropic alignment materials to said substrate is by the method of screen printing.

9. The method of claim 1, wherein the curing of the solid film in step (e) is thermal curing.

10. The method of claim 1, wherein the curing of the solid film in step (e) is photo-curing.

11. The method of claim 1, wherein the mechanical treatment in step (f) is mechanical rubbing by a piece of fabric in a fixed direction.

12. The method of claim 1, wherein the mechanical treatment in step (f) is irradiation by an ion beam in a vacuum in a fixed direction at a fixed incident angle.

13. The method of claim 1, wherein the substrate is indium tin oxide coated glass.

14. The method of claim 13, wherein the indium tin oxide is patterned into rows and columns in a passive matrix display.

15. The method of claim 1, wherein the substrate has an active matrix thin film transistor array.