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Title:
METHOD FOR MANUFACTURING A POLARIZER
Document Type and Number:
WIPO Patent Application WO/2006/070318
Kind Code:
A2
Abstract:
A method for manufacturing a polarizer is disclosed. The method comprises: providing a liquid crystalline mixture comprising reactive molecules, non-reactive molecules, dichroic dye molecules, and a polymerization initiator; providing a uniaxially oriented film of said mixture; polymerizing said film; and removing at least part of said non-reactive molecules. Also disclosed are polarizers manufactured by such a method, as well as optical devices comprising such polarizers.

Inventors:
HIKMET RIFAT A M (NL)
Application Number:
PCT/IB2005/054367
Publication Date:
July 06, 2006
Filing Date:
December 21, 2005
Export Citation:
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Assignee:
KONINKL PHILIPS ELECTRONICS NV (NL)
HIKMET RIFAT A M (NL)
International Classes:
G02B5/30; C09B69/00; C09K19/38
Foreign References:
EP1462485A12004-09-29
EP1256602A12002-11-13
US6245399B12001-06-12
US20040135117A12004-07-15
Attorney, Agent or Firm:
Damen, Daniel M. (AA Eindhoven, NL)
Download PDF:
Claims:
CLAIMS:
1. A method for manufacturing a polarizer comprising: providing a liquid crystalline mixture comprising reactive molecules, nonreactive molecules, dichroic dye molecules, and a polymerization initiator; providing a uniaxially oriented film of said mixture; polymerizing said film; and removing at least part of said nonreactive molecules.
2. A method according to claim 1, wherein said uniaxially oriented film is formed on a layer which induces planar uniaxial orientation.
3. A method according to claim 2, wherein said layer which induces planar uniaxial orientation is a rubbed polymer layer.
4. A method according to claim 3, wherein said rubbed polymer layer is a reactive polyimide layer.
5. A method according to claim 2, wherein said layer which induces planar uniaxial orientation is a photoalignment layer.
6. A method according to any one of the preceding claims, wherein the weight ratio of said reactive molecules and said nonreactive molecules is in the range of 1:10 to 10:1.
7. A method according to any one of the preceding claims, wherein the concentration of said dichroic molecules in said mixture is in the range of 0.1 10% by weight.
8. A method according to any one of the preceding claims, wherein said reactive molecule is a liquid crystalline diacrylate, epoxy, vinyl ether or thiolene system.
9. A method according to any one of the preceding claims, wherein said reactive molecule is the molecule of formula (XIII):.
10. A method according to any one of the preceding claims, wherein said polymerization is initiated by ultraviolet radiation, gamma rays, xrays, electron beams, or heat.
11. A method according to any one of the preceding claims, wherein said at least part of said nonreactive molecules are removed by heating or solvent extraction.
12. A polarizer obtainable by a method according to any one of the claims 1 to 11.
13. An optical device comprising a polarizer according to claim 12.
Description:
Method for manufacturing a polarizer

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing a polarizer. It also relates to polarizers manufactured by such a method as well as to optical devices comprising such polarizers.

BACKGROUND OF THE INVENTION

Polarizers are widely used in many optical devices, such as liquid crystal displays and emissive displays, in order to enhance contrast. The most common commercially available polarizers are made of highly uniaxially oriented polymeric materials containing dichroic dye molecules or crystals. They are then laminated onto the surfaces.

In some cases, there is a need for polarizers which can be coated onto patterned surfaces. For this purpose, polarizers which can be brought onto a surface from a lyotropic liquid crystal phase has been suggested (Optiva). Another possibility is to use dye molecules which can be mixed into a monomeric reactive liquid crystal mixture. Such a liquid mixture can be brought onto a surface at high speeds followed by polymerization using UV light.

One problem with the currently used techniques is that the solubility of dichroic dyes in liquid crystals is rather low. Thus, thick films are needed in order to obtain effective polarizers. However, it is desirable that a display including a polarizer is as thin as possible, and therefore it is desirable to have a thin efficient polarizer. This is particularly the case for reliable and/or flexible displays.

SUMMARY OF THE INVENTION

One object of the invention is to provide a way to produce effective polarizers having reduced thickness.

This object is achieved by a method for manufacturing a polarizer comprising: providing a liquid crystalline mixture comprising reactive molecules (1), non-reactive molecules (2), dichroic dye molecules (3), and a polymerization initiator; providing a uniaxially oriented film of said mixture;

polymerizing said film; and removing at least part of said non-reactive molecules.

By the polymerization of such a liquid crystalline mixture, a gel is obtained, which is composed of cross-linked polymer networks and dye molecules, and is swollen by the non-reactive molecules. Because of this swelling, the gel can take a relatively high concentration of dichroic dye molecules.

However, it was found that removing the non-reactive molecules essentially leaves the dichroic dye molecules behind in the network, so that a very thin polarizer can be formed. The method according to the invention therefore allows higher dye loading at a given thickness, and can result in a thinner polarizer with maintained dichroic ratio.

The uniaxially oriented film may be formed on a layer which induces planar uniaxial orientation (4). For example, the film may be formed on a rubbed polymer layer, such as a reactive polyimide layer, which induce uniaxial macroscopic orientation in liquid crystal molecules. The uniaxially oriented film may also be formed on a photoalignment layer.

For example, the weight ratio of the reactive and non-reactive molecules is in the range of 10:1 to 1:10, and the concentration of dichroic molecules is in the range of 0.1 - 10% by weight.

The reactive molecules may be selected from the molecules disclosed in table 1. For example, the reactive molecule is a liquid crystalline diacrylate, epoxy, vinyl ether or thiolene system. A specific example of a reactive molecule, C6M, is shown in table 3.

The polymerization may be induced by ultraviolet (UV) radiation, gamma rays, x-rays, electron beams, or heat.

The removal of non-reactive molecules may be effected by heating or solvent extraction.

The invention also relates to a polarizer obtainable by the above-described method, as well as to an optical device comprising such a polarizer.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. Ia-Ic is a schematic illustration of the method for manufacturing a polarizer according to the invention, showing cross-sections of the manufactured polarizer,

Fig. 2 and Fig. 3 show the results from the examples 1 and 2, where the absorbance corresponding to parallel and perpendicular arrangement of the polarization direction with respect to molecular alignment is shown,

Figs. 4 to 7 show the properties of various networks containing dye molecules, Fig. 8 shows the dichroic ratio (absorbance parallel/absorbance perpendicular) as a function of wavelength for different materials.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present inventors have found that advantageously an anisotropic gel system may be used for producing thin polarizers.

The method comprises the following main steps: providing a liquid crystalline mixture comprising reactive molecules (1), non-reactive molecules (2), dichroic dye molecules (3), and a polymerization initiator; providing a uniaxially oriented film of said mixture; polymerizing said film; and removing at least part of said non-reactive molecules.

In the first step, a liquid crystalline mixture, containing reactive molecules (1) and non-reactive molecules (2), is made. The reactive molecule (2) may be a cross-linkable mesogen, e.g. a liquid crystal diacrylate. As used herein, the expression "reactive molecule" relates to a molecule with at least one reactive group which, in the presence of a suitable initiator, leads to polymerization of the molecule upon exposing it to high energy radiation, such as ultraviolet light, electron beams, x-rays, or simply by heating. Suitable examples of reactive molecules are shown in table 1. More specifically, the reactive molecule is suitably a liquid crystalline diacrylate, epoxy, vinyl ether or thiolene system. For example C6M, the structural formula of which is shown in table 3, may be used.

The non-reactive molecule (2) may be a non-reactive mesogen, and may be selected from any conventional LC-molecules, such as those used in liquid crystal display cells. As used herein, the expression "non-reactive molecule" relates to a molecule which does not have reactive groups and does not get polymerized.

A mesogen is a compound that under suitable conditions can exist as a mesophase (liquid crystal phase), i.e. a phase occurring within the mesomorphic state. The mesomorphic state is a state in which the degree of molecular order is intermediate between the perfect three-dimensional, long-range positional and orientational order found in solid crystals and the absence of long-range order found in isotropic liquids, gases, and amorphous solids. In thermotropic liquid crystals a mesophase can be observed in a certain temperature range.

In the liquid crystalline mixture, a maximum amount of dichroic dye molecules (3) is dissolved. The concentration of the dichroic molecules in the mixture is suitably in the range of 0.1 to 10 % by weight, or in the range of 2-6% by weight. Examples of suitable dichroic molecules are shown in table 2. As used herein, the expression "dichroic" relates to a property of a substance in which one polarized component of incident light is absorbed to a different extent than the other polarization component. The amount of reactive and non-reactive molecules shall be effective to dissolve the dichroic dye. The weight ratio of the reactive and non-reactive mesogens may be in the range of 10:1 to 1 : 10, or in the range of 4:1 to 1:4. The mixture is also provided with an initiator for polymerization (not shown in

Fig. 1). Such an initiator can produce suitable species (ions, radicals) to induce polymerization when heated or subjected to radiation. Examples of polymerization initiators are peroxides, such as benzoyl peroxide, and azo compounds, such as AIBN. Photo initiators, such as α,α- dimethoxydeoxybenzø, can also be used, as well as cationic initiators, such as diphenyliodonium tetraiuluoroborate.

Subsequently, the mixture shall be formed into an uniaxially oriented film (Fig. Ia). This is effected by the use of a layer which induces planar uniaxial orientation (4). The uniaxially oriented film may for example be formed on a uniaxially rubbed polymer layer, such as a polyimide layer, which induce uniaxial macroscopic orientation in liquid crystal molecules.

For example, the uniaxially oriented film may be obtained by placing said mixture in a cell produced by placing glass plates provided with uniaxially rubbed polyimide layers on top of each other separated by 10 μm thick spacers. When the liquid crystal mixture comes in contact with uniaxially rubbed polyimide layers, uniaxial macroscopic orientation in the direction of the rubbing is induced within the liquid crystal. However, uniaxial orientation can also be induced in a thin liquid crystal film by simply placing it on top of a rubbed polyimide layer.

A uniaxially oriented film can also be obtained on so-called photo-orientable layers, i.e. photo alignment layers. Such layers are obtained upon exposure to polarized light. They tend to induce macroscopic orientation in liquid crystal molecules brought on top of them. Various other techniques are also known to those working in the field.

As used herein, the expression "uniaxially oriented film" relates to a molecule system where the long axis of the molecules is orientated macroscopically on average in the same direction.

Thereafter, the system is polymerized in the oriented state to form a polymer network containing free molecules. Thereby, a uniaxially oriented polymerized film comprising uniaxially oriented dye molecules and non-reactive molecules are obtained (Fig. 2b). The polymerization is suitably effected by ultraviolet radiation. However, other polymerization methods may be used, e.g. gamma rays, x-rays, electron beam, or thermal polymerization.

The polymerization may e.g. be induced by ultraviolet(UV) radiation at room temperature. Under the influence of UV radiation the photo initiator present in the mixture forms radicals which start the polymerization of the reactive groups. The mixture according to the invention shows a stable liquid crystal phase with a high order parameter making material processing at room temperature possible. There are many liquid crystal phases reported in the literature. Nematic, Smectic-A, smectic-B are the most known phases where the long axis of the molecules are oriented along a common average axis. Nematic phase has a lower order parameter than smectic-A phase and smectic-B phase shows the highest order. By the polymerization of the mixture, an anisotropic gel is obtained, i.e. a gel having properties which vary depending on the angle of observation. In liquid crystals, this is due to the alignment and the shape of the molecules. The gel is composed by cross-linked anisotropic networks and dye molecules swollen by the non-reactive molecules. After polymerization, the system is heated to evaporate the free liquid crystalline molecules, i.e. the non-reactive mesogens, leaving the network and the dye molecules behind (Fig. Ic). The thickness of the polymerized film is thereby reduced to form the thin film dichroic polarizer. According to the present invention, a shrinkage of the film in the magnitude of 75% may be obtained. The non-reactive molecules may also be removed by extraction using a suitable solvent. The heating may be performed under a protecting atmosphere, e.g. under a nitrogen atmosphere, in order to prevent degradation of the dye molecules.

Suitably, the reduction of the film is uniaxial. In order to obtain a uniaxial shrinkage, i.e. only the thickness of the gel decreases, the adhesion between the surface and the gel should be high. High adhesion is obtained by forming the gel film on a reactive polyimide layer. When the layer which induces planar uniaxial orientation is made of a material which polymerizes with the molecules of the gel the adhesion is increased, and a uniaxial shrinkage of the gel is promoted, i.e. only the thickness of the gel decreases.

wherein :

X is selected from the group consisting of:

wherein p is an integer in the range of 1-20.

R is selected from the group consisting of H, CH 3 and Cl, and M is selected from the group consisting of:

wherein the rings can be benzene or cyclohexane, which might have substitutions .

EXAMPLES The invention will now be further explained in the following examples. These examples are only intended to illustrate the invention and should in no way be considered to limit the scope of the invention.

Example 1 and 2 A liquid crystalline mixture was made by mixing the eutectic mixture E44

(consisting a mixture of cyano biphenyl and terphenyl compounds, obtained from BDH/ Merck) with a liquid crystalline diacrylate C6M (table 3) in a 1 :4 ratio. The mixture was also provided with 0.5% photo initiator Irgacure 651 (Ciba Geigy.) Fig. 2 shows the absorbance corresponding to parallel and perpendicular arrangement of the polarization direction with respect to molecular alignment for the first experiment. Fig. 3 shows the result of the second example. Here again it can be seen that thin and effective polarizers can be made.

Example 3 In order to compare properties of various networks containing dye molecules, the following experiments were performed.

Example 3:1

A reactive acrylate C6M containing 4% dye molecules (S-409) was prepared and polymerized at 100 0 C. The spectra are shown in Fig 4.

Example 3:2

The mixture described in example 3:1 (4% dye in C6M) was mixed with E7 (obtained from BDH/MERCK, consisting a mixture of cyano biphenyl and terphenyl

compounds) to a weight ratio of 1 : 1. The mixture was than polymerized. The absorbance of the polymerized gel is shown in Fig 5.

Example 3:3 The non reactive molecules (E7) were extracted by gradual heating of the gel to 190 0 C. This was done under nitrogen atmosphere in order to reduce/prevent degradation of the dye molecules. The spectra of the extracted gel is shown in fig. 6.

Example 3:4 During the evaporation described in example 3:3 it could be seen that the gel shrunk. In order to prevent this a reactive polyimide layer was used. In this way shrinkage could be prevented. The absorbance of extracted gel is shown in fig 7.

Example 3:5 In order to compare various materials we looked the dichroic ratio

(Absorbance parallel/absorbance perpendicular) as a function of wavelength for different materials and the results are shown in figure 8.

In figure 8 it can be seen that the especially when a reactive poyimide is used the order parameter of dye in the extracted network is almost the same as in the polymer network (obtained by polymerization of dye in C6M without an additive).

Example 4

The network obtained by polymerization of pure C6M was compared with the network obtained after extraction of E7 from the gel. This was done by measuring the weight gain of the networks after soaking them in chloroform. In the case of a pure network a weight increase of 3% was observed after soaking it in chloroform. In the case of a network obtained by extraction, a weigh increase of 83% was observed. This result clearly shows that the network obtained according to the present invention is very different from the network obtained by polymerization of pure C6M. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent for one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.