GB2266599A

GB2266599A - Optical phase-retardation compensating film

Info

Publication number: GB2266599A
Application number: GB9309047A
Authority: GB
Inventors: Jong-Cheon Lee
Original assignee: Samsung Electron Devices Co Ltd
Current assignee: Samsung SDI Co Ltd
Priority date: 1992-04-30
Filing date: 1993-04-30
Publication date: 1993-11-03
Anticipated expiration: 2013-04-30
Also published as: TW225546B; JPH0627323A; GB2266599B; FR2690762A1; GB9309047D0; DE4314349A1; FR2690762B1; CN1078049A

Abstract

An optical phase-retardation compensating film using a polymer dispersed liquid crystal includes a polymer layer (20) having liquid crystal (10) in a globular state or contiguously mixed therein, a protection element for protecting the polymer layer, in which the liquid crystal has a preset optical phase-retardation compensating value. Also, transparent electrodes (50 and 50') are provided on both sides of the polymer layer (20) to thereby compensate for the orientation of the liquid crystal by a compensating voltage supplied to the electrodes (50 and 50'). When applied to a liquid crystal display, the characteristic of the optical phase-retardation compensating film is adjusted corresponding to the change of optical-control of the liquid crystal display due to the ambient temperature, so that the optical phase retardation can be effectively compensated. <IMAGE>

Description

OPTICAL PHASSRETARDATION COMPENSATING FILM The present invention relates to a film for compensating optical phase retardation, and more particularly to an optical phase-retardation compensating film using polymer-dispersed liquid crystal.

Conventionally, high molecular film is stretched in one direction to induce anistropy in the molecular arrangement, so that the stretched film is employed as an optical phaseretardation compensating film having optical anisotropy. Also, liquid crystalline polymer (LCP) is orientated to form a twisted configuration, thereby using the twisted LCP as an optical phase-retardation compensating film.

As shown in FIG. 1, of the accompanying drawings, known optical phase-retardation compensating film is formed such that a high molecular film 1 having birefringence properties is attached on a rear plate 3 via an adhesive layer 4, and then, a faceplate 2 is joined thereon. Such a conventional optical phase-retardation compensating film affects the phase retardation variation with respect to transmitted light by means of the high molecular film generally composed of poly-carbonate or polystrene in one direction in order to induce anisotropy of the molecules, so that the berefringence is obtained.

This conventional optical phase-retardation compensating film is mainly employed in liquid crystal displays so as to contrive improved picture quality. However, since the conventional optical phase-retardation compensating film has different physical properties from those of the liquid crystal in a display apparatus employing the film, a desired purpose cannot be sufficiently accomplished when put into practical use. This is due to the fact that, while the birefringence of the liquid crystal being the heart of the display apparatus is varied by light wavelength and temperature variations, the conventional optical phase-retardation compensating film has a pre-designed compensating degree of optical phase retardation, thereby to impede the compensation of its own phase retardation caused by environmental changes.

Accordingly, the optical phase retardation cannot be effectively compensated in correspondence to the birefringent variation of the liquid crystal.

Additionally, the latter LCP is obtained by dissolving an LCP having a cholesteric phase in a solvent at high temperature and then coating the obtained LCP on a substrate whose surface is pre-oriented so as to make the thickness of the LCP filmlike. This LCP has drawbacks in that its optical characteristic which changes in accordance with temperature variation is difficult to match with that of a super twisted nematic liquid crystal, and to possess the same optical characteristic as the liquid crystal molecules due to the difference of molecular weights.Moreover, since a single substrate isa requisfte, the thickness cannot he thinned below a certain extent It is an object of the present invention to provide an optical phase-retardation compensating film capable of effectively compensating optical phase retardation, corresponding to the birefringent variation of a liquid crystal display.

It is another object of the present invention to provide a variable optical phase retardation compensating film capable of effectively compensating optical phase retardation, corresponding to the birefringent variation of a liquid crystal display.

According to one aspect of the present invention, there is provided an optical phase-retardation compensating film comprising a polymer layer having liquid crystal dispersed in a globular state or interlinked with each other in a polymer, and protection means for protecting the polymer layer, wherein the liquid crystal is oriented in a predetermined direction thereby to have a preset optical phase-retardation compensating value.

According to another aspect of the invention there is provided an optical phaseretardation compensating film comprising a polymer layer having liquid crystal dispersed in a globular state; protection means for protecting the polymer layer; and transparent electrodes provided on both sides of the polymer layer, whereby the orientation of the liquid crystal is compensated by a compensating voltage supplied to the electrodes.

In embodiments of the optical phase-retardation compensating film according to the present invention, the liquid crystal globules may be separately dispersed thereby to cause disclination or may be interlinked with adjacent globules thereby to be formed as a network.

Also, after being dispersed in the monomer layer for the use of the polymer layer, the liquid crystal can be oriented in one direction by an ordinary post-processing for polymerizing the monomer. The post-processing is carried out such that the polymer layer is stretched to induce simultaneously the anistropy of the polymer and the orientation of the liquid crystal.

in another embodiment, a rubbed orientation layer, e.g. a polyimide, is provided on both sides of the polymer layer. Therefore, the liquid crystal is oriented in a predetermined direction without performing the post-processing, thereby allowing the liquid crystal to have a predetermined optical phase-retardation compensation value. This can be performed together with the stretching of the polymer.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: FIG. 1 is a schematic section view showing a conventional optical phaseretardation compensating film; FIG. 2 is a schematic section view showing one embodiment of a fixed optical phase-retardation compensating film according to the present invention; FIG. 3 is a schematic section view showing another embodiment of the fixed optical phase-retardation compensating film according to the present invention; FIGs. 4 and 5 are schematic section views showing yet other embodiments of the fixed optical phase-retardation compensating film according to the present invention; FIG. 6 is a schematic section view of one embodiment of a variable optical phase-retardation compensating film according to the present invention;; FIG. 7 is a schematic section view showing another embodiment of the variable optical phase-retardation compensating film according to the present invention; FIG. 8 is a schematic section view showing yet another embodiment of the variable optical phase-retardation compensating film according to the present invention; and FIGs. 9 and 10 are graphs representing the optical transmittance of the optical phase-retardation compensating film of the present invention and that of a general liquid crystal.

An optical phase-retardation compensating film according to the present invention is classified into two types: one is a fixed type wherein the phase compensating value is fixed, and the other is a variable type wherein the phasecompensating value is varied. Therefore, these will be separately described below.

Fixed Optical Phase-retardation compensatin film FIG. 2 illustrates an embodiment of the fixed optical phase-retardation compensating film according to the present invention, wherein the pre-orientation of a liquid crystal can be attained by its structural nature. Here, a polymeric liquid crystal is applied which is obtained by dispersing a small quantity of liquid crystal in a polymer and generally used for display apparatuses. The polymer is stretched in one direction to induce anisotropy in the aspect of molecular configuration, when required.

Through this process, the liquid crystal can be oriented. First, orientation layers 30 and 30' of polyimide and high molecular films 40 and 40' are sequentially stacked on both the top and bottom, respectively, of apolymer layer20, wherein liquid crystal 10 is dispersed in a globular state. -The lower high molecular film 40' is attached on a rear plate 70 via an adhesive layer 50, and a faceplate 60 is stacked on the upper portion of upper molecular film 40.

In this structure, the orientation layer is selectively adopted when, as described above, the polymer layer is stretched thereby structurally to orient the liquid crystal, and can be utilized altogether. Also, the orientation layer determines the orientation direction of each molecule of liquid crystal 10, which is based upon a value for compensating for an optical phase retardation. If the polymer layer is stretched as described above, the orientation direction must be parallel to the stretched direction.

FIGs. 3. 4 and 5 briefly illustrate other embodiients of the present invention. Here, FIG. 3 shows an optical phase-retardation compensating film wherein faceplate 60 of the FIG. 2 embodiment is omitted. FIG. 4 is based on polymer networked-liquid crystal (PNLC) configuration wherein liquid crystal 10 is interlinked in the polymer as a network. In FIG. 5, liquid crystal 10 and polymer 20 are mixed as a gel.

Variable Optical Phase-retardation compensatinz film FIG. 6 schematically illustrates an embodiment of the variable optical phaseretardation compensating film according to the present invention.

Liquid crystal 10 is dispersed in a polymer layer 20 in a globular state, and electrodes 50 and 50' composed of a transparent conductive material, e.g., indium-tin oxide (ITO), light-transmitting faceplate and rear plate 40 and 40' are respectively stacked on the front and rear portions of polymer layer 20.

Since the aligned angle of liquid crystal in the polymer layer is changed by a variable voltage applied to electrodes 50 and 50', the phase of light passing through the polymer layer can be varied. Further, polymer layer 20 may be stretched in one direction for the orientation of the liquid crystal. In this case, even though a voltage is not supplied to electrodes 50 and 50', a certain fixed optical phase compensating value can be obtained.

Also, the fixed optical phase compensating value becomes varied when applying a predetermined voltage to the electrodes thereby to obtain a desired compensating value.

FIGs. 7 and 8 illustrate further embodiments of the variable optical phaseretardation compensating film according to the present invention. Here, orientation layers 30 and 30' are added to the structure of the embodiment of FIG. 6, for fixed optical phase compensation of the liquid crystal like the stretching of the polymer layer.

At this time, of course, the stretching of the polymer layer is selective. The stretching of the polymer layer along with the addition of an orientation layer facilitates the adjustment of the fixed optical phase compensating value.

Here, the orientation layer determines the orientation direction of each molecule of liquid crystal 10 in the globular state, which is based upon a value for compensating for an optical phase retardation. If the polymer layer is stretched as described above, the orientation direction must be parallel to the direction of the stretching.

In the variable optical phase-retardation compensating film of the present invention, the liquid crystal dispersed in the polymer layer may be separately dispersed as noncontiguous globules, or interlinked with each other to thus form a PNLC as the embodiment shown in FIG. 4. Also, the liquid crystal may be in a gel state as shown in FIG. 8.

Manufacturing Method Hereinbelow, a method for manufacturing above described embodiments of the optical phase-retardation compensating film according to the present invention will be described.

To begin with, a small quantity of polymer having the same configuration as a liquid crystal molecule is added to the liquid crystal so as to induce a certain molecular arrangement. A liquid crystal orientation layer is formed on at least one side of and outside a liquid crystal layer to induce the orientation of the liquid crystal molecules, and the small quantity of polymer is also induced to have the same orientation. The orientation layer formation method uses the same method as that for a conventional liquid crystal orientation layer, and, a pretilt angle has a range of O - 90" with respect to the normal line of the orientation layer, even though the pretilt angle differs according to the LCD which adopts the same.All liquid crystals, whether they have a negative or a positive anisotropic dielectricity, can be applied; especially, biaxial nematic liquid crystal can also be utilized. The polymer is formed such that a monomer is mixed with the, liquid crystal and a small quantity of photo-initiator, the mixture is injected into the space of a cell, and then ultraviolet rays are applied, thereby achieving the polymerization.

The mixture ratio of the polymer is 0.1~20wt% of the liquid crystal, and the photo-initiator is properly added according to the amount of the monomer. If an electric field or magnetic field having a proper intensity is applied to be across the liquid crystal layer while irradiating the ultraviolet rays, the orientation direction of liquid crystal can be selected freely. The thickness of the liquid crystal layer is 1 - 30corm, and typical minute balls or rod-shaped spacers are mixed to maintain the uniform thickness. The liquid crystal orientation layer is formed on a transparent glass, a glass coated with ITO, poly-carbonate film, poly-acetate film, a polymer film coated with ITO, etc.

If the monomer and the liquid crystal are properly heated during mixing, the mixing is more rapidly carried out. Moreover, when the mixture is properly heated at the time of its injection into the cell, the injection can be accelerated while preventing the mixture from being separated. The polymerization of the monomer can be adjusted by heating the cell during the application of ultraviolet rays. In general, the polymerization is accelerated at a high temperature, and the materials can be consistently mixed.

When only one side is oriented, the orientation direction is regularly maintained in one direction, and the orientation of the other side must be weakly treated as such.

At this time, the arrangement of the liquid crystal molecules is not so good as compared with the case when both sides are oriented. A polymer rubbing, LB-PI and SiO2 deposition are given as an examp]e of the orientation method. There is no method to absolutely weaken the orientation, but methods using an unrubbed polymer, an etching with a weak acid solution and coating with an uncrystallized inorganic material, are suggested to comparatively weaken the orientation.

When both sides are oriented, the liquid crystal orientation direction of the sides should be set to 0 - 1800, which is determined by the rubbing direction of an orientation material such as polyimide. In this state, the liquid crystal and polymer molecules are arranged parallel in one direction. Thus, an anisotropy of an optical refraction occurs due to the anisotropy of the molecular arrangement. Generally, the birefringence index is smaller than that of nematic liquid crystal, and is varied by a polymer content, manufacturing circumstances, an interaction between liquid crystal molecules, etc. At this time, preferably thebirefringence of the liquid crystal is ina range of 0.005 - 0.35.

If the orientation direction of both sides has a certain angle (not zero), a chiral dopant is added to the nematic liquid crystal to give a desired twisting power, wherein the twisted amount is determined by the thickness of the film and the angle formed by the orientation directions of both sides. It should be noted that the difference between the natural twisted angle of the nematic liquid crystal by the chiral dopant and the angle formed by two orientation directions must be smaller than 90" and greater than -90 . (The angle formed by the two orientation directions may be more than 360 .) If the difference is not in this range, disclination occurs in the liquid crystal configuration.

Example 1 After coating a rubbed glass surface of a plate with PI (SE-150) to a thickness of 800A, the obtained resultant is hardened by heating. Then, rubbing is performed with a piece of soft cloth at a rate of 12m/sec. 100 to 200 minute balls of approximately 6pm in diameter, are distributed per one square millimeter. Thereafter, using a sealing material hardened by ultraviolet rays, two glass plates are fixed parallel to each other by a predetermined distance, leaving an inlet for injection, such that the glass plates are opposed so as to allow their rubbing directions to be parallel to each other, thereby forming a hollow cell.

4,4'-bisacryloyl-biphenyl 3wt% which is an ultraviolet-hardened monomer is put in liquid crystal (E7, Merk Co.), and 1 wt% of a photo-initiator (Darocure-1173) is added, which are then properly mixed by heating at a temperature of 60 C. Then, the mixture is injected into the hollow cell using a vacuum injection method at a temperature of 40 C.

After finishing the injection, end-sealing is performed using an ultraviolet-hardened material, and ultraviolet rays are irradiated with an intensity of 3mw/cm2 for an hour while maintaining the temperature of 40"C.

Example 2 Under the state of the Example 1, the upper and lower glass plates are adhered to each other such that their rubbing orientations are twisted to have an angle of 240C with respect to each other, thereby forming a vacuum cell. Then, 0.85wt% of S-811 (Merk Co.) is added to the liquid crystal as a chiral dopant.

Example 3 A glass coated with ITO electrodes is utilized in place of the rubbed glass plates employed in Examples 1 and 2.

Example 4 A voltage of 3V is applied to the ITO electrode when hardening with ultraviolet rays in Example 3.

Example 5 In place of the glasses used in Examples 1 to 4, a transparent polymer film or ITOcoated transparent polymer film is utilized. Here, polyimide coating is not performed, but the polymer film is directly rubbed without coating polyimide.

FIG. 9 illustrates the transmittance of light of the cell completely composed of liquid crystal and an optical phase-retardation compensating film according to an embodiment of the present invention manufactured by the above-described methods. Here, the transmittance of a laser beam (He-Ne) is measured while rotating an analyzer under the state that the thickness of the cell is 6,um, the twisted angle is 90 , the rubbing orientation forms horizontality, and a polarizer is set to 00, assuming the rubbing orientation as a reference of 90". The horizontal axis represents the angle of the analyzer, and the vertical axis represents the transmittance and arbitrary scales.Here, the solid line denotes a cell composed of liquid crystal, and the dotted line denotes the characteristic variations of the cell according to the embodiment of the present invention.

The result of FIG. 9 shows that, although the transmittance characteristic of the cell having the polymer is slightly inferior to that of the cell ccmposed of the liquid crystal, its molecular configuration is twisted to 900 and the cell having the polymer has birefringent characteristics.

FIG. 10 illustrates the transmittance characteristic of the cell manufactured by being supplied with 3 volts when applying ultraviolet rays to the monomer. Here7 the solid line denotes the characteristic of a cell formed without the applied voltage, and the dotted line denotes the characteristics of a cell wherein the rnonomer is hardened while supplying the 3 volts.

During the above-described polymerizing process, the liquid crystal is arranged in the orientation direction by the interaction with the orientation layer, maintain the state. Here, the liquid crystal globules are separately dispersed or interlinked with one another to thereby form a network configuration.

optical phase-retardation compensating film according to the present invention manufactured as above performs the optical phase-retardation compensation via the interaction between the polymer layer and liquid crystal. Here, if the refraction of the dispersed liquid crystal corresponds to that of the polymer layer, light is not scattered, to thereby form a transparent film. Moreover, since the liquid crystal oriented by the orientation layer has its own birefringence, the light passing through the liquid crystal exhibits a phase retardation. The phase retardation of the transmitted light which is varied according to the characteristic of liquid crystal can be easily adjusted by the thickness of the polymer layer, and accordingly, can be easily adapted to the characteristic of an applied liquid crystal display.

If the optical control state of a liquid crystal display is changed due to ambient temperature variations when the optical phase-retardation compensating film according to the present invention is applied to the liquid crystal display, its characteristic is varied corresponding to these changes, so that optical phase-retardation is effectively compensated. Additionally, since a liquid crystal of the same characteristic can be used as the optical phase-retardation filter of the liquid crystal display, the matching of characteristics with respect to the corresponding liquid crystal display is greatly facilitated.

Furthermore, the variable optical phase-retardation compensating film according to the present invention has an optical control layer wherein liquid crystal globules are dispersed in the polymer. Here, the liquid crystal is treated to be oriented toward one direction, and an electrode for actively compensating for the optical phase retardation is provided on both sides of the optical control layer by compensating for the oriented degree of the liquid crystal. Therefore, the birefringence can be optionally adjusted to match the optical control characteristics of the liquid crystal display, thereby enabling an effective optical phase-retardation compensation. For example, by monitoring ambient temperature, humidity or the wavelength of incident light, an adjusting voltage is applied which permits the liquid crystal to have a desired birefringence according to the result of the monitoring.

While the present invention has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be effected therein without departing from the scope of the invention.

Claims

1. An optical phase-retardation compensating film comprising a polymer layer having liquid crystal dispersed in a globular state or interlinked with each other in the polymer, and protection means for protecting said polymer layer, wherein said liquid crystal has a preset optical phase-retardation compensating value.

2. An optical phase-retardation compensating film comprising a polymer layer stretched in one direction, liquid crystal dispersed in a globular state in said polymer layer, and protection means for protecting said polymer layer, wherein said liquid crystal is oriented in a predetermined direction to have a preset optical phase-retardation compensating value.

3. An optical phase-retardation compensating film as claimed in claim 1 or 2, wherein a plurality of liquid crystal globules are noncontiguously dispersed.

4. An optical phase-retardation compensating film as claimed in claim 1 or 2, wherein said liquid crystal globules are interlinked with one another thereby to form one network in said polymer layer.

5. An optical phase-retardation compensating film as claimed in any preceding claim, wherein an orientation layer for orienting said liquid crystal is provided on both sides of said polymer layer.

6. An optical phase-retardation compensating film as claimed in claim 5, wherein said orientation layer comprises rubbed polyimide.

7. An optical phase-retardation compensating film as claimed in any preceding claim, wherein the thickness of said polymer layer is 1 - 30m.

8. An optical phase-retardation compensating film as claimed in any preceding claim, wherein the birefringence index of said liquid crystal has a value within a range of 0.005 - 0.35.

9. An optical phase-retardation compensating film as claimed in any preceding claim, wherein said liquid crystal has a dielectric anisotropy of either positive or negative.

10. An optical phase-retardation compensating film as claimed in any preceding claim, wherein the mixture ratio of said polymer to said liquid crystal is 0.1 - 20wt%.

11. An optical phase-retardation compensating film as claimed in any preceding claim, wherein said liquid crystal is a nematic type, and the difference of a natural twisted angle and an angle formed by two orientation directions is smaller than 90C and greater than e 9or .

12. An optical phase-retardation compensating film as claimed in any preceding claim, wherein the pretilt angle of said liquid crystal has a range of 0 - 900.

13. A variable optical phase-retardation compensating film comprising a polymer layer having liquid crystal dispersed in a globular state; protection means for protecting said polymer layer; and transparent electrodes on both sides of said polymer layer, whereby the orientation of said liquid crystal is adjusted by a compensating voltage supplied to said electrodes.

14. A variable optical phase-retardation compensating film as claimed in claim 13, wherein said polymer layer is stretched, so that said liquid crystal dispersed in said polymer layer is oriented in a predetermined direction.

15. A variable optical phase-retardation compensating film as claimed in claim 13 or 14 wherein an orientation layer for orientating said liquid crystal is provided on each side of said polymer layer.

16. A variable optical phase-retardation compensating film comprising: a polymer layer having liquid crystal dispersed in a globular state; protection means for protecting said polymer layer; transparent electrodes provided on both sides of said polymer layer for adjusting the orientation of said liquid crystal by means of a compensating voltage supplied to said both electrodes; and orientation layers on the inner surface of said transparent electrodes directly in contact with said polymer layer for allowing said liquid crystal dispersed in said polymer layer to be oriented in a predetermined direction.

17. A variable optical phase-retardation compensating film as claimed in claim 16, wherein said polymer layer is stretched in one direction and the liquid crystal dispersed in the stretch polymer layer is oriented in a predetermined direction.

18. A variable optical phase-retardation compensating film as claimed in any of claims 15 to 17, wherein said orientation layer comprises rubbed polyimide.

19. A variable optical phase retardation compensating film as claimed in any one of claims 13 to 18, wherein the liquid crystal globules are interlinked with one another thereby to form a network of said liquid crystal in said polymer layer.

20. A variable optical phase-retardation compensating film as claimed in any of claims 13 to 19, wherein the thickness of said polymer layer is 1 - 30yam.

21. A variable optical phase-retardation compensating film as claimed in any of claims 13 to 20, wherein the birefringence index of said liquid crystal has a value within a range of 0.005 - 0.35.

22. A variable optical phase-retardation compensating film as claimed in any of claims 13 to 21, wherein said liquid crystal has a dielectric anistropy of either positive or negative.

23. A variable optical phase-retardation compensating film as claimed in any of claims 13 to 22, wherein the mixture ratio of said polymer to said liquid crystal is 0.1 20wit%.

24. A variable optical phase-retardation compensating film as claimed in any of claims 13 to 23, wherein said liquid crystal is a nematic type, and the difference of a natural twisted angle and an angle formed by two orientation directions is smaller than 90" and greater than -90".

25. A variable optical phase-retardation compensating film as claimed in any of claims 13 to 24, wherein the pretilt angle of said liquid crystal has a range of 0 - 90".

26. An optical phase-retardation compensating film substantially as hereinbefore described with reference to any of FIGs. 2 to 8 of the accompanying drawings.

27. A variable optical phase-retardation compensating film substantially as hereinbefore described with reference to any of FIGs. 6 to 8 of the accompanying drawings.