Optical compensator, process of its fabrication, and liquid crystal display

The present invention relates to a multilayer optical compensator for a liquid crystal display containing polymer A and polymer B layers exhibiting specific birefringence properties. The present invention also relates to such compensators and methods for manufacturing liquid crystal displays using such compensators.

Liquid crystals are widely used for electronic displays. In these display systems, the liquid crystal cell is generally located between a pair of polarizers and an analyzer. The incident light polarized by the polarizer passes through the liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be changed by applying a voltage applied to the cell. The deflected light enters the detector. By using this principle, it is possible to control the transmission of light from an external light source including ambient light. The energy required to achieve this control is generally much less than that required for fluorescent materials used in other display types such as cathode ray tubes (CRTs). Therefore, liquid crystal technology is used for many electronic imaging devices including, but not limited to, digital clocks, computers, portable computers, and electronic games, where light weight, low power consumption, and long life are important characteristics.

Contrast, color reproduction, and stable grayscale intensity are important quality attributes for electronic displays using liquid crystal technology. A major factor limiting the contrast of a liquid crystal display (LCD) is the tendency for light to "leak" through liquid crystal elements or cells that are in a dark or "black" pixel state. Moreover, this leak, and thus the contrast of the liquid crystal display, also depends on the direction in which the display screen is viewed. In general, optimal contrast is observed only within a narrow viewing angle region that is concentrated around vertical incidence on the display, and drops sharply as the viewing angle moves away from the display perpendicular. In color displays, this leakage problem not only results in poor contrast, but also causes color or hue shifts with associated poor color reproduction.

LCDs are quickly replacing CRTs as monitors for desktop computers and other office or household appliances. It is also expected that the number of LCD TV monitors with large screen sizes will increase sharply in the near future. However, if viewing angle dependence issues such as coloration, contrast degradation, and brightness inversion are not resolved, the use of LCDs as a replacement for conventional CRTs is limited.

Vertically oriented liquid crystal displays (VA-LCDs) provide extremely high contrast ratios for vertically incident light. 2A and 2B are schematic views of the VA liquid crystal cell in the OFF 201 and ON 203 states. In its off state, the liquid crystal optical axis 205 is approximately perpendicular to the substrate 207, FIG. 2A. When a voltage is applied, the liquid crystal optical axis 205 tilts away from the cell vertical direction, Fig. 2B. In the off state, the light does not show birefringence in the vertical direction 209, giving a dark state close to that of an orthogonal cross-polarizer. However, the light 211 propagating diagonally encounters a phase delay and causes light leakage. This results in a poor contrast ratio in some viewing angle regions.

The bend-oriented nematic liquid crystal display, also called an adaptive optics bend liquid crystal display (OCB-LCD), uses a nematic liquid crystal cell based on a symmetrically bent state. In its actual operation, the brightness of a display using a bend-oriented nematic liquid crystal cell is controlled by an applied voltage or electric field that results in different angles of bend within the cells shown in FIGS. 3A (off) 301 and 3B (on) 303. Will be done. In both states, the liquid crystal optical axis 305 is symmetrically bent around the cell central plane 307. In the on state, the optical axis is substantially perpendicular to the cell plane except near the cell substrate 309. OCB mode provides a faster reaction rate suitable for liquid crystal display television (LCD-TV) applications. It also has the advantage of viewing angle characteristics (VAC) over conventional displays such as twisted nematic liquid crystal displays (TN-LCDs).

The two modes mentioned above are expected to dominate high-end applications such as LCD-TVs due to their superiority over conventional TN-LCDs. However, practical applications for both OCB and VA-LCD require optical compensating means for optimizing VAC. In both modes, due to the birefringence of the liquid crystal and the cross-polarizer, the VAC has poor contrast when viewing the display from an tilt angle. It has been proposed to use biaxial film to compensate for OCB (US Pat. No. 6,108,058) and VA (Japanese Patent Laid-Open No. 11-95208) LCDs. In both modes, the liquid crystal is oriented sufficiently perpendicular to the plane of the cell in the on (OCB) or off (VA) state. This state is positive R_thTherefore, the compensation film has a negative R large enough for satisfactory optical compensation._thMust have. Big R_thThe need for biaxial films with is also common to ultra-twisted nematic liquid crystal displays (STN-LCDs).

Sufficiently negative R suitable for compensating LCD modes such as OCB, VA and STN_thSeveral methods have been proposed for producing biaxial films with.

U.S. Pat. No. 2001/0026338 discloses the use of delayed-increasing agents in combination with triacetyl cellulose (TAC). The delayed increasing agent is selected from aromatic compounds having at least two benzene rings. Stretcher-doped TAC, generally by R_thAnd R_inCan produce both. The problem with this method is the amount of drug doping. R_thAnd R_inThe amount of drug required to produce the desired effect is high enough to cause coloration. In this way, R_thAnd R_inIt is difficult to control the value of.

Sasaki et al. Have proposed the use of cholesteric liquid crystals placed on a thermoplastic resin substrate that is positively birefringent (US Patent Application Publication No. 2003/0086033). The pitch of the cholesteric liquid crystal (CHLC) is shorter than the wavelength of visible light, so a properly oriented CHLC has a negative R._thShows the original birefringence that gives. R_inIs controlled by adjusting the amount of stretching of the thermoplastic resin substrate. This method is R_thAnd R_inCan be adjusted individually. However, the use of short pitch CHLC not only increases manufacturing costs, but also complicates the process due to the orientation operation.

Japanese Patent Application Laid-Open No. 2002-210766 discloses the use of propionyl or butyryl-substituted TAC. They show a higher birefringence than normal TAC. Therefore, in general, R by biaxially stretching the substituted TAC film._inAnd R_thTo cause. This method does not require any additional coatings or layers, but it is R_inAnd R_thDifficult to control the independence of.

<p> Therefore, an independently controlled R that can be easily manufactured._thAnd R_inIt is a problem to be solved to provide a multilayer optical compensator having the above.</p>

<p> The present invention is a multilayer compensator containing one or more polymer A layers and one or more polymer B layers: the A layer is even less negative than -0.01 out-of-plane (Δn)._th) Contains a polymer with a birefringence; the B layer contains an amorphous polymer with an out-of-plane birefringence that is even more negative than -0.01; and the overall in-plane delay (R) of the multilayer compensator._in) Is greater than 20 nm, and the out-of-plane delay (R) of the multilayer compensator_th) Provides a multi-layer compensator that is even more negative than -20nm. The present invention also provides a method for preparing an LCD and a compensator of the present invention.</p><p> The multilayer optical compensator of the present invention is easily manufactured and R._inAnd R_thProvide the required value of.</p><p> The present specification is concluded in a claim that specifically points out and explicitly claims the subject matter of the invention, but if the invention is taken into account in conjunction with the accompanying drawings, it will be better understood from the following description. There will be.</p>

The following definitions apply to the description herein. The "optical axis" refers to the direction in which the propagating light does not see birefringence. The "on and off states" refer to states in which the voltage applied to the liquid crystal cell is present or absent. In-plane phase delay, R of layer 101 shown in FIG._inIs quantitatively defined by (nx-ny) d. Here, nx and ny are the refractive indexes in the x and y directions. The x-axis is taken as the direction of the maximum index of refraction in the xy plane, and the y-direction is perpendicular to the x-axis. For a stretched positive birefringent layer, x corresponds to the direction of primary stretching. The xy plane is parallel to the plane 103 of the layer. d is the layer thickness in the z direction. Quantity (nx-ny), in-plane birefringence, Δn_inThat is. Δn_inAnd R_inThe value of is subsequently given at the wavelength λ = 550 nm.

Out-of-plane phase delay, R of layer 101 shown in FIG._thIs quantitatively defined herein by [nz-(nx + ny) / 2] d. nz is the index of refraction in the z direction. Quantity [nz-(nx + ny) / 2], out-of-plane birefringence, Δn_thThat is. If nz&gt; (nx + ny) / 2, Δn_thIs positive, so the corresponding R_thIs also positive. If nz &lt;(nx + ny) / 2, Δn_thIs negative and R_thIs also negative. Δn_thAnd R_thThe value of is subsequently given at the wavelength λ = 550 nm.

Polymer "inherent birefringence" Δn_intIn this specification, (n_e-n_{o o}) Is the quantity defined by. Where n_eAnd n_{o o}Are the anomalous refractive index and the normal refractive index of the polymer, respectively. The intrinsic birefringence is determined by factors such as the polarizability of the functional groups and the angle of bond to their polymer chains. Actual birefringence of the polymer layer (in-plane Δn_inOr out-of-plane Δn_th) Is the way it is generated, and thus the order parameter, and Δn_intIt depends on.

"Amorphous" means lack of long-range order. Therefore, amorphous polymers do not show long-range order even when measured by techniques such as X-ray diffraction. "Chromophore" means an atom or group of atoms that serves as a unit in light absorption (Modern Molecular Photochemistry Nicholas J. Turro Editor, Benjamin / Cummings Publishing Co., Menlo Park, CA (1978) Pg77). Common color group groups include vinyl, carbonyl, amide, imide, ester, carbonate, aromatic (ie, heteroaromatic or alicyclic aromatic such as phenyl, naphthyl, biphenyl, thiophene, bisphenol group), sulfone. , And azo groups or combinations of these groups.

"Invisible chromophore" means a chromophore having a maximum absorption value outside the range of 400 to 700 nm. By "adjacent" is meant that the articles are in contact with each other. In two adjacent layers, one layer is in direct contact with the other. Therefore, when the polymer layer is formed on the substrate by coating, the substrate and the polymer layer are adjacent to each other.

The present invention is a multilayer compensator containing one or more polymer A layers and one or more polymer B layers: the A layer is even less negative than -0.01 out-of-plane (Δn)._th) Contains a polymer with a birefringence; the B layer contains an amorphous polymer with an out-of-plane birefringence that is even more negative than -0.01; and the overall in-plane delay (R) of the multilayer compensator._in) Is greater than 20 nm, and the out-of-plane delay (R) of the multilayer compensator_th) Is a multi-layer compensator that is even more negative than -20nm. The A layer is made of a polymer film other than those containing a chromophore group in the main chain. Layer A has a positive intrinsic birefringence, Δn_intHave. Examples of such polymers include TAC, cellulose butyrate acetate (CAB), cyclic polyolefins, polycarbonates, polysulfonates, and other polymers known to those of skill in the art. These polymeric materials can be in film form by solvent casting, thermal extrusion, or other methods. R over 20 nm in layer A_inAny feasible method can be used to produce, but the most commonly performed operation is stretching.

Layer A is positive Δn_intSince it is made from a polymer film having, the refractive index in the plane of the layer satisfies nx&gt; ny, where "x" is the primary stretching direction and "y" is the direction perpendicular to x. By stretching the polymeric material, the individual polymeric chain segments are predominantly oriented in the primary stretching direction, resulting in increased birefringence of the polymeric layer. Since it is necessary to orient the polymer segments, stretching must take place above the glass transition temperature of the polymeric material. Therefore, the polymer film is T_gIt is heated and stretched beyond.

Another method is to stretch the film while introducing the solvent into the film. By this method, the film can be stretched immediately after the polymer is solvent cast into film form. The film can be uniaxially or biaxially stretched. In uniaxial stretching, the film is stretched in one direction. However, it is difficult to control the trirefractive index, nx, ny and nz of the film (where nz is the refractive index in the perpendicular direction of the film) by uniaxial stretching. This is especially true if the stretch of the film in the x direction is large enough to have shrinkage in the y direction. This shrinkage efficiently provides stretching in the film perpendicular direction z, resulting in an increase in nz. In biaxial stretching, when the bistretching directions x and y are perpendicular to each other, the undesired shrinkage caused by stretching in the primary stretching direction (ie, in the x direction) is prevented by simultaneous stretching in the second direction (y). Will be done. As a result, the increase in nz can be effectively prevented.

The means for stretching is not particularly limited as long as the stretched film has sufficient uniformity at 3 refractive indexes. Polymer A layer is less negative than -0.01 Δn_thHave. The polymer A layer of the multilayer compensator has an overall in-plane delay (R) of the multilayer compensator._in) Is suitablely greater than 20 nm, preferably between 30 and 200 nm, and conveniently between 30 nm and 150 nm.

The polymer B layer is generally solvent coated on the A layer. This solvent coating could be achieved by spin coating, hopper coating, gravure coating, wire bar coating, or other coating methods known to those of skill in the art. The applied layer B is adjacent to the layer A.

Layer B is applied from a solution containing a polymer that produces a high negative birefringence, which is even more negative than -0.01 when solvent coated. Negative Δn_th(Or R_th) To produce a positive Δn_intA polymer having the above is used. Such polymers usually contain invisible chromogenic groups such as vinyl, carbonyl, amide, imide, ester, carbonate, sulfone, azo and aromatic (ie, benzene, naphthalate, biphenyl, bisphenol A) groups in the polymer main chain. To do. Examples of such polymers include polyester, polycarbonate, polyimide, polyetherimide, and polythiophene. Also, in general, fillers and non-polymer molecules could be added to these polymers for the second layer.

Desirably, the polymer intended for use in layer B does not have a chromophore away from the main chain. An example of an undesired polymer having a chromophore in and away from the main chain would be polyallylate with a fluorene group. Glass transition temperature of polymer used in layer B (T_g) Is important. It is preferably above 180 ° C to achieve the desired result. The polymers used in layer B could be synthesized by a variety of techniques: condensation, addition, anion, cation, or other common synthetic methods.

The thickness of each B layer is preferably less than 30 μm. In general, it is preferably 0.1 μm to 20 μm. Conveniently, it is preferably 1.0 μm to 10 μm. Desirably, it is preferably 2 μm to 8 μm.

The total thickness of the multilayer optical compensator is preferably less than 200 μm. In general, it is preferably 40 μm to 150 μm. Desirably, it is preferably 80 μm to 110 μm.

The B layer is preferably thick enough so that the out-of-plane delay of the B layer is even more negative than -20 nm. In general, it is preferably -600 nm to -60 nm. Conveniently, it is preferably -500 nm to -50 nm. Desirably, it is -400 nm to -50 nm.

Now, the various elements of the present invention are numerically displayed with reference to the drawings in which the present invention is studied so that those skilled in the art can make and use the present invention. It will be appreciated that elements not specifically indicated or described can take various forms well known to those of skill in the art.

4A, 4B and 4C are elevational schematic views of a typical multilayer optical compensator according to the present invention. The compensator 401 in FIG. 4A has a structure in which the B layer 409 is arranged on the A layer 407. Layer A 407 and Layer B 409 are adjacent. It is also possible to have two B layers 413, 415 arranged on one A layer 411, as in the compensator 403 in FIG. 4B. In the other case 405, one B layer 417 is sandwiched by two A layers 419, 421. The compensator 405 can be formed, for example, by laminating adjacent layers of A421 and B417 and a single layer of A419. Lamination is performed at the interface between layer B 417 and layer A 419, and the two layers 417 and 419 may or may not be adjacent depending on the method of lamination. Those skilled in the art will be able to come up with even more complex structures.

In the LCD 501 shown in FIG. 5A, the liquid crystal cell 503 is placed between the polarizer 505 and the analyzer 507. The transmission axes of the polarizer 509 and the analyzer 511 form an angle of 90 ° ± 10 °, therefore the pair of polarizers 509 and the analyzer 511 are said to be "cross-polarizers". The multilayer optical compensator 512 is placed between the polarizer 505 and the liquid crystal cell 503. It can also be placed between the liquid crystal cell 503 and the analyzer 507. The LCD 513 schematically shown in FIG. 5B has two multilayer optical compensators 515, 517 placed on each surface of the liquid crystal cell 503. FIG. 5C shows an application example of the multilayer optical compensator in the reflective LCD 519. The liquid crystal cell 503 is located between the polarizer 505 and the reflector 521. In the figure, the multilayer compensator 523 is placed between the liquid crystal cell 503 and the polarizer 505. However, it can also be placed between the reflector 521 and the liquid crystal cell 503.

Compared to the prior art, embodiments of the present invention avoid delay-increasing agents that cause coloration, do not require the use of liquid crystal compounds and manipulation of their placement, and are functionally enhanced optical of relatively thin (&lt;200 μm) structures. It provides compensation and is easily manufactured.

As a further attribute, the embodiment is primarily the responsibility of layer A, R._inAllows control of, but on the other hand R_thControl is primarily the responsibility of Layer B. In the prior art, R_inAnd R_thAre often paired and are not controlled independently.

The present invention will be further described by the following non-limiting examples.<u style="single">Polymer 1 (Synthetic):</u> 4,4'-Hexafluoroisopropyridene diphenol (23.53 g, 0.07 mol), 4,4'-(2-norbornenilidene) bisphenol (8.4 g, 0.03 mol) and triethylamine in methyl ethyl ketone (100 mL) at 10 ° C. To a stirred mixture of (22.3 g, 0.22 mol) was added a solution of terephthaloyl chloride (16.23 g, 0.8 mol) and isophthaloyl chloride (4.08 g, 0.2 mol) in methyl ethyl ketone (60 mL). After the addition, the temperature was left to rise to room temperature and the solution was stirred under nitrogen for 4 hours, during which time triethylamine hydrochloride precipitated in gelatinous form and the solution viscous. The solution was then diluted with toluene (160 mL), washed with diluted hydrochloric acid (200 mL of 2% acid), and then washed 3 times with water (200 mL). The solution was then poured into ethanol with vigorous stirring to precipitate the white bead-like polymer, which was collected and dried under vacuum at 50 ° C. for 24 hours. The glass transition temperature of this polymer was measured at 265 ° C by differential scanning calorimetry.

<chemistry num="1"><img file="JP4636622B2_D0001.tif" /></chemistry>

Poly (4,4'-hexafluoroisopropylidene-bisphenol-co-4,4'-(2-norbornenilidene) bisphenol) terephthalate-co-isophthalate

<u style="single">Polymer 1</u> Polymer 1 was spin cast onto both glass slides and stretched polymer substrate samples (8% solids in 80% propyl acetate and 20% toluene) and R._thAnd R_inThe analysis was performed using an ellipsometer (model M2000V, JA Woollam Co.) at a wavelength of 550 nm. These values are listed in Table I.

<tables num="1"><img file="JP4636622B2_D0002.tif" /></tables>

The layer of polymer 1 also showed no signs of long-range order, so it was measured that the layer consisted of an amorphous polymer.

<figref num="1">FIG. 1 is a diagram of a typical layer with a thickness d and an xyz coordinate system attached to the layer.</figref><figref num="2A">FIG. 2A is a schematic diagram showing a general on (ON) and off (OFF) state of the VA liquid crystal cell.</figref><figref num="2B">FIG. 2B is a schematic diagram showing a general on (ON) and off (OFF) state of the VA liquid crystal cell.</figref><figref num="3A">FIG. 3A is a schematic diagram showing a general ON (ON) and OFF (OFF) state of the OCB liquid crystal cell.</figref><figref num="3B">FIG. 3B is a schematic diagram showing a general ON (ON) and OFF (OFF) state of the OCB liquid crystal cell.</figref><figref num="4A">FIG. 4A is an elevational schematic view of the multilayer optical compensator of the present invention.</figref><figref num="4B">FIG. 4B is an elevational schematic view of the multilayer optical compensator of the present invention.</figref><figref num="4C">FIG. 4C is an elevational schematic view of the multilayer optical compensator of the present invention.</figref><figref num="5A">FIG. 5A is a schematic view of a liquid crystal display having the multilayer optical compensator of the present invention.</figref><figref num="5B">FIG. 5B is a schematic view of a liquid crystal display having the multilayer optical compensator of the present invention.</figref><figref num="5C">FIG. 5C is a schematic view of a liquid crystal display having the multilayer optical compensator of the present invention.</figref>

Code description

101 film 103 film plane 201 VA LCD cell in the off state 203 VA LCD cell in the on state 205 LCD optical axis 207 LCD cell substrate 209 Light propagating vertically in the cell 211 Light propagating diagonally OCB LCD cell in the 301 off state OCB LCD cell with 303 on 305 LCD optical axis 307 cell center plane 309 cell boundary

401 Multi-layer optical compensator 403 Multi-layer optical compensator 405 Multi-layer optical compensator 407 A layer 409 B layer 411 A layer 413 B layer 415 B layer 417 B layer 419 A layer 421 A layer 501 LCD 503 LCD cell 505 Polarizer 507 Photon 509 Polarizer transmission axis 511 Transmitter transmission axis 512 Multi-layer optical compensator 513 LCD 515 Multilayer optical compensator 517 Multilayer optical compensator 519 LCD 521 reflector 523 Multilayer optical compensator

Refractive index in the nx x direction Refractive index in the ny y direction Refractive index in the nz z direction no Normal index of refraction ne abnormal refractive index Δn_th Out-of-plane birefringence Δn_ln In-plane birefringence Δn_lnt Polymer-specific birefringence Layer or film thickness R_th Out-of-plane phase delay R_in In-plane phase delay λ wavelength T_g Glass-transition temperature