OPTICAL COMPENSATOR AND LIQUID CRYSTAL DISPLAY

Abstract: The invention relates to an optical compensator for liquid crystal displays comprising: at least one O plate retarder (3, 3'); at least one DAC film (4, 4') having the optical properties of a negative C plate retarder, and further relates to a liquid crystal display comprising such a compensator.

Description: Optical Compensator and Liquid Crystal Display Field of the Invention The invention relates to an optical compensator for liquid crystal displays and to a liquid crystal display comprising such a compensator. Background and Prior Art Optical compensators are used to improve the optical properties of liquid crystal displays (LCD), such as the contrast ratio and the grey scale representation at large viewing angles. For example in uncompensated displays of the TN or STN type at large viewing angles often a change of the grey levels and even grey scale inversion, as well as a loss of contrast and undesired changes of the colour gamut are observed. An overview of the LCD technology and the principles and methods of optical compensation of LCDs is given in US 5,619,352, the entire disclosure of which is incorporated into this application by way of reference. As described in US 5,619,352, to improve the contrast of a display at wide viewing angles a negatively birefringent C-plate compensator can be used, however, such a compensator does not improve the greyscale representation of the display. On the other hand, to suppress or even eliminate grey scale inversion and improve the grey scale stability US 5,619,352 suggests to use a birefringent O- plate compensator. An O-plate compensator as described in US 5,619,352 includes an O-plate, and may additionally include one or more A-plates and/or negative C-plates. The terms 'O-plate', 'A-plate' and 'C-plate' as used in US 5,619,352 and throughout this invention have the following meanings. An 'O- plate' is an optical retarder utilizing a layer of a positively birefringent (e.g. liquid crystal) material with its principal optical axis oriented at an oblique angle with respect to the plane of the layer. An 'A-plate' is an optical retarder utilizing a layer of uniaxially birefringent material with its extraordinary axis oriented parallel to the plane of the layer, and its ordinary axis (also called ^a-axis') oriented perpendicular to the plane of the layer, i.e. parallel to the direction of normally incident light. A 'C-plate' is an optical retarder utilizing a layer of a uniaxially birefringent material with its extraordinary axis (also called 'c-axis') perpendicular to the plane of the layer, i.e. parallel to the direction of normally incident light. As an O-plate retarder for example an optical retardation film (hereinafter abbreviated as ORF) comprising a layer of a liquid crystal or mesogenic material with tilted or splayed structure can be used. As an A-plate retarder for example a uniaxially stretched polymer film, like for example a stretched polyvinylalcohol (PVA) or polycarbonate (PC) film, can be used. Alternatively, an A-plate retarder may comprise for example a layer of a positively birefringent liquid crystal or mesogenic material with planar orientation. As a negatively birefringent C-plate retarder for example a uniaxially compressed polymer film can be used. Alternatively, a negatively birefringent C-plate may comprise for example a layer of a liquid crystal or mesogenic material with a planar orientation and a negative birefringence. Typical examples of negatively birefringent liquid crystal materials are various kinds of discotic liquid crystal compounds. In addition to US 5,619,352, optical compensators comprising one or more O plates are described in prior art in WO 97/44409, WO 97/44702, WO 97/44703 and WO 98/12584, the entire disclosure of which is incorporated into this application by way of reference. WO 97/44703 and WO 98/12584 further suggest to use O plates in combination with a A plate. WO 97/44703 reports that the use of a compensator comprising a O plate in combination with a A plate, wherein the principal optical axes of both ORFs are oriented at right angles to each other, allows particularly good compensation of a TN-LCD, as it simultaneously reduces the angle dependence of the contrast and the grey scale inversion in the display. However, when using compensators as described in the above mentioned prior art in combination with liquid crystal displays, especially TN or STN-displays, the improvements of the optical properties of the display, like contrast at wide viewing angles, grey scale level stability, and suppression of grey scale inversion, are still far from sufficient for most applications. Therefore, it is desirable to have available improved optical compensators to further improve the optical performance of LCDs. Definition of Terms In connection with optical polarization, compensation and retardation layers, films or plates as described in the present application, the following definitions of terms as used throughout this application are given. For the sake of simplicity, the term 'liquid crystal material' is used hereinafter for both liquid crystal materials and mesogenic materials, and the term 'mesogen' is used for the mesogenic groups of the material. The terms 'tilted structure' or 'tilted orientation' means that the optical axis of the film is tilted at an angle θ between 0 and 90 degrees relative to the film plane. The term 'splayed structure' or 'splayed orientation' means a tilted orientation as defined above, wherein the tilt angle additionally varies monotonuousiy in the range from 0 to 90 °, preferably from a minimum to a maximum value, in a direction perpendicular to the film plane. The term 'low tilt structure' or 'low tilt orientation' means that the optical axis of the film is slightly tilted or splayed as described above, with the average tilt angle throughout the film being between 1 and 10 °. The term 'planar structure' or 'planar orientation' means that the optical axis of the film is substantially parallel to the film plane. This definition also includes films wherein the optical axis is slightly tilted relative to the film plane, with an average tilt angle throughout the film of up to 1 °, and which exhibit the same optical properties as a film wherein the optical axis is exactly parallel, i.e. with zero tilt, to the film plane. The average tilt angle θ ave is defined as follows wherein θ'(d') is the local tilt angle at the thickness d' within the film, and d is the total thickness of the film. The tilt angle of a splayed film hereinafter is given as the average tilt angle θ ave> unless stated otherwise. The term 'helically twisted structure' relates to a film comprising one or more layers of liquid crystal material wherein the mesogens are oriented with their main molecular axis in a preferred direction within molecular sublayers, with this preferred orientation direction in different sublayers being twisted around a helix axis that is substantially perpendicular to the film plane, i.e. substantially parallel to the film normal. This definition also includes orientations where the helix axis is tilted at an angle of up to 2 ° relative to the film normal. The term 'homeotropic structure' or 'homeotropic orientation' means that the optical axis of the film is substantially perpendicular to the film plane, i.e. substantially parallel to the film normal. This definition also includes films wherein the optical axis is slightly tilted at an angle of up to 2 ° relative to the film normal, and which exhibit the same optical properties as a film wherein the optical axis is exactly parallel, i.e. with no tilt, to the film normal. For sake of simplicity, an optical film with a tilted, splayed, low tilted, planar, twisted and homeotropic orientation or structure is hereinafter being shortly referred to as 'tilted film', 'splayed film', 'low tilt film', 'planar film', 'twisted film' and 'homeotropic film', respectively. Throughout this invention, both a tilted and a splayed film will also be referred to as 'O plate'. A planar film will also be referred to as 'A plate' or 'planar A plate'. A low tilt film will also be referred to as 'low tilt A plate'. A twisted film will also be referred to as 'twisted A plate'. In tilted, planar and homeotropic optical films comprising uniaxially positive birefringent liquid crystal material with uniform orientation, the optical axis of the film as referred to throughout this invention is given by the orientation direction of the main molecular axes of the mesogens of the liquid crystal material. In a splayed film comprising uniaxially positive birefringent liquid crystal material with uniform orientation, the optical axis of the film as referred to throughout this invention is given by the projection of the orientation direction of the main molecular axes of the mesogens onto the surface of the film. The term 'film' as used in this application includes self-supporting, i.e. free-standing, films that show more or less pronounced mechanical stability and flexibility, as well as coatings or layers on a supporting substrate or between two substrates. The term 'liquid crystal or mesogenic material' or 'liquid crystal or mesogenic compound' should denote materials or compounds comprising one or more rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e. groups with the ability to induce liquid crystal phase behaviour. The compounds or materials comprising mesogenic groups do not necessarily have to exhibit a liquid crystal phase themselves. It is also possible that they show liquid crystal phase behaviour only in mixtures with other compounds, or when the mesogenic compounds or materials, or the mixtures thereof, are polymerized. Brief Description of the Drawings Figure 1 and 2 depict uncompensated prior art TN-LCD devices according to comparison example 1. Figure 3 and 4 depict compensated TN-LCD devices with compensators according to example 3 and 4 of the present invention. Figure 5a and 6a are isocontrast plots of uncompensated prior art TN-LCD devices according to comparison example 1 and 2, respectively. Figures 5b, 6b and 5c, 6c are grey level diagrams of uncompensated prior art TN-LCD devices according to comparison example 1 and 2, respectively in horizontal (b) and vertical (c) viewing planes. Figures 7a and 8a are isocontrast plots of compensated TN-LCD devices according to examples 3 and 4, respectively. Figures 7b, 8b and 7c, 8c are grey level diagrams of compensated TN-LCD devices according to example 3 and 4, respectively, in horizontal (b) and vertical (c) viewing planes. Summary of the Invention One aim of the present invention is to provide an optical compensator which has improved performance for compensation of LCDs, is easy to manufacture, in particularly for mass production, and does not have the drawbacks of prior art compensators as described above. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description. The inventors of the present invention have found that the above drawbacks can be overcome, and an optical compensator with superior performance for compensation of the optical properties of liquid crystal displays can be obtained by using a combination of at least one O plate retarder and at least one diacetylcellulose (DAC) film exhibting the optical properties of a negative C plate retarder. The use of DAC films as compensators for TN and STN displays has been reported in prior art in US 4701 ,028 (Clerc). However, there is no suggestion in these documents to use the DAC film in combination with a tilted or splayed optical retarder. When using an optical compensator according to the present invention in an LCD, the contrast at large viewing angles and the grey level representation of the display are considerably improved, and grey scale inversion is suppressed. In case of coloured displays, the colour stability is considerably improved and changes of the colour gamut are suppressed. Furthermore, a compensator according to the present invention is particularly suitable for mass production. One object of the present invention is an optical compensator for liquid crystal displays, characterized in that it comprises - at least one O plate retarder, - at least one diacetylcellulose (DAC) film having the optical properties of a negative C plate retarder. Another object of the invention is a liquid crystal display device comprising the following elements - a liquid crystal cell formed by two transparent substrates having surfaces which oppose each other, an electrode layer provided on the inside of at least one of said two transparent substrates and optionally superposed with an alignment layer, and a liquid crystal medium which is present between the two transparent substrates, - a polarizer arranged outside said transparent substrates, or a pair of polarizers sandwiching said substrates, and - at least one optical compensator according to the present invention, being situated between the liquid crystal cell and at least one of said polarizers, it being possible for the above elements to be separated, stacked, mounted on top of each other or connected by means of adhesive layers in any combination of these means of assembly. Detailed Description of the Invention Preferred embodiments of the present invention relate to an optical compensator comprising at least one O plate and at least one DAC film as described above, wherein - the average tilt angle θ ave in said O plate is from 2 to 88 °, preferably from 30 to 60 ° - the tilt angle θ in the O plate varies monotonuousiy in a direction perpendicular to the plane of the film, - the tilt angle θ in the O plate varies from a minimum value θ mιn at one surface of the film to a maximum value θ max at the opposite surface of the film, - θ min in the O plate is from 0 to 80°, preferably from 1 to 20°, - θ max in the O plate is from 10 to 90°, preferably from 40 to 90°, - the thickness d of the O plate is from 0.1 to 10 μm, in particular from 0.2 to 5 μm, very preferably from 0.3 to 3 μm, - the optical retardation of the O plate is from 6 to 300 nm, in particular from 10 to 200 nm, very preferably from 20 to 120 nm, - the O plate comprises a linear or crosslinked liquid crystalline polymer, - the thickness of the DAC film is from 10 to 300 μm, in particular from 20 to 200 μm, very preferably from 50 to 150 μm, - the on-axis optical retardation of the DAC film , i.e. the retardation for light at normal incidence, is from 2 to 100 nm, in particular from 3 to 50 nm, very preferably from 5 to 20 nm, - the optical retardation of the DAC film for light with an angle of incidence of 60 ° is from 20 to 250 nm, in particular from 30 to 200 nm, very preferably from 45 to 150 nm, Especially preferred is an optical compensator comprising one O plate and one negatively birefringent DAC film as a negative C plate. A further preferred embodiment of the present invention relates to a liquid crystal display comprising a liquid crystal cell, a pair of polarizers sandwiching the cell, and one inventive compensator as described above and below located on each side of the liquid crystal (LC) cell between the cell and the polarizer. Especially preferred are displays wherein • the LC cell is a twisted nematic or supertwisted nematic cell, • the O plate is facing the polarizer and the DAC film is facing the LC cell, • the optical axis of the O plate is parallel or at right angles to the optical axis of the liquid crystal medium at the nearest surface of the liquid crystal cell and to the polarization directon of the polarizers, • the O plate is situated with its low tilt surface facing the LC cell, The inventive optical compensators can be used for compensation of conventional displays, in particular those of the twisted nematic or super twisted nematic mode, such as TN, HTN, STN or AMD-TN displays, in displays of the IPS (in plane switching) mode, which are also known as 'super TFT' displays, in displays of the DAP (deformation of aligned phases) or VA (vertically aligned) mode, like e.g. ECB (electrically controlled birefringence), CSH (colour super homeotropic), VAN or VAC (vertically aligned nematic or cholesteric) displays, in displays of the bend mode or hybrid type displays, like e.g. OCB (optically compensated bend cell or optically compensated birefringence), R-OCB (reflective OCB), HAN (hybrid aligned nematic) or π-cell displays. Especially preferably the compensators are used for compensation of TN, HTN and STN displays. In the following, the invention will exemplarily be described in detail for compensation of a TN display. Fig. 1 depicts an uncompensated standard type TN display device in its off-state, i.e. when no voltage is applied, comprising a TN cell 1 with a liquid crystal layer in the twisted nematic state sandwiched between two transparent electrodes (which are not shown here), and a pair of linear polarizers 2,2'. The twisted nematic orientation of the liquid crystal layer is schematically depicted by the mesogens 1a. The dashed lines 1b and 1c represent the orientation direction of the mesogens 1a that are adjacent to the cell walls of the TN cell 1. In the display device shown in Fig. 1 , the polarization axes of the linear polarizers 2,2' are oriented at right angles to the optical axis 1 b, 1c of the liquid crystal medium at the nearest surface of the liquid crystal cell 1 , respectively. This orientation of the polarizers relative to the TN cell is hereinafter also generally referred to as Ε mode'. Fig. 2 depicts an uncompensated standard type TN display device like that of Fig. 1, but wherein the polarization axes of the linear polarizers 2,2' are oriented parallel to the optical axis 1b,1c of the liquid crystal medium at the nearest surface of the liquid crystal cell 1 , respectively. This orientation of the polarizers relative to the TN cell is hereinafter also generally referred to as 'O mode'. Fig. 3 and Fig. 4 schematically depict compensated TN-LCD devices according to a preferred embodiment of the present invention in the off-state, with Fig. 3 showing a device in the O mode and Fig. 4 showing a device in the E mode, as explained above. The devices in both Fig. 3 and Fig. 4 consist of a TN cell 1 with a liquid crystal layer in a twisted nematic state sandwiched between two transparent electrodes (which are not shown here), a pair of linear polarizers 2,2' and two compensators, each compensator consisting of a splayed O plate 3,3' and a negatively birefringent DAC film having the optical performance of a negative C plate. In the devices examplarily shown in Fig. 3 and Fig. 4 the O plates 3,3" are provided directly on the DAC films 4,4' which serve as substrates for the O plates. The stacks of optical components in the devices shown in Fig. 1 to 4 are symmetrical, hence incoming light may enter the device from either side. The DAC films 4,4', as an example, are generally uniaxially compressed in a direction perpendicular to the film plane as a consequence of the manufacturing process to give a negative C plate optical structure. The optical axis of the DAC films is then parallel to the compression direction. The O plates 3,3' consist, as an example, of a layer of polymerized liquid crystal material with splayed structure. The splayed structure is schematically depicted by the mesogens 3a and 3a' which are oriented with their main molecular axis tilted at an angle θ relative to the plane of the layer, wherein the tilt angle θ increases in a direction normal to the film, starting with a minimum value θ min on the side of the O plate 3,3' facing the TN cell 1. The dashed lines 3b and 3b' represent the projection of the orientation directions of the mesogens 3a and 3a', respectively, in different regions of the O plates 3,3' onto the surfaces of the respective O plates 3,3'. The dashed lines 3b,3b" are also identical with the principal optical axis of the respective O plates 3,3'. In the devices shown in Fig. 3 and 4, the principal optical axes of the O plates 3,3' are oriented parallel to the polarization direction of the respective adjacent linear polarizer 2,2', and parallel to the respective adjacent orientation direction 1 b, 1c of the mesogens 1a in the TN cell 1. In the devices shown in Fig. 3 and 4, the mesogens at the surface of the O plate 3,3' facing the TN cell 1 exhibit a planar orientation, i.e. the minimum tilt angle θ min is substantially 0 degrees. However, other values of θ mjn are also possible. In the O plate according to the preferred embodiments as shown e.g. in Fig. 3 and 4, the minimum tilt angle θ min is preferably from 0 to 80°, in particular from 1 to 20°, very preferably from 1 to 10° and most preferably from 1 to 5°. The maximum tilt angle θ max in an O plate according to these preferred embodiments is preferably from 10 to 90°, in particular from 20 to 90°, very preferably from 30 to 90°, most preferably from 40 to 90°. Apart from the preferred embodiments as depicted in Fig. 3 and 4, other combinations and stack formats of the O plates and DAC films are also possible. For example, in the preferred devices shown in Fig. 3 and 4, the O plate 3 and the adjacent DAC film 4, and/or the O plate 3' and the adjacent DAC film 4', are mutually exchangeable with each other. Furthermore, the compensators or entire ORF stacks on one side of the TN cell are mutually exchangeable with the compensators or entire film stacks on the opposite side of the TN cell. In the inventive devices exemplarily shown in Fig. 3 and 4, the optical axes 3b,3b' of the O plates 3,3' are either parallel or at right angles to the orientation direction 1 b, 1c of the mesogens 1a in the TN cell 1 and to the polarization direction of the polarizers 2,2'. In another preferred embodiment of the present invention, the optical axes 3b,3b' of the O plates 3,3' are twisted within the film plane at an angle δ relative to the optical axes of the the orientation direction 1 b, 1c of the mesogens 1a in the TN cell 1 and to the polarization direction of the polarizers 2,2'. The absolute value of said twist angle δ is preferably froml to 15 °, very preferably from 5 to 10 °. Further to the preferred embodiments shown in Fig. 3 and 4, a compensator according to the present invention may also comprise more than one O plate and/or more than one DAC film. In case the inventive compensator comprises two or more O plates, the optical axes of the O plates can be parallel one to another, or be oriented at an angle with one another. Preferably the optical axes of the O plates are oriented either parallel or at right angles to each other. In case an inventive compensator comprises two or more O plates, each O plate can be arranged relative to the closest successive O plate such that their respective surfaces with minimum tilt angle θ mιn are facing each other, or such that their respective surfaces with maximum tilt angle θ max are facing each other, or such that the surface of a first O plate with minimum tilt angle θ mιn is facing the surface of the closest successive O plate with maximum tilt angle θ max . Further preferred arrangements of two or more O plates in an inventive compensator are those as described in WO 98/12584, in particular those according to the preferred embodiments described in WO 98/12584 on pages 8-11 and in Figures 1a, 1 b and 1c. The devices shown in Fig. 3 and 4 comprises splayed O plates. Alternatively, it is possible to use tilted, but not splayed, O plates instead of, or in addition to splayed O plates in the inventive LC displays. Preferably, however the inventive LC displays do comprise one or more splayed O plates. The DAC films are preferably negatively birefringent films, which can be obtained by the uniaxial compression associated with the manufacturing process of commercially available DAC film. Negatively birefringent DAC films are commercially available e.g. from Clarifoil, Derby, UK. As an O plate for the inventive compensator it is possible to use an optical film comprising a polymerized liquid crystal material with tilted or splayed structure, as described in the US 5,619,352, WO 97/44409, WO 97/44702, WO 97/44703 or WO 98/12584, with the entire disclosure of these documents being incorporated into this application by way of reference. As an O plate, it is also possible to use a multilayer film comprising two or more sublayers of polymerized liquid crystal material, with each sublayer having a tilted structure with constant tilt angle, wherein said tilt angle increases or decreases monotonuousiy from one sublayer to the next sublayer throughout the multilayer. In a preferred embodiment of the invention, the O plate is a tilted or splayed optical retardation film (ORF) film as described in WO 98/12584, or a film prepared in analogy to the methods disclosed therein. According to the WO 98/12584, an ORF with tilted or splayed structure can be obtained by coating a layer of a polymerizable mesogenic material onto a substrate or between two substrates, aligning the material into a tilted or splayed orientation, and polymerizing the material by exposure to heat or actinic radiation. Alternatively it is possible to use as an O plate a liquid crystal film as described in WO 96/10770, which is prepared from a polymerizable liquid crystal material with a smectic A or smectic C phase and a nematic phase at higher temperatures. The polymerizable liquid crystal material is applied in its nematic phase onto a substrate that is e.g covered with an alignment layer of obliquely deposited SiO, and lowering the temperature into smectic C phase of the material. This leads to an increase of the tilt angle, as the material adopts its naturally tilted smectic C structure, which is then fixed by polymerization of the liquid crystal material. The above preparation method and possible variations thereof are described in detail in WO 96/10770, the entire disclosure of which is incorporated into this application by way of reference. It is also possible to use as an O plate an inorganic thin film with a tilted microstructure, which can be obtained by oblique vapor deposition of an inorganic material, e.g. Ta 2 0 5 , as described in WO 96/10773. Particularly preferred are inventive compensators wherein the O plate is provided directly on the DAC film, or sandwiched between two DAC films, which serve as substrates for the O plate. As linear polarizer, a standard type commercially available polarizer can be used. In a preferred embodiment of the present invention the linear polarizer is a low contrast polarizer. In another preferred embodiment of the present invention the linear polarizer is a dichroic polarizer, like a dyed polarizer. The optical compensator may also comprise one or more retardation films selected from planar A plates, low tilted A plates and highly twisted A plates and homeotropic films. The individual optical components in the inventive compensators and displays, such as the liquid crystal cell, the individual retarders and the linear polarizers, can be separated or can be laminated to other components. They can be stacked, mounted on top of each other or be connected e.g. by means of adhesive layers. It is also possible that stacks of two or more retarders are prepared by coating the liquid crystalline material of a retarder directly onto an adjacent retarder, the latter serving as substrate. The optical compensator and/or the display device according to the present invention may further comprise one or more adhesive layers provided to the individual optical components like the liquid crystal cell, the polarizers and the different retarders. In case the polymerized liquid crystal material in the O plate is a polymer with high adhesion, separate adhesive layers may also be omitted. Highly adhesive polymers are for example liquid crystal polyepoxides. Furthermore, liquid crystal linear polymers or crosslinked polymers with low degree of crosslinking show higher adhesion than highly crosslinked polymers. The above highly adhesive liquid crystal polymers are therefore preferred for specific applications, especially for those which do not tolerate additional adhesive layers. The inventive compensator may also comprise one or more protective layers provided on the surface of the individual optical components described above. In case of the O plate, the polymerizable material preferably consists essentially of achiral polymerizable mesogenic compounds. Preferably a polymerizable mesogenic material is used that comprises at least one polymerizable mesogen having one polymerizable functional group and at least one polymerizable mesogen having two or more polymerizable functional groups. In another preferred embodiment the polymerizable material comprises polymerizable mesogenic compounds having two or more polymerizable functional groups (di- or multireactive or di-or multifunctional compounds). Upon polymerization of such a mixture a three-dimensional polymer network is formed. An optical retardation film made of such a network is self-supporting and shows a high mechanical and thermal stability and a low temperature dependence of its physical and optical properties. By varying the concentration of the multifunctional mesogenic or non mesogenic compounds the crosslink density of the polymer film and thereby its physical and chemical properties such as the glass transition temperature, which is also important for the temperature dependence of the optical properties of the optical retardation film, the thermal and mechanical stability or the solvent resistance can be tuned easily. The polymerizable mesogenic mono-, di- or multireactive compounds used for the instant invention can be prepared by methods which are known per se and which are described, for example, in standard works of organic chemistry such as, for example, Houben-Weyl, Methoden der organischen Chemie, Thieme-Verlag, Stuttgart. Typical examples are described for example in WO 93/22397; EP 0 261 712; DE 19504224; DE 4408171 and DE 4405316. The compounds disclosed in these documents, however, are to be regarded merely as examples that do not limit the scope of this invention. Examples representing especially useful monoreactive polymerizable mesogenic compounds are shown in the following list of compounds, which should, however, be taken only as illustrative and is in no way intended to restrict, but instead to explain the present invention: P-(CH 22 ) ) o_ / / Vcoo Examples of useful direactive polymerizable mesogenic compounds are shown in the following list of compounds, which should, however, be taken only as illustrative and is in no way intended to restrict, but instead to explain the present invention - - 1 0 In the above formulae, P is a polymerizable group, preferably an acryl, methacryl, vinyl, vinyloxy, propenyl ether, epoxy or stytryl group, x and y are each independently 1 to 12 , A is 1 ,4-phenylene that is optionally mono- di or trisubstituted by L 1 or 1 ,4- cyclohexylene, v is 0 or 1 , Z° is -COO-, -OCO-, -CH 2 CH 2 - or a single * r bond, Y is a polar group, R° is an unpolar alkyl or alkoxy group, and L 1 and L 2 are each independently H, F, CI, CN or an optionally halogenated alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl or alkoxycarbonyloxy group with 1 to 7 C atoms. Q The term 'polar group' in this connection means a group selected from F, CI, CN, N0 2 , OH, OCH 3 , OCN, SCN, an optionally fluorinated carbonyl or carboxyl group with up to 4 C atoms or a mono- oligo- or polyfluorinated alkyl or alkoxy group with 1 to 4 C atoms. 5 The term 'unpolar group' means an alkyl group with 1 or more, preferably 1 to 12 C atoms or an alkoxy group with 2 or more, preferably 2 to 12 C atoms. The polymerizable mesogenic material is coated onto substrate, aligned into a uniform orientation and polymerized according to a process as described in WO 98/12584 or GB 2,315,072, thereby permanently fixing the orientation of the polymerizable mesogenic material. e As a substrate for example a glass or quarz sheet or a plastic film or sheet can be used. It is also possible to put a second substrate on top of the coated mixture prior to and/or during and/or after polymerization. The substrates can be removed after polymerization or not. When using two substrates in case of curing by actinic radiation, at least one substrate has to be transmissive for the actinic radiation used for the polymerization. Isotropic or birefringent substrates can be used. In case the substrate is not removed from the polymerized film after polymerization, preferably isotropic substrates are used. Preferably at least one substrate is a plastic substrate such as for example a film of polyester such as polyethyleneterephthalate (PET), of polyvinylalcohol (PVA), polycarbonate (PC) or triacetylcellulose (TAC), especially preferably a PET film or a TAC film. As a birefringent substrate for example an uniaxially stretched plastic film can be used. For example PET films are commercially available from ICI Corp. under the trade name Melinex. In a preferred embodiment, a negatively birefringent DAC film is used as substrate for preparation of the O plate, with the substrate, or in case two substrates are used at least one of the substrates, not being removed after polymerization. The polymerizable mesogenic material can also be dissolved in a solvent, preferably in an organic solvent. The solution is then coated onto the substrate, for example by spin-coating or other known techniques, and the solvent is evaporated off before polymerization. In most cases it is suitable to heat the mixture in order to facilitate the evaporation of the solvent. Planar alignment can be achieved for example by shearing the material, e.g. by means of a doctor blade. It is also possible to apply an alignment layer, for example a layer of rubbed polyimide or sputtered SiO x , on top of at least one of the substrates. Planar alignment of the polymerizable mesogenic material can also be achieved by directly rubbing the substrate, i.e. without applying an additional alignment layer. This is a considerable advantage as it allows a significant reduction of the production costs of the optical retardation film. In this way a low tilt angle can easily be achieved. For example rubbing can be achieved by means of a rubbing cloth, such as a velvet cloth, or with a flat bar coated with a rubbing cloth. In a preferred embodiment of the present invention rubbing is achieved by means of a at least one rubbing roller, like e.g. a fast spinning roller that is brushing across the substrate, or by putting the substrate between at least two rollers, wherein in each case at least one of the rollers is optionally covered with a rubbing cloth. In another preferred embodiment of the present invention rubbing is achieved by wrapping the substrate at least partially at a defined angle around a roller that is preferably coated with a rubbing cloth. The polymerizable composition according to the the present invention may also comprise one or more surfactans to improve planar alignment. Suitable surfactants are described for example in J. Cognard, Mol.Cryst.Liq.Cryst. 78, Supplement 1 , 1-77 (1981 ). Particularly preferred are non-ionic surfactants, such as the commercially available fluorocarbon surfactants Fluorad 171 (from 3M Co.), or Zonyl FSN (from DuPont). Preferably the polymerizable mixture comprises 0.01 to 5 %, in particular 0.1 to 3 %, very preferably 0.2 to 2 % by weight of surfactants. The orientation of the mesogenic material depends, inter alia, on the film thickness, the type of substrate material, and the composition of the polymerizable mesogenic material. It is therefore possible, by changing these parameters, to control the structure of the ORF, in particular specific parameters such as the tilt angle and its degree of variation. Thus, for the preparation of the O plate, it is possible to adjust the alignment profile in the direction perpendicular to the film plane by appropriate selection of the ratio of monoreactive mesogenic compounds, i.e. compounds with one polymerizable group, and direactive mesogenic compounds, i.e. compounds with two polymerizable groups. For an O plate with strong splay, i.e. a large variation of the tilt angle throughout the thickness of the film, preferably the ratio of mono- to direactive mesogenic compounds should be in the range of 6:1 to 1 :2, preferably 3:1 to 1 :1 , especially preferably about 3:2. Another effective means to adjust the desired splay geometry is to use a defined amount of dielectrically polar polymerizable mesogenic compunds in the polymerizable mesogenic material. These polar compounds can be either monoreactive or direactive. They can be either dielectrically positive or negative. Most preferred are dielectrically positive and monoreactive mesogenic compounds. The amount of the polar compounds in the mixture of polymerizable mesogenic material is preferably 1 to 80 %, especially 3 to 60 %, in particular 5 to 40 % by weight of the total mixture. Polar mesogenic compound in this connection means a compound with one or more polar groups as defined above. Especially preferred are monoreactive polar compounds selected from formulae la to lc given above. Furthermore, these polar compounds preferably exhibit a high absolute value of the dielectric anisotropy Δε, which is typically higher than 1.5. Thus, dielectrically positive compounds preferably exhibit Δε > 1.5 and dielectrically negative polar compounds preferably exhibit Δε < -1.5. Very preferred are dielectrically positive polar compounds with Δε > 3, in particular with Δε > 5. Polymerization of the polymerizable mesogenic material takes place by exposing it to heat or actinic radiation. Actinic radiation means irradiation with light, like UV light, IR light or visible light, irradiation with X-rays or gamma rays or irradiation with high energy particles, such as ions or electrons. Preferably polymerization is carried out by UV irradiation. As a source for actinic radiation for example a single UV lamp or a set of UV lamps can be used. When using a high lamp power the curing time can be reduced. Another possible source for actinic radiation is a laser, like e.g. a UV laser, an IR laser or a visible laser. The polymerization is carried out in the presence of an initiator absorbing at the wavelength of the actinic radiation. For example, when polymerizing by means of UV light, a photoinitiator can be used that decomposes under UV irradiation to produce free radicals or ions that start the polymerization reaction. When curing polymerizable mesogens with acrylate or methacrylate groups, preferably a radical photoinitiator is used, when curing polymerizable mesogens vinyl and epoxide groups, preferably a cationic photoinitiator is used. It is also possible to use a polymerization initiator that decomposes when heated to produce free radicals or ions that start the polymerization. As a photoinitiator for radical polymerization for example the commercially available Irgacure 651 , Irgacure 184, Darocure 1173 or Darocure 4205 (all from Ciba Geigy AG) can be used, whereas in case of cationic photopolymerization the commercially available UVI 6974 (Union Carbide) can be used. The polymerizable mesogenic material preferably comprises 0.01 to 10 %, very preferably 0.05 to 5 %, in particular 0.1 to 3 % of a polymerization initiator. UV photoinitiators are preferred, in particular radicalic UV photoinitiators. The curing time is dependening, inter alia, on the reactivity of the polymerizable mesogenic material, the thickness of the coated layer, the type of polymerization initiator and the power of the UV lamp. The curing time according to the invention is preferably not longer than 10 minutes, particularly preferably not longer than 5 minutes and very particularly preferably shorter than 2 minutes. For mass production short curing times of 3 minutes or less, very preferably of 1 minute or less, in particular of 30 seconds or less, are preferred. In addition to polymerization initiators the polymerizable material may also comprise one or more other suitable components such as, for example, catalysts, stabilizers, chain-transfer agents, co-reacting monomers or surface-active compounds. In particular the addition of stabilizers is preferred in order to prevent undesired spontaneous polymerization of the polymerizable material for example during storage. As stabilizers in principal all compounds can be used that are known to the skilled in the art for this purpose. These compounds are commercially available in a broad variety. Typical examples for stabilizers are 4-ethoxyphenol or butylated hydroxytoluene (BHT). Other additives, like e.g. chain transfer agents, can also be added to the polymerizable material in order to modify the physical properties of the inventive polymer film. When adding a chain transfer agent, such as monofunctional thiol compounds like e.g. dodecane thiol or multifunctional thiol compounds like e.g. trimethylpropane tri(3- mercaptopropionate), to the polymerizable material, the length of the free polymer chains and/or the length of the polymer chains between two crosslinks in the inventive polymer film can be controlled. When the amount of the chain transfer agent is increased, the polymer chain length in the obtained polymer film is decreasing. It is also possible, in order to increase crosslinking of the polymers, to add up to 20% of a non mesogenic compound with two or more polymerizable functional groups to the polymerizable material alternatively or in addition to the di- or multifunctional polymerizable mesogenic compounds to increase crosslinking of the polymer. Typical examples for difunctional non mesogenic monomers are alkyldiacrylates or alkyldimethacrylates with alkyl groups of 1 to 20 C atoms. Typical examples for non mesogenic monomers with more than two polymerizable groups are trimethylpropanetrimethacrylate or pentaerythritoltetraacrylate. In another preferred embodiment the mixture of polymerizable material comprises up to 70%, preferably 3 to 50 % of a non mesogenic compound with one polymerizable functional group. Typical examples for monofunctional non mesogenic monomers are alkylacrylates or alkylmethacrylates. It is also possible to add, for example, a quantity of up to 20% by weight of a non polymerizable liquid-crystalline compound to adapt the optical properties of the optical retardation film. In some cases it is of advantage to apply a second substrate to aid alignment and exclude oxygen that may inhibit the polymerization. Alternatively the curing can be carried out under an atmosphere of inert gas. However, curing in air is also possible using suitable photoinitiators and high UV lamp power. When using a cationic photoinitiator oxygen exclusion most often is not needed, but water should be excluded. In a preferred embodiment of the invention the polymerization of the polymerizable mesogenic material is carried out under an atmosphere of inert gas, preferably under a nitrogen atmosphere. To obtain a polymer film with the desired molecular orientation the polymerization has to be carried out in the liquid crystal phase of the polymerizable mesogenic material. Therefore, preferably polymerizable mesogenic compounds or mixtures with low melting points and broad liquid crystal phase ranges are used. The use of such materials allows to reduce the polymerization temperature, which makes the polymerization process easier and is a considerable advantage especially for mass production. The selection of suitable polymerization temperatures depends mainly on the clearing point of the polymerizable material and inter alia on the softening point of the substrate. Preferably the polymerization temperature is at least 30 degrees below the clearing temperature of the polymerizable mesogenic mixture. Polymerization temperatures below 120 °C are preferred. Especially preferred are temperatures below 90 °C, in particular temperatures of 60 °C or less. The invention is further explained by the following examples. Therein, the following abbreviations are used: θ tilt angle [degrees] φ twist angle [degrees] p helical pitch [nm] n e extraordinary refractive index (at 20 °C and 589 nm) n 0 ordinary refractive index (at 20 °C and 589 nm) En dielectric constant parallel to the long molecular axis (at 20 °C and 1 kHz) ε ± dielectric constant perpendicular to the long molecular axis (at 20 °C and 1 kHz) Kn first elastic constant K 22 second elastic constant K 33 third elastic constant V on threshold voltage [V] V off saturation voltage [V] d layer thickness [μm] Example 1 - Comparison Example An uncompensated standard type TN-LCD device of the E mode as depicted in Fig. 1 , comprising a TN cell 1 and a pair of linear polarizers 2,2' has the following parameters n e 1.5700 n 0 1.4785 ε± 3.5 ε ll 10.8 K 22 5.7 d 5.25 μm pre- tilt 2 ° Voπ 4.07 V v off 1.56 V Fig. 5a depicts the isocontrast plot of the display, showing ranges of identical contrast in steps of 10 %. The isocontrast plots are measured as luminance at V on / luminance at V off . Fig. 5b and 5c show 8 grey levels (given as transmission versus viewing angle), on a linear luminance scale in horizontal and vertical viewing planes, respectively. Ideally, the grey level lines should be parallel, where they cross, grey level inversion occurs. The latter is a serious disadvantage especially for the darker grey levels. It can be seen in Fig. 5b that levels 7 and 8 are very poor even at low angles such as 30 ° in horizontal direction, and in Fig. 5c that the levels cross at angles of 30 ° and higher in vertical direction. The polarisers can be any standard polariser used in normal LCD displays. V oriι V off correspond to values generally adopted in TN and STN-LCD displays. Example 2 - Comparison Example An uncompensated standard type TN-LCD device of the O mode as depicted in Fig. 2, comprising a TN cell 1 and a pair of linear polarizers 2,2', has the parameters as given in example 1. Fig. 6a depicts the isocontrast plot of the display, showing ranges of identical contrast in steps of 10 %. The isocontrast plots are measured as luminance at V on / luminance at V 0ff . Fig. 6b and 6c show the grey levels in horizontal and vertical viewing planes, respectively. It can be seen in Fig. 6b that levels 7 and 8 are very poor even at low angles such as 30 ° in horizontal direction, and in Fig. 6c that the levels cross at angles of 30 ° and higher in vertical direction. Example 3 A compensated TN-LCD device of the E mode according to the present invention as depicted in Fig. 3 consists of a TN cell 1 with a liquid crystal layer in a twisted nematic state, a pair of linear polarizers 2,2', two splayed O plates 3,3' and two DAC films 4,4' having the optical properties of a negative C plate and serving as substrates for the O plates 3,3'. The TN cell 1 and the polarizers 2,2" are as defined in example 1. The O plates 3,3' exhibit a splayed structure with the tilt angle θ ranging from θ min on one surface to θ max on the opposite surface. The parameters of the O plates 3,3' are as follows θ r 88 o "ave 45 ° n e 1.610 n 0 1.495 d 1.2 μm retardation 69 nm The parameters of the DAC films 4,4' are as follows n x 1.4816 n y 1.4817 n 2 1.4797 d 120 μm retardation 12 nm ( on-axis) 76 nm ( at 60 c ' incidence) wherein the x and y are directions parallel and z is the direction perpendicular to the plane of the DAC film. Fig. 7a shows the isocontrast plot of the display, Fig. 7b and 7c show the grey levels (transmission vesus viewing angle) in horizontal and vertical directions respectively. In the isocontrast plot Fig. 7a it can be seen that the display has a viewing angle that is significantly larger in horizontal direction, compared to the uncompensated display of example 1 , and is also improved in vertical direction. In Fig. 7b and 7c it can be seen that the grey levels 7 and 8 in vertical direction are improved at negative angles, compared to the uncompensated display of example 1 , and are also improved in horizontal direction, where they cross at angles of -45 ° and + 45 °. Example 4 A compensated TN-LCD device in the O mode according to the present invention as depicted in Fig. 4 consists of a TN cell 1 with a liquid crystal layer in a twisted nematic state, a pair of linear polarizers 2,2', two splayed O plates 3,3' and two DAC films 4,4" having the optical properties of a negative C plate and serving as substrates for the O plates 3,3'. The TN cell 1 and the polarizers 2,2' are as defined in example 1. The parameters of the O plates 3,3' are as follows "min 2 ° "max 88 ° "ave 45 ° n e 1.610 n 0 1.495 d 1.281 μm retardation 74 nm The parameters of the DAC films 4,4' are as follows n x 1.4803 n y 1.4892 n z 1.4777 d 930 μm retardation 12 nm ( on-axis) 71 nm ( at 60 c ' incidence) wherein the x and y are directions parallel and z is the direction perpendicular to the plane of the DAC film. Fig. 8a shows the isocontrast plot of the display, Fig. 8b and 8c show the grey levels in horizontal and vertical directions respectively. In the isocontrast plot Fig. 8a it can be seen that the display has a viewing angle that is significantly larger in vertical direction, compared to the uncompensated display of example 2, and is also improved in horizontal direction. In Fig. 8b and 8c it can be seen that the grey levels 7 and 8 in vertical direction are improved at negative angles, compared to the uncompensated display of example 2, and are also improved in horizontal direction, where they cross at angles of - 45 ° and + 45 °. The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples. From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various conditions and usages.

Claims: WO 01/20395 - 32 - PCT/EPOO/08935 Patent Claims 1. Optical compensator for liquid crystal displays comprising - at least one O plate retarder, 5 - at least one diacetyl cellulose (DAC) film having the optical properties of a negative C plate. 2. Optical compensator according to claim 1 , characterized in that ι n the average tilt angle θ ave in said O plate retarder is from 2 to 88°. 3. Optical compensator according to at least one of claims 1 and 2, characterized in that the tilt angle in said O plate retarder varies monotonuousiy in a direction perpendicular to the plane of the <l 5 film from a minimum value θ min at one surface of the film to a maximum value θ max at the opposite surface of the film. 4. Optical compensator according to claim 3, characterized in that θ min is from 0 to 80°. 0 5. Optical compensator according to claim 3 or 4, characterized in that θ max is from 10 to 90°. 6. Optical compensator according to at least one of claims 1 to 5, 5 characterized in that the thickness of said O plate is from 0.1 to 10 μm. 7. Optical compensator according to at least one of claims 1 to 6, characterized in that the optical retardation of said O plate is Q from 6 to 300 nm. 8. Optical compensator according to at least one of claims 1 to 7, characterized in that the thickness of said DAC film is from 20 to 200 μm. 5 9. Optical compensator according to at least one of claims 1 to 8, characterized in that the on-axis optical retardation of said DAC film is from 2 to 100 nm. 10. Optical compensator according to at least one of claims 1 to 9, characterized in that said O plate comprises a linear or crosslinked polymerized liquid crystalline material with a tilted or splayed structure. 11. A liquid crystal display device comprising the following elements a liquid crystal cell formed by two transparent substrates having surfaces which oppose each other, an electrode layer provided on the inside of at least one of said two transparent substrates and optionally superposed with an alignment layer, and a liquid crystal medium which is present between the two transparent substrates, a polarizer arranged outside said transparent substrates, or a pair of polarizers sandwiching said substrates, and - at least one optical compensator according to at least one of claims 1 to 10 being situated between the liquid crystal cell and at least one of said polarizers, it being possible for the above elements to be separated, stacked, mounted on top of each other, coated on top of each other or connected by means of adhesive layers. 12. A liquid crystal display device according to claim 11 , characterized in that it is a TN, HTN or STN display.

Inventors: COATES DAVID, LE MASURIER PETER, PARRI OWAIN LLYR, VERRALL MARK, COATES, DAVID, LE MASURIER, PETER, PARRI, OWAIN, LLYR, VERRALL, MARK
Applicants: COATES DAVID, LE MASURIER PETER, MERCK PATENT GMBH, PARRI OWAIN LLYR, VERRALL MARK, COATES, DAVID, LE MASURIER, PETER, PARRI, OWAIN, LLYR, VERRALL, MARK
Application year: 2000
Main classification: G02F1
IPC codes: G02B5, G02F1