GB2355315A

GB2355315A - Circular polariser with optically anisotropic and isotropic layers

Info

Publication number: GB2355315A
Application number: GB0022563A
Authority: GB
Inventors: Mark Verrall; Kim Slaney
Original assignee: Merck Patent GmbH
Current assignee: Merck Patent GmbH
Priority date: 1999-09-17
Filing date: 2000-09-14
Publication date: 2001-04-18
Anticipated expiration: 2020-09-14
Also published as: DE10033916B4; GB2355315B; DE10033916A1; GB0022563D0

Abstract

A circular polarizer comprises at least one layer of an optically anisotropic material with helically twisted molecular structure and planar alignment eg a linear or crosslinked polymerised cholesteric liquid crystal material, and at least one layer of an optically isotropic material preferably being adjacent to the layer of optically anisotropic material, wherein said isotropic material may be selected such that its refractive index matches the mean refractive index of said anisotropic material and may be a polymerised material. The polariser may be used in a liquid crystal display device.

Description

2355315 Circular Polarizer

Field of the Invention

The invention relates to a circular polarizer comprising at least one layer of an optically anisotropic material with helically twisted molecular structure and planar alignment, characterized in that it further comprises at least one layer of an optically isotropic material preferably being adjacent to the layer of optically anisotropic material, 10 wherein said isotropic material is selected such that its refractive index matches the mean refractive index of said anisotropic material. The invention further relates to the use of such a circular polarizer in liquid crystal display devices and to a liquid crystal display comprising a liquid crystal cell and such a circular polarizer. 15 Brief Description of the Drawinq Figure 1 illustrates the definition of the ellipticity of the electric field of 20 circularly polarized light as used in prior art. Figure 2 illustrates the definition of the ellipticity of the intensity of polarized light transmitted by an inventive polarizer as used in the present invention. 25 Figure 3 and 4 show the ellipticity of the intensity of light transmitted by inventive circular polarizer films versus the refractive index of the isotropic layer. 30 Background and Prior Art Circular polarizers are used in optical and electrooptical applications in order to create circularly polarized light of a single handedness 35 with a defined wavelength band. They typically consist of a layer of cholesteric liquid crystal material with helically twisted molecular structure and planar alignment.

The term "planar alignment" as used in this application in connection with a layer of a liquid crystal material with helically twisted molecular structure means that the material exhibits an orientation wherein the axis of the molecular helix is oriented substantially normal to the plane of the layer.

When unpolarized light is incident on such a cholesteric polarizer, 50 % of the intensity of a selective band of wavelengths are reflected as circularly polarized light of the same handedness as the cholesteric helix, whereas the other 50 % are transmitted as circularly polarized light of the opposite handedness. The light with a wavelength outside the selective band of wavelengths is transmitted as unpolarized light.

Circular polarizers comprising cholesteric liquid crystal materials have been proposed in prior art e.g. for use as broadband or notch polarizers or colour filters in displays or projection systems.

For example, circular polarizers or colour filters having a narrow notch bandwidth or even transmitting monochromatic light, are described e.g. in the EP 0 643 121 and EP 0 407 830. Stacked or multilayer circular polarizers are described in the WO 95/43022.

In particular broadband cholesteric polarizers, which are transmitting circularly polarized light of a broad wavelength band covering a large part of the visible spectrum, are suitable as polarizers for backlit liquid crystal displays. They are used in combination with a reflector that redirects the light reflected by the circular polarizer onto the circular polarizer, where it is again partially transmitted and partially reflected. Thus, the reflected light of the 'wrong' polarization sense is also utilized, so that in total the light emitted from the backlight unit is very efficiently transformed into polarized light. Broadband circular polarizer are therefore especially useful in liquid crystal displays in replacement of conventional linear absorption polarizers.

Broadband choiesteric polarizers are described e.g. in the WO 97/35219, EP 0 634 674, EP 0 606 940 and WO 97/16762.

The broadband cholesteric polarizers, described in prior art are preferably used in combination with a quarter wave retardation plate, which transforms circularly polarized light transmitted by the polarizer into linearly polarized light. 10 However, when using circular polarizers of prior art in practical applications, they show several disadvantages, For example, it is observed that light transmitted by a state of the art circular polarizer is not perfectly circularly, but elliptically polarized, which negatively affects the contrast ratio of the polarizer. 15 This undesired transformation of unpolarized light into elliptically polarized light in a circular polarizer typically occurs for light passing through the polarizer at large viewing angles. However, it can sometimes even be observed for light of normal incidence. 20 Elliptically polarized light can be described by the degree of 'ellipticity' of the electric field, as defined in prior art e.g. by G. Snatzke, Angew. Chem. 80, 1, 15-26 (1980), and as illustrated in Figure 1. 25 In Figure 1, the field vector E0 of a ray of, in this case right-handed, elliptically polarized light is shown as the sum of the field vectors Er and t 1 of two circularly polarized light beams with the same frequency and wavelength, but with different field strength. The 30 rotation sense of one of these field vectors is right-handed (Er) and the rotation sense. of the other one is left-handed (Ej. The field vector E 0 of the propagating elliptically polarized light ray periodically rotates on an ellipsoid having the long axis a = I Er + 35 E, I and the short axis b = I 1 r - E I I. The ellipticity of the light ray is then usually defined as the angle y, with

I Cirpol P arctan b/a Thus, according to this definition linearly polarized light has an ellipticity of xV = 0, whereas perfectly circularly polarized light has an ellipticity of kV = 45 0 (which is at the same time the largest possible value of W).

Another drawback of state of the art circular polarizers besides their ellipticity is the fact that their contrast ratio often depends strongly on the wavelength of transmitted light, which can lead to a decrease of the contrast ratio and to the appearance of undesired colouration effects.

Thus, there still exists a considerable demand for circular polarizers; which transmit circularly polarized light of a single handedness with a specific wavelength band with high efficiency, i.e. with the ellipticity Y being close to or, in the ideal case, approximately 45 0, and which can be used in liquid crystal display devices.

The circular polarizers having such high efficiency should be suitable as polarizers transmitting narrow selective wavelength bands, such as notch polarizers or even monochromatic polarizers. There is, however, also a need for broadband cholesteric polarizers having such advantageous properties that transmit large bandwidths which may even cover the full visible spectrum.

One aim of the present invention is to provide circular polarizers fulfilling the above requirements. Another aim of the present invention is to provide a method of preparing such circular polarizers.

Other aims of the invention are immediately evident to the skilled in the art from the following description.

It was now found that the above aims can be achieved by providing a circular polarizer that comprises an optically anisotropic layer of a polymerized cholesteric liquid crystal material having planar alignment, and further comprises at least one layer of an optically C1TCP0! isotropic material being adjacent to the anisotropic layer, wherein the refractive index of the isotropic material matches the mean refractive index of the anisotropic material.

Such a circular polarizer transmits circularly polarized light of a defined wavelength band with a high efficiency, i.e. with the ellipticity Y being close to or, in the ideal case, approximately 45 ", at normal incidence. Detailed Description of the Invention 10 One object of the present invention is a circular polarizer comprising at least one layer of an optically anisotropic; material with helically twisted molecular structure and planar alignment, characterized in that it further comprises at least one layer of an optically isotropic 15 material adjacent to the layer of optically anisotropic material, wherein said isotropic material is selected such that its refractive index matches the mean refractive index of said anisotropic material. The layer of optically isotropic material should be in optical conctact 20 with the layer of optically anisotropic material. It is also possible to apply intermediate layer, e.g. a layer of adhesive, between the isotropic and anisotropic layers, provided the intermediate layer is in intimate contact with these layers. 25 Another object of the present invention is the use of such a circular polarizer in liquid crystal display devices. Another object of the present invention is a liquid crystal display comprising a liquid crystal cell and such a circular polarizer. 30 As mentioned above, the term 'ellipticity' in prior art is used in relation to the ellipse formed by the electric field vector of a propagating polarized light ray, and is given by the angle y (see Figure 1). In contrast to that, in this invention another, more practical quantity is 35 used to define the 'ellipticity' of polarized light, i.e. a quantity related to the intensity of the light transmitted by the circular polarizer. It is

CjTCPOI defined as the ratio of the intensity Tx of the short axis to the intensity TY of the long axis of the ellipse formed by the polarized light, as schematically illustrated in Figure 2b. The ellipticity thus defined can be easily measured as follows:

An inventive circular polarizer is illuminated with unpolarized light of a given wavelength, and the intensity of light transmitted by the circular polarizer is measured through a rotating linear polarizer, e.g. a dichroic polarizer. By rotating the linear polarizer, a minimum transmission T., can be observed at a particular angle of rotation, and the corresponding position of the optical axis of the linear polarizer is defined as the short axis of the ellipse. The long axis is then defined as the axis perpendicular to the short axis, and the transmission in this direction defined as Ty. 15 The polarizer used to determine E according to the method described above is a standard stretched polarizer as used in LC displays, based on stretched PVA with iodine type dye provided between two layers of TAC. Alternatively, however, a prismatic 20 polarizer (e.g. Glann-Thomson) can be used. The ellipticity of the intensity is then given by the ratio Tx/Ty and can. be quoted as a percentage E = (Tx/Ty) - 100 %. According to this definition an ellipticity of E = 100 % corresponds to perfectly circularly 25 polarized light (as shown in Figure 2a), whereas an ellipticity of E = 0 % corresponds to linearly polarized light (as shown in Figure 2c). The absolute values of Tx and TY are depending on the intensity of the incident light and usually also on the polarization characteristics 30 of the linear polarizer used for their measurement. For some types of linear polarizers, however, it is possible that even the ratio of Tx/Ty and thus the value of E are also depending on the polarization characteristics of the linear polarizer. 35 Assuming that the dependence of E on the polarization characteristics of the linear polarizer used for measuring Tx and Ty is CircP011 neglectable, a state of polarization wherein E = 100 % would correspond to a value of y = 45 ', and a state of polarization wherein E = 0 % would correspond to a value of y = 0, with y being the ellipticity of the electric field as defined above. It is, nevertheless, obvious that the scales of y and E are not directly proportional to each other.

The inventive circular polarizers are characterized by a high ellipticity E as defined above and by a high contrast ratio that has a low dependency on the wavelength.

Thus, another object of the present invention is a circular polarizer comprising at least one layer of an optically anisotropic material with helically twisted molecular structure and planar alignment, characterized in that it further comprises at least one layer of an optically isotropic material contacting said layer of optically anisotropic material and causing an increase of the ellipticity of said layer of optically anisotropic material such that the following equation is fulfilled (E-E)/(l-E)!! 0.3 wherein E is the ellipticity of said at least one layer of optically anisotropic material alone, and E is the ellipticity of the circular polarizer comprising said at least one layer of optically isotropic and said at least one layer of optically anisotropic material, with E and E being the ellipticity of the intensity as defined above.

In a preferred embodiment of the present invention, the circular polarizer essentially consists of a layer of a first layer of optically anisotropic material, preferably a cholesteric liquid crystal material, and a second and. optionally a third layer both of optically isotropic material directly adjacent to said first layer, wherein said second and, if present, said third layer are located on different sides of said first layer.

Further preferred embodiments of the invention relate to a circular polarizer, wherein the layer of optically anisotropic material comprises a linear or crosslinked polymerized cholesteric liquid crystal material with macroscopically uniform planar alignment as defined above, at least one of said layers of optically isotropic material comprises an isotropic polymer material, at least one of said layers of optically isotropic comprises a linear or crosslinked polymerized liquid crystalline material with random orientation, said polymerized liquid crystalline material with random orientation is a nernatic liquid crystalline material, said polymerized liquid crystalline material with random orientation is a cholesteric liquid crystalline material, said linear or crosslinked polymerized liquid crystalline material is obtained by polymerization and/or crosslinking in the isotropic phase of the liquid crystalline material, 9 the difference between the mean refractive index of said layer of optically anisotropic material and the refractive index of said at least one layer of optically isotropic material is 0.3 or less, the circular polarizer exhibits an ellipticity E of at least 0.8, with E being as defined above, the thickness of the isotropic layer is at least 0.3 gm, 9 the thickness of the isotropic layer is from 0.1 to 50 Lrn, preferably from 0.3 to 10 im, very preferably from 0.3 to 5 Lrn, 9 the central wavelength (half width half maximum, HWHM) of the waveband transmitted by the circular polarizer is from 300 to 1800 nm, in particular from 300 to 1200 nm, very preferably from 310 to 800 nm, the bandwidth of the wavelength band transmitted by the circular polarizer is from 10 to 200 nm, the circular polarizer transmits monochromatic light with a 5 wavelength from 300 to 1800 nm, in particular from 300 to 1200 nm, very preferably from 310 to 800 nm (half width half maximum, HWHM). Another object of the invention is a method of increasing the ellipticity 10 of a circular polarizer comprising at least one layer of an optically anisotropic material with helically twisted molecular structure and planar alignment, wherein said layer of anisotropic material is cpmbined with at least one layer of optically isotropic material and wherein the following equation is fulfilled 15 (E-E-)/(1-E-)! 0.3 with E and E being as defined above. The inventive method of increasing the ellipticity can be applied to a cholesteric polarizer with a narrow bandwidth of transmitted polarized 20 light, which is suitable e.g. as a notch or monochromatic polarizer or as a colour filter. The inventive method can also be applied to a broadband cholesteric polarizers transmitting circularly polarized light of a broad wavelength band covering a large part of the, or even the full, visible spectrum. 25 Thus, the term 'circular polarizer' as used the foregoing and the following encompasses broadband cholesteric polarizers as described e.g. in the WO 97/35219, EP 0 634 674, EP 0 606 940 or WO 97/16762, circular polarizers, or colour filters with a narrow notch 30 bandwidths and monochromatic circular polarizers, as described e.g. in the EP 0 643 121-Al, EP 0 407 830-Al, furthermore stacked or multilayer circular polarizers as described in the WO 95/43022 with the entire disclosure of these documents being incorporated into this application by way of reference. 35 -Cir 10- In the following, the present invention is exemplarily described in detail for a circular polarizer wherein the anisotropic layer comprises a polymerized cholesteric liquid crystalline material with planar alignment. The invention is, however, by no means limited to such types of polarizers.

An important aspect of the present invention is the finding that the ellipticity of polarized light passing through a circular polarizer comprising optically anisotropic (birefringent) cholesteric liquid crystal material even at normal incidence is significantly affected by the optically isotropic medium which is directly in contact with the surface of the polarizer.

For example, in practical applications a circular polarizer is often adjacent to an optically isotropic medium, such as an adhesive layer or a protection film, or simply to air.

It was now found that the ellipticity of polarized light transmitted by the circular polarizer depends upon the refractive index of the isotropic medium directly adjacent to the polarizer, and reaches a maximum when the refractive index of the isotropic material substantially matches the average of the refractive indices of the anisotropic (birefringent) material of the polarizer.

The mean refractive index n, of a cholesteric liquid crystal material is given by the average of the refractive indices parallel and perpendicular to the long axis of the liquid crystal molecules, which are corresponding to the nematic refractive indices. Thus, nr, is defined as nr, = 1/2(ne + n.) with ne being the ordinary and n. the extraordinary refractive index of the liquid crystal material in the nernatic phase.

Thus, if a very thin layer of a cholesteric liquid crystal material has the mean refractive index nm, the ellipticity of the polarized light passing through the cholesteric layer into an isotropic medium will be at an optimum when the refractive index ni of the isotropic medium is approximately the same as the mean refractive index nm of the cholesteric material according to the equation ni = nm =1/2(ne + n,,) In case the cholesteric layer is adjacent to air (having a refractive index of 1) the ellipticity of the transmitted light will be at a minimum.

By using a further layer of isotropic material with a refractive index ni it is possible to achieve high values of the ellipticity and therewith a high contrast of the cholesteric layer.

According to the present invention, circular polarizers are realized that have an ellipticity of the intensity E of 70 % or larger. Circular polarizers are preferred that exhibit an ellipticity E! 80%, in particular E t 90 %, very preferably E! 95 %, with E being as defined above.

It is also possible to use a stack of different anisotropic layers with different ellipticity that are in contact with each other, e.g. a stack of cholesteric liquid crystal layers reflecting red, green and blue light, wherein the top layer in the stack is covered by an isotropic layer. The important point of the present invention is that the interface between the anisotropic layer (or, in case of a stack of anisotropic layers, the top anisotropic layer in the stack) and air (or another low refractive index medium) is optically improved by the isotropic layer.

Particularly preferably the material used in the optically anisotropic layer and the material used in the optically isotropic layer are selected from liquid crystalline compounds having similar chemical structure and thereby similar values of refractive indices. Especially preferably the materials for the anisotropic and isotropic layers are selected from the same type of liquid crystalline compounds.

Inventive circular polarizers are preferred wherein the difference I nm _ ni I between the mean refractive index nm of said at least one layer of optically anisotropic material and the refractive index of at least one of said layers of optically isotropic material ni is 0.3 or less, in particular 0.2 or less, very preferably 0.1 or less.

The thickness of the optically anisotropic layer is preferably from 1 to 20 jam, very preferably from 2 to 8 gm.

The thickness of the optically isotropic layer is preferably from 0.1 to 50 gm, very preferably from 0.3 to 10 gm.

The inventive circular polarizers can be manufactured from a cholesteric liquid crystal material by methods that are known in prior art, and are described e.g. in the WO 97/35219, EP 0 634 674, EP 0 606 940, WO 97/16762, EP 0 643 121-Al, EP 0 407 830-Al and WO 95/43022. 15 According to a preferred embodiment of the present invention, the optically anisotropic layer is obtainable by a process wherein a mixture of a polymerizable mesogenic material, preferably a cholesteric material, comprising 20 a) at least one achiral polymerizable mesogenic compound, b) at least one chiral compound that can in addition be polymerizable and/or mesogenic, c) a polymerization initiator, is coated on a substrate or between two substrates in form of a layer, aligned in a planar orientation so that the axis of the molecular helix extends transversely to the layer, and polymerized in its mesophase by exposure to heat or actinic radiation. Optionally the substrates are 30 removed from the polymerized material. In a preferred embodiment of the present invention, component b) of the polymerizable mesogenic material is essentially consisting of polymerizable chiral compounds, preferably polymerizable chiral 35 mesogenic compounds.

citcP01 In another preferred embodiment of the present invention, component b) of- the polymerizable mesogenic material essentially consists of non- polymerizable chiral mesogenic compounds, like for example commercially available chiral dopants. 5 Especially preferred are chiral dopants with a high helical twisting power (HTP), in particular those disclosed in WO 98/00428. Further typically used chiral dopants are e.g. the commercially available S 1011, R 811 or CB 15 (from Merck KGaA, Darmstadt, Germany). 10 Especially preferred are chiral non-polymerizable dopants selected from formula I and 11 Z0j.V R coo 0 t_ - - 0 00C z R (R.S) H H 0 0 R J-a ZO E OL_z R V V 0 H (R,R) including the (R,S), (S,R), (R,R) and (S,S) enantiorners not shown, wherein E and F are each independently 1,4-phenylene or trans-1, 4cyclohexylene, v is 0 or 1, Zo is -COO-, -OCO-, -CH2CH2- or a single bond, and R is alkyl, alkoxy or alkanoyl with I to 12 C atoms. The compounds of formula I and their synthesis are described in WO 35 98/00428. The compounds of formula 11 and their synthesis are described in GB 2,328,207.

Cirpol The above chiral compounds of formula VII and VIII exhibit a very high helical twisting power (HTP), and are therefore particularly useful for the purpose of the present invention.

Particularly preferably component b) consists essentially of chiral dopants of formula I or 11.

Further preferred embodiments of the invention relate to a circular polarizer wherein the optically anisotropic layer is obtainable from a process as described above, characterized in that at least one of said substrates is a plastic film.

the polymerizable mesogenic material contains at least one chiral polymerizable mesogenic compound having one polymerizable group and at least one achiral polymerizable mesogenic compound having one polymerizable group.

the polymerizable mesogenic material contains at least one chiral polymerizable mesogenic compound having one polymerizable group and at least one achiral polyrnerizable mesogenic compound having two or more polymerizable groups.

the polymerizable mesogenic material contains at least one non polymerizable chiral compound and at least one achiral polymerizable mesogenic compound having one or two polymerizable groups.

The optically isotropic layer is preferably coated, laminated or fabricated directly onto the anisotropic layer. It can comprise an isotropic fluid, e.g. a refractive index fluid or an isotropic linear, cyclic or crosslinked polymer, such as polymers selected from the class of polyacrylates, polymethacrylates, polyvinylalcohols, polyesters, and copolymers thereof. The material of the isotropic layer has to be selected such that its refractive index fulfills the conditions for E and E given above. For example, as isotropic layer commercially available plastic films can be used.

15- Isotropic layers showing a slight birefringence, which is due e.g. to uniaxial compression or stretch associated with the manufacturing process of the commercially available plastic-film, can still be tolerated. Preferably, however, the isotropic layer should exhibit zero or very low birefringence. Thus, the optical retardation dAn of the isotropic layer is preferably smaller than 30 nm, very preferably smaller than 10 nm, with d being the thickness and An the birefringence.

In a preferred embodiment of the invention, the isotropic layer is prepared from a polymerizable mesogenic material that is similar to the material used for the preparation of the anisotropic layer, which is then polymerized in its isotropic phase. In this preferred embodiment, the isotropic layer is especially preferably obtainable from a polymerizable mesogenic. material comprising at least one achiral polymerizable mesogenic compound having one polymerizable group and at least one polymerizable mesogenic compound having two polymerizable groups, 9 a polymerizable mesogenic material comprising at least two achiral polymerizable mesogenic compounds having one polymerizable group, a polymerizable mesogenic material comprising at least one achiral polymerizable mesogenic compound and at least one non-mesogenic polymerizable compound having two polymerizable groups.

In a further preferred embodiment of the present invention, a polymerizable cholesteric liquid crystalline material is used for the preparation of both the isotropic and the anisotropic layer. Therein, the anisotropic layer is prepared by polymerizing the cholesteric material in its uniformly oriented cholesteric phase, and the isotropic layer is prepared by polymerizing the cholesteric material in its unoriented isotropic phase.

The polymerizable mesogenic material used for the preparation of the isotropic and anisotropic layers can comprise polymerizable CirCPO:

compounds with one polymerizable group (monofunctional) and compounds with two or more polymerizable groups (di- or multifunctional).

By varying the concentration of monfunctional and di- or multifunctional polymerizable 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 polarizer, 10 the thermal and mechanical stability or the solvent resistance can be tuned easily. The polymerizable mesogenic material used for the preparation of the anisotropic layer comprises preferably at least one achiral and at 15 least one chiral compound. By changing the ratio of chiral and achiral compounds the pitch lengths and thus the central wavelength of the transmitted wavelength band of the circular polarizer can be varied. In case of 20 the preparation of a broadband circular polarizer, preferably the ratio of the chiral and achiral mesogenic compound is selected such that the waveband transmitted by the circular polarizer is covering a substantial part of the spectrum of visible light. 25 The terms polymerizable mesogen, polymerizable mesogenic compound or polymerizable liquid crystal or liquid crystalline compound as used in the foregoing and the following comprise compounds with a rod-shaped, board-shaped or disk-shaped mesogenic group (i.e. a group with the ability to induce mesophase 30 behaviour in a compound comprising such a group). These compounds do not necessarily have to exhibit mesophase behaviour by themselves. In a preferred embodiment of the present invention they show mesophase behaviour only upon admixturewith other compounds or upon polymerization of the polymerizable mesogenic 35 compounds or the mixtures comprising them.

CiTCPOI Suitable polymerizable mesogenic compounds and mixtures that are preferably used in the inventive process can be found in the WO 98/04651 -A, WO 98/00475 and WO 98/12584-A, with the entire disclosure of these documents being incorporated into this application by way of reference.

Furthermore, suitable polymerizable mesogenic compounds can be found in WO 93/22397; EP 0 261 712; DE 195 04 224; DE 44 08 171 or DE 44 05 316. The compounds disclosed in these documents, however are to be regarded merely as examples that should not limit the scope of this invention.

Further typical examples representing polymerizable mesogenic compounds are shown in the following list of compounds, which is, however, to be understood only as illustrative without limiting the scope of the present invention:

L 1 L 2 P(CH 2).0 coo OCO -a O(CH 2)y P 0 L L 2 P(CH2).O CH 2 CH 2 i6 CH 2 CH 2 -a O(CH 2)yP (2) L 2 0 C02 02C 0 P pl"- (3) P-(CH 2),0 -<:t coo FV 0Y P-(CH 2)'0 --a coo -0-Y (5) clrcpo P-(CH C 0 0 R 0 2)xo +,& P-(CH2)10 coo +041& R 0 P-(CH2),O-C>-Z a+o-zqv-&R 0 (8) CH2=CHCOO(CH 2).0 _9v R 0 (9) P-(CH2)10 CH=CH-Coo -a R 0 P-(CH2)10 coo CH2CH(CH 3)C2H5 L P-(CH2))Io -a COO COO CH2CH(CH3)C2H5 (12) P-(CH2),o COO-Ter P-(CH 2)xo COO-Chol P-(CH2)),o_&coO (15) Cimpol In the above formulae, P has one of the meanings of formula I and its preferred meanings as mentioned above, x and y are each independently 1 to 12, A and D are 1,4-phenylene or 1,4 cyclohexylene, v is 0 or 1, Zo is -COO-, -OCO-, -CH2CH2- or a single bond, Y is F, Cl, CN, N02, OH, OCH3, 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, Ro is an alkyl group with 1 to 12 C atoms or an alkoxy group with 2 to 12 C atoms, Ter is a terpenoid radical like e.g. menthyl, Choi is a cholesteryl group, and Ll and L2 are each independently H, F, Cl, CN or an optionally halogenated alkyl, alkoxy or carbonyl or group with 1 to 7 C atoms.

The polymerizable mesogenic compounds shown above 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. Some specific methods of preparation can be taken from the examples.

The achiral polymerizable mesogenic compounds of component a) are preferably selected of the above formulae 1, 2 and 4 to 10, wherein Y and Ro are achiral groups.

The chiral polymerizable mesogenic compounds of component b) are preferably selected of the above formulae 3 and 11 to 15, or of formula 4 to 10 wherein Y and Ro are chiral groups.

According to the above described preparation of the isotropic and anisotropic layers of an inventive circular polarizer, a mixture of a polymerizable mesogenic material is coated on a substrate or between two substrates, aligned into a uniform planar orientation and cured by exposure to heat or actinic radiation in the presence of 35 an initiator. A detailed description of this method can be found e.g. in D.J.Broer et al., Makromol.Chem. 190, pp. 2255 ff. (1989).

Cirpol As substrates for example a glass or quarz sheet as well as plastic films or sheets can be used. 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.

In particular for mass production it is suitable to use plastic films as substrates, like e.g. polyester films such as polyethylene terephthalate (PET), polyvinylalcohol (PVA), polycarbonate (PC), di or triacety1cellulose (DAC/TAC). As a birefringent substrate for example an uniaxially stretched plastic film can be used. Preferably at least one substrate is a plastic substrate, especially preferably a PET film or a TAC film. PET films are commercially available e.g.

from ICI Corp. under the trade name Melinexo.

The substrates can be removed after polymerization or not. At least one substrate has to be transmissive for the actinic radiation used for the polymerization.

The polymerizable mesogenic material is coated on the substrate or between the substrates in form of a thin layer. This can be done by conventional techniques that are known to the skilled in the art.

It is also possible to dissolve the polymerizable mesogenic material in a suitable solvent. This solution is then coated onto the substrate and the solvent is evaporated off before curing.

For this purpose, for example standard organic solvents can be used, such as ketones like e.g. methyl ethyl ketone or cyclohexanone, aromatic solvents like e.g. toluene or xylene, halogenated hydrocarbons like e.g. di- or trichloromethane, or alcohols such as e.g.

methanol, ethanol or isopropyl alcohol. It is also possible to use binary, ternary or higher mixtures of the above solvents.

Cimpol To prepare the anisotropic layer, the coated layer of the polymerizable mesogenic material is aligned to give a planar orientation, i.e. wherein the axis of the molecular helix extends transversely to the layer.

A planar orientation 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 SiOx, on top of at least one of the substrates and/or to give rubbing treatment to at least one of the substrates. 10 Furthermore, planar alignment with uniform orientation can be improved by applying an electric or magnetic field and/or by adding one or more surfactants to the polymerizable mesogenic material. 15 In another preferred embodiment, the shearing caused by putting together two substrates is sufficient to give good alignment. The preparation of the isotropic layer is preferably carried out in the isotropic, unoriented phase of the polymerizable mesogenic material. 20 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, I R light or visible light, irradiation with X-rays or gamma rays or irradiation with high energy particles, 25 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. Another possible source for actinic 30 radiation is a laser, like e.g. a UV laser, an IR laser or a visible laser.

For mass production short curing times:5 3 minutes, very preferably:5 1 minute, in particular:!! 30 seconds are preferred.

The polymerization is carried out in the presence of an initiator absorbing at the wavelength of the actinic radiation. For example, Cirpol 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 %, 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.

In some cases a second substrate is used that does not only aid alignment of the polymerizable comp ' osition, but also excludes 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 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.

Cimpol In addition to the polymerization initiators mentioned above, the polyrnerizable 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 a preferred embodiment of the invention, the polymerizable material comprises a stabilizer that is used to prevent undesired spontaneous polymerization for example during storage of the composition. Stabilizers are commercially available in a broad variety. Typical examples for stabilizers are 4-ethoxyphenol or butylated hydroxytoluene (BHT). The amount of the stabilizer in the polymerizable mixture is preferably from 1 to 1000 ppm, especially preferably from 10 to 500 ppm.

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 mixture 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 alky1diacrylates or alkyldimethacrylates with alkyl groups of 1 to 20 C atoms. Typical examples for non mesogenic monomers with more than two polymerizable groups are trim ethyl pro panetri methacryl ate or pentaerythritoltetraacryl ate. In another preferred embodiment the polymerizable mixture comprises up to 70%, preferably 3 to 50 % of a non mesogenic compound with one polymerizable functional group. Typical 30 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 liq u id-crystal line compound to adapt 35 the optical properties of the inventive optical retardation film.

Cimpol To obtain an anisotropic layer with the desired molecular orientation the polymerization has to be carried out in the liquid crystal phase of the polymerizable mesogenic mixture. 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 one to reduce the polymerization temperature, which makes the polymerization process easier and is a considerable advantage especially for mass production.

The material for the isotropic and the anisotropic layer are preferably selected from the same types of compounds and mixtures, in order to achieve a good index match between the layers.

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. In case of the anisotropic layer, preferably the polymerization temperature is at least 30 'C below the clearing temperature of the polymerizable mesogenic mixture. In case of the isotropic layer, the polymerization temperature is preferably not more than 30 ' C above the clearing temperature of the polymerizable mesogenic mixture.

Polymerization temperatures below 120 C are generally preferred.

For the anisotropic layer, especially preferred are temperatures below 90 'C, in particular temperatures of 60 'C or less.

The inventive circular polarizer can b e used in optical or electrooptical applications where circularly polarized light is needed.

Thus, it can be used for example as notch polarizer, light shutter or colour filter in displays or projection systems.

An inventive circular polarizer with a broad bandwidth can be used in backlit liquid crystal displays in order to transform unpolarized light emitted from the backlight into polarized light. In this case, it is preferably combined with a quarter wave retardation film (QWF) that converts the circularly polarized light transmitted by the circular cirpol polarizer into linearly polarized light. In this case, a high ellipticity of the inventive circular polarizer is especially advantageous, as the rate of conversion of circularly into linearly polarized light, and thus the yield of light emitted from the backlit is significantly increased. This leads to a lower energy consumption of the display, and, e.g. in case of laptop computers, to a longer lifetime of the electric power supply.

Where an inventive circular polarizer is used in liquid crystal display devices, it may be combined, apart of QWFs, also with other optical components, such as linear polarizers, optical compensators of different geometry, diffusors or mirrors.

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 usages and conditions.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to ist fullest extent. The following examples are, therefore, to be construed as merely illustrative and not limitative of the remainder of the disclosure in any way whatsoever.

In the foregoing and in the following examples, unless otherwise indicated, all temperatures are set forth uncorrected in degrees Celsius and all parts and percentages are by weight.

The following abbreviations are used to illustrate the liquid crystalline phase behaviour of the compounds: K = crystalline; N = nematic; S = smectic; Ch = cholesteric; I = isotropic. The numbers between these symbols indicate the phase transition temperatures in degree Celsius.

Cirpol Example 1

A 6 ptm thick cholesteric layer of the nernatic liquid crystal mixture E7 (from Merck KGaA; Darmstadt, Germany) doped with a chiral compound to a cholesteric pitch of 300 nm was investigated by optical modelling based on a 4x4-matrix technique as described by D.W. Berreman, J. Opt. Soc. Am. 62/4, 502-510 (1972).

The refractive index of an external isotropic medium adjacent to the modelled cholesteric layer was varied from 1.0 to 1.7, and the ellipticity of polarized light with a wavelength of 462 nm (centre of the reflection band) transmitted by the cholesteric layer and the adjacent isotropic medium was calculated using the above modelling method.

Figure 3 depicts the ellipticity thus calculated versus the refractive index of the external isotropic medium.

Example 2

The following mixture was formulated compound (1) 22.0% compound (2) 25.0% compound (3) 50.0% compound (4) 2.0% Irgacure 184 1.0% CH2=CHC00(CH2).0 -a coo CH2CH(CHdC2H5 CH 2 =CHCOO(CH2)60 -C- coo CH2CH(CHdC2 H5 (2) circP01 CH 3 CH2 =CHC02 (CH 2)30 COO -6 OCO O(CH 2)3 0,CCH=CH 2 (3) 5 H 13 C 6 O-U COO H 0 0 H 00C -a Oc 6 H 13 (4) The compounds (1) and (2) can be prepared as described in the DE 195 04 224. The direactive compound (3) can be prepared as described in the WO 93/22397. The preparation of compound (4) is described in the WO 98/00428. Irgacure 184 is a photoinitiator commercially available from Ciba Geigy AG.

The mixture shows a blue reflective cholesteric texture up to 80C.

A film of approximately 10 pm thickness of the mixture was polymerized in its cholesteric phase at 25 OC between two polyester sheets (100 gm Melinex 40 1, available from ICI Corp.) using a metal halide UV lamp with a lamp power of 5 MW/CM2. The polyester sheets were removed leaving a cholesteric film with a reflective band from 430 to 470 nm. The ellipticity was measured in a spectro photometer using depolarized light at 450 nm wavelength.

The light transmitted by the cholesteric film has an ellipticity of 58 % when measured in air. A selection of isotropic liquids with various refractive indices were carefully applied to the surface of the film adjacent to the detector, forming a thin layer. The ellipticity was shown to depend upon the refractive index ni of the isotropic layer as shown in table 1.

Cirpol Table 1 - Ellipticity of an inventive cholesteric film adoacent to various isotropic layers Isotropic layer ni of isotropic layer Ellipticity Air 1.00 58 1% Decon solution 1.32 82 Refractive index fluid') 1.40 90 Refractive index fluid') 1.50 95 Refractive index fluid') 1.60 97 Refractive index fluid') 1.70 94 Cargille certified refractive index liquids, from Cargille Laboratories Inc. (UK) The results are depicted in Figure 4. From Figure 4 it can be seen that the ellipticity of the cholesteric film has a maximum when the refractive index of the isotropic layer is about 1.6, thus the index match between the mean refractive index nm of the cholesteric film and the refractive index ni of the isotropic layer is best at this value. 20 Example 3 The cholesteric mixture of example 2 was polymerized between two 25 100 pm thick sheets of isotropic triacety1cellulose (TAC). The ellipticity of the cholesteric film was measured before and after removing the TAC sheets adjacent to the detector. Then a layer of pressure sensitive adhesive (PSA, mainly consisting of polybutylacrylate with an estimated refractive index of 1.45) was applied to the open surface of 30 the cholesteric film, a sheet of TAC was laminated on to the PSA, and the ellipticity was measured with the detector adjacent ot the TAC sheet. The results are shown in table 2. 35 Table 2 - Ellipticity of an inventive cholesteric film adiacent to various isotropic layers Isotropic layer ni of isotropic layer Ellipticity Air 1.00 64 TAC 1. 5 94 PSA + TAC 1. 4 5 88 It can be seen that the index match and thus the ellipticity is best for a combination of the cholesteric film and a TAC sheet.

Example 4

The following mixture was prepared compound (5) 99.0% Irgacure 184 1.0% CH2 =CHC02(CH2)40 -C- coo -a O(CH2)402CCH=CH2 (5) The synthesis of the diacrylate compound (5) is described in the WO 93/22397, The mixture shows a nematic phase up to 39 "C.

A number of films of the above mixture with approximately 3 gm in thickness were prepared between two sheets of polyester (Melinex 401, 100 tm) and polymerized in the isotropic phase of the mixture at 50 'C under 1 MW/CM2 UV irradiation at 50 C to give a polymer network. In each sample one of the polyester sheets was removed, leaving a bilayer consisting of an optically isotropic film of the crosslinked polymerized diacrylate (5) and a single polyester sheet.

a Cirpol The cholesteric mixture of example 2 was then coated between two such bilayers with the mixture being in intimate contact with the isotropic layer of polymerized diacrylate (5). The cholesteric mixture was polymerized in its cholesteric phase at 25 'C using 5 MW/CM2 UV irradiation (metal halide lamp).

The outer polyester sheets were removed, leaving a cholesteric film coated on both sides with an isotropic layer of the polymerized diacrylate (5). The composite film showed a reflective band from 430 to 470 nm. The ellipticity was measured in a spectrophot6meter using depolarized light at 450 nm wavelength.

The composite film exhibits a high ellipticity of 97 %. Thus, the composite film transmits essentially perfectly circularly polarized light, since even the depolarized light used for the measurement did not show a higher ellipticity (measured typically 96 - 98 % ellipticity).

The estimated refractive index of the isotropic layer of diacrylate (5) is 1.55 and matches the mean refractive index of the cholesteric layer.

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

1 A circular polarizer comprising at least one layer of an optically anisotropic material with helically twisted molecular structure and 5 planar alignment, characterized in that it further comprises at least one layer of an optically isotropic material preferably being adjacent to the layer of optically anisotropic material, wherein said isotropic material is selected such that its refractive index matches the mean refractive index of said anisotropic material. 10

2. A circular polarizer comprising at least one layer of an optically anisotropic material with helically twisted molecular structure and planar alignment, characterized in that it further comprises at least one layer of an optically isotropic material preferably being adjacent 15 to said layer of optically anisotropic material and causing an increase of the ellipticity of said layer of optically anisotropic material such that the following equation is fulfilled (E-E-)/(1-E-): 0.

3 wherein 20 E is the ellipticity of the intensity of said at least one layer of anisotropic material, and E is the ellipticity of the intensity of the circular polarizer comprising said at least one layer of optically isotropic and said at least one layer of optically anisotropic material, 25 with E and E respectively being defined as the ratio of the intensity Tx of the short axis to the intensity Ty of the long axis of the ellipse formed by the light transmitted by said at least one anisotropic layer or by said circular polarizer respectively. 3o 3. A circular polarizer as claimed in claim 1 or claim 2, characterized in that said layer of optically anisotropic material essentially consists of a linear or crosslinked polymerized cholesteric liquid crystal material with macroscopically uniform planar alignment.

4. A circular polarizer as claimed in any of claims 1 to 3, characterized

5 in that at least one layer of optically isotropic material essentially consists of a polymerized material. 5. A circular polarizer as claimed in any of claims 1 to 4, characterized in that the difference Inm - nil between the mean refractive index nm 10 of said layer of optically anisotropic material and the refractive index ni of said at least one layer of optically isotropic material is 0.3 or less.

6. A circular polarizer as claimed in any of claims 1 to 5, characterized 15 in that it exhibits an ellipticity E of at least 0.8, wherein E is as defined in claim 2.

7. A circular polarizer as claimed in any of claims 1 to 6, characterized in that the thickness of the isotropic layer is at least 0. 3 Rm. 20

8. A circular polarizer substantially as hereinbefore described in the foregoing examples.

9. A circular polarizer substantially as hereinbefore described with 25 reference to the accompanying drawings.

10. Use of a circular polarizer as claimed in any of claims I to 9 in a liquid crystal display device.

11. A liquid crystal display device comprising a circular polarizer as claimed in any of claims 1 to 9.

12. A liquid crystal display device comprising a liquid crystal cell and a circular polarizer as claimed in any of claims 1 to 9.

13. Use of a liquid crystal cell and a circular polarizer as claimed in any 5 of claims 1 to 9 in a liquid crystal display device.

14. A method of increasing the ellipticity of a circular polarizer comprising at least one layer of an optically anisotropic material with helically twisted molecular structure and planar alignment, 10 wherein said layer of anisotropic material is combined with at least one layer of optically isotropic material and wherein the following equation is fulfilled (E-E- )/(1-E-)2: 0.3 with E and E being as defined in claim 2. 15

15. A method of increasing the ellipticity of a circular polarizer substantially as hereinbefore described in the foregoing examples.

16. A method of increasing the ellipticity of a circular polarizer 20 substantially as hereinbefore described with reference to the accompanying drawings.