WO2004044631A1

WO2004044631A1 - Cholesteric color filter and method of manufacturing such

Info

Publication number: WO2004044631A1
Application number: PCT/IB2003/004624
Authority: WO
Inventors: Rene T. Wegh; Dirk J. Broer; Johan Lub
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-11-13
Filing date: 2003-10-20
Publication date: 2004-05-27
Also published as: AU2003269372A1

Abstract

The invention pertains to a method of manufacturing a layer of a cholesterically ordered polymer material, in which the material is oriented in such a way that the axis of the molecular helix of the cholesterically ordered material extends transversely to the layer, wherein the method comprises the following steps: a) providing a layer comprising a cholesterically ordered material, which material comprises a quantity of a convertible compound, the conversion of said compound being inducible by radiation; b) irradiating the layer to convert at least a part of the convertible compound in the irradiated parts of the layer; c) optionally at least partially polymerizing and/or cross-linking the cholesterically ordered material to form a three-dimensional polymer; d) applying a diffusion agent onto the layer to transversely swell the layer as a function of the concentration variation of the diffusion agent; e) fixing the diffusion agent to the polymer to freeze in the formed structure.

Description

Cholesteric color filter and method of manufacturing such

The invention pertains to cholesteric color filter and a method of manufacturing such.

Cholesteric color filters (CCF) can be used for reflective and transmissive liquid crystal displays and provide color selection by means of reflection instead of absorption as is the case for color filters based on dyes. The light reflected by the cholesteric color filter may be recycled to be offered again to the filter thus rendering the cholesteric color filter highly efficient. Moreover, the cholesteric color filter is polarization-selective in that it the reflected light is circularly polarized. In WO 00/34808 a cholesteric color filter is disclosed which comprises a polymerized and/or cross-linked cholesteric layer in which the material is oriented in such a way that the axis of the molecular helix of the cholesterically ordered material extends transversely to the layer. The layer is patterned to comprise first, second and third regions which comprise a quantity of a convertible compound which in its non-converted and in its converted state determines the pitch of the cholesterically ordered material to a different extent, the conversion of said compound being inducible by radiation, and the layer substantially absorbs said radiation.

The process of making these filters consists of applying a solution of a mixture of nematic diacrylates, a photosensitive chiral compound and a photo-initiator on a substrate by a coating technique (e.g. spin-coating). Color formation is done by irradiation through a mask. The colors are fixed by photo-polymerization under nitrogen upon which the color filter is obtained as a stable cross-linked film.

A disadvantage of the said known cholesteric color filter is that, when used in a reflective LCD, the filter is not efficient across the entire visible wavelength range. In particular at the high and low wavelength range of the visible spectrum light incident on the filter is not reflected very efficiently.

When used in a transmissive LCD, the cholesteric color filter has the disadvantage of exhibiting a pronounced viewing angle dependency in terms of the color perceived by a viewer. It is an object of the invention to obviate these drawbacks. In accordance with the invention this object is achieved by a cholesteric color filter comprising a polymerized and or cross-linked cholesterically ordered layer comprising a convertible compound and a converted compound, the convertible compound being convertible to the converted compound by radiation and the convertible compound and the converted compound having different helical twisting power; the cholesteric layer being patterned to comprise a first, a second and a third region in which the ratio of convertible to converted compound is selected for reflecting light in a wavelength band corresponding to a red, a green and a blue color respectively, the wavelength band corresponding to the first, the second and the third region having a bandwidth of at least 90 nm.

The inventors of the present invention have observed that the reflection bandwidth of the prior art cholesteric color filter (CCF) is limited to about 50 nm in the blue and about 70 nm in the red. This is determined by the birefringence Δn of the materials, which is about 0.15 for the mixtures currently used. This implies that three reflection bands centered in the blue, green, and red together do not cover the complete visible spectrum (400- 700 nm). h comparison, the transmission bands of absorbing color filters are about 100 nm wide. Consequently, the combined red green and blue reflection bands do not cover completely the visible spectral range of about 400 to about 700 nm. Accordingly, when used in a reflective LCD the known cholesteric color filters is less bright than a reflective LCD with absorbing color filters. The present invention provides a CCF which has red green and blue reflection bands which are sufficiently wide to cover the complete visible range, viz. bandwidths of at least 100 nm. Such wide bandwidths are also advantageous to reduce the viewing angle dependency of the color perceived by a viewer when the CCF is used in a transmissive LCD.

This is because a backlight will always provide to some extent an angular distribution of emitted light. The reflection band of a cholesteric layer shifts to shorter wavelength under oblique angle. Consequently, to reflect the red, green, and blue light under all emission angles, the CCF needs to exhibit broad reflection bands. Thus, for application in reflective as well as transmissive LCD broadening of the reflection bands of the CCF is important. In the context of the invention, reflection bandwidth is defined as wavelengths at which transmission is 50 % or less.

In a preferred embodiment the combined bandwidth of the first second and third regions is at least 290 nm. In another preferred embodiment the bandwidth of the blue region extends to about 400 nm and/or the bandwidth of the red region to about 700 nm.

In order to be able to manufacture the cholesteric color filter in accordance with the invention in a simple manner, a particular embodiment of the cholesteric color filter in accordance with the invention is characterized in that the polymerized cholesteric layer comprises a pitch-modifying diffusion agent distributed in the cholesteric layer in accordance with a concentration gradient along a direction transverse to the layer where the concentration gradient is substantially commensurate with a concentration gradient obtainable by allowing a substance to diffuse into a semi-infinite solid while keeping the surface concentration of the substance constant. The diffusion agent is pitch-modifying meaning capable of modifying, more particular enlarging, the pitch of the molecular helix of the cholesteric layer when admixed to the cholesteric layer, the degree to which the pitch is modified being proportional to the local concentration of the diffusion agent. Consequently, since the diffusion agent is distributed with a concentration gradient a pitch gradient is established which in turn leads to reflection band broadening. Band broadening may be achieved by introducing a variation in pitch transverse to the cholesteric layer. One way to obtain a transverse pitch variation is by applying a gradient in rate of photo-polymerization by a UN absorption gradient. However, this method does not combine optimally with the method of manufacturing the red, green and blue regions. A CCF wherein a pitch-modifying compound is introduced in the cholesteric layer by means of a diffusion process allows incorporation of both transverse and lateral pitch variations in layers of cholesterically ordered polymer material in a convenient manner. Diffusion into a semi-infinite solid while keeping the surface concentration of the diffusing species constant is well-known process. The analytical solution of the corresponding diffusion equation is an elementary result of diffusion theory.

In a preferred embodiment, the cholesteric color filter is provided with a cover layer comprising polymerized pitch-modifying diffusion agent, polymerized pitch-modifying diffusion agent also being dispersed in the cholesteric layer in accordance with a concentration gradient, the which is also dispersed in the cholesteric layer. If a polymerizable pitch-modifying diffusion agent is allowed to diffuse into the cholesteric layer, surplus polymerizable diffusion agent may be left in place to provide, after polymerization, a cover layer which protects the cholesteric layer.

In another aspect, the present invention provides a method in which a pitch of the molecular helix can be made to vary transversely to the layer in combination with a lateral pitch variation. More particularly, the invention provides a method in which the layer can be patterned at the same temperature and in which relatively large pitch differences through the layer can be realized with any cholesterically ordered polymer in a simple manner. It is a further object of the invention to provide a cholesteric color filter having a patterned layer of a cholesterically ordered material, manufactured by means of this method. In accordance with the invention, it is a method comprising the steps of: a. providing a layer comprising a cholesterically ordered material, which material comprises a quantity of a convertible compound, the conversion of said compound being inducible by radiation; b. irradiating the layer so that at least a part of the convertible compound in the irradiated parts of the layer is converted; c. optionally at least partially polymerizing and/or cross-linking the cholesterically ordered material to form a three-dimensional polymer; d. applying a diffusion agent onto the layer to transversely swell the layer as a function of the concentration variation of the diffusion agent; e. fixing the diffusion agent to the polymer to freeze in the formed structure.

Methods of obtaining cholesterically ordered layers with transversal pitch differences are known per se. For example, a gradient in the pitch transverse to the layer could be obtained by a method described in WO 97/23580, which discloses a method of manufacturing a switchable cholesteric filter by providing an inhomogeneous polymeric network wherein a mixture of polymerizable liquid crystalline molecules is provided between two substrates and a pitch variation is obtained by adding a photo-stabilizing compound. This compound causes the formation of an inhomogeneous polymeric network, resulting in a variation in the pitch of the molecular helix in the direction transverse to the polymer layer, which molecular helix is formed by the polymer. This variation of the pitch provides the optically active layer with a large bandwidth, the value of which is proportional to the value of the variation in pitch. Also this method has the previously indicated drawbacks. For example, it cannot or can hardly be combined with a lateral pitch variation. In addition, the speed of the process is governed by the diffusion of the polymerizable molecules, which is an inherently slow process.

It has been found that by making use of diffusion of molecules into the polymerized or partly polymerized layer according to the invention, that patterned layers of cholesterically ordered, liquid crystalline material can be manufactured in a simple way at the same temperature, with the maximum pitch difference in the layer being relatively large. Such layers can very suitably be used as color filters.

This method leads to swelling of the upper part of the layer, resulting in an enlargement of the pitch in this region. Preferably, the layer comprises laterally sections with varying main reflection wavelengths. In this manner suitable CCF's with transversely and laterally varying pitch can be made. The process to make a CCF according to the present invention is very simple. By (partly or completely) fixing, preferably by polymerizing the diffusion agent in the layer, the pitch of the molecular helix in the layer is fixed. In this way, a patterned layer of cholesterically ordered material with broadened reflection bands can be manufactured in a simple manner. In a preferred embodiment, a conventional single-pitch CCF layer is made by subsequent coating, photo-isomerization, and partly or completely photo-polymerization. Then, the diffusion agent is applied on top of the CCF. After waiting for the appropriate time, swelling occurs by diffusion of the diffusion agent, and the diffusion agent is then preferably polymerized to freeze in the formed structure (i.e. to freeze in the transverse pitch variation).

There is a number of parameters that can be used as tools to tune the speed and the degree of swelling and thus the broadening of the reflection band, such as:

- cross-link density of the single-pitch CCF (the term 'single pitch' means that the pitch has a single value transverse to the layer; the pitch may differ laterally);

- size and chemical properties of the diffusing molecules;

- chemical properties of the single-pitch CCF;

- time of the diffusion process

- temperature of the diffusion process; - thickness of the CCF layer

A suitable way to fix the diffusion agent to the polymer network is to provide the diffusion agent with a polymerizable group and to polymerize the compound to an at least partly polymerized network. A preferred way to make a mixture to manufacture CCF, comprises acrylate- or memacrylate-functional monomers, preferably acrylate- or methacrylate-functional monomers having at least two acrylate or methacrylate groups. The mixture most preferably used to make CCF's mainly consists of diacrylates, leading to a very high cross-link density. Other polymerizable groups than acrylate groups can equally well be used. Preferably, the polymerization is performed by a photo-polymerization step followed by a post-curing step. In order to enhance the diffusion, the cross-link density can be lowered either by interrupting the photo-polymerization, or by replacing part of the diacrylates by mono-acrylates. Moreover, the post-curing step, for instance at about 150°C, can be performed after the diffusion process.

A preferred diffusion agent comprises at least one of a liquid crystalline material, an acrylate- or methacrylate-functional monomer, and a photo-initiator. When acrylate- or methacrylate-functional monomers are used, preferably acrylate- or methacrylate-functional monomers having at least two acrylate or methacrylate groups, also a photo-initiator may be added. In a particularly preferred embodiment according to the invention the method comprises that the diffusion agent contains a mixture of acrylate- and/or methacrylate-functional monomers, each monomer having a different diffusion coefficient, such that a controlled gradient of diffused monomer is formed resulting in a controlled variation of the cholesteric pitch. Thus the diffusion agent is preferably a monomer or a mixture of monomers, that can be coated by e.g. spin-coating on the CCF and can be photo-polymerized after diffusion. Suitable monomers are for example 2-phenoxyethyl acrylate, 2-phenoxyethyl methacrylate, ethoxylated phenol monoacrylate, 2-(p- chlorophenoxy)ethyl acrylate, p-chlorophenyl acrylate, phenyl acrylate, 2-phenylethyl acrylate, 2-(l-naphthyloxy)ethyl acrylate, o-biphenyl methacrylate, o-biphenyl acrylate and mixtures thereof. Preferred monomers containing at least two (meth)acrylate groups are pentaerythritol triacrylate, trimethylolpropane triacrylate, ethoxylated bisphenol A diacrylate, 1,6-hexanediol diacrylate, 1,4-butanediol diacrylate, diethylene glycol methacrylate, trimethylolpropane trimethacrylate, tetraethylene glycol dimethacrylate, triethylene glycol dimethacrylate, 1,6-hexanediol dimethacrylate, 1,4-butanediol dimethacrylate, 1,14- tetradecanediol dimethacrylate, and mixtures thereof. When the monomer is a diacrylate, the polymerization leads to a higher cross-link density and therefore a better thermal stability. Moreover, if the amount of monomer applied is more than needed for diffusion, an extra protection layer on top of the CCF will be formed. It is known that a topcoat layer improves the stability of the CCF considerably. Thus, the coated monomer does not only act as diffusion agent, but can also form a passivation and planarization layer after polymerization. The two-fold function and the ease of the process are major advantages of this method of reflection band broadening.

It is also possible to remove the excess amount of diffusion agent by e.g. washing with xylene and thus polymerize only the molecules diffused into the layer.

The diffusion agent is possibly a nematic monomer, or a mixture of nematic monomers. In that way the birefringence Δn in the cholesteric layer remains maximal, leading to maximum broadening of the reflection band. In this way, it is in principle possible to use the excess amount of diffusion agent as quarter wave film, which is needed to convert the polarization state of transmitted light from circular to linear. It is also possible to polymerize the excess amount in the isotropic state, so that it does not change the polarization state of the transmitted light.

The broadening of the reflection band is shown in the Figures.

Fig. 1 shows the transmission spectrum of the blue, green, and red regions of a CCF with broadened reflection bands due to a cholesteric layer according to the invention.

Fig. 2 shows the transmission spectrum of the blue, green, and red regions of a CCF with normal reflection bands due to a conventional layer.

The conversion of the convertible compound is effected by irradiation with energy in the form of, for example, electro-magnetic radiation, nuclear radiation or an electron beam. Preferably said conversion is effected by means of UN radiation. Preferably, the method according to the invention is carried out using actinic radiation in the wavelength region between 300 and 600 nm or using electron beam irradiation.

In accordance with a preferred embodiment of the invention, part of the layer has sections with different pitches, for instance an upper layer with more or less the same concentration of the diffusion agent and a certain pitch and a lower layer without the diffusion agent and another pitch. Thus according to this invention such layers are made wherein the upper part of the layer of the cholesterically ordered polymer material contains a certain concentration of the diffusion agent and has a certain pitch, and the lower part of the layer is substantially free from the diffusion agent and has another pitch. In another embodiment the filter is characterized in that the pitch of the molecular helix increases substantially continuously from a minimum value at one surface of the layer to a maximum value at the other surface of the layer. By means of this particular configuration it is attained that the helical structure of the cholesteric material, viewed in the direction of the normal to the layer, changes gradually. This precludes the occurrence of material stresses in the optical layer and has a favorable effect on the strength of said layer. It is also possible to vary the degree of polymerization in step c laterally by means of a photo mask, so that some areas are less accessible for the diffusion agent than other areas which results in a lateral pattern of narrow and broader reflection bands of the cholesteric layer.

Another favorable embodiment of the filter according to the invention is characterized in that the polymer material forms a three-dimensional network. Optically active layers, which consist of such three-dimensional networks are exceptionally mechanically and thermally stable. Such layers are pre-eminently suitable as color filters, which also belong to the present invention. Thus it is also an object of the invention to provide a cholesteric color filter comprising a patterned layer of a fixed cholesterically ordered polymer network, and said layer further optionally comprising monomers that may be polymerized, said layer having a pitch difference in both transversal and lateral directions and having at 50% transmission a reflection bandwidth that is at least 30%, preferably at least 50%, broader than the reflection bandwidth of said color filter without a pitch difference in the transversal direction. Preferably, such color filter is made by a method wherein the lateral pitch difference is obtained by irradiation of a layer comprising a cholesterically ordered material, which material comprises a quantity of a convertible compound, the conversion of said compound being inducible by radiation; and irradiating the layer to convert at least a part of the convertible compound in the irradiated parts of the layer.

Apart from the color filters per se, the invention further pertains to displays comprising such color filters. Displays according to the invention comprise in particular liquid crystalline displays (LCDs). An LCD consists of two substrates of which one is coated with the color filter of the invention. Furthermore, both substrates contain transparent electrodes and an orientation layer. Liquid-crystalline molecules are sandwiched between the substrates, which are switched by applying a voltage over the LC cell. Addressing of the individual pixels can be done by passive or active matrix addressing methods. Furthermore, the LCD comprises at least one polarizer and optionally retarder films, an illumination system, a reflector, and/or a diffusor. The color filter of the invention can also be used in other display types based on modulation of white light (e.g. electrophoretic displays).

The invention is illustrated by the following non-limitative examples.

Example 1

On a clean glass surface polyimide was applied by spin-coating followed by baking and rubbing.

2.0 g of a mixture of 7.0% of diacrylate I, 36.4% of diacrylate II (ex Merck GmbH, Darmstadt), 18.2% of diacrylate III (ex Merck GmbH, Darmstadt), 36.4% of a 1 : 1 mixture of mono-acrylates IN and N prepared as described herein below, 1.0% of 2-(Ν- ethylperfluorooctanesulfonamido)-ethylacrylate (ex Acros) , and 1.0% of Darocur® 4265 (ex Ciba) (photo-initiator) were mixed with 1.8 g of xylene containing 100 ppm of 4-methoxy- phenol (inhibitor).

Diacrylate I

Diacrylate II

Diacrylate III

Mono-acrylate IV

Mono-acrylate N

The homogeneous solution was filtered and spin-coated for 30 sec at 1000 rpm (BLE) on the polyimide surface. The film was irradiated by UN light from a Philips HPA lamp (4.5 mW/cm² at 365 nm) in air for 0, 5, and 10 seconds to obtain a blue, green and red region, respectively. Subsequently, the film was annealed at 70°C for 1 minute and photo- polymerized under nitrogen by irradiation with the same lamp for 8 minutes. After an oxygen plasma treatment to improve the wetting, 1,6-hexanediol diacrylate (HDD A) was spin-coated for 30 sec at 2000 rpm (BLE) on top of the cholesteric layer. After 30 minutes the layer was photo-polymerized under nitrogen for 10 minutes. The polymerization was finalized by post- curing for 90 minutes at 150°C in nitrogen. The transmission spectra for the three regions of this sample show a broadening of the reflection bands to about 100 nm due to diffusion of HDDA into the top part of the CCF layer. The diffusion process was stopped before HDD A reached the bottom of the CCF layer, so that the pitch lengths at the top and the bottom are different, resulting in reflection band broadening. The reflection bandwidth of the sample as prepared via the above method approaches the reflection bandwidth most desired for CCF for transmissive and reflective LCD. This is shown in Figs. 1 and 2. Diacrylate I was prepared as follows:

Synthesis of 4-(6-acryloyloxy-hexyloxy)-cinnamic acid 6-(S)-[4-(6- acryloyloxy-hexyloxy)-cirmamoyloxy}-hexahydrofuro[3,2-b]furan-3(R)-yl ester (I)

A mixture of 3.18 g (0.01 mole) of 4-(6-acryloyloxyhexyloxy)-cinnamic acid, 0.73 g (5 mmole) of isosorbide and 122 mg (1 mmole) of 4-N,N-dimethylaminopyridine in 40 ml of dichloromethane was cooled in an ice/water bath under nitrogen while stirring. Then 2.06 g (0.01 mole) of N,N'-dicyclohexyl carbodiimide were added to the mixture. The mixture was stirred overnight. The mixture was filtered over silica and the dichloromethane was evaporated to give 3.06 g of a semi-solid, which was dissolved in 10 ml of dichloromethane, 20 ml of ethanol were added and the dichloromethane was then removed in vacuum (40°C, 500 mbar). The cloudy solution (14.2 g) was put in the refrigerator. The solid formed was filtered off and dried in a dessicator over silica. 2.12 g of a white solid (I) (56%) were obtained with mp 77°C.

The 1 : 1 mixture of mono-acrylates IN and N was prepared as shown in the following scheme:

A: Synthesis of a mixture of (3-methyl-4-hydroxyphenyl)-4-hexyloxy-benzoate and (2- methyl-4-hydroxyphenyl)-4-hexyloxy-benzoate, (IX) .

45.4 g of N,N'-dicyclohexyl carbodiimide (0.22 mole) were added to a solution of a mixture of 46 g of 2-methyl-4-(tetrahydro-pyran-2-yloxy)-phenol and 3-methyl- 4-(tetrahydro-pyran-2-yloxy)-phenol (VI, see Lub et al., Liquid Crystals. 1998, 24, 375), 48.9 g of 4-hexyloxybenzoic acid (Nil) and 2.69 g of 4-Ν,Ν-dimethylaminopyridine in 450 ml of dichloromethane, and stirred under nitrogen atmosphere in an ice bath. Then the mixture was stirred at room temperature overnight. The solution was filtered through silica and the dichloromethane was evaporated. The remaining oil was mixed with 5 g of pyridinium 4- toluenesulfonate and dissolved in 400 ml of ethanol. After stirring the solution for two hours at 55°C it was added dropwise to 400 ml of water and 200 g of ice under vigorous stirring. A sticky solid was obtained which was washed with 400 ml of a 1 : 1 solution of water and ethanol and then dissolved in 200 ml of dichloromethane. The solution was washed twice with 200 ml of water, dried over magnesium sulfate and evaporated at 80°C. 54 g (yield 75%) of a brown sticky solid was obtained which was pure enough to be used in the next step.

B: Synthesis of a mixture of 4-(4-hexyloxybenzoyloxy)-3-methylphenyl 4-(3-acryloyl- oxypropyloxy) benzoate (IN) and 4-(4-hexyloxybenzoyloxy)-2-methylphenyl 4-(3- acryloyloxypropyloxy)benzoate (V).

33.93 g of Ν.Ν'-dicyclohexyl carbodiimide were added to a mixture of 41.15 g of 4-(3-acryloyloxypropyloxy) benzoic acid (VIII), 54 g of mixture (IX), 1.95 g of 4-Ν,Ν- dimethylaminopyridine and 400 ml of dichloromethane, and stirred under nitrogen and cooled in an ice bath. The mixture was stirred at room temperature overnight, filtered through silica and evaporated. The solid product was washed twice with 250 ml of ethanol and then dried over silica in a desiccator. 63.11 g (yield 70 %) of a white solid was obtained with mp 67°C and cp 153°C (from the nematic phase).

Example 2 On a clean glass surface polyimide was applied by spin-coating followed by baking and rubbing. 2.0 g of a mixture of 6.5% of diacrylate I, 36.6% of diacrylate II, 18.3% of diacrylate III, 36.4% of a 1:1 mixture of mono-acrylates IV and V, 1.0% of 2-(N- ethylperfluorooctanesulfonamido)-ethylacrylate (ex Acros) , and 1.0% of Darocur® 4265 (ex Ciba) (photo-initiator) were mixed with 1.6 g of xylene containing 100 ppm of 4-methoxy- phenol (inhibitor).

The homogeneous solution was filtered and spin-coated for 30 sec at 1000 rpm (BLE) on the polyimide surface. The film was irradiated by UN light from a Philips HPA lamp (4.5 mW/cm at 365 nm) in air for 0, 6, and 12 seconds to obtain a blue, green, and red region, respectively. Subsequently, the film was annealed at 70°C for 1 minute and photo- polymerized under nitrogen by irradiation with the same lamp for 4 minutes. After an oxygen plasma treatment to improve the wetting, 1,4-butanediol diacrylate (BDDA) was spin-coated for 30 sec at 1000 rpm (BLE) on top of the cholesteric layer. After 5 minutes the layer was photo-polymerized under nitrogen for 10 minutes. The polymerization was finalized by post- curing for 90 minutes at 150°C in nitrogen. The transmission spectra for the three regions of this sample are similar to those of Example 1.

Example 3 On a clean glass surface polyimide was applied by spin-coating followed by baking and rubbing. 2.0 g of amixture of 7.0% of diacrylate I, 72.8% of diacrylate II, 18.2% of diacrylate III, 1.0% of 2-(N-ethylperfluorooctanesulfonamido)-ethylacrylate (ex Acros), and 1.0% of Darocur® 4265 (ex Ciba) (photo-initiator) were mixed with 1.6 g of xylene containing 100 ppm of 4-methoxyphenol (inhibitor). The homogeneous solution was filtered and spin-coated for 30 sec at 1200 rpm

(BLE) on the polyimide surface. The film was irradiated by UN light from a Philips HPA lamp (4.5 mW/cm² at 365 nm) in air for 0, 12, and 24 seconds to obtain a blue, green, and red region, respectively. Subsequently, the film was annealed at 70°C for 1 minute and photo- polymerized under nitrogen by irradiation with the same lamp for 10 minutes. After an oxygen plasma treatment to improve the wetting, diacrylate II in the molten state was applied on the CCF at 80°C. The excess amount of diacrylate II was removed immediately by washing with xylene. The layer was photo-polymerized under nitrogen for 10 minutes. The polymerization was finalized by post-curing for 90 minutes at 150°C in nitrogen. The transmission spectra for the three regions of this sample are similar to those of Example 1.

Claims

CLAIMS:

1. A cholesteric color filter comprising a polymerized and/or cross-linked cholesterically ordered layer comprising a convertible compound and a converted compound, the convertible compound being convertible to the converted compound by radiation and the convertible compound and the converted compound having different helical twisting power; the cholesteric layer being patterned to comprise a first, a second and a third region in which the ratio of convertible to converted compound is selected for reflecting light in a wavelength band corresponding to a red, a green and a blue color respectively, the wavelength band corresponding to the first, the second and the third region having a band-width of at least 90 nm.

2. A cholesteric color filter as claimed in claim 1, wherein the combined bandwidth of the first second and third regions is at least 290 nm.

3. A cholesteric color filter as claimed in claim 1 or 2, wherein the bandwidth of the blue region extends to about 400 nm and/or the bandwidth of the red region to about 700 nm.

4. A cholesteric color filter as claimed in claim 1, 2 or 3, wherein the polymerized cholesteric layer comprises a pitch-modifying diffusion agent distributed in the cholesteric layer in accordance with a concentration gradient along a direction transverse to the layer where the concentration gradient is substantially commensurate with a concentration gradient obtainable by allowing a substance to diffuse into semi-infinite solid while keeping the surface concentration of the substance constant.

5. A cholesteric color filter as claimed in claim 1, 2, 3 or 4, wherein the cholesteric color filter is provided with a cover layer comprising polymerized pitch- modifying diffusion agent, the polymerized pitch-modifying diffusion agent also being dispersed in the cholesteric layer in accordance with a concentration gradient.

6. A method of manufacturing a layer of a cholesterically ordered polymer material, in which the material is oriented in such a way that the axis of the molecular helix of the cholesterically ordered material extends transversely to the layer, wherein the method comprises the following steps: a. providing a layer comprising a cholesterically ordered material, which material comprises a quantity of a convertible compound, the conversion of said compound being inducible by radiation; b. irradiating the layer to convert at least a part of the convertible compound in the irradiated parts of the layer; c. optionally at least partially polymerizing and/or cross-linking the cholesterically ordered material to form a three-dimensional polymer; d. applying a diffusion agent onto the layer to transversely swell the layer as a function of the concentration variation of the diffusion agent; e. fixing the diffusion agent to the polymer to freeze in the formed structure.

7. The method according to claim 6, wherein the diffusion agent comprises a polymerizable group.

8. The filter in accordance with claim 2 or method according to claim 7, wherein the diffusion agent comprises at least one of a liquid crystalline material, an acrylate- or methacrylate-functional monomer, and a photo-initiator.

9. The method according to claim 8, wherein at least part of the acrylate- or methacrylate-functional monomer comprises at least two acrylate or methacrylate groups.

10. The method according to any one of claims 7, wherein the diffusion agent comprises a mixture of acrylate- and/or methacrylate-functional monomers, each monomer having a different diffusion coefficient.

11. The method according to any one of claims 7 to 10, wherein step c is performed and the polymerized and/or cross-linked cholesterically ordered three-dimensional polymer obtained has a uniform pitch in the transversal direction of the layer.

12. The method according to any one of claims 7 to 11, wherein the upper part of the layer of the cholesterically ordered polymer material contains a certain concentration of the diffusion agent and has a certain pitch, and the lower part of the layer is substantially free from the diffusion agent and has another pitch.

13. The method according to any one of claims 7 to 12, wherein the layer of the cholesterically ordered polymer material is oriented in such a way that the axis of the molecular helix of the cholesterically ordered material extends transversely to the layer, and the pitch of the molecular helix increases substantially continuously from a minimum value at one surface of the layer to a maximum value at the other surface of the layer.

14. The method according to any one of claims 7 to 13, wherein the layer is irradiated with laterally varying irradiation intensities to convert the convertible compound laterally to a different extent as to provide a layer comprising lateral sections with different main reflection wavelengths and a transversal pitch difference.

15. The method according to any one of claims 7 to 14, for providing a layer with a coating comprising the diffusion agent, by applying an amount of the diffusion agent that is more than needed for diffusion.

16. A cholesteric color filter comprising a patterned layer of a fixed cholesterically ordered polymer network, and said layer further optionally comprising monomers that may be polymerized, said layer having a pitch difference in both transversal and lateral directions and having at 50% transmission a reflection bandwidth that is at least 30%, preferably at least 50%», broader than the reflection bandwidth of said color filter without a pitch difference in the transversal direction.

17. A method of making the cholesteric color filter of claim 16, wherein the lateral pitch difference is obtained by irradiation of a layer comprising a cholesterically ordered material, which material comprises a quantity of a convertible compound, the conversion of said compound being inducible by radiation; and irradiating the layer to convert at least a part of the convertible compound in the irradiated parts of the layer.

18. A display comprising the cholesteric color filter of claim 1, 2, 3, 4, 5, 6 or 16 or manufactured in accordance with a method in accordance with any one of the claims 7 to 15 or 17.