GB2355720A - Process of preparing a CLC reflective film comprising a photodegradable chiral compound - Google Patents

Abstract

A process for preparing a reflective film by coating a polymerizable cholesteric liquid crystal (CLC) material onto a substrate, aligning the material into planar orientation, and polymerizing the material by exposure to actinic radiation, characterized in that the polymerizable material comprises at least one photodegradable chiral compound that loses its chirality when being exposed to actinic radiation. Also disclosed is the use of said reflective film in optical, electroophical, information storage, decorative and security applications, a liquid crystal display device, and a photodegradable compound of formulae IX - XII <EMI ID=1.1 HE=136 WI=123 LX=494 LY=1036 TI=CF> <PC>wherein MG is a mesogenic group and R is H, halogen or an organic group having up to 30 C atoms.

Description

2355720 Process of Preparing a Reflective Film The invention relates to a

process of preparing a reflective film by using a photode gradable chiral compound. The invention also relates to a reflective film obtainable by the inventive process, to photodegradable chiral compounds used in the inventive process, to the use of an inventive reflective film in optical, electrooptical, information storage, decorative and security applications, and to a liquid crystal display comprising an inventive reflective film.

Reflective films comprising polymerized cholesteric liquid crystal material have been proposed in prior art for a variety of uses, such as broadband or notch polarizers, colour filters in liquid crystal displays or projection systems, and decorative purposes like for example the preparation of coloured image films or cholesteric pigment flakes.

These reflective films usually comprise one or more layers of a polymerized cholesteric liquid crystal material with planar orientation, i.e. wherein the axis of the cholesteric helix is oriented substantially perpendicular to the plane of the layer. As a consequence, the film exhibits selective reflection of light.

The bandwidth AX of the waveband reflected by a reflective film as described above is dependent upon the birefringence of the mesogenic material An and the pitch of the molecular helix p according to the equation AX = An x p. Thus, the bandwidth among other factors is determined by the birefringence of the material.

For some of the above mentioned uses, in particular for an application as broadband reflective polarizer in liquid crystal displays, it is desirable that the bandwidth of the reflective film should comprise a substantial portion of the visible wavelength range, whereas for other applications, for example as notch polarizer or as coloured reflective film in decorative or security applications, often films are desired that exhibit a specific reflection colour.

In particular broadband reflective polarizers, also known as circular 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.

Methods described so far for the preparation of broadband reflective films from liquid crystalline materials do have various drawbacks.

EP 0 606 940 discloses a method of preparing a circular reflective polarizer with a bandwidth of up to 400 nm. According to this method, a polymerizable cholesteric liquid crystalline mixture is coated on a substrate as a thin layer and subsequently polymerized by UV irradiation. During polymerization, diffusion of polymerizable 15 mesogenic compounds with different reactivity and chirality within the coated layer cause the formation of a pitch gradient throughout the layer, as described in Broer et al. Nature, Vol. 378, pp. 467 (1995). However, this process is rather slow and in some cases even takes 20 several minutes to complete. This is incompatible with most methods of large scale production of circular polarizer, which is usually done on continuously moving substrates. Also, it is difficult to apply the method described in EP 0 606 940 to plastic substrates. 25 WO 97/16762 describes a method of preparing a broadband reflective polarizer with a reflection bandwidth of up to 2000 nm. According to this method a layer of a material comprising a polymerizable cholesteric liquid crystal and a non-polymerizable nernatic liquid crystal is polymerized by UV irradiation. During 30 polymerization of the material phase separation occurs, with the nonpolymerizable nernatic liquid crystal segregating from the polymerizing cholesteric liquid crystal, leading to sites with different concentration of nematic liquid crystal, and thus with different pitch, throughout the material. 35 However, since this process also relies on material diffusion during film preparation, the preparation time is a critical issue. Furthermore, the polymerization temperature and the layer thickness as disclosed for the preparation of broadband films in WO 97/16762 are both very high, which is also a disadvantage.

WO 97/35219 describes a process for the production of reflective films on plastic substrates. This process is completed in the order of to 30 seconds, and is thus faster than that used by Broer et al., however, it is still relatively difficult by this process to control the resultant reflection wavelength and bandwidth of the reflective polarizer.

Furthermore, the methods described in EP 0 606 940, WO 97/35219 and WO 97/16762 produce reflective films with uniform optical characteristics along the lateral dimensions of the film. On the other hand, there are also applications where it is desired to have a reflective film with reflection characteristics that are varying along the lateral dimensions of the film, such as patterned films wherein different regions of the film exhibit different reflection colours. These films are useful inter alia for information storage or as multicoloured images.

GB 2,315,760 discloses a thermochromic polymeriza6le mesogenic composition, i.e. a composition showing a change of the reflection colour upon temperature variation, and also discloses a method to prepare a multicoloured reflective film by coating the thermochromic composition as a thin, oriented layer onto a substrate, selectively heating different regions of the layer to different temperatures (e.g.

by means of a laser), so that they exhibit different reflection colours, and curing the different regions to fix the respective colour.

The method described in the GIB 2,315,760, however, is still relatively complicated and time-consuming, as several heating and curing steps are required.

Consequently there was a need for a method of preparing reflective films with better and more easy control both of the reflection wavelength and the bandwidth of the film, as well as for a method to produce patterned reflective films, i.e. having reflection characteristics that vary along the lateral dimensions of the film, to obtain films that are suitable as reflective circular polarizers, colour filters or coloured films for information storage, decorative or security applications.

One aim of the invention is to provide a method of manufacturing a reflective film that does not have the above mentioned drawbacks in an efficient and cost-effective manner, and which is in particular suitable for mass production. Other aims of the invention are immediately evident to a person skilled in the art from the following description.

The inventors have found that the above mentioned aims can be achieved and the drawbacks of prior art can be overcome with a process of preparing a reflective film according to the present invention.

The process according to the present invention utilizes a polymerizable cholesteric liquid crystal material comprising a light sensitive chiral component that changes its chirality upon irradiation with light.

Chiral materials that change their chirality upon photoirradiation are known in prior art, and are also labelled 'photoisomerizable' or '(photo)tunable' chiral materials. US 5,668,614 discloses a tunable chiral material (TCM) that can be 30 changed from chiral to achiral or to a racemic mixture by irradiating with, for example, UV light or a high energy source such as laser. Further disclosed is a light modulating liquid crystal cell comprising a light modulating chiral nernatic liquid crystal material including a TCM, wherein different regions of the liquid crystal material exhibit 35 different reflection wavelengths. The cell is prepared by partially exposing the liquid crystal material with the TCM to photo-irradiation, e.g. through a photomask, leading to a change of the chirality of the TCM and thus to a change of the helical pitch in the exposed parts of the chiral nernatic liquid crystal material.

WO 98/57223 discloses a multi domain liquid crystal display device comprising a layer of nematically ordered liquid crystalline material containing a chiral dopant sandwiched between two substrates. The liquid crystal layer comprises at least two types of sub-pixels in which the twist senses of the liquid crystalline material are mutually 10 opposite, and the composition of the chiral dopant in the different types of subpixels is different. The device is manufactured by sandwiching between the substrates a layer of liquid crystalline material containing an isomensable chiral dopant with a first twist sense and a non-isomerisable chiral dopant with an opposite twist 15 sense, and photoirradiating the layer through a photomask. This causes the isomerisable dopant in the exposed parts of the layer to convert its chirality and thus its twist sense, leading to a change of the helical pitch in the exposed parts. 20 However, in order to achieve photoisomerization in chiral materials as described in US 5,668,614 and WO 98/57223 irradiation with UV light of high intensity and long duration is required. Therefore high lamp powers and long irradiation times are needed, which is a serious drawback for mass production. 25 This is especially disadvantageous in case the isomerizable chiral compound is used in a photocurable or photopolymerizable liquid crystal mixture, where the light used to induce a change in the isomerizable chiral compound also has the undesirable effect of 30 inducing a premature polymerization process in the mixture. Furthermore, UV irradiation of the mixture with high radiation doses (i.e. high radiation intensities and long radiation periods) can cause undesired degradation of the other components of the liquid crystal mixture. 35 The inventors have found that the above problems can be overcome by using a mixture comprising one or more photodegradable chiral compounds that irreversebly degrade when exposed to photoirradiation, e.g. to irradiation with UV light.

A class of compounds that is especially useful in the inventive process are photoinitiators having a chiral centre attached to the photoactive part of the initiator molecule. If the initiator is exposed to photoradiation, e.g. with UV light, the chirality of the molecule is irreversibly destroyed, causing a change of the chirality in the mixture. These chiral initiators have increased sensitivity to the photoradiation, compared to the photoisomerizable chiral materials of prior art, therefore lower radiation doses can be applied and premature polymerization can be prevented. Alternatively, it is possible to select the chiral photoinitiator such that it produces free radicals when decomposing, which initiate the polymerization.

Furthermore, the amount of degradation of the photodlegradable compound is proportional to light intensity, so patterned films with different colours can be produced using a polymerizable liquid crystal mixture containing the chiral photosensitive initiators, depending on the intensity and/or duration of radiation.

One object of the present invention is a process of preparing a reflective film by coating a polymerizable cholesteric liquid crystal (CLC) material onto a substrate, aligning the material into planar orientation, and polymerizing the material by exposure to actinic radiation, characterized in that the polymerizable material comprises at least one photodegradable chiral compound that loses its chirality when being exposed to actinic radiation.

Preferably the photodegradable compound irreversebly decomposes and loses its chirality when being exposed to the actinic radiation used for polymerizing the CLC material.

Another object of the present invention is a reflective film obtainable by the inventive process.

Another object the present invention are chiral photodegradable compounds used in the inventive process.

Another object of the invention is to the use of a reflective film obtainable by the inventive process in optical, electrooptical, information storage, decorative and security applications Another object the present invention is a liquid crystal display comprising - 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 reflective film according to the present invention being situated between the liquid crystal cell and at least one of said polarizers.

A first preferred embodiment of the present invention relates to the process of preparing of a reflective film comprising a polymerized CLC material with planar orientation, exhibiting a broad waveband of reflection. Such a film is particularly useful as a broadband reflective polarizer for liquid crystal displays.

The polymerizable CLC material used in this preferred embodiment comprises polymerizable achiral and/or chiral mesogenic compounds, a photoinitiator, and one or more photodegradable chiral compounds that lose their chirality upon irradiation with light. The photodegradable material is preferably selected such that it -a- decomposes and loses its -chirality upon irradiation with light used for polymerizing the CLC material.

The CLC material is coated onto a substrate, aligned and polymerized by exposure to UV light. The CLC material is preferably selected to exhibit a reflection peak in the blue region of visible light or in or close to the UV region, but preferably outside the wavelength range used for polymerization.

Upon exposure, the photodegradable chiral material at the top of the layer decomposes and loses its chirality first, thus reducing the effective concentration of chiral molecules and thereby the pitch of the CLC material, whereas the photodegradable material at the bottom of the layer will not degrade as much, resulting in the formation of a pitch gradient and thereby a broadening of the reflection band.

Concurrent with this process, the photoinitiator, which is exposed to the same UV intensity as the photodegradable material, will polymerize the film.

In the above described process, there will be a balance between the polymerization rate caused by the photoinitiator and the pitch' change caused by the photodegradable material upon UV irradiation.

Therefore, the pitch gradient and thereby the bandwidth of the reflection band can be controlled by careful selection of the amount, type and ratio of the photoinitiator and the photodegradable chiral material.

Thus, with inventive method broadband polarizers with a broad reflection band as well as notch polarizers with a reflection band of limited bandwidth can be prepared.

In case of a broadband reflective polarizer, the bandwidth of the reflected wavelength band is larger than 220 nm, particularly preferably larger than 320 nm, very particularly preferably larger than 420 nm.

In case of a reflective narrowband or notch polarizer, the bandwidth of the reflected wavelength band is from 40 to 250 nm, particularly preferably from 50 to 200 nm, very preferably from 70 to 150 nm.

In a particularly preferred embodiment, the above effect is further enhanced by adding a dye to the CLC material having an absorption maximum adjusted to the wavelength of the radiation used for polymerization, e.g. a UV dye. Preferably, a dye is used whose absorption maximum lies outside the reflection wavelength range of the reflective film in order to exclude undesired absorptions during the use of the film.

The dye produces a intensity gradient of the radiation in the thickness direction of the film, thus the difference of the rate and extent of degradation of the photodegradable material between the top and the bottom of the film is increased, leading to an increased pitch gradient.

A second preferred embodiment of the present invention relates to the preparation of a reflective film comprising a polymerized CLC material with planar orientation, wherein the reflection wavelength varies in lateral directions along the film, e.g. in form of a regular pattern. Such a film is particularly suitable e.g. as colour filter for liquid crystal displays or projection systems, or as -multicolour image for decorative and security uses.

According to this embodiment, a polymerizable CLC material as described for the first preferred embodiment above is coated onto a substrate and aligned. The CLC material is preferably selected to exhibit a reflection peak in the blue region of visible light or in or close to the UV region, but preferably outside the wavelength range used for polymerization.

In a first step, the aligned CLC layer is then partially exposed to UV light of moderate intensity, e.g. by covering the layer with a photomask. In the exposed parts of the layer, the effective concentration of chiral material is reduced, as the photodegradable chiral material decomposes and loses its chirality, therefore the selective reflection wavelength in this part of the film changes for example to green.

In case the photodegradable chiral material is a chiral photoinitiator, the degraded material produces e.g. radicals which can induce a polymerization process in the CLC material. However, it is also possible to add a conventional photoinitiator to the CLC material.

In a second step, the process of the first step is repeated, but with a different shaped photomask that covers different parts of the layer.

The UV radiation dose is preferably higher than in the first step, which has the effect of further decreasing the effective concentration of chiral molecules in the exposed part of the mixture, so that the selective reflection wavelength in this part changes for example to red.

The process can be repeated several times, with different UV radiation doses, i.e. different intensities and exposure times.

It is also possible to use a photomask that comprises different parts having different transmissivity for the actinic radiation used for polymerization. For example, it is possible to use as photomask a black and white photocopy or photograph exhibiting different grey shades, which is printed or copied onto a transparent material.

Alternatively, it is possible to irradiate selected parts of the CLC material by means of a finely focussed radiation source like, for example, a laser beam. In this case, the photodegradable material and the photoinitiator have to be selected to show absorption of the emission wavelength of the laser used for polymerization.

In a third preferred embodiment, the photodegradable chiral material is a chiral photoinitiator which decomposes under irradiation to produce free radicals or ions that start the polymerization reaction, and at the same time loses its chirality, causing a change of the chirality in the mixture. In this case, an additional photoinitiator is not necessarily required.

In a fourth preferred embodiment, the CLC material further comprises one or more additional chiral compounds which do not show a substantial change of chirality, but instead retain their chirality, under the same conditions where the photodegradable chiral compound loses its chirality. Thus, the additional chiral compounds should retain their chirality when irradiated with the same radiation dose as the photodegradable compounds. These additional chiral compounds are hereinafter also referred to as 'non-photodeg rad able' compounds. By adding a selected amount of one or more non-photodegradable chiral compounds to the CLC material, it is possible to control the position of the reflection band of the resulting polymer film.

Thus, when preparing for example a broadband reflective polarizer as described in the first preferred embodiment above, for example a CLC material is used that comprises polymerizable components and one or more non-photodegradable chiral compounds, the amount and helical twisting power (HTP) of which is selected such that the CLC mixture comprising only the non-degradable chiral compounds shows reflection of red light. To this mixture are then added one or more photodegradable chiral compounds, the amount and helical twisting power (HTP) of which is selected such that the total mixture, comprising both degradable and non-degradable chiral components, shows selective reflection of blue light. A polymer film is then prepared as described for the first preferred embodiment above. The resulting film will have a bandwidth extending from a minimum reflection wavelength in the blue region to a maximum reflection wavelength in the red region.

This method can be applied analoguously to the preparation of patterned films as described for the second preferred embodiment.

Thus, the different colours in the different regions of the resulting patterned polymer film can be controlled by appropriate selection of the amount and HTP of the degradable and non-degradable chiral compounds.

In a particularly preferred embodiment, the CLC material comprises degradable and non-degradable chiral materials having opposite twist sense. The resulting pitch length of the CLC material is then given by the amount and helical twisting power (HTP) of the two materials with different handedness. In this case, decomposition of the degradable chiral compound having a given handedness upon photo-irradiation will lead to an increase of the effective concentration of the non-degradable material with the opposite handedness, leading to a decrease of the resulting pitch and thus to a decrease of the reflection wavelength of the CLC material.

The thickness of a reflective film prepared by the inventive method is preferably from 1 to 30 Iim, in particular from 1.5 to 20 [Lm, very preferably from 2 to 10 im. In case of reflective films with a pitch variation in the direction perpendicular to the plane of the film, the thickness is also influencing the bandwidth of the reflective film.

Depending on the band position and the bandwidth, the thickness of the reflective film is preferably from 5 to 30 jam. For bandwidths of about 300 nm or more, a thickness of 10 to 20 tm is particularly preferred. For reflective films with smaller bandwidths e.g. in the range from 100 to 200 nm a thickness of 1.5 to 10 pm is preferred.

The polymerizable CLC mixture preferably comprises at least one polymerizable mesogenic compound having two or more polymerizable functional groups (di- or multireactive or di- or multifunctional compound). Upon polymerization of such a mixture a three-dimensional polymer network is formed. A polymer film made of such a network is self-supporting and shows a high mechanical and thermal stability and a low temperature dependence of its liquid crystal properties properties. In the ideal case the liquid crystal properties of the polymer film are temperature independent.

By varying the concentration of the multifunctional mesogenic or non mesogenic compounds, the crosslink density of the resulting 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 film, the thermal and mechanical stability or the solvent resistance can be tuned easily.

Especially preferably the polymerizable CLC mixture comprises 5 to 100 %, in particular 25 to 80 %, very preferably 45 to 70 %, by weight of multireactive mesogenic compounds. Direactive mesogenic compounds are particularly preferred.

The polymerizable components of the CLC material according to the present invention are preferably selected of formula I P-(SP-X)n-MG-Rl I wherein 0 P is CH2=CW-COO-, CH2=CH-O- or WH C CH- with W being H, CH3 or CI and k being 0 or 1, Sp is a spacer group having 1 to 25 C atoms, X is -0-, -S-, -CO-, -COO-, -OCO-, -CO-NH-, -NH-CO-, -CH2CH2-, -OCH2-, -CH20-, -SCH2-, -CH2S-, -CH=CH-, -CH=CH-COO-, -OCO-CH=CH-, -C=-C-, or a single bond, n is 0 or 1, MG is a mesogenic group, and R' is H, CN, halogen or a straight-chain or branched alkyl radical with up to 25 C atoms which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH2 groups to be replaced, in each case independently from one another, by -0-, -S-, -NH-, -N(CH3)-, - CO-, -COO-, -OCO-, -OCO-O-, -S-CO-, -CO-S- or -C-=C- in such a manner that oxygen atoms are not linked directly to one another, or alternatively R is denoting P-(Sp-X),-.

MG in formula I is preferably selected of formula 11 -A'-Z'_(A2_Z2_)m-A 310 wherein Z' and Z2 are each independently -COO-, - OCO-, -CH2CH2-, -OCH2-, -CH20-, -CH=CH-, -CH=CH-COO-, -OCO-CH=CH-, 15 -C- =C-, or a single bond, A', A 2 and A3 are each independently 1,4- phenylene in which, in addition, one or more CH groups may be replaced by N, 1,4cyclohexylene in which, in addition, one or two non-adjacent CH2 20 groups may be replaced by 0 and/or S, 1,4-cyclohexenylene, 1,4-bicyclo(2, 2,2)octylene, piperidine-1,4-diyl, naphthalene-2,6diyl, decahydronaphthalene-2,6-diyl, or 1,2,3,4-tetrahydronaphthalene-2,6-diyl, it being possible for all these groups to be unsubstituted, mono- or polysubstituted with F,Cl, OH, CN, N02 25 or alkyl, alkoxy or alkanoyl groups having 1 to 7 C atoms wherein one or more H atoms may be substituted by F or Cl, and m is 0, 1 or 2.

A smaller group of preferred mesogenic groups of formula 11 is listed below. For reasons of simplicity, Phe in these groups is 1,4-phenyiene, PheL is a 1,4-phenylene group which is substituted by 1 to 4 groups L, with L being F, Cl, CN, OH, N02 or an optionally fluorinated alkyl, alkoxy or alkanoyl group with 1 to 7 C atoms, and Cyc is 1,435 cyclohexylene. The following list of preferred mesogenic groups is comprising the subformulae 11-1 to 11-24 and their mirror images -Phe-Z-Phe- 11-1 -Phe-Z-Cyc- 11-2 -Cyc-Z-Cyc- 11-3 -PheL-Z-Phe- 11-4 -PheL-Z-Cyc- 11-5 -PheL-Z-PheL- 11-6 -Phe-Z-Phe-Z-Phe- 11-7 -Phe-Z-Phe-Z-Cyc- 11-8 -Phe-Z-Cyc-Z-Phe- 11-9 -Cyc-Z-Phe-Z-Cyc- 11-10 -Phe-Z-Cyc-Z-Cyc- 11-11 -Cyc-Z-Cyc-Z-Cyc- 11-12 -Phe-Z-Phe-Z-PheL- 11-13 -Phe-Z-PheL-Z-Phe- 11-14 -PheL-Z-Phe-Z-Phe- 11-15 -PheL-Z-Phe-Z-PheL- 11-16 -PheL-Z-PheL-Z-Phe- 11-17 -PheL-Z-PheL-Z-PheL11-18 -Phe-Z-PheL-Z-Cyc- 11-19 -Phe-Z-Cyc-Z-PheL- 11-20 -Cyc-Z-Phe-Z-PheL11-21 -PheL-Z-Cyc-Z-PheL- 11-22 -PheL-Z-PheL-Z-Cyc- 11-23 -PheL-Z-Cyc-Z-Cyc- 11-24 -Cyc-Z-PheL-Z-Cyc- 11-25 Particularly preferred are the subformulae 11-1, 11-2, 11-4, 11-6, 11-7, 11-8, 11-11, 11-13,11-14,11-15 and 11-16.

In these preferred groups Z in each case independently has one of the meanings of Z' as given in formula 1. Preferably Z is -COO-, - OCO-, -CH2CH2-, -C=C-or a single bond.

Very preferably the mesogenic group MG is selected from the following formulae and their mirror images (L)r (L)r Ila (L)r Ilb 10 Ilc (L)r (L)r COO-& lid (L)r (L)r CH2CH2 Ile (L)r (L)r (L)r (L)r (L)r IIg (L)r (L)r (L)r coo IIh 35 (L)r (L)r coo (L)r (L)r (L)r COO-& OOC-6- Ilk (L)r (L)r (L)r coo coo < - Ilm (L)r (L)r (L)r CH 2 CH2 -5 CH2CH2 -6- lin (L)r (L)r (L)r 00C - wherein L has the meaning given above and r is 0, 1 or 2.

(L)r The group in these preferred formulae is very preferably L L L L L denoting or furthermore 0 - t L with L having each independently one of the meanings given above.

Particularly preferred are the subformulae lid, 11g, Ilh, Ili, Ilk and lio, in particular the subformulae lid and Ilk.

L is preferably F, Cl, CN, OH, N02, CH3, C2H5, OCH3, OC21-15, COCH3, COC21-15, COOCH3, COOC21-15, CF3, OCF3, OCHF2, OC21F5, in particular F, Cl, CN, CH3, C21-15, OCH3, COCH3 and OCF3, Most preferably F, Cl, CH3, OCH3and COCH3.

In the compounds of formula I R1 may be a polar or unpolar group.

In case R' is a polar group, it is preferably selected from CN, N02, halogen, OCH3, OCN, NCS, COR', COOR 3 or a mono- oligo- or polyfluorinated alkyl or alkoxy group with 1 to 4 C atoms. R 3 is optionally fluorinated alkyl with 1 to 4, preferably 1 to 3 C atoms.

Halogen is preferably F or Cl. Especially preferably R1 in these compoundsis selected of F, Cl, CN, N02, OCH3, COCH3, COC2H5, COOCH3, COOC2H5, CF3, C2F5, OCF3, OCHF2, andOC2F5, in particular of F, Cl, CN, OCH3and OCF3 In case R1 is an unpolar group, it is preferably alkyl with up to 15 C atoms or alkoxy with 2 to 15 C atoms.

If R1 is an alkyl or alkoxy radical, i.e. where the terminal CH2 group is replaced by -0-, this may be straight-chain or branched. It is preferably straight-chain, has 2, 3, 4, 5, 6, 7 or 8 carbon atoms and accordingly is preferably ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy, heptoxy, or octoxy, furthermore methyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, nonoxy, decoxy, undecoxy, dodecoxy, tridecoxy or tetradecoxy, for example.

Oxaalkyl, i.e. where one CH2 group is replaced by -0-, is preferably straight-chain 2-oxapropyl (= meth oxym ethyl), 2- (=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3-, or 4-oxapentyl, 2-, 3-, 4-, or 5 oxahexyl, 2-, 3-, 4-, 5-, or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl or 2-, 3-, 4-, 5-, 6-,7-, 8- or 9 oxadecyl, for example.

In the compounds of formula I R1 may be an achiral or a chiral group.

In case of a chiral group it is preferably selected according to the following formula III:

1 2 _X _Q -CH-Q I Q 3 5 wherein X1 is -0-, -S-, -CO-, -COO-, -OCO-, -OCOOor a single bond, Q1 is alkylene or alkylene-oxy with 1 to 10 C atoms or a single bond, Q2 is an alkyl or alkoxy group with 1 to 10 C atoms which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH2 groups to be replaced, in each case independently from one another, by -C=-C-, -0-, -S-, -NH-, -N(CH3)-, -CO-, -COO-, -OCO-, -OCO-O-, -S-CO- or -CO-S- in such a manner that oxygen atoms are not linked directly to one another, Q3 is halogen, a cyano group or an alkyl or alkoxy group with 1 to 4 C atoms different from Q2.

In case Q1 in formula III is an alkylene-oxy group, the 0 atom is preferably adjacent to the chiral C atom.

Preferred chiral groups R' are 2-butyl (=l-methylpropyl), 2 methylbutyl, 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2 propylpentyl, 2-octyl, in particular 2-methylbutyl, 2-methylbutoxy, 2 methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 1-methylhexoxy, 2 octyloxy, 2-oxa-3-methylbutyl, 3-oxa-4-methylpentyl, 4-methylhexyl, 2-nonyl, 2-decyl, 2-dodecyl, 6-methoxyoctoxy, 6-methyloctoxy, 6 methyloctanoyloxy, 5-methylheptyloxycarbo-nyl, 2-methylbutyryloxy, 3-methylvaleroyloxy, 4-methylhexanoyloxy, 2-chlorpropionyloxy, 2 chloro-3-methylbutyryloxy, 2-chloro-4-methylvaleryloxy, 2-chloro-3 methylvaleryloxy, 2-methyl-3-oxapentyl, 2-methyl-3-oxahexyl, 1- methoxypropyl-2-oxy, 1-ethoxypropyl-2-oxy, 1-propoxypropyl-2-oxy, 1 -butoxypropyl-2-oxy, 2-fluorooctyloxy, 2-fluorodecyloxy for example.

In addition, compounds of formula I containing an achiral branched group R1 may occasionally be of importance, for example, due to a reduction in the tendency towards crystallization. Branched groups of this type generally do not contain more than one chain branch.

Preferred achiral branched groups are isopropyl, isobutyl (=m ethyl propyl), isopentyl (=3-methylbutyl), isopropoxy, 2-methyl propoxy and 3-methylbutoxy.

Another preferred embodiment of the present invention relates to compounds of formula I wherein R1 is denoting P-(SP-X)n-.

P in formula I is preferably an acrylate group, a methacrylate group, a vinyloxy group or an epoxy group, in particular an acrylate or epoxy group. As for the spacer group Sp in formula I all groups can be used that 20 are known for this purpose to the skilled in the art. The spacer group Sp is preferably a linear or branched alkylene group having 1 to 20 C atoms, in particular 1 to 12 C atoms, in which, in addition, one or more non-adjacent CH2groups may be replaced by -0-, -S-, -NH-' -N(CH3)-, - CO-, -0-CO-, -S-CO-, -0-COO-, -CO-S-, -CO-O-, 25 -CH(halogen)-, -CH(CN)-, -CH=CH- or -C=-C-. Typical spacer groups are for example -(CH2).-, (CH2CH20)p-CH2CH2, -CH2CH2-S-CH2CH2-or -CH2CH2-NH-CH2CH2-,with o being an integer from 2 to 12 and p being an integer from I to 3. 30 Preferred spacer groups are ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene, dodecylene, octadecylene, ethyleneoxyethylene, methyleneoxybutylene, ethylenethioethylene, ethylene-N-methyl35 iminoethylene, 1-methylalkylene, ethenylene, propenylene and butenylene for example.

Especially preferred are inventive compounds of formula I wherein Sp is denoting an alkyl or alkoxy group with 2 to 8 C atoms. Straightchain alkyl or alkoxy groups are especially preferred. 5 In another preferred embodiment of the invention the chiral compounds of formula I comprise at least one spacer group Sp that is a chiral group of the formula IV:

W -Q 1-CH-Q 4_ 1 Q IV wherein Q1 and Q3 have the meanings given in formula 111, and Q4 is an alkylene or alkylene-oxy group with 1 to 10 C atoms or a single bond, being different from Q1.

In the event that R is denoting P-Sp-X-, the two spacer groups Sp in the compounds of formula I may be identical or different.

Of the preferred compounds described above particularly preferred are those wherein n is 1.

Further preferred are compounds comprising both a group P-(SP-X)nwherein n is 0 and a group P-(Sp-X),,- wherein n is 1.

The compounds of formula I can be synthesized according to or in analogy to methods which are known per se and which are described in standard works of organic chemistry such as, for example, Houben-Weyl, Methoden der organischen Chemie, Thieme-Veriag, Stuttgart. Some specific methods of preparation can be taken from 35 the examples.

Examples of suitable polymerizable mesogenic compounds that can be used. as components of the polymerizable CLC material, are disclosed for example in WO 93/22397; EP 0,261,712; DE 195,04,224; WO 95/22586 and WO 97/00600. The compounds disclosed in these documents, however, are to be regarded merely as examples that shall not limit the scope of this invention.

Preferably the polymerizable CLC mixture comprises at least one polymerizable mesogenic compound having one polymerizable functional group and at least one polymerizable mesogenic compound having two or more polymerizable functional groups.

Examples of especially useful monoreactive chiral and achiral 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-(CH2)xO --<Df coo pva Y (Va) P-(CH2)xO coo -0-0-y (Vb) ID-(CH coo Ro 2)X0 +V& - (VC) P-(CH2) X 0 -0 coo +CHV-& Ro (Vd) P-(CH2)xo -a CH=CH - COO -a Ro (Ve) CH2 =CHCOO(CH2),, 0 - R 0 (Vf) P-(CH2).o __C coo CH2CH(CH3)C2H. (Vg) Li P-(CH2).o -&coo Z_COO_& CH2&(CH3)C2H. (Vh) P-(CH2).O -a COO-Ter P-(CH2).O -a COO-Choi (Vk) P-(CH2).o -a c 0 0 (Vm) wherein, P has one of the meanings of formula I and its preferred meanings as mentioned above, x is an integer from 1 to 12, A is 1, 4phenylene or 1,4-cyclohexylene, v is 0 or 1, Y is a polar group as defined for R1 above, Ro is an unpolar alkyl or alkoxy group as defined for R1 above, Ter is a terpenoid radical like e.g. menthyl, Choi is a cholesteryl group, and Ll and L2 are each independently H, F, Cl, OH, CN, N02 or an optionally halogenated alkyl, alkoxy or carbonyl group with 1 to 7 C atoms.

The term unpolar group in this connection preferably denotes an alkyl group with 1 or more, preferably 1 to 15 C atoms or an alkoxy group with 2 or more, preferably 2 to 15 C atoms.

Examples of useful direactive chiral and achiral 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 L 1 L 2 P(CH2).O -a coo OCO O(CH2)y P (Vla) L I L 2 P(CH2).O -C- CH2CH, i -6 CH2CH2 O(CH2)YP (Vlb) L 2 P C02 OC (VIC) 15 P(CH2).O CH=CHCOQ H OOCCH=CH O(CH2)YP (Vid) H 0 0 P(CH2).o D O(CH2)yP o' V H (Vle) wherein P, x, D, Ll and L have one of the meanings given above and y is an integer from 1 to 12 the same as or different from x.

A polymerizable CLC material according to the first preferred embodiment as described above comprises one or more chiral dopants which themselves do not necessarily have to show a liquid crystalline phase and give good planar alignment themselves, in particular non-polymerizable chiral dopants.

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). 5 Especially preferred are chiral non-polymerizable dopants selected from formula VII H R [a etvC co t 0 0 E 00C_V z R (R,S) H V Vil and/or formula VI I I H 0 0 R Z E 0 zo V R V 0 H (R,R) Vill including the (S,S) enantiomer of formula Vill 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 1 to 12 C atoms.

The compounds of formula Vil and their synthesis are described in WO 98/00428. The compounds of formula Vill and their synthesis are described in GIB 2,328,207. 35 The above chiral compounds of formula Vil and Vill exhibit a very high helical twisting power (HTP), and are therefore particularly useful for the purpose of the present invention.

The photodegradable chiral material used in the inventive process is preferably sensitive to the same radiation as used for polymerizing the CLC material. Particularly suitable photodegradable chiral materials that are sensitive to UV light are the compounds of the following formulae 0 CH3 Ix R-MG- H C N) 3 0 x R-MG OCH 3 F13C CH2 CH3 0 H X1 R-MG I N 0 %\'10 ---<,' H C CH 3 'A 3 CH2 CH 3 PH3 x1l CH CH 3 2 R-MGZ N \--/ 0 0 wherein 30 MG is a mesogenic group as defined above, and R is H, halogen or an organic group with up to 30 C atoms. 35 In the above compounds MG has preferably one of the meanings of formula 11 and its preferred subformulae. R has preferably one of the meanings of R1 in formula I and its preferred meanings given above.

Particularly preferred are compounds of the following formulae 0 CH3 IX-1 R I H, C NN 3 0 X-1 OCH3 U r'% H3C CH2CH3 0 H XI-3 R 0 F13C CH3 CH2CH3 PH3 XI 1-4 CH CH Z:' 3 2: N /--\ 0 wherein R has the meaning given above, in particular wherein R is alkyl or alkoxy with 1 to 12 C atoms.

The chiral compounds of formula IX - X11 can be synthesized according to or in analogy to methods which are known per se and which are described in standard works of organic chemistry such as, for example, Houben-Weyl, Methoden der organischen Chemie, Thieme-Verlag, Stuttgart.

The compounds of formula IX can be prepared according to or in analogy to reaction scheme 1. Scheme 1 shows the synthesis of a compound where the chirality is derived from the amino acid proline, 'Scheme 1 CH 0 + 1 3,0 D&S 0 Ny,,C-CH N I H 3 C \ r H OH CH 3 50 hrs -0 step 1 LIDA step 2 Hexane 0 CHI Et,,) <C H3 0 CF 3 <CH 3 0 0 Mel 01-i N N N OH step 4 OH 0 H step 3 CFI"O 3 step 5 Oxalyl chloride CH 2 C12 Y <C H 3 &R- kC H 3 -R1 CN 0 CH2C12 r _C I N CF,"O Cl AICI 3 1 0 3 CF,"'O step 6 3 step 7 NH3 /MeOH kCI-13C:-R' Mel / THF kCH3C -R' CN r < CN step 8 CH3 0 (IX-1a) H 0 wherein R'is R'-A' _Zl_(A2_Z2_)M_, R1 has the meaning of formula 1, and Z1, Z2, A', A2 and m have the meanings of formula 11.

The method of scheme 1 involves taking the readily available amino acid proline, methylating the (x-carbon without loss of chirality (steps 1 3), and subsequent protection of the free amine group with a trifluoro acetyl group (step 4). After conversion of the free acid to an acid chloride (step 5) this protected molecule is used as a reagent in a Friedel-Crafts acylation reaction (step 6). In the Friedel-Crafts reaction (step 6), a calamitic mesogenic core is attached to the chiral methyl proline group.

The compound IX-1 a is selected to have a reasonable helical twisting power HTP, in particular between 3 and 7 Lrn-, and breaks down by splitting along the chiral carbon-carbonyl bond to give two radicals. Thereby, the chirality of the molecule is changed. In addition the radicals can initiate a polymerization reaction, depending on the reaction conditions. However, the radicals must not necessarily act as initiator.

Further novel compounds can be synthesized from compound X-1 a as an intermediate, wherein these novel compounds have improved mesogenic or liquid crystalline properties, higher HTP and improved photochemical properties, such as a loss of chirality already when irradiated with UV light of lower power or at different wavelength.

Especially preferably these compounds are selected of the following formulae H 3 C IX-2 N-CH 3 N R-0 R 0 no N-CH H C - N/-'-] IX-3 3 3 F13C ICH 3 0 0 The compounds of formula XII can be prepared according to or in analogy to reaction scheme 2. In scheme 2 an achiral photosensitive compound is derived from an a-amino nitrile, and in the last step is resolved into its two enantiomers.

Scheme 2 CH 0 aq. solution 1 3,CH3 + 0). + NaCN >- NE-EE I N stir (24 hrs) C21-15 CH3 (XII-4') toluene reflux CH3 CH3CH2 N 0 R-- 0 (XII-4") Resolution with R-camphor sulfonic PH3 acid F CH3CH2 N 0 0 (XII-4a) CH3 CH CH 32",,, 0 (XII-4b) wherein R has one of the meanings of formula 1.

In the route depicted in scheme 2, a Strecker synthesis is used to generate the (x-amino nitrile XII-4', which can be obtained in good yield and purified by distillation. The nitrile is then reacted with organometallic carbanions to give the coupled compound XII-4", which subsequently has to be resolved with R-camphor sulphonic acid to give the two enantionmers XII-4a and XII-4b.

Preferably the CLC material comprises 0.1 to 20 %, in particular 0.2 to 15 %, very preferably 0.5 to 10 % by weight of one or more photodegradable chiral compounds. The CLC material preferably comprises one to six, very preferably-one to three, in particular one or two photodegradable chiral compounds. Most preferably a single photodegradable. chiral compound is used.

The above described novel photosensitive chiral compounds of formulae IX to XII, their preferred subformulae and their preparation are further objects of the present invention.

In a preferred embodiment the polymerizable mixture contains 0 to 20 %, preferably 0. 1 to 10 %, in particular 0.5 to 5 % by weight of a dye having an absorption maximum adjusted to the wavelength of the radiation used for polymerization. As a suitable dye for example an IR dye, a UV dye or a dye absorbing in the visible wavelength range can be used. A dichroic dye can also be used.

Preferably, a dye is used whose absorption maximum lies outside the reflection wavelength range of the reflective film in order to exclude undesired absorptions during the use of the film.

Suitable dye compounds are described in the literature and can be chosen, for example from the group of benzophenones, benzotriazoles, cinnamic and salicylic acids and their respective derivatives, anthraquinone and azo or azoxy dyes, organic Ni complex compounds, natural substances such as, for example, umbelliferon and yellow dyes like, for example, methyl yellow.

Particularly preferred are dyes showing absorption of UV light, like e.g. the following dye compounds O-N=N-&N(CH3)2 Q-N=N-Q-NH2 methyl yellow CH3 CH 3 0 S-Phe 011 NZ I'll I 1 11-11 Q 6 Phe phenyl Phe-S 0 0 C6 H 13 S I I O(CH 60H 0 2) and in particular UV dyes having an absorption maximum between 320 and 440 nm, preferably having a structural element of the following formulae HO 0 (:::::N -aL' =N \ N N kNI Commercially available UV dyes are for example the Tinuvin @) (from Ciba Geigy, Switzerland). An especially preferred UV dye is CLID 2 with the following formula 0 0-(CH 670-CO-CH 2) 3 H C-S 3 11 0 The examples of dye compounds given above should be taken only as illustrative and are in no way intended to restrict, but instead to explain the present invention.

The dye component is preferably comprising one to six, very preferably one to three, in particular one or two dye compounds.

Most preferably a single dye compound is used.

A particularly preferred polymerizable' CLC mixture according to the first preferred embodiment comprises a) 5 to 80 %, preferably 10 to 65 % by weight of one or more monoreactive polymerizable mesogenic compounds, b) 10 to 90 %, preferably 25 to 75 % by weight of one or more direactive polymerizable mesogenic compounds, c) 0.2 to 20 %, preferably 0.5 to 10 % by weight of one or more photodegradable chiral compounds as defined above, d) 0 to 15 %, very preferably 0.2 to 9 % by weight of one or more 5 non-polymerizable, non- photodegradable chiral compounds as defined above, e) 0 to 10 %, very preferably 0 to 5 % by weight of a photoinitiator, The compounds of component a) are preferably selected from the above formulae Va to Vm, in particular from formulae Va to Vf. The compounds of component b) are preferably selected from the above formulae Via to Vie, in particular from formulae Via and Vlb. 15 The photodegradable chiral compounds of component c) are preferably selected from the above formulae IX to XII. The non-polymerizable, non-photodegradable chiral compounds of 20 component d) are preferably selected from the above formulae VII and VIII. According to the inventive method, the polymerizable CLC material is coated onto a substrate and aligned into a uniform orientation. It is 25 then subsequently polymerized to permanently fix the orientation of the cholesteric material. The in-situ polymerization of polymerizable liquid crystalline compounds and compositions is described in detail by D. J.Broer et 30 al., Makromol.Chem. 190, 2255ff. and 3202ff. (1989). As a substrate for example a glass or quarz sheet or a plastic film or sheet can be used. In case of polymerizable matetrials, the substrates can be removed after polymerization or not. In case the 35 substrate is not removed from the polymerized film after polymerization, preferably an isotropic substrate is used.

Preferably the substrate is a plastic substrate such as for example a film of polyester such as polyethyleneterephthalate (PET), of polyvinylalcohol (PVA), polycarbonate (PC) or triacetylcel I u lose JAC), 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.

The CLC 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. In most cases it is suitable to heat the mixture in order to facilitate the evaporation of the solvent.

In addition to the methods described above, planar alignment in the coated layer of the CLC material can further be enhanced 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 20. polyimide or sputtered SiO, on top of the substrate, or alternatively to directly rub the substrate, i.e. without applying an additional alignment layer.

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 material 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.

Polymerization of the polymerizable CLC material takes place by exposing it to 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.

As a photoinitiator, it is preferred to used an initiator 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.

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 CLC mixture preferably comprises 0.01 to 10 %, very preferably 0.05 to 5 %, in particular 0.1 to 3 0/6 of a photoinitiator. 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. trim ethyl propane 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 alky1diacrylates or alky1dimethacrylates with alkyl groups of 1 to 20 C atoms. Typical examples for non mesogenic monomers with more than two polymerizable groups are trim ethylpropa netrimethacrylate or pentaerythritoltetraacrylate.

In another preferred embodiment the polymerizable CLC mixture 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 a preferred embodiment of the invention the polymerization of the polymerizable CLC mixture is carried out under an atmosphere of inert gas, preferably under a nitrogen atmosphere.

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.

The reflective film prepared by the inventive process can be used as reflective broadband or notch polarizer or as a multicoloured film or image in liquid crystal displays, as colour filter, in effect pigments, for decorative or security applications, like e.g. false-proof security markings on banknotes, identity or credit cards, passports or other documents of value.

Claims (16)

Claims

1 A process for preparing a reflective film by coating a polymerizable cholesteric liquid crystal (CLC) material onto a substrate, aligning 5 the material into planar orientation, and polymerizing the material by exposure to actinic radiation, characterized in that the polymerizable material comprises at least one photodegradable chiral compound that loses its chirality when being exposed to actinic radiation. io

2. A process as claimed in claim 1, characterized in that the photodegradable chiral compound irreverebly degrades and loses its chirality when exposed to the actinic radiation used for polymerizing the CLC material. 15

3. A process as claimed in claim 1 or claim 2, characterized in that the photodegradable chiral compound is a chiral photoinitiator.

4. A process as claimed in any of claims 1 to 3, characterized in that the polymerizable CLC material comprises at least one chiral 20 compound that does not substantially change its chirality when exposed to the actinic radiation used for degrading the photodegradable chiral compounds and for polymerizing the CLC material. 25

5. A process as claimed in any of claims 1 to 4, characterized in that the polymerizable CLC material comprises at least one dye showing absorption of the actinic radiation used for polymerizing the CLC material. 3o

6. A process as claimed in any of claims 1 to 5, characterized in that the polymerizable CLC material comprises a) 5 to 80%, preferably 10 to 65% by weight of one or more monoreactive polymerizable mesogenic compounds, b) 10 to 90%, preferably 25 to 75% by weight of one or more direactive polymerizable mesogenic compounds, c) 0.2 to 20%, preferably 0.5 to 10% by weight of one or more photodegradable chiral compounds, d) 0 to 15%, very preferably 0.2 to 9% by weight of one or more non-polymerizable, non-photodegradable chiral compounds, e) 0 to 10%, very preferably 0 to 5% by weight of a photoinitiator.

7. A process substantially as hereinbefore described as being in accordance with the present invention.

8. A reflective film comprising polymerized cholestric liquid crystal (CLC) material with planar orientation, being obtainable by a process as claimed in any of claims 1 to 7.

9. A reflective film as claimed in claim 8, characterized in that the helical pitch varies asymmetrically in a direction perpendicular to the plane of the film.

10. A reflective film as claimed in claim 8 or claim 9, characterized in that the helical pitch varies in lateral directions across the film.

11. A photodegradable chiral compound of the following formula 0 CH3 Ix R-MG H C 3 0 x R-MG-: OCH3 1-13C CH2CH3 0 H- X1 - R-MG 1 0 N H CH 3 -, A% 3 CH2CH3or CH3 x1i CH CH 3 2 N 0 R-MGZ 0 wherein MG is a mesogenic group and R is H, halogen or an organic group having up to 30 C atoms.

12. A photodegradable chiral compound substantially as hereinbefore described as being in accordance with the present invention.

13. Use of a reflective film as claimed in any of claims 8 to 10 and of a compound as claimed in claim 11 or claim 12 in optical, electrooptical, information storage, decorative and security applications.

14. An optical, electrooptical, information storage, decorative or security application comprising a reflective film as claimed in any of claims 8 to 10 and a compound as claimed in claim 11 or claim 12.

15. A liquid crystal display device comprising 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 10 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 15 substrate, and at least one reflective film as claimed in any of claims 8 to 10 being situated between the liquid crystal cell and at least one of said polarizers. 20

16. Use of 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 25 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 30 substrates, and at least one reflective film as claimed in any of claims 8 to 10 being situated between the liquid crystal cell and at least one of said polarizers, in a liquid crystal display device.