Process for broadening cholesteric reflection bands of photopolymerizable cholesteric liquid crystals, and optical elements produced by this process

Description

process of broadening the reflection bands of photopolymerizablelesteric liquid crystals, and to optical elements, such as coloredters, reflectors, polarizers and pigments, produced by this process.Liquid-crystalline materials having cholesteric properties,reviated to cholesteric liquid crystals, are substances having aical arrangement of the molecules. These materials are usuallypared as a thin layer between two suitable substrates in such a 15that the helix axis is perpendicular to the substratefaces. The pitch of the helix is material- dependent and is constantr the layer thickness. Such optically anisotropic layers are able tolect a circular light component fully if the direction of 20ation and light wavelength [err] in the material correspond to theection of rotation and pitch p of the cholesteric helixolesteric reflection). By contrast, the second circular lightponent having the opposite direction of rotation is transmitted 25ly. The cholesteric reflection occurs here in a spectrald between the wavelengths [err] = p*n[err] and [err] = p*n[err],re n[err] and n[err] denote the extraordinary and ordinaryractive indices of the material. This 30 reflection band can beracterized by two parameters, the central wavelength [err] and theth [err]. The central wavelength [err] depends on the meanractive index and pitch p of the material. The width [err] of thelesteric reflection band is dependent on the 35 birefringencer]n = n[err]-n[err] of the material in accordance with theation [err] = p*(n[err]-n[err]). In practice, the birefringence oft cholesteric materials in the visible spectral region is restrictedvalues lowerh polarization directions (right-handed circular and left-handedircular, i.e. unpolarized) is transmitted fully. The reflected ornsmitted circular-polarized light can, if desired, be converted intoear-polarized light by means of an additional quarter-waveardation layer. An essential prerequisite for the use10 cholesteric materials is adequate thermal and mechani- calbility of the layers. This stability can be achieved by fixing thegnment state by polymeriza- tion or by rapid cooling to temperaturesow the glass transition temperature. Stable cholesteric layers 15 ofs type are described, for example, by R. Maurer et al. under thele "Polarizing Color Filters made from Cholesteric LC Silicones" in90 Digest, 1990, pp. 110 - 113. Owing to saidical and mechanical 20 properties, cholesteric materials aretable both as polarizing and color-selective reflectors and asarizing and color-selective optical filters. They have the greatantage over filters made from absorbent materials that heating offilter material 25 is substantially avoided. Given a corresponding bandth of the cholesteric reflection, these materials can also be usedso-called reflective polarizers, for example in liquid-crystalplays: If unpolarized light from a light source 30 locatedween a cholesteric layer and a mirror (metal) hits the cholestericer, circular-polarized light having a direction of rotation oppositethat of the layer helix passes through the layer, while theaining fraction having the same direction of 35 rotation is reflected.s component hits the mirror and experiences inversion of theection of rotation of the circular polarization, with thesequence that this light component can then likewise pass throughlight source, mirror and absorptive polarizer, it is possible toble the 5 light yield of the illumination unit of a liquid- crystalplay. At the same time, the absence of absorption means that heatingbleaching of the polarizer is avoided (S.V. Belayev, M. Schadt,. Barnik, J. Funfschilling, N.V. Malimoneko and K. 10 Schmitt, Jpn.Appl. Phys. 29, L273 (1990)). Photopolymerizable cholestericerials can also be photostructured. This is described, formple, by R. Maurer et al. "Cholesteric Reflectors with a Colortern" in SID 94 Digest, 1994, pp. 399 - 15 402. The material describedrein exhibits pronounced thermochromicity, i.e. a strong dependencethe reflection color on temperature. The desired color canrefore be set by means of the temperature of the sample and fixed byosure to UV through a mask. The 20 color of the unexposed areas of thelesteric layer can be modified by subsequent temperature change.s color is permanently fixed by a second exposure to UV, ifired again through a mask. This operation can be repeated atferent temperatures with further masks 25 to produce multicoloreductured filters and reflectors. Such structured filters andlectors can be used, for example, in color projectors and inuid-crystal displays. A further application of cholestericerials 30 is as pigments produced by grinding and screeninglesteric films. Suitable materials and their production arecribed, for example, in EP 0 601 483. The actual achievementthese potential applications has hitherto been greatly restricted bythe limited width of the reflection bands. For industrial, it is in addition desirable for both the central wavelengths ofreflection band and the width of the reflection band to be freelylection band to cover the entire visible spectral region, i.e. forcholesteric layer to have 5 a band width of greater than 300 nm.problem of inadequate band width can in principle be solved bystructing the optical element from a plurality of layers havingferent central wavelengths. This is described in the above-mentionedarticle by R. Maurer et al. However, this method is very expensivehas the disadvantage that the optical quality of the opticalment decreases with each additional layer owing to scattering atws and inhomogeneities. 15 Another process of solving theve-mentioned problem is to broaden the reflection band by means of adient in the helix pitch (pitch gradient). This approach has alreadyn known for some time from theoretical studies (for example, S.kedian, S. 20 Melone, F. Rustichelli, J. Physique 37, 731 (1976) and. Hajdo, A.C. Erigen, J. Opt. Soc. Am. 36, 1017 (1979)).process described in EP 0 606 940 A2 uses a mixture of chiral andatic monomers having different 25 reactivity with respect to theirymerization proper- ties which additionally contains dye whoseorption properties are matched to the UV radiation used for thetopolymerization. During the photopolymerization, the dye absorbst of the UV light, generating a 30 strong intensity gradient within thelesteric layer. Owing to the different reactivity of the nematic andral monomers, a diffusion process takes place, generating theired pitch gradient. In EP 0 606 940 A2, this is a linear pitchdient, where the smallest 35 pitch occurs on the side facing the UVrce. The process described is furthermore characterized bytinuous UV exposure to low intensities for a long period.hus requires complex and expensive material synthesis. A furtheradvantage is that the ultra-violet expo- sure must be kept constanta relatively long time, in the order of 10 minutes. In thetinuous production process, in which the optical layer is 10lied continuously to or between films and photo- polymerized, ag, homogeneously illuminated exposure zone is therefore necessary.long residence time greatly restricts the achievable throughput ofduced film. The admixture of the UV dye also results in some 15advantages. For example, the absorption of the dye, as described inexample of EP 0 606 940 A2, results in a undesired restriction ofband width in the short-wave spectral region. In addition, theming associated with dye absorption can result in impairment 20 orn destruction of the optically active layer. A furthercess which likewise has the object of generating a pitch gradientbeen published by Faris et al., "A Single-Layer Super Broadbandlec- tive Polarizer" in SID 96 Digest, 1996, pp. 111 - 113. 25 Thiscess is based on a mixture of a photocrosslinkablelesteric polysiloxane with a non- crosslinkable low-molecular-weightatic compound. Here too, slow photocrosslinking is carried out with-intensity UV exposure, with phase separation 30 between thesslinkable polysiloxane and the non- crosslinkable nematic compounding place during the UV polymerization. As a consequence of thisse separation, the segregated molecules can diffuse within theer and generate a concentration gradient, which 35 in turn results in ach gradient. As in the previous process, this process alsothe principal disadvantage that at least two different startingponents must be synthesized. This process is likewise based on slowsslinking beingvide a process of broadening the cholesteric reflection bandsphotopolymerizable cholesteric liquid crystals which enables thetral wavelength and band width of the cholesteric reflection band ofpolymerized material to be adjusted independently 10 of one anotherat the same time allows the above- mentioned disadvantages, inticular the extended UV exposure, to be avoided. Theect is achieved by a process which comprises the following threeps: 15 1) partial polymerization of a layer containingtopolymerizable cholesteric liquid crystals by exposure toinic light for a defined brief period at a defined temperature,2) observance of a defined waiting time withoutosure (dark phase) at a defined temperature, 3) fixing of theultant layer by further exposure to actinic light at a definedperature. 25 In contrast to known processes, the processording to the invention makes it possible to start from only oneuid-crystalline substance having a cholesteric phase. However, it iso possible to use mixtures of liquid-crystalline substancesing 30 cholesteric phases or to admix further components in orderoptimize other desired properties. The novel process has thenificant advantage over known processes that it is not necessary tory out extended exposure. 35 Suitable starting materialsthe novel process are photopolymerizable materials havinglesteric properties. Such materials are disclosed, formple, inomers, oligomers or polymers with chiral monomers or mixtures oflesteric monomers, 5 oligomers or polymers with achiral monomers ortures of cholesteric oligomers with chiral and achiral monomersmixtures of achiral monomers, oligomers or polymers havinguid-crystalline phases with chiral monomers. 10 Preferencegiven to cholesteric polysiloxane-based oligomers.ticular preference is given to cholesteric polysiloxane-basedgomers which contain cholesterol derivatives or isomeric cholesterolivatives as 15 chiral species. Such materials are disclosed, formple, in US 5,211,877. It is known that liquid crystalss) consisting of organosiloxane skeletons carryingogenic side groups are distinguished over non- 20 siloxane-containingsystems by the possibility of varying the molecular weight simplyto a virtually unlimited degree through the choice of the organo-oxane backbone. This enables the liquid-crystalline properties, suchfor example, phase behavior, glass 25 transition temperature andaring point, or, for example, also the viscosity to beched to requirements in broad ranges. The novel processbe used for materials from the material class described above whiche 30 left- or right-handed rotation. The photopolymerization of thestances is facilitated by addition of a photoinitiator.the novel process, the photopolymerizable material is preferablyd in the form of a layer, for 35 example in the form of a film.film is prepared by methods known from the literature. Thetopolymerizable material is pre- pared, for example, as a thinogeneous layer on axible carriers or combinations thereof. The smooth surface 5 andnsparency of glass plates or films makes them particularly suitable.ference is given to optically isotropic substrates, since they dochange the state of polarization of the light. For filters andlectors 10 which are intended to generate linear polarizationtead of circular, a particularly suitable carrier is an opticallyaxial, birefringent substrate which has an optical retardation of5 times the wavelength in the wavelength range used. Such arter-wave 15 retardation layer, abbreviated to [err] / 4 retardationer, is produced, for example, by defined stretching of aycarbonate, polyethylene terephthalate or polypropylene film.ernatively, the substrate used can also be a laminate of twoferent birefringent 20 films whose directions of stretching areentially aligned perpendicular to one another. Owing to theferent dispersions of the two films, the overall retardation oflaminate changes with the wavelength. The film material anddegree of 25 stretching should be selected so that an overallardation of 0.25 times the wavelength occurs if possible over theire wavelength range used by the filter or reflector. Itof course also possible subsequently to 30 combine a [err] / 4ardation layer with the novel cholesteric layer. Itadvantageous to use alignment layers on the substrate side facingcholesteric layer, for example in the form of rubbed polyimide orbed 35 polyvinyl alcohol layers. These alignment layers favor goodgnment of the cholesteric helix axis perpen- dicular to thestrate surface. When films are used, unidirectional rubbing of them surface can alsomple with the aid of a knife coater or roller or by spin coating. 5layer thickness applied is preferably 3 - 60 [err]m, particularlyferably from 5 to 40 [err]m, and can be set, for example, by meansa spacer or by an application method having a defined layerckness. The macroscopic alignment of the layer is 10ried out at a temperature at which the material has a cholestericse and is achieved by methods known per se, such as, for example,aring of the material or application of electric or magnetic fields.application and alignment of the liquid-crystalline 15stances can be carried out fully continuously, semi- continuously orcontinuously. An aligned, but as yet unpolarized layer produced ins way has constant pitch over the layer thickness. Infirst process step necessary in 20 accordance with the invention,aligned cholesteric film is exposed to actinic light. Actinic lightphotochemically active light, for example UV light, X- rays,ma radiation or irradiation with high-energy particles, such asctrons or ions. 25 Preference is given to irradation withlight. The irradiation is carried out in such a wayt only some of all possible polymerizable molecules are polymerizeder exposure. The proportion of 30 polymerized molecules after exposureuld preferably be between 0.1% and 69%, particularly preferablyween 1 and 50%, of the polymerizable molecules. If thisportion is lower than stated, the resultant polymer structure is notficiently stable 35 for the subsequent process steps. This is evident,example, from a large temperature change resulting in a shiftthe central wavelength and not in a broadening of the reflectiond. If, by contrast, the firstventional exposure, which is taken to mean exposure which results ine than 70% of the 5 polymerizable molecules being polymerized.s proportion of polymerized molecules is determined, for example, byal exposures with subsequent extraction or by calorimetricermination of the time / conversion curve. 10 The proportionpolymerized molecules is controlled by the incident exposurergy per unit area. Preference is given to intense exposure of thertest possible duration. The requisite exposure energy depends ontype of irradiation used, on the 15 material used, on thetoinitiator and on the layer thickness. Preferredosure energies per unit area during the first exposure are in thege from 1 to 500 mJ / cm[err] (UV-A region), particularly preferablythe 20 range from 10 to 50 mJ / cm[err] (UV-A). By comparison,ventional exposure resulting in polymerization of > 70% of theymerizable molecules uses exposure energies of greater than 500cm[err]. The temperature at which the first exposure is 25ried out can be selected within the cholesteric phase range ofmaterial used. It is preferably in the range from 0[err]C to[err]C. This temperature also affects the centralelength of the broadened reflection band. This 30 temperatureection allows, for example in the case of the preferred materials,central wavelength to be varied over the entire visible spectralion. The time period for the dark phase can be selected in thege from a few seconds to a number of days. The 35 exposure in the firstp (pre-exposure) is followed, as the second step, by a dark phase,. a period without exposure. The dark phase canceed at the samep by up to [err] 100[err]C relative to the temperature in the firstcess 5 step. The temperature in the dark phase can be usedcontrol the rate of broadening of the reflection band. Aperature increase results in faster 10 broadening of the reflectiond. The maximum possible temperature for the dark phase istricted by the clearing point of the partially polymerized layer.temperature selected is preferably between the temperature in thest 15 process step and this clearing point. The essentialameters for setting the desired band width of the reflection bandthe material having cholesteric properties are the exposurergy of the exposure in the first process step and 20 the duration andperature of the dark phase in the second process step. Atsame temperature and duration of the dark phase, the width of thelection band of the LC material increases with decreasing exposurergy in 25 the first process step. On the other hand, a longeration of the dark phase at the same exposure energy in the firstcess step results in increasing broadening of the reflection band ofLC material. The choice of a minimal pre-exposure energy anda correspondingly long dark phase allow band widths of greater thannm to be achieved by means of the novel process. Thek phase is followed, as the third process step, by a second exposureactinic light. 35 This second exposure has the object of polymerizingrever possible all as yet unpolymerized molecules. This results ining and stabilization of the cholesteric layer. Preference isin given toferent temperature to the dark phase. The temperature rangecribed for the dark phase applies. The simplification of the processns that this third process step is preferably carried out at thee temperature as during the dark phase. 10 Compared witherials produced by conventional exposure, the materialsduced by the novel process exhibit broadening of the cholestericlection band by at least 10 nm on both sides. These materialsferably exhibit a reflection band having a 15 width of greater than 100The invention thus also relates to photo- polymerizedlesteric liquid crystals produced by the novel process and having alesteric reflection band which is broadened by at least 10 nmpared with 20 photopolymerized cholesteric liquid crystals preparedconventional exposure. The greatest pitch in the materialording to the invention occurs on the material side facing theosure lamp. 25 The novel process can also be used forlection bands in the near-UV and in the infra-red spectral region.this end, materials may be used whose central wavelength is in thisctral region. For industrial implementation, it is desirablefor the novel process to be suitable for a continuous productioncess on a coating machine. A continuous production processthis type preferably proceeds as follows: a carrier film coatedh a polymerizable liquid crystal and laminated with 35 a cover film isjected, in a first process step at a defined temperature, to a firstrt exposure, which results, as already stated above, in partialymer- ization of the polymerizable material. The shortthe dark phase) for a correspondingly short period and the thirdcess step (the second exposure). It is advantageous for all processps to be carried out in a single pass. For very broadlection bands, a longer 10 duration of the second process step (thek phase) may be necessary. This could be achieved by interimrage of the exposed film, if desired at a temperature whichfers from the temperature of the first process step. Subsequently,the third process 15 step, the second exposure is carried out in aner known per se, and the material is finally stabilized. Thisarate third process step can, if desired, be combined with furthercess steps, for example application of an adhesive layer. In spitethe 20 interim storage, this procedure ensures high materialoughput, since the machine passes can take place at high speed owingthe short exposure times. The novel process is alsotable, for example, for the production of photostructuredcholesteric filters and reflectors having broadened reflectionds. To this end, the process is carried out as described above, withdifference that at least the exposures of the material (processps 1 and 3 according to the invention) take place through a 30k. The first mask is then moved or replaced by a second mask, andcess steps 1 to 3 according to the invention are repeated with ange in at least one parameter in steps 1 or 2, so that an as yetxposed part of the material is irradiated. 35 The phaseth a change in at least one parameter in steps 1 or 2" isen to mean that, when repeating the process, a different reflectionor is set for the material region now irradiated by means of aferent temperature during the exposure in theadiated is in each 5 case set as desired. If desired, thecess is repeated as often as necessary with as yet unexposed regionsthe material. In this way, a multicolored photostructured filterreflector can be produced whose individual 10 colors are freelyustable through the respective choice of central wavelength andd width of the reflection band. In detail, thelowing procedure, for example, can also be followed: 15 1.tostructured materials having reflection bands of differentd widths and the same central wavelength are obtained ifcholesteric material 20 a) is exposed at the sameperature, but at different pre-exposure doses for theividual structures in the first process step in each case,the other process steps are each carried out at the sameperature and for the same duration 25 of the dark phase, or b)exposed at the same temperature and the same pre-exposure dosethe individual structures in the first process step in eache, and the other process steps are carried out at different 30peratures and / or different durations of the dark phase forrespective structure. 2. Photostructured materials havinglection bands of different band widths and different centralelengths are obtained if the cholesteric 35 material a)exposed at different temperatures with in each case different-exposure doses for the individual structures in the firstcess step in each case, and the other process steps are eachuctures in 5 the first process step in each case, and the othercess steps are carried out at different temperatures / or duration of the dark phases for the respective structure.Photostructured materials having reflection bands 10 of thee band width and different central wavelength areained if the cholesteric material is exposed at differentperatures with the same pre-exposure dose for the individualuctures in the first process step in each case, 15 and the othercess steps are each carried out at the same temperature and forsame duration of the dark phase. Depending onsystem actually selected, 20 optimization of the initial dose and ofduration and temperature of the dark phase may be necessary.order to accelerate and to simplify the production process, thee process steps for the various structures, instead of beingried out at 25 separate locations and / or times, can, if desired, alsocarried out at the same locations and / or times. The controlthe central wavelength and width of the reflection band which issible by means of the novel process allows the desired photometricproperties of optical elements, such as polarizers, coloredters, pigments or reflectors, in particular also of structuredters and reflectors for left- handed or right-handedcular-polarized light, to be adjusted in a simple manner. 35invention therefore also relates to optical elements, for example,ters, reflectors and polar- izers, which include layers containingerials having cholesteric properties which have been produced by theel process.lection band together with the substrates in the form of a laminatein the form of a layer free on one side or even as a free film afteroval of the substrate(s). This layer or laminate can beered by 10 further individual cholesteric layers or by otherers, for example retardation films (for example a [err] / 4ardation layer), absorptive polymerization films, colored filmsan adhesive layer. However, it is also possible to use theel 15 process to produce optical elements, such as filters,lectors and polarizers, in which the carrier substrate for thetopolymerizable LC material is a [err] / 4 retardation layer.term "[err] / 4 retardation layer" is taken to 20 mean a layer which hasretardation value 0.25 times the particular wavelength, if possibler the entire wavelength range used by the optical element.novel layer can even be used as optical element in comminuted formflakes. EP 0 601 483 Al 25 describes how cholesteric pigments whichlect visible light can be produced by detaching aymerized cholesteric film from the substrate and then comminutingrough pieces obtained in this way. The novel films can bed in an analogous 30 manner to produce cholesteric flakes having aadened reflection band. Owing to their broader reflection band,h cholesteric pigments exhibit greater light reflection andrefore achieve better brightness. In addition, specificallyadened reflection bands allow 35 new shades and effects to be achieved.o interesting are cholesteric pigments whose reflection band coversentire visible spectral region. They can be produced in anlogous manner from a novel cholesteric broad-band film. Suchhly reflectivee a colored impression when observed at an angle differing from thependicular. The pigments can subsequently be incorporatedlogously to EP 0 685 749 A1 into a suitable binder system.ference is given to binder systems which, 10 after curing, appearorless in visible light and have a similar mean refractive index tocholesteric flakes. To this end, the cholesteric flakes are stirredo the still-liquid binder. The alignment of the flakes parallel tosurface is achieved as 15 described, for example, in EP 0749 Al on application of a thin layer of the pigment / binderture to a substrate or on extrusion of the mixture. Depending on theticular requirements the film can be detached from the substrateer the binder has 20 cured or combined with further layers.s invention likewise relates to devices containing the layersduced by the novel process in the form of filters, reflectors orarizers. Such devices are, for examples, projectors, projectiondisplays and liquid-crystal displays. For example, aad-band polarizer produced by this process can be used as reflectivearizer in the illumination unit of a liquid-crystal display. Thisows the light yield of the liquid-crystal display to 30 be improved byto 100%. The following examples serve to illustrate theention in greater detail: Starting materials: 35 All substancested below were admixed with 2% by weight of the photoinitatoracure 907 (Ciba-Geigy AG, Switzerland).stance 2 was obtained by mixing 96.2% by weight of substance 1 with% by weight of the chiral component isosorbide(4-allyloxybenzoate). This chiral compo- 10 nent was prepared aslows: 24 mmol of isosorbide and 48 mmol of an [err]-propenyl-benzoyl chloride are dissolved in 40 mmol of toluene, and theture is refluxed for 12 hours. The toluene is removed by vacuumtillation, and the 15 crude product is recrystallized fromanol or isopropanol. Substance 3 (50% of TC blue + 50% of TC) Substance 3 was obtained by mixing substance 1 and a 20 furthert-handed helical oligomer prepared as described in Example 1b)EP 0 661 287 (corresponding to Example 1b) of US-A-5,605,649) in theio 1:1. Substance 4 (74.3% of CC blue + 17.1% of Machol + 8.6% 25MAABH) Substance 4 was obtained by mixing 74.3% by weight of thelesteric oligomer obtained according to Example 1A of DE 42 40 743rresponding to Example 1A of US-A-5,362,315) with 17.1% by weight ofhacrylic 30 acid cholesteryl ester and 8.6% by weight ofethacryloyloxyphenyl 4-allyloxybenzoate. Substance 5 (77.7% of CCe rh + 19.4% of MAHBEP + 2.9% of ABIM) 35 Substance 5 was obtainedmixing 19.4% by weight of 4'-ethylphenyl 4-methacryloyloxybenzoateh 2.9% by weight of isomannide bis(4-allyloxybenzoate) and 77.7%406,978): 2.78 g of doristeryl 4-(propen-2-oxy)benzoate (CAS No.:235-15-1), 0.88 g of 4'-(4"-methoxyphenylcarbonyl- 5 oxy)phenylpropen-2-oxy)benzoate (prepared by a conventional preparationcess) and 0.95 g of tetra- methylcyclotetrasiloxane were dissolved20 ml of dry toluene, 46 [err]l of a solution ofyclopenta- dienylplatinum dichloride (1% strength by weight in 10hylene chloride) were added, and the mixture was warmed at[err]C for 1.5 hours. The solution was cooled to 50[err]C, 2.46 g ofmethacryloyloxyphenyl 4-(propen- 2-oxy)benzoate (CAS No.:235-16-2), 500 ppm of Q1301 (alternatively 3,000 ppm of-di-tert-butyl-4-methyl- 15 phenol) and a further 27 [err]l of thealyst solution were added. This solution was stirred at 70-80[err]C2 hours. When the reaction was complete, the product wasrred with 150 mg of sodium hydrogencarbonate and filtered, and theduct was reprecipitated from 20 ethanol. The preparation ofmannide bis(4-allyloxybenzoate) was carried out as follows: 24l of isomannide and 48 mmol of an [err]- propenyloxybenzoyloride were dissolved in 40 mmol 25 of toluene, and the solution wasluxed for 12 hours. The toluene was removed by vacuum distillation,the crude product was recrystallized from ethanol orpropanol. 30 Working examples The films were produced as follows,ess stated otherwise: Two glass plates were provided with ayimide alignment layer, which was rubbed unidirectionally with 35 avet cloth. A small amount of the melted substance was applied to thegnment layer of one of the platesroscopic alignment. The light source used was a mercury arc lampdel 68810, L.O.T.-Oriel GmbH) whose shutter can be controlled byns of a time switch. The exposure powers in the UV-A region 5 weresured using a UV Power Puck (EIT Inc., USA). The resultantlection and transmission spectra were determined using a Perkiner Lambda 19 UV / VIS spectrometer. The measurements in thecular- polarized ray path were carried out using a combination 10Fresnel rhombus and Glan-Thompson polarizer as achromaticcular-polarizing analyzer. Working Example 1 A) Startingstance 1 was introduced, as described 15 above, by meltparation at 90[err]C between two polyimide-coated glass plates,jected to shear and exposed at 33 mW / cm[err] (UV-A) aterr]C for 0.8 second. The layer thickness of the material was[err]m. After a waiting time of 30 minutes at 90[err]C 20 in thek, the second exposure was carried out, likewise at 90[err]C,a power of 33 mW / cm[err] (UV-A), this time for 60 seconds.band width of the transmission band at the plateau (i.e. atof minimum transmission) is 25 68 nm. B) Substance 2 wasd to prepare a layer as described in Example 1A), whichexposed by the same process steps. A band width of 120 nm isained. 30 C) Substance 3 was used to prepare a layer ascribed in Example 1A), which was exposed by the same processps. A band width of 107 nm is obtained. D) Substance 4used to prepare a layer as 35 described in Example 1A),ch was exposed by the- were carried out as in Example A), with the onlyference being that the exposures and the dark phase wereried out at 85[err]C instead of at 90[err]C. A band width of 93is obtained. 5 Comparative Example 1 A) Starting substance 1introduced, as described above, by melt preparation aterr]C between two polyimide-coated glass plates andjected to 10 shear. The layer thickness was 15 [err]m. This layerirradiated conventionally at 33 mW / cm[err] (UV-A) at 90[err]C60 seconds, i.e. only a single exposure, but at high exposurergy, was carried out. A band width of 34 nm is measured in 15nsmission. B) Substance 2 was used to prepare a layer ascribed in Comparative Example 1A), which was exposed by thee process step. A band width of 39 nm is obtained. 20 C)stance 3 was used to prepare a layer as described inparative Example 1A), which was exposed by the same processp. A band width of 30 nm is obtained. D) Substance 4 wasd to prepare a layer as 25 described in Comparative Example, which was exposed by the same process step. A band width ofnm is obtained. E) Substance 5 was used to prepare a layerdescribed in Comparative Example 1A), which was 30osed by 33 mW / cm[err] (UV-A) at 85[err]C for 60 seconds. A bandth of 45 nm is obtained. The central wavelengths andths of the reflection bands from Working Examples 1A to 1E and thed. The first four substances are- left-handed helical mixtures, while substance 5 is aht-handed helical mixture. Here too, use of novel process results inroadening of the reflection band. 5 Table 1:s- Central Band Broad- tance wave-th for ened length conv. ex.d [nm] [nm] [nm]parative Example 1A 1 386 34 Working Example 1A386 68 Comparative Example 1B 2 504Working Example 1B 2 504 120parative Example 1C 3 490 30 Working Example 1C490 107 Comparative Example 1D 4 443Working Example 1D 4 443 83parative Example 1E 5 445 45 Working Example 1E445 93 Working Example 2 A) A smallunt of substance 2 was prepared in the melt at 96[err]C ascribed above between two 10 polyimide-coated glass plates. Theer thickness was 12 [err]m. The first exposure at 96[err]Cted 1.5 seconds at an exposure power of 33 mW / cm[err] (UV-Aion). After the exposure, the film was cooledsurement in the unpolarized ray path using the 20 UV / VISctrometer gave the reflection and associated transmissionds shown in Fig. 1. The band width of the transmission band atplateau (i.e. 90% of the minimum transmission) is 32 nm.3 - B) A layer was prepared as in Example 2A) and exposedthe same process steps as in Example 2A), with the onlyference being that the time for the first exposure was 1.2onds. The associated 5 reflection and transmission bands areewise shown in Fig. 1. A band width of 48 nm isained. C) A layer was prepared as in Example 2A) and exposedthe same process steps as in Example 2A), with 10 the onlyference being that the time for the first exposure was 1ond. The associated reflection and transmission bands areewise shown in Fig. 1. A band width of 59 nm isained. 15 D) A layer was prepared as in Example 2A) and exposedthe same process steps as in Example 2A), with the onlyference being that the time for the first exposure was 0.8ond. The associated reflection and transmission bands areewise 20 shown in Fig. 1. A band width of 108 nm isained. Comparative Example 2 For comparison, substance 2 wasd to prepare a layer 25 having a thickness of 12 [err]m as described inking Example 2. This layer was irradiated conventionally at 33cm[err] (UV-A) at 96[err]C for 60 seconds, i.e. only aa from Working Example 2 and Comparative Example 2 are shown inle 2: the exposure energy is obtained from the product of theosure power (in the 35 UV-A region) times the exposure time.4 - Table 2 1st Exposured exposure energy in 1st widthexposure [nm] / cm[err]] Comparative Example 2 60 1.980 29king Example 2A) 1.5 49.5 32 Working Example1.2 39.6 48 Working Example 2C) 159 Working Example 2D) 0.8 26.4 108king Example 3 5 A) The substance 2 was used to prepare a layer ascribed in Working Example 2. The layer thickness was[err]m. The first exposure at 33 mW / cm[err] (UV-A) wasried out at 96[err]C for an exposure time of 0.8 second. Afteraiting time 10 of 1 minute at 96[err]C, the second exposurecarried out (96[err]C, 33 mW / cm2 for 10 seconds). Ad width of 36 nm is obtained. B) A layer was prepared as inmple 3A) and exposed by the same process steps as in Example, with 15 the only difference being that the waiting time4 minutes at 96[err]C. A band width of 60 nm isand width of 86 nm is obtained. D) A further layer waspared as in Example 3A) and exposed by the same process stepsin Example 25 3A), with the only difference being that theting time was 15 minutes at 96[err]C. A band width of 115 nm isained. - 25 - The datathe resultant transmission bands in the unpolarized ray path forking Examples 3A) to 3D) are shown in Table 3 and Fig. 2. As theting time increases, so does the broadening of the 5lection band. Table 3 1st exposure Darkd [s] phase widthn] [mm] Working Example 3A) 0.8 1 36king Example 3B) 0.8 4 60 Working Example 3C)7 86 Working Example 3D) 0.8 15The following three working examples show how 10 theel process can be used to adjust the central wavelength and the bandth independently of one another using the same material.king Example 4 15 A layer of substance 2 is prepared as described inking Example 2 and exposed at 96[err]C for 0.8 second, thenditioned at 96[err]C for 4 minutes and finally exposed at 33 mW / cm296[err]C for 10 seconds. The centrale material prepared conventionally at 96[err]C. 25 Working ExampleA layer of substance 2 was prepared as described in Workingmple 2A and, after shearing at 96[err]C, was cooled to 70[err]C.first exposure at 33 mW / cm[err] was carried out at this temperaturea period of 0.6 30 second. The sample was then heated to 100[err]C atrr]C / min and conditioned at this temperature for 20 minutes. The6 - second exposure was also carried out at 100[err]C (10onds at 33 mW / cm[err]). A central wavelength of 550 nm isained for the reflection band of the material treated in this 5 way,the band width of the reflection band is about 160 nm. The opticalperties of the layers produced by the novel process are shown in. 3. This figure shows the transmission and reflection in the left-ded and right-handed circular-polarized ray path. 10 The ratioween right-handed and left-handed circular-polarizednsmission is greater than 10:1. The ratio of left-handed toht-handed circular- polarized reflection is better than 100:1. 15king Example 6 A layer of substance 2 was prepared as described inking Example 2A and, after shearing at 96[err]C, was cooled aterr]C. The first exposure (33 mW / cm[err], for 3 seconds) wasried out after a conditioning time of 20 15 minutes at 45[err]C. Theple was then heated to 100[err]C at about 5[err]C / min. This waslowed by a waiting time of 5 minutes, before the sample was exposedthe second time (33 mW / cm[err], for 60 seconds). Owing to theinitial temperature in the first exposure, thepath are 30 shown in Fig. 4. -- Table 4 1st dark 2nd centrald exposure phase exposure wavelength widthking 0.8 s 4 min 10 s 468 nm 60 nm Example 4err]C 96[err]C 96[err]C Working 0.6 s 20 min 10 snm 160 nm Example 5 70[err]C 100[err]C 70[err]Cking 3 s 5 min 60 s 630 nm 115 nm Example 6err]C 100[err]C 100[err]C The following twoking examples show how 5 reflection bands which cover the entireible spectral region can be produced with the aid of the novelcess: Working Example 7 10 A layer of substance 2 waspared at 96[err]C as described in Example 2A and then cooled toerr]C. The layer thickness was 30 [err]m. The first exposure wasried out at 33 mW / cm2 (UV-A) for 0.8 second at 85[err]C. Thelesteric layer was then heated to 100[err]C. After a 15 waiting time120 minutes at 100[err]C, the second exposure was carried out33 mW / cm[err] (UV-A) at 100[err]C for a period of 60 seconds.resultant reflection and transmission bands are shown in Fig. 5.reflection band extends from 20 370 nm to 750 nm.er was exposed at 33 mW / cm[err] (UV-A) for 0.8 second, heated toerr]C and then conditioned at this temperature for 120 minutes. Theond exposure was carried out at 33 mW / cm[err] (UV-A) at 95[err]C foreriod of 30 60 seconds. - 28 -resultant reflection band extends from 360 to 700 nm. Workingmple 9 5 A novel film was prepared between glass plates as describedWorking Example 1D). The central wavelength was at 443 nm, andwidth of the reflection band was 83 nm. The glass plates weresequently separated. The cholesteric film was 10 scraped off thess substrate using a knife blade. The particles remaining wereund to a mean particle diameter of about 100 [err]m and mixed with anish in the ratio 1:10 parts by weight. The clear varnish useda two-component polyurethane-based topcoat 15 (Standox,berts). The varnish mixture was knife- coated onto black board in a-film thickness of 120 [err]m with the aid of a film applicator anddried at 80[err] for one hour. The resultant board showed aght blue-green coloration which shifted to blue with 20 increasingwing angle. Co 9701partial polymerization of a layer containingtopolymerizable cholesteric liquid crystals by 10 exposure toinic light for a defined brief period at a definedperature, 2) observance of a defined waiting time withoutosure (dark phase) at a defined temperature, 3) fixing of theultant layer by exposure to 15 actinic light at a definedperature. 2. A process as claimed in claim 1, wherein thetopolymerizable material having cholesteric proper- ties isected from the group consisting of cholesteric monomers,gomers or polymers, mixtures 20 of cholesteric monomers, oligomers orymers with chiral monomers, mixtures of cholesteric monomers,gomers or polymers with achiral monomers, mixtures of cholestericgomers with chiral and achiral monomers, and mixtures of achiralomers, oligomers 25 or polymers having liquid-crystalline phasesh chiral monomers. 3. A process as claimed in claim 2,rein the cholesteric oligomer used is a liquid-crystallineoxane. 30 4. A process as claimed in any of claims 1 to 3,rein a temperature change in the range from -100 to 100[err]C hasen place between the first process step (the pre-exposure) and theond process step. 5. A process for the production oftostructured 35 cholesteric filters and reflectors having broadenedlection bands from cholesteric liquid-crystalline materials, whichprises carrying out the exposures of the liquid-crystalline materialmeans of process steps 1 to 3 as claimed in claim 1 through a maskas yet 5 unexposed part of the material is irradiated. 6. Atopolymerized cholesteric liquid crystal prepared by a process asimed in any one of claims 1 to 5, having a cholesteric reflectiond which is broadened by at least 10 nm compared with a photo- 10ymerized cholesteric liquid crystal prepared by conventionalosure. 7. An optical element having a broadenedlesteric reflection band, which comprises at least one layertaining material having a broadened 15 cholesteric reflection bandpared by a process as claimed in any one of claims 1 to 6. 8.optical element as claimed in claim 7, wherein the reflectiond has a band width of greater than 100 nm. 20 9. An opticalment as claimed in claim 7 or 8, which additionally includes ar] / 4 retardation layer. 10. A device containing at least oneical element as claimed in claim 7, 8 or 9.

Claims

Claims1.      A process for broadening the cholesteric reflection bands of photopolymerizable cholesteric liquid crystals, which comprises the following three process steps:1)   partial polymerization of a layer containing photopolymerizable cholesteric liquid crystals by exposure to actinic light for a defined brief period at a defined temperature,2)   observance of a defined waiting time without exposure (dark phase) at a defined temperature,3)   fixing of the resultant layer by exposure to actinic light at a defined temperature.

2. A process as claimed in claim 1, wherein the photopolymerizable material having cholesteric properties is selected from the group consisting of cholesteric monomers, oligomers or polymers, mixtures of cholesteric monomers, oligomers or polymers with chiral monomers, mixtures of cholesteric monomers, oligomers or polymers with achiral monomers, mixtures of cholesteric oligomers with chiral and achiral monomers, and mixtures of achiral monomers, oligomers or polymers having liquid-crystalline phases with chiral monomers.

3. A process as claimed in claim 2, wherein the cholesteric oligomer used is a liquid-crystalline siloxane.

4. A process as claimed in any of claims 1 to 3, wherein a temperature change in the range from -100 to 100°C has taken place between the first process step (the pre-exposure) and the second process step.

5. A process for the production of photostructured cholesteric filters and reflectors having broadened reflection bands from cholesteric liquid-crystalline materials, which comprises carrying out the exposures of the liquid-crystalline material by means of process steps 1 to 3 as claimed in claim 1 through a mask andthen moving the mask or replacing the mask by a second mask, and repeating process steps 1 to 3 as claimed in claim 1 with a change in at least one parameter in process steps 1 and 2 in such a way that an as yet 5 unexposed part of the material is irradiated.

6. A photopolymerized cholesteric liquid crystal prepared by a process as claimed in any one of claims 1 to 5, having a cholesteric reflection band which is- broadened by at least 10 nm compared with a photo-10 ' polymerized cholesteric liquid crystal prepared by conventional exposure.

7. An optical element having a broadened cholesteric reflection band, which comprises at least one layer containing material having a broadened15 cholesteric reflection band prepared by a process' as claimed in any one of claims 1 to 6.

8. An optical element as claimed in claim 7, wherein the reflection band has a band width of greater than 100 nm.20  9. An optical element as claimed in claim 7 or 8,which additionally includes a 1 / 4 retardation layer.

10. A device containing at least one optical element as claimed in claim 7, 8 or 9.

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