EP1629309A1

EP1629309A1 - Polymer film with a helical molecular structure

Info

Publication number: EP1629309A1
Application number: EP04723197A
Authority: EP
Inventors: Eckhard Hanelt
Original assignee: Consortium fuer Elektrochemische Industrie GmbH
Current assignee: Wacker Chemie AG
Priority date: 2003-06-05
Filing date: 2004-03-25
Publication date: 2006-03-01
Also published as: WO2004109348B1; JP4567671B2; DE10325610A1; WO2004109348A1; US7311952B2; US20060083866A1; TWI361911B; CN1798991A; TW200500660A; CN100526921C; KR100757824B1; KR20060014439A; JP2006526797A

Abstract

The invention relates to a polymer film with adjustable gradients of the pitch of a helical molecular structure and to a method for the production and use thereof.

Description

Polymer film with a helical molecular structure

The invention relates to a polymer film with an adjustable gradient of the pitch of a helical molecular structure, a process for its production and its use.

Many optical applications use films that convert unpolarized light into polarized light by absorbing a polarization component of the light. In the ideal case, a maximum of 50% of the light will be let through such a polarizing film. In addition to the loss of light output, this method has the further disadvantage that the polarizer is strongly heated by the absorption, particularly in the case of bright light sources. Both disadvantages are avoided with reflective polarizers, which reflect the portion of the undesired polarization back into the light source. Depending on the respective arrangement of the optical components of the light source, this portion can change its polarization by reflection or scattering into the desired polarization and thus at least partially contribute to increasing the luminous efficiency.

No. 5,235,443 describes one possibility for realizing a reflective polarizer in the form of films made of cholesteric liquid crystals. Cholesteric liquid crystals are substances with a helical orientation of the molecules. A thin layer of it can be prepared between two suitable substrates so that the helix axis is perpendicular to the substrate surfaces. The pitch of the helix depends on the material and is constant over the layer thickness. Such a layer reflects a circular light component almost completely if the direction of rotation and wavelength λ of the light in the material match the direction of rotation and the pitch p of the cholesteric helix and the layer thickness is a multiple of the pitch (cholesteric reflection). The second circular light component with the opposite direction of rotation and light components with other wavelengths are ideally however, completely let through. If required by the application, the circularly polarized light component can be converted into linearly polarized light by an additional quarter-wave retardation layer.

The cholesteric reflection occurs in a spectral band between two wavelengths λ] _ = Pn ₀ and 2 ~ P ' ⁿ e, where n _e and n ₀ denote the extraordinary and orderly refractive index of the material. This band is characterized by the center wavelength λ ₀ = pn, which depends on the average refractive index n = (n ₀ + n _e ) / 2 and the pitch p of the material, and the width Δλ = p-Δn, which is given at a given pitch p is determined by the birefringence Δn = n _e -n ₀ . In practice that is

Birefringence of most cholesteric materials in the visible spectral range limited to values less than 0.3. This results in a maximum possible bandwidth of about 100 nm, but usually only 30 nm to 50 nm are reached. The intensity of the light reflected in the cholesteric band increases with the number of pitches λ ₀ / n in the layer and reaches the maximum value for unpolarized incident light at 50% of the incident intensity. A reflection can only be observed from a layer thickness of about three pitches. The minimum required layer thickness is therefore a few μ for most cholesteric materials in the visible spectral range.

A sufficient thermal and mechanical stability of the helical molecular structure is a prerequisite for the use of liquid crystalline materials in applications such as a reflective polarizer or LC pigments. This stability can be achieved by fixing the orientation state by polymerization or by rapid cooling to temperatures below the glass point. Such stable cholesteric layers are described, for example, by R. Maurer et al. in "Polarizing Color Filters made from

Cholesteric LC Silicones "in SID 90 Digest, 1990, pp. 110-113. In addition to the use as a polarization film, other uses of cholesteric films in various optical elements are described in the literature, which are not to be discussed further here. In all applications, the center wavelength λ ₀ and the width Δλ of the cholesteric band should be adjustable. For the special application as a reflective polarizer, which is used, for example, to improve the brightness of an LCD, it is particularly necessary that the reflection band covers the entire visible spectral range, ie that the bandwidth Δλ should have more than 300 nm. However, the reflective properties of such films and in particular the polarization of the transmitted light depend on the flashing angle relative to the normal to the film. The band of a reflective polarizer should cover at least the range from 450 nm to 600 nm for all relevant viewing angles. In order to use the method described by S. Ishihara et al. in "Preparation and properties of optical notch filters of cholesteric liquid crystals" in Polymer, Vol. 29, 1988, pp. 2141-2145 to compensate for the effect of color shift as a function of the viewing angle, the band should therefore, for example, for a desired viewing angle range up to 45 ° at least from 450 nm to 850 nm when viewed vertically. In addition, the films should also be as thin as possible in order to minimize the viewing angle dependence of the polarization. For broadband LC pigments, which can produce special color effects in the visible range, for example, there are smaller bandwidths around 100 nm - but these should be achieved in layers less than 6 μm thick.

Polymer films with a bandwidth Δλ which is greater than the value λ ₀ - (n _e -n ₀ ) / n corresponding to the liquid-crystalline material can, as, inter alia, in the above-cited article by R. Maurer et al. Be described that the optical element is made up of several cholesteric layers with different center wavelengths. However, this method is very expensive and has the disadvantage that the optical quality with every additional layer decreases due to scatter at imperfections and inhomogeneities. This method cannot be used for LC pigments either, because total thicknesses below 6 μ are difficult to achieve with several even thinner individual layers. A more suitable method for producing a broadband cholesteric polarizer is the sequence of individual layers with a constant pitch of the helical

Replace molecular structure with a single layer with a continuously increasing or decreasing pitch. The broadening of the reflection band by a gradient in the pitch of the helix (pitch gradient) has long been known from theoretical studies (e.g. S. Mazkedian, S. Melone, F. Rustichelli, J. Physique 37, 731 (1976) or LE Hajdo, AC Eringen, J. Opt. Soc. Am. 36, 1017 (1979)). According to the current state of the art, there are several technical processes that make it possible to produce such a layer with a pitch gradient. They can essentially be divided into the following three categories:

1. Generation of a crosslinking density gradient with diffusion of the uncrosslinked components, 2. Laying of at least two layers with different chemical compositions and subsequent diffusion and 3. Depth-dependent extraction of unpolymerized parts from a partially polymerized film.

Each of these processes requires special material mixtures that are matched to the respective process.

European patent application EP 0 606 940 A2 describes a process in which a mixture of chiral and nematic monomers with different reactivity with regard to their polymerization properties is polymerized with a low UV dose over a longer period of time, so that the monomers diffuse during the polymerization occurs, which then generates a pitch gradient due to the mixed material composition. The The driving force for diffusion is a gradient in the crosslinking density, which is caused by a gradient of the UV intensity in the material. The decrease in UV intensity in the film is controlled by adding a dye, but this has disadvantages for the stability and the optical

Has properties of the film. Because of the requirement that the film must be completely polymerized in order to achieve good mechanical stability, high UV absorption is not possible and therefore the UV gradient in the film is not particularly strong. This makes the entire process relatively slow, so that it is poorly suited for the industrial manufacture of a broadband cholesteric polarizer on a continuously running substrate such as a plastic film. The weak UV gradient also has an effect on the pitch gradient, so that the minimum thickness of the layer is not sufficient for a required width of the reflection band for applications in which thin layers below 10 μm are important. In the examples of EP 0 606 940 A2, the layer thicknesses of the films are 20 μm.

Also, the method described in the European patent application EP 0982605 Al USER ^'t the absorption of UV radiation in the cholesteric layer to produce a pitch gradient. In the LC mixtures used, a transition from the cholesteric to a smectic phase takes place during the polymerization, which causes the helical structure to unscrew. This process is potentially faster than a pure diffusion process, but in the examples given broadband films are not achieved under an exposure time of 30 s. Another disadvantage of this method is that the reorientation of the mesogens into the smectic phase leads to the formation of small domains. The light scatter caused by the domain boundaries reduces the degree of polarization of the transmitted light.

Another method, which is described in European patent application EP 0 885 945 AI, overcomes the disadvantage of slow diffusion process, which is caused by the weak crosslink density gradient in the process according to EP 0 606 940 A2, thereby,. that the UV exposure process is divided into two steps, between which there is a change in the temperature of the film. In this way, the thermochromic property of the cholesteric phase is used, which leads to a quick reorientation of the mesogens and thus a faster adjustment of the pitch in the differently cross-linked zones of the film. However, this method has the disadvantage that the necessary bandwidth of over 300 nm is poorly achieved under the conditions necessary for industrial production.

US Pat. No. 6,099,758 A claims a method in which the crosslinking density gradient is increased in that, in addition to the UV gradient, a further gradient occurs due to the inhibiting influence of the substrates used. Although this process is faster than that described in EP 0 606 940 A2, broadband films are not achieved under an exposure time of 30 s in the examples given. The thermal and mechanical stability of the films are low due to the low total UV doses obtained from the exposure times and UV intensities indicated. The production process becomes technically complex in that the cholesteric film must either be produced between two different substrates, or in that an additional oxygen barrier layer on the substrate is necessary when using a single substrate and polymerizing in an air atmosphere.

Also described in EP 0 606 940 A2 is a method in which two layers with different chemical compositions, one of which is cholesterically oriented, are brought into contact. The cholesteric layer is swollen by diffusion, so that a pitch gradient is created. Finally the film is polymerized. In another embodiment of this method in European patent application EP 0 881 509 A2 two cholesteric Laminated films with different pitch and fused by controlled diffusion at elevated temperature so that a continuous _. There is a transition between the two pitches. Disadvantages of these methods are the complex production of at least two different layers and the technically difficult control of the diffusion process between the two layers.

In European patent application EP 0 875 525 A1 it is described that a partially polymerized cholesteric film is immersed on a substrate in baths of organic solvents which cause an extraction of unpolymerized parts in the film. Since this effect is strongest on the surface of the film, a depth-dependent shrinkage and thus a pitch gradient is generated after the film has been dried and tempered. In order to obtain reproducible results, large amounts of solvents have to be exchanged constantly, which is not desirable from an environmental point of view. The necessary purification of these solvents and the complex process control also increase the costs considerably.

In the known patent literature it is discussed with partly contradictory results how a cholesteric polarizer manufactured according to the above-described method has to be arranged in an LCD in order to achieve the highest possible luminous efficiency with the least possible dependence on the viewing angle. In the arrangement of the cholesteric polarizer between the light source and the electrically switchable liquid crystal cell, either the side with a shorter pitch or with an asymmetrical film with a continuous pitch gradient, which is described in the known examples as a constant or in the mathematical sense monotonically increasing or decreasing gradient face the side with the longer pitch towards the light source. In the case of the symmetrical film hypothetically described in US 6,099,758 A, there is no preferred direction. However, our own studies show that for a Low viewing angle dependence of the luminous efficacy and the color, an arrangement is advantageous in which a film is used, the pitch distribution of which has a sequence of short, long and medium pitches which approximately correspond to the reflection colors blue, red and green, preferably the short pitch of the light source is closest. According to the prior art, such a distribution of the pitch heights could only be produced with a multilayer structure of the film, which, however, as described above, would require a complex process.

It was one of the objects of the present invention to provide a polymer film which, for example when used in an LCD, increases the brightness as much as possible with the least possible change in the relative

Viewing angle dependence of brightness and color of the LCD. This object is achieved by a polymer film with a helical molecular structure, which contains a sequence of a short, long and medium pitch of the helical structure in a direction perpendicular to the film surface.

In order to enable a simple production process, a film could be developed which is made from a single layer of a mixture of polymerizable liquid crystalline material. In this film the transition between short and long pitch preferably takes place in less than ten turns of the helical structure, a fast transition in less than five turns is very preferred. The invention also relates to other ways of using this film.

The known production processes for cholesteric polymer films with a gradient of the pitch have the specific advantages and disadvantages described above. So far, none of these processes has been able to achieve a fully continuous industrial production of a broadband reflective polarizer or broadband LC pigments suitable for an LCD, and this with a single one Coating process on a single substrate. Another object of the present invention is a polymer film with an adjustable gradient of the pitch of a helical molecular structure produced by a process comprising the steps:

Applying a mixture of polymerizable liquid-crystalline material which a) contains monomers or oligomers, each having at least one mesogenic group and at least one polymerizable functional group, and b) a chiral compound, on a substrate,

Orientation of the material so that the mesogenic groups are arranged in a helical structure, the axis of which is transverse to the layer, partial polymerisation of this layer in one for the

Polymerization inhibiting environment by

Exposure to actinic radiation, and - final fixing after the material in the partially polymerized structure has reoriented, by completely polymerizing the film or cooling the

Films in the glass state.

Furthermore, a method for producing a polymer film with an adjustable gradient of the pitch of a helical molecular structure comprises the steps:

Applying a mixture of polymerizable liquid-crystalline material which a) monomers or oligomers, each having at least one mesogenic group and at least one polymerizable functional group, and b) a chiral compound, in one layer on a substrate, - orienting the material, see above that the mesogenic groups are arranged in a helical structure, the axis of which is transverse to the layer, Partial polymerisation of this layer in an environment which inhibits the polymerisation by brief exposure to actinic radiation, and final fixing after a defined waiting period during which the material is partly polymerised

Structure reoriented by fully polymerizing the film or cooling the film to the glass state, object of this invention.

With the method according to the invention, a method for the cost-effective, industrial production of a polymer film with a broad reflection band is now available, with which the disadvantages of the prior art listed above are avoided.

Surprisingly, it was found that the polymer films with helical molecular structure, which were produced by this method, have a significantly broader reflection band than films made of the same material, which were polymerized between two substrates or under a nitrogen atmosphere. As in EP 0 606 940 A2, a gradient in the

Crosslink density of the cholesteric layer is generated, which, however, is not due to an intensity gradient of actinic radiation, but to a gradient of the inhibiting influence of the environment. The inhibitory influence of the environment is based on the diffusion of molecules from the environment into the cholesteric layer, which act as radical scavengers there. Due to the brief exposure to actinic radiation, a defined amount of radicals is formed in the layer, which are only partially neutralized by the radical scavengers already contained in the cholesteric layer. At this point in time, the distribution of the radicals and radical scavengers is approximately constant over the entire layer thickness. The remaining radicals start a polymerization, which is stopped again by the molecules diffusing into the layer from the environment. This diffusion is slow enough to produce a much steeper gradient than the absorption of the actinic light. The gradient is set by duration and intensity of the short-term irradiation are matched to the diffusion constant and the polymerization rate. Long exposure as in US 6,099,758 A, EP 0 606 940 A2 and the other methods of the prior art is therefore not necessary.

The gradient of the pitch of the helical structure dp / dx (x = position coordinate perpendicular to the surface of the layer) is preferably set so that the intensity of the reflected light is as high as possible and the thickness of the polymer film is as small as possible. Assuming a layer thickness of at least three pitches for a sufficient intensity in the reflection band, this results in a maximum gradient of dp / dx <0.33-Δn / n. However, even steeper gradients are possible if the associated loss of reflected light intensity can be tolerated in the respective application. It is a particular advantage of this method compared to the prior art that such steep gradients can be realized in the pitch of the helical structure of a single layer. This enables the production of polymer films with widened

Reflection bands with minimal layer thicknesses. The layer thicknesses of the polymer films according to the invention are preferably thicker than 2 / Δn times and thinner than 20 / Δn times the desired width of the reflection band. For use as a reflective polarizer, the polymer films according to the invention are particularly preferably thicker than 3 / Δn times and thinner than 6 / Δn times the desired width of the reflection band. Polymer films according to the invention that meet this criterion show a greatly reduced viewing angle dependence of the polarization of the light transmitted in the reflection band. For a polymerizable liquid-crystalline material with Δn of about 0.2 and a desired width of the reflection band of 300 nm, 5 μm to 10 μm thick polymer films are sufficient to produce a reflective polarizer. The requirement for broadband LC pigments with layer thicknesses below 6 μm can also be met with this process. In other methods known from the prior art, which are based on a generation a crosslink density gradient with subsequent diffusion of the uncrosslinked components (for example DE 198 42 701 AI, EP 0 606 940 A2), but mostly only layer thicknesses of 15 μm and more are obtained.

The partial polymerization is started by a defined irradiation with actinic light at a temperature in a phase range with a helical orientation of the mesogens. It is not necessary that, as described in EP 0 606 940 A2, an intensity of the actinic radiation which varies over the layer thickness acts on the polymerizable liquid-crystalline material. On the contrary, it is advantageous for the stability and the optical properties of the film that no additional UV-absorbing dyes have to be added to the material. In the method described here, the intensity profile of the actinic light in the layer can be regarded as almost constant, because the absorption of the actinic light is small over the layer thickness of a few μm. Accordingly, such films do not show broadening of a cholesteric band without exposure to an inhibitory environment (Examples Id and Le). In contrast, polymer films, for example according to the teaching of DE 198 42 701 AI, show a broadening of the cholesteric band. If the polymerizable liquid-crystalline material used according to the invention nevertheless has a significant absorption for the actinic radiation, this is not a hindrance for the method described here, but can optionally be used to optimize the bandwidth.

Another advantage of this method compared to continuous exposure to UV radiation of low intensity as in DE 198 42 701 AI and EP 0 606 940 A2 is that the orientation state achieved is fixed in a separate, final step, so that a high thermal and mechanical stability of the film can be guaranteed. The long-term stability of a reflective, for example, is critical in application Polarizers on the warm backlight of an LCD. It has now surprisingly been possible to show that a polymer film produced by the process according to the invention has a sufficiently high stability for use in LCDs in a stability test at elevated temperature (Example 1c). If the final fixing of the layer takes place by means of a polymerization reaction, this reaction is preferably initiated by exposure to high-intensity actinic light, electron radiation or radical-forming thermal initiators such as peroxides. The inhibitory effect of the environment is preferably reduced at the same time. The final polymerization is particularly preferably carried out in an inert gas atmosphere, for example a nitrogen atmosphere. However, it is also possible to laminate an impermeable polymer film onto the partially polymerized film after the pre-exposure and a short waiting time in order to reduce the inhibiting effect of the environment during the final fixing.

Since oxygen is an inhibitor for polymerization reactions, a gas containing oxygen is preferably used as the inhibiting environment. Air or oxygen-enriched air are particularly preferred. Another suitable gas with an inhibiting effect is, for example, nitrogen monoxide, which, like oxygen, can also be used in a gas mixture. In order to stabilize the LC mixture against premature polymerization and to control the inhibiting effect of the oxygen, the polymerizable liquid crystal mixture preferably contains a radical scavenger which is activated by oxygen, for example 2,6-ditet. -butyl-4-methylphenol (BHT). The amount of this radical scavenger is preferably 1-5000 ppm, particularly preferably 100-3000 ppm. As an alternative to a gaseous inhibiting environment, an inhibiting liquid or solid can also be brought into temporary contact with the polymerizable material, preferably in the form of a thin film. It is also possible for this purpose to use a film with high permeability to apply inhibitory substances to the polymerizable liquid-crystalline material.

Due to the brief exposure to actinic light with a subsequent waiting period during which the material is reoriented, the oxygen barrier layer on the inner surface of the substrate, which is absolutely necessary in DE 198 42 701 A1, is not necessary. A substrate without an additional barrier layer is preferably used. The waiting time is preferably chosen until large domains are in the

Form Grandjean texture so that the proportion of scattered light caused by the domain boundaries is less than a few percent of the total intensity. An asymmetrical pitch gradient is formed, the longest pitch of which is not necessarily as required in DE 198 42 701 AI

Surface of the film that faced the air during manufacture. On the contrary, in examples 1c) and 2c) of the process according to the invention, pitch distributions are present which have a longer pitch on the side of the PET substrate than in the vicinity of the surface of the film which faced the air during the production. During the waiting time, the ambient temperature of the film can be changed in order to additionally use the thermochromism of the cholesteric material, as in EP 0 885 945 A1. The process is preferably carried out at a constant temperature in the region of the liquid-crystalline phase.

For a suitable mixture of the polymerizable liquid-crystalline material, by adjusting the dose of the actinic radiation in the first exposure step and the concentration of the inhibiting molecules in the environment acting on the radiation, the course of the crosslinking density in the film can be adjusted such that a distribution is achieved through the subsequent diffusion process the pitch of the helical structure arises, which has a sequence of short, long and medium pitch. The short pitch is approximately the pitch of the mixture of the polymerizable liquid-crystalline material in the oriented Condition which is only slightly or not polymerized on the open surface after the first exposure step due to the active inhibition. The long pitch is caused by the swelling of the partially polymerized material near the open surface and the mean pitch is that

Pitch of the somewhat more polymerized and therefore less swollen material on the side of the substrate.

The pitch heights required for a polymer film according to the invention are calculated from the required

Reflection wavelengths / average refractive index of the liquid-crystalline material in the polymerized state, the average pitch according to this formula corresponds to a wavelength in the middle of the reflection band and the short and long pitches to a shorter or longer wavelength, which is preferably more than 10% of the mean Distinguish wavelength and are particularly preferably on the edge of the required cholesteric bandwidth.

The polymerizable liquid-crystalline material preferably contains mixtures of monomers or oligomers which have mesogenic groups and polymerizable functional groups and contain at least one chiral component, these monomers or oligomers differing in their reactivity with respect to the polymerization. Polymerizable cholesteric liquid crystals are particularly preferred

Monomer or oligomer (A), each of the monomers or oligomers (A) having at least two polymerizable functional groups which are selected from the group comprising (meth) acrylate ester, epoxy and vinyl ether groups, liquid-crystalline monomer or oligomer (B), wherein each of the monomers or oligomers (B) has exactly one polymerizable functional group which are selected from the group containing (meth) acrylate ester, epoxy and vinyl ether groups, 1 to less than 50% by weight, based on the polymerizable mixture (A + B + C + D), of a monomer (C) which does not contain a group which corresponds to the polymerizable functional groups of the monomers or oligomers (A) and ( B) can react

Monomer or oligomer (D) with a chiral group.

For the purposes of this invention, mesogenic groups are those chemical groups which can produce liquid-crystalline properties in a molecule. Chemical compounds that contain mesogenic groups usually have a Kalamitic or discotic configuration. They do not have to have a liquid-crystalline phase themselves, but it is also sufficient if they contribute to a liquid-crystalline phase in a mixture with other mesogenic compounds. In principle, all mesogenic groups known in the literature are suitable for components (A), (B) and (C). For example, a regularly updated collection of known esogenic groups by V. Vill et al. published as a database under the name LiqCryst (available from LCI Publisher GmbH, Eichenstr. 3, D-20259 Hamburg). Those mesogenic groups are preferably used which are easily accessible synthetically on an industrial scale and which give compounds which ensure sufficient stability over a long period of time for use as a polymer film. Examples of this are chemical structural elements such as carboxylic acid esters and alcohols based on phenyl, biphenyl, cyanobiphenyl, naphthyl and cyanonaphthyl derivatives and combinations of these groups.

A preferred embodiment is a polymer film according to the invention, which is characterized in that it has a reflection band widened by at least 50% compared to the unpolymerized state.

Another preferred embodiment relates to a structured polymer film with a helical molecular structure, which is characterized in that the partial Polymerization and / or during the final fixing, the action of the actinic radiation takes place through a mask and that the mask is subsequently changed or replaced by a second mask and this process step is repeated, if necessary with the change of further process parameters, such that a part of the material that has not yet been finally fixed is irradiated with actinic radiation. Such a structured polymer film with a helical molecular structure is particularly suitable, for example, as a color filter for an LCD display, since the bandwidth of the color pixels can be specifically adjusted in order to optimize the color tone and the brightness of the display.

Another preferred embodiment of the invention relates to LC pigments with a helical molecular structure, which are produced by comminuting a polymer film according to the invention in a subsequent process step.

The invention also relates to a method for producing a polymer film with a helical molecular structure. Various continuous or discontinuous processes for producing optically anisotropic polymer films from polymerizable mixtures are known from the literature. The polymerizable mixture is applied to a substrate, oriented and then fixed in the glass state by a chemical reaction or by cooling.

The polymerizable mixtures can be applied to the substrate surface in solution or as a solvent-free melt above the glass point of the mixture, for example by spin coating, with a doctor blade or a roller. If a solvent is used for application, it must be removed in a subsequent drying step. The thickness of the dry LC layer on the substrate determines the number of pitches of the helical structure and thus the shape of the reflection band. The preparation of unilaterally open polymer films in melt application and from a solution are, for example, in European ones Publications EP 0 358 208 AI and EP 0 617 111 AI described. In order to improve the wetting of the substrate or the orientation of the mesogens on the open surface, the polymerizable mixtures, as shown in EP 0 617 111 A1, can contain small amounts of surface-active substances as additives, as are known, for example, for improving the flow from paint manufacture. Particularly suitable surface-active substances are organosiloxanes, which are used, for example, as paint auxiliaries. The organosiloxanes themselves can also have mesogenic properties as oligomers. If the polymer film is to remain on the substrate after crosslinking, the adhesion to the substrate can be improved, depending on the nature of the substrate surface, by means of suitable adhesion promoters in the polymerizable mixture, which are also state of the art. An improvement in the adhesion of the polymer film to the substrate can also be achieved by a suitable pretreatment of the substrate, for example a corona treatment. If the additives themselves have no mesogenic properties, they are only added in such a small amount that they do not impair the formation of the liquid-crystalline phase.

The polymerizable liquid-crystalline material is oriented in such a way that the mesogenic groups are arranged in a helical structure, the axis of which is transverse to the layer. Crosswise to the layer means that the axis is preferably inclined at an angle of less than 20 ° to the surface normal to the surface. The axis of the helical structure is particularly preferably perpendicular to the film surface. The orientation of the mesogens in the polymerizable mixture takes place, for example, by shearing the material during application or, after application, by the interaction of the mesogens with the appropriately selected substrate surface or by an electrical or magnetic field. This is preferably done in a temperature range from above the glass point or melting point to below the start of clarification of the polymerizable liquid-crystalline material. To one To enable a simple technical process, the composition of the polymerizable mixtures is preferably adjusted so that the optimal orientation temperature is between 20 ° C and 120 ° C. Because the pitch of cholesteric liquid crystals is generally temperature-dependent, this orientation temperature also influences the center wavelength of the reflection band. If the orientation of the mesogens is to take place through an interaction with the substrate surface, then a suitable orientation layer can be applied to the substrate by known coating, printing or dipping methods described in the literature in order to improve the orientation effect. The orientation layer or the substrate can be given a surface structure which favors orientation by additional treatment, for example rubbing. A location-dependent change in the orientation direction is possible, for example, using known methods for structuring an orientation layer in the μm to mm range by means of exposure to polarized UV light through a mask. Suitable methods for achieving an inclination between the mesogens of a liquid-crystalline phase and their interfaces are also described in the literature, for example exposure to polarized UV light or the vapor deposition of inorganic materials at an oblique angle. The orientation layer can also contain an optically uniaxial birefringent medium, for example an oriented and polymerized layer made of a liquid crystal mixture. Such a layer is particularly preferred which has an optical delay of 0.25 times the wavelength in the wavelength range used.

The substrates can be flat or curved. Substrates which are thermally and mechanically stable in the production, processing and use of the polymer film are particularly preferably used. Very particularly preferred substrates are glass or quartz plates, polymer films, such as, for example, polycarbonates, polysulfones, polyalkylene terephthalates, polyalkylene naphthalates, cellulose triacetate and polyimides. at If required, the substrate can be provided with an additional orientation aid, for example a layer of polyimide, polyamide, polyvinyl alcohol, a silicon oxide or a layer of a polymerized liquid crystal. If the polymer film with a helical molecular structure is to remain on the substrate after its production, then those materials are preferably suitable as substrates which are also used for the production of other optical elements known according to the prior art. Particularly preferred are substrates that are transparent or semi-transparent in the wavelength range relevant to the respective application, like many organic or inorganic substrates. If the polymer film is used as a reflective polarizer for linearly polarized light, then a particularly preferred substrate is an optically uniaxial birefringent substrate which has an optical delay of 0.25 times the wavelength in the wavelength range used. Such a quarter-wave retardation layer, or λ / 4 retardation layer for short, is produced, for example, by defined stretching of a polycarbonate, a polyethylene terephthalate or a polypropylene film or from a nematic LC polymer. Alternatively, a laminate of two different birefringent foils can be used as the substrate, the stretching directions of which are oriented at an angle to one another. Because of the different dispersion of the two

Foils also change the overall retardation of the laminate with the wavelength. The film material and the degree of stretching are to be selected such that a total delay of 0.25 times the wavelength occurs over the entire wavelength range used by the filter or reflector.

Of course, a λ / 4 delay layer can also be subsequently combined with the cholesteric layer according to the invention. These requirements for the optical properties of the substrate do not apply if the polymer film is detached from the substrate after production. In this case, such

Substrates are particularly preferred which enable the polymerizable mixture to be oriented well and which impart only slight adhesion to the surface. Daylight should be avoided when applying the polymerizable mixtures to the substrate surface and during the subsequent orientation of the mesogens, since the low UV radiation contained in daylight can already result in a small amount of polymerization of the liquid-crystalline mixtures, which leads to an increase in viscosity and thus slows the orientation of the mesogen. The application of the mixture and the subsequent orientation of the mesogens are therefore preferably carried out with the exclusion of UV radiation. Particles cause a disturbance in the pitch structure in the film, which are visible as clear inhomogeneities in polarized light. To avoid this, the polymerizable mixtures should not contain any particles larger than the thickness of the dry film. The polymerizable mixtures particularly preferably contain no particles whose longest diameter is greater than 20% of the film thickness. This is ensured by filtering the polymerizable mixture or its constituents or the solutions containing these constituents before application and cleaning the substrate surfaces under clean room conditions. Filtration, application, orientation and polymerization of the polymerizable liquid-crystalline materials are also preferably carried out under clean room conditions.

After orientation, the polymerizable mixture is briefly exposed to the action of actinic radiation at unchanged temperature and partially polymerized or copolymerized. Actinic radiation is photochemically active radiation, for example UV radiation, X-rays, gamma radiation or the irradiation with high-energy particles such as electrons or ions. The use of UV-A radiation is preferred. The irradiation is carried out in such a way that only a part of all possible polymerizable molecules is polymerized after the irradiation. The proportion of the polymerized molecules after the irradiation should preferably be between 0.1% and 70%, are particularly preferably between 1% and 50% of the polymerizable molecules. If the proportion of the polymerized molecules is too small, the crosslinking density gradient is insufficient and only a shift in the center wavelength and no broadening of the reflection band is observed after the final fixing of the layer. If, on the other hand, too many groups are polymerized during the first exposure, the pitch of the helical structure is fixed so much from the start that the formation of a pitch gradient is prevented. This case applies in particular to the conventional single exposure method, in which more than 70% of the polymerizable molecules are integrated into the network. The proportion of polymerized molecules can be determined, for example, by trial exposures and subsequent extraction in a suitable solvent. It is controlled by the irradiated exposure energy per unit area and unit of time. The irradiated exposure energy and also its temporal distribution are thus important parameters for setting the width of the reflection band of the polymer film according to the invention. For example, short, intensive exposures to UV-A radiation in air show good results for the polymerizable mixtures used in Examples 1 to 3. The necessary exposure energy depends on the type of radiation used, the material used, the photoinitiator and the layer thickness. For the first exposure, the preferred exposure energies per unit area for UV-A radiation are in the range from 1 to 500 mJ / cm ² , particularly preferably in the range from 10 to 50 mJ / cm ² . The duration of the exposure is preferably shorter than 30 s, particularly preferably shorter than 10 s. In comparison, exposure energies of more than 500 mJ / cm ² are used in conventional exposure, which leads to a polymerization of over 70% of the polymerizable molecules.

Following the short-term irradiation in an environment which has an inhibiting effect on the polymerization, the film is exposed to actinic radiation for a defined waiting time during which the material is in the partially polymerized structure reoriented. This waiting time is adapted to the requirements of the process depending on the composition of the polymerizable liquid-crystalline material, the inhibitory effect of the environment, the film thickness and the temperature. It is preferably in the range of

10 s to 60 s, but can be extended if the procedure requires. The waiting time can expire at the same temperature as the exposure in the first step or at a different temperature than the exposure in the first step. For example, it is possible to change the temperature during the waiting time by up to 100 ° C compared to the temperature in the first orientation phase in order to influence the speed of the reflection band broadening. The maximum possible temperature in the waiting time is limited by the clearing point of the polymerized layer. A temperature in the range from the temperature of the first orientation phase to 10 ° C. below this clearing point is preferably selected. In order to decouple the process of partial polymerization and the final fixation, the waiting time can, however, also be extended and / or the temperature can be reduced during the waiting time.

In addition to the irradiated energy of the exposure in the first process step, the duration and the temperature of the waiting time are the most important parameters for setting the desired width of the reflection band of the polymer film according to the invention. With the same temperature and duration of the waiting time and the same exposure time, the width of the reflection band increases with increasing exposure energy in the first process step up to a maximum value and then decreases again until the reflection band takes on the original shape of a conventionally polymerized film. On the other hand, a longer duration of the waiting time with the same exposure energy in the first process step leads to an increasing broadening of the reflection band. By choosing a suitable pre-exposure energy and a correspondingly long waiting time, bandwidths over 300 nm can be realized with the method according to the invention. The ^'waiting time, a further step follows, in which the orientation condition of the film achieved is finally fixed. For this purpose, the film can be polymerized completely or the film is cooled to the glass state. If the fixation by a

Polymerization reaction takes place, this reaction is preferably initiated by exposure to actinic light of high intensity, electron radiation or radical-forming thermal initiators such as peroxides. However, the crosslinking can also be carried out using crosslinkers containing hydrogen atoms bonded directly to silicon, with catalysis

Platinum metal catalysts can be effected or it can also be 'cationic or anionic. Crosslinking by UV light with an energy dose of more than 500 mJ / cm ² is particularly preferred. At the same time, the inhibiting effect of the pre-exposure environment is preferably reduced, for example by carrying out the polymerization in an inert gas atmosphere such as nitrogen. The oxygen content of the inert gas atmosphere is particularly preferably less than 1%. The partially polymerized film can also be covered with a film that serves as a barrier layer against the inhibiting influence of the environment.

The resulting polymer film can be used together with the substrate in the form of a laminate or after removal of the substrate as a free film. Another preferred application of the polymer film with a helical molecular structure is LC pigments, which are produced in further process steps by comminuting the polymer films, grinding and sieving. EP 0 601 483 A1 describes how pigments with a liquid-crystalline structure with a chiral phase, which reflect colored light, are produced by detaching a polymerized cholesteric film from the substrate and then comminuting the raw clods obtained in this way. The pigments can then be incorporated into a suitable binder system and applied to a substrate. For LC pigments in particular Particle size is an important parameter for many applications. With the same form factor, thinner pigments result in smaller particles, which are characterized, for example, by a more homogeneous visual impression and, in coatings, also by a lower topcoat level. At different

Printing process, only a maximum particle size is possible. The process according to the invention for the production of LC pigments has the advantage that, because of the adjustable steep gradient of the pitch, the smallest possible pigment platelets with broadened reflection bands can be produced. In many

Applications are preferred LC pigments with a thickness of 1 μm to 10 μm, the thickness 1 μm to 6 μm is particularly preferred.

LC pigments with a broadened reflection band show a higher light reflection due to their wider reflection band and therefore achieve a better brilliance. In addition, with specifically broadened reflection bands compared to the classic LC pigments (Helicone "HC; Wacker-Chemie GmbH / Munich) new shades and effects can be achieved. If the concentration of the chiral agents is chosen so that the reflection band is at least partially visible Wavelength range, then these LC pigments are ideally suited for decorative applications. Also highly interesting are highly reflective, color-neutral LC pigments, whose reflection bands cover the entire visible spectral range and thus produce metallic effects. For the production of security markings to protect against counterfeiting, for example Banknotes, security prints, documents or in brand protection can be used particularly advantageously for LC pigments, since they can usually be integrated into the printing or other coating processes that already exist in these applications with relatively little effort good protection against unauthorized copying is achieved due to the color effects and the polarization of the reflected light. A particular advantage of the LC pigments according to the invention is that because of the wider reflection bands when viewed through right- and left-helical circular polarizers or in Polarization-sensitive detectors enable a higher brightness contrast and are therefore even easier to recognize. IR-reflective LC pigments are suitable for producing markings that are invisible to the human eye and which can be registered by devices with IR detectors because of their good reflection in the IR range. Such LC pigments, which are obtained with a low concentration of chiral agents, are preferably transparent and colorless in the visible light range. The wavelength of the lower band edge of the reflection band is preferably above 750 nm. In all

The LC pigments according to the invention can also be used for the production of optically imaging, wavelength- and polarization-selective elements on curved substrates (EP 0 685 749 A1).

In order to apply the LC pigments to a substrate, they are incorporated into a suitable binder system as described, for example, in EP 0 601 483 A1 or EP 0 685 749 A1. The required properties of the binder systems, in particular the optical properties, depend on the intended use of the LC pigments. It is preferred to use binders which are optically transparent at least in the region of the reflection wavelength. Binder systems are preferably used for optical elements, the average refractive index of which after curing is the middle one

Refractive index of the LC pigments is similar. Curable binder systems are preferably suitable for producing permanent layers which contain LC pigments. However, non-hardenable binders such as oils and pastes can also be used for special applications. Binder systems which do not change the physical properties of the LC pigments or only change them in a defined manner are particularly preferred. Suitable binder systems are, for example, polymerizable resins (PU resins, silicone resins, epoxy resins), dispersions, solvent-based lacquers or

Water-based paints or all transparent plastics, for example polyvinyl chloride, polymethyl methacrylate, polycarbonate. In addition to these isotropic binders, liquid crystalline ones can also be used Systems are used as binders, for example liquid-crystalline polymers or polymerizable liquid-crystalline resins. To produce films with special anisotropic optical properties, the LC pigments are stirred into a liquid binder. The

Orientation of the platelets parallel to the surface of the layer occurs when a thin layer of the pigment-binder mixture is applied to a substrate, when the mixture is extruded or when drying. Depending on the requirements of the respective application and the properties of the binder, the film can be detached from the substrate after curing.

Through the control of the center wavelength and the width of the reflection band of a polymer film with helical, which is possible with the method according to the invention

Molecular structure allows the desired photometric properties of optical elements such as polarizers, color filters, pigments or reflectors, in particular also of structured filters and reflectors for left or right circularly polarized light, to be set in a simple manner. The invention therefore also relates to optical elements such as filters, reflectors and polarizers which contain at least one layer of a polymer film according to the invention with a helical molecular structure. These layers in the optical elements have reflection bands with preferably more than 1.5 times the bandwidth Δλ = λ- (n _β -n ₀ ) / n, which would correspond to the polymerizable liquid-crystalline material solely because of its refractive index anisotropy. The polymer film according to the invention, together with the substrate or after removal of the substrate, is also suitable as a free film as an optical element or part of an optical element. Further polymer films with a helical molecular structure or other layers, for example homeotropically or planarly oriented retardation layers (for example a λ / 4 retardation layer), absorbing polarization foils, colored foils or adhesive layers, can be applied to this polymer film or the substrate. However, it is also possible to use optical elements using a method according to the invention To produce, in which a retardation layer is used as a carrier substrate for the polymerizable liquid-crystalline material, which has, for example, a retardation value of a quarter of the respective wavelength in the entire wavelength range used.

In optical devices that work with polarized light, such as, for example, the liquid crystal displays, the polymer films according to the invention are advantageously used as a reflective circular polarizer for colored or white light. Further examples of applications in optics are filters (EP 0 302 619 A2) and optically imaging, wavelength- and polarization-selective elements for the entire wavelength range from infrared to near UV (EP 0 631 157 AI). Possible forms of application of these optical elements are, for example, beam splitters, mirrors and lenses. The invention also relates to the use of a polymer film according to the invention as a reflective polarizer in liquid crystal displays, as described, for example, in EP 0 606 939 A1 and EP 0 606 940 A2. Other devices which contain at least one layer of a polymer film according to the invention with a helical molecular structure and / or LC pigments according to the invention are also the subject of this invention. Such devices are, for example, projectors, projection displays and lamps which enable low-glare lighting by means of polarized light.

In addition to the optical applications in the visible

In the wavelength range, the polymer films according to the invention are particularly preferably suitable for the production of filters which reflect circularly polarized light in the infrared (IR) range at a low concentration of the chiral agents. By using curved substrates, the production of optically imaging, polarization-selective elements in the IR range is made possible in particular. IRs of this type are particularly preferred for many applications. reflective polymer films and LC pigments that are completely transparent to visible light. For this purpose, the wavelength of the lower band edge is preferably above 750 nm. Applications of such IR layers are, for example, machine-readable inscriptions or markings which are invisible to the human eye, for example security markings on security prints or in brand protection. In these applications, the circular polarization of the reflected IR radiation is particularly advantageous, since this is a security feature that can only be reproduced with difficulty. Another application of the IR-reflecting polymer films and LC pigments are colorless and transparent protective layers against thermal radiation, for example for thermal insulation glazing of buildings or vehicles. Since it matters in this application to reflect the entire heat radiation as possible, are here preferably two layers with a broad reflection band as possible and with opposite directions of rotation of the cholesteric ^'helix combined to reflect left- and rechtshelikale polarization.

Digital or analog optical storage media, which are based on a local change in the helical structure, can be produced by locally changing the orientation of the mesogenic groups before the final polymerization.

This can be achieved, for example, by local UV radiation through a mask that is impermeable to UV radiation if the external forces or the temperature of the polymer films are changed between the individual exposure steps, or by local heating of the

Polymer films, for example by a laser, or by local change in the HTP (helical twisting power) of the chiral contained, e.g. by UV-induced isomerization.

Cholesteric liquid crystals are thermochromic in the unpolymerized state, ie when the temperature in the cholesteric phase changes, the pitch of the helical molecular structure changes and thus the reflection wavelength. In US 4,637,896 and R. Maurer et al. "Cholesteric Reflectors with a Color Pattern", SID International Symposium Digest of Technical Papers, Vol. 25, San Jose, June 14-16, 1994, pp. 399-402, describes how this effect can be used, a flat To achieve color structuring of an LC polymer film by successively irradiating individual areas of the film at different temperatures through a mask with actinic light, if the temperature range of the liquid-crystalline phase is wide enough, color and polarization filters with red, green can be used and blue pixels that can be used, for example, as LCD color filters. Polymer films with a width of the reflection bands of approximately 80-110 nm are particularly preferred for this application, since this bandwidth enables maximum brightness and color saturation of the individual color pixels. In combination With a circular polarizer for white light reversed helicity, such a structured Color filters are also used to further increase the brightness of an LCD or other optical device that uses polarized light.

To produce structured polymer films with a helical molecular structure with broadened reflection bands, the method described above is modified in such a way that individual exposures of the material to actinic radiation take place through a mask. Subsequently, the mask is shifted or replaced by a second mask and the exposure of the material to actinic radiation is repeated, if necessary changing other parameters of the preceding steps, in such a way that an unexposed part of the film is irradiated and / or a part of the film that has not yet been finally fixed Material is irradiated again. A change in a parameter of the preceding steps is to be understood to mean that when the method is repeated, for example by changing the temperature during the exposure in the first

A different reflection color is set for the now irradiated material area or by a corresponding selection of temperature and / or duration of the process step Waiting time in the following process step, the bandwidth of the reflection band for the now irradiated material area is set differently. If necessary, this process is repeated as often as necessary with areas of the material that have not yet been irradiated. In this way, for example, a multicolored photostructured filter or reflector can be produced, the individual colors of which can be freely adjusted via the respective choice of the center wavelength and width of the reflection band. Examples of the detailed procedure for setting the center wavelengths and widths of the

Reflection bands of different pixels are given in EP 0 885 945 AI.

Examples: The invention is to be illustrated by the following examples, without being restricted to these. In these examples, unless stated otherwise, all quantities and percentages are based on weight. The mixture components used in the examples and their syntheses are known in the prior art.

Example 1: a) Preparation of a liquid-crystal mixture (mixture 1): 25 g of the polymerizable mesogenic compound hydroquinone bis (4-acryloylbutoxy) benzoate (prepared by the process from EP 1 059 282 A1), 45 g of the polymerizable mesogenic compound 4 - (4 ^Λ -Acryloylbutoxy) -benzoic acid- (4 ^X - cyanobiphenyl) ester (... prepared according to M. Portugall et al, Macromol Chem 183 (1982) 2311), 30 g of the mesogenic compound 4-allyloxybenzoate (4 '-cyanobiphenyl) ester (prepared according to EP 0 446 912 Al), 8.2 g of the polymerizable chiral 2.5- bis- [4- [(l-oxo-2-propenyl) oxy] benzoate] -1.4: 3, 6-dianhydro-D-glucitol (according to EP 1 046 692 AI), 90 mg silicone oil AF98 / 300 (Wacker-Chemie GmbH / Munich) and 220 mg stabilizer 2,6-di-tert-butyl-4-methylphenol ( BHT) were in one

Solvent mixture of 110 g tetrahydrofuran (THF) and 220 g toluene dissolved. Before use, the solution was filtered to exclude particles larger than 1 μm and it became 2.2 g Photoinitiator 2-methyl-1- [4- (methylthio) phenyl] -2-morpholinopropanone-1 (Irgacure® 907; Ciba Specialty Chemicals G bH / Lampertsheim) added. b) Comparative example: Mixture 1 was placed in a coating installation

Clean room conditions and exclusion of UV radiation in yellow ambient light are applied to a continuously moving polyethylene terephthalate (PET) film. The wet film was dried in hot air at 110 ° C for about 1 minute. The result was a sticky film which was cloudy at room temperature and had a homogeneous thickness of about 6 μm. The film was annealed in the absence of UV radiation in air at 90 ° C for two minutes, whereby it became completely clear and then at 90 ° C in a low-oxygen nitrogen atmosphere (oxygen content <0.5%) with a high UV dose of approx 650 mJ / cm ² irradiated and thus cross-linked.

The resulting tack-free and scratch-resistant film had a blue color when viewed vertically in daylight. The measurement of the transmission in a UV / VIS spectrometer showed an almost box-shaped band at 425 nm with a half width of 70 nm. C) Production of a polymer film according to the invention:

Mixture 1 was applied to a continuously moving polyethylene terephthalate (PET) film in a coating installation, with the exclusion of UV radiation, as in comparative example 1b). The wet film was dried in hot air at 110 ° C for 1 minute. The result was a sticky film which was cloudy at room temperature and had a homogeneous thickness of about 6 μm. The film was annealed in air at 90 ° C. for a further minute, during which it became completely clear and then exposed to UV-A radiation (total dose 25 mJ / cm ² ) for 0.3 s. After a further minute at unchanged temperature with the exclusion of UV radiation, the film was irradiated at 90 ° C. in a low-oxygen nitrogen atmosphere (oxygen content <0.5%) with a high UV dose of approx. 650 mJ / cm ² and thus finally networked.

The resulting tack-free and scratch-resistant film had a silvery color when viewed vertically in daylight. The Light reflected from the film was circular to the right and the light transmitted through the film was polarized to the left. The measurement of the transmission of left-hand circularly polarized light in a UV / VIS spectrometer (Spectro 320 from Instrument Systems) showed an almost box-shaped band from 450 nm to 710 nm (measured at 50% transmission for left-hand circularly polarized light).

A cross section of the film was taken with a scanning electron microscope and the pitch was measured thereon. The average pitch of the first three helices on the side facing the PET substrate was 320 nm. This corresponds to an average reflection wavelength of 530 nm with an average refractive index of 1.66, which was measured with a prism coupler (Metricon Model 2010) The pitch increases almost linearly up to the 8th turn to 420 nm (corresponding to a wavelength of 700 nm) and then remains constant up to the 10th turn. Between the 10th and 12th turns, the pitch suddenly drops to 270 nm (corresponding to a wavelength of 450 nm) and retains this value until the 14th turn. On the open surface of the film, which begins with the 15th turn, a slight unscrewing of the helix is observed to an orientation of the mesogens inclined towards the surface. To evaluate the light amplification effect when used as a reflective polarizer, the LC polymer film, a retardation film with λ / 4 property and a linear polarizer in 45 "alignment with the axis of the λ / 4 film were placed on a commercial backlight for LCD displays and the The same measurement was carried out with an arrangement ^• of LCD backlight with linear polarizer without LC polymer film and without λ / 4 film. When comparing the two measurements, it was found that the brightness of the arrangement with LC polymer film when viewed perpendicularly was 42% brighter than without an LC polymer film and averaged over all viewing angles up to 60 °, the gain in brightness was 30%.

To test the stability of the film, it was stored in a heating cabinet at 80 ° C. for 8 days and then again Transmission measured in the UV / VIS spectrometer. The film, which was unchanged after visual inspection, showed an almost box-shaped band of 450 nm to 710 nm in transmission. D) Comparative example:

Mixture 1 was applied to a continuously moving polyethylene terephthalate (PET) film in a coating installation, with the exclusion of UV radiation, as in comparative example 1b). The wet film was dried in hot air at 110 ° C for 1 minute. It resulted in a sticky, too

Room temperature cloudy film with a homogeneous thickness of approx.

6 μm. A second PET film was laminated onto this film as a top layer. The laminated film was annealed in air at 90 ° C. for one minute as in comparative example 1c), it becoming completely clear and then exposed to UV-A radiation (total dose 25 mJ / cm ² ) for 0.3 s. After a further minute at unchanged temperature with the exclusion of UV radiation, the film was irradiated at 90 ° C. in a low-oxygen nitrogen atmosphere (oxygen content <0.5%) with a high UV dose of approx. 650 mJ / cm ² and thus finally networked.

The resulting tack-free and scratch-resistant film had a blue color when viewed vertically in daylight. The measurement of the transmission in a UV / VIS spectrometer showed an almost box-shaped band at 430 nm with a half width of 75 nm. E) Comparative example:

In the production of a polymer film according to Comparative Example ld), after drying the LC film, a cellulose triacetate film (TAC) was laminated on instead of a polyethylene terephthalate film (PET) as a cover layer. The non-tacky and scratch-resistant film after the final crosslinking also had a blue color when viewed vertically in daylight. The measurement of the transmission in a UV / VIS spectrometer showed, as in comparative example ld), an almost box-shaped band at ^' 430 nm with a half width of

75 nm. Example 2: a) Preparation of a liquid crystal mixture (mixture 2): 79 g of the polymerizable mesogenic compound hydroquinone bis (4-acryloylbutoxy) benzoate (prepared by the process from EP 1 059 282 A1), 79 g of the polymerizable mesogenic

Compound 4- (4-acryloylbutoxy) benzoic acid 4-biphenyl ester (manufactured according to US 4,293,435), 13 g of the polymerizable chiral 2,5-bis- [4- [(l-oxo-2-propenyl) oxy] benzoate] - 1,4: 3,6-dianhydro-D-glucitol (according to EP 1 046 692 AI), 110 mg silicone oil AF98 / 300 (Wacker-Chemie GmbH / Munich) and 350 mg

Stabilizer 2, 6-di-tert-butyl-4-methylphenol (BHT) was dissolved in a mixed solvent of 120 g tetrahydrofuran (THF) and 240 g toluene. Before use, the solution was filtered to exclude particles larger than 1 μm, and 3.5 g of photoinitiator 2-methyl-1- [4- (methylthio) phenyl] -2-morpholinopropanone-1 (Irgacure® 907; Ciba Specialty Chemicals GmbH / Lampertsheim) added. b) Comparative example:

Mixture 2 was applied to a continuously moving polyethylene terephthalate (PET) film in a coating system under clean room conditions and with the exclusion of UV radiation in yellow ambient light. The wet film was dried in hot air at 110 ° C for 1 minute. The result was a sticky film which was cloudy at room temperature and had a homogeneous thickness of about 6 μm. The film was annealed in the absence of UV radiation in air at 95 ° C for two minutes, whereby it became completely clear and then at 95 ° C in a low-oxygen nitrogen atmosphere (oxygen content <0.5%) with a high UV dose of approx 650 mJ / cm ² irradiated and thus cross-linked.

The resulting tack-free and scratch-resistant film had a blue color when viewed vertically in daylight. The measurement of the transmission in a UV / VIS spectrometer showed an almost box-shaped band at 470 nm with a half width of 55 nm. C) Production of a polymer film according to the invention:

Mixture 2 was applied as in comparative example 2b) in a coating installation with the exclusion of UV radiation continuously moving polyethylene terephthalate film applied. The wet film was dried in hot air at 110 ° C for 1 minute. The result was a sticky film which was cloudy at room temperature and had a homogeneous thickness of about 6 μm. The film was annealed in air at 95 ° C for a further minute, whereby it became completely clear, and then exposed to UV-A radiation (total dose 25 mJ / c ² ) for 0.3 s. After a further minute at unchanged temperature with the exclusion of UV radiation, the film was irradiated at 95 ° C. in a low-oxygen nitrogen atmosphere (oxygen content <0.5%) with a high UV dose of approx. 650 mJ / cm ² and thus finally networked.

The resulting tack-free and scratch-resistant film had a golden color when viewed vertically in daylight. The light reflected from the film was polarized to the right in a circular fashion. The measurement of the transmission in the UV / VIS spectrometer showed a broadened reflection band from 500 nm to 680 nm (measured at 50% transmission for left circularly polarized light). The pitch distribution was asymmetrical and had the longest pitch on the side of the PET substrate. d) Production of LC pigments:

The LC polymer film from Example 2c) was scraped off the PET film with a blade, comminuted and then ground in an Alpine 200 LS laboratory universal mill. In this way, particles with an average grain diameter of approximately 50 μm were produced. The powdery material obtained was then sieved using an analytical sieve with a mesh size of 50 μm and then incorporated into a conventional alkyd-melamine resin binder system (Sacolyd ^• F410 / Sacopal M110; Kolms Chemie / Krems, Austria). The

The viscosity of the binder system was adjusted to a run-out time of approximately 80 seconds using a diluent (mixture of aromatic hydrocarbons and methyl isobutyl ketone) in a DIN-4 flow cup. The mixture of LC pigments and binder thus obtained was applied onto a black-primed sheet using a film puller (from Erichsen) in a wet film thickness of 120 μm. Then the sheet became one Dried at 80 ° C for one hour. Under vertical supervision, the sheet showed a strong, shiny, silvery-gold color after drying, which changed to a silvery-green color at flatter viewing angles. By viewing with right and left circular polarizers, one became special. striking light-dark contrast in daylight and also in artificial light, which is more visually apparent than with Heliconen ^® (Wacker-Chemie GmbH / Munich), which are made from a polymer film with a helical molecular structure without widening the cholesteric band.

Example 3:

A glass plate was provided with an orientation layer made of polyimide, which was rubbed with a velvet cloth unidirectionally. Mixture 2 was applied to the polyimide layer by spin coating with the exclusion of UV radiation in yellow ambient light and then dried for a few minutes at room temperature until a cloudy, sticky film with a thickness of approximately 5 μm remained. The film was then placed in a heated case with a quartz window filled with air. The temperature of the case was set at 55 ° C. After the film became clear, half of the film (Zone 2) was covered with an aluminum foil and then the uncovered half (Zone 1) through the quartz window in air for 0.8 s with UV-A radiation (total dose approx. 26 mJ / cm ² ) exposed. A mercury arc lamp (model 68810, LOT-Oriel GmbH) was used as the exposure source, the shutter of which is controlled by a timer. The temperature in the housing was then raised to 95 ° C. The aluminum foil was removed and after the film was clear again, it was exposed to UV-A radiation (total dose 26 mJ / cm ² ) again in air for 0.8 s. The housing was then filled with nitrogen. After one minute at unchanged temperature, the film was finally crosslinked in the nitrogen atmosphere at 95 ° C. with a UV dose of approx. 2 J / cm ² .

The resulting tack-free and scratch-resistant film had a copper-red color when viewed vertically in daylight Zone 1 and green-yellow color in Zone 2. The light reflected from the film was polarized to the right in a circular shape. The measurement of the transmission in the UV / VIS spectrometer showed a broadened reflection band from 550 nm to 710 nm in zone 1 (measured at 50% transmission for left-hand circularly polarized light) and from 480 nm to 650 nm in zone 2. The pitch distribution was asymmetrical with the longer pitch on the side of the glass plate.

Claims

claims

1. Polymer film with a helical molecular structure which contains a sequence of a short, long and medium pitch of the helical structure in a direction perpendicular to the film surface.

2. Polymer film according to claim 1, characterized in that it is made from a single layer of a mixture of polymerizable liquid-crystalline material and that the change between short and long

Pitch in less than five turns of the helical

Structure takes place.

3. Polymer film with an adjustable gradient of the pitch of a helical molecular structure produced by a process comprising the steps:

Orientation of the material so that the mesogenic groups are arranged in a helical structure, the axis of which is transverse to the layer, partial polymerization of this layer by the action of actinic radiation in a for the

Polymerization inhibiting environment, and final fixation after the material has reoriented in the partially polymerized structure, by completely polymerizing the film or cooling the film in the glass.

4. Polymer film according to at least one of claims 1 to 3, characterized in that it has a reflection band widened by at least 50% compared to the unpolymerized state.

5. Polymer film according to at least one of claims 1 to 4, characterized in that the thickness is 2 to 20 times the width of the reflection band divided by (n _e -n ₀ ), where n _e and n _{0 are} the refractive indices for the extraordinary (n _e ) and the ordinary (n ₀ ) beam are in a birefringent medium.

6. Polymer film according to at least one of claims 1 to 5, characterized in that the substrate is a polymer film which contains no additional oxygen barrier layer.

7. Polymer film according to at least one of claims 1 to 6, characterized in that the partial polymerization takes place at a temperature in a phase region with a helical orientation of the mesogens.

8. Polymer film according to at least one of claims 1 to 7, characterized in that the inhibitory environment is formed during the first, partial polymerization by a gas which contains oxygen.

9. Polymer film according to at least one of claims 1 to 8, characterized in that the mixture of polymerizable liquid-crystalline material contains monomers or oligomers which differ in their reactivity with respect to the polymerization.

10. Polymer film according to at least one of claims 1 to 9, characterized in that the partial Polymerization, during the final fixation or both, the action of the actinic radiation takes place through a mask and that the mask is subsequently changed or replaced by a second mask and this process step is repeated, if necessary with the modification of further process parameters, in such a way that a part of the process which has not yet been fixed Material is irradiated with actinic radiation.

11. LC pigments with a helical molecular structure, characterized in that a polymer film according to one of claims 1 to 10 is comminuted in a subsequent process step.

12. A method for producing a polymer film with an adjustable gradient of the pitch of a helical molecular structure comprising the steps:

Applying a mixture of polymerizable liquid-crystalline material which a) monomers or oligomers, each having at least one mesogenic group and at least one polymerizable functional group, and b) a chiral compound, in one layer on a substrate, - orienting the material, see above that the mesogenic

Groups are arranged in a helical structure, the axis of which is transverse to the layer, partial polymerisation of this layer by exposure to actinic radiation in an environment which inhibits the polymerization, and final fixing after the material has reoriented in the partly polymerised structure, completely Polymerize the film or cool the film to glass.

13. The method according to claim 12, characterized in that the application of a mixture of polymerizable liquid crystalline material and the subsequent orientation of the mesogens is carried out under clean room conditions and exclusion of UV radiation.

14. Wavelength-selective or polarization-selective optical elements for electromagnetic radiation from the near ultraviolet to the infrared range, characterized in that they contain at least one layer of a polymer film with a helical molecular structure according to one of claims 1 to 10 or LC pigments according to claim 11.

15. Reflective linear polarizer, characterized in that a polymer film according to one of claims 1 to 10 is combined with a λ / 4 retardation film.

16. Use of a polymer film according to one of claims 1 to 10 or of LC pigments according to claim 11 as a reflector or polarizer in liquid crystal displays, visible 'and invisible markings, security elements, reflector for heat or infrared radiation or for decorative purposes.

17. A projector, projection display or lighting fixture containing at least one layer of a polymer film with a helical molecular structure according to one of claims 1 to 10 or LC pigments according to claim 11.

18. Liquid crystal device with a liquid crystal material arranged between two electrodes, optionally further optical components selected from the group comprising light source, mirror, polarizer, retarder, color filter and at least one film for producing or increasing the yield of polarized light, characterized in that it is the polarizing film is a polymer film according to any one of claims 1 to 10.

9. Liquid crystal device with one between two

Electrode-arranged liquid-crystalline material, optionally further optical components selected from the group comprising light source, mirror, polarizer, retarder, color filter and at least one film for the selection of colored light, characterized in that the color filter is a polymer film according to one of Claims 1 to 10 acts.