WO2006085741A1

WO2006085741A1 - Process for preparing a polymeric relief structure

Info

Publication number: WO2006085741A1
Application number: PCT/NL2005/000106
Authority: WO
Inventors: Cees Bastiaansen; Dick Broer; Carlos Sanchez
Original assignee: Stichting Dutch Polymer Institute
Priority date: 2005-02-09
Filing date: 2005-02-09
Publication date: 2006-08-17
Also published as: JP4995739B2; JP2008530612A; US20080131626A1; WO2006085757A3; EP1846803A2; WO2006085757A2; US8927178B2

Abstract

The present invention relates to a process for the preparation of a polymeric relief structure by a) coating a substrate with a first coating composition comprising one or more radiation-sensitive ingredients, d) locally treating the coated substrate with electromagnetic radiation having a periodic or random radiation-intensity pattern, forming a latent image, e) polymerizing and/or crosslinking the resulting coated substrate to a first coating. This process is improved by applying a second coating composition on top of the first coating composition, said second coating composition comprising either an organic compound (Co) of a monomeric nature and wherein Co is also polymerized during the process, or wherein said second coating comprises a dissolved polymer (Cp). As a result a polymeric relief structure is obtained, where a substrate is coated with a functional, stacked, bi-layer, in which each layer exhibits a specific, and from eachother differing function.

Description

PROCESS FOR PREPARING A POLYMERIC RELIEF STRUCTURE

The present invention relates to a process for the preparation of a polymeric relief structure by a) coating a substrate with a first coating composition comprising one or more radiation-sensitive ingredients, b) locally treating the coated substrate with electromagnetic radiation having a periodic or random radiation-intensity pattern, forming a latent image, c) polymerizing and/or crosslinking the resulting coated substrate to a first coating. Such a process, hereinafter also to be called "photo-embossing", is known from "Photo-embossing as a tool for complex surface relief structures", De Witz, Christiane; Broer, Dirk J., Abstracts of Papers, 226^th ACS National Meeting, New York, NY, United States, September 7-11 , 2003.

Polymers in use in optical systems for data transport, storage and displays are nowadays of great interest. By structuring the surface of a polymer film or layer, light that passes these layers can be controlled. For instance if the surface structure contains small semi-sphere like elements, a lens array is obtained that may focus transmitted light. Such an element is for instance useful in a backlight of a liquid crystal display in order to focus light on the transparent area of the display. For these types of applications it is often necessary to control the shape of the surface profiles down to the micrometer region. Also regular patterns of surface structures may diffract light such that a single beam, upon transmission, is split up in multiple beams, that for instance can be used as beam splitter in telecommunication devices. Surface structures are also important to control reflection of light. This can successfully be applied to suppress specular reflection of a surface. This so-called anti-glare effect is for instance applied on the front screen of a television set, but can also be used for applications such as glazing, car finishes, etc. A polymer film, with well-defined surface profiles, may be provided with a conformal reflective film, such as evaporated aluminum or sputtered silver. Incident light falling on this mirror is, upon reflection, distributed in space in a very controlled way. This is for instance used to make internal diffusive reflectors for reflective liquid crystal displays. Another application of surface profiles is for creating anti-fouling structures known as the "Lotus effect". Thereto surface profiles with dimensions smaller than 1 micrometer are needed. Electromagnetic-radiation induced polymerization, like UV photo- polymerization, is a method to prepare devices from e.g. a mixture of two (meth)acrylate monomers and a photo-initiator. The polymerization reaction is initiated only in those regions where the UV light can activate the photo-initiator. In addition, it is possible to vary the light intensity spatially and vary the polymerization speed accordingly. Differences in the monomer reactivity, size or length, cross-linking ability, and energetic interaction, result in gradients in the monomer chemical potentials. These chemical potentials form the driving force for monomer migration and for polymer swelling in the illuminated regions. The monomer diffusion coefficients determine the time-scale on which this migration takes place. Subsequently, uniform UV illumination with a higher intensity than during the patterned UV illumination is used to polymerize the entire film.

Patterned UV photo-polymerization of a mixture of two liquid monomers thus results in a polymer structure. This can be done holographically or lithographically. For holography, the interference pattern of two coherent light beams generates regions of high and low light intensity. For lithography, a photo-mask is used to produce these intensity differences. If for instance a striped mask is used, a grating is produced. If a mask with circular holes is used, a microlens structure is formed. Differences in the refractive index are caused by lateral variations of the monomer-unit concentrations in the polymer.

A better method to create a surface structure is to use a blend that basically consists of polymer(s), monomer(s) and initiator(s). The polymer can be a single polymeric material, but may also be a blend of more than one polymer. Similarly the monomer may be a single compound, but may also be consisting of several monomeric materials. The initiator preferably is a photoinitiator, but sometimes is a mixture of a photoinitiator and a thermal initiator. This mixture is generally dissolved in an organic solvent in order to enhance processing, e.g. formation of thin films by spin coating. The blending conditions as well as the properties of the polymer(s) and monomer(s) are chosen such that after evaporation of the solvent a solid film is formed. In general this allows that, upon patterned exposure with UV light, a latent image is formed. The latent image can be developed into a surface profile by heating, as a result of which polymerization and diffusion occur simultaneously, thus increasing the materials volume at the exposed area or visa versa, which results in a surface deformation. An aim of the present invention is the preparation of a functional bi- layer on a substrate, having at least a relief structure on the interface of the two layers, and in which the two layers are substantially connected to each other.

As a result thereof, a polymeric relief structure is obtained, where a substrate is coated with a functional, stacked, bi-layer, in which each layer exhibits a specific, and from eachother differing function. So can a first layer, closest to the substrate and carrying a relief structure, have anti-glare properties, whereas a second layer on top of said first layer have anti-reflective properties.

The application of an as-is polymeric layer on top of the first layer with a relief structure does not result in an acceptable bi-layer structure, as the second layer will not accurately enough follow the contours of the relief structure of the first layer.

Therefore, there is a need for a process in which a bi-layer structure is made, in which structure the two layers substantially integrally adhere to each other. A weakness of a photo-embossing process is that the resulting relief structure has a rather low aspect ratio. The aspect ratio (AR) is hereinafter defined as the ratio between the relief height and the distance (or pitch) between neighboring reliefs. The edges of such a relief structure are not sharp or not accurately reproduced, as a result of which the optical function or other functionality that is aimed at is less optimal. Moreover, often it is desired to introduce a second optical, electrical or electro- optical functionality to the structured film such as specific alignment characteristics for reactive or non-reactive mesogens, electro-luminescence, fluorescence and anti-reflex properties. In a specific embodiment of the present invention, this problem is also dealt with.

The present invention provides an improved process for preparing a polymeric relief structure, and is characterized in that a second coating composition is applied on top of the first coating composition, said second coating composition comprising either an organic compound (C₀) of a monomeric nature and wherein C₀ is also polymerized during the process, or wherein said second coating comprises a dissolved polymer (C_p). The present invention involves a process for preparing a polymeric relief structure in a material that is applied as a coating onto the photoembossing layer.

The photoembossing layer provides the surface profile transmitted to the layer on top.

The top layer provides another function that is combined with the function of the added surface relief. Examples of functions of the toplayer are: - antireflex as caused by a surface nanostructure created by nanoparticles added - A -

to the second coating composition. The profile added by the photoembossing layer will enhance the antiglare properties of the film. In this way the unique combination of anti-reflex and anti-glare is established. - Controlled optical retardation by a liquid crystalline monomer in the second coating composition. In this case the relief in the photoembossing film provides an alignment of the liquid crystal monomer that is subsequently frozen in by polymerization. The polymerized liquid crystal provides the optical retardation. The top layer can also be an alignment film for liquid crystals. In that case the relief of the photoembossed layer is transmitted to a relief in the alignment layer that may affect the orientation of a liquid crystal that is applied on it in a later stage. The alignment layer itself also has an effect on the liquid crystal alignment. The combination of both alignment effects allows exact tuning of the liquid crystal alignment. For instance two stable liquid crystal orientations can be achieved by choosing the alignment directions not to be parallel to each other. The alignment layer can be a rubbed polymer film (polyimide, polyvinylalcohol, nylon, etc.) or a film that has its alignment ability by photochemical processes (polyvinyl cinnamates, etc.). Various strategies can be followed to create a surface structure in the top film:

1. The photoembossed layer is applied and dried. Then the top layer is coated on top of this either from a polymer solution or as a monomer

(solution) and dried or cured. Subsequently, the photoembossed layer is exposed by e.g. UV light through a mask or by interference (holographic setup) and developed by a heating step. The photoembossed layer deforms and the top layer takes over this deformation. 2. The photoembossed layer is applied, dried and e.g. UV exposed through a mask or by interference. After this exposure the film is still flat and is coated with the top layer that is dried or cured. Subsequently the surface is deformed by the heating step.

3. As in the previous two cases, but the heating step is performed in the situation that the top layer is still 'wet' (=not cured in case of the use of a monomer, or solvent containing in case of a polymer). In that case the free surface of the top layer does not take over the surface profile of the photoembossing layer but remains flat.

In a specific embodiment, compound C₀ of a monomeric nature, or compound C_p reduces the interfacial tension of the coated substrate of step b). As a result, relief structures with an enhanced relief aspect ratio (the improvement typically showing an increase of a factor 2), as well as much sharper edged relief, are obtained.

The monomer compound C₀, when used to reduce the interfacial surface tension, can be applied in at least two distinct ways. The first way is in a process, wherein C₀ is applied to the coated substrate resulting from step b) of the present process, after which step c) is executed. The second way is in a process, in which the C₀ is already present in the coating used in step a) of the present process. As a result hereof, C₀ is present in step b) as well as in step c). The coating composition used in step a) of the present process comprises one or more radiation sensitive ingredients, which in general are C=C unsaturated monomers, polymerizable via electromagnetic radiation. These ingredients can be used as such, but also in the form of a solution.

This first coating composition may be applied onto the substrate by any process known in the art of (wet) coating deposition. Examples of suitable processes are spin coating, dip coating, spray coating, flow coating, meniscus coating, doctor's blading, capillary coating, and roll coating.

Typically, the radiation sensitive ingredient(s) are mixed, preferably with at least one solvent and, optionally, crosslinking initiator, to prepare a mixture that is suitable for application to the substrate using the chosen method of application.

In principle, a wide variety of solvents may be used. However, the combination of the solvents and all other materials present in the mixture should preferentially form stable suspensions or solutions.

Preferably, the solvent used is evaporated after applying the first coating composition onto the substrate. In the process according to the invention, optionally the first coating composition may, after application to the substrate, be heated or treated in vacuum to aid evaporation of the solvent. As a result, the first coating is a solid film.

Examples of solvents that are suitable are 1 ,4-dioxane, acetone, acetonitrile, chloroform, chlorophenol, cyclohexane, cyclohexanone, cyclopentanone, dichloromethane, diethyl acetate, diethyl ketone, dimethyl carbonate, dimethylformamide, dimethylsulphoxide, ethanol, ethyl acetate, m-cresol, mono- and di-alkyl substituted glycols, N,N-dimethylacetamide, p-chlorophenol, 1 ,2-propanediol, 1-pentanol, 1-propanol, 2-hexanone, 2-methoxyethanol, 2-methyl-2-propanol, 2-octanone, 2-propanol, 3-pentanone, 4-methyl-2-pentanone, hexafluoroisopropanol, methanol, methyl acetate, butyl acetate, methyl acetoacetate, methyl ethyl ketone, methyl propyl ketone, n-methylpyrrolidone-2, n-pentyl acetate, phenol, tetrafluoro-n- propanol, tetrafluoroisopropanol, tetrahydrofuran, toluene, xylene and water. Alcohol, ketone and ester based solvents may also be used, although the solubility of acrylates may become an issue with high molecular weight alcohols. Halogenated solvents (such as dichloromethane and chloroform) and hydrocarbons (such as hexanes and cyclohexanes) are also suitable.

The first coating composition comprises one or more radiation- sentitive ingredients, being a compound that upon exposure to actinic radiation generates a reactive species, i.e. free-radicals or cationic species that initiates the polymerization. Examples of monomers suitable for use as polymerizing ingredient and having at least two crosslinkable groups per molecule include monomers containing (meth)acryloyl groups (such as trimethylolpropane tri(meth)acrylate, pentaerythritol (meth)acrylate), ethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, 1 ,4-butanediol di(meth)acrylate, 1 ,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, polybutanediol di(meth)acrylate, tripropyleneglycol di(meth)acrylate, glycerol tri(meth)acrylate, phosphoric acid mono- and di(meth)acrylates, C₇-C_2O alkyl di(meth)acrylates, trimethylolpropanetrioxyethyl (meth)acrylate, tris(2-hydroxyethyl)isocyanurate tri(meth)acrylate, tris(2-hydroxyethyl)isocyanurate di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol monohydroxy pentacrylate, dipentaerythritol hexacrylate, tricyclodecane diyl dimethyl di(meth)acrylate and alkoxylated versions, preferably ethoxylated and/or propoxylated, of any of the preceding monomers, and also di(meth)acrylate of a diol which is an ethylene oxide or propylene oxide adduct to bisphenol A, di(meth)acrylate of a diol which is an ethylene oxide or propylene oxide adduct to hydrogenated bisphenol A, epoxy (meth)acrylate which is a (meth)acrylate adduct to bisphenol A of diglycidyl ether, diacrylate of polyoxyalkylated bisphenol A, and Methylene glycol divinyl ether, adduct of hydroxyethyl acrylate, isophorone diisocyanate and hydroxyethyl acrylate (HIH), adduct of hydroxyethyl acrylate, toluene diisocyanate and hydroxyethyl acrylate (HTH), and amide ester acrylate.

Examples of suitable monomers having only one crosslinking group per molecule include monomers containing a vinyl group, such as N-vinyl pyrrolidone, N-vinyl caprolactam, vinyl imidazole, vinyl pyridine; isobornyl (meth)acrylate, bornyl (meth)acrylate, tricyclodecanyl (meth)acrylate, dicyclopentanyl (meth)acrylate, dicyclopentenyl (meth)acrylate, cyclohexyl (meth)acrylate, benzyl (meth)acrylate, 4-butylcyclohexyl (meth)acrylate, acryloyl morpholine, (meth)acrylic acid, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, amyl (meth)acrylate, isobutyl

(meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, caprolactoπe acrylate, isoamyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, tridecyl (meth)acrylate, undecyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, butoxyethyl (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, methoxyethylene glycol (meth)acrylate, ethoxyethyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, methoxypolypropylene glycol (meth)acrylate, diacetone (meth)acrylamide, beta-carboxyethyl (meth)acrylate, phthalic acid (meth)acrylate, isobutoxymethyl (meth)acrylamide, N,N-dimethyl (meth)acrylamide, t-octyl (meth)acrylamide, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, butylcarbamylethyl (meth)acrylate, n-isopropyl (meth)acrylamide fluorinated (meth)acrylate, 7-amino-3,7-dimethyloctyl (meth)acrylate, N,N-diethyl (meth)acrylamide, N,N-dimethylaminopropyl (meth)acrylamide, hydroxybutyl vinyl ether, lauryl vinyl ether, cetyl vinyl ether, 2-ethylhexyl vinyl ether; and compounds represented by the following formula (I)

CH₂=C(R⁶)-COO(R⁷O)_m-R⁸ (I)

wherein R⁶ is a hydrogen atom or a methyl group; R⁷ is an alkylene group containing 2 to 8, preferably 2 to 5 carbon atoms; and m is an integer from 0 to 12, and preferably from 1 to 8; R⁸ is a hydrogen atom or an alkyl group containing 1 to 12, preferably 1 to 9, carbon atoms; or, R⁸ is a tetrahydrofuran group- comprising alkyl group with 4- 20 carbon atoms, optionally substituted with alkyl groups with 1-2 carbon atoms; or R⁸ is a dioxane group-comprising alkyl group with 4-20 carbon atoms, optionally substituted with methyl groups; or R⁸ is an aromatic group, optionally substituted with C₁-C₁₂ alkyl group, preferably a C₈-C₉ alkyl group, and alkoxylated aliphatic monofunctional monomers, such as ethoxylated isodecyl (meth)acrylate, ethoxylated lauryl (meth)acrylate, and the like.

Oligomers suitable for use as a radiation sensitive ingredient in the coating composition of step a) are for example aromatic or aliphatic urethane acrylates or oligomers based on phenolic resins (ex. bisphenol epoxy diacrylates), and any of the above oligomers chain extended with ethoxylates. Urethane oligomers may for example be based on a polyol backbone, for example polyether polyols, polyester polyols, polycarbonate polyols, polycaprolactone polyols, acrylic polyols, and the like. These polyols may be used either individually or in combinations of two or more. There are no specific limitations to the manner of polymerization of the structural units in these polyols. Any of random polymerization, block polymerization, or graft polymerization is acceptable. Examples of suitable polyols, polyisocyanates and hydroxylgroup-containing (meth)acrylates for the formation of urethane oligomers are disclosed in WO 00/18696, which is incorporated herein by reference.

Combinations of compounds that together may result in the formation of a crosslinked phase and thus in combination are suitable to be used are for example carboxylic acids and/or carboxylic anhydrides combined with epoxies, acids combined with hydroxy compounds, especially 2-hydroxyalkylamides, amines combined with isocyanates, for example blocked isocyanate, uretdion or carbodiimide, epoxies combined with amines or with dicyandiamides, hydrazinamides combined with isocyanates, hydroxy compounds combined with isocyanates, for example blocked isocyanate, uretdion or carbodiimide, hydroxy compounds combined with anhydrides, hydroxy compounds combined with (etherified) methylolamide ("amino-resins"), thiols combined with isocyanates, thiols combined with acrylates or other vinylic species (optionally radical initiated), acetoacetate combined with acrylates, and when cationic crosslinking is used epoxy compounds with epoxy or hydroxy compounds.

Further possible compounds that may be used as a radiation sensitive ingredient are moisture curable isocyanates, moisture curable mixtures of alkoxy/acyloxy-silanes, alkoxy titanates, alkoxy zirconates, or urea-, urea/melamine-, melamine- formaldehyde or phenol-formaldehyde (resol, novolac types), or radical curable (peroxide- or photo-initiated) ethylenically unsaturated mono- and polyfunctional monomers and polymers, e.g. acrylates, methacrylates, maleate/vinyl ether), or radical curable (peroxide- or photo-initiated) unsaturated e.g. maleic or fumaric, polyesters in styrene and/or in methacrylates. Preferably the ingredient(s) in the radiation-sensitive first coating composition is/are selected from the group comprising (meth-) acrylates, epoxies, oxetanes, vinyl ethers, styrenes, and thiol-enes. Preferably, the applied coating composition used in step a) also comprises a polymer. Preferably, this polymer has a weight-averaged molecular weight (Mw) of at least 20,000 g/mol. Such a polymer, when used in the coating composition of step a), preferably has a glass transition temperature of at least 300 K. Preferably, the polymer in the coating composition used in step a) is dissolved in the monomer(s), present in the radiation sensitive coating composition of step a), or in the solvent used in the coating composition of step a) of the process of the present invention.

A wide variety of inorganic or organic substrates may be used as a substrate in the process according to the invention. Suitable substrates are for example flat or curved, rigid or flexible polymeric substrates, including films of for example polycarbonate, polyester, polyvinyl acetate, polyvinyl pyrollidone, polyvinyl chloride, polyimide, polyethylene naphthalate, polytetrafluoro-ethylene, nylon, polynorbornene; or amorphous solids, for example glass or crystalline materials, such as for example silicon or gallium arsenide. Metallic substrates may also be used. Preferred substrates for use in display applications are for example glass, polynorbornene, polyethersulfone, polyethyleneterephtalate, polyimide, cellulose triacetate, polycarbonate and polyethylenenaphthalate.

An initiator may be present in the first coating composition to initiate the crosslinking reaction. The amount of initiator may vary between wide ranges. A suitable amount of initiator is for example between above 0 and 5 wt% with respect to total weight of the compounds that take part in the crosslinking reaction.

When UV-crosslinking is used to initiate crosslinking, the mixture preferably comprises a UV-photo-initiator. A photo-initiator is capable of initiating a crosslinking reaction upon absorption of light; thus, UV-photo-initiators absorb light in the Ultra-Violet region of the spectrum. Any known UV-photo-initiator may be used in the process according to the invention. Preferably the polymerization initiator comprises a mixture of a photo initiator and a thermal initiator.

In step b) of the process of the present invention the coated substrate resulting form process step a) is locally treated with electromagnetic radiation having a periodic or latent radiation intensity pattering as a result of which a latent image is formed. In one preferred embodiment, this treatment is performed using UV- light in combination with a mask. In another preferred embodiment, this treatment is performed by the use of light interference/ holography. Still another embodiment is by the use of electron beam lithography. Any cross-linking method that may cause the first coating composition to polymerize and/or crosslink so that a final first coating is formed, is suitable to be used in the process according to the invention. Suitable ways to initiate crosslinking are for example electron beam radiation, electromagnetic radiation (UV, Visible and Near IR), thermally, and by adding moisture, in case moisture-curable compounds are used. In a preferred embodiment crosslinking is achieved by UV- radiation. The UV-crosslinking may take place through a free radical mechanism or by a cationic mechanism, or a combination thereof. In another preferred embodiment the crosslinking is achieved thermally.

In the present invention a second coating composition is applied on top of the first composition, wherein said second coating composition comprises either an organic compound (C₀) of a monomeric nature or said second coating composition comprises a dissolved polymer (C_p). In the case of a polymer, it is preferable to apply the polymer by a coating process using a volatile solvent. In case of the use of C₀, it may also be in the form of a monomer/polymer mixture/solution or as a monomer dissolved in a volatile solvent, said solvent being evaporated after coating. In the case that an organic compound of a monomeric nature is used; said compound is polymerized later on in the process. In both cases (C₀ and C_p) it is preferred that a bi- layer structure is formed after coating, i.e. mutal dissolution of the components of the first and second coating is preferably avoided as much as possible. In a specific embodiment of the present invention, the organic compound (C₀) of a monomeric nature reduces the interfacial tension between the photo-polymer of step and its surroundings, and C₀ is polymerized after step c). For the term "Interfacial tension", reference is given to the publication "Polymer Surfaces" from F. Garbossi et. al, Wiley 1994, pages 183-184, where a description is given how to determine the interfacial tension of a solid with air by using a Zisman-plot. In order to achieve and evaluate the benefits of the present invention, it is advisable to first determine the interfacial tension of a coated substrate (with air), obtained with all the ingredients except C₀, and compare the so-obtained value with the interfacial tension of the coated substrate obtained with all the ingredients (thus including C₀) (See Fryer et al. Macromolecules, 2001 , 34, page 5627-5634). Preferably, the C₀ reduces the interfacial tension with at least 10 mJ/m².

C₀ can be of the same nature as the monomer(s) applicable in the coating composition of step a), as described before. Preference is given to C₀ being selected from the group comprising (meth-) acrylates, epoxies, oxetanes, vinyl esters, styrenes, and thiol-enes. For specific applications it is preferred that a liquid crystalline, polymerizable monomer is used. Another preference is that C₀ or C_p also comprise an inorganic filler (as such known in the art, like talcum). Another preference is for C₀ being selected from the group of polymerizable monomers, which after polymerization produce an inorganic material. In order to be polymerized in step c), C₀ preferably also comprises one or more polymerization-initiators; preferred in the form of a thermal initiator or a mixture of a photo-initiator and a thermal initiator. Said polymerization of C₀ preferably is performed with heat, UV-light, e-beam irradiation, X-ray irradiation, ion beam irradiation, visible light irradiation, or IR-light.

C₀ may also comprise a polymer, next to the organic compound of monomeric nature. Preferably said polymer is selected from the group comprising: a) thermoplastic semi-crystalline or amorphous polymers, b) thermoplastic elastomers (TPE, TPV), and c) chemically crosslinkable rubbers.

The use of C₀ or C_p is preferably in an amount of 0.01-5 times the amount of the first coating; more preferred, said amount is in the range of 0.05-2.5. The conditions under which the process steps a)- c) have to be performed, are as such known in the art of radiation polymerization. As temperatures for said process steps preferably a temperature of between 175 and 375 K is used for step b), and preferably a temperature of between 300 and 575 K is used for step c). The conditions are of course selected such that they advocate both the polymerization of the ingredients in the first coating composition as well as, when relevant, the polymerization of C₀.

The second coating composition can also comprise a dissolved polymer (C_p), application of which results in a second coating layer. The selection of C_p is such that it adds a second functionality to the polymeric relief structure. In the process of the present invention, C_p is applied in the form of a solution of the polymer in a suitable solvent. The solvent has to be selected as such that it dissolves the polymer (C_p) to be used in the second coating composition, but on the other hand has (hardly) any influence on the ingredients of the first coating composition or on the polymer formed there from. C_p can be of the same nature as the polymer described above when used in conjunction with the use of C₀ in the second coating composition. After the application of the second coating composition as a solution of the polymer C_p, the solvent is removed, preferably by evaporation, by as such known techniques. Preferably, the conditions under which the process a)-c) are performed ensure that the generation of the relief structure is not/hardly influenced by the second layer. In practice, a preference is given to systems in which the first (photo- embossed) coating and the second layer are multually insoluble and/or hardly interact at the interface. Typically, a very hydrophobic nature is selected for C₀ or C_p if the first layer is hydrophilic, either before or after polymerization. The same applies visa versa. The polymeric relief structures of the present invention can have an improved aspect ratio as well as an improved sharpness, especially at the interface between the first and the second layer; this in case when C₀ reduces the interfacial tension of the first coating. The aspect ratio (AR, being the ratio between the relief height, and the distance between neighbouring reliefs, both in μm) of the reliefs of the invention is then in general at least 0.075, and more preferably at least 0.12; even more preferably, the AR is at least 0.2. The sharpness of the relief structure can be quantified by the maximum absolute value of the curvature k. The procedure to derive the curvature k from AFM measurements is as follows: (i) the shape of the relief structure is fitted with, for instance, a Boltzmann fit, (ii) the first and second derivative of the fit are calculated, (iii) the curvature k is calculated with : _{k =} /"(*)

^~ (i +/'W²)^3/2

The absolute maximum value for the curvature ( I k_max I ) of the relief structures according to the invention is at least 0.35 and more preferably at least 0.45 and even more preferably 0.65 μm^'1 most preferred at least 0.7 μm^'1. Both parameters (aspect ratio and sharpness) are to be determined via atomic force microscopy (AFM). The thickness of the second layer can be controlled, for instance, by the deposition conditions. Consequently, the end result might be a film with a relief structure at the air-interface which is (almost) identical to the relief structure at the interface between the first and second layer. Alternatively, the second layer can be so thick that it forms a smooth surface, despite the presence of a relief structure at the interface between the first and second layer. Of course all kinds of intermediate structures can also be generated; it's usefulness is determined by the desired functionalty/application. Of course, the enhancement of the aspect ratio and sharpness of the surface structure is reduced if a rather thick monomer or polymer layer is used in the second coating. The polymeric relief structures of the present invention are applicable in optical components, preferably in light management applications. Preferred embodiments thereof are refractive-, diffractive- or holographic-optical elements or antireflective and/or antiglare layers. Another preference of the use of the polymeric relief structure according to the present invention is the use as a structured electrode, a fluorescent layer, an electro-luminescent layer, a structured metallic, or a ceramic layer. Another, and preferred embodiment is the use of the polymeric relief structure as a master for replication purposes in organic or inorganic matter. Still another preference is the use of the polymeric relief structure of the present invention as an orientation layer for a second species. Preferably, said second species is a monomer, a polymer, a liquid crystalline monomer, a liquid crystalline polymer, a electroluminescent monomer, a electro-luminescent polymer, a polymeric alignment layer, a self assembled monolayer and/or mixtures thereof.

Claims

I . Process for the preparation of a polymeric relief structure by a) coating a substrate with a first coating composition comprising one or more radiation-sensitive ingredients, b) locally treating the coated substrate with electromagnetic radiation having a periodic or random radiation-intensity pattern, forming a latent image, c) polymerizing and/or crosslinking the resulting coated substrate to a first coating, wherein a second coating composition is applied on top of the first coating composition, said second coating composition comprising either an organic compound (C₀) of a monomeric nature and wherein C₀ is also polymerized during the process, or wherein said second coating comprises a dissolved polymer (C_p). 2. Process according to claim 1 , wherein C₀ or C_p is applied to the resulting coated substrate of step b).

3. Process according to claim 1 , wherein C₀ is already present in the coating composition used in step a).

4. Process according to anyone of claim 1-3, wherein C₀ reduces the interfacial tension of the first coating.

5. Process according to anyone of claims 1-4, wherein the radiation-sensitive ingredient(s) in step a) comprise(s) one or more monomers, in combination with one or more polymerization initiators.

6. Process according to anyone of claims 1-5, wherein the first coating composition also comprises a polymer.

7. Process according to claim 5, wherein the polymerization initiator is a mixture of a photo-initiator and a thermal initiator.

8. Process according to anyone of claims 1-7, wherein the first coating is a solid film after evaporation of the used solvent. 9. Process according to anyone of claims 1-8, wherein in step b) a lithographic mask is used in direct contact with the photo-polymer film. 10. Process according to anyone of claims 1-9, wherein the electromagnetic radiation is UV-light in combination with a mask.

I 1. Process according to anyone of claims 1-10, wherein the treatment in step b) is by the use of light interference/ holography. 12. Process according to anyone of claims 1-10, wherein the electromagnetic radiation in step b) is in the form of e-bean, ion beam, X-ray, or gamma-ray.

13. Process according to anyone of claims 1-12, wherein the substrate comprises a polymer. 14. Process according to claim 6, wherein the polymer in the coating composition of step a) has a weight averaged molecular weight ( Mw) of at least 20,000 g/mol.

15. Process according to anyone of claims 6 or 14, wherein the polymer in the coating composition of step a) has a glass transition temperature of at least 300 K.

16. Process according to anyone of claims 6, and 14-15, wherein the polymer is dissolved in the monomer(s) of the radiation-sensitive coating composition used in step a).

17. Process according to anyone of claims 1-16, wherein the ingredient(s) in the radiation-sensitive first coating composition is/are selected from the group comprising (meth-) acrylates, epoxies, oxetanes, vinyl ethers, styrenes, and thiol-enes.

18. Process according to anyone of claims 1-17, wherein C₀ reduces the interfacial tension of the first coating with at least 10 mJ/m². 19. Process according to anyone of claims 1-18, wherein C₀ or C_p is applied in an amount of from 0.05 - 5 times the amount of the first coating. 20. Process according to anyone of claims 1-19 wherein C₀ is selected from the group comprising (meth-) acrylates, epoxies, oxetanes, vinyl ethers, styrenes, and thiol-enes. 21. Process according to anyone of claims 1-19, wherein C₀ is selected from the group of polymerizable monomers, which after polymerization produce an inorganic material. 22. Process according to claim 20, wherein C₀ comprises a liquid crystalline, polymerizable monomer. 23. Process according to anyone of claims 1-22, wherein C₀ also comprises one or more polymerization-initiators.

24. Process according to claim 23, wherein the polymerization initiator is a thermal initiator or a mixture of a photo-initiator and a thermal initiator.

25. Process according to anyone of claims 1-24, wherein the polymerization of C₀ is performed with heat, UV-light, e-beam irradiation, X-ray irradiation, ion beam irradiation, visible light irradiation, or IR-light.

26. Process according to anyone of claims 1-24, wherein C₀ also, and C_p comprises a polymer selected from the group comprising: a) thermoplastic semi-crystalline or amorphous polymers, b) thermoplastic elastomers (TPE, TPV), or c) chemically crosslinkable rubbers.

27. Process according to anyone of claims 1-26, wherein C₀ or C_p also comprises an organic filler.

28. Polymeric relief structure comprising a first and second layer and a relief structure between the first and second layer, obtainable through a process according to anyone of claims 1-27.

29. Polymeric relief structure according to claim 28, wherein also a second relief structure on top of the second layer is present, said second relief structure also being obtainable through a process according to anyone of claims 1-27.

30. Polymeric relief structure comprising several layers, wherein at the interface of any layer combination and possibly also on the top a relief structure is present, each relief structure being obtainable through a process according to anyone of claims 1-27.

31. Polymeric relief structure according to anyone of claims 28-30, wherein the aspect-ratio of the relief structure of the first layer (AR) is at least 0.12, the AR being the ratio between the relief height and the distance between neighboring reliefs.

32. Polymeric relief structure according to anyone of claims 28-31 , wherein the maximum absolute value of the curvature of the relief structure of the first layer I k_maχ I is at least 0.35, more preferably at least 0.45, and even more preferably at least 0.65 μm^'1

33. Polymeric relief structure according to claim 31 , wherein AR is at least 0.2.

34. Polymeric relief structure according to claim 32, wherein I k_max I is at least 0.7 μm^'1.

35. Process according to anyone of claims 1-27, wherein step b) is performed at a temperature between 175 and 375 K.

36. Process according to anyone of claims 1-27 and 35, wherein step c) is performed at a temperature of between 300 and 575 K.

37. Use of a polymeric relief structure according to anyone of claims 28-34, or prepared in a process according to anyone of claims 1-27 or 35-36 in light- management applications. 38. Use according to claim 37 in refractive-, diffractive- or holographic-optical elements.

39. Use according to claim 37 in antireflective and/or antiglare layers.

40. Use of a polymeric relief structure according to anyone of claims 28-34 or prepared in a process according to anyone of claims 1-27 or 35-36 as a master for replication purposes in organic or inorganic matter.

41. Use of a polymeric relief structure according to anyone of claims 28-34 or prepared in a process according to anyone of claims 1-27 or 35-36 as an orientation layer for a second species. 42. Use according to claim 41 , wherein the second species is a monomeric and/or polymeric liquid crystal.

43. Use of a polymeric relief structure according to anyone of claims 28-34 or prepared in a process according to anyone of claims 1-27 or 35-36 as a structured electrode, a fluorescent layer, or a electro-luminescent layer, or a structural metallic or ceramic layer.