Constructed materials

Abstract

The present invention relates to a structured material, particularly for optical applications, and to a method for producing the same, in which a composition comprising a photoinitiator and an azo compound is cured with the formation of gas bubbles. The method can also be carried out in a multi-step process involving irradiation and heating.

Description

[Technical field]

[0001] The present invention relates to the production of structured materials, and more particularly optical materials having fine bubbles.

[0002] Nanoporous materials have always attracted scientific attention for various applications, such as low-cost lightweight materials, membranes, thermal or heat insulators, etc. Nanoporous materials can also prevent crack propagation and reduce the refractive index of polymeric materials.

[0003] The strategy to produce nanoporous materials is seemingly simple: (i) introduction of ultrafine bubbles (nanobubbles) into a monomer or oligomer solution, (ii) stabilization of existing bubbles, and (iii) polymerization and structural fixation while the bubbles retain their size and shape. However, none of these steps are easy to implement. For a single bubble in a solution, the internal pressure of the bubble, i.e., the Laplace pressure, increases significantly as the bubble size decreases (Equation 1: Pressure difference of a spherical bubble in a liquid). △P=2 / r·γ (Equation 1) In the equation, ΔP is the pressure difference between the inside and outside of a spherical bubble with radius r, and γ is the surface tension.

[0004] Even if the solution is previously supersaturated, even a slight disturbance will result in a positive or negative loop, destabilizing the bubbles. This is the so-called Laplace pressure bubble catastrophe (LPBC), which means that small bubbles, more specifically ultrafine bubbles, can never be thermodynamically stable. For this reason, the existence and stability of ultrafine bubbles is still under debate and has not been unambiguously resolved. Furthermore, the fixation of bubbles during the polymerization process is an additional obstacle compared to other processes with nanoparticles. It is difficult to keep bubbles stable, since polymerization necessarily involves successive phase transitions from liquid to solid, associated volume shrinkage, and changes in surface tension.

[0005] Thus, gas from the bubble diffuses into the surrounding medium and the positive loop causes the bubble to continue shrinking until it disappears.

[0006] Therefore, most of the current research on the production of nanoporous materials is focused on template-based techniques such as block copolymers (BCP) (Non-Patent Document 1, Non-Patent Document 2), aerogels (Non-Patent Document 3, Non-Patent Document 4) and high internal phase emulsion polymerization (HIPE) (Non-Patent Document 5, Non-Patent Document 6, Non-Patent Document 7), where the unwanted parts are later removed to leave a porous structure. Although these techniques offer clear possibilities for the production of ordered nanostructures, the main drawback is the long process times of assembly, precipitation, and drying. Extraction of sacrificial components by heating or specific chemicals, such as organic solvents, is generally necessary.

[0007] An alternative template-free technique for producing porous materials is the thermoplastic foaming method, which uses physical or chemical blowing agents. Supercritical CO2 is one of the most popular physical blowing agents. The foaming process is carried out by rapid pressure reduction in an autoclave, or by blowing the material over the glass transition temperature (T g) into the tank. Due to the absorbency under different external pressures or different external temperatures, the supersaturated CO2 in the polymer matrix is ​​physically desorbed and starts to generate a bubble structure. However, it has been shown that the average diameter of expanded polystyrene obtained by rapid release of 380 bar pressure at 80 °C is in the range of 2 μm to 3 μm (Non-Patent Document 8). The size of the bubbles is too large to scatter visible light, so the sample becomes opaque. Furthermore, releasing the pressure of 380 bar in a short time is very dangerous. Two-stage foaming or mixing of the polymer with materials with different CO2 affinity allows further reduction in size to the sub-micro range (Non-Patent Document 9, Non-Patent Document 10, Non-Patent Document 11). In that case, when the pressure is released from the outside, a mass fraction of CO2 absorption of about 30% is required, which corresponds to almost 180 times the volume of the polymer matrix at STP (Non-Patent Document 12, Non-Patent Document 13). So far, there have been no reports on practical techniques for generating ultrafine bubbles. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] NA Lynd, AJ Meuler and MA Hillmyer, Progress in Polymer Science, 2008, 33, 875-893 [Non-Patent Document 2] DA Olson, L. Chen and MA Hillmyer, Chemistry of Materials, 2008, 20, 869-890 [Non-Patent Document 3] ND Hegde and A. Venkateswara Rao, Applied Surface Science, 2006, 253, 1566-1572 [Non-Patent Document 4] T. Shimizu, K. Kanamori, A. Maeno, H. Kaji, CM Doherty, P. Falcaro and K. Nakanishi, Chemistry of Materials, 2016, 28, 6860-6868

Non-licensed Document 5

Non-licensed Document 6

Non-licensed Document 7

Non-licensed literature 9

Non-licensed literature 10

Non-licensed Document 11

[0009] The object of the present invention is to identify a method which makes it possible to generate fine gas bubbles in a polymer, a material containing such gas bubbles, and its uses.

[0010] This object is achieved by the invention having the features of the independent claims. Advantageous developments of the invention are characterized in the dependent claims. The text of all claims is hereby made part of this description by reference. The invention also encompasses all reasonable combinations of the independent and / or dependent claims, in particular all specified combinations.

[0011] The individual method steps are described in detail below: The steps do not necessarily have to be performed in the specified order, and the methods described may include additional steps that are not mentioned. [Means for solving the problem]

[0012] The object of the present invention is to provide a method for producing a structured material, comprising the steps of: a) a1) at least one monomer having at least one group suitable for non-condensative chain polymerization or polycondensation, a2) at least one initiator for the non-condensative chain polymerization or polycondensation of monomers; a3) at least one azo compound; providing a curable composition comprising: b) curing the composition, including at least one irradiation, to form a cellular structured material; This is achieved by a method comprising:

[0013] Surprisingly, it has now been found that the combination of initiator and azo compound allows precise control of the size and formation of bubbles.Therefore, for example, it is possible to control the moment of decomposition of azo compound, and therefore the release of nitrogen that generates bubbles during the curing process.This is especially true when azo compound decomposes relatively slowly or does not decompose at all under the activation condition of initiator.

[0014] The provided compositions may be curable by any desired curing mechanism, for example, physical curing, thermal curing, chemical curing or radiation curable, preferably the compositions are chemically curable or radiation curable, more preferably radiation curable.

[0015] The least preferred physical cure refers to the simple evaporation of the solvent.

[0016] In the present invention, the composition is preferably a radiation-curable composition that is cured by radiation.

[0017] In this specification, radiation curing refers to, for example, (N)IR light in the wavelength range of λ = 700 nm to 1200 nm, preferably 700 nm to 900 nm, and / or UV light in the wavelength range of λ = 200 nm to 700 nm, preferably λ = 200 nm to 500 nm, more preferably λ = 250 nm to 400 nm, and / or an electron beam in the range of 150 keV to 300 keV, more preferably at least 80 mJ / cm 2 , preferably 80 mJ / cm 2 ~3000mJ / cm 2This means that radical polymerization of the polymerizable compound is initiated as a result of electromagnetic radiation and / or particle radiation of the electron beam at a radiation dose of 100 nm to 150 nm.

[0018] In the case of curing by (N)IR and / or UV light, it should be ensured in this case that a photoinitiator is present in the composition as initiator, which photoinitiator can be decomposed by the light of the irradiating wavelength to give radicals that can thereby start a radical polymerization.

[0019] In the case of curing by electron beam, on the other hand, the presence of such a photoinitiator is not necessary.

[0020] This means that radiation-curable monomers are present in the composition, which are preferably monomers for acrylate, methacrylate and / or unsaturated polyester resins.

[0021] Unsaturated polyester resins are known per se to those skilled in the art.

[0022] The radiation curable monomers are preferably monomers for methacrylate, acrylate, hydroxy, polyester, polyether, carbonate, epoxy or urethane (meth)acrylates, and also (meth)acrylated polyacrylates, optionally partially amine modified.

[0023] These monomers include at least one, for example 1 to 4, preferably 1 to 3, more preferably 1 or 2, very preferably exactly 1 radiation-curable monomer having at least one radically polymerizable group.

[0024] Polyester (meth)acrylates are the corresponding esters of α,β-ethylenically unsaturated carboxylic acids, preferably (meth)acrylic acid, more preferably acrylic acid, with polyester polyols.

[0025] Polyether (meth)acrylates are the corresponding esters of α,β-ethylenically unsaturated carboxylic acids, preferably (meth)acrylic acid, more preferably acrylic acid, with polyetherols.

[0026] The polyetherols are polyethylene glycols having a molar mass of 106 to 2000, preferably 106 to 1500, more preferably 106 to 1000, poly-1,2-propanediols having a molar mass of 134 to 1178, poly-1,3-propanediols having a molar mass of 134 to 1178, and polyetherols having a number average molecular weight M in the range of about 500 to 4000, preferably 600 to 3000, more particularly 750 to 2000. n It is preferred that the copolymer comprises polytetrahydrofuran diol having the formula:

[0027] Urethane (meth)acrylates can be obtained, for example, by reaction of polyisocyanates with hydroxyalkyl (meth)acrylates and, optionally, chain extenders such as diols, polyols, diamines, polyamines or dithiols or polythiols. Urethane (meth)acrylates that are dispersible in water without the addition of emulsifiers further contain ionic and / or non-ionic hydrophilic groups that are introduced into the urethane by synthesis components such as, for example, hydroxycarboxylic acids.

[0028] Epoxy (meth)acrylates can be obtained by reacting epoxides with (meth)acrylic acid. Examples of possible epoxides include epoxidized olefins, aromatic or aliphatic glycidyl ethers, preferably aromatic or aliphatic glycidyl ethers.

[0029] The epoxidized olefin may be, for example, ethylene oxide, propylene oxide, isobutylene oxide, 1-butene oxide, 2-butene oxide, vinyloxirane, styrene oxide or epichlorohydrin, preferably ethylene oxide, propylene oxide, isobutylene oxide, vinyloxirane, styrene oxide or epichlorohydrin, more preferably ethylene oxide, propylene oxide or epichlorohydrin, and very preferably ethylene oxide and epichlorohydrin.

[0030] Aromatic glycidyl ethers are, for example, bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, bisphenol B diglycidyl ether, bisphenol S diglycidyl ether, hydroquinone diglycidyl ether, alkylation products of phenol / dicyclopentadiene, such as 2,5-bis[(2,3-epoxypropoxy)phenyl]octahydro-4,7-methano-5H-indene) (CAS number [13446-85-0]), tris[4-(2,3-epoxypropoxy)phenyl]methane isomer) (CAS number [66072-39-7]), phenol-based epoxy novolacs (CAS number [9003-35-4]) and cresol-based epoxy novolacs (CAS number [37382-79-9]).

[0031] Aliphatic glycidyl ethers are, for example, 1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether, trimethylolpropane triglycidyl ether, pentaerythritol tetraglycidyl ether, 1,1,2,2-tetrakis[4-(2,3-epoxypropoxy)phenyl]ethane (CAS number [27043-37-4]), diglycidyl ether of polypropylene glycol (α,ω-bis(2,3-epoxypropoxy)poly(oxypropylene) CAS number [16096-30-3]), and diglycidyl ether of hydrogenated bisphenol A (2,2-bis[4-(2,3-epoxypropoxy)cyclohexyl]propane, CAS number [13410-58-7]).

[0032] The epoxy (meth)acrylates preferably have a number average molar weight M of 200 g / mol to 20000 g / mol, more preferably 200 g / mol to 10000 g / mol, very preferably 250 g / mol to 3000 g / mol. n The (meth)acrylic group content is preferably 1 to 5, more preferably 2 to 4, per 1000 g of epoxy (meth)acrylate (measured by gel permeation chromatography using polystyrene as a standard and tetrahydrofuran as an eluent).

[0033] (Meth)acrylated polyacrylates are the corresponding esters of α,β-ethylenically unsaturated carboxylic acids, preferably (meth)acrylic acid, more preferably acrylic acid, with polyacrylate polyols, obtainable by esterification of the polyacrylate polyols with (meth)acrylic acid.

[0034] Carbonate (meth)acrylates are similarly available with a variety of functional groups.

[0035] Number average molecular weight M of carbonate (meth)acrylate n is preferably less than 3000 g / mol, more preferably less than 1500 g / mol, more preferably less than 800 g / mol (measured by gel permeation chromatography using polystyrene as standard, tetrahydrofuran solvent).

[0036] Carbonate (meth)acrylates can be obtained in a simple manner by transesterification of carbonic acid esters with polyhydric, preferably dihydric, alcohols (diols, e.g. hexanediol) and subsequent esterification of the free OH groups with (meth)acrylic acid or else by transesterification with (meth)acrylic acid esters, as described, for example, in EP-A-0 092 269. They can also be obtained by reaction of phosgene, urea derivatives with polyhydric, e.g. dihydric, alcohols.

[0037] Also contemplated are the (meth)acrylates of polycarbonate polyols, such as the reaction products of one of the abovementioned diols or polyols with carbonate esters, and also (meth)acrylates containing hydroxyl groups.

[0038] Suitable carbonates are, for example, ethylene, 1,2- or 1,3-propylene carbonate, dimethyl, diethyl or dibutyl carbonate.

[0039] Suitable (meth)acrylates containing hydroxyl groups are, for example, 2-hydroxyethyl (meth)acrylate, 2- or 3-hydroxypropyl (meth)acrylate, 1,4-butanediol mono(meth)acrylate, neopentyl glycol mono(meth)acrylate, glyceryl mono- and di(meth)acrylate, trimethylolpropane mono- and di(meth)acrylate, and pentaerythritol mono-, di- and tri(meth)acrylate.

[0040] Particularly preferred monomers are methacrylates, acrylates, and their modified and unmodified esters. Examples include, for example, methacrylic acid esters or acrylic acid esters. Examples of (meth)acrylates containing hydroxyl groups include, for example, 2-hydroxyethyl (meth)acrylate, 2- or 3-hydroxypropyl (meth)acrylate, 1,4-butanediol mono(meth)acrylate, neopentyl glycol mono(meth)acrylate, glyceryl mono- and di(meth)acrylate, trimethylolpropane mono- and di(meth)acrylate, and pentaerythritol mono-, di-, and tri(meth)acrylate.

[0041] The composition comprises at least one initiator, preferably a photoinitiator, more particularly a UV photoinitiator, for the non-condensative chain polymerization or polycondensation of monomers.

[0042] The UV photoinitiator may be, for example, a photoinitiator known to the person skilled in the art, for example those described in "Advances in Polymer Science", Volume 14, Springer Berlin 1974 or KK Dietliker, Chemistry and Technology of UV- and EB-Formulation for Coatings, Inks and Paints, Volume 3, Photoinitiators for Free Radical and Cationic Polymerization, PKT Oldring (Ed.), SITA Technology Ltd, London.

[0043] Possible examples include phosphine oxides, benzophenones, α-hydroxyalkylaryl ketones, thioxanthones, anthraquinones, acetophenones, benzoins and benzoin ethers, ketals, imidazoles or phenylglyoxylic acids and mixtures thereof. Preferred photoinitiators are those that do not generate any gaseous components such as nitrogen on decomposition.

[0044] Phosphine oxides are, for example, mono- or bisacylphosphine oxides, as described, for example, in EP-A-7508, EP-A-57474, DE-A-19618720, EP-A-0495751 or EP-A-0615980, such as, for example, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, ethyl 2,4,6-trimethylbenzoylphenylphosphineate or bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.

[0045] Benzophenone is, for example, benzophenone, 4-aminobenzophenone, 4,4'-bis(dimethylamino)benzophenone, 4-phenylbenzophenone, 4-chlorobenzophenone, Michler's ketone, o-methoxybenzophenone, 2,4,6-trimethylbenzophenone, 4-methylbenzophenone, 2,4-dimethylbenzophenone, 4-isopropylbenzophenone, 2-chlorobenzophenone, 2,2'-dichlorobenzophenone, 4-methoxybenzophenone, 4-propoxybenzophenone or 4-butoxybenzophenone, α- Hydroxyalkylaryl ketones are, for example, 1-benzoylcyclohexan-1-ol (1-hydroxycyclohexyl phenyl ketone), 2-hydroxy-2,2-dimethylacetophenone (2-hydroxy-2-methyl-1-phenylpropan-1-one), 1-hydroxyacetophenone, 1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one or polymers containing 2-hydroxy-2-methyl-1-(4-isopropen-2-ylphenyl)propan-1-one in copolymerized form.

[0046] Xanthones and thioxanthones are, for example, 10-thioxanthenone, thioxanthen-9-one, xanthen-9-one, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, 2,4-diisopropylthioxanthone, 2,4-dichlorothioxanthone or chloroxanthenone.

[0047] Anthraquinone is, for example, β-methylanthraquinone, tert-butylanthraquinone, anthraquinone carboxylic acid ester, benz[de]anthracen-7-one, benz[a]anthracene-7,12-dione, 2-methylanthraquinone, 2-ethylanthraquinone, 2-tert-butylanthraquinone, 1-chloroanthraquinone or 2-amylanthraquinone.

[0048] Examples of acetophenone include acetophenone, acetonaphthoquinone, valerophenone, hexanophenone, α-phenylbutyrophenone, p-morpholinopropiophenone, dibenzosuberone, 4-morpholinobenzophenone, p-diacetylbenzene, 4'-methoxyacetophenone, α-tetralone, 9-acetylphenanthrene, 2-acetylphenanthrene, 3-acetylphenanthrene, 3-acetylindole, 9-fluorenone, 1-indanone, 1,3,4-triacetylbenzene, 1- Acetonaphthone, 2-acetonaphthone, 2,2-dimethoxy-2-phenylacetophenone, 2,2-diethoxy-2-phenylacetophenone, 1,1-dichloroacetophenone, 1-hydroxyacetophenone, 2,2-diethoxyacetophenone, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one, 2,2-dimethoxy-1,2-diphenylethan-2-one or 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butan-1-one.

[0049] Benzoins and benzoin ethers are, for example, 4-morpholino-deoxybenzoin, benzoin, benzoin isobutyl ether, benzoin tetrahydropyranyl ether, benzoin methyl ether, benzoin ethyl ether, benzoin butyl ether, benzoin isopropyl ether or 7H-benzoin methyl ether.

[0050] The ketal is, for example, acetophenone dimethyl ketal, 2,2-diethoxyacetophenone, or a benzil ketal, for example benzyl dimethyl ketal.

[0051] Phenylglyoxylic acid is described, for example, in DE 19826712 A1, DE 19913353 A1 or WO 98 / 33761.

[0052] Further photoinitiators which can be used are, for example, benzaldehyde, methyl ethyl ketone, 1-naphthaldehyde, triphenylphosphine, tri-o-tolylphosphine or 2,3-butanedione.

[0053] Typical mixtures include, for example, 2-hydroxy-2-methyl-1-phenylpropan-2-one and 1-hydroxycyclohexyl phenyl ketone, bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide and 2-hydroxy-2-methyl-1-phenylpropan-1-one, benzophenone and 1-hydroxycyclohexyl phenyl ketone, bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide and 1-hydroxycyclohexyl phenyl ketone, 2,4,6-trimethylbenzoyldiphenylphosphine oxide and 2-hydroxy-2-methyl-1-phenylpropan-1-one, 2,4,6-trimethylbenzophenone and 4-methylbenzophenone, or 2,4,6-trimethylbenzophenone and 4-methylbenzophenone and 2,4,6-trimethylbenzoyldiphenylphosphine oxide.

[0054] Preferred among these photoinitiators are 2,4,6-trimethyl-benzoyldiphenylphosphine oxide, ethyl 2,4,6-trimethylbenzoyl-phenylphosphinate, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, benzophenone, 1-benzoylcyclohexan-1-ol, 2-hydroxy-2,2-dimethylacetophenone and 2,2-dimethoxy-2-phenylacetophenone.

[0055] The composition further comprises at least one azo compound. This is a compound that releases nitrogen when activated. Preferably, it is a radical photoinitiator based on azonitrile. Depending on their structure, these compounds show little absorption in the UV range and little decomposition under UV irradiation. In particular, therefore, they are not affected by the activation of the initiator, more specifically the photoinitiator. However, it is also possible to be activated by correspondingly strong UV irradiation.

[0056] Examples of such azo compounds include azobisisobutyronitrile (2,2'-azobis(2-methylpropioninitrile) (AIBN), azobiscyanovaleric acid (ACVA), 2,2'-azobis(2,4-dimethyl)valeronitrile (ABVN), 2,2'-azobis(2-methylbutyronitrile) (AMBN), 1,1'-azobis(cyclohexane-1-carbonitrile) (ACCN), 1-((cyano-1-methylethyl)azo)formamide (CABN), 2,2'-azobis(2-methylpropionamido) dihydrochloride (MBA), dimethyl-2,2'-azobis(2-methylpropionate) (AIBME), 2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride (MBA), dimethyl-2,2'-azobis(2-methylpropionate) (AIBME), dimethyl-2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride (MBA ... Examples of suitable azonitrile-based compounds include azobis(2,4-dimethylpentanenitrile) dihydrochloride (AIBI), 2,2'-azobis(2,4-dimethylpentanenitrile), 1,1'-azobis(cyclohexanecarbonitrile) (ACHN), 2,2'-azobis(2-methylpropaneimidamide) dihydrochloride, 2,2'-azobis(2-acetoxypropane;2-tert-butylazo)isobutyronitrile, 2(tert-butylazo)-2-methylbutanenitrile, 1-(tert-butylazo)cyclohexanecarbonitrile, or a mixture of two or more of these compounds. Particularly preferred are compounds based on azonitriles, more particularly AIBN and ABVN. Of these, ABVN is more reactive and is particularly suitable for decomposition by irradiation.

[0057] In particular, upon heating up to 120° C., and more particularly up to 100° C., decomposition begins with the release of nitrogen.

[0058] When using an azo compound that also decomposes under UV irradiation, this decomposition preferably begins at a different UV wavelength than the UV photoinitiator used.The photoinitiator preferably requires a longer wavelength for activation than the azo compound.For example, the photoinitiator can be activated by irradiation with wavelengths greater than 400 nm, while the azo compound only decomposes at wavelengths less than 400 nm, preferably less than 380 nm.

[0059] Thus, one preferred composition according to the invention comprises at least one radical UV photoinitiator and at least one azonitrile-based radical initiator.

[0060] In one preferred embodiment, the composition further comprises at least one surface active agent, more particularly at least one stabilizer, more particularly a surfactant.

[0061] Specific examples of surfactants are non-ionic surfactants, for example polyoxyethylene alkyl ethers such as polyoxyethylene lauryl ether, polyoxyethylene stearyl ether, polyoxyethylene cetyl ether and polyoxyethylene oleyl ether, polyoxyethylene alkyl aryl ethers such as polyoxyethylene octylphenol ether, polyoxyethylene alkylaryl ethers such as polyoxyethylene octylphenyl ether and polyoxyethylene nonylphenyl ether, polyoxyethylene or polyoxypropylene block copolymers, sorbitan fatty acid esters such as sorbitan monolaurate, sorbitan monopalmitate, sorbitan monostearate, sorbitan monooleate, sorbitan trioleate and sorbitan tristearate, and also polyoxyethylene-sorbitan fatty acid esters such as polyoxyethylene sorbitan monolaurate, polyoxyethylene-sorbitan monopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan trioleate and polyoxyethylene sorbitan tristearate; fluorosurfactants, for example Megaface. F-171, F-173, R-08, R-30, R-40, F-553 and F-554 (trade names, products of DIC Corporation), Fluorad FC-430, FC-4430 and FC-431 (trade names, products of 3M Advanced Materials Division), AsahiGuard AG710 and Surflon S-382, SC101, SC102, SC102, SC103, SC104, SC105 and SC106 (trade names, products of AGC Seimi Chemical Co. Ltd.); and organosiloxane polymers such as BYK-302, BYK-307, BYK-322, BYK-323, BYK-330, BYK-333, BYK-370, BYK-375 and BYK-378 (trade names, products of BYK-Chemie Wesel products), more specifically based on polyether-modified dimethylpolysiloxane.

[0062] The at least one surfactant is preferably used in an amount of 0% to 2% by weight, more particularly 0% to 1% by weight, especially 0% to 0.5% by weight, based on the total composition.

[0063] Additionally, the compositions that can be used in the method of the present invention may further contain from 0% to 10% by weight of further additives.

[0064] Further additives used may be activators, fillers, pigments, dyes, thickeners, thixotropic agents, viscosity modifiers, plasticizers or chelating agents.

[0065] Preferably, all of the components are soluble in each other and form a homogeneous composition.

[0066] Additionally, the composition may further comprise a solvent, but is preferably solvent-free.

[0067] In a preferred embodiment, the composition comprises from 0.01% to 5% by weight, preferably from 0.01% to 1% by weight, more particularly from 0.01% to 0.5% by weight of at least one initiator, relative to the total composition.

[0068] In a preferred embodiment, the composition comprises from 0.01% to 20% by weight, preferably from 1% to 10% by weight, more particularly from 2% to 8% by weight of at least one azo compound, relative to the total composition.

[0069] In a preferred embodiment, the composition comprises, relative to the total composition, 0.01% to 1% by weight of at least one initiator and 0.01% to 20% by weight, preferably 1% to 10% by weight, more particularly 2% to 8% by weight, relative to the total composition, of at least one azo compound.

[0070] In one embodiment of the present invention, the composition comprises 50% to 99% by weight of at least one monomer, 0.01% to 5% by weight of at least one initiator, 0.01% to 20% by weight of at least one azo compound, and 0% to 2% by weight of at least one surfactant.

[0071] In one embodiment of the invention, the composition comprises 50% to 99% by weight of at least one monomer, 0.01% to 1% by weight of at least one initiator, 1% to 10% by weight of at least one azo compound, and 0% to 1% by weight of at least one surfactant.

[0072] In a further embodiment of the invention, the amount and nature of the azo compound is used such that the volume of maximum releasable nitrogen to the volume of the composition (at STP) is not more than 50:1, more particularly not more than 20:1, especially not more than 10:1.

[0073] Providing the composition preferably involves applying the composition to a surface as a coating or introducing it into a mould.

[0074] The composition is then cured to form the structured material, which includes at least one irradiation to form the structured material containing the bubbles.

[0075] Drying may be carried out, if necessary, below the decomposition temperature of the components of the composition.

[0076] The composition can be cured under an oxygen-containing atmosphere or under an inert gas.

[0077] Radiation curing is carried out with high energy light, for example (N)IR, VIS or UV light, or with an electron beam, preferably UV light.

[0078] Suitable radiation sources for radiation curing are, for example, low and medium pressure mercury emitters, high pressure emitters, and also fluorescent tubes, pulsed emitters, metal halide emitters, LEDs or excimer emitters. Radiation curing is carried out by exposure to high energy radiation, i.e. UV radiation or sunlight, preferably light in the wavelength range λ=200 nm to 700 nm, more preferably λ=200 nm to 500 nm, very preferably λ=250 nm to 400 nm. Examples of radiation sources used are high pressure mercury vapor lamps, lasers, pulsed lamps (flashlights) or halogen lamps or excimer emitters. The dose is preferably 80 mJ / cm 2 2 ~3000mJ / cm 2 , preferably 2700 mJ / cm 2 It is.

[0079] Of course, it is also possible to use several, for example two to four, radiation sources for curing.

[0080] Each of these may emit in a different wavelength range.

[0081] For example, irradiation can be carried out in multiple steps at different wavelengths to selectively control the decomposition of the azo compound. Thus, for example, it is conceivable to initiate only or mainly the photoinitiator in a first step, and thus the polymerization of at least one monomer, by irradiation with different wavelengths, and to initiate the decomposition of only the azo compound in a second step.

[0082] Irradiation can also be carried out in the absence of oxygen, for example under an inert gas atmosphere. Suitable inert gases are preferably nitrogen, noble gases, carbon dioxide, or combustion gases. Furthermore, irradiation can also be carried out using a coating material covered with a transparent medium. The transparent medium is, for example, a polymer film or glass.

[0083] In that case, the transparency of the coating to the radiation used must be matched to the photoinitiator used.

[0084] Thus, for example, PET is transparent to radiation having wavelengths less than 300 nm. Examples of photoinitiators that may generate radicals under such radiation include 2,4,6-trimethylbenzoyldiphenylphosphine oxide, ethyl 2,4,6-trimethylbenzoylphenylphosphinate, and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide.

[0085] In a particularly preferred embodiment, curing is performed by at least one of the following methods: 1. Complete polymerization of the composition followed by decomposition of the azo compound by heating; 2. Partial polymerization of the composition by excitation of the initiator and subsequent decomposition of the azo compound by heating with simultaneous irradiation; 3. Complete polymerization of the composition and decomposition of the azo compound by single- or multi-step irradiation.

[0086] The advantage of the two-step process is that it allows better optimization of the process to obtain a defined bubble density or size, especially since the bubble formation can be observed over the temperature and time of heating. This is especially true for simultaneous heating and irradiation. As a result, when the nitrogen is released, the material is not yet fully polymerized.

[0087] During irradiation, it is also possible to change the intensity and / or wavelength of the irradiation in several stages, for example to initiate decomposition of the azo compound only after the initial partial polymerization. This is particularly preferred when no heating is performed as in technique 3. Thus, for example, in a first step, polymerization can be initiated by irradiation with a first wavelength. Decomposition of the azo compound is then performed by irradiation with a second wavelength. This second irradiation can also be performed in addition to the first irradiation. The intensity of the irradiation in the wavelength range of the azo compound can also be lower than that of the photoinitiator. At least multiple stages, more specifically two stages of irradiation, are preferred.

[0088] The process of curing, during which the azo compound decomposes, is continued until bubbles of the desired size are present.

[0089] Depending on the respective method, the constituents of the composition, monomers, photoinitiators and azo compounds, and preferably stabilizers, if present, can likewise be adapted accordingly.

[0090] As a result of at least partial polymerization in advance, excessive expansion of bubbles or leakage of nitrogen from the composition is prevented. At the same time, violent foaming is avoided, and instead fine bubbles are obtained. Thus, the polymerization of the monomer increases the viscosity of the composition, which reduces the bubble growth caused by the azo compound.

[0091] The addition of a surfactant also reduces the surface tension of the composition, which promotes the formation of air bubbles and also reduces the diameter of the air bubbles.

[0092] In one preferred embodiment of the invention, the composition is heated to less than 120° C., more particularly less than 100° C., more particularly less than 70° C., so that it is possible to apply the structured coating according to the invention even to temperature-sensitive substrates.

[0093] The heating time is preferably less than 1 minute, more particularly less than 40 seconds.

[0094] The irradiation time is preferably at most 10 minutes, preferably at most 5 minutes. This applies to single-stage and multi-stage methods. In the case of simultaneous heating, the irradiation can last as long as the heating.

[0095] The bubbles that are formed have a refractive index of approximately 1 and may therefore help reduce scattering.

[0096] Curing is preferably carried out until bubbles are obtained with an average diameter of less than 1 μm, more particularly less than 500 nm, more particularly less than 300 nm, which can be determined by determining the average diameter of at least 40 bubbles by SEM in a cross-section of the material.

[0097] In particular, transparent structured materials can be obtained, especially if small air bubbles are embedded.

[0098] As a result of complete polymerization of the composition, the air bubbles are fixed and the structures present within the material remain stable.

[0099] The present invention therefore relates to a structured material produced by the method of the present invention.

[0100] Such materials comprise a polymer matrix containing a large number of closed cavities (air bubbles) with a diameter of less than 1 μm, more particularly less than 500 nm, in particular less than 300 nm. These cavities are preferably distributed within the material. This size preferably applies to at least 50%, more particularly at least 60%, in particular at least 80% of the cavities. These details preferably apply to the largest diameter of the cavities. The determination is preferably made by SEM, based on 100 cavities in a cross-section of the material, with the cavities evaluated in the cross-section being not more than 20 μm apart from each other.

[0101] In a preferred embodiment of the invention, the structured material is optically transparent. The presence of bubbles can be verified, inter alia, by scattering of laser light.

[0102] The materials of the present invention can be used in many, particularly optical, applications, particularly where an optically transparent material with scattering properties is required.

[0103] The material may be applied as a coating to a smooth or structured surface or it may itself be structured, for example by impression.

[0104] The structured material of the present invention is particularly suitable for producing or coating optical elements.These optical elements are particularly suitable for holographic applications, light management foils, diffusers, planar gradient index lenses in imaging optics, head-up displays, head-down displays, optical waveguides, especially in optical communication and transmission technology, and optical data stores.Examples of optical elements that can be produced include security holograms, image holograms, digital holograms for information storage, systems with components for processing optical wavefronts, planar waveguides, beam splitters, and lenses.

[0105] Thus, the material of the invention can be produced, for example, on embossed optical structures. The bubbles in this case are generated with such fineness that the material remains optically transparent. The presence of the bubbles can then be verified by scattering of laser light. The internal structure of the material cannot be replicated, and therefore the properties themselves cannot be replicated by normal means.

[0106] Further details and features are evident from the following description of preferred exemplary embodiments in conjunction with the dependent claims, in which case the respective features can be realized on their own or in combination with one another in any combination. The possibilities for achieving the objectives are not limited to the exemplary embodiments.

[0107] Thus, for example, the specification of a range always includes all unmentioned intermediate values ​​and all possible subintervals.

[0108] Exemplary embodiments are illustrated diagrammatically in the figures, in which identical reference numbers in the different figures indicate identical or functionally identical elements or which correspond to one another with regard to their functionality. [Brief description of the drawings]

[0109] [Figure 1] FIG. 13 represents a PHEMA film after heating at 120° C. for 1 hour (a) without AIBN (b) with AIBN). [Diagram 2] Figure 14 depicts micrographs (in situ) of UV-cured PHEMA film during subsequent heating (a) after 2 min at 100° C., (b) after 3 min at 100° C., (d) after 30 s at 110° C., (e) after 40 s at 110° C., (g) after 15 s at 120° C., (h) after 20 s at 120° C. Figure 15 depicts SEM micrographs of cross-sections of foamed PHEMA film after (c) 100° C., 3.5 min, (f) 110° C., 1 min, (i) 120° C., 30 s, where the sample turned white. [Diagram 3] Figure 13 represents SEM micrographs of cross-sections of foamed PHEMA films (a) with 0.4 wt% AIBN and BYK378 at 110 °C for 45 s; (HEMA (10 g), AIBN (0.32 g), Irgacure 819 (0.02 g) + BYK378 (0.4 wt%)), (b) enlarged middle detail of a), (c) 0.4 wt% ABVN and BYK378 at 110 °C for 10 s, (d) 30 s; detail shows enlarged detail). All films were prepolymerized with UV light and then cured on a hotplate with the formation of nanobubbles. [Figure 4] Figure 13 represents SEM micrographs of PHEMA film (HEMA (10 g), ABVN (0.65 g), Irgacure819 (0.03 g) + BYK378 (0.4 wt%)) with nanobubbles prepolymerized by UV light heating on a hotplate at 110 °C for 10 seconds ((a) low magnification, b) high magnification of center of a); c), d), e) magnifications of surface, middle and bottom marked in a). [Diagram 5]Figure 13 represents SEM micrographs of PHEMA film (HEMA (10 g), ABVN (0.65 g), Irgacure819 (0.03 g) + BYK378 (0.4 wt%)) with prepolymerized nanobubbles by UV light heating on a hotplate at 70 °C for 30 seconds in combination with UV light (a) low magnification, b) high magnification of the center of a); c), d), e) magnifications of the surface, middle and bottom marked in a). [Figure 6] FIG. 1 depicts a schematic diagram of an area with a security hologram, embossed structures 110, and nanobubbles 120 between two glass plates 100. [Figure 7] (a) A transparent embossed security marking (blue circle) and (b) an image of the diffraction pattern of the security marking on a black surface with a red laser pointer; SEM micrograph of the film: c) the interface between the embossed structure (embossed) and the porous region (nanobubbles); d) an enlarged detail of (c) (red rectangle); d), e) an enlarged view of the nanobubbles and the embossed region of (c). [Figure 8] (a) PLA-glass fiberglass with ultrafine bubbles (white spots), (b) light extraction through the bubbles, (c) diagram of a control sample without bubbles; SEM micrograph: (d) cross-section of a coated fiber with bubbles (arrows indicate the location of enlarged micrographs e), f) and g). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0110] Experiments and Materials 2-HEMA (2-hydroxyethyl methacrylate, 98%) and AIBN (2,2'-azobis(2-methylpropionitrile), 98%) were purchased from Sigma Aldrich. The UV initiator IRGACURE819 was purchased from Ciba Spezialitaetenchemie AG. V-65 (2,2'-azobis(2,4-dimethylvaleronitrile)) was purchased from FUJIFILM Wako Chemicals, Europe GmbH. BYK-378 surfactant was purchased from BYK (BYK Additives and Instruments, Germany). All materials were used without further purification.

[0111] Two different UV lamps (M405LP1-C5, THORLABS and Thermo-Oriel (1000W). Intensity: UV-A 20800mW / cm 2 , UV-B 18200mW / cm 2 , UVC 2894mW / cm 2 , total 86300mW / cm 2 ) was used to initiate the polymerization and the process after bubble formation, respectively.

[0112] General manufacturing method for PHEMA film with bubbles A mixture of HEMA monomer (10 g), AIBN (0.32 g) and Irgacure 819 (0.02 g) was stirred at room temperature for 1 h. Using a 200 μm masking tape as a spacer, the mixture was introduced between two glass substrates (one of them treated with a non-stick silane treatment) and then irradiated with a UV lamp (wavelength 405 nm) for 5 min. After UV irradiation, the non-stick glass was removed. For bubble generation, the film on the glass substrate was transferred to a hot plate at different temperatures above the transition temperature (Tg) of PHEMA and cooled to room temperature. Various foaming conditions, i.e., temperature and time, were experimentally determined when the film became white (opaque). During foaming, the sample on the hot plate was placed under an optical microscope to measure the nucleation and growth of bubbles in situ. An additional film was produced as a reference, without AIBN, containing only the monomer and Irgacure.

[0113] Generation of ultrafine bubbles Foaming from fully cured HEMA BYK378 surfactant (0.4 wt%) was added to the mixture. The foaming on the hot plate was closely monitored until the film just started to turn white (opaque). Additionally, AIBN was replaced with ABVN.

[0114] Foaming from partially cured HEMA The mixture was partially but not fully cured under a UV lamp for 2 minutes. The thick, partially cured HEMA was then transferred to a hotplate at 70° C., i.e., below the Tg of PHEMA, and irradiated together with UV radiation (1000 W) for an additional 2 minutes.

[0115] analysis The pre-UV cured PHMEA films remained transparent in both cases, both with and without AIBN, as shown in Figure 1(a). The polymerization was mainly induced by the photoinitiator Irgacure 819 and UV radiation at a wavelength of 405 nm, so the AIBN, which has its main absorption maximum at 350 nm, remained mostly unreacted.

[0116] However, if the film is then heated above the decomposition temperature of AIBN and the glass transition temperature of PHEMA, it was possible to use only AIBN as the chemical blowing agent and supply nitrogen gas for the formation of bubbles. Thus, as bubbles begin to grow in the PHEMA film, the film becomes opaque (white), as shown in Figure 1(b). Conversely, without AIBN, the film remains transparent after heating. Other studies have reported the possibility of using azo initiators as CBAs (chemical blowing agents) (M. Pradny, M. Slouf, L. Martinova and J. Michalek, e-Polymers, 2010, 10, 1 Article number 043 (ISSN 1618-7229); L.-Z. Guo, X.-J. Wang, Y.-F. Zhang and X.-Y. Wang, Journal of Applied Polymer Science, 2014, 131, 40238. DOI: 10.1002 / app.40238).

[0117] However, in contrast to the two-step method, the pyrolysis starts from a low-viscosity liquid monomer solution. Therefore, the pores were elongated vertically due to the driving effect, or the pore sizes were in the range of 50 μm to 100 μm. It must be noted that the amount of azo initiator not only affects the amount of nitrogen gas produced, but also the free radicals that change the polymerization rate. This is the main drawback of using CBA in previous reports.

[0118] An additional advantage of the two-stage foaming is that after pre-curing and subsequent heating of the PHEMA film containing AIBN, the nucleation and growth of bubbles can be observed under an optical microscope. Figure 2 shows how nucleation begins and the nuclei grow at different temperatures. AIBN can be pyrolyzed more quickly at high temperature (120°C), so a larger number of nuclei were formed in a short time compared to other films heated at lower temperatures (100°C). Interestingly, it was observed that the growth of existing bubbles and the nucleation of others occur simultaneously. At a certain point, the film becomes opaque due to light scattering caused by the bubbles (and the film is stretched vertically if heating continues for longer than 1 hour, because the shape of the bubbles changes from spherical to elliptical). The size and density of the bubbles could be controlled with the foaming temperature and foaming time, but it was not possible to obtain ultrafine bubbles.

[0119] 0.32 g of AIBN (0.002 mol) was added to 10 g of HEMA monomer. According to the ideal gas law, 1 mole of nitrogen gas at STP (standard temperature and pressure, 0 °C and 1 atm) occupies 22.4 L, so 0.002 mol of AIBN corresponds to 44.8 mL of N2 (0.002 mol x 22.4 L / mol = 44.8 mL), assuming all AIBN decomposes during heating. At 100 °C, the volume can further increase up to 61 mL (44.8 x (1 + 100 / 273) = 61.21 mL). The volumes of the HEMA monomer and PHEMA polymer are 9.35 mL each (density: 1.07 g / cm). 3 ) and 8.70mL (density: 1.15g / cm 3 ). Therefore, the volume ratio of nitrogen gas to PHEMA film is about 7:1. Considering supercritical CO2 foaming, where the volume ratio of CO2 to PMMA is up to 180:1, the generation of ultrafine bubbles by conventional chemical foaming is virtually unattainable without further modification.

[0120] Figure 3(a) shows the effect of surfactants on the size of bubbles. To reduce the surface tension and free energy required to obtain an interface between the bubbles and the surrounding matrix, Byk378 was added to the solution (10 g HEMA monomer, 0.32 g AIBN, 0.02 g Irgacure819, 0.4 wt% BYK378). By carefully controlling the foaming temperature and foaming time, it was possible to obtain only ultrafine bubbles. The bubbles were clearly verified using SEM, as shown in Figure 3(b). AIBN was then replaced by ABVN. Reportedly, ABVN produces nitrogen gas at least three times faster than AIBN. Thus, it was possible to significantly reduce the foaming time from 45 to 10 seconds, while at the same time increasing the density of the bubbles, as shown in Figure 3(c). However, if the bubble formation continues for longer than 20 seconds, the bubbles start to age, similar to the previous results. Despite bubble aging, ultrafine bubbles remained among the microbubbles (Figure 3(d)).

[0121] The importance of the heating time is also evident from Figures 4 and 5. At 100°C, heating times longer than 20 seconds yielded only microbubbles.

[0122] When combined with UV light and heating to only 70°C, bubble formation ceased after 20 seconds (Figure 5). Only nanobubbles were obtained.

[0123] Optical Device Manufacturing Embossed holographic security mark PHEMA with embossed structures was produced using a commercially available stamping foil as a master. A mixture of HEMA and Irgacure 819 was used to replicate the structure from the master foil and was fully cured by UV radiation. A further mixture of HEMA, Irgacure 819, BYK 378 and ABVN was introduced into the embossed structure and placed between two slides. The foaming process was carried out as previously optimized, i.e., 2 min of UV radiation (405 nm), followed by 1 min of a combination of thermal heating at 70 °C and powerful UV radiation (1000 W). Additionally, samples were tested to determine whether specific diffraction patterns were observed for laser passing through the structured areas. The microstructure and distribution of nanobubbles were characterized using SEM.

[0124] Light extraction scattering points in optical waveguides A PLA (polylactic acid) optical waveguide with a diameter of 400 μm was used. The tip of the PLA wire was manually dip-coated at certain points with a mixture of HEMA, Irgacure 819, BYK 378 and ABVN. The dip-coated PLA wire was transferred to a N2 flow chamber and held horizontally. The coated area was directly irradiated with UV radiation (1000 W) while the wire was continuously rotated at room temperature. The extraction efficiency was verified in qualitative terms by coupling a green laser into the fiber, demonstrating the scattering effect. The samples were characterized with respect to bubble size and distribution using SEM.

[0125] analysis The security hologram is shown diagrammatically in Figure 6. In the holographic security mark, the coating of the security marking containing ultrafine bubbles in the embossed structure was transparent, and the object behind the coating was easily visible, as shown in Figure 7(a). Furthermore, the linear structure of the master embossed structure was difficult to recognize with the naked eye. In appearance, the coating appears to be uniform and transparent. However, when red laser light passed through the sample, a line-shaped diffraction pattern appeared on the screen, as shown in Figure 7(b). From the SEM micrograph, it is clear that the microstructure and contrast of the two regions are different. In the upper part of the porous PHEMA layer, ultrafine bubbles were generated in the 50nm-100nm region, as shown in Figures 7(c)-7(f), while no bubbles were observed in the lower dense region. The refractive index of each dense and porous individual PHEMA layer was 1.51 or 1.44. This kind of "invisible security marking" could show different diffraction patterns due to the change in structure. Furthermore, since the information comes from the "internal microstructure", it is not possible to reproduce this information with a camera or a copier. Compared to previous techniques for producing porous materials, the method of the present invention is a very cost-effective technique.

[0126] Moreover, the ultrafine bubbles in the PHEMA film act efficiently as scattering points, as shown in Figure 8. In the case of the green laser demonstration, there was a noticeable difference between the fiber with nanobubbles and the fiber without the outcoupling coating. This phenomenon could be explained by the SEM images of the porous PHEMA coating. The amount of scattered light could be further tailored by changing the thickness of the outcoupling coating or by changing the size of the bubbles, which would result in a tailored light-extracting fiber system.

[0127] Sample characterization The bubbles were characterized using optical microscopy (Nikon, Eclipse LV100ND) and scanning electron microscopy (SEM, FEI, Quanta400f). For SEM measurements, the surface was sputtered with gold at 20 mA for 60 s (JEOL, JFC-1300, Autofine Coater). The refractive index of the film was measured by ellipsometry (EC-400, JA Woollam Co. Inc.). The bubble sizes in the OM and SEM images were analyzed using the ImageJ program.

[0128] It is expected that this new approach can be used for broad-spectrum UV-curable polymer systems such as PMMA, which may open new doors to various areas of materials science. Finally, this work will also provide more insight into the thermodynamic debate on the existence and stability of metastable or isolated ultrafine bubbles in polymers. In particular, the clear verification of the generation of ultrafine bubbles by successive processes such as aging and matrix expansion provides comprehensive information for the verification of individual foaming processes.

[0129] A new technique for generating micro- and ultrafine bubbles in transparent PHEMA using azo initiators is described. It is concluded that both the reduction of the surface tension of the matrix and the increase of the degree of supersaturation are crucial factors for the generation of ultrafine bubbles. It was possible to show that the foaming process can be carried out under slightly different conditions, e.g. (a) thermal heating only, (b) a combination of thermal heating and UV radiation, and (c) UV radiation only at room temperature.

[0130] References NA Lynd, AJ Meuler and MA Hillmyer, Progress in Polymer Science, 2008, 33, 875-893. DA Olson, L. Chen and MA Hillmyer, Chemistry of Materials, 2008, 20, 869-890. N. D. Hegde and A. Venkateswara Rao, Applied Surface Science, 2006, 253, 1566-1572. T. Shimizu, K. Kanamori, A. Maeno, H. Kaji, C. M. Doherty, P. Falcaro and K. Nakanishi, Chemistry of Materials, 2016, 28, 6860-6868.M. Paljevac, K. Jerabek and P. Krajnc, Journal of Polymers and the Environment, 2012, 20, 1095-1102. S. Kovacic, D. Stefanec and P. Krajnc, Macromolecules, 2007, 40, 8056-8060; R. Butler, I. Hopkinson and A. I. Cooper, Journal of the American Chemical Society, 2003, 125, 14473-14481. I. Tsivintzelis, A. G. Angelopoulou and C. Panayiotou, Polymer, 2007, 48, 5928-5939. J. Pinto, J. Reglero Ruiz, M. Dumon and M. Rodriguez-Perez, Journal of Supercritical Fluids The, 2014, 94. J. Pinto, M. Dumon, M. Pedros, J. Reglero Ruiz and M. Rodriguez-Perez, Chemical Engineering Journal, 2014, 243, 428-435.J. Pinto, M. Dumon, MA Rodriguez-Perez, R. Garcia and C. Dietz, The Journal of Physical Chemistry C, 2014, 118, 4656-4663.H. Guo and V. Kumar, Polymers, 2014, 57; H. Guo, A. Nicolae and V. Kumar, Polymer, 2015, 70, 231–241.M. Pradny, M. Slouf, L. Martinova and J. Michalek, e-Polymers, 2010, 10, 1. L.-Z. Guo, X.-J. Wang, Y.-F. Zhang and X.-Y. Wang, Journal of Applied Polymer Science, 2014, 13, 131, 40238. DOI: 10.1002 / app.40238.

[0131] That is, the present invention provides a method for producing a structured material as follows. Section 1. 1. A method of making a structured material, comprising: a) a1) at least one monomer having at least one group suitable for non-condensative chain polymerization or polycondensation, a2) at least one initiator for the non-condensative chain polymerization or polycondensation of the monomers; a3) at least one azo compound; providing a curable composition comprising: b) curing the composition, including at least one irradiation, to form a structured material containing gas bubbles; A method comprising: Section 2. Item 1. The method according to item 1, wherein the initiator a2) is a photoinitiator. Section 3. 3. The method according to item 1 or 2, wherein the composition is a radiation-curable composition. Section 4. 4. The method according to any one of items 1 to 3, wherein the azo compound is an azonitrile-based radical photoinitiator. Section 5. 5. The method according to any one of items 2 to 4, wherein the photoinitiator is a UV photoinitiator. Section 6. 6. The method according to any of items 1 to 5, characterized in that the composition further comprises at least one surface-active agent, preferably at least one surfactant. Section 7. The curing is performed by one of the following methods: 1. Complete polymerization of the composition followed by decomposition of the azo compound by heating; 2. Partial polymerization of the composition by excitation of the initiator and subsequent decomposition of the azo compound by heating with simultaneous irradiation; 3. Complete polymerization of the composition and decomposition of the azo compound by single or multiple step irradiation; 7. The method according to any one of items 1 to 6, characterized in that it is carried out by: Section 8. A structured material obtained by the method according to any one of items 1 to 7. Section 9. Item 9. The structured material of item 8, comprising a polymer matrix comprising a multiplicity of closed cavities having a diameter of less than 1 μm. Item 10. Use of the structured material according to item 8 or 9 for optical applications.

Claims

1. 1. A method of making a structured material, comprising: a) a1) at least one monomer having at least one group suitable for non-condensative chain polymerization or polycondensation; a2) at least one initiator for the non-condensative chain polymerization or polycondensation of the monomers; a3) at least one azo compound; and providing a curable composition comprising: b) curing the composition, including at least one irradiation, to form a structured material containing gas bubbles; A method comprising:

2. 2. The method according to claim 1, characterized in that the initiator a2) is a photoinitiator.

3. 3. The method of claim 1 or 2, characterized in that the composition is a radiation curable composition.

4. 4. The method according to claim 1, wherein the azo compound is an azonitrile-based radical photoinitiator.

5. The method according to any one of claims 2 to 4, characterized in that the photoinitiator is a UV photoinitiator.

6. The method according to any one of claims 1 to 5, characterized in that the composition further comprises at least one surfactant.

7. The method according to claim 1, wherein the composition further comprises at least one surfactant.

8. The curing is performed by one of the following methods:

1. Complete polymerization of the composition followed by decomposition of the azo compound by heating; 2. Partial polymerization of the composition by excitation of the initiator and subsequent decomposition of the azo compound by heating with simultaneous irradiation; 3. Complete polymerization of the composition and decomposition of the azo compound by single or multiple irradiation steps; The method according to any one of claims 1 to 7, characterized in that it is carried out by

9. A structured material obtained by the method according to any one of claims 1 to 8.

10. 10. The structured material of claim 9 comprising a polymer matrix containing a multiplicity of closed cavities having diameters less than 1 μm.

11. 11. Use of a structured material according to claim 9 or 10 for optical applications.

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