RU2422197C2 - Method to produce oxygen-absorbing elements of protective coating in form of microcapsules - Google Patents

Abstract

FIELD: nanotechnologies.

SUBSTANCE: invention relates to the field of nanotechnology, namely, to development of protective thin-film materials with specified properties for micro- and nano-devices: organic LED, micro- and nano-sensors, chips, photon crystals, etc. A method is proposed to produce oxygen-absorbing elements of a protective coating. The method consists in the fact that a polymer coating is applied onto a polymer shell by means of layerwise adsorption of oppositely charged polyelectrolytes, such as sodium polystyrene sulfonate and polydiallyl dimethyl ammonia chloride. Nuclei are removed from produced microcapsules by complex formation with a disodium salt of ethylenediaminetetraacetic acid. Produced microcapsules are placed into aqueous solution of fluorescent dye of tris-2,2-bipyridyl ruthenium dichloride at pH=9-10 and soaked until microcapsules turn bright-orange, same as the solution colour. Nuclei of calcium carbonate are monodisperse spherical colloid particles of calcium carbonate.

EFFECT: method provides for intensification of protective properties in thin-film coatings against penetration of atmospheric moisture and oxygen in photoactive layer of organic light-emitting diodes of micro- and nano-devices.

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Description

Technical field

The invention relates to the field of nanotechnology, namely to the development of protective thin-film materials with desired properties for micro- and nanodevices: OLED, micro- and nanosensors, chips, photonic crystals, etc.

State of the art

The problem of protecting materials from oxygen and water for many years remains relevant, requiring new scientific developments. The review below describes the modern approaches proposed for such protection, as well as compounds - oxygen carriers and desiccants, which can be used as components of protective systems.

Known oxygen-absorbing packaging of products [US 20010003235], allowing them for a long time not to deteriorate and not to change the original color. To absorb oxygen, metal iron powder is used, which in the amount of 5 wt.% Or less is included in the composition of the breathable packaging material.

To protect the material from water and moisture, coatings containing nanoparticles of silicon, titanium, aluminum, iron, calcium and magnesium oxides are used (ALGoodman, E.T. Bernard, V.N. Grassian. Spectroscopic Study of Nitric Acid and Water Adsorption on Oxide Particles: Enhanced Nitric Acid Uptake Kinetics in the Presence of Adsorbed Water. // J. Phys. Chem. A. 2001. V.105. P.6443-6457). In this work, it was shown that the treatment of γ-Fe ₂ O ₃ , CaO, and MgO particles with nitric acid, which leads to the formation of a nitrate layer, increases the adsorption of water by particles.

In the work [M. Vogt, R. Hauptmann. Plasma-deposited passivation layers for moisture and water protection. // Surface & coatings technology. 1995. V.74: 1-31-3. P.676-681] describes the protective properties of films of silicon oxide, silicon oxynitride and silicon nitride, obtained by the method of resonant electron-cyclotron plasma deposition, from water and moisture, depending on the application parameters: flow, microwave power and pressure in the reactor. Although silicon nitride is slightly better soluble in water than SiO ₂ , lower moisture penetration and high electrical stability when exposed to a humid atmosphere make it more attractive for various applications as a passivating material.

A known method of producing a multilayer film absorbing oxygen [US 6746772], which consists of the following layers:

1) the outer layer is a thermoplastic resin;

2) a layer of epoxy resin, which is a gas barrier due to the content of 30 wt.%, And preferably from 40 to 90 wt.% Units of xylylenediamine;

3) oxygen absorbing layer - a thermoplastic resin with grains of iron-based oxygen-absorbing agent included in its volume;

4) an intermediate layer of oxygen-permeable thermoplastic resin.

Epoxy resins of the described composition and corresponding to the above layers were applied to a polypropylene or nylon film using a corona discharge, after which an oxygen-absorbing multilayer film was obtained by lamination.

The multilayer film should have a total thickness of not more than 120 microns, because this is optimal for oxygen absorption, gas impermeability, strength, film stiffness, production efficiency and ease of manufacture of packaging for food, pharmaceutical and other products.

In the protective layer, built on the basis of such grains, there is a risk of “poisoning” of the layer due to the diffusion of oxides from the surface of the grains throughout the intergrain space. The direct incorporation of active absorbent components into the protective coating can lead to their deactivation, destruction of the polymer matrix or peeling of the coating from the surface of the protected material.

Disclosure of invention

The objective of the invention is the creation of elements for flexible thin-film coatings that protect micro- and nanodevices from the penetration of atmospheric moisture and oxygen into the photoactive layer of organic light-emitting diodes.

The technical result of the invention is to enhance the protective properties of coatings made from oxygen-absorbing elements obtained by the proposed method, for which it is proposed to place the active component in a nano- or microcontainer, which is proposed to use polyelectrolyte capsules. The protective capsule prevents the diffusion of oxides and significantly slows down the "poisoning" of the protective layer.

The problem is solved by the method of producing oxygen-absorbing protective coating elements, which consists in applying a polymer shell to the calcium carbonate nuclei by layer-by-layer adsorption of oppositely charged polyelectrolytes, using sodium polystyrenesulfonate and polydiallyldimethylammonium chloride, and removing the nuclei from the obtained microcapsules by dissolving them. place the microcapsules in an aqueous solution of the fluorescent dye tris-2,2-bipyridyl ruthenium dichloride at a pH of 9-10 and incubated until iobreteniya microcapsules bright orange color corresponding to the color of the solution.

In this case, monodispersed spherical colloidal particles of calcium carbonate are used as nuclei from calcium carbonate, and the dissolution of nuclei from calcium carbonate is carried out by extracting microcapsules in an aqueous solution of disodium salt of ethylenediaminetetraacetic acid (EDTA).

The problem of protecting materials from oxygen, while remaining relevant for many years, requires new scientific approaches. To reduce the negative effect of deactivation of active absorbing components, destruction of the polymer matrix or peeling of the coating from the surface of the protected material, it is proposed to place the active absorbing component in a nano- or microcontainer, which can be used as polyelectrolyte capsules. Permeable to small molecules of oxygen or water, the walls of the capsules do not let out the reaction products of the contents of the capsules with oxygen (water). Thus, neither highly active components nor the products of their reaction with oxygen or water “poison” the working layer.

Consider a specific example of the implementation of this invention. Using the method of sequential adsorption of oppositely charged polyelectrolytes on the surface of colloidal particles, microcapsules of polydiallyldimethylammonium and polystyrenesulfonate are obtained. The synthesized calcium carbonate microspherulites were used as nuclei. The inclusion of a fluorescent oxygen-sensitive dye tris-2,2-bipyridyl ruthenium in the shell of polyelectrolyte capsules and the method of fluorescence spectroscopy demonstrated the binding of such dissolved oxygen capsules to water. The developed capsules can be considered in the future as a functional element of the protective antioxidant coating. A possible technique for applying thin films that can be used to create such coatings is proposed.

Consider the methodology in more detail.

The technique for the formation of polyelectrolyte capsules consists in sequential adsorption of oppositely charged macromolecules of polyelectrolytes on the surface of colloidal particles (nuclei) of various nature. Then the core is usually removed by dissolution. Such capsules can be used as microcontainers, as well as microreactors. The size of the capsule is determined by the size of the core. The advantages of polyelectrolyte capsules over other similar systems are their monodispersity with a range of preset sizes from hundreds of nanometers to tens of microns, ease of regulation of their permeability, the possibility of a wide choice of wall material [De Geest B. G., Sanders NN, Sukhorukov GB, Demeester J., De Smedt SC // Chem. Soc. Rev. 2007. V.36 P.636].

It is proposed to use a ruthenium-based fluorescent dye as an oxygen binding compound. Due to the charge, regulated by pH, compounds of this type can be included in the composition of polyelectrolyte layers, and the fluorescent properties will allow controlling their binding to oxygen. For this purpose, a commercial fluorescent dye tris-2,2-bipyridyl dichloride ruthenium (hexahydrate) was selected, which is characterized by a stable surface charge and high solubility in water. The structural formula of ruthenium tris-2,2-bipyridyl dichloride hexahydrate is given below.

In the works of Lvov with co-authors [Chang-Yen D.A., Lvov Y., McShane M.J., Gale B.K. // Sensors and Actuators B. 2002. V.87. P.336; McShane M.J., Brown J.Q., Guice K.B., Lvov Y.M. // J. Nanoscience & Nanotechnology. 2002. V.2. P.411] this compound was included in the composition of layered polyelectrolyte films, as well as the shells of polyelectrolyte capsules formed on cores of melamine formaldehyde, in order to create a fluorescent biosensor, the functioning of which would not be interfered with by oxygen dissolved in biological and biomedical samples.

In the proposed method, colloidal particles of calcium carbonate were used as capsule nuclei. For polymer nuclei, there is a problem of removing decay products after their destruction. For example, when using melamine formaldehyde latex particles as nuclei, it was shown that such nuclei do not completely dissolve - a certain amount of melamine formaldehyde always remains inside the capsules [McShane MJ, Brown JQ, Guice KB, Lvov YM // J. Nanoscience & Nanotechnology. 2002. V.2. P.411]. In addition, for application purposes, the use of organic solvents may be undesirable. In the case of inorganic colloidal particles, no organic solvents are required for decomposition and there is no need to fear the toxic effects of decay products. The positive side is the extreme low cost of CaCO ₃ . In the decomposition of this mineral core, kinetic obstacles to the reaction induced by a complexing agent such as EDTA are also less difficult than kinetic obstacles in the case of polymers. In addition, dry calcium calcium carbonate is extremely hygroscopic, therefore capsules with undissolved or partially dissolved cores can also be used as a moisture-absorbing element of the protective coating.

Polyelectrolyte capsules containing tris-2,2-bipyridyl dichloride ruthenium were prepared, the binding of oxygen dissolved in water with such capsules was demonstrated, and a film deposition technique that could be used to create a protective anti-oxygen coating using such capsules was demonstrated.

Fabrication of CaCO ₃ cores. CaCO ₃ microspherulites were obtained by mixing solutions of calcium chloride and sodium carbonate by the reaction:

CaCl ₂ + Na ₂ CO ₃ = CaCO ₃ + 2NaCl.

The technique for producing monodisperse spherical colloidal CaCO ₃ particles is presented in [Volodkin DV, Petrov AI, Prevot M., Sukhorukov GB // Langmuir. 2004. V.20. P.3398; Volodkin DV, Larionova NI, Sukhorukov GB // Biomacromolecules. 2004. V.5. P.1962]. The amorphous precipitate of CaCO ₃ formed upon rapid mixing of CaCl ₂ and Na ₂ CO ₃ solutions, as a result of colloidal aggregation, passes into ordered micron-sized spherulites. A great influence on the dispersion of particles, their size and morphology is exerted by the process conditions: concentration of reagents, temperature, intensity of mixing of the reaction mixture, its duration. By varying these parameters, it is possible to obtain microspherulites with an average diameter of 2 to 15 μm with a fairly narrow size distribution. An equal volume of 0.33 M aqueous solution of Na ₂ CO _{3 was} quickly added to a 0.33 M aqueous solution of CaCl ₂ intensively stirred on a 0.33 M aqueous solution. Particles with an average diameter of 4.5 μm were obtained by stirring the reaction mixture at a speed of 500 rpm for 30 s. After that, the resulting suspension was kept for 10-15 minutes at room temperature without stirring.

After completion of the formation process, the obtained CaCO ₃ particles were thoroughly washed from Na ⁺ and Cl ^- ions with distilled water and dried in an oven for 1 h at 60 ° С. Thorough washing and drying are of fundamental importance, since in the wet state the spherulites gradually transform into classical rhombohedral polycrystals, and in the presence of NaCl the process is noticeably accelerated. When dry, CaCO ₃ microspherulites can be stored indefinitely.

Obtaining polyelectrolyte capsules. To create a polymer shell on CaCO ₃ nuclei, the technique of layer-by-layer adsorption of polyelectrolytes was used [Donath E., Sukhorukov GB, Caruso F. et al. // Angew. Chem. 1998. V.37 P.2202]. Sodium polystyrenesulfonate (PSS) (M _cp = 70,000) and polydiallyldimethylammonium chloride (PDADMAC) (M _cp = 275000) were used as materials for preparing the multilayer film. To 0.015 g of cores, 2 ml of a PSS solution (concentration of 2 mg / ml) in a 0.5 M NaCl solution was added. The suspension was stirred for 15 min using a mini-shaker, then centrifuged for 1 min with an acceleration of 330 g, after which the substrate was taken, and the particles were washed three times with water (precipitation by centrifugation, 330 g, 1 min). Then, the same procedure was carried out using a PDADMAC solution with a concentration of 2 mg / ml in a 0.5 M NaCl solution. Then, in the same way, layers of oppositely charged macromolecules were adsorbed alternately on colloidal particles, resulting in a polyelectrolyte shell of the composition (PSS / PDADMAX) ₄ . To prevent particle aggregation during the application of the first two layers, tubes with suspension were placed for 10 s in an ultrasonic bath (35 kHz).

Obtaining hollow polyelectrolyte shells - permeable capsules - was carried out by dissolving the nuclei. Particles of CaCO _{3 were} dissolved by adding an aqueous solution of EDTA disodium salt - calcium is removed from the capsule due to the formation of a stable complex of this metal with EDTA. For this, a 0.2 M salt solution was adjusted to pH 7.5, then it was poured twice into centrifuged capsules and kept under stirring on a mini-shaker for 20 minutes. After the nuclei were dissolved, the suspension of the capsules was washed three times with water (centrifugation, 330 g, 3 min).

Inclusion of tris-2,2-bipyridyl ruthenium in polyelectrolyte capsules. First, to encapsulate the selected compound inside the polyelectrolyte capsules, an encapsulation method using adsorption on nuclei was used. Microspherulites of calcium carbonate have a porous surface and increased adsorption ability for various kinds of compounds, therefore, one of the effective encapsulation methods was the adsorption of the selected compound by the developed surface of the nuclei. Then the core is dissolved, and the necessary compound remains inside the shell due to either the large size of the encapsulated molecules (larger than the pores of the capsule shell) or the interaction with the oppositely charged inner layer of the shell.

It was assumed that positively charged tris-2,2-bipyridyl ruthenium would be retained inside the capsule due to interaction with the negatively charged inner layer of the shell, the PSS polyanion. However, it was found that the selected dye is not adsorbed on CaCO ₃ nuclei.

Then, the inclusion of the selected compound in the composition of the polyelectrolyte capsule shells was used. In articles [Chang-Yen DA, Lvov Y., McShane MJ, Gale BK // Sensors and Actuators B. 2002. V.87. P.336; McShane MJ, Brown JQ, Guice KB, Lvov YM // J. Nanoscience & Nanotechnology. 2002. V.2. P.411], it was shown that direct adsorption of dye molecules onto a polyelectrolyte layer leads to charge neutralization instead of its inversion, as in the case of an oppositely charged polyelectrolyte. Therefore, further adsorption of the layers does not occur, and the amount of the dye thus incorporated is small. To avoid this, the dye was adsorbed by the formed hollow shell (PSS / PDADMAX) ₄ due to a change in the charge ratio at an increased pH of the subphase.

In the alkaline pH region of the MSS, the shell has a large number of uncompensated negative charges, as follows from the above literature, and has the ability to adsorb small strongly charged positive ions of tris-2,2-bipyridyl ruthenium freely penetrating into the shell.

To implement this approach, the capsules were placed in an aqueous solution of tris-2,2-bipyridyl ruthenium dichloride with a concentration of 0.16 mg / ml, the pH of the solution was adjusted to a value of 9-10 by adding a 25% solution of NH ₄ OH. The suspension was kept under stirring on a mini-shaker for 20 minutes. And left for 2-3 hours. As a result, the capsules acquired a bright orange color corresponding to the dye solution. In a fluorescence microscope, an orange glow of the obtained capsules is observed (see Fig. 1).

Figure 1 shows the fluorescence image of polyelectrolyte capsules with tris-2,2-bipyridyl ruthenium in the composition of the shell (microscope Nikon E200).

Binding of oxygen by polyelectrolyte capsules to ruthenium tris-2,2-bipyridyl. For the potential use of polyelectrolyte capsules with tris-2,2-bipyridyl ruthenium in the shell as an oxygen absorbing element of the protective coatings, the possibility of binding oxygen dissolved in water by such a system was demonstrated. It is known that the binding of oxygen by a fluorescent compound leads to quenching of fluorescence. When functionalized capsules are found in an aqueous suspension in air, they absorb oxygen dissolved in water. When passing (sparging) through such an inert gas system (usually the time of such an operation is about 15 minutes), the oxygen concentration decreases to trace values, which leads to a reversible increase in the dye fluorescence intensity. The fluorescence value of the system was measured using a Fluorolog-tau-3 fluorimeter. In a homogeneous aqueous solution, such an increase in intensity is 25%. When passing argon through a suspension of capsules with a dye, the glow intensity also increases (see figure 2), although to a lesser extent (approximately 14%).

Figure 2 shows the fluorescence spectra of an aqueous suspension of functionalized capsules (1) before purging with argon, (2) after purging with argon.

The obtained experimental result confirms the possibility of using microcapsules filled with the proposed dye, as a possible absorbing element in protective films.

The method of applying films using the obtained microcapsules

To create thin films on solid substrates from a variety of methods, the most convenient method of self-organization is self-assembling, and the method of irrigation, in particular spincotting.

The self-organization method consists of several stages - treatment of the substrate with a charged polymer, for example, by immersion in an aqueous solution of polyethyleneimine, 2-3 rinsing cycles with distilled water, then immersion in a suspension of capsules, rinsing and applying a fixing layer of polyethyleneimine.

To create a coating by irrigation, a water-soluble polymer matrix, for example sodium alginate, a copolymer of acrylic acid, etc., was introduced into the suspension of capsules, mixed until a homogeneous state was applied, a certain amount of a colloidal solution was applied to the substrate, uniformly distributed and dried. A more uniform distribution of the suspension is provided by centrifugal force - as in the classical centrifugation method.

Theoretical modeling of the barrier layer with absorption

To describe the penetration of gas into a layer with oxygen absorption, the authors developed a model that allows the calculation of two-layer protective barriers taking into account the gas permeability and absorption properties of the indicator layer.

Consider the system of protective layers shown in figure 3, which corresponds to the possible use of the previously described absorbing layer (it corresponds to the plot δ ₁ in the figure) for the protection of acid.

Figure 3 presents a graph of the distribution of concentration in a stationary mode in a three-layer system. The leftmost section corresponds to the atmosphere: n ₀ is the concentration of the gas from which the protection (analyte) occurs in the atmosphere. Section δ corresponds to the first barrier layer, in which there is no absorption. Section δ ₁ corresponds to the second protective layer in which the analyte is absorbed. Section δ ₂ corresponds to a photoactive (indicator) layer, in which there is also an analyte absorption.

An external optically transparent protective layer serves to preliminarily retain oxygen. It is characterized by very low diffusion and lack of absorption. In this case, Al ₂ O ₃ (Vitex System) is used as the material.

Within the framework of the model, for the density q ₀ of the gas flow into the photosensitive layer (OLED), the expression

Where

D is the diffusion coefficient of the analyte in the corresponding layer, δ is its thickness, h is the enthalpy per molecule of gas inside this layer, and c is the absorption coefficient of the analyte in the region δ ₁ . It can be shown that

where σ is the solubility of the analyte in the layer, p _a is the standard atmospheric pressure.

It means that

where P is the gas permeability of the corresponding layer.

Moreover, if

those. the analyte is completely absorbed in the polymer layer and the analyte does not reach the indicator layer. In this case, the flow into the indicator layer vanishes. Based on this condition, it is possible to calculate the absorption, which should ensure the complete absorption of the analyte:

Below is a calculation of the layer developed by Vitex System [http://www.vitexsys.com/new/barix.htm]. Inorganic layers there are obtained by sputtering a thin (several nm) layer A1 onto an organic layer. There are no reliable data on the diffusion coefficient of oxygen in A1. This is due to the fact that the diffusion and solubility of oxygen largely depends on the number of defects in the metal, its degree of purity, etc., therefore, a special measurement must be made for each technology for preparing the barrier layer. However, gas permeability assessment can be done on the basis of data provided by the Vitex System, namely, the diffusion coefficient D = 10 ^-20 cm ² / s. The solubility of oxygen can be estimated as 0.1 atm ^-1 according to the data of [Deshman S. Scientific principles of vacuum technology. M.: Mir, 1964]. Then P = 10 ^-25 m ² / (s · atm). (In this case, the gas permeability of the organic polymer approximately corresponds to PEN (P = 0.1 cm ⁴ / (m ² · day · atm), 10 ^-14 m ² / (s · atm)).

Thus, for the ratio of gas permeability of the organic and inorganic layers, we obtain a value of the order of 10 ¹¹ . With this in mind, formula (6) can be rewritten in the form

The density of the flow absorbed by the polymer layer, in this case, is

м/с, что отвечает типичным требованиям к проникновению кислорода в ОСИД: эта величина не должна превышать W=10^-5 см³/(м²·сут) или, в системе СИ, как раз 10^-16 м/с.In this case, the rate of analyte delivery through an inorganic optically transparent layer 1 nm thick is

m / s, which meets the typical requirements for the penetration of oxygen into acid: this value should not exceed W = 10 ^-5 cm ³ / (m ² · day) or, in the SI system, just 10 ^-16 m / s.

For a typical operating time of a layer of 10,000 h, 1 cm ^{2 of the} coating will absorb about 5 · 10 ^-7 cm ^{3 of} analyte (or 10 ^-11 mol). If the absorbing microcapsules with a size of the order of 1 μm are located with a maximum density, then each of them will have about 10 ^-17 mol or about 10 ⁷ analyte molecules, which corresponds to the absorbing capacity.

Claims (2)

1. A method of producing oxygen-absorbing protective coating elements, which consists in applying a polymer shell to the calcium carbonate nuclei by adsorbing layer-by-layer adsorption of oppositely charged polyelectrolytes, using sodium polystyrenesulfonate and polydiallyldimethylammonium chloride, removing the nuclei from the obtained microcapsules by complexation with disodium diamide disodium ethoxide acids, place the microcapsules in an aqueous solution of the fluorescent dye tris-2,2-bipyridyl ruthenium dichloride at pH 9-10 and maintained until acquisition microcapsules bright orange color corresponding to the color of the solution. 2. The method according to claim 1, characterized in that monodispersed spherical colloidal particles of calcium carbonate are used as nuclei from calcium carbonate.

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