Patent Application
20120082831
Antireflection coatings are important for all lens based imaging systems but some substrates, wavelengths and environments present greater challenges than others. The simplest design for an optical antireflection coating for a given substrate and a narrow band of wavelengths centered on λ calls for a single thin film whose refractive index n=√(substrate index) with thickness d such that nd=λ/4. When the substrate index is relatively low however, such as for polymethylmethacrylate (PMMA) with index=1.49, it is difficult to identify lower index materials with a target index n=√1.49=1.22, especially if such materials must also be robust and stable in a space environment. The coating material itself must be transparent at the wavelengths of use, which relatively easy to achieve in the near-IR or visible but challenging in the UV spectrum. Finally it must lend itself to a well controlled deposition process capable of very uniform, conformal coatings over large areas. Very few stable materials have such low refractive indices. One approach is to incorporate air into the film (porosity) but this must be done in such a way as to achieve a stable, solid and very smooth film on a nm scale.
This problem confronts space applications that require the use of large plastic Fresnel lenses (2.5 m) for orbiting cosmic ray telescopes which gather light in the 300-400 nm UV band from Cerenkov radiation in the atmosphere. Previous attempts at coating PMMA with AR films resulted in poor adhesion. Commercial processes for plastic optics, such as the use of Teflon coatings for ophthalmic lenses, are not ideal for PMMA since the Teflon index, n=1.31, is too high to provide a single layer solution (the best possible AR coating would have reflectance not less than 0.5%). In addition to the optical efficiency, NASA desires that the coating on large PMMA optics should not require vacuum deposition, is sufficiently conformal to provide uniform coverage of angled wedges comprising the Fresnel structure, has excellent adhesion and environmental stability in space, is low in film stress so as not to distort the optic, and is capable of multi-layer designs. Finally, low scattering is important—which for a film thickness of only 61 nm for a quarterwave film means smooth and defect free surfaces on the scale of 1-2 nm.
There is a need to provide antireflection coatings for some substrates, wavelengths and environments.
There is also a need for methods for depositing multiple layers of nanoporous coatings for systems that implement those methods.
Methods for depositing multiple layers of nanoporous coatings and systems that implement those methods are disclosed hereinbelow.
In one embodiment of the methods of these teachings, a method for depositing successive layers in order to produce nanoporous multilayer coatings on a substrate includes depositing on the substrate a polyelectrolyte solution and a nanoparticle solution, repeating the deposition for each successive layer, rinsing the deposited layers, and drying the rinsed deposited layers.
In one instance the method also includes preparing the substrate by dissolving a predetermined block copolymer in a predetermined solvent; the dissolving resulting in a block copolymer solution, immersing the thermoplastic substrate in the block copolymer solution for a predetermined soaking time, the predetermined soaking time being selected such that a monolayer of block copolymer is formed on a surface of the thermoplastic substrate, annealing the thermoplastic surface with the block copolymer monolayer at a predetermined annealing temperature for a predetermined annealing time, the predetermined annealing temperature and annealing time being selected such that block copolymer moeties are integrated into the surface and negatively charged moieties are located on the surface.
In one embodiment, the system of these teachings apparatus for producing nanoporous multilayer coatings on a substrate includes at least one atomizing mist delivery component receiving a liquid and a gas, the liquid comprising a polyelectrolyte solution and/or a nanoparticle solution when used in a coating operation and comprising a rinsing solution when used in a cleaning operation. The atomizing mist delivery component delivers a coating mist when used in a coating operation, a rinsing solution when used in a cleaning operation and a pressurized gas when used in a drying operation. The embodiment also includes at least one thermoplastic substrate disposed to receive fluid from the one or more atomizing mist delivery components.
In one instance, the thermoplastic substrate has at least one monolayer of block copolymer formed on a surface of the thermoplastic substrate, block copolymer moeties integrated into the surface and negatively charged moieties located on the surface.
In one instance, an anti-reflection coating is obtained by practicing the above described embodiment of the method of these teachings.
Other embodiments of the method and of the system of these teachings are also disclosed.
For a better understanding of the present teachings, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims.
a-3d are graphical diagrammatic representations of structure of components of the coating of these teachings;
The following detailed description is of the best currently contemplated modes of carrying out these teachings. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of these teachings, since the scope of these teachings is best defined by the appended claims.
As used herein, the singular forms “a,” “an,” and “the” include the plural reference unless the context clearly dictates otherwise.
Except where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not to be considered as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding conventions.
Additionally, the recitation of numerical ranges within this specification is considered to be a disclosure of all numerical values and ranges within that range. For example, if a range is from about 1 to about 50, it is deemed to include, for example, 1, 7, 34, 46.1, 23.7, or any other value or range within the range. However, any ranges not cited in the claims are only intended for illustration and not for limiting the range covered by our invention. Numerical values are cited for exemplary reasons and to identify embodiments and are not intended to limit the general concept of these teachings.
Methods for depositing multiple layers of nanoporous coatings and systems that implement those methods are disclosed hereinbelow.
As described in JERI' ANN HILLER, JONAS D. MENDELSOHN AND MICHAEL F. RUBNER “Reversibly erasable nanoporous anti-reflection coatings from polyelectrolyte multilayers” Nature Materials, VOL 1, SEPTEMBER 2002, 59-63, and in US Patent Application Publication No. 20030215626, Nanoporous Coatings, both of which are Incorporated by reference herein in their entirety and for all purposes, nanoporous thin film coatings comprised of nanoparticles of glass can be engineered to be used as UV AR coatings. These coatings are created by using a layer-by-layer process involving the alternate adsorption of negatively charged nanoparticles and a positively charged polymer. The assembly conditions can be optimized to produce a thin film coating that is loaded with a high density of nanopores. Since the nanopores lower the refractive index of the coating, it functions as a high performance anti-reflection coating; when applied to glass, the transmission of light increases from about 92% to as high as 99.8%. However, the nano structured coatings had only been previously applied using lab-based layer-by-layer self-assembly methods, which have low yield and limited capability for large areas, and their AR capability had only been demonstrated in the visible wavelength range.
Whether by the method disclosed in US Patent Application Publication No. 20030215626 or by the method of these teachings, the coating process can be substantially optimized through tuning the size of nanoparticles, type of polyelectrolyte, pH and concentration of the polyelectrolyte and nano-particle solutions.
One example of a substantially optimized coding, not a limitation of these teachings, is provided in Table 1 below.
In one embodiment, the method of these teachings for preparing a thermoplastic substrate for layer by layer depositions, the substrate being prepared in order to improve adhesion of the deposited layers to the substrate, includes preparing the substrate by: dissolving a predetermined block copolymer in a predetermined solvent; the dissolving resulting in a block copolymer solution, immersing the thermoplastic substrate in the block copolymer solution for a predetermined soaking time, the predetermined soaking time being selected such that a monolayer of block copolymer is formed on a surface of the thermoplastic substrate, annealing the thermoplastic surface with the block copolymer monolayer at a predetermined annealing temperature for a predetermined annealing time, the predetermined annealing temperature and annealing time being selected such that block copolymer moeties are integrated into the surface and negatively charged moieties are located on the surface.
One exemplary embodiment, not a limitation of these teachings, of the surface modification process is shown in
In another instance, an adhesion layer is inserted between the coating and substrate to improve robustness. The surface modification of substrate is intended to introduce amine groups on the surface. These amine groups will chemically react with the coating materials such as PAA, leading to chemical bonds formation between the substrate and coating layer. The interaction of the interface is shown in
In one embodiment of the methods of these teachings, a method for depositing successive layers in order to produce nanoporous multilayer coatings on a substrate includes depositing on the substrate a polyelectrolyte solution and a nanoparticle solution, repeating the deposition for each successive layer, rinsing the deposited layers, and drying the rinsed deposited layers.
In one instance the method also includes preparing the substrate by dissolving a predetermined block copolymer in a predetermined solvent; the dissolving resulting in a block copolymer solution, immersing the thermoplastic substrate in the block copolymer solution for a predetermined soaking time, the predetermined soaking time being selected such that a monolayer of block copolymer is formed on a surface of the thermoplastic substrate, annealing the thermoplastic surface with the block copolymer monolayer at a predetermined annealing temperature for a predetermined annealing time, the predetermined annealing temperature and annealing time being selected such that block copolymer moeties are integrated into the surface and negatively charged moieties are located on the surface.
The rinsing step substantially cleans the deposited layers and may be performed after each deposition. After the rinsing step, there may be rinsing liquid residues left on the substrate. These residues can dilute concentrations of nanoparticle or polymer solutions to be applied subsequently, or change the solution chemistry. These changes could change the absorption of nanoparticles or polymers. In one embodiment, the drying step is realized by blowing the surface using a high pressure carry gas. If one pass of blowing does not dry the surface completely, additional passes of blowing will be needed. The dying step removes some or all of the residual rinsing liquid, producing a more reproducible and reliable coating.
In one instance, depositing the polyelectrolyte solution and the nanoparticle solution includes alternate deposition of negatively charged nanoparticles and positively charged polyelectrolyte.
In one instance, the deposition is substantially optimized by selecting the size of nanoparticles (the selected size being referred to as a predetermined size), selecting pH and concentration of the nanoparticle solution, selecting a polyelectrolyte and selecting pH and concentration of the polyelectrolyte solution, where the optimization steps can be taken in conjunction or separately or in groups.
A variety of selections of nanoparticles, sizes and polyelectrolytes are within the scope of these teachings. For example, the selections provided in Table I are within the scope of these teachings. Similarly, charged (+SiO2) and negatively charged (−SiO2) nano-particles, and positively and negatively charged poly-electrolytes (PAH, and poly (acrylic acid) (PAA)), whose molecular structures are shown in
In one instance, the thermoplastic substrate is Poly(methyl methacrylate) (PMMA). In another instance, the block copolymer used in preparing the substrate (also referred to as the predetermined block copolymer) is poly(methylmethacrylate(-b-acrylic acid (PMMA-b-PAA).
In one embodiment, the system of these teachings apparatus for producing nanoporous multilayer coatings on a substrate includes at least one atomizing mist delivery component receiving a liquid and a gas, the liquid comprising a polyelectrolyte solution and/or a nanoparticle solution when used in a coating operation and comprising a rinsing solution when used in a cleaning operation. The atomizing mist delivery component delivers a coating mist when used in a coating operation, a rinsing solution when used in a cleaning operation and a pressurized gas when used in a drying operation. The embodiment also includes at least one thermoplastic substrate disposed to receive fluid from the one or more atomizing mist delivery components. The at least one atomizing mist delivery component and the thermoplastic substrate are displaceable with respect to each other.
In one instance, the thermoplastic substrate has at least one monolayer of block copolymer formed on a surface of the thermoplastic substrate, block copolymer moeties integrated into the surface and negatively charged moieties located on the surface.
One exemplary embodiment, not a limitation of these teachings, of the system of these teachings is shown in
Referring to
Referring to
The method and systems of these teachings can be used to produce anti-reflective coatings. Using thinner coatings (about 60-70 nm), the transmittance maximum of PAH/SiO2 based nanoporous coatings is shifted to the UV range.
In order to better illustrate the present teachings, the following results from exemplary embodiments are presented hereinbelow. These teachings are not limited only to this exemplary embodiments.
Table 2 shows the tested sample sizes and applied coating solutions, coating methods. Most of PMMA samples were coated by using an impact ultrasonic nozzle system (wide and fan-shaped spray pattern). The flow rate was almost 20 ml/min and syringe dispenser system was used in case of impact nozzle. The nozzle moving speed was 7 cm/sec. For 1×3″ PMMA sheet coating, 5 times and 10 times passed of nozzle were tested and, for 3×3″ samples, nozzle path was programmed as “2D scan” and 5 time passes as shown in
In order to test the efficiency the coating system shown in
In order to compare with traditional dipping coating method, sample 1 was prepared by others and other samples were fabricated by using the automated spray coating system of
In reflectance spectra test, coating uniformity also was excellent; it was similar result with transmission as shown in
UV-Vis spectra were obtained for the wiping test results. The wiping test was conducted at same spot on one side of the sample. The spray coating sample seems relatively stable up to 10 wiping tests thus, wiping test up to 80 circles to check out the limitation was carried out. As a result, around 94% of original transmission was maintained and reflectance was increased almost two times under the 80 times wiping test at 350 nm.
Using the system of
For the purposes of describing and defining the present teachings, it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.
Although the invention has been described with respect to various embodiments, it should be realized these teachings are also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims. What is claimed is:
This application claims priority of U.S. Provisional Patent Application 61/344,774, Nano-Porous Coatings and Making Methods, filed on Oct. 4, 2010, which is incorporated by reference herein in its entirety for all purposes.
This invention was made partially with U.S. Government support from the N.A.S.A. under contract number NNM08AA03C. The U.S. Government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61344774 | Oct 2010 | US |
