Patent Description:
Zinc sulfide is a material used in a variety of optical elements. For some applications, the performance of zinc sulfide optical elements can be improved by coating or treating the zinc sulfide with organic polymers. However, due to the fundamental chemical differences between the ionic salt-like structure of the zinc sulfide and the non-polar nature of the polymers, the organic polymers poorly adhere to the zinc sulfide surfaces. These differences in the chemical nature of the two materials can result in poor wetting of the zinc sulfide, which can lead to poor adhesion.

In other systems, adhesion promoters or primers can be used to couple the organic polymers to the inorganic surfaces. For example, in glasses, silanes can be used as such a promoter/primer.

<NPL>, demonstrates a colour distinctive, ratiometric pH sensor using pH responsive and fluorescent (PyMMP-b-P2VP) deblock copolymer coated CdSe/ZnS QDs. <CIT> describes a high-refractive index material that includes semiconductor nanocrystal compositions. <CIT> describes a polymeric organic coating for electroluminescent lamp components. <CIT> describes an optical device comprising an optical surface transmissive in the infrared wavelength range and a polymer coating.

According to the invention as defined in claim <NUM>, a method for making an optical element is provided, the optical element comprising: a zinc sulfide layer; a polymer adhered to a surface of the zinc sulfide layer;.

Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings.

Disclosed herein are coupling agents to promote polymer adhesion to zinc sulfide. In one embodiment, an article of manufacture includes a zinc sulfide layer and a coupling agent adhered to the zinc sulfide layer by a disulfide bond. The coupling agent comprises a functional group extending from the zinc sulfide layer. In another embodiment, an article of manufacture includes a zinc sulfide layer and a polymer adhered to a surface of the zinc sulfide layer. A coupling agent forms a disulfide bond and is disposed between the zinc sulfide layer and the organic polymer. Yet, in another embodiment, a method of making an article of manufacture includes attaching an organic polymer to a zinc sulfide surface. A coupling agent extends from the organic polymer and forms a disulfide bond with the zinc sulfide surface.

The articles of manufacture are optical elements, wherein the optical elements are lenses, optical windows, sensors, detectors, domes, and beam splitters.

As used herein, the articles "a" and "an" preceding an element or component are intended to be nonrestrictive regarding the number of instances (i.e. occurrences) of the element or component. Therefore, "a" or "an" should be read to include one or at least one, and the singular word form of the element or component also includes the plural unless the number is obviously meant to be singular.

As used herein, the terms "invention" or "present invention" are non-limiting terms and not intended to refer to any single aspect of the particular invention but encompass all possible aspects as described in the specification and the claims.

As used herein, the term "about" modifying the quantity of an ingredient, component, or reactant employed refers to variation in the numerical quantity that can occur, for example, through typical measuring and liquid handling procedures used for making concentrates or solutions. Furthermore, variation can occur from inadvertent error in measuring procedures, differences in the manufacture, source, or purity of the ingredients employed to make the compositions or carry out the methods, and the like. In one aspect, the term "about" means within <NUM>% of the reported numerical value. In another aspect, the term "about" means within <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>% of the reported numerical value.

As used herein, the term "coupling agent" means a compound having a sulfur atom that can form a disulfide bond with zinc sulfide and a functional group that can form a contact product with or bond with a polymer.

Zinc sulfide is a durable material that is intrinsically transparent to relatively long electromagnetic wavelengths in the far-infrared (IR) range. Such properties contribute to its use in applications which require IR transmission capability, such as in IR detectors and missile domes. Zinc sulfide articles can be produced by chemical vapor deposition (CVD), hot pressing techniques, and hot-isostatic pressing (HIP) techniques. Methods of forming a zinc sulfide surface are known in the art. The conditions, such as temperature and vacuum pressure, under which a zinc sulfide object is formed are also known in the art. The transmission, thermal stability, and strength can depend on the substrate upon which the zinc sulfide is deposited. Depending on the substrate used and conditions employed, a zinc sulfide surface can be suitable to withstand conditions encountered by missiles, projectiles, satellites, and related devices. The present disclosure is not limited to any zinc sulfide deposition or forming method, article shape, or application.

Coupling agents disclosed herein react with zinc sulfide to provide a substantially non-polar surface or a polymer-reactive surface. The modified zinc sulfide surface can be used to provide robust adherent interactions between polymers and zinc sulfide. Polymeric coatings provide improved mechanical properties, stability and protection from handling damage.

The coupling agents have a first sulfur atom containing functional group that reacts with the zinc sulfide surface to form a stable covalent bond, a disulfide bond. The coupling agents also include a second functional group that reacts with the polymer. Accordingly, the coupling agent forms a chemical bond between the zinc sulfide surface and the polymer.

The disulfide-forming coupling agents improve various properties of the zinc sulfide surface. For example, when the coupling agent is bonded to the zinc sulfide surface, the adhesion strength between the polymer and the zinc sulfide is greater than an untreated, like zinc sulfide surface without the coupling agent (see Example <NUM> below). The coupling agent also improves polymer wetting of the zinc sulfide surface (see Example <NUM> below).

<FIG> is an exemplary diagram <NUM> of a polymer <NUM> attached to a zinc sulfide surface <NUM> with a coupling agent <NUM>. Although the zinc sulfide surface <NUM> is shown as a flat surface in <FIG>, the zinc sulfide surface <NUM> can be any shape. In a non-limiting example, the zinc sulfide surface <NUM> can be curved, such as concave or convex. The coupling agent <NUM> can be any sulfur-containing compound that has a reactive sulfur group that can form a disulfide bond <NUM> with a sulfur atom on the zinc sulfide surface <NUM>. The coupling agent <NUM> also includes a functional group (R) that extends from the surface of the zinc sulfide layer <NUM> and can interact with the polymer <NUM> of interest. The functional group (R) can be any group that can chemically bond, adhesively bond, or interact with the polymer. The functional group (R) can form a contact product with the polymer.

The coupling agent can be a thiol, a thiolate, a thioglycolic acid, or a thioglycolic acid salt. The coupling agent can include any additional functional groups or substitutions, provided that it includes a reactive sulfur-containing group that can form a disulfide bond with the zinc sulfide surface. The coupling agent can have a carbon chain, such as an alkyl, alkenyl, or alkynyl chain extending between the disulfide bond and the functional group. The alkyl, alkenyl, or alkynyl chain can have any number of carbons, depending on the desired polymer and application. In one aspect, the coupling agent has a carbon chain with between about <NUM> and about <NUM> carbons. In another aspect, the coupling agent has a carbon chain with between about <NUM> and about <NUM> carbons. Yet in another aspect, the coupling agent has a carbon chain with between about <NUM> and about <NUM> carbons. Still yet, in another aspect, the coupling agent has a carbon chain with about or in any range between about <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> carbons.

The functional group on the coupling agent can be any group, depending on the polymer of interest. Non-limiting examples of the functional group include an acrylate group, an acyl halide group, an amide group, an amine group, a carboxylate group, a carboxylate thiol group, an epoxy group, an ester group, an ether group, a halogen, a hydroxamic acid group, a hydroxyl group, a nitrate group, a nitrile group, a phosphate group, a phosphine group, a phosphonic acid group, a silane group, a sulfate group, a sulfide group, a sulfite group, a thiolate group, an alkyl group, an alkenyl group, an alkynyl group, an aryl group, an azide group, an acetal group, an aldehyde group, a diene group, a cycloalkyl group, a cycloaryl group, a polycycloaryl group, a substituted cycloaryl group, or any combination thereof.

The polymer interacting with or bonding with coupling agent depends on the application. The polymers included are an acrylic polymer, an acrylate polymer, a styrene polymer, a urethane polymer, an epoxy polymer, a polyester polymer, a polyvinyl polymer, a phenoxy polymer, a nylon polymer, a melamine polymer, a fluorinated polymer, a polyvinyl alcohol polymer, a silicone polymer, or any combination thereof. The polymers can be homopolymers or copolymers and can include any additional substitutions or functional groups. The thickness of the polymer layer is not intended to be limited. The thickness depends on the target application.

The optimal conditions for forming a disulfide bond with the zinc sulfide surface will depend on the particular coupling agent used. The solvents, temperatures, and reactant concentration can be tailored accordingly. A basic pH and oxidizing conditions can be used.

The order in which the zinc sulfide surface, the coupling agent, and the polymer are combined can vary. For example, the coupling agent can be first reacted with the zinc surface to form a disulfide bond, and then the polymer can be reacted with the coupling agent. Alternatively, the coupling agent and polymer can be combined and reacted to form a coupling agent-polymer conjugate, and the coupling agent-polymer conjugate can be reacted with the zinc sulfide surface. For example, the coupling agent can be reacted with the polymer to form a zinc-sulfide reactive polymer before attaching the polymer to the zinc sulfide surface.

Isothiouronium salts are used to couple the polymer to the zinc sulfide surface. Isothiouronium can have the general formula: RSC(NH<NUM>)<NUM>]+, wherein R = alkyl, aryl. The hydrogen (H) groups can be substituted with alkyl or aryl groups. The isothiouronium also has a functional group to interact or bind the polymer, as described below.

In one example, isothiouronium salts can have the following structure:
<CHM>
wherein n is an integer from about <NUM> to <NUM>, X is a functional group that can interact with or bond to the polymer, and Y- is an anion that can form a salt with the isothiouronium group.

Other suitable isothiouronium salts can have the following structure:
<CHM>
wherein R is an alkyl chain, alkenyl chain, alkynyl chain, or aryl chain containing any number of carbons and optionally substituted with any functional groups; X is a functional group that can interact with or bond to the polymer; and Y- is an anion that can form a salt with the isothiouronium group.

Other non-limiting examples of suitable isothiouronium salts include those having the following structure:
<CHM>
wherein R<NUM>, R<NUM>, R<NUM>, R<NUM>, and R<NUM> are each independently a hydrogen, an alkyl chain, an alkenyl chain, an alkynyl chain, or an aryl chain containing any number of carbons, for example from about <NUM> to about <NUM>, and can be substituted with any functional groups; X is a functional group that can interact with or bond to the polymer; and Y- is an anion that can form a salt with the isothiouronium group. Non-limiting examples for each of R<NUM>, R<NUM>, R<NUM>, and R<NUM> are methyl groups, ethyl groups, propyl groups, butyl groups, or any combination thereof.

Any method known in the art can be used to make isothiouronium salts. Scheme <NUM> below is an exemplary non-limiting method using an SN<NUM> substitution reaction:
<CHM>
wherein n is any integer, for example between <NUM> and <NUM>; X is a functional group that can interact with or bond to a polymer of interest; and Y- is a leaving group that can be replaced in an SN<NUM> substitution reaction. As shown, thiourea (<NUM>) reacts with compound (<NUM>) with the formula: X(CH<NUM>)nY. Y is substituted for the sulfur in thiourea to form the isothiouronium salt (<NUM>).

Non-limiting examples of X, as described above, include an acrylate group, an acyl halide group, an amide group, an amine group, a carboxylate group, a carboxylate thiol group, an epoxy group, an ester group, an ether group, a halogen, a hydroxamic acid group, a hydroxyl group, a nitrate group, a nitrile group, a phosphate group, a phosphine group, a phosphonic acid group, a silane group, a sulfate group, a sulfide group, a sulfite group, a thiolate group, an alkyl group, an alkenyl group, an alkynyl group, an aryl group, an azide group, an acetal group, an aldehyde group, a diene group, a cycloalkyl group, a cycloaryl group, a polycycloaryl group, a substituted cycloaryl group, or any combination thereof.

Non-limiting examples of the leaving group, Y, include a protonated hydroxide group, an amino group, an alkoxide group, a fluoride, a chloride, a bromide, an iodide, a methyl sulfate group, a methanesulfonate (mesylate) group, a trifluoromethanesulfonate (triflate) group, a <NUM>-methanebenzenesulfonate (tosylate) group, a dialkylether group, a nitrate group, a phosphate group, an ammonium group, an inorganic ester group, a carboxylate group, a phenoxide group, or an amide group. A hydroxyl group can react with, for example, isocyante to form a urethane. A hydroxyl group can also react with an epoxy polymer.

As shown in Scheme <NUM> below, the isothiouronium salt (<NUM>) is then hydrolyzed with any basic compound (<NUM>) to provide the reactive thiol (<NUM>) that can form the disulfide bond with zinc sulfide (see <FIG>).

Any of the above described isothiouronium salts can be hydrolyzed and bonded to a zinc sulfide surface and a polymer of interest. The base or basic compound can be any compound, reactant, or solution with a pH value sufficient to hydrolyze the isothiouronium salt. For example, the pH of the base or basic compound can be greater than <NUM>. Non-limiting examples of the base or basic compound include a carbonate compound, a hydroxide compound, a cyanide compound, a borate compound, a phosphate compound, a pyrophosphate compound, a sulfite compound, a sulfide compound, or any combination thereof. The base or basic compound can be a silicate of an alkali metal, including sodium, potassium, lithium, rubidium and cesium; a carbonate compound, a hydroxide compound, a cyanide compound, a borate compound, or a sulfide of ammonia; an alkoxide of an alkali metal; a quaternary ammonium hydroxide, or any combination thereof.

One advantage of using an isothiouronium salt is that it provides a protective group, the isothiouronium group, which allows for introduction of a variety of functional groups (X) into the coupling agent that might otherwise prematurely intra-molecularly interact with a reactive thiolate group. Thus, the protective group allows for formation of the disulfide bond when desired or appropriate.

The isothiouronium salts can be first reacted with the polymer to form an isothiouronium salt-polymer conjugate. The isothiouronium salt-polymer conjugate can then hydrolyzed with a base and then disposed onto the zinc sulfide surface to form a disulfide bond. However, the isothiouronium salt-polymer conjugate can be first disposed onto the zinc sulfide surface and then hydrolyzed with a base at high pH to form the disulfide bond.

Alternatively, the isothiouronium salt can be first hydrolyzed and disposed onto the zinc sulfide surface to form a disulfide bond. Then, the isothiouronium salt bonded to the zinc sulfide surface can be hydrolyzed with a base and then reacted with the polymer.

The coupling agents can be prepared by any method or procedure known in the art. In one non-limiting example, isothiouronium salts can be prepared by reacting thiourea and the corresponding compound having an appropriate leaving group, such as X(CH<NUM>)nY, in an inert, polar, organic solvent (e.g., ethanol) at a temperature of from about <NUM>° to about <NUM> for about <NUM> to <NUM> hours. If a precipitate forms upon cooling, it can be filtered and recrystallized from ethanol. If no precipitate forms upon cooling, then the ethanol is removed by rotary evaporation and the residue is employed without further purification.

Equimolar amounts of thiourea and bromododecanol were reacted to form <NUM>-dodecanol-<NUM>-isothiouronium bromide. <NUM>-dodecanol-<NUM>-isothiouronium bromide was reacted with a <NUM> sodium hydroxide to provide a reactive thiolate. An optical lens having a zinc sulfide surface was treated with the reactive thiolate.

As shown in <FIG>, a "dot" of epoxy resin was disposed onto the treated/coupled zinc sulfide surface. The contact angle, or angle formed between the zinc sulfide surface <NUM> and the epoxy resin "dot" <NUM> was <NUM>°.

For comparison, <FIG> shows a cured "dot" of epoxy resin disposed onto an untreated (without the coupling agent) zinc sulfide surface. The contact angle between the untreated zinc sulfide surface <NUM> and the epoxy "dot" was <NUM>°. These results demonstrated improved wetting of the zinc sulfide surface on a treated lens.

Claim 1:
A method for making an optical element comprising:
a zinc sulfide layer;
a polymer adhered to a surface of the zinc sulfide layer;
wherein a coupling agent forming a disulfide bond is disposed between the zinc sulfide layer and the polymer;
wherein the coupling agent has a functional group that forms a contact product with the polymer,
wherein the polymer is an organic polymer which is an acrylic polymer, an acrylate polymer, a styrene polymer, a urethane polymer, an epoxy polymer, a polyester polymer, a polyvinyl polymer, a phenoxy polymer, a nylon polymer, a melamine polymer, a fluorinated polymer, a polyvinyl alcohol polymer, or a silicone polymer; and
wherein the optical element is a lens, an optical window, a sensor, a detector, a dome, or a beam splitter;
the method comprising attaching the organic polymer to the zinc sulfide layer; and further comprising treating an isothiouronium salt with a basic compound to form the coupling agent.