PRIORITY DATA

This patent application is a continuation application of U.S. patent application Ser. No. 17/187,826, filed on Feb. 28, 2021, which is hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention generally relates to broadband-transparent materials, such as infrared-transparent polymers.

BACKGROUND OF THE INVENTION

Broadband-transparent materials are materials that are transparent over a broad electromagnetic band including at least two of ultraviolet (UV), visible (Vis), and infrared (IR). Broadband-transparent materials may be used in a variety of applications, such as coatings, windows, and lenses.

Combining visual transparency and IR transparency in a single material is challenging due to requirements such as a high degree of homogeneity, a low variation in index of refraction throughout the material, and the requirement for bonds that do not absorb in the visual spectrum and the IR spectrum. Many IR-transparent materials are classified as semiconductors and possess band gaps in excess of the energies of IR light. However, most of these band gaps are not sufficiently large to pass visible light, thus rendering the IR-transparent materials visually opaque. The few materials that are transparent across both visible and IR spectrums are difficult or impossible to fashion into lenses or windows. Additionally, these materials suffer the drawback of typically being rigid and prone to breakage. Materials with good UV transmission (such as quartz) or good IR transmission (such as inorganic semiconductors) tend to be hard, brittle materials that are difficult and expensive to process and shape. IR-transparent windows and lenses have significant commercial value but their cost and fragility have been a drawback.

Industrial materials are often exposed to the environment and intense UV radiation. Durability to extended exposure in these environments is critical. However, common UV stabilizers are typically highly absorptive in the IR band, thus rendering their addition unacceptable when an IR-transparent material is desired. When the IR-transparent material is also UV-transparent, UV absorption and consequent degradation is reduced or avoided.

There are potential advantages to broadband-transparent materials in the form of broadband-transparent polymers. In particular, polymers have the potential to provide good toughness, durability, and cost. However, transparent polymeric materials tend to be limited in the wavelength ranges they can pass. For example, polycarbonate and poly (methyl methacrylate) are visually transparent but infrared-opaque. Polyethylene is infrared-transparent but visually opaque. Broadband transparency, particularly in the UV and mid-wave to long-wave IR spectrum, is an uncommon feature of polymeric materials. Polymer crystallinity and inhomogeneity at the micrometer scale scatters light, thereby limiting visual transmission and image quality across the Vis-IR spectrum. In addition, polymer softness produces poor abrasion resistance and limits durability in aggressive environments.

A combination of the toughness and durability of typical engineering plastics (such as polycarbonate) with broadband transmission properties is a longstanding trade in the materials space and is a serious problem in the areas of IR detectors and lenses, for example. An improvement upon the heretofore known structures would be the formation of a curable network over convenient times and temperatures, creating a crosslinked structure that is transparent across a wide wavelength range and is robust against environmental exposure.

Some specific applications of broadband-transparent polymers are durable, transparent protective coatings for automotive integrated light detection and ranging (LIDAR); optics for UV sources; quartz window alternatives for intense UV sanitation; and low-cost, increased-durability materials (windows and lenses) for IR vision systems. Additional commercial opportunities exist in the area of 3D printing, see-through inspection panels, low-emissivity coatings, and compound lens stacks, for example. Other applications relate to dual-band imaging, such as visible—LWIR imaging or near infrared (NIR)—LWIR imaging.

Transparency specifically within the infrared band is important for many applications. Infrared radiation is emitted or absorbed by molecules when they change their rotational and vibrational movements, and as a result of thermal heating. Infrared radiation is used in industrial, scientific, military, law enforcement, and medical applications. Applications include target acquisition, surveillance, night vision, homing, tracking, thermal efficiency analysis, environmental monitoring, industrial facility inspections, remote temperature sensing, short-range wireless communication, and weather forecasting. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected or with additional illumination at night. Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space such as molecular clouds, detect objects such as planets, and view highly red-shifted objects from the early days of the universe. Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, observe changing blood flow in the skin, and detect overheating of electrical apparatus. Materials with infrared transparency play a critical role in enabling new optical devices and systems for thermal-imaging applications, such as heat sensing and night-vision optics.

The medium-wavelength infrared (MWIR) and the long-wavelength infrared (LWIR) bands find applications in infrared thermography for military or civil applications, e.g. target signature identification, surveillance, and non-destructive evaluation. The MWIR band may be preferred when inspecting high-temperature objects, and the LWIR band may be preferred when working with near-room-temperature objects. Other criteria for band selection include the operating distance, indoor versus outdoor operation, object temperature, background temperature, and emissivity of the bodies of interest.

Modern infrared cameras operate over multiple bands in both the MWIR and LWIR spectrums. In order to function successfully, the camera's detector surface is usually enclosed in an IR-transparent window or domed enclosure. When used in service on an automotive or aerospace vehicle, the enclosure must not only be IR-transparent but also able to withstand considerable environmental exposure in the form of temperature extremes along with high-speed erosion causes including wind, rain, ice, dust, sand, and dirt. Such conditions will rapidly degrade soft, non-durable transmission windows through erosion and surface etching (such as abrasion by sand particles).

Other applications utilizing LWIR optics require environmental durability and mechanical strength, in addition to infrared optical transparency. That is, the materials must simultaneously serve as optical and structural materials. The dual requirements make this a particularly difficult materials problem. There is a need in the art to reduce the cost and increase the durability of materials that function in the MWIR and LWIR bands. Tough, low-cost IR windows and lenses are particularly sought.

Inorganic semiconductors and metal oxides are known IR-transparent materials but suffer from various deficiencies, such as being prone to environmental damage during use. While highly transparent across various bands of the IR spectrum, inorganic semiconductors are brittle and lack durability required of many applications such as placement on the exterior of a high-speed vehicle or when exposed to extended periods of sand, rain, and wind in the environment. Environmental damage can be caused by cracking or scratching, for example. Additionally, the bandgaps of inorganic semiconductors typically cause them to be opaque in the visible spectrum.

Aluminum oxynitride (AlON) is a ceramic that is IR-transparent at wavelengths from 0.25 μm to 4.5 μm. It has a density of 3.7 g/cm³and an index of refraction of 1.8. AlON is damage-resistant as well as thermally stable to temperatures of 1900° C. However, AlON is a very expensive material.

Sapphire is IR-transparent at wavelengths from 0.15 μm to 5 μm. It has a density of 4 g/cm³and an index of refraction of 3.9. Sapphire is damage-resistant as well as thermally stable to temperatures of 1800° C. However, sapphire is a precious gemstone and is very expensive.

Germanium (Ge) is IR-transparent at wavelengths from 3 μm to 12 μm. It has a density of 2.3 g/cm³and an index of refraction of 1.7. Germanium is not damage-resistant, is not thermally stable above 200° C. and starts to lose IR transparency above 100° C., and does not scale-up well. Germanium is a very expensive material.

Zinc sulfide (ZnS) and zinc selenide (ZnSe) are IR-transparent at wavelengths from 3 μm to 12 μm. ZnS has a density of 4.1 g/cm³and an index of refraction of 2.2, while ZnSe has a density of 5.3 g/cm³and an index of refraction of 2.4. These materials are only somewhat damage-resistant, and they do not scale-up well. Both ZnS and ZnSe are extremely expensive and are not stable above 200° C.

Advanced chalcogenide alloys (e.g., Ge₂₈Sb₁₂Se₆₀or As₄₀Se₆₀) also suffer from low work of fracture and do not scale-up well. Chalcogenide glasses require extreme processing requirements for melt fabrication of optical elements. Chalcogenide glasses are also very brittle.

Inorganic salts are another option with excellent visual and IR broadband transparency, but water solubility and brittleness limit their utility outside of very benign environments.

Inorganic-organic materials such as sulfur-based glasses made by inverse vulcanization have shown some promise in MWIR transparency, but not broadband transparency. Sulfur-based materials are susceptible to numerous complications from high-temperature processing, narrow processing parameters, and poor mechanical properties. Also, these materials are usually visibly opaque.

In the area of polymeric materials, many visually transparent and environmentally tough materials can be found, but the tendency of polymers is to be opaque across the IR spectrum due to vibrational absorption frequencies of common organic bonds.

Polyethylene is a polymer that is IR-transparent at wavelengths from 4 μm to 6.5 μm and from 7.5 μm to 12 μm. Polyethylene has a density of about 1 g/cm³and an index of refraction of 1.53. Although polyethylene is a low-cost material, it is soft and not damage-resistant which limits its applications for IR-transparent materials. Polyethylene cannot normally be crosslinked to make it a hard (thermoset) polymer. The polymer is thus soft and easily scratched, resulting in a material that will scatter light when damaged. Also, polyethylene crystallinity creates scattering and prevents visual transparency.

Polytetrafluoroethylene (PTFE) is a polymer that is IR-transparent at wavelengths from about 3 μm to 7.5 μm and from about 9 μm to about 15 μm. PTFE has a density of about 2.2 g/cm³and an index of refraction of about 1.4. Although PTFE is a relatively low-cost material, it is not damage-resistant which limits its applications for IR-transparent materials. PTFE cannot normally be crosslinked to make it a hard (thermoset) polymer. The polymer is thus soft, resulting in a material that will scatter light when damaged.

Many common polymers such as polyesters, polyethers, and polysiloxanes exhibit characteristic LWIR absorptions due to single bonds to oxygen such as C—O or Si—O bonds. Other aromatic polymers, such as polystyrene, exhibit LWIR absorptions due to aromatic C—H bonds. These absorptions essentially disqualify any aromatic material containing C—O or Si—O single bonds, when LWIR transparency is needed.

Olefin polymers are often capable of LWIR transmission. While polyethylene and polypropylene can be LWIR-transparent, polyethylene suffers from visual opacity as noted above. In addition, both polyethylene and polypropylene suffer from chemical inertness that makes it difficult to (a) functionalize the polymers with desirable functional groups, (b) bond to surfaces, or (c) crosslink the polymers. Cyclic olefin copolymers typically have narrow IR transparency bands.

Clearly, it has proven heretofore very challenging to produce tough, broadband-transparent films. There is a desire for a broadband-transparent window material for many applications, such as (but by no means limited to) broadband cameras functional in both visual and IR frequencies along with the durability to operate in harsh, real-world environments while protecting the underlying imaging equipment.

SUMMARY OF THE INVENTION

The present invention addresses the aforementioned needs in the art, as will now be summarized and then further described in detail below.

Some variations provide a polyoxalamide polymer comprising:

- (a) one or more first segments containing at least one repeat unit that includes (i) a branched, aliphatic hydrocarbon species and (ii) first amide groups at internal ends of the repeat unit, wherein the first amide groups are part of oxalamide groups;
- (b) one or more polymer end groups containing second amide groups that are each covalently bonded directly to one of the first amide groups, wherein the second amide groups are also part of the oxalamide groups, and wherein the oxalamide groups contain —N—C(═O)—C(═O)—N— sequences; and
- (c) a reacted form of one or more multifunctional amine chain extenders or crosslinkers with an amine functionality of 3 or greater.

In some embodiments, the branched, aliphatic hydrocarbon species has a degree of branching β of at least 0.05, such as at least 0.1 or at least 0.2.

In some embodiments, the first segments contain at least 2 repeat units, such as at least 10 repeat units.

In some embodiments, the —N—C(═O)—C(═O)—N— sequences are —N(H)—C(═O)—C(═O)—N(H)— sequences.

The branched, aliphatic hydrocarbon species may have a number-average molecular weight from about 50 g/mol to about 5,000 g/mol, for example.

The branched, aliphatic hydrocarbon species may be derived from a branched monomer selected from the group consisting of (12E,15E)-N-[(21E,24E)-hexatriaconta-21,24-dienyl]hexatriaconta-12,15-dien-1-amine, 2,2,4-trimethyl hexamethylene diamine, 2,4,4-trimethyl hexamethylene diamine, 2-methyl-1,5-pentanediamine, isophorone diamine, chemical analogues thereof, oligomers thereof, and combinations of the foregoing.

In some embodiments, the N—C(═O)—C(═O)—N— sequences are reaction products derived from starting oxalate species comprising dialkyl oxalates. The dialkyl oxalates may be selected from the group consisting of dimethyl oxalate, diethyl oxalate, dibutyl oxalate, di-tert-butyl oxalate, chemical analogues thereof, and combinations of the foregoing.

The multifunctional amine chain extenders or crosslinkers may be selected from the group consisting of 4-(aminomethyl)octane-1,8-diamine, 3-(aminomethyl)octane-1,8-diamine, 2-(aminomethyl)octane-1,8-diamine, 1-(aminomethyl)octane-1,8-diamine, tetraethylenetriamine, tetraethylenetetramine, tetraethylenepentamine, N-(hydroxyethyl)diethylenetriamine, chemical analogues thereof, and combinations of the foregoing.

In some embodiments, the multifunctional amine chain extenders or crosslinkers have an amine functionality of 4 or greater.

In some embodiments, the polyoxalamide polymer has a degree of polymer crosslinking χ of at least 0.5, such as at least 0.9.

In certain embodiments, the polyoxalamide polymer does not contain sulfur crosslinks. In certain embodiments, the polyoxalamide polymer does not contain ether bonds. In certain embodiments, the polyoxalamide polymer does not contain ester bonds.

Some variations provide a polyoxalamide polymer composition comprising a polyoxalamide polymer containing:

- (a) one or more first segments containing at least one repeat unit that includes (i) a branched, aliphatic hydrocarbon species and (ii) first amide groups at internal ends of the repeat unit, wherein the first amide groups are part of oxalamide groups;
- (b) one or more polymer end groups containing second amide groups that are each covalently bonded directly to one of the first amide groups, wherein the second amide groups are also part of the oxalamide groups, and wherein the oxalamide groups contain —N—C(═O)—C(═O)—N— sequences; and
- (c) a reacted form of one or more multifunctional amine chain extenders or crosslinkers with an amine functionality of 3 or greater.

In some embodiments, the polyoxalamide polymer composition further contains one or more additives selected to adjust light-transmission properties, mechanical properties, and/or adhesion properties. The additives may be selected from the group consisting of polypropylene, polyethylene, ethylene-propylene copolymer, high-refractive-index nanoparticles, sulfur, mineral oil, inorganic pigments, and combinations thereof, for example. In certain embodiments, the high-refractive-index nanoparticles have an average particle size from about 5 nanometers to about 100 nanometers. In these or other embodiments, the high-refractive-index nanoparticles may be selected from the group consisting of titanium dioxide, silicon, germanium, zinc oxide, silicon carbide, silicon dioxide, hafnium oxide, zinc selenide, zinc sulfide, sapphire, and combinations thereof. Alternatively, or additionally, the high-refractive-index nanoparticles may be selected from salts, such as calcium fluoride (CaF₂), sodium fluoride (NaF), lithium fluoride (LiF), cesium fluoride (CsF), cesium bromide (CsBr), potassium bromide (KBr), potassium iodide (KI), or combinations thereof.

The polyoxalamide polymer composition may be sprayable and/or solution-castable, for example.

In various embodiments, the polyoxalamide polymer composition is present as or in a lens, a window, a coating, a film, a film adhesive, a protective material, or a combination thereof.

The polyoxalamide polymer composition may be characterized by a transparency selected from the group consisting of UV transparency, visual transparency, NIR transparency, MWIR transparency, LWIR transparency, and combinations thereof (e.g., both UV transparency and NIR transparency, or NIR/MWIR/LWIR transparency, etc.).

Some variations provide a polyoxalamide prepolymer comprising:

- (a) one or more first segments containing at least one repeat unit that includes (i) a branched, aliphatic hydrocarbon species and (ii) amide groups at internal ends of the repeat unit; and
- (b) one or more prepolymer end groups containing ester groups that are each covalently bonded directly to one of the amide groups to form —N—C(═O)—C(═O)—O— sequences.

In some polyoxalamide prepolymers, the branched, aliphatic hydrocarbon species have a degree of branching β of at least 0.05, such as at least 0.1.

In some embodiments, the first segments contain at least 2 repeat units.

The —N—C(═O)—C(═O)—O— sequences may be —N(H)—C(═O)—C(═O)—O— sequences, in the polyoxalamide prepolymer.

In some embodiments, the branched, aliphatic hydrocarbon species has a number-average molecular weight from about 50 g/mol to about 5,000 g/mol, in the polyoxalamide prepolymer.

In some polyoxalamide prepolymers, the branched, aliphatic hydrocarbon species is derived from a branched monomer selected from the group consisting of (12E,15E)-N-[(21E,24E)-hexatriaconta-21,24-dienyl]hexatriaconta-12,15-dien-1-amine, 2,2,4-trimethyl hexamethylene diamine, 2,4,4-trimethyl hexamethylene diamine, 2-methyl-1,5-pentanediamine, isophorone diamine, chemical analogues thereof, oligomers thereof, and combinations of the foregoing.

The —N—C(═O)—C(═O)—O— sequences in the polyoxalamide prepolymer may be reaction products derived from starting oxalate species comprising dialkyl oxalates. The dialkyl oxalates may be selected from the group consisting of dimethyl oxalate, diethyl oxalate, dibutyl oxalate, di-tert-butyl oxalate, chemical analogues thereof, and combinations of the foregoing.

In certain embodiments, the polyoxalamide prepolymer does not contain ether bonds. In certain embodiments, the polyoxalamide prepolymer does not contain ester bonds except for an ester bond contained in an ester group bonded to an amide group.

Some variations provide a polyoxalamide prepolymer composition comprising a polyoxalamide prepolymer, and further comprising one or more multifunctional amine chain extenders or crosslinkers with an amine functionality of 3 or greater.

Other variations of the invention provide a method of making a polyoxalamide polymer, the method comprising:

- (a) providing a first reactant comprising a branched, aliphatic hydrocarbon species that is amine-terminated;
- (b) providing a second reactant comprising an oxalate species;
- (c) reacting the first reactant with the second reactant, under effective prepolymerization conditions and optionally in the presence of a first solvent, to generate a polyoxalamide prepolymer; and
- (d) reacting the polyoxalamide prepolymer with a multifunctional amine chain extender or crosslinker having an amine functionality of 3 or greater, under effective polymerization conditions and optionally in the presence of a second solvent, to generate a polyoxalamide polymer.

The method may further comprise introducing one or more additives to the polyoxalamide polymer, wherein the additives may be selected to adjust light-transmission properties, mechanical properties, and/or adhesion properties. In some embodiments, additives are selected from the group consisting of polypropylene, polyethylene, ethylene-propylene copolymer, high-refractive-index nanoparticles, sulfur, mineral oil, inorganic pigments, and combinations thereof. The high-refractive-index nanoparticles may be selected from the group consisting of titanium dioxide, silicon, germanium, zinc oxide, silicon carbide, silicon dioxide, hafnium oxide, zinc selenide, zinc sulfide, sapphire, and combinations thereof. The high-refractive-index nanoparticles may be selected from salts, such as CaF₂, NaF, LiF, CsF, CsBr, KBr, KI, or combinations thereof.

In some methods, the branched, aliphatic hydrocarbon species has a degree of branching β of at least 0.05.

In some methods, the first reactant is selected from the group consisting of (12E, 15E)-N-[(21E,24E)-hexatriaconta-21,24-dienyl]hexatriaconta-12,15 dien-1-amine, 2,2,4-trimethyl hexamethylene diamine, 2,4,4-trimethyl hexamethylene diamine, 2-methyl-1,5-pentanediamine, isophorone diamine, chemical analogues thereof, oligomers thereof, and combinations of the foregoing.

In some methods, the oxalate species are selected from dialkyl oxalates, such as dialkyl oxalates selected from the group consisting of dimethyl oxalate, diethyl oxalate, dibutyl oxalate, di-tert-butyl oxalate, chemical analogues thereof, and combinations of the foregoing.

In some methods, the multifunctional amine chain extender or crosslinker is selected from the group consisting of 4-(aminomethyl)octane-1,8-diamine, 3-(aminomethyl)octane-1,8-diamine, 2-(aminomethyl)octane-1,8-diamine, 1-(aminomethyl)octane-1,8-diamine, tetraethylenetriamine, tetraethylenetetramine, tetraethylenepentamine, N-(hydroxyethyl)diethylenetriamine, chemical analogues thereof, and combinations of the foregoing.

The multifunctional amine chain extender or crosslinker may have an amine functionality of 4 or greater.

In some methods, the polyoxalamide polymer has a degree of polymer crosslinking χ of at least 0.5 or at least 0.9.

The method may further comprise casting or spraying a mixture of the polyoxalamide prepolymer and the multifunctional amine chain extender or crosslinker, optionally with the second solvent, onto a substrate. Alternatively, or additionally, the method may comprise casting or spraying the polyoxalamide polymer, optionally with a third solvent, onto a substrate.

The first solvent and/or the second solvent may be selected from the group consisting of toluene, xylenes, ethanol, butanol, tetrahydrofuran, methyl ethyl ketone, methyl isobutyl ketone, and combinations thereof, for example.

In some methods, effective prepolymerization conditions include a temperature selected from about 10° C. to about 100° C. In some methods, effective polymerization conditions include a temperature selected from about 10° C. to about 100° C.

In certain methods, effective polymerization conditions include two-stage curing with a first curing stage conducted at a first temperature from about 10° C. to about 50° C. for a first time from about 30 minutes to about 2 hours, and a second curing stage conducted at a second temperature of about 50° C. to about 100° C. for a second time from about 2 hours to about 5 hours.

The method may further comprise utilizing the polyoxalamide polymer to fabricate a lens, a window, a coating, a film, a film adhesive, a protective material, or a combination thereof. The polyoxalamide polymer may be characterized by a transparency selected from the group consisting of UV transparency, visual transparency, NIR transparency, MWIR transparency, LWIR transparency, and combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.

FIG. 1 depicts an overall reaction scheme for producing a polyoxalamide polymer, in some embodiments, including Example 7.

FIG. 2 shows DMA plots illustrating thermomechanical properties of the polyoxalamide polymer of Example 1 (green) in comparison to a commercial polyethylene film (blue).

FIG. 3 displays a camera image of a 10-mil film of the Example 7 polyoxalamide polymer.

FIG. 4 displays a camera image of a 10-mil film of the Example 8 polyoxalamide polymer.

FIG. 5 displays a camera image of a 10-mil film of the Example 9 polyoxalamide polymer.

FIG. 6 shows a storage modulus versus temperature sweep of the PBH-121 polyoxalamide thermoset film in Example 11.

FIG. 7 shows thermal cycling tests of the PBH-121 polyoxalamide thermoset film in Example 11.

FIG. 8 reveals % transmittance of light versus wavelength, for UV, Vis, and IR bands, for four different film thicknesses (3 mil, 4 mil, 7 mil, and 10 mil), in Example 11.

FIG. 9A displays a camera image of an unprotected Ge window before any abrasion (no exposure to glass beads), in Example 13.

FIG. 9B displays a camera image of an unprotected Ge window after 5 seconds of abrasion with glass beads, in Example 13.

FIG. 9C displays a camera image of an unprotected Ge window after 30 seconds of total exposure to glass beads, in Example 13.

FIG. 10A displays a camera image of a polyoxalamide-protected Ge window before any abrasion (no exposure to glass beads), in Example 13.

FIG. 10B displays a camera image of a polyoxalamide-protected Ge window after 5 seconds of abrasion with glass beads, in Example 13.

FIG. 10C displays a camera image of a polyoxalamide-protected Ge window after 30 seconds of total exposure to glass beads, in Example 13.

FIG. 11A presents a light-microscope image (photomicrograph) of an unprotected Ge window before any abrasion (no exposure to glass beads), in Example 13.

FIG. 11B presents a light-microscope image of an unprotected Ge window after 5 seconds of abrasion with glass beads, in Example 13.

FIG. 11C presents a light-microscope image of an unprotected Ge window after 30 seconds of total exposure to glass beads, in Example 13.

FIG. 12A presents a light-microscope image of a polyoxalamide-protected Ge window before any abrasion (no exposure to glass beads), in Example 13.

FIG. 12B presents a light-microscope image of a polyoxalamide-protected Ge window after 5 seconds of abrasion with glass beads, in Example 13.

FIG. 12C presents a light-microscope image of a polyoxalamide-protected Ge window after 30 seconds of total exposure to glass beads, in Example 13.

FIG. 13 displays a long-wave infrared (LWIR) camera image capturing LWIR transmission through a 1-inch, unprotected Ge window before and after 30 seconds of window abrasion with glass beads, in Example 13.

FIG. 14 displays a long-wave infrared (LWIR) camera image capturing LWIR transmission through a 1-inch, polyoxalamide-protected Ge window before and after 30 seconds of window abrasion with glass beads, in Example 13.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The principles, materials, compositions, and methods of the present invention will be described in detail by reference to various non-limiting embodiments.

This description will enable one skilled in the art to make and use the invention, and it describes several embodiments, adaptations, variations, alternatives, and uses of the invention. These and other embodiments, features, and advantages of the present invention will become more apparent to those skilled in the art when taken with reference to the following detailed description of the invention in conjunction with the accompanying drawings.

As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly indicates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this invention belongs.

Unless otherwise indicated, all numbers expressing conditions, concentrations, dimensions, 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 at least upon a specific analytical technique.

The term “comprising,” which is synonymous with “including,” “containing,” or “characterized by” is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. “Comprising” is a term of art used in claim language which means that the named claim elements are essential, but other claim elements may be added and still form a construct within the scope of the claim.

As used herein, the phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. When the phrase “consists of” (or variations thereof) appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. As used herein, the phrase “consisting essentially of” limits the scope of a claim to the specified elements or method steps, plus those that do not materially affect the basis and novel characteristic(s) of the claimed subject matter.

With respect to the terms “comprising,” “consisting of,” and “consisting essentially of,” where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms, except when used in Markush groups. Thus in some embodiments not otherwise explicitly recited, any instance of “comprising” may be replaced by “consisting of” or, alternatively, by “consisting essentially of.”

Some variations of the invention are predicated on the discovery of durable, transparent polymer films. This disclosure describes durable polymeric films and windows capable of transmitting across a broad spectrum of light, such as within the ultraviolet (UV, 200-400 nm), visual (Vis, 400-800 nm), near infrared (NIR, 0.8-3 μm), medium-wave infrared (MWIR, 3-5 μm), and/or long-wave infrared (LWIR, 8-12 μm) wavelengths of light. Reference may also be made to light wavenumbers, which are the number of waves per unit distance (e.g., x-axis of FIG. 8). The wavenumber is the inverse of wavelength. For instance, a wavelength of 5 μm corresponds to a wavenumber of 2000 cm⁻¹. The wavenumber times the speed of light is the wave frequency.

In this disclosure, “broadband” refers to a plurality of wavelengths that encompass, or are within, at least of two of UV, Vis, NIR, MWIR, and LWIR. In some embodiments, a broadband wavelength refers to wavelengths that encompass, or are within, at least of three, at least four, or all five of UV, Vis, NIR, MWIR, and LWIR.

In this disclosure a “polymer” means a large molecule, or macromolecule, composed of at least two repeat subunits. The number of repeat subunits may be in principle any number greater than 2, but typically is limited to about 10,000. In various embodiments, the number of repeat subunits (also known as the degree of polymerization) is 2, 3, 4, 5, about 10, about 50, about 100, about 200, about 500, about 1,000, about 2,000, or about 5,000. The polymer may be linear, branched, cyclic, crosslinked, or a combination thereof. The polymer is typically a solid but can also be a liquid, depending on molecular weight, degree of crosslinking, and external conditions such as temperature. A “crosslink” is a bond that links one polymer chain to another. The crosslinking bonds may be covalent bonds, ionic bonds, or a combination thereof.

The present inventors believe that a combination of visible and IR transparency with mechanical durability is unprecedented for polymers. While organic, polymeric materials are frequently tough and durable, the polymers absorb broadly in the infrared (IR) spectrum due to absorption characteristic of organic bonds as well as scattering caused by crystallinity or inhomogeneity that limits visible transmission. However, the present inventors have now discovered polyoxalamide compositions and corresponding processing techniques as routes to durable, broadband-transparent (in particular, visible-transparent and IR-transparent) polymer films.

Polyoxalamides are a known class of materials and the IR transparency of the structural oxalamide unit is also known. See Dong et al., “Preparation and properties of poly (hexamethylene oxamide) copolymers containing ether moiety”, Polymer Engineering & Science Volume 58, Issue 9 (2017), which is incorporated by reference. However, the ability to process polyoxalamide materials into transparent films across a broad range of frequencies has heretofore been frustrated by (1) monomers that are absorbing in the IR spectrum; (2) the tendency of the polymer to crystallize or otherwise be inhomogeneous, rendering it opaque; and (3) the rapid reaction rate of the materials, making it difficult to cast or spray monoliths or films.

Polyoxalamides show promising LWIR transparency with characteristic absorbance peaks just outside of the LWIR band. Polyoxalamides have not been highly investigated due to commercial abundance of polyurethane and polyurea-type resins with similar mechanical properties. However, urethane and urea motifs (in commercial resins) absorb more in the LWIR band than do oxalamides, typically because of the density of urethane and urea bonds and their combination with polyester or polyether LWIR-absorbing species.

Polyoxalamides with broadband transparency are not known in the art due to high crystallinity (visible opaqueness) in systems with low-molecular-weight aliphatic species and infrared absorption in systems with common soft segments (LWIR opaqueness). Low-molecular-weight, aliphatic polyoxalamide systems tend to become semi-crystalline and non-processable. In order to be processable, polyoxalamide resins conventionally incorporate polyester, polyether, polysiloxane, or other types of soft segments—which tend to absorb in the LWIR.

Alkyl-rich polyoxalamides break the aforementioned trades by providing a broadband-transparent material with durability and toughness that far exceeds current IR-transparent materials, thereby enabling use in environmentally demanding applications. For example, a broadband-transparent window material enables broadband cameras that are effective in both visual and IR frequencies. The polymer durability allows operation in harsh real-world environments while protecting the underlying imaging equipment.

In particular, in some variations, oxalate precursors are combined with alkyl amine species to form IR-transparent networks arising from low crystallinity and relatively IR-transparent species, such as branched aliphatic monomers. Crystallization strongly scatters light and renders the material opaque across a variety of wavelengths. Judicious selection of monomers can suppress crystallization and support amorphous structures that maintain excellent visual transparency in the materials. The hydrogen-bonding character of oxalamide groups provides mechanical durability in the polymer, in the form of high tensile strength and elongation.

Polyoxalamides may generally be synthesized by the bulk reaction of amine reactive groups with dioxalate groups, forming an alcohol condensate. Amine reactive groups may be small-molecule diamines or oligomers endcapped with amines, for example. Small-molecule diamines generate crystallizable polyoxalamide systems due to proximity of hydrogen bonding from the oxalamide. Oxalamide hydrogen bonding is beneficial to mechanical properties, such as toughness, but the crystallinity is not desirable for broadband-transparent materials. Flexible oligomeric soft-segment amines decrease polymer crystallization and reduce the efficacy of hydrogen bonding from the oxalamide.

The present inventors have found an aliphatic, branched diamine with high carbon content that does not crystallize when polymerized with dioxalate groups. By exploiting the non-crystalline nature, the material forms reactive polyoxalamide prepolymers containing dioxalate end groups that can be thermosetted with amine curatives. The pot life of the polyoxalamide prepolymers and amine curative is typically limited, but the polyoxalamide prepolymers may be rapidly processed into thin films. Polyoxalamide prepolymers are miscible with various solvents and optionally olefinic fillers to aid in processing and extend pot life. Ultimately, the polyoxalamide prepolymers react with amine curatives to form amorphous, crosslinked, thermoset films with broadband transparency. Branched aliphatic polyoxalamides provide tough, broadband-transparent films unlike any other known materials.

Some variations provide a polyoxalamide polymer comprising:

- (a) one or more first segments containing at least one repeat unit that includes (i) a branched, aliphatic hydrocarbon species and (ii) first amide groups at internal ends of the repeat unit, wherein the first amide groups are part of oxalamide groups;
- (b) one or more polymer end groups containing second amide groups that are each covalently bonded directly to one of the first amide groups, wherein the second amide groups are also part of the oxalamide groups, and wherein the oxalamide groups contain —N—C(═O)—C(═O)—N— sequences; and
- (c) a reacted form of one or more multifunctional amine chain extenders or crosslinkers with an amine functionality of 3 or greater.

The branched, aliphatic hydrocarbon species is characterized by a degree of branching β. “Branching” refers to the departure from a linear hydrocarbon species. A linear (unbranched) hydrocarbon species is one in which each carbon atom is bonded to only one other carbon atom, in addition to hydrogen atoms or hydrogen substitutions (e.g., halogen atoms).

The degree of branching β is a parameter that is 0 when there is no branching and 1 when there is maximum branching. A hydrocarbon species with β=0 means that the species is unbranched. A hydrocarbon species with β>0 means that the species is branched. One test to determine whether a hydrocarbon species is branched is that for a given branched hydrocarbon species, there will exist an isomer that is a linear, unbranched molecule with the same total number of carbon atoms but in a longer main chain (more carbon atoms) than the main chain of the branched hydrocarbon species. If no such isomer exists, then the hydrocarbon species is considered to be unbranched (β=0). Note that for purposes of this disclosure, cyclic hydrocarbon species are branched.

In some embodiments, the degree of branching β of a hydrocarbon species may be calculated as the number of carbon branch points in the hydrocarbon species divided by the total number of carbon atoms. Terminal carbon atoms are included in the calculation of β even though they cannot themselves be branched, by definition. As an illustration of calculating β, 2,2-dimethylpropane, a branched alkane with five carbon atoms, has a central carbon and four terminal carbons. Its degree of branching β=⅕=0.2. Although the central carbon is actually double-branched, it counts as one branch point for purposes of calculating β. This constraint is necessary to ensure the highest possible value of β for a general carbonaceous molecule is 1 (e.g., in the case of graphite where all carbons are branch points).

In some embodiments of the invention, the branched, aliphatic hydrocarbon species has a degree of branching β of about, at least about, or at most about 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, or 0.20, including all intervening ranges. In certain embodiments, the branched, aliphatic hydrocarbon species has a degree of branching β of about, at least about, or at most about 0.1, 0.2, 0.3, 0.4, or 0.5, including all intervening ranges. As one example, Priamine 1074 (FIG. 1) has 36 carbon atoms and 4 carbon branch points, and thus β= 4/36=0.11.

In this disclosure, “aliphatic” means that the hydrocarbon species contains no carbon-carbon aromatic bonds (delocalized conjugated π system with resonance bond order of 1.5). Aliphatic hydrocarbon species may contain carbon-carbon single bonds, carbon-carbon double bonds, and/or carbon-carbon triple bonds. Preferably, aliphatic hydrocarbon species contain carbon-carbon single bonds. Aliphatic hydrocarbon species may be linear, cyclic (but not aromatic), or branched. In an aliphatic hydrocarbon species, any hydrogen atom may be substituted with another element or functional group, such as Cl, NH₂, OH, etc.

In some embodiments, the first segments contain at least 2 repeat units, such as at least 10 repeat units. In various embodiments, the first segments contain 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 50, 75, 100, or more repeat units.

In some embodiments, the —N—C(═O)—C(═O)—N— sequences are —N(H)—C(═O)—C(═O)—N(H)— sequences.

The branched, aliphatic hydrocarbon species may have a number-average molecular weight from about 50 g/mol to about 5,000 g/mol, for example.

The branched, aliphatic hydrocarbon species is preferably derived from a branched monomer that is an amine-terminated, branched, aliphatic monomer. The branched, aliphatic hydrocarbon species may be derived from a branched monomer selected from the group consisting of (12E,15E)-N-[(21E,24E)-hexatriaconta-21,24-dienyl]hexatriaconta-12,15-dien-1-amine, 2,2,4-trimethyl hexamethylene diamine, 2,4,4-trimethyl hexamethylene diamine, 2-methyl-1,5-pentanediamine, isophorone diamine, chemical analogues thereof, oligomers thereof, and combinations of the foregoing.

In other (less-preferred) embodiments that employ an unbranched, aliphatic hydrocarbon species, such species may be derived from a linear monomer selected from the group consisting of ethylenediamine, propanediamine, butanediamine, pentanediamine, hexanediamine, heptanediamine, octanediamine, dibutylamine (or other dialkylamines), tetraethylethylenediamine (or other tetraalkylethylenediamines), and combinations thereof, for example.

In some embodiments, the N—C(═O)—C(═O)—N— sequences are reaction products derived from starting oxalate species comprising dialkyl oxalates. The dialkyl oxalates may be selected from the group consisting of dimethyl oxalate, diethyl oxalate, dibutyl oxalate, di-tert-butyl oxalate, chemical analogues thereof, and combinations of the foregoing.

The multifunctional amine chain extenders or crosslinkers may be selected from the group consisting of 4-(aminomethyl) octane-1,8-diamine, 3-(aminomethyl)octane-1,8-diamine, 2-(aminomethyl) octane-1,8-diamine, 1-(aminomethyl)octane-1,8-diamine, tetraethylenetriamine, tetraethylenetetramine, tetraethylenepentamine, N-(hydroxyethyl) diethylenetriamine, chemical analogues thereof, and combinations of the foregoing.

In certain embodiments, the multifunctional amine chain extenders or crosslinkers have an amine functionality of 2. Preferably, the multifunctional amine chain extenders or crosslinkers have an amine functionality of 3 or more. In some embodiments, the multifunctional amine chain extenders or crosslinkers have an amine functionality of 4 or greater.

In some embodiments, the polyoxalamide polymer is crosslinked and may be defined by a degree of polymer crosslinking χ. The degree of polymer crosslinking χ is a parameter that is 0 when there is no crosslinking and 1 when there is maximum crosslinking within the polymer. In some embodiments, degree of polymer crosslinking χ is defined by the extent of the crosslinking reaction of prepolymer with chain extender or crosslinker. If no reaction takes place, then χ=0. If all available functional groups within the chain extender or crosslinker fully react with all possible polymer chains, then χ=1. In some embodiments, the degree of polymer crosslinking χ is at least 0.5 or at least 0.9. In various embodiments, the degree of polymer crosslinking χ is about, or at least about, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 0.99, or 1, including all intervening ranges.

In certain embodiments, the polyoxalamide polymer does not contain sulfur crosslinks. In certain embodiments, the polyoxalamide polymer does not contain ether bonds. In certain embodiments, the polyoxalamide polymer does not contain ester bonds. In certain embodiments, the polyoxalamide polymer does not contain siloxane bonds.

Some variations provide a polyoxalamide polymer composition comprising a polyoxalamide polymer containing:

- (a) one or more first segments containing at least one repeat unit that includes (i) a branched, aliphatic hydrocarbon species and (ii) first amide groups at internal ends of the repeat unit, wherein the first amide groups are part of oxalamide groups;
- (b) one or more polymer end groups containing second amide groups that are each covalently bonded directly to one of the first amide groups, wherein the second amide groups are also part of the oxalamide groups, and wherein the oxalamide groups contain N—C(═O)—C(═O)—N— sequences; and
- (c) a reacted form of one or more multifunctional amine chain extenders or crosslinkers with an amine functionality of 3 or greater.

In some embodiments, the polyoxalamide polymer composition further contains one or more additives selected to adjust light-transmission properties, mechanical properties, and/or adhesion properties. The additives may be selected from the group consisting of polypropylene, polyethylene, ethylene-propylene copolymer, high-refractive-index nanoparticles, sulfur, mineral oil, inorganic pigments, and combinations thereof, for example.

“High-refractive-index nanoparticles” are nanoparticles with a refractive index of at least about 1.5, such as at least about 2, at least about 2.5, or at least about 3. In certain embodiments, the high-refractive-index nanoparticles have an average particle size from about 5 nanometers to about 100 nanometers, such as from about 5 nanometers to about 50 nanometers. The high-refractive-index nanoparticles may be selected from the group consisting of titanium dioxide, silicon, germanium, zinc oxide, silicon carbide, silicon dioxide, hafnium oxide, zinc selenide, zinc sulfide, sapphire, and combinations thereof. Alternatively, or additionally, the high-refractive-index nanoparticles may be selected from salts, such as CaF₂, NaF, LiF, CsF, CsBr, KBr, KI, or combinations thereof.

In some embodiments, the polyoxalamide polymer composition further includes one or more filler particles physically blended with the polymer. The concentration of filler particles may vary, such as from about 0.1 wt % to about 50 wt % on the basis of the overall polyoxalamide polymer composition.

Filler particles for use in the compositions of the present disclosure preferably exhibit high broadband transparency and small particle size relative to incident radiation wavelength in order to increase transparency and limit scattering. In particular, for example, the filler particles material may be chosen so as to be transparent in the MWIR and LWIR bands, and/or other bands. However, the present invention is not limited to filler particles (when present) that are transparent in any particular bands. If the concentration of filler particles is relatively low (e.g., less than about 30 wt %, 20 wt %, 10 wt %, 5 wt %, or 1 wt %), the filler particles may lack broadband transparency, while the overall material still maintains acceptable transparency.

Suitable filler particles include, but are not limited to, germanium, CaF₂, NaCl, KCl, KBr, diamond, sapphire, Si, CsI, MgO, MgF₂, LiF, NaF, BaF₂, ZnS, ZnSc, PbS, PbSe, PbF₂, LiYF₄, NaYF₄, thallium bromoiodide (ThBrI), thallium bromochloride (ThBrCl), and combinations thereof.

Filler particles for use in the compositions of the present disclosure may be selected in the nanoparticle size range. In some embodiments, the filler particles have an average discrete particle size ranging from about 10 nm to about 1000 nm, such as from about 100 nm to about 500 nm. Particles sizes may be measured by a variety of techniques, including dynamic light scattering, laser diffraction, image analysis, or sieve separation, for example. Dynamic light scattering is a non-invasive, well-established technique for measuring the size and size distribution of particles typically in the submicron region, and with the latest technology down to 1 nanometer. Laser diffraction is a widely used particle-sizing technique for materials ranging from hundreds of nanometers up to several millimeters in size. Exemplary dynamic light scattering instruments and laser diffraction instruments for measuring particle sizes are available from Malvern Instruments Ltd., Worcestershire, UK. Image analysis to estimate particle sizes and distributions can be done directly on photomicrographs, scanning electron micrographs, or other images.

The polyoxalamide polymer composition may be sprayable and/or solution-castable, for example.

In various embodiments, the polyoxalamide polymer composition is present as or in a lens, a window, a coating, a film, a film adhesive, a protective material, or a combination thereof.

The polyoxalamide polymer composition may be characterized by a transparency selected from the group consisting of UV transparency, visual transparency, NIR transparency, MWIR transparency, LWIR transparency, and combinations thereof (e.g., both UV transparency and NIR transparency, or NIR/MWIR/LWIR transparency, etc.).

By “transparent” in this disclosure, it is not necessarily meant completely transparent; some absorption of radiation may occur. In particular, “transparent” means a sheet of polymer with known thickness (such as 25 microns) transmits at least 50%, preferably about 60% or more, more preferably about 70% or more, and most preferably about 80% or more, of incident light at wavelengths of interest. Incident light is normal (perpendicular) to the surface of the polymer sheet. If a wavelength range is used, the transmission is averaged over that range.

One of ordinary skill in the art will understand that transmission generally can include regular transmission and diffuse transmission, both of which can contribute to total transmission (and thus transparency). The average transmission values in this specification include only regular transmission, which is also referred to as transmittance (e.g., see FIG. 8). Note also that the average transmission (transparency) is an average over a range of wavelengths and may be calculated, for example, as the average value of the transmission (transparency) within a selected range of wavelengths.

Regular transmission of light, and therefore transparency, at a selected wavelength can be determined for a sample 25 microns (about 0.001 inch) thick, or for another thickness of interest, as I(t)/I₀, where I₀is the incident intensity and I(t) is the intensity as a function of thickness. I(t)/I₀at a given polyoxalamide polymer thickness can be experimentally determined, for example, using a Perkin Elmer UV/Vis/IR spectrometer.

Some embodiments provide an polyoxalamide polymer composition with at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% average transparency through a 100-micron-thick film for incident light within the UV band and the Vis band (averaged over both bands).

Some embodiments provide an polyoxalamide polymer composition with at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% average transparency through a 100-micron-thick film for incident light within the Vis band and the NIR band (averaged over both bands).

Some embodiments provide an polyoxalamide polymer composition with at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% average transparency through a 100-micron-thick film for incident light within the UV band, the Vis band, and the NIR band (averaged over all three of these bands).

Some embodiments provide an polyoxalamide polymer composition with at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% average transparency through a 100-micron-thick film for incident light within the Vis band, the NIR band, and the MWIR band (averaged over all three of these bands).

Some embodiments provide an polyoxalamide polymer composition with at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% average transparency through a 100-micron-thick film for incident light within the NIR band, the MWIR band, and the LWIR band (averaged over all three of these bands).

Some embodiments provide an polyoxalamide polymer composition with at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% average transparency through a 100-micron-thick film for incident light within the Vis band, the NIR band, the MWIR band, and the LWIR band (averaged over all four of these bands).

Some embodiments provide an polyoxalamide polymer composition with at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% average transparency through a 100-micron-thick film for incident light within the UV band, the Vis band, the NIR band, and the MWIR band (averaged over all four of these bands).

Certain embodiments provide an polyoxalamide polymer composition with at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% average transparency through a 100-micron-thick film for incident light within the UV band, the Vis band, the NIR band, the MWIR band, and the MWIR band (averaged over all five of these bands). See, for example, FIG. 8.

Other film thicknesses may be used to determine average transparency. Certain embodiments provide an polyoxalamide polymer composition with at least 50%, at least 60%, at least 70%, or at least 80% average transparency through a 250-micron-thick film for incident light within the UV band, the Vis band, the NIR band, the MWIR band, and the MWIR band (averaged over all five of these bands). See, for example, FIG. 8 (10 mil is about 250 microns).

Some variations provide a polyoxalamide prepolymer comprising:

- (a) one or more first segments containing at least one repeat unit that includes (i) a branched, aliphatic hydrocarbon species and (ii) amide groups at internal ends of the repeat unit; and
- (b) one or more prepolymer end groups containing ester groups that are each covalently bonded directly to one of the amide groups to form —N—C(═O)—C(═O)—O— sequences.

In some polyoxalamide prepolymers, the branched, aliphatic hydrocarbon species have a degree of branching β of at least 0.05, such as at least 0.1.

In some embodiments, the first segments contain at least 2 repeat units.

The —N—C(═O)—C(═O)—O— sequences may be —N(H)—C(═O)—C(═O)—O— sequences, in the polyoxalamide prepolymer.

In some embodiments, the branched, aliphatic hydrocarbon species has a number-average molecular weight from about 50 g/mol to about 5,000 g/mol, in the polyoxalamide prepolymer.

In some polyoxalamide prepolymers, the branched, aliphatic hydrocarbon species is derived from a branched monomer selected from the group consisting of (12E, 15E)-N-[(21E,24E)-hexatriaconta-21,24-dienyl]hexatriaconta-12,15-dien-1-amine, 2,2,4-trimethyl hexamethylene diamine, 2,4,4-trimethyl hexamethylene diamine, 2-methyl-1,5-pentanediamine, isophorone diamine, chemical analogues thereof, oligomers thereof, and combinations of the foregoing.

The —N—C(═O)—C(═O)—O— sequences in the polyoxalamide prepolymer may be reaction products derived from starting oxalate species comprising dialkyl oxalates. The dialkyl oxalates may be selected from the group consisting of dimethyl oxalate, diethyl oxalate, dibutyl oxalate, di-tert-butyl oxalate, chemical analogues thereof, and combinations of the foregoing.

In certain embodiments, the polyoxalamide prepolymer does not contain ether bonds. In certain embodiments, the polyoxalamide prepolymer does not contain ester bonds except for an ester bond contained in an ester group bonded to an amide group.

Some variations provide a polyoxalamide prepolymer composition comprising a polyoxalamide prepolymer, and further comprising one or more multifunctional amine chain extenders or crosslinkers with an amine functionality of 2 or greater, preferably 3 or greater.

The multifunctional amine chain extenders or crosslinkers may be selected from the group consisting of 4-(aminomethyl)octane-1,8-diamine, 3-(aminomethyl)octane-1,8-diamine, 2-(aminomethyl)octane-1,8-diamine, 1-(aminomethyl)octane-1,8-diamine, tetraethylenetriamine, tetraethylenetetramine, tetraethylenepentamine, N-(hydroxyethyl)diethylenetriamine, chemical analogues thereof, and combinations of the foregoing. In the polyoxalamide prepolymer composition, the multifunctional amine chain extenders or crosslinkers are present in unreacted form until the crosslinking reaction is initiated.

In some embodiments, no sulfur is used and the materials may be cured at room temperature (about 25° C.) rather than via aggressive heating (e.g., 150° C.). Sulfur is undesirable because it creates a tinted material that does not have good visual transparency. Also, the use of sulfur during synthesis tends to create noxious fumes that evolve at high temperatures.

In some embodiments, no organometallic monomers containing carbon-metal bonds are employed.

In some embodiments, there are no polyesters, polyethers, or polysiloxanes in the polyoxalamide polymer composition.

Other variations of the invention provide a method of making a polyoxalamide polymer, the method comprising:

- (a) providing a first reactant comprising a branched, aliphatic hydrocarbon species that is amine-terminated;
- (b) providing a second reactant comprising an oxalate species;
- (c) reacting the first reactant with the second reactant, under effective prepolymerization conditions and optionally in the presence of a first solvent, to generate a polyoxalamide prepolymer; and
- (d) reacting the polyoxalamide prepolymer with a multifunctional amine chain extender or crosslinker having an amine functionality of 3 or greater, under effective polymerization conditions and optionally in the presence of a second solvent, to generate a polyoxalamide polymer.

For example, FIG. 1 depicts an overall reaction scheme, such as the scheme employed in Example 7.

The method may further comprise introducing one or more additives to the polyoxalamide polymer, wherein the additives may be selected to adjust light-transmission properties, mechanical properties, and/or adhesion properties. In some embodiments, additives are selected from the group consisting of polypropylene, polyethylene, ethylene-propylene copolymer, high-refractive-index nanoparticles, sulfur, mineral oil, inorganic pigments, and combinations thereof. The high-refractive-index nanoparticles may be selected from the group consisting of titanium dioxide, silicon, germanium, zinc oxide, silicon carbide, silicon dioxide, hafnium oxide, zinc selenide, zinc sulfide, sapphire, and combinations thereof. Alternatively, or additionally, the high-refractive-index nanoparticles may be selected from salts, such as CaF₂, NaF, LiF, CsF, CsBr, KBr, KI, or combinations thereof.

In some methods, the branched, aliphatic hydrocarbon species has a degree of branching β of at least 0.05, such as at least 0.1.

In some methods, the first reactant is selected from the group consisting of (12E,15E)-N-[(21E,24E)-hexatriaconta-21,24-dienyl]hexatriaconta-12,15 dien-1-amine, 2,2,4-trimethyl hexamethylene diamine, 2,4,4-trimethyl hexamethylene diamine, 2-methyl-1,5-pentanediamine, isophorone diamine, chemical analogues thereof, oligomers thereof, and combinations of the foregoing.

In some methods, the oxalate species are selected from dialkyl oxalates, such as dialkyl oxalates selected from the group consisting of dimethyl oxalate, diethyl oxalate, dibutyl oxalate, di-tert-butyl oxalate, chemical analogues thereof, and combinations of the foregoing.

In certain embodiments, the oxalate species are selected from polyfunctional oxalates, such as trifunctional oxalates. A polyfunctional oxalate is a species with a central core and three or more arms. Along each arm, there is an oxalate group. Therefore, in a polyfunctional oxalate, there are three or more —C(O)—C(O)— groups able to react with amines or a partially reacted group, —NH—C(O)—C(O)—, to also react with amines.

In some methods, the multifunctional amine chain extender or crosslinker is selected from the group consisting of 4-(aminomethyl) octane-1,8-diamine, 3-(aminomethyl)octane-1,8-diamine, 2-(aminomethyl) octane-1,8-diamine, 1-(aminomethyl)octane-1,8-diamine, tetraethylenetriamine, tetraethylenetetramine, tetraethylenepentamine, N-(hydroxyethyl) diethylenetriamine, chemical analogues thereof, and combinations of the foregoing.

The multifunctional amine chain extender or crosslinker may have an amine functionality of 4 or greater.

In some methods, the polyoxalamide polymer has a degree of polymer crosslinking χ of at least 0.5 or at least 0.9.

The kinetic reaction rate between amines and oxalates is quite rapid, leaving very short pot lives between the two reactants that makes it difficult to cast or spray materials as in traditional two-component systems (urethanes, silicones, etc.). This shortcoming can be overcome by rapid processing of cast films or dual-spray head techniques, for example.

The method may further comprise casting or spraying a mixture of the polyoxalamide prepolymer and the multifunctional amine chain extender or crosslinker, optionally with the second solvent, onto a substrate. Alternatively, or additionally, the method may comprise casting or spraying the polyoxalamide polymer, optionally with a third solvent, onto a substrate. Spraying may utilize a spray gun with dual spray heads. Casting may be performed with a doctor blade or drawdown bar, for example.

The first solvent, second solvent, and third solvent may independently be selected from the group consisting of toluene, xylenes, ethanol, butanol, tetrahydrofuran, methyl ethyl ketone, methyl isobutyl ketone, and combinations thereof, for example. Various solvent dilutions may be used, such as from about 1 wt % to about 100 wt % of the material being diluted within the overall solution.

In some methods, effective prepolymerization conditions include a temperature selected from about 10° C. to about 100° C. In certain methods, the prepolymerization temperature is selected from about 15° C. to about 50° C., such as from about 20° C. to about 30° C. Mixing, such as centrifugal mixing, may be employed to assist the prepolymerization reaction(s).

In some methods, effective polymerization conditions include a temperature selected from about 10° C. to about 100° C. In certain methods, the polymerization temperature is selected from about 15° C. to about 50° C., such as from about 20° C. to about 30° C. Mixing, such as centrifugal mixing, may be employed to assist the polymerization reaction(s).

In certain methods, effective polymerization conditions include two-stage curing with a first curing stage conducted at a first temperature from about 10° C. to about 50° C. for a first time from about 30 minutes to about 2 hours, such as from about 45 minutes to about 90 minutes; and a second curing stage conducted at a second temperature of about 50° C. to about 100° C., such as from about 75° C. to about 85° C., for a second time from about 2 hours to about 6 hours, such as from about 3 hours to about 5 hours.

The method may further comprise utilizing the polyoxalamide polymer to fabricate a lens, a window, a coating, a film, a film adhesive, a protective material, or a combination thereof. The polyoxalamide polymer may be characterized by a transparency selected from the group consisting of UV transparency, visual transparency, NIR transparency, MWIR transparency, LWIR transparency, and combinations thereof.

Applications of the present invention include, but are not limited to, night vision systems and LiDARs that function in the MWIR and LWIR. Tougher and less breakage-prone windows and lenses are useful for highly durable IR vision systems. Infrared windows and lenses are useful for military applications, replacing damage-prone semiconductor or oxide windows. Sheets of the broadband-transparent polymer as disclosed may function as IR-transparent anti-fouling appliques for MWIR and LWIR LiDARs.

In this detailed description, reference has been made to multiple embodiments and to the accompanying drawings in which are shown by way of illustration specific exemplary embodiments of the invention. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that modifications to the various disclosed embodiments may be made by a skilled artisan.

Where methods and steps described above indicate certain events occurring in certain order, those of ordinary skill in the art will recognize that the ordering of certain steps may be modified and that such modifications are in accordance with the variations of the invention. Additionally, certain steps may be performed concurrently in a parallel process when possible, as well as performed sequentially.

All publications, patents, and patent applications cited in this specification are herein incorporated by reference in their entirety as if each publication, patent, or patent application were specifically and individually put forth herein.

The embodiments, variations, and figures described above should provide an indication of the utility and versatility of the present invention. Other embodiments that do not provide all of the features and advantages set forth herein may also be utilized, without departing from the spirit and scope of the present invention. Such modifications and variations are considered to be within the scope of the invention defined by the claims.

EXAMPLES
Materials.

Dibutyl oxalate, 1,5-diamino-2-methylpentane, toluene, xylenes, and paraffin oil are purchased from Sigma Aldrich (St. Louis, Missouri, USA) and used as received. Priamine™ 1074 is procured from Croda Inc. (Edison, New Jersey, USA) and used as received. Priamine 1074 is (12E, 15E)-N-[(21E,24E)-hexatriaconta-21,24-dienyl]hexatriaconta-12,15-dien-1-amine (International Union of Pure and Applied Chemistry, IUPAC name) and its chemical structure is shown in FIG. 1. Hexatran 100 is procured from Ascend Performance Materials (Houston, Texas, USA) and used as received. Hexatran™ 100 is 4-(aminomethyl)octane-1,8-diamine (IUPAC name) and its chemical structure is shown in FIG. 1. Dibutyl oxalate is also known as the dibutyl ester of ethanedioic acid, and its chemical structure is shown in FIG. 1.

In these Examples, PBH-121, DBH-121, and PBD-121 indicate the three components used based on their single-letter abbreviations (P=Priamine 1074, B=dibutyl oxalate, H=Hexatran 100, and D=1,5-diamino-2-methylpentane) and the corresponding mole ratios of the components. For example, PBH-121 refers to a material made from Priamine 1074, dibutyl oxalate, and Hexatran 100 in a 1:2:1 molar ratio, respectively.

Example 1: Fabrication of PBH-121 Polyoxalamide Polymer

Priamine 1074 (5.00 g, 9.36 mmol, 1 eq), dibutyl oxalate (3.79 g, 18.72 mmol, 2 eq), and toluene (2.00 g) are added to a 100 mL Flacktek (Landrum, South Carolina, USA) container. The container is sealed and mixed in a centrifugal mixer at 2000 rpm for 1 minute. Hexatran 100 (1.08 g, 6.24 mmol, 1 eq) and toluene (1.00 g) are added to a 100 mL Flacktek container. The container is sealed and mixed in a centrifugal mixer at 2000 rpm for 1 minute. The polyoxalamide prepolymer is stored at 5° C. for 10 minutes before prepolymer and curative solution are combined and mixed in a centrifugal mixer at 2000 rpm for 10 seconds. The mixture is cast onto Mylar film using a doctor blade set at 10 mil and allowed to set for 1 hour at room temperature (about 25° C.). The crosslinked film is then heated to 80° C. for 12 hours to remove residual toluene solvent. Films with thicknesses from 2 mil to 10 mil are fabricated.

Example 2: Fabrication of PBH-341 Polyoxalamide Polymer

Priamine 1074 (6.00 g, 11.24 mmol, 3 eq), dibutyl oxalate (3.03 g, 14.98 mmol, 4 eq), and toluene (2.00 g) are added to a 100 mL Flacktek container. The container is sealed and mixed in a centrifugal mixer at 2000 rpm for 1 minute. The prepolymer curative solution is prepared by adding Hexatran 100 (0.87 g, 2.48 mmol, 1 eq) and toluene (1.00 g) to a 100 mL Flacktek container. The container is sealed and mixed in a centrifugal mixer at 2000 rpm for 1 minute. The prepolymer curative is stored at 5° C. for 10 minutes before prepolymer and curative solution are combined and mixed in a centrifugal mixer at 2000 rpm for 10 seconds. The mixture is cast onto Mylar film using a doctor blade set at 10 mil and allowed to set for 1 hour at room temperature (about 25° C.). This thermosetting reaction crosslinks the polymer. The crosslinked film is then heated to 80° C. for 12 hours to remove residual toluene solvent. Films with thicknesses from 2 mil to 10 mil are fabricated.

Example 3: Spray-Coated Polyoxalamide Thermoset

Polyoxalamide prepolymer from Example 1 is diluted with toluene (10.35 g) and poured into a spray cup. The polyoxalamide prepolymer curative solution from Example 1 is diluted with toluene (15.50 g) and poured into a spray cup. Using two airbrushes attached in parallel to allow for spraying of dual streams, the two components are sprayed simultaneously onto a Mylar substrate. The sprayed film is allowed to set for 1 hour at room temperature (about 25° C.). This thermosetting reaction crosslinks the polymer. The crosslinked film is then heated to 80° C. for 12 hours to remove residual toluene solvent. The final film thickness is about 4 mil (about 100 μm).

Example 4: Fourier Transform Infrared Spectroscopy (FT-IR)

FT-IR transmission spectra are obtained for the polyoxalamide polymer of Example 1 using a Nicolet iS50 FT-IR spectrometer from Thermo Fisher Scientific (Waltham, Massachusetts, USA). Spectra are recorded over the 400-8000 cm⁻¹wavenumber range by cumulating 32 scans at 4 cm⁻¹resolution, corrected for baseline.

Example 5: Dynamic Mechanical Analysis (DMA)

Dynamic mechanical analysis is carried out in oscillatory tension mode at 1 Hz, 50 μm amplitude, and 3° C./min to reveal the behavior of storage modulus versus temperature from −100° C. to 200° C. FIG. 2 shows the DMA plots, illustrating thermomechanical properties of the polyoxalamide polymer of Example 1 (green) in comparison to a commercial polyethylene film (blue).

Example 6: Tensile Testing

Tensile experiments are performed on an Instron® 5969 testing system from Instron (Norwood, Massachusetts, USA). Dogbone specimens of the polyoxalamide material from Example 1 and from Example 2 are cut for testing. The tensile testing is conducted at 10 mm/min. The results are shown in Table 1 below.

TABLE 1

Tensile Properties of Polyoxalamide Thermoset Films

Compared to a Commercial Polyethylene Film.

Stress at
Strain at

Break
Break
Modulus

Example
(MPa)
(%)
(MPa)

1
78
187
118

2
56
235
75

Polyethylene
254
433
259

Example 7: Fabrication of PBH-121 Polyoxalamide Polymer

To fabricate a PBH-121 prepolymer, dibutyl oxalate (2 equivalents, 3.50 g) is added at once to Priamine 1074 (1 equivalent, 4.62 g) in a plastic container and mixed in a Flacktek for 1 minute at 2000 rpm. The mixture is then left to cool off for 5 minutes at room temperature (about 25° C.) while curative is weighed. Hexatran 100 (1 equivalent, 1.00 g) is quickly added to the prepolymer. The mixture is immediately mixed at 2000 rpm for 6 seconds. The contents are quickly cast on a sheet of Mylar on top of a glass plate. This step is completed within 2 minutes to ensure that the film can be cast smoothly before crosslinking occurs. The film is left at room temperature for 1 hour and then placed in an 80° C. oven for at least 6 hours to completely evaporate butanol condensate.

FIG. 1 depicts the overall reaction scheme employed in Example 7.

Example 8: Fabrication of PBD-121 Polyoxalamide Polymer

To fabricate a PBD-121 prepolymer, dibutyl oxalate (2 equivalents, 3.48 g) is quickly added at once to Priamine 1074 (1 equivalent, 4.59 g) in a plastic container and mixed in a Flacktek for 1 minute at 2000 rpm. The mixture is then left to cool off for 5 minutes at room temperature (about 25° C.) while curative is weighed. 1,5-Diamino-2-methylpentane (1 equivalent, 1.00 g) is quickly added to the prepolymer. The mixture is immediately mixed at 2000 rpm for 6 seconds. The contents are quickly cast on a sheet of silanized Mylar on top of a glass plate. This step is completed within 2 minutes to ensure that the film can be cast smoothly before crosslinking occurs. The film is left at room temperature for 1 hour and then placed in an 80° C. oven for at least 6 hours to completely evaporate butanol condensate.

Example 9: Fabrication of DBH-121 Polyoxalamide Polymer

To fabricate a DBH-121 prepolymer, 1,5-diamino-2-methylpentane (1 equivalent, 2.01 g) is quickly added at once to dibutyl oxalate (2 equivalents, 7.00 g) in a plastic container and mixed in a Flacktek for 1 minute at 2000 rpm. The mixture is then left to cool off for 5 minutes at room temperature (about 25° C.) while curative is weighed. Hexatran 100 (1 equivalent, 2.00 g) is quickly added to the prepolymer. The mixture is immediately mixed at 2000 rpm for 6 seconds. The contents are quickly cast on a sheet of Mylar on top of a glass plate. This step is completed within 2 minutes to ensure that the film can be cast smoothly before crosslinking occurs. The film is left at room temperature for 1 hour and then placed in an 80° C. oven for at least 6 hours to completely evaporate butanol condensate.

Example 10: Comparison of PBH-121 Polyoxalamide Polymer, PBD-121 Polyoxalamide Polymer, and DBH-121 Polyoxalamide Polymer

FIGS. 3, 4, and 5 show camera images of 10-mil films of Example 7 (PBH-121), Example 8 (PBD-121), and Example 9 (DBH-121), respectively. These color images qualitatively reveal the degree of visual-spectrum transparency through each of the polymer films. As can be observed, the PBH-121 polyoxalamide polymer, when placed over a live scene from Malibu, California, USA, is significantly transparent to ordinary light. By contrast, the PBD-121 polyoxalamide polymer, when placed over the same live scene, generates a blurry camera image, indicating lack of visual transparency. The DBH-121 polyoxalamide polymer, when placed over an ink-bearing HRL Laboratories card, shows significant visual opacity.

Table 2 below summarizes mechanical properties of the polymers of Example 7 (PBH-121), Example 8 (PBD-121), and Example 9 (DBH-121). Polyoxalamide polymer PBH-121 exhibits excellent mechanical properties. Polyoxalamide polymer PBD-121 exhibits a low modulus. Polyoxalamide polymer DBH-121 exhibits a very high modulus, but according to FIG. 5, inferior visual transparency. PBH-121 is a preferred amorphous, branched aliphatic composition because this optimized polyoxalamide polymer simultaneously provides a combination of high visual clarity and good mechanical performance.

TABLE 2

Mechanical Properties of Polyoxalamide Thermoset

Filmsin Examples 7, 8, and 9.

Instantaneous

Elongation
Failure Stress
Modulus

Example
(%)
(MPa)
(MPa)

7 (PBH-121)
203 ± 3
35.2 ± 1.1
120

8 (PBD-121)
69 ± 23
11.7 ± 1.2
5.8

9 (DBH-121)
6 ± 2
10.1 ± 1.1
2200

Example 11: Mechanical Properties and Broadband Transparency of PBH-121 Polyoxalamide Thermoset Films

FIG. 6 is a storage modulus versus temperature sweep of the PBH-121 polyoxalamide thermoset film provided in Example 7. In FIG. 6, storage modulus versus temperature is shown in green, and tan 8 versus temperature is shown in blue. The DMA settings are the same as in Example 5.

FIG. 7 shows thermal cycling tests of the PBH-121 polyoxalamide thermoset film provided in Example 7. In FIG. 7, storage modulus versus time is shown in green, and temperature versus time is shown in blue.

FIG. 8 is a plot of % transmittance of light versus wavelength, for UV, Vis, and IR bands, for four different film thicknesses (3 mil, 4 mil, 7 mil, and 10 mil). According to FIG. 8, the PBH-121 polyoxalamide polymer exhibits greater than 90% transparency within the UV band, in the Vis band, and within much of the NIR band. The transparencies within the MWIR and LWIR bands vary more with film thickness. For a 4-mil (about 100 μm) film thickness, the peak MWIR transmittance is about 75%, and the peak LWIR transmittance is about 40%.

Example 12: Bonding of PBH-121 Film on a 1-Inch Ge Window

A 1-inch-diamter disc is cut from a PBH-121 sheet (100 microns thick) using a 1-inch circular punch. This disc is then placed on a Mylar sheet coated with a release layer. Using a pipette, 3-4 drops of freshly mixed PBH-121 solution in 50 wt % xylenes is placed on the previously cured PBH-121 disc. A 1-inch germanium (Ge) window is promptly pressed onto the uncured PBH-121 adhesive to bond it to the previously cured film. A glass plate is placed on top of the Ge window to apply pressure while the bonding process proceeds for 1 hour at room temperature (about 25° C.) before transfer to an 80° C. oven for 6 hours. After curing, excess polyoxalamide polymer is cut off the edges of the Ge disc with a razor blade.

Example 13: Accelerated Abrasion of Ge Windows With and Without Protective PBH-121 Film

35-micron glass beads from Comco Inc. (Burbank, California, USA) are loaded in a microbead blaster from Comco Inc. The system pressure is set to 80 psi and a bead blasting nozzle mounted in a configuration that aims the nozzle at the center of the 1-inch Ge window. The 1-inch Ge window is loaded into the clamp and positioned 4.25 inches from the nozzle exit. The particles are fired at the window for a fixed duration.

An unprotected Ge window and a Ge window protected with 100 μm of PBH-121 film are both abraded for 5 seconds, and then another 25 seconds, for a total of 30 seconds exposure each.

FIG. 9A shows a camera image of the unprotected Ge window before any abrasion (no exposure to glass beads). FIG. 9B shows a camera image of the unprotected Ge window after 5 seconds of abrasion with glass beads. FIG. 9C shows a camera image of the unprotected Ge window after 25 additional seconds of abrasion with glass beads (30 seconds of total exposure to glass beads). It is estimated that after 5 seconds of abrasion, there is about 0.1% mass loss of Ge, and that after 30 total seconds of abrasion, there is about 0.9% mass loss of Ge. The significant mass loss and surface roughening are easily observed visually in FIG. 9B and especially FIG. 9C.

FIG. 10A shows a camera image of the polyoxalamide-protected Ge window before any abrasion (no exposure to glass beads). FIG. 10B shows a camera image of the polyoxalamide-protected Ge window after 5 seconds of abrasion with glass beads. FIG. 10C shows a camera image of the polyoxalamide-protected Ge window after 25 additional seconds of abrasion with glass beads (30 seconds of total exposure to glass beads). It is observed that after 5 seconds of abrasion, there is no mass loss of Ge, and that after 30 total seconds of abrasion, there is no mass loss of Ge, from the polyoxalamide-protected Ge window. The surface is only lightly foggy after exposure to glass beads.

FIG. 11A shows a light-microscope image (photomicrograph) of the unprotected Ge window before any abrasion (no exposure to glass beads). FIG. 11B shows a light-microscope image of the unprotected Ge window after 5 seconds of abrasion with glass beads. FIG. 11C shows a light-microscope image of the unprotected Ge window after 25 additional seconds of abrasion with glass beads (30 seconds of total exposure to glass beads). FIGS. 11B and 11C reveal significant surface roughness caused by the physical impingement of the glass beads onto the Ge surface.

FIG. 12A shows a light-microscope image of the polyoxalamide-protected Ge window before any abrasion (no exposure to glass beads). FIG. 12B shows a light-microscope image of the polyoxalamide-protected Ge window after 5 seconds of abrasion with glass beads. FIG. 12C shows a light-microscope image of the polyoxalamide-protected Ge window after 25 additional seconds of abrasion with glass beads (30 seconds of total exposure to glass beads). Very little surface roughness is caused by the physical impingement of the glass beads onto the polyoxalamide-protected Ge surface.

FIG. 13 shows a long-wave infrared (LWIR) camera image capturing LWIR transmission through a 1-inch, unprotected Ge window before and after 30 seconds of window abrasion with glass beads. Following abrasion, there is no clear image capable of being transmitted due to window damage.

FIG. 14 shows a long-wave infrared (LWIR) camera image capturing LWIR transmission through a 1-inch, polyoxalamide-protected Ge window before and after 30 seconds of window abrasion with glass beads. The Ge window is protected with a bonded film with thickness of 4 mil (about 100 μm). Following abrasion, the LWIR transmission quality is nicely maintained.