UV-curable thiol-ENE system for broadband infrared transparent objects

Abstract

An ultraviolet (UV)-curable composition. The composition includes a first precursor compound comprising more than one alkene groups and a second precursor compound including more than two thiol groups. The first precursor compound may consist of carbon, hydrogen, and optionally sulfur. The second precursor compound may consist of carbon, hydrogen, and sulfur. The composition may be used to form the polymeric material into substrates via 3D printing and UV-curing.

Description

UV-CURABLE THIOL-ENE SYSTEM FOR BROADBAND INFRAREDTRANSPARENT OBJECTSCROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims benefit of U.S. Provisional Application No. 63 / 605,168 filed December 1, 2023, the specification of which is incorporated herein in its entirety by reference.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under Grant No. CA268190 awarded by the National Institutes of Health. The government has certain rights in the invention.FIELD OF THE INVENTION

[0003] The present invention is directed to a moldable, 3D-printable, photo-curable, and UV-curable material capable of visual, middle-wave infrared (MWIR), and long-wave infrared (LWIR) transparency.BACKGROUND OF THE INVENTION

[0004] Optics in the infrared (IR) region play a critical role in various applications, including cancer diagnosis, autopilot sensors, defense, aerospace, etc. The most common materials used for IR windows and optics include ZnS, ZnSe, GeO2, Ge, and Si, but most of these inorganic materials are brittle, have high melting points, and are costly to fabricate compared to most polymeric materials. Furthermore, many chalcogenide glasses are not transparent to visible light, which limits their applications. In contrast, inorganic salts like NaCl, KBr, and CaF2can be used to fabricate IR windows with relatively low-cost methods, such as high-pressure compression. However, they are sensitive to moisture in the air, limiting their applicability in certain environments. Compared to most inorganic materials, organic polymeric materials are known for their good flexibility, extensibility, and plasticity at relatively low temperatures, which make processing easier compared to inorganic materials. Polymeric materials like poly(methyl methacrylate), epoxy resins, polycarbonate, and silicone have been widely used in different applications such as lens material, lens coating, and bandpass filters. However, the majority of these polymers cannot be used for applications that require transparency in the MWIR (3-5 pm) or LWIR (7-14 pm) region due to the complex absorption peaks caused by covalent bonds in the polymer structure, resulting in a lack of transparency, especially when the polymer structure contains not only hydrocarbons but also oxygen, nitrogen, or other atoms that introduce dipole moments. This effect is more pronounced in the LWIR region since it is located in the fingerprintregion of the IR spectrum. Poly(ethylene) (PE) shows good transparency not only in MWIR but also in LWIR due to its simple structure with only hydrocarbons. However, because of its thermoplastic properties, this material has low thermal resistance. Furthermore, its semi-crystalline nature causes it to lose transparency in the visible light region.

[0005] To overcome the limitations of PE, some cross-linked polymers prepared using sulfur and vinyl compounds were developed. Elemental sulfur has a high refractive index and good transparency in both the MWIR and LWIR regions, making incorporating a high ratio of sulfur into polymer chains a simple but effective route to fabricate IR-transparent materials. Early reports describe utilizing inverse vulcanization with sulfur and diisopropenylbenzene (DIB) to form cross-linked copolymers that are transparent in the MWIR region. However, the presence of both methyl and aromatic rings in the network results in poor transparency of the material in the LWIR region. Compared to thermal curing, photo-curing shows advantages in curing efficiency and convenience. Moreover, the rapid development of photo-based 3D printing techniques further enhances the potential of photo-curable systems in the fabrication of objects with tiny and complex structures. Optics with complex structures have been fabricated through different types of photo-based 3D printing techniques. However, no material has been reported that is transparent in both MWIR and LWIR regions and can be 3D printed through a photo-curing mechanism. Therefore, it would be desirable to develop a photo-curable resin that is transparent in multiple IR regions.

[0006] Thiol-ene click reaction is famous for its high efficiency. This reaction mechanism, operating through a free radical process, offers versatility by being triggered through either thermal or photo initiation, both with or without initiators. Such a feature not only makes the thiol-ene system a powerful tool in polymer synthesis and biosynthesis, but also makes it widely used in material science including coating, molding, and additive manufacturing. Notably, the majority of thiol-ene materials exhibit favorable transparency within the visible light range, rendering them a prime choice for fabricating optics characterized by diverse mechanical and optical properties. However, most thiol-ene systems are unsuitable for infrared imaging applications, especially within the LWIR region. This is primarily due to the presence of molecular structures, such as ether and carbonyl groups, which lead to IR absorption across both MWIR and LWIR spectra, resulting in diminished transparency. Thus, there exists a present need for a moldable, 3D-printable, photo-curable, and UV-curable material capable of visual, middle-wave infrared (MWIR), and long-wave infrared (LWIR) transparency.BRIEF SUMMARY OF THE INVENTION

[0007] It is an objective of the present invention to provide compositions that allow for a moldable, 3D-printable, photo-curable, and UV-curable material capable of visual, middle-wave infrared (MWIR), and long-wave infrared (LWIR) transparency, as specified in the independent claims. Embodiments of the invention are given in the dependent claims. Embodiments of the present invention can be freely combined with each other if they are not mutually exclusive.

[0008] Conventional infrared transparent materials, including inorganic ceramic, glass, and sulfur-rich organic materials, are usually processed through thermal or mechanical progress. The present invention features a photo-curable liquid material based on a specially designed thiol-ene strategy, where the multi thiols and divinyl oligomers were designed to contain only C, H, and S atoms. This approach ensures transparency in a wide range spectrum from visible light to mid-wave infrared (MWIR), and to long-wave infrared (LWIR). The refractive index, thermal properties, and mechanical properties of samples prepared by this thiol-ene resin were characterized. Objects transparent to LWIR and MWIR were fabricated by molding and two-photon 3D printing techniques. The potential of the material in a range of applications is shown, including the fabrication of IR optics with high imaging resolution and the construction of micro-reactors for temperature monitoring. This UV-curable thiol-ene system provides a rapid and convenient alternative for the fabrication of thin, IR-transparent objects.

[0009] One of the unique and inventive technical features of the present invention is the implementation of a thiol-ene mechanism for developing a compound comprising only carbon, sulfur, and hydrogen groups. Without wishing to limit the invention to any theory or mechanism, it is believed that the technical feature of the present invention advantageously provides for an easily formable and 3D-printable material capable of visual, MWIR, and LWIR transparency. None of the presently known prior references or work has the unique inventive technical feature of the present invention.

[0010] Any feature or combination of features described herein are included within the scope of the present invention provided that the features included in any such combination are not mutually inconsistent as will be apparent from the context, this specification, and the knowledge of one of ordinary skill in the art. Additional advantages and aspects of the present invention are apparent in the following detailed description and claims.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

[0011] The features and advantages of the present invention will become apparent from a consideration of the following detailed description presented in connection with theaccompanying drawings in which:

[0012] FIGs 1 A-1E show molecular structures of the multithiol-divinyl oligomer (DVO) system and the long-wave infrared (LWIR) transmission results for pre-screening. FIG. 1A shows a scheme of monomer and resin preparation. FIG. IB shows the proposed di-vinyl oligomers(DVO) that will be selected. FIG. 1C shows the model compounds that are used in computation. FIG. ID shows an experimental LWIR transmission percentage of samples (500 pm thickness) prepared using DVO(2-10) and polySH. FIG. IE shows the simulated LWIR intensity of compounds in FIG. 1C. The shaded regions refer to potential windows transparent to LWIR.

[0013] FIGs 2A-2H shows broad band IR transmission results of samples with different thicknesses. FIG. 2A shows long-wave infrared (LWIR) transmission of samples prepared using TetraSH with divinyl oligomer (DVO2). FIG. 2B shows long-wave infrared (LWIR) transmission of samples prepared using polySH with DVO2. FIG. 2C shows long-wave infrared (LWIR) transmission of samples prepared using diurethane dimethacrylate (DUDMA). FIG. 2D shows the long-wave infrared (LWIR) transmission of samples prepared using pentaerythritol tetraacrylate (PETA) with different thicknesses. FIG. 2D shows mid-wave infrared (MWIR) transmission of samples prepared using tetraSH with DVO2. FIG. 2E shows mid-wave infrared (MWIR) transmission of samples prepared using polySH with DVO2. FIG. 2G shows mid-wave infrared (MWIR) transmission of samples prepared using DUDMA. FIG. 2H shows mid-wave infrared (MWIR) transmission of samples prepared using PETA.

[0014] FIGs 3A-3H show thermal and mechanical properties of cured tetraSH-divinyl oligomer (DVO2) and polySH-DVO2. FIG. 3A shows differential scanning calorimetry (DSC) of samples after thermally post-curing treatment. FIG. 3B shows dynamic mechanical analysis (DMA) of samples after thermally post-curing treatment. FIG. 3C shows a stress-strain curve of samples prepared with tetraSH and DVO2. FIG. 3D shows a stress-strain curve of samples prepared with polySH and DVO2. FIG. 3E shows a comparison of samples’ ultimate stress before and after thermally post-curing. FIG. 3F shows a comparison of samples’ ultimate strain before and after thermally post-curing. FIG. 3G shows cyclic tensile testing of samples prepared with tetraSH and DVO2. FIG. 3H shows cyclic tensile testing of samples prepared with polySH and DVO2.

[0015] FIGs 4A-4L show mid-wave infrared (MWIR) imaging performance of lenses fabricated by molding or 3D printing. FIG. 4A shows a scheme of setup for MWIR imaging experiments with transmission mode and set up for imaging experiments of USAF 1951 targets with reflection mode. FIGs 4B-4E show imaging taken with a commercial lens, lens molded with tetraSH and divinyl oligomer (DVO2) resin, lens molded with polySH and DVO2 resin, and lens molded withPETA, respectively. All the molded lenses have a diameter of 6 mm and a thickness of 180 pm. FIG. 4F shows an image of a human face obtained with a MWIR sensor and a commercial lens. FIG. 4G shows an image of the human face obtained with a MWIR sensor and molded lens. FIG. 4H shows the reflection imaging result of a USAF 1951 target as a reflective target. The main body of the USAF target is made of glass and the numbers and bars on this target were coated with chrome with high reflectivity. FIG. 41 shows a scheme of fabrication of the lens array for the experiment. A 3X3 lens array was printed on a NaCl plate by two-photon polymerization using the resin prepared with tetraSH and DVO2. A polymer mask containing 3X3 holes matched to the lens array was 3D printed by a commercial digital light processing (DLP) printer and a commercial resin to block light not passing through the lens array. FIG. 4J schemes of the imaging systems for experiments. The printed lens array and molded lens were assembled in a frame to maximize the imaging quality. Both the transmission mode (top) and reflection mode(bottom) were employed during the experiments. FIG. 4K shows a transmission image of steel masks with a single hole (left, 5 mm diameter), vertical grille (middle, 2 mm width), and mesh grille (right, 2x2 mm for every single lattice). FIG. 4L shows reflection images of USAF targets (Group -2, left and middle) and a steel ruler (right) as reflection targets.

[0016] FIGs 5A-5G show the long-wave infrared (LWIR) imaging performance of lenses fabricated by molding. FIG. 5A shows a scheme of assembling an LWIR camera using the molded lens. The assembled camera was connected to a cellphone to capture images or record video using an APP provided by Seek Thermal. FIG. 5B shows an LWIR image of a mesh grille using a lens molded using tetraSH and divinyl oligomer (DVO2). FIG. 5C shows an LWIR image of the same mesh grille using a lens molded using pentaerythritol tetraacrylate (PETA). FIG. 5D shows a scheme of adjusting imaging distance for LWIR imaging using the 3D-printed mounting structure. FIGs 5E-5G show the field of view (FOV) and magnification change as the imaging distance increases. The imaging experiments were conducted in reflection mode.

[0017] FIGs 6A-6F show the performance of a 3D-printed micro-reactor for temperature monitoring. FIG. 6A shows a scheme of using a long-wave infrared (LWIR) camera to monitor temperature change inside of a printed micro-reactor. The IR transparent wall faced the camera. The wall thickness was 100 pm. FIG. 6B shows the 5M NaOH mixed with DI water. The temperature change was not significant enough to be detected with the setup. FIG. 6C shows the 5M NaOH mixed with 5M HC1. The reactor turned brighter in the LWIR range right away after the NaOH was added. FIG. 6D shows a NaOH particle added to the 5M HC1. The reactor became much brighter in the LWIR range. FIG. 6E shows the IR untransparent wall facing the camera.The wall thickness was 100 pm. FIG. 6F shows the temperature change inside the reactor cannot be detected when the diurethane dimethacrylate (DUDMA) wall faces the camera.

[0018] FIGs 7A-7D show the steps of synthesizing multi thiols. FIG. 7A shows synthesis of 1 -chi oro-3 -(hydroxy ethylthio)-2-propanol (CHTEP). FIG. 7B shows the synthesis of tetraol. FIG. 7C shows the synthesis of tetrathiol. FIG. 7D shows the synthesis of polythiol.DETAILED DESCRIPTION OF THE INVENTION

[0019] Referring to the figures, the present invention features an ultraviolet (UV)-curable composition. In some embodiments, the composition may comprise a first precursor compound comprising more than one alkene groups, and a second precursor compound comprising more than two thiol groups. The first precursor compound may consist of carbon, hydrogen, and optionally sulfur. The second precursor compound may consist of carbon, hydrogen, and sulfur. In some embodiments, the alkene groups may be vinyls. In some embodiments, the first precursor compound may comprise a carbon chain or a cycloalkyl. In some embodiments, the first precursor compound may comprise a di-vinyl oligomer, or a vinylic polymer. In some embodiments, the second precursor compound may comprise a polythiol or a tetrathiol. In some embodiments, the first precursor compound and the second precursor compound may be mixed and UV-cured to form a polymeric material that is transparent to mid-wave infrared (MWIR) and to long-wave infrared (LWIR). In some embodiments, the composition may be used to form the polymeric material into substrates via 3D printing and UV-curing.

[0020] According to some embodiments, the present invention features an optical element comprising any embodiment of the polymer described above. In some embodiments, the optical element may be MWIR and LWIR transparent. In some embodiments, the optical element may be a lens. According to some embodiments, the present invention features a method of preparing an optical element. In some embodiments, the method may comprise forming any embodiment of the composition described above into a shape of the optical element, and UV-curing said composition. In some embodiments, the composition may be formed via 3D printing or by a molding process. In some embodiments, the optical element may comprise a substrate. In some embodiments, the optical element may comprise a lens, a prism, a grating, a filter, a window, an optical flat, a polarizer, a beamsplitter, a wave plate, a fiber optic, or a combination thereof. In some embodiments, the optical element may be formed by a photon-assisted or thermal -assisted compression or press molding process.

[0021] According to some embodiments, the present invention features a polymeric material that is transparent to mid-wave infrared (MWIR) and to long-wave infrared (LWIR). In someembodiments, said polymer may consist of carbon, hydrogen, and sulfur. Said polymer may be a product of ultraviolet (UV) curing a composition. In some embodiments, said composition may comprise a first precursor compound comprising more than one alkene group. The composition may further comprise a second precursor compound comprising more than two thiol groups. The first precursor compound may consist of carbon, hydrogen, and optionally sulfur. The second precursor compound may consist of carbon, hydrogen, and sulfur. In some embodiments, the alkene groups may be vinyls. In some embodiments, the first precursor compound may comprise a carbon chain or a cycloalkyl. In some embodiments, the first precursor compound may comprise a di-vinyl oligomer, or a vinylic polymer. In some embodiments, the second precursor compound may comprise a polythiol or a tetrathiol.

[0022] According to some embodiments, the present invention features an optical element that is transparent to mid-wave infrared (MWIR) and to long-wave infrared (LWIR). In some embodiments, the optical element may be formed via a 3D printing or molding process. In some embodiments, the optical element may comprise a polymer consisting of carbon, hydrogen, and sulfur. Said polymer may be a product of ultraviolet (UV) curing a composition. The composition may comprise a first precursor compound comprising more than one vinyls. The composition may further comprise a second precursor compound comprising more than two thiol groups. The first precursor compound may consist of carbon, hydrogen, and optionally sulfur. The second precursor compound may consist of carbon, hydrogen, and sulfur. In some embodiments, the first precursor compound may comprise a carbon chain or a cycloalkyl. In some embodiments, the first precursor compound may comprise a di-vinyl oligomer, or a vinylic polymer. In some embodiments, the second precursor compound may comprise a polythiol or a tetrathiol. In some embodiments, the optical element may comprise a substrate. In some embodiments, the optical element may comprise a lens, a prism, a grating, a filter, a window, an optical flat, a polarizer, a beamsplitter, a wave plate, a fiber optic, or a combination thereof. In some embodiments, the optical element may be formed by a photon-assisted or thermal-assisted compression or press molding process.EXAMPLE

[0023] The following is a non-limiting example of the present invention. It is to be understood that said example is not intended to limit the present invention in any way. Equivalents or substitutes are within the scope of the present invention.

[0024] As previously mentioned, PE has excellent IR transparency in both the LWIR and MWIR ranges. This is due to the PE molecule consisting only of C and H atoms, which avoids thevibration caused by bonds between C (or H) and other hetero atoms, such as C=O, C-O, C-N, C-X, N-H, O-H, etc. Additionally, saturated hydrocarbons avoid vibrations caused by alkenes, alkynes, or aromatic rings. Moreover, the highly branched molecular structure of low-density polyethylene (LDPE) forces the methylene chains apart, making the CH2peaks of LDPE in the fingerprint region simpler than the CH2peaks of high-density polyethylene (HDPE) in the fingerprint region, resulting in better transparency in the LWIR than HDPE. Some previously reported LWIR transparent materials contain not only C and H, but also S and Tin. The absorption peaks caused by C-S and C-Sn are not as complicated as the peaks caused by C-0 stretching in the fingerprint region, which helps avoid massive peaks in the LWIR region. Therefore, to achieve IR transparency in both the MWIR and LWIR range, the molecular design followed two basic principles: 1. The element (atom) types in the final molecular structure were kept to a minimum; 2. The asymmetric aromatic rings were avoided in the final molecular structure. As UV-curing was a desired feature of the target material, the thiol-ene curing system was selected because it allowed avoiding the use of acrylate, methacrylate, or vinyl benzene, which were also UV-curable but introduced chemical bonds that significantly affect the MWIR and LWIR transparency.

[0025] To enable the thiol-ene system for 3D printing, multifunctional building blocks with more than two functional groups per molecule were required to form the crosslinked network during the printing process. However, commonly used candidates in thiol-ene curing systems, such as glyoxal bis(diallyl acetal), pentaerythritol tetrakis(3-mercaptopropionate), Trimethylolpropane tris(3 -mercaptopropionate), and l,3,5-Triallyl-l,3,5-triazine-2,4,6(lH,3H,5H)-trione were not selected for preparing the IR transparent resin, as they all contain C-O, C=O, or C-N bonds. Instead, tetrathiol and polythiol, which only contain C, H, and S, were chosen as the source of thiol. The di-vinyl oligomers (DVO) were synthesized using two equivalents of 5-vinyl-2-norbornene and one equivalent of dithiols. Due to the reactivity difference between the vinyl in the norbornene ring and the terminal vinyl outside of the ring, the thiol group preferred to react with the vinyl in the norbomene ring, leaving the terminal vinyl unreacted. The tetrathiol (tetraSH) and polythiol (polySH) were then synthesized and utilized as the multi-thiol source in the thiol-ene reaction during UV curing. FIG. 1A shows the basic route designed to prepare the thiol-ene system for the infrared (IR) transparent resin.

[0026] To prepare the DVO required for the thiol-ene reaction, four different dithiols, namely 2,2'-thiodiethanol, 1,6-hexanedithiol, 1,8-octanedithiol, and 1,10-decanedithiol, were chosen as bridge molecules to react with 5-vinyl-2-norbornene, yielding DVO2, DVO6, DVO8, andDVOIO, respectively (FIG. IB). The crosslinked material prepared with DVO containing longer hydrocarbon bridges showed lower transmission percentages in the LWIR region. Specifically, samples prepared with DV06, DV08, and DVOIO, when measured with a 500 pm thickness window, exhibited very low transparency with no peak higher than 5% of transmission in the LWIR region (FIG. ID).

[0027] Since the samples used were not purified, computations of some model compounds were also conducted to predict the IR spectra. Several model compounds were designed based on the structure of different dithiols, and the simulated IR spectra of the gas phase were generated through a standard computational procedure. The different dithiol molecules were connected through two norbomane rings to simplify the modeling molecules. Spectra of compounds that have 3, 12, and 20 carbons between S atoms were also simulated. Since most of the MWIR region referred to wavenumber higher than 1500 cm’1, which is out of the fingerprint region, the absorption peaks within the MWIR region were easily predicted from the molecule structure, and all the proposed candidates had similar absorption peaks in the MWIR region. Therefore, only the computational results of the LWIR region were compared. FIGs 1B-1C show the correspondence between the potential molecules designed for IR transparent resin and the model compounds used in the simulation.

[0028] The gas phase IR spectra predicted by computational simulations did not necessarily reflect the real transmission spectra of condensed phase samples, particularly when the samples were relatively thick. Gas phase spectra represented extremely dilute samples, while in the condensed phase, intermolecular interactions between molecules led to more complex absorption peaks that cannot be ignored. Therefore, the main purpose of simulating gas phase spectra was to identify a window containing zero or weak IR signals. The fewer and weaker the peaks are, the greater the likelihood of achieving high transmission for the corresponding wavelength when the sample was in a condensed phase with a relatively thick thickness.

[0029] The simulated IR spectra showed that when there were only two carbons between sulfur atoms, four small windows were identified that did not contain significant absorption peaks (the shaded region in FIG. IE). These windows aligned with the high transmission region (FIG. 1C) observed in the actual sample in FIG. ID. However, as the number of carbons increased to three, new absorption peaks appeared in the 800 cm’1- 860 cm’1window, the 720 cm’1window, and 1360 cm’1- 1425 cm’1window became narrower. The introduction of new hydrocarbon bonds, which differed from those directly connected to the S atoms, caused this effect. For example, the new peak at around 1350 cm’1could have belonged to out-of-plane -CH2- rocking vibrationswhen there were more than 3 carbons between two sulfur atoms. Further increases in the number of carbons to 6 and 8 introduced more complex C-H vibrations in the fingerprint region, causing the 1360 cm’1- 1425 cm’1window to decrease in size and the 1050 cm-1-1080 cm’1and 800 cm-1-860 cm’1windows to disappear. Increasing the number of carbons to 10, 12, and 20 did not lower the signal intensity in the LWIR region. 2,2'-thiodiethanol (TDE) was chosen for the synthesis of both DVO and the multi-thiols for preparing IR transparent films and optics.

[0030] The synthesis of DVO was achieved by mixing TDE and 5-Vinyl-2-norbornene, which then reacted with each other. The thiol group exhibited a preference for reacting with the norbomene ring due to the difference in reactivity between the vinyl in the ring and the terminal vinyl outside the ring. As a result, the terminal vinyls were left unreacted, although a small portion of the thiol group still reacted with them. The 1H Nuclear Magnetic Resonance (NMR) spectrum indicated that during the synthesis of DVO2, approximately 75% of ring vinyls (peaks from 6.15 ppm to 5.90 ppm) and 25% of terminal vinyls (peaks from 5.80 ppm to 4.75 ppm) had reacted with thiols.

[0031] To prepare the resin for two-photon printing and UV curing, the vinyl and thiol concentrations of DVO, polySH, and tetraSH were determined by NMR or titration, as they were not highly pure reagents. Then, DV02 was mixed with either polySH or tetraSH. 0.1 wt% of pyrogallol inhibitor and 2 wt% of 2,4-diethyl-9H-thioxanthen-9-one photo-initiator were added. 2,4-diethyl-9H-thioxanthen-9-one was found to be a suitable photo-initiator as it did not contain amine groups and had acceptable initiation efficiency despite its lower absorption efficiency to 380 nm light compared to other initiators.

[0032] To assess the infrared (IR) transparency of samples prepared with tetraSH or polySH, two sets of test windows with thicknesses of approximately 150 pm and 550 pm were fabricated on KBr crystals. The 150 pm samples demonstrated good transparency in the LWIR region, as two large windows (7 - 8 pm and 8.5 - 14 pm) with transmittance as high as 70% were observed (FIGs 2A-2B). However, with a thickness of 500 pm, the transmittance decreased. Both samples exhibited three main transparent windows in the LWIR region, namely, 1313 cm’1- 1401 cm’1, 1124 cm’1- 960 cm’1, and 954 cm’1- 760 cm’1, with transmittance between 10% and 20%. At a thickness of around 900 pm, both materials demonstrated only minor transmission peaks in the LWIR region, showing nearly no transparency to LWIR at this thickness. For comparison, two additional windows were produced using diurethane dimethacrylate (DUDMA, FIG. 2C) and pentaerythritol tetraacrylate (PETA, FIG. 2D), which were commonly utilized as monomers or crosslinkers in 3D printable resins. Due to strong absorption by C-O, the transparency ofwindows fabricated using these two molecules in the LWIR region was considerably poorer (FIGs 2C-2D). With a thickness of 150 pm, DUDMA demonstrated only a narrow window from 10.5 pm to 14 pm with a peak transmittance of 30%, while PETA exhibited better performance, with a peak transmittance of around 55%, but the transparency window was still much narrower than that of samples produced using tetraSH and polySH in conjunction with DVO2. At higher thicknesses (-500 pm), both cured DUDMA and PETA demonstrated almost no transparency in the LWIR region.

[0033] In addition to the LWIR transparency of the tetraSH and polySH samples, it is also worth noting that they exhibited adequate transparency in the MWIR region (3-5 pm). To evaluate the MWIR transparency, samples prepared with tetraSH or polySH together with DVO2, DUDMA, and PETA were compared. Since all those materials have much fewer absorption peaks between 3-6.5 pm compared to the absorption peaks in the LWIR region, they all showed better transparency in the MWIR region with a thickness of 500 pm. However, DUDMA and PETA contained high amounts of C=O which severely affected their transparency in the MWIR region. Both the tetraSH-DVO2 and polySH-DVO2 samples showed a maximum transmission of over 60% (FIGs 2E-2F), while the DUDMA and PETA samples only showed transmission of less than 30% and 20%, respectively (FIGs 2G-2H). Even at a thickness of 2 mm, the tetraSH and polySH samples exhibited a maximum transmission of close to 20%, whereas the DUDMA and PETA samples showed almost no transmission. These results highlighted the advantages of using the thiol-ene system for MWIR transparency compared to conventional UV-curable materials.

[0034] Prior to performing 3D printing or molding, the mechanical and thermal properties of the UV-cured resins were characterized. It was observed that both samples prepared using tetraSH or polySH exhibit chemical stability at temperatures below 300°C. Upon UV curing, the resins displayed significant elastic behavior under external pressure, which indicated that they were likely elastomers with glass transition temperatures (Tg) lower than room temperature. The Tgof the two resin systems was determined using differential scanning calorimetry (DSC, FIG. 3A) and dynamic mechanical analysis (DMA, FIG. 3B). The results from both methods indicated that the resin prepared using tetraSH or polySH had slightly different Tg, with a slightly higher Tgobserved for the polySH-prepared sample. Meanwhile, the discrepancy between the DSC and DMA results was due to the difference in thermal transportation in the two technologies, as well as the difference in sample size used in DMA and DSC. However, both sets of results indicated that the cured resin was considered an elastomer at room or higher temperatures.

[0035] It is important to note that during some early DMA testing, it was observed that the storage modulus increased at temperatures higher than 140 °C, suggesting the possibility of additional crosslinking during the measurement. This phenomenon was more pronounced in samples prepared using polySH. Correspondingly, a broad exothermic peak was also observed in the DSC measurement of cured polySH-DVO2 samples at temperatures higher than 150 °C, which disappeared upon the second run of the same sample. However, this peak was not significant in the DSC measurement of cured tetraSH-DVO2 samples, indicating that tetraSH-DVO2 samples have higher monomer conversion during UV curing. Fourier-transform infrared spectroscopy (FTIR) of the UV-cured sample also revealed the presence of unreacted vinyl and thiol.

[0036] To enhance the comprehension of the curing kinetics and optimize thiol-ene conversion, FTIR was employed to monitor the consumption of both thiols and alkenes during UV exposure. These findings indicated that in both the tetraSH and polySH systems, the conversion of thiol rapidly stabilizes at approximately 70% (thiol) for tetraSH and 50% (thiol) for polySH within the initial 60 seconds of standard UV exposure. These conversions were maintained over the subsequent 9-minute period. In order to investigate the potential for post-UV curing to further enhance thiol-ene conversion, two additional post-UV curing steps were conducted. Initially, the UV exposure was extended by another 10 minutes while maintaining the same power. The tetraSH system exhibited consistent conversion rates for both thiol and alkene, whereas the polySH system demonstrated a slightly increased conversion (to -55% for thiol). Subsequently, the samples were subjected to additional curing using a Formlabs® post-curing machine (at room temperature for 12 hours). The tetraSH system's conversion remained unchanged, whereas the polySH system's thiol-ene conversion significantly increased to approximately 90% (thiol), highlighting distinctive UV curing kinetics for these two systems. This difference can be attributed to structural dissimilarities between tetraSH and polySH, as the latter contains only primary thiols while the former comprises both primary and secondary thiols.

[0037] During the initial 10-minute curing period, the tetraSH shows a higher reaction rate in comparison to the polySH system which might be due to the lower molecular weight and viscosity of tetraSH compared to polySH. However, the lower reactivity of secondary thiols in tetraSH compared to primary thiols becomes a limiting factor. As the curing process progresses, the constrained mobility impedes the remaining secondary thiols from reacting effectively with alkenes under UV conditions. Conversely, in the polySH system, unreacted primary thiols continue to react with alkenes, albeit at a relatively slower rate. This continuous reactioncapability enables the conversion to reach 87% through overnight UV exposure. Nevertheless, while the DSC results for both the polySH-DVO2 and tetraSH-DVO2 systems display a nearly flat curve between 100 to 200 °C, DMA reveals a slight elevation in E' values for both systems at temperatures exceeding 160 °C.

[0038] Besides UV curing, it is well-known that thiol-ene systems can be cured by thermal curing with and without initiators. To further maximize thiol-ene conversion, both systems were subjected to a post-curing thermal treatment at 170 °C for 12 hours. After post-curing, both DMA and DSC measurements showed stable properties of the samples at temperatures from 100 to 200 °C, indicating that the reachable thiols and vinyls had reacted (FIGs 3A-3B). The storage modulus also increased from 3.8 MPa to 8.3 MPa for tetraSH-DVO2 samples and from 1.3 MPa to 13.2 MPa for polySH-DVO2 samples, respectively. Correspondingly, the thiol conversion for tetraSH-DVO2 and polySH-DVO2 was increased to 91% and 98%, respectively, as the thiol and alkene peaks became almost invisible in FTIR spectra. Here, the thiol conversion of tetraSH-DVO2 also increased, indicating that the higher temperature helped enhance molecule mobility, making the secondary thiol easier to react with the alkene. Although FTIR still showed unreacted thiol and vinyl peaks for both systems after post-curing, these vinyls and thiols are probably trapped and not reachable from each other since they are embedded in a solid. The thermal post-curing slightly increased the transmission in the LWIR region, but this difference was not significant enough to improve the imaging quality in the later imaging test. Thermally post-curing in the air caused oxidation of the sample, leading to brown color and decreased transparency in the visible light region. This color change was more noticeable for the sample prepared using polySH and DV02.

[0039] In order to assess the mechanical properties of the resin after UV curing, coupons molded into American Society for Testing and Materials® (ASTM) D638 type IV were prepared using either tetraSH or polySH. These coupons were then subjected to stress-strain testing, both with and without thermal post-curing. The stress-strain curves exhibited elastomeric behavior. Although both tetraSH-DVO2 (FIG. 3C) and polySH-DVO2 (FIG. 3D) samples had similar ultimate stress values, the samples made using polySH showed a much higher ultimate strain of 240% compared to tetraSH's ultimate strain of around 90%. This was attributed to the longer backbone of the polybutadiene used for the synthesis of polySH, which resulted in a more flexible final network. Additionally, the stress-strain results showed that the samples prepared with polySH were tougher than those prepared with tetraSH. Subsequently, both sets of samples underwent post-curing, resulting in the formation of more rigid networks. The ultimate stress wasincreased (from 0.6 MPa to 0.77 MPa for tetraSH-DVO2 and from 0.6 MPa to 3.57 MPa for polySH-DVO2), while the ultimate strain was decreased (from 90% to 76% for tetraSH-DVO2 and from 240% to 105% for polySH-DVO2). These results were consistent with DMA and DSC findings and confirmed that more thiols and vinyls reacted during the post-curing process. Table 1 lists the major thermal and mechanical properties of both tetraSH-DVO2 and polySH-DVO2 systems. Furthermore, cyclic tensile testing was performed on both tetraSH-DVO2 (FIG. 3G) and polySH-DVO2 (FIG. 3F) samples. TetraSH-DVO2 samples show good recovery ability after 200 cycles when the maximum strain was below 30%. However, larger strains, such as 50%, caused the samples to weaken and ultimately break, likely due to tiny defects introduced during the 3D printing of the ASTM® D638 type IV molds or during coupon molding. Meanwhile, PolySH-DVO2 samples show a slightly decreasing maximum force as the cycles increased with some small damages observed. This could be due to the fact that the overall monomer conversion of polySH-DVO2 is lower than the monomer conversion of tetraSH-DVO2 making it easier to be damaged under external stress.

[0040] Both molding and two-photon printing methods were used to fabricate optics with low surface roughness (< 5nm) and high surface accuracy (average error < 1 pm) (see methods section for more fabrication details). An MWIR camera was used to evaluate the MWIR imaging performance of both molded and printed optics. The original IR lens of the camera was replaced with the molded lens. To evaluate the imaging performance of the molded lenses, a laser-cut PMMA as an IR mask with a thickness of approximately 1 mm was used to block most of the MWIR signal from the 40 °C blackbody (FIG. 4A, top). The imaging results for the lenses molded by either tetraSH-DVO2 or polySH-DVO2 are shown in FIGs 4C and 4D, respectively. These results demonstrated the ability of the molded lenses to capture MWIR images with sharp edges, as evidenced by the capture of the “A” logo and the tiny laser-cutting structure. Although both tetraSH-DVO2 and polySH-DVO2 lenses exhibited lower contrast compared to the commercial lens (FIG. 4B), they exhibited much better imaging quality compared to the lens made with PETA (FIG. 4E), which absorbed much more MWIR and led to a blurry image result. Additionally, FIG. 4G demonstrates the competitive imaging resolution of the molded lens compared to the commercial IR lens (FIG. 4H), albeit with slightly lower contrast. These results suggest that this material has great potential for fabricating compact imaging systems, as all these images were taken with molded lenses of only 6 mm diameter and 180 pm thickness. Furthermore, materials transparent to visible light provide an additional advantage overconventional IR lens materials, allowing for more compact designs in applications that require detecting both visible and IR light.

[0041] To demonstrate the imaging resolution of the molded IR lenses, the U.S. Air Force (USAF) 1951 target was utilized to reflect the thermal radiation from an 80 °C heat plate (FIG. 4A, bottom). The USAF target body was made of glass that absorbed MWIR, while all the numbers and bars on the target were coated with chrome, which reflected MWIR with high efficiency. Therefore, all the numbers and bars show a much brighter color than other areas of this target in an MWIR image. FIG. 4H clearly shows that both the 3rd vertical and horizontal elements in Group 3 were identified, which have a line width of 49.61 pm, indicating excellent imaging resolution in MWIR.

[0042] The performance of the 3D-printed lens array was also evaluated. To prevent light from passing through the gaps between each singlet lens in the lens array, a plastic mask was also 3D printed using commercial printing resin to cover the lens array. This mask absorbed MWIR light and improved the imaging quality (FIG. 41). Furthermore, a black mask was also used to cover the entire NaCl substrate except the lens array region to ensure that the IR light only passed through each singlet lens in the lens array. To further enhance the imaging quality, the printed lens array was assembled with the molded lens (tetraSH-DVO2) to create an optical system (FIG. 4J). To demonstrate the imaging capabilities of the assembled imaging system, both transmission and reflection modes were employed (FIG. 4J). Various steel masks with different shapes were utilized to evaluate the imaging quality of the system (FIG. 4K). The results demonstrated that objects with millimeter-sized dimensions could be captured with reasonable quality. To quantitatively assess the imaging resolution of the assembled imaging system, the USAF target was employed as a positive target to reflect the radiation from a heat plate (FIG. 4L). Analysis revealed that element 2 (1.78 mm line width) in Group -2 could be identified, highlighting the high-resolution capability of the system. In addition, a steel ruler was also used as a negative reflection target to conduct the same experiment, as the numerical numbers on this ruler absorbed MWIR light. Both “3” and “0” in FIG. 4L had widths of around 3 mm and could be observed through the assembled imaging system.

[0043] To demonstrate the LWIR imaging ability of the optics fabricated using multi thiol-D VO resin, a 3D printed mount was utilized to assemble the molded lens onto an LWIR sensor from Seek Thermal®, which is sensitive to 7.8 - 14 pm light. This mounting structure facilitates easy lens changes within the imaging system (FIG. 5A). Images captured by a molded tetraSH-DVO2 lens and a PETA lens with the same thickness of 180 pm are shown in FIGs 5B-5C, respectively.The tetraSH-DV02 lens was able to depict the shape of the steel mesh as well as the contour of the heat plate behind it, whereas the PETA lens only captured a blurred shape of the steel mesh and failed to identify the heat plate. This comparison indicates that the conventional resins used for 3D printing or UV curing are not suitable for imaging applications in the LWIR region even with a thin thickness, but the multithiol-DVO resins offer clear advantages.

[0044] The 3D-printed mounting structure also enables easy tuning of the imaging distance (FIG. 5D), allowing for a different field of view (FOV) to be achieved without replacing the imaging lens. As demonstrated in FIGs 5E-5G, the FOV gradually decreased as the imaging distance increased, resulting in an increase in imaging magnification and the ability to observe smaller objects with greater detail. In FIG. 5G, all the elements in Group 3 on the USAF 1951 resolution target can be seen under high magnification, indicating that the imaging resolution can reach 35 pm. This resolution is highly valuable for LWIR imaging applications, considering that the pixel size of the sensor is only 12 x 12 pm.

[0045] In addition to IR imaging, the potential of the IR transparent thiol-ene resin was explored for monitoring temperature changes in micro-reactors through LWIR detection. The ability to monitor temperature changes during reactions in micro-reactors or microfluidic systems is critical for many applications. Typically, conventional methods use thermocouples, thermistors, or other custom-built sensors to measure the temperature at a specific location. These methods provide accurate temperature measurements but have limitations in measuring temperature change over a region. The limitation is that the nanoparticles need to be mixed into the solution. Herein, the IR transparent resin offers an alternative for monitoring temperature dynamics within reactors or channels, obviating the need for nanoparticles. The material's 3D printability further lends itself to the customization of micro-reactors or microfluidic channels. To demonstrate the temperature monitoring capability, a stitch-free printing method was first employed to create a micro-reactor with three walls made of DUDMA and one wall made of tetraSH-DVO2, as shown in FIG. 6A, and then positioned the tetraSH-DVO2 wall facing the LWIR camera and filled the micro-reactor with either 0.6 pL water or 0.6 pL 5M HC1.

[0046] 0.6 pL of 5M NaOH solution was then added to the reactor using a syringe. As shown in FIG. 6B, when there was only water in the reactor, the heat generated by the dilution of NaOH was not high enough to be detected. However, when 5M NaOH was mixed with 5M HC1, the acid-base neutralization released a larger amount of heat, which could be detected by the custom LWIR camera (middle picture in FIG. 6C). When the 5M NaOH was replaced with a NaOH particle, both the dissolving and neutralization generated much more energy in a shorter period,resulting in a brighter region in the LWIR image (FIG. 6D). Conversely, when the wall printed with DUDMA faced the LWIR camera (FIG. 6E), no significant changes were observed when NaOH was mixed with HC1 (FIG. 6F), indicating that most of the IR signal was absorbed by the cured DUDMA wall. Thus, it was demonstrated that it was possible to monitor temperature changes through a thin layer of the multithiol-DVO material using an LWIR camera, providing an alternative solution for monitoring reactions in microreactors for microfluidic systems.

[0047] In summary, a UV-curable thiol-ene system with multithiols and divinyl oligomers only based on S, C, and H atoms with maximum transparency higher than 60% in the MWIR range and 20% in the LWIR range at the thickness of 500 pm has been developed. It has been demonstrated that besides the functional groups, the hydrocarbon structure (chain length of CH2between S) also plays an important role in affecting materials’ LWIR transparency. The reported molecular design strategy ensures the resin shows impressive transparency in both MWIR and LWIR ranges compared to other commercial or reported UV-curable resins for photo-polymerization additive manufacturing. Thermal and mechanical studies indicate that the monomer conversion and the mechanical properties of photo-cured resin can be improved by the thermal post-curing process. It has been demonstrated that optics with low surface roughness (< 5nm) and high surface accuracy (average error < 1 pm) can be fabricated by both UV-assisted molding and photo-polymerization 3D printing methods. High imaging resolution (< 50 pm) was reached for both MWIR and LWIR imaging applications.

[0048] Materials: Epichlorohydrin, 2-mercaptoethanol, 5-Vinyl-2-norbomene, polybutadiene(90% 1,2-vinyl), 1,6-hexanedithiol, 1,8-octanedithiol, PETA, DUDMA, pyrogallol, and 2,4-diethyl-9H-thioxanthen-9-one were purchased from Sigma-Aldrich®. Bis(2-mercaptoethyl) sulfide and 1,10-Decanedithiol were purchased from TCI America®. All the chemicals were used as received. The synthesis procedure was modified based on the procedure in previous literature. Epichlorohydrin (10.00g, 0.108 mol), borax (4.12g, 0.011 mol), and DI water (50 mL) were mixed in a 250 mL round bottom flask. 2-mercaptoethanol (8.44g, 0.108 mol) was then added to the flask dropwise with stirring at room temperature. The mixture became a homogenous solution gradually during the addition. This solution was stirred at room temperature for 4 hours. The solution was then extracted using chloroform (100 mL x 3). The combined organic layer was then washed with brine (50 mL) and dried using MgSO4. The solvent was then removed using rotary evaporation to obtain a colorless and viscous oil (10.38 g, 56 % yield) which was used in the next step without further purification. 'H NMR (500 MHz, CDC13) 5 = 4.01-3.95 (m, 1H), 3.80(t, 2H), 3.68-3.61(m, 2H), 2.88-2.70(m, 4H);13C NMR (500MHz, CDC13) 5 = 70.63, 61.22, 47.91, 36.27, 35.92.

[0049] The synthesis procedure was modified based on the published literature. In a 250 mL round bottom flask, ethanol (50 mL), NaOH (2.15 g, 0.054 mol), and bi s(2 -mercaptoethyl) sulfide (4.146 g, 0.0269 mol) were mixed and stirred for 10 min. After that, CHTEP (9.219 g, 0.0537 mol) was added dropwise. A white precipitate was formed during the addition. The suspension was stirred at room temperature overnight. After this period, 36% HC1 (7.68 g) was added slowly to the suspension and all the precipitate was filtered off. The solvent in the obtained solution was then removed under vacuum to get a colorless and viscous oil (8.055 g, 70.8% yield) which was used in the next step without further purification. 'H NMR (500 MHz, MeOD-d6) 5 = 3.88-3.81 (m, 2H), 3.69(t, 4H), 2.84-2.64(m, 20H);13C NMR (500 MHz, MeOD-d6) 5 = 70.84, 61.18, 37.58, 37.27, 34.75, 32.60, 31.77.

[0050] The synthesis procedure follows the published literature. Tetrol (8.055 g, 0.0191 mol), 36% HC1 (17.9 g), and thiourea (8.1 g, 0.106 mol) were added to a 3-neck round bottom flask. The mixture was heated to 110 °C for 1 hour. After this period, the solution was cooled to room temperature, and the 50 wt% NaOH solution (17.88 g) was added under N2atmosphere. The mixture was then stirred at room temperature for 24 h. After that, the aqueous layer was extracted using toluene (100 mL x 4). The combined organic layer was then washed with 1 M HC1 (100 mL), water (100 mL), brine (100 mL), and dried with MgSO4. The toluene was then removed with reduced pressure to obtain a colorless oil (5.81 g, 62.4% yield). This tetrathiol was used without further purification. 'H NMR (500 MHz, CDC13) 5 = 3.15-2.60(m, 26H), 1.82-1.66(m, 4H); 13C NMR (101 MHz, CDC13) 5 = 51.6, 48.9, 37.1, 36.1, 35.7, 28.8, 28.7, 28.2, 25.1, 24.9; LRMS (ESI): calculated [M + Na]+: 508.87, found: 508.97.

[0051] To a 100 mL round bottom flask, polybutadiene (1,2 addition, 0.50 g, 0.0083 mol of vinyl group), 2,2 -thiodi ethanethiol (1.27 g, 0.0083 mol, 1 equiv. to vinyl group), benzophenone (20 mg, 0.1 mmol) and 50 mL of THF were added. This solution was exposed to UV for 1.5 h with stirring at room temperature. After that, the solvent was removed using rotary evaporation and then a high vacuum with a cold trap filled with liquid N2to obtain a highly viscous colorless oil. 'H NMR and13C NMR show that all the vinyl groups from polybutadiene were consumed. To a 25 mL round bottom flask, 5-vinyl-2-norbornene (2.00 g, 0.017 mol,) was added. This flask was then purged with N2for 5 min. Then, Bis(2-mercaptoethyl) sulfide (0.008 mol) was added dropwise under N2. The solution was allowed to be stirred at room temperature for 24 hours. After that period, the vinyl concentration of the final liquid product was determined using NMR with MEHQ as a calibrator. The diene oligomers prepared using other dithiols (1,6-hexanedithiol,1,8-octanedithiol, and 1,10-decanedithiol) were prepared using a similar method.

[0052] In general, in a 250 mL Erlenmeyer flask, around 0.15g polythiol and 15 mL pyridine were mixed together to form a solution. A 5 mL silver nitrate solution (0.4 M) was then added to this solution to form a yellow suspension. This suspension was allowed to sit for 5 min. After that period, 100 mL H2O and 1 drop of phenolphthalein (1% solution) were added to the suspension. This solution was then titrated using 0.1 M NaOH to light pink as the endpoint. Each batch of polythiol was titrated individually to get the accurate thiol concentration. In general, the UV-curable thiol-ene resin was prepared by mixing diene oligomer and tetraSH (or polySH) together based on the vinyl concentration of diene oligomers and the thiol concentration of tetraSH (or polySH). TetraSH was firstly mixed with 2,4-diethyl-9H-thioxanthen-9-one (initiator, 2 wt% calculated based on the total mass of TetraSH and DVO) and pyrogallol (inhibitor, 0.1 wt% calculated based on the total mass of TetraSH and DVO). DVO was then added to the vial. This mixture was stirred until a homogenous resin was formed.

[0053] To prepare the comparison resin containing acrylate and methacrylate groups, pentaerythritol tetraacrylate or diurethane dimethacrylate was mixed with initiator (2 wt% to resin), respectively. The resin can be cured under UV irradiation. The calculation of IR spectra of model compounds was conducted using Gaussian® 16. For each molecule, the energy was firstly optimized using B3LYP / 6-31*. Then, the frequency calculation was done with the energy-optimized configuration using the B3LYP / 6-31* set. The predicted IR spectra of the gas phase can be obtained from the frequency calculation output using GaussSum™ with a scaling factor of 0.98. The printing system contains a 780 nm femtosecond fiber laser with 150 fs pulse, 77MHz, and a maximum power of 130 mW. The full-width half maximum (FWHM) of the beam was 5 mm, 83% filling the objective (NA=0.6). The component was printed with a 1.17 nJ pulse energy and 16900 pm / s on the Sodium-chloride substrate. After printing, the uncured resin was washed using THF for 15 minutes and post-cured under UV exposure for 10 minutes. When thermal post-curing of the UV-cured sample was needed, the UV-cured samples were placed in a sealed oven. Nitrogen was purged for 15 min before increasing the temperature. After that period, the samples were heated to 170 °C and kept for 12 hours in the N2atmosphere, and cooled to room temperature before they were removed from the oven.

[0054] To demonstrate the imaging capabilities of the developed IR transparent resin, molding and two-photon direct ink writing methods were both utilized to fabricate optics for testing imaging performance in the MWIR and LWIR ranges. The refractive index (RI) of both tetraSH- DVO2 and polySH-DVO2 were measured before the lens design. TetraSH-DVO2 shows aslightly higher RI (-1.55 from MWIR to LWIR) compared to polySH-DVO2 (-1.54 from MWIR to LWIR). This was mainly because the S ratio in tetraSH (59 wt%) was higher than it in polySH (46 wt%). The molding process involved curing the resin directly on a NaCl substrate using a glass mold (-180 pm Sagitta length). The high manufacturing efficiency of this process was evident in the quick curing of the mm-sized lens within a minute. After UV curing, the molded lens was thermally cured together with the mold and the NaCl substrate at 170 °C for 12h. The molded lens exhibited a smooth surface with a surface roughness of only 4.9 nm. For the two- photon printing process, a 3X3 lens array with lenses’ diameter of 450 pm and a thickness of 25 pm was directly printed onto a NaCl substrate equipped with the pre-printed polymer mask, showcasing the printing quality that reached a surface roughness as low as 4.3 nm. The lens array was not thermally cured before the imaging test. The results indicate that the designed shape can be well-maintained by either the molding method or the 3D printing method. It should be noted that the printed lens array had some swell behavior during the washing process using THF, which could be due to the relatively low monomer conversion after 3D printing compared to commercial resin. This could potentially be solved by using a more efficient initiator or increasing the laser power during printing. Another problem that has been noticed is that although both the tetraSH-DVO2 and polySH-DVO2 resins are 3D printable, the polySH-DVO2 resin has even slower curing efficiency and lower printing quality, making it difficult to precisely control the surface shape of printed optics. This result agrees with the results of the thiol conversion that polySH-DVO2 reached around 50% thiol conversion during the first 10 mins of UV exposure. Therefore, only the tetraSH-DVO2 resin was used for 3D printing optics.

[0055] To fabricate the micro-reactor with 3 walls made of DUDMA and 1 wall made of tetraSH-DVO2, DUDMA was first used with an initiator to print 3 walls and the bottom structure by two-photon polymerization. The wall thickness was set as 100 pm. After washing off the uncured DUDMA, one drop of tetraSH-DVO2 resin was dropped to surround the printed structure. The focal point of the laser was carefully located in the desired position. The IR transparent wall was then printed with a thickness of 100 pm. The printed micro-reactor had a width and length of 1.1 mm and a height of 900 pm. It can be observed that the printing resolution of tetraSH-DVO was lower than the printing resolution of DUDMA.

[0056] To explore the printing capability for the fabrication of thin channel structures using tetraSH-DVO2 resin, structures that contained microchannels were designed and printed. It was found that the smallest diameter of the micro-channels printed using tetraSH-DVO2 is around 10 pm. IR spectra were obtained with a ThermoFisher Scientific Nicolet iS50R®. The transmissionspectra of different samples were measured with the transmission mode. The conversions during material curing were measured with the Attenuated Total Reflectance (ATR) mode. Nuclear magnetic resonance (NMR) was obtained using a Bruker® DRX 500 MHz. Scanning Electron Microscope (SEM) images were taken using Field Electron and Ion Company® (FEZ) Inspect Scanning Electron Microscope. The surface profile was measured by Zygo® Newview® 8300 white light interference microscope. The thermogravimetric analysis (TGA) was carried out with TA Instruments® TGA 5500 (10 °C / min, 35°C to 800 °C) and the differential scanning calorimetry (DSC) was carried out using TA DSC 2500 (20 °C / min, -60°C to 200 °C).

[0057] For UV curable resin, on a KBr sample card (Real Crystal®), the prepared UV curable resin was dropped to fully fill the space above the KBr crystal. To minimize the potential deformation caused by shrinkage during the curing, a glass slide coated with Polydimethylsiloxane (PDMS) was covered above the thiol-ene resin. The resin was then cured using Omni Cure® S2000 (~3W / cm2source power, 10 cm distance between resin and light source, 10 min exposure time) to obtain a thin window for the FTIR transmission test. The thickness of each window is between 500 to 550 pm. For LDPE and HDPE, the polymer pallets were placed on a PDMS-coated glass slide. A heating plate was placed below the glass slide to melt the polyethylene (PE) polymer. A KBr sample card was then covered and pressed above the PE polymer to form an LDPE (or HDPE) window with a thickness between 500 to 550 pm.

[0058] To measure the thiol (alkene) conversion vs. UV exposure time, a series of thin samples ~lmm thick were cured under UV with the same curing conditions (OmniCure® S2000,~3W / cm2source power, 10 cm distance between resin and light source) except for curing time (from 20s to 20 min for difference samples). The cured samples were measured by FTIR-ATR to obtain the spectra. To calculate and compare the conversion, the obtained spectra were normalized based on the alkane peaks (-2900 cm'1). The conversion of thiols and alkene was then calculated based on the integration change of the peak near 2500 cm'1and the peak between 3000 and 3100 cm'1. For a long time post UV curing, the samples cured for 20 min by OmniCure® were transferred to a Formlabs® curing machine and cured for 12 h at room temperature. After that, the conversion was calculated based on the results from FTIR-ATR. ASTM® D638 type IV coupons were firstly 3D printed. Then, the printed coupons were used as a positive mold to fabricate a negative mold using PDMS. With the PDMS negative mold, thiol-ene resins were poured into the mold and exposed under UV to finally obtain the ASTM® D638 type IV coupons for the tensile test. The tensile testing was carried out using an Instron® tensile machine with tensile mode (1 mm / min to break). When the cyclic tensile testing was conducted, the samples were stretched to 30% straincompared to the original length and then gradually pushed back to the original length at the rate of 1 mm / min. For both tetraSH-DVO2 and polySH-DVO2 with and without thermal post-curing, 6 samples were tested in tensile testing.

[0059] To prepare the samples for the DMA test with compression mode, the thiol-ene resin was filled in a hollow cylinder with a 3 mm inner diameter and 1 mm thickness between two glass slides coated with PDMS. The resin was then cured using OmniCure® S2000 (~3W / cm2source power, 10 cm distance between resin and light source, 10 min exposure time). The DMA test was carried out using DMA 242 E under a compression model. The temperature was set from -50 °C to 200 °C with an increasing rate of 3 °C / min. A TFProbe® InfraRed Spectroscopic Ellipsometry (IRSE) tool (made by Angstrom Sun Technologies Inc.®) was used to characterize the refractive index of materials in the IR range. To characterize the material in the infrared range (350 - 7400 cmA-l), an infrared light source input was used inside a Michelson interferometer setup. A liquid nitrogen-cooled MCT detector was used to receive the IR signal after it passed through the ellipsometer’s polarizing optics and reflected off the test sample. The IRSE measurement was taken at an angle of incidence (AOI) of 70 degrees. The measurement was taken and analyzed using the TFProbe® 3.3 software. A bare substrate and the sample of interest were both measured, and the analysis was done by creating a model to fit onto the measured data. A dispersion-type fitting algorithm was used to characterize and calculate both the thickness and optical constants (NK) of the samples. For improved measurement accuracy, a bare substrate of the sample was first measured and modeled to obtain the NK constants and subsequently used within the model for the sample of interest.

[0060] Although there has been shown and described the preferred embodiment of the present invention, it will be readily apparent to those skilled in the art that modifications may be made thereto which do not exceed the scope of the appended claims. Therefore, the scope of the invention is only to be limited by the following claims. In some embodiments, the figures presented in this patent application are drawn to scale, including the angles, ratios of dimensions, etc. In some embodiments, the figures are representative only and the claims are not limited by the dimensions of the figures. In some embodiments, descriptions of the inventions described herein using the phrase “comprising” includes embodiments that could be described as “consisting essentially of’ or “consisting of’, and as such the written description requirement for claiming one or more embodiments of the present invention using the phrase “consisting essentially of’ or “consisting of’ is met.

Claims

WHAT IS CLAIMED IS:

1. An ultraviolet (UV)-curable composition comprising: a. a first precursor compound comprising more than one alkene groups; and b. a second precursor compound comprising more than two thiol groups; wherein the first precursor compound consists of carbon, hydrogen, and optionally sulfur; wherein the second precursor compound consists of carbon, hydrogen, and sulfur.

2. The composition of claim 1, wherein the alkene groups are vinyls.

3. The composition of claim 1, wherein the first precursor compound comprises a carbon chain or a cycloalkyl.

4. The composition of claim 1, wherein the first precursor compound comprises a di -vinyl oligomer, or a vinylic polymer.

5. The composition of claim 1, wherein the second precursor compound comprises a polythiol or a tetrathiol.

6. The composition of claim 1, wherein the first precursor compound and the second precursor compound are mixed and UV-cured to form a polymeric material that is transparent to mid-wave infrared (MWIR) and to long-wave infrared (LWIR).

7. The composition of claim 6, wherein the composition is used to form the polymeric material into substrates via 3D printing and UV-curing.

8. An optical element comprising the polymer of claim 6, wherein the optical element is MWIR and LWIR transparent.

9. The optical element of claim 8, wherein the optical element comprises a lens, a prism, a grating, a filter, a window, an optical flat, a polarizer, a beamsplitter, a wave plate, a fiber optic, or a combination thereof.

10. A method of preparing an optical element, the method comprising forming a composition according to claim 1 into a shape of the optical element, and UV-curing said composition.

11. The method of claim 10, wherein the composition is formed via 3D printing or by a molding process.

12. A polymeric material that is transparent to mid-wave infrared (MWIR) and to long-wave infrared (LWIR), wherein said polymer consists of carbon, hydrogen, and sulfur, wherein said polymer is a product of ultraviolet (UV) curing a composition comprising: a. a first precursor compound comprising more than one alkene groups; and b. a second precursor compound comprising more than two thiol groups; wherein the first precursor compound consists of carbon, hydrogen, and optionally sulfur,wherein the second precursor compound consists of carbon, hydrogen, and sulfur.

13. The polymeric material of claim 12, wherein the alkene groups are vinyls.

14. The polymeric material of claim 12, wherein the first precursor compound comprises a carbon chain or a cycloalkyl.

15. The polymeric material of claim 12, wherein the first precursor compound comprises a di-vinyl oligomer, or a vinylic polymer.

16. The polymeric material of claim 12, wherein the second precursor compound comprises a polythiol or a tetrathiol.

17. An optical element that is transparent to mid-wave infrared (MWIR) and to long-wave infrared (LWIR), formed via a 3D printing or molding process, the optical element comprising a polymer consisting of carbon, hydrogen, and sulfur, wherein said polymer is a product of ultraviolet (UV) curing a composition comprising: a. a first precursor compound comprising more than one vinyls; and b. a second precursor compound comprising more than two thiol groups; wherein the first precursor compound consists of carbon, hydrogen, and optionally sulfur, wherein the second precursor compound consists of carbon, hydrogen, and sulfur.

18. The optical element of claim 17, wherein the first precursor compound comprises a carbon chain or a cycloalkyl.

19. The optical element of claim 17, wherein the first precursor compound comprises a di-vinyl oligomer, or a vinylic polymer.

20. The optical element of claim 17, wherein the second precursor compound comprises a polythiol or a tetrathiol.

Images