Thermal Imaging and Clandestine Surveillance using Low‐Cost Polymers with Long‐Wave Infrared Transparency

With increasing use of infrared imaging in medical diagnostics, military and civilian surveillance, and navigation of autonomous vehicles, there is a need for low‐cost alternatives to traditional materials used in infrared optics such as germanium. Sulfur‐rich copolymers hold promise, as they are made from low‐cost feedstocks and have a high refractive index. In this report, cyclopentadiene is copolymerized with sulfur to provide a plastic with the highest long‐wave infrared transparency reported to date for this class of materials. Diverse lens architectures are accessible through melt casting or reactive injection molding. The featured copolymer is black, which enabled its use as an infrared‐transparent blind for protection of thermal imaging equipment and clandestine surveillance. These findings portend expanded use of sulfur copolymers in infrared optics.


Introduction
Infrared imaging is a rapidly growing field with established uses in surveillance, [1] and in the medical, [2] agricultural, [3] and civil [4] and electrical engineering sectors. [5] More recently, infrared imaging has become integral to navigation of drones [6] and autonomous vehicles, [7] as well as space exploration. [8] As the applications for this technology increase, there is a need for DOI: 10.1002/adom.202300058 low-cost and easily processible alternatives to the conventional materials used for lenses and other infrared optics devices. This is particularly true for midwave (MWIR) and long-wave (LWIR) infrared cameras, as they rely on components made from expensive germanium or chalcogenide glasses. [9] Additionally, many of the commonly used chalcogenide glass materials used for infrared optics contain toxic elements such as selenium and arsenic, which complicate manufacture and lifetime management of these materials. [10] Sulfur-rich polymers have recently emerged as attractive, low-cost alternatives to semiconductor and chalcogenide glass components for infrared imaging. [1] Pioneering contributions from Pyun and collaborators have shown that polymers containing more than 50% sulfur by mass have high refractive indices (n = 1.7-1.9) and useful transmission in the MWIR region (3-5 μm). [11] In addition to these optical properties, the S-S bonds in these polysulfide materials can be broken and reformed, enabling melt-processing and repair not possible with traditional IR optics components. [11][12] The incorporation of selenium into these polymers can further increase the refractive index, which is important for focusing power of lenses for thermal imaging. [13,14] For LWIR imaging using polymer-based optics, Boyd and coworkers have reported copolymers made from tetravinyltin and sulfur, [15] while Pyun and coworkers have designed sulfur copolymers with simplified fingerprint regions of the infrared spectrum for increased transparency of 7-14 μm wavelength light. [16] Despite these creative advances, these materials still have limitations such as toxic components and limited stability in the case of Boyd's tin-containing polymer [15] and the requirement for multistep synthesis and relatively low overall LWIR transmittance in the case of Pyun's designed polymers. [16] There remains a need for inexpensive and processible polymers that have a high refractive index (n > 1.7) and high transparency of LWIR radiation. These properties are particularly important for imaging humans because the maximum intensity of infrared light emitted at normal body temperature is located in the LWIR region at a wavelength of ≈10 μm. In this study, new polymerization techniques and materials are reported to meet this need. Specifically, gaseous cyclopentadiene and molten sulfur were copolymerized to provide a high refractive index material with the highest LWIR transparency reported for a synthetic polymer. The material is processible by melt casting or injection Figure 1. A) Cyclopentadiene (CPD) was prepared through a retro-Diels-Alder reaction of dicyclopentadiene (DCPD). B) Copolymerization of CPD with elemental sulfur to form polysulfide polymers. C) Representative structure of 67-poly(S-r-CPD) in which the average sulfur rank is ≈2. D) Poly(S-r-CPD) prepared using the reflux method. The 1-mm thick windows were obtained after curing in a mold for 24 h at 140°C. E) Poly(S-r-CPD) prepared using the gas phase method. The left flask contains CPD and is connected by a tube to the polymerization vessel (right flask). A pump circulates the gaseous CPD through the molten sulfur via a needle submerged in the sulfur (inset, top right). The 1-mm thick windows were obtained after curing in a mold for 24 h at 140°C. molding, which allows access to complex lens architectures. And while the LWIR transparency of this polymer is high, it absorbs visible light-a property that was leveraged for new applications in clandestine surveillance in low-light environments.
Cyclopentadiene (CPD) was selected as a target comonomer for reaction with sulfur because it is low-cost and readily prepared from dicyclopentadiene (DCPD) via a retro-Diels-Alder reaction ( Figure 1A). Furthermore, CPD is a simple diene that can provide a densely cross-linked polymer on reaction with sulfur ( Figure 1B). The resulting sulfurized cyclopentane repeating unit was envisioned to impart shape persistence and provide limited modes of vibration for increased LWIR transparency.

Results and Discussion
Two methods of copolymerization of molten sulfur and cyclopentadiene were evaluated. The first method simply involved adding liquid CPD at a variety of feed ratios to molten sulfur at 140°C through a reflux condenser ( Figure 1D). At a scale of 3.0 g of molten sulfur, the CPD had to be added in 150 μL portions every 3 min or the sulfur would cool too rapidly, crystallize, and interfere with stirring. The condenser was effective in recycling the volatile CPD and returning it to the polymerization vessel. Using this method, the sulfur and CPD were heated for a total of 60 min before the black liquid product was transferred to a silicone mold and cured in an oven at 140°C for 24 h. This method was used to produce polymers ranging from 40 to 80 wt% sulfur; however, for most experiments, 1.5 g of CPD was used, corresponding to 67 wt% sulfur and a sulfur to CPD molar ratio of ≈4. This poly-mer is referred to as 67-poly(S-r-CPD) where the first number corresponds to the weight percent of sulfur in the reaction and r denotes a random polymerization of the sulfur (S) and cyclopentadiene (CPD) comonomers. The final sulfur content for this and other polymers was determined by combustible analysis, and in some cases varied slightly from the initial sulfur in the reaction (see Supporting Information).
The second polymerization method involved circulating gaseous CPD through the molten sulfur using a pump. In this method, two flasks were connected by a tube and a reflux condenser was attached to the one serving as the CPD reservoir ( Figure 1E). Freshly distilled CPD was added to the flask with the condenser while sulfur was added to the other flask. Equal masses of CPD and sulfur were used (3.0 g of each). The inlet of a gas pump was inserted into the headspace of the flask with sulfur, while the outlet was submerged in the sulfur. Both the CPD and sulfur vessels were then heated to 140°C and the CPD was pumped through the molten sulfur at an approximate rate of 500 mL min −1 . After 60 min of reaction, the resulting viscous black liquid was poured into a silicone mold and cured for 24 h at 140°C in an oven to provide the final polymer for evaluation in thermal imaging. It is worth noting that any DCPD formed in the CPD reservoir (left flask, Figure 1E) cannot enter the gas phase and react with the sulfur because the boiling point of DCPD is 170°C. And while ultrathin sulfur polymer films have recently been made using gaseous monomers, [17] the process in Figure 1E constitutes the first bulk copolymerization of sulfur with a gaseous monomer [18] -a process previously thought to be infeasible. [16] Recently, another method for inverse vulcanization Adv. Optical Mater. 2023, 11,2300058 www.advancedsciencenews.com www.advopticalmat.de of gaseous monomers was also reported that relies on photochemical activation. [19] The synthesized polymers were first characterized by differential scanning calorimetry (DSC) to determine the glass transition temperature. For the 50-poly(S-r-CPD) polymer prepared using the reflux method, a glass transition temperature of 41°C was measured. This decreased to 3°C when the sulfur content was increased to 67 wt%. The polymer prepared using gaseous CPD possessed the lowest glass transition temperature at -12°C . These results are likely due to a number of factors. First, increasing sulfur content appears to result in a lower T g for these specific polymers. Second, the reflux method results in the regeneration of some DCPD that can be incorporated into the polymer. DCPD is known to increase the T g when used as a comonomer for this class of polymers. [20] Indeed, in control experiments with the CPD system, adding exogenous DCPD as a comonomer dramatically increased the glass transition temperature of the polymer. Even with as little as 10% DCPD content by mass, the glass transition temperature increased to over 120°C. In contrast, the 50-poly(S-r-CPD) made using the gas phase method had the lowest T g because no DCPD was incorporated into the polymer, as it will remain in liquid form in the CPD reservoir in the reactor at 140°C and not be circulated into the molten sulfur. Relatedly, the compression modulus is higher when the reflux method is used (up to 12 MPa), while the compression modulus is lower when the polymer is made using the gas phase method (<2 Mpa). This difference was attributed to the amount of DCPD incorporated into the polymer ( Figure S12, Supporting Information).
Importantly, using the optimized polymerization methods, no unreacted sulfur was detected by DSC and no sulfur blooming was observed after more than 1 year of storage of 50-poly(Sr-CPD)-a problem previously reported by Boyd in other sulfur polymer systems, which can cause scattering of infrared radiation. [15] Raman spectroscopy, however, was used to detect crystalline S 8 in some batches of 67-poly(S-r-CPD), which we regard as the practical upper limit of sulfur content for this system.
Due to the poor solubility of these polymers, many common solution-based characterization techniques such as gel permeation chromatography or solution-based NMR were not possible. Therefore, a cross-polarization, magic angle spinning carbon-13 NMR spectrum was obtained for three polymers: 50-poly(S-r-CPD) made using the gas phase method; 67-poly(S-r-CPD) using the reflux method; and 50-poly(S-r-DCPD) for comparison. While the peaks are broad, the copolymer made from sulfur and DCPD shows a clear peak for unreacted alkenes at 132 ppm (Figure 2A). This peak is absent or significantly reduced in both polymers made by the direct reaction of sulfur and CPD. These results suggest that there is high alkene conversion in the polymerization. Reduction of the polymers with sodium borohydride, followed by GC-MS analysis, resulted in the detection of the tetrathiol product shown in Figure 2B, which is consistent with the proposed repeating unit. Combustion analysis indicated 66 wt% sulfur in the polymer, which was consistent with the feed ratio of the polymerization.
Infrared spectroscopy was used next to assess transparency in the MWIR and LWIR regions ( Figure 2C). Free standing polymer windows were prepared at a thickness of 1 mm in triplicate for all polymers by melt casting the polymer into a silicone mold before curing. Important for precision optics, the sample surfaces were smooth, with an average surface roughness of 2.68 ± 0.47 nm for 50-poly(S-r-CPD) and 1.96 ± 0.79 nm for 67-poly(S-r-CPD), as determined by AFM. These windows were tested for transmission between 2 and 20 μm and compared with simulated infrared spectra for model polymers ( Figure 2D). The simulated spectra, generated using DFT methods, show good agreement in both the MWIR region and LWIR regions and corroborate the proposed structure. These simulations-the first for this class of sulfur copolymers-also reveal the contribution of each potential chemical and structural feature of the polymer to its IR spectrum (see Supporting Information). As expected, there was an absorbance due to C-H stretching in all polymers at a wavelength of around 3.3 μm, but the rest of the MWIR region (3-5 μm) showed very little absorption. The 67-poly(S-r-CPD) polymer, for instance, transmitted >50% of MWIR radiation. This transparency was calculated by integrating over the transmission spectrum region and dividing by the wavelength range. The transmittance of 67poly(S-r-CPD) in the MWIR region is among the highest reported for synthetic polymers. For comparison, the previously reported poly(S-r-DIB) polymer system [11] prepared at either the same mass or molar feed ratios exhibited ≈25% transmittance (Figures S18-S19, Supporting Information). Importantly, the LWIR transparency for the 67-poly(S-r-CPD) was also excellent for both the reflux and gas phase synthesis methods. For example, 67-poly(Sr-CPD) made using the gas phase synthesis was found to have an average transmission of 9.0% in the LWIR range (7-14 μm) for 1-mm thick windows. When comparing directly to previously described synthetic polymers, [11,[15][16]21] this is the highest LWIR transparency reported to date for a window of this thickness ( Figure 2E). Additional LWIR and MWIR transmission data and how it varies with sulfur feed ratio was tabulated in Figure S15 (Supporting Information).
The refractive index is an important property for infrared optics applications. For most polymers, a refractive index of n = 1.6 is a typical limit, where sulfur-rich copolymers can have a refractive index as high as n = 1.9. [1,17] For 67-poly(S-r-CPD) made using the reflux method, the refractive index was found to be in the range of n = 1.85-1.90 across the MWIR and LWIR region, as determined by specular reflectance spectroscopy ( Figure 2F and Figure S12, Supporting Information). The reflectance ranged from 0.088 to 0.1 over the MWIR and LWIR regions due to the high refractive index of the material.
Consistent with the high LWIR transmittance of 67-poly(S-r-CPD), thermal imaging through a 1-mm thick window of the polymer was possible with an infrared camera operating in the 7.5-13 μm range. Notably, human subjects could clearly be imaged through the polymer window ( Figure 2G,H), which is only possible with high LWIR transmittance. In a new application for this class of material, a polymer sheet was made by hot-pressing the powdered polymer at 140°C and 30 MPa for 10 min, [22] for subsequent use as a protective barrier that conceals thermal imaging equipment in low light (Figure 3). The 67-poly(S-r-CPD) absorbs visible light and appears black, which is distinct from other sulfur-based polymers for infrared imaging that are partially transparent to visible light and appear yellow, orange, or red. [1] This unique property of 67-poly(S-r-CPD) enables protection of the thermal imaging equipment from the environment and concealment of the camera (Figure 3). Such capabilities may Figure 2. A) Cross-polarization, magic angle-spinning carbon-13 NMR spectrum of 50-poy(S-r-DCPD) (top), 50-poly(S-r-CPD) made using the gas phase method (middle), and 67-poly(S-r-CPD) made using the reflux method (bottom). B) Reaction of polymer with sodium borohydride and a key degradation product observed in GC-MS, consistent with the proposed repeating unit. C) FTIR transmission spectra from 2 to 20 μm of 1-mm thick windows of 67poly(S-r-CPD). The MWIR (3-5 μm) and LWIR (7-14 μm) regions are highlighted. D) Simulated IR spectrum for model polymer and measured spectrum for 67-poly(S-r-CPD). The "saturated" model polymer is one in which all alkenes have reacted with sulfur. The "unsaturated" model polymer is one that contains unreacted alkenes. There was good agreement between the simulated and experimental spectra in the LWIR and MWIR regions; however, a peak at ≈1700 cm −1 in the synthesized 67-poly(S-r-CPD) could not be assigned. E) Average LWIR (7-14 μm) transmittance of 67-poly(S-r-CPD) is higher than all other reported sulfur-based synthetic polymers (organic monomer shown). F) The refractive index in the MWIR and LWIR region for 67-poly(Sr-CPD) was n = 1.85-1.90 and the reflectance was between 0.088 and 0.1. G,H) LWIR images of a human subject taken through a 1-mm thick window of 67-poly(S-r-CPD) using a FLIR E6 thermal camera (7.5-13 μm).   . C) LWIR image of a 150°C hot plate with a Flinders University mask viewed with and without the plano convex lens, allowing for letters to be read which were previously too small to be seen clearly. D,E) Top and side view of melt cast plano concave lens made from the 67-poly(S-r-CPD). F) LWIR image of a 100°C oil bath viewed with and without the plano concave lens. Reduction of image size is labeled. G,H) Top and side view of a Fresnel lens prepared made from the 67-poly(S-r-CPD) polymer using injection molding. I) LWIR image of a 250°C soldering iron, magnified by use of a Fresnel lens. All LWIR images are taken with a FLIR E6 thermal camera over a wavelength range of 7.5-13 μm. Video S1 (Supporting Information) further illustrates the operation of the Fresnel lens.
find use in clandestine surveillance for wildlife monitoring, and defense and security applications.
The 67-poly(S-r-CPD) polymer was also used in the preparation of LWIR transparent lenses. Glass plano convex and plano concave lenses were used as negatives to form silicone molds. A custom form for injection molding was made to prepare a Fresnel lens ( Figure S25, Supporting Information). Polymer lenses were then made by pouring the molten precursor to 67-poly(S-r-CPD) into these molds and curing for 24 h at 140°C. As shown in Figure 4, the plano convex lens was used to image a hotplate through a mask with the expected magnification, while the plano concave lens generated a reduced image of the same hotplate. Finally, the Fresnel lens operated satisfactorily in magnifying the image generated by the camera operating in the LWIR region. The Fresnel lens is notable in that it is lightweight, and a complex architecture not easily accessible through the CNC-milling techniques typically used in the manufacture of germanium lenses. This first report of reactive injection molding of a sulfur-rich polymer is therefore a promising advance in the manufacture of lenses for infrared optics.

Conclusion
This report described the first use of a gaseous monomer in a bulk copolymerization with molten sulfur, providing novel materials with high MWIR and LWIR transparency. The low-cost feedstocks and versatile processing methods allowed manufacture of diverse, complex lenses for infrared thermal imaging, with the highest LWIR transparencies reported for this class of sulfurbased materials. The featured polymer could also be converted into a sheet useful for the protection and concealment of thermal imaging equipment due to its unique absorption of visible light and transmission of LWIR light. Computational methods were also devised that provided insight into how the infrared spectrum of these sulfur copolymers is influenced by specific chemical and structural features. Such insights will help guide the design and synthesis of these materials to further improve their performance in thermal imaging. Future studies will expand on the reaction of volatile monomers with sulfur, clarify mechanistic steps of this copolymerization, and provide new materials for use in infrared optics.

Experimental Section
All experimental protocols, characterization, and computational methods are provided in detail in the Supporting Information.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.