Chalcogenide Hybrid Inorganic/Organic Polymers (CHIPs): Plastics for Infrared Imaging and Photonics

Abstract

This dissertation contains five chapters describing recent advances in the use of a new class of polymeric materials (Chalcogenide Hybrid Inorganic/Organic Polymers (CHIPs)) for infrared imaging and photonics. Plastics are generally not used for these applications given their typically high optical losses at these wavelengths and low refractive indices. CHIPs are a revolutionary class of materials that are composed of a high loading of chalcogen moieties that are copolymerized with organic monomers into the backbone of the polymer. The large chalcogen content in CHIPs enables IR transparency and high refractive indices (n), whereas the organic comonomers afford solution and melt processablility making these materials highly attractive for use in the aforementioned applications. The last chapter focuses on the development of a novel photopolymer for use in photonic interconnects operating at telecommunication wavelengths. This chapter lays the groundwork for using this photopolymer in “smart print” technologies for optical communications. The first chapter is a review on the fundamentals and applications of infrared imaging, the current materials used as optical elements in those systems, and examples of polymeric materials that have been developed for the same purpose. IR imaging has numerous applications ranging from medical diagnostics to autonomous vehicle technologies, and is used frequently in the military for night vision. There is a growing consumer market for IR cameras as well, now that the cost of certain components has been reduced. However, expensive semiconductor materials are still used as the optical components. Consequently, there is a desire to replace semiconductors with cheaper, easier to work with plastics. CHIPs address this need as they are more IR transparent than conventional plastics and are amenable to conventional thermoplastic reforming techniques. These unique features of CHIPs are a result of the chemistry termed “inverse vulcanization” used to prepare them. The inverse vulcanization process uses elemental sulfur as a comonomer and reaction medium. At high temperatures, elemental sulfur is homolytically ring opened, resulting in a high concentration of sulfur radicals which then react with other chalcogens/organic vinylic comonomers. The resulting polymeric material contains a high content of IR transparent polysulfur bonds which is the basis for IR transparency in CHIPs. The second chapter describes the synthesis of a new organic comonomer for inverse vulcanization designed to improve the thermomechanical properties of CHIPs. Early CHIPs materials prepared using 1,3-di-isopropenylbenzene (DIB) suffered from low glass transition temperatures resulting in a narrow use range. To address this issue 1,3,5-tri-isopropenyl benzene (TIB) was synthesized and used in inverse vulcanization reactions. The higher favg (number of crosslinkable moieties per monomer unit) associated with TIB afforded a more densely crosslinked material according to dynamic rheological experiments. As expected, this increased crosslink density resulted in CHIPs with dramatically improved glass transition temperatures (approximately twice that of their DIB based analogues) exceeding Tg > 100 °C. Despite being highly crosslinked, this new material still exhibited the self-healing properties previously demonstrated for DIB based CHIPs. Most importantly, this new CHIPs material was similarly transparent in the mid wave IR to DIB based CHIPs. This work demonstrated that the thermomechanical properties of CHIPs could be adjusted to fit the demands of practical applications without sacrificing desirable optical properties. The third chapter explores the method used to further increase the refractive index of CHIPs materials. Refractive index is a volume averaged bulk property, thus higher loadings of sulfur result in higher refractive indices for CHIPs. However, the maximum refractive index possible is limited by the amount (~85 wt%) of sulfur that can be efficiently incorporated into CHIPs. Therefore, a higher refractive index chalcogen must be used to realize higher n. Tellurium is mildly toxic and exceedingly expensive, so selenium was chosen instead. The enabling discovery was that elemental selenium reacts with ring opened sulfur species, generating a reactive mixed chalcogen in situ, that copolymerizes with organic crosslinkers. This enabled preparation of CHIPs materials with n ≤ 2.1 demonstrating a marked increase over previously synthesized CHIPs materials. The expected mid wave IR transparency was observed and evaluated by IR imaging experiments as well. Most significantly, certain compositions were found to be solution processable. Access to soluble CHIPs with high refractive indices is critical for the solution-based fabrication of various photonic devices. Only one such device will be discussed in this dissertation, but other examples are currently under development. The preparation and characterization of 1D photonic crystals (1D PhCs) utilizing selenium containing CHIPs is the topic of the fourth chapter. 1D PhCs are dielectric devices assembled with alternating layers of high and low refractive index material. Layer thickness determines the λmax of the photonic bandgap while the difference in refractive index (Δn) between layers affects the magnitude of reflection. In this design, larger Δn values facilitate highly reflective mirrors assembled with only a few layers. Consequently, various combinations of metal oxides are often used to achieve large Δn. However, entirely inorganic 1D PhCs are brittle, limiting their use in certain applications, and why wholly polymeric 1D PhCs are of great interest as an alternative design. Fabrication of such devices is complicated by the need to generate a large number of layers to offset the typically small Δn values observed in polymeric 1D PhCs. This issue is addressed by employing ultra-high refractive index CHIPs as one of the layers. 1D PhCs were prepared by spin coating alternating layers of selenium containing CHIPs (high refractive index layer) and cellulose acetate (low refractive index layer). Devices fabricated in this manner were shown to possess >90% reflection with just 11 bilayers, and whose band gap was tuned across the near-infrared (1000-2000 nm). The fifth chapter is devoted to the preparation of long wave IR (LWIR) transparent CHIPs. Imaging in the LWIR is significantly cheaper than in the mid wave IR, and thus attractive for a variety of consumer applications. CHIPs are transparent in the mid wave IR as a result of low C-H bond content, but this does not necessarily translate to LWIR transparency. This is because the LWIR overlaps with the fingerprint region of the infrared spectrum which encompasses a broader range of vibrational absorption modes. Consequently, even the small amount of organic comonomer present in CHIPs results in opaque materials at longer wavelengths. Interestingly, some organic polymers, like polyethylene, are remarkably transparent over much of the LWIR. Polyethylene itself is not suited towards use as a broadband IR optic due to its polycrystallinity, but indicated polymeric materials could in principle be designed with adequate transparency in this region. To this end, simulated FTIR spectra of various model compounds representative of the corresponding polymeric repeat unit were used to guide the design of LWIR transparent CHIPs. Ultimately, a dimer based on the [2+2] cycloaddition of 2,5-norbornadiene (resulting in the monomer termed NBD2) emerged as a monomer amenable to inverse vulcanization chemistry and expected to possess improved LWIR transparency. Melt cast windows of CHIPs containing NBD2 or DIB as the organic crosslinker were then prepared with the same thickness and compared in LWIR imaging experiments. The DIB based CHIPs material was opaque at these wavelengths, while the new NBD2 based CHIPs material afforded significantly improved transparency. In the future, this methodology will be expanded to develop more CHIP materials that are suitable for LWIR imaging applications. The sixth chapter represents a departure from an emphasis on mid-IR transparent materials. Instead, the focus is development of materials for photonics applications at wavelengths relevant to the telecommunications industry (1310 and 1550 nm). In this work, a novel photopolymer (poly(SBOC)) was prepared and demonstrated as a material for fabrication of photonic interconnects. These devices require a high refractive index medium through which light propagates to be surrounded by lower refractive index material. Such patterning is typically achieved through photolithography and a number of solvent development/etching steps. Poly(SBOC) simplifies this process since permanent refractive index changes with high resolution are generated by direct laser writing in dry polymer films. Optical power attenuation through the device (propagation loss) is an important metric of photonic interconnects. To assess this for poly(SBOC), waveguides were fabricated and characterized by the cut back method. The propagation losses observed were ~2 dB/cm and commensurate to values typically observed for hydrocarbon based waveguides. The promising results from this project have resulted in a collaboration with the AIM Photonics program in the College of Optical Sciences at The University of Arizona to develop “smart write” photonic interconnects.