100th Anniversary of Macromolecular Science Viewpoint: High Refractive Index Polymers from Elemental Sulfur for Infrared Thermal Imaging and Optics

Abstract

Optical technologies in the midwave and long wave infrared spectrum (MWIR, LWIR) are important systems for high resolution thermal imaging in near, or complete darkness. While IR thermal imaging has been extensively utilized in the defense sector, application of this technology is being driven toward emerging consumer markets and transportation. In this viewpoint, we review the field of IR thermal imaging and discuss the emerging use of synthetic organic and hybrid polymers as novel IR transmissive materials for this application. In particular, we review the critical role of elemental sulfur as a novel feedstock to prepare high refractive index polymers via inverse vulcanization and discuss the fundamental chemical insights required to impart improved IR transparency into these polymeric materials.

One-step vapor-phase synthesis of transparent high refractive index sulfur-containing polymers

“…The unique advantage of HRIPs over other high-n materials lies in facile processing to create bespoke components, such as molded lenses, coatings, and stacks. Most of all, although n > 1.8 across the IR spectrum can be achieved from the one-step, bulk inverse vulcanization reaction (12,13), the materials show strong absorption in the lower wavelength region of the visible range, rendering them brownish red in color and thus not suitable for optical applications that require high transparency in visible range. Developing methods using elemental sulfur directly toward HRIPs with full transparency over the entire visible spectrum and n > 1.8 still remains unsolved.…”

Using conjugated system from natural sources for the synthesis of sulfur copolymers by bi‐function catalysts at mild temperatures

“…Concerning Principles of Green Chemistry, 9 a huge amount of the abundant, oversupply and inexpensive byproducts must be smartly exploited toward profitable industrial applications. After the concept 'inverse vulcanization' was proposed by Pyun et al, 10 the potential applications 11–13 of elemental sulfur have been researched including cathode material for rechargeable batteries, 14–16 materials with high refractive indexes, 17,18 IR optical materials, repairable materials, 19–21 adhesives, 21,22 fertilizers, 23,24 and composite materials 25,26 …”

Scalable High Refractive Index polystyrene-sulfur nanocomposites via in situ inverse vulcanization

“…Further enhancement in RI was achieved by adding inorganic selenium in the copolymer, the addition of selenium into the inverse vulcanized sulfur copolymers significantly improved the RI (n > 2) and showed excellent IR transparence 31 . More recently, Kleine, et al, developed chalcogenide hybrid inorganic/organic polymers with enhanced long-wave infrared (LWIR) spectrum (7–14 µm), this low organic content terpolymers showed superior IR transparence and demonstrated the ability to take highly resolved thermal images in near or complete dark environment 32 , 33 . Similar chemistry of introducing S–S bond in the copolymer to improve the refractive index was later adopted by many researchers are prepared various HRI copolymers 34 – 38 .…”