Structural Effect of Cyclic Olefin Cross-Linkers on Long-Wave Infrared-Transmitting Sulfur Polymers

The structural characteristics of the organic cross-linker greatly affect the potential infrared (IR) optics applications of sulfur polymers synthesized through the inverse vulcanization reaction. Unlike elemental sulfur, organic cross-linkers induce various absorptions in the IR region and leave organic moieties that affect the IR transmittance, even after inverse vulcanization. Most of the cyclic olefin cross-linkers [e.g., dicyclopentadiene (DCPD)] investigated so far are expected to produce side reactions and form byproducts during inverse vulcanization. The side reactions caused by differences in reactivity between the reaction sites can result in the deterioration of the optical properties of sulfur polymers. In this study, thiol groups were introduced as a cross-linker to effectively control the side reactions and byproducts that may occur in inverse vulcanization. The sulfur and thiol cross-linkers reacted rapidly and uniformly at all reaction sites to form sulfur polymers without unwanted side reactions. Optical windows prepared with the sulfur polymer exhibited enhanced IR transparency and achieved thermal imaging of the human body in the long-wave IR (LWIR) region. By design of a cross-linker for LWIR transparent sulfur polymers, these results provide a useful solution for IR optics applications.