The Polymer Society of Korea

Abstract

Infrared (IR) transmissive polymers must balance high refractive index, IR transparency, and thermal stability, which is particularly challenging in the long-wave infrared (LWIR) region due to strong molecular absorption. To address this, a sulfur-based copolymer was developed using inverse vulcanization of 1,3,5-benzenetrithiol (BTT) and elemental sulfur. BTT was selected for its symmetric structure and relatively simple IR absorption, while sulfur is largely IR inactive. The resulting material formed ~1 mm thick optical windows exhibiting an ultrahigh refractive index (average n > 1.9) along with high mid-wave and LWIR transmission, without compromising thermal properties such as glass transition temperature. This sulfur copolymer demonstrates performance competitive with conventional inorganic and polymeric IR optical materials, extending the potential of organic materials for LWIR optical applications.