Synthesis and characterization of novel, soluble sulfur-containing copolyimides with high refractive indices

High refractive index, colorless polymers with high temperature resistancy are of great industrial interest. In this study, new sulfur-containing copolyimides with high refractive indices have been synthesized via polycondensation reaction using 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride (DSDA) as dianhydride monomers, and 4,4′-diaminodiphenyl sulfide (4,4′-SDA) and 2,3,5,6-tetramethyl-1,4-phenylene diamine (4MPD) as well as 3,3′-diaminodiphenyl sulfone (3,3′-DDS) as diamino monomers. The resulting copolyimides are amorphous and soluble in common organic solvents such as dimethylacetamide and N-methylpyrrolidone. They also exhibit very good optical properties such as high transparency (>88% at 589.6 nm) for samples with a thickness of 120–150 μm. Furthermore, higher refractive indices ranging from 1.64 to 1.72 at visible wavelengths compared to currently applied polymer materials, which show refractive indices between 1.5 and 1.6, are found. In addition, the copolyimides synthesized in this study show high thermal stability with glass transition temperatures between 273 and 306 °C.