High Refractive Index Polyimides Derived from 2,7-Bis(4-aminophenylenesulfanyl)thianthrene and Aromatic Dianhydrides

High refractive aromatic polyimides (PIs) containing a thianthrene-2,7-disulfanyl moiety in their main chain have been developed. The PIs were prepared from a newly developed aromatic diamine 2,7-bis(4-aminophenylenesulfanyl)thianthrene (APTT) and several aromatic dianhydrides by a two-step polycondensation procedure. The relationships between the structures and the thermal, mechanical, and optical properties of the flexible, tough, transparent, and amorphous PI films were established. Introduction of the thianthrene-2,7-disulfanyl group endowed the PIs with good combined properties, including high thermal stability with 10% weight loss temperatures (T10%) higher than 500 °C in nitrogen and the glass transition temperatures (Tgs) higher than 200 °C, good transparency with transmittances higher than 80% at 500 nm, and refractive indices higher than 1.73 at 632.8 nm. Because of higher sulfur content, the refractive indices of the PIs we synthesized are higher than those of PIs derived from the other sulfur-containing diamines such as 4,4'-(p-phenylenedisulfanyl)dianiline (2SPDA) and 4,4'-thiobis[(p-phenylenesulfanyl)aniline] (3SDA). The PI derived from APTT and 4,4'-[p-thiobis(phenylenesulfanyl)]diphthalic anhydride (3SDEA) has the highest sulfur content (23.2%) in its repeating units, and hence, it exhibits the highest refractive index, i.e., 1.7600.