Synthesis and characterization of thianthrene-containing preimidized soluble polyimide resins and the derived films with high refractive indices and good optical transparency

A series of organo-soluble polyimide (PI) resins were prepared from a newly designed and synthesized aromatic thianthrene-containing diamine, 2,7-bis(3-aminophenoxy)thianthrene (APOT) and various aromatic dianhydrides via a one-step high temperature polycondensation procedure with N-methyl-2-pyrrolidone (NMP) as the solvent. The derived PI resins were soluble in polar aprotic solvents, including NMP and N,N- dimethylacetamide (DMAc). For comparison, several unsoluble sulfur-containing PIs were prepared from anotherpara-substituted thianthrene-containing diamine, 2,7-bis(4-aminophenylenesulfanyl)thianthrene (APTT) and aromatic dianhydrides by a two-step polycondensation procedure via poly(amic acid) (PAA) precursors at elevated temperatures as high as 300 °C. Flexible and tough PI films with the tensile strength higher than 88 MPa were successfully cast from the APOT-PI resin solution at a relatively low curing temperature of 250 °C. The meta-substituted molecular skeleton in the APOT- PIs endowed the films good optical transparency with the transmittance higher than 83% at a thickness around 10 um, which was apparently higher than those of the para-substituted APTT-PI analogues. The sulfur-rich thianthrene moiety with high molar refraction endowed the APOT-PI films high refractive indices up to 1.7147 at 632.8 nm and low birefringence in the range of 0.0064–0.0133. In addition, the PI films showed good thermal stability with the glass transition temperatures (Tg) higher than 198.3 °C and 5% weight loss temperatures higher than 510 °C in nitrogen.