Synthesis and Characterization of Thioether and Pyridine-Bridged Aromatic Polyimides with High Refractive Indices and High Glass Transition Temperatures

In this study, a novel aromatic diamine containing thioether and pyridine linkages, 2,6-bis[4-(4-aminophenylenesulfanyl)phenyl]-2-phenylpyridine (BASPP), was synthesized and characterized. The BASPP, along with several other sulfur-containing diamines, including 4,4′-( p-phenylenedisulfanyl) dianiline (2SPDA), 4,4′-bis( p-aminophenylsulfanyl)biphenyl (2SBPA), 4,4′-sulfonylbis[( p-phenylenesulfanyl)aniline] (BADPS), and 2,7-bis( p-aminophenylenesulfanyl)thianthrene (APTT) were polymerized with a sulfur-containing dianhydride, 4,4′-thiodiphthalic anhydride (TDPA), via a two-step polycondensation procedure to obtain five polyimides (PI-1 to PI-5). Flexible, tough, and transparent polyimide (PI) films with tensile strengths that ranged from 78.6 to 96.3 MPa were successfully cast from their poly(amic acid) (PAA) precursors. The PI films exhibited good thermal stability with glass transition temperatures ( T g values) as high as 265°C and 5% weight loss temperatures > 505 °C. The high sulfur contents and flexible thioether linkages endowed the PI films with refractive indices ( n av ) as high as 1.7166 at 1310 nm and birefringence ( Δn) < 0.01. The PI-4, containing both pyridine and thioether moieties in the main chain, exhibited the best combined properties, including T g of 265 °C, n av of 1.6966, Δn of 0.0078, and optical transmittance of 88% at 450 nm at the film thickness of 10 µ m.