Synthesis and Characterization of Highly Refractive Polyimides Derived from Thiophene-Containing Aromatic Diamines and Aromatic Dianhydrides

Aromatic thiophene-containing diamines, 2,5-bis(4-aminophenylsulfanyl)thiophene (APST) and 2,5-bis{2'-[5'-(4 ''-aminophenyl)sulfanyl]thienyl}thiophene (APSTT), were synthesized and polymerized with a sulfur-containing dianhydride, 4,4'-[p-thiobis(phenylenesulfanyl)]diphthalic anhydride (3SDEA) and 4,4'-oxydiphthalic anhydride (ODPA), respectively, to afford four poly(amic acid)s (PAAs) with inherent viscosities of 0.41 to 1.68 dL/g. Flexible and tough polyimide (PI) films obtained from the PAA precursors showed good thermal, mechanical, and optical properties. The glass-transition temperatures (T-g) of the PIs were in the range of 157-220 degrees C, determined by differential scanning calorimetry (DSC), and 160-216 degrees C by dynamic mechanical analysis (DMA), depending on the dianhydride used. The 10% weight loss temperatures were in the range of 354-456 degrees C, showing relatively high thermally resistant characteristics of the PI films. The PI films also showed good optical transparency above 500 nm, which agreed well with the calculated absorption spectra using the time-dependent density functional theory. The average refractive indices (n(av)) measured at 633 nm were 1.7228 to 1.7677, and the in-plane/out-of-plane birefringences (Delta n) were 0.0067 to 0.0074. In particular, the n(av) of PI derived from 3SDEA and APSTT exhibited the highest refractive index, that is, 1.7677 at 633 nm, in the high-refractive-index polyimides. the high refractive indices originate from the high sulfur content, dense molecular packing, and the absence of bulky Structures. The relatively small birefringence mainly results from the flexible and bent thioether linkage structures of the diamine.