Refractive indices and thermo-optic coefficients of aromatic polyimides containing sulfur atoms

Abstract

Aromatic polyimides (PIs) containing thioether (-S-) or disulfide (-S-S-) groups in their molecular structures were prepared to attain high refractive index (nav) and large thermo-optic coefficient (dnav/dT). These Pis exhibit higher nav than the respective ether (-O-) containing PIs, which is due to the large polarizability of sulfur atoms. On the other hand, the values of in-plane/out-of-plane birefringence (Δn) are as small as the other amorphous Pis since the flexible -S- and -S-S- linkages cause small degrees of chain orientation. The values of dnav/dT (-88 to -91 ppm/K) are significantly larger than the other amorphous Pis (-52 to -76 ppm/K). Increases in |dnav/dT| are accounted by high nav and relatively large thermal volume expansion. On the other hand, polarization dependence in thermo-optic coefficients (d(Δn)/dT) of sulfur-containing PIs are relatively large because of large stress birefringences. © 2005 TAPJ.