Refractive Indices and Thermo-Optic Coefficients of Aromatic Polyimides Containing Sulfur Atoms

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 (∆n) 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(∆n)/dT) of sulfur-containing PIs are relatively large because of large stress birefringences.