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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.

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