Highly refractive thiazole-containing polyimides: a structural property comparison

Highly refractive, thermally stable, and solution processable polyimides (PIs) were synthesized by the introduction of thiazole units, thioether linkages, and phenyl or nitrophenyl groups into the polymer backbones. These PIs were prepared via the polycondensation of two diamine monomers, 5,5′-thiobis(2-amino-4-phenyl-thiazole) (DA-1) or 5,5′-thiobis(2-amino-4-(3-nitrophenyl)thiazole) (DA-2), with various aromatic dianhydrides(a–d). The bulky pendant phenyl or nitrophenyl units as well as the non-coplanar conformations because of ortho-sulfide linkages endowed the resulting PIs with excellent solubility in organic solvents. These PIs showed outstanding thermal stability, with 10% weight loss temperatures exceeding 492 °C under nitrogen and 475 °C in air atmosphere, while their glass transition temperatures were in the range of 239–278 °C. In general, the synergic effects of thiazole groups, phenyl or nitrophenyl substituents, and thioether linkages provided PIs with very high refractive indices of up to 1.7646 at 632.8 nm, along with small birefringences (<0.0085) and high Abbe’s numbers. The structure–property relationships of the analogous PIs containing phenyl or nitrophenyl substituents groups were also studied in detail by comparing the results.