Coloration of Aromatic Polyimides and Electronic Properties of Their Source Materials

The relationships between the color intensities of polyimide films and the electronic properties of their source materials-aromatic diamines and aromatic tetracarboxylic dianhydrides-are discussed. The arrangement of the diamine moieties in order of color intensity of polyimides shows fairly good agreement with the order of the electron-donating properties of the diamines estimated from 15N NMR chemical shifts (bN). On the other hand, the arrangement of the dianhydrides moieties in order of color intensity of polyimides agrees with the order of the electron-accepting properties of the dianhydrides estimated from experimental and calculated electron affinity (EA) although systematic inconsistencies are observed for the dianhydrides having-CF3 groups and a benzophenone carbonyl group. These results are consistent with the formation of charge transfer complex (CTC) and indicate that the electron-donating properties of diamines and electron-accepting properties of dianhydrides are retained to a significant extent even in polyimide molecular chains.