Preparation and characterization of highly transparent and colorless semi-aromatic polyimide films derived from alicyclic dianhydride and aromatic diamines

A novel meta-substituted aromatic diamine containing trifluoromethyl and sulfonyl groups in the backbone, i.e., 2,2′-bis[4-(3-amino-5-trifluoromethylphenoxy)phenyl]sulfone (m-6FBAPS), was synthesized and characterized. A series of semi-aromatic polyimides were prepared from 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA) and various aromatic diamines. These polymers exhibited excellent solubility, high thermal stabilities and good mechanical properties. All semi-aromatic polyimide films showed considerably improved optical transparency as comparing with traditional aromatic ones due to the incorporation of alicyclic moieties. They gave the UV cutoff wavelengths of 292-314 nm, transmittance at 450 nm > 91% and yellowness indices <3.8. The extremely transparent and entirely colorless films were obtained from the polymers incorporated with bulky electron-withdrawing trifluoromethyl and sulfonyl groups in diamine moieties, such as PI-2, PI-5 and PI-6. Their excellent optical properties are attributed to the distorted molecular conformation combined with the weakened electron-accepting and electron-donating properties of dianydride and diamines, which significantly restrained the formation of inter-/intra-molecular charge transfer interactions.