Preparation of robust, flexible, transparent films from partially aliphatic copolyimides

Robust, flexible, and transparent films have been prepared from partially aliphatic copolyimides, and their structure-property relationship was systematically studied. Spiro-type rel-(1'R,3S,5'S)-spiro[furan-3(2H),6'-[3]oxabicyclo[3.2.1]octane]-2,2',4',5(4H)-tetrone (DAn) and ladder-type 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA) were used as cycloaliphatic dianhydride. Each dianhydride and an aromatic dianhydride, benzophenone-3,3',4,4'-tetracarboxylic dianhydride (BTDA), were polymerized with an aromatic diamine, 4,4'-oxydianiline (ODA). Molar feed ratio of the aliphatic and aromatic dianhydrides was varied, and the molar ratios of dianhydride units in the obtained poly(amic acid)s were in good agreement with the molar feed ratios. Highly flexible and transparent films were obtained from the copolyimides. The relationship between structure and properties such as film flexibility, solubility, optical transparency, and thermal stability is explained by intermolecular interactions, degree of intramolecular conjugation, intermolecular charge transfer complex formation and backbone flexibility including the measurement of d-spacing values by wide-angle X-ray diffraction (WAXD) analysis. The polyimide prepared with the molar feed ratio of DAn:BTDA:ODA of 0.9:0.1:1 gave thin films having high flexibility, transparency, and colorlessness coupled with good solubility, thermal stability, and mechanical properties.