Effect of the chemical structure of aromatic-cycloaliphatic copolyimide films on their surface morphology, relaxation behavior and dielectric properties

Aromatic - cycloaliphatic copolyimides are synthesized by polycondensation reaction of dimethylsubstituted diamine-diphenyl methane with a mixture of an aromatic dianhydride namely benzophenonetetracarboxylic dianhydride (BTDA) with a cycloaliphatic dianhydride namely 1, 2, 3, 4-cyclopentanetetracarboxylic dianhydride (CPDA) taken in various molar ratios. The resulted copolyimides are characterized with regard to their solubility, inherent viscosity, molecular weight distribution and thermal stability. The quality of free-standing films prepared from the polymers is thoroughly studied by atomic force microscopy (AFM) technique. Dielectric properties of four representative films are investigated in detail in a large frequency range, from 1 to 10⁶ Hz, and in a wide temperature domain, from -150 to 300 C, and their variations are correlated with polymer structure and composition.