Abstract
Aromatic polyimides were prepared by polycondensation reaction of two aromatic diamines, such as 4,4'-diaminodiphenylmethane (DDM) and 3,3'-dimethyl-4,4'-diaminodiphenylmethane (MDDM), with aromatic dianhydrides, such as 4,4'-isopropylidene-diphenoxy-bis(phthalic anhydride) (6HDA), benzophenonetetracarboxylic dianhydride (BTDA) and hexafluoroisopropylidene-bis (phthalic anhydride) (6FDA). These polymers are soluble in polar aprotic solvents and can be cast into thin films from such solutions. The polyimides show high thermal stability, with decomposition temperature being above 430 degrees C in air, and high glass transition temperature being in the range of 200-287 degrees C. The free standing films, having the thickness of tens of micrometers, exhibited good mechanical and electrical insulating properties. The dielectric constant, molecular mobility and AC conductivity of thin films prepared from these polymers were investigated in detailed. The study of their dielectric behavior evidenced low dielectric constant values, in the range of 2.88-3.48 at 1 Hz at room temperature, and three relaxation processes (gamma, beta(1) and beta(2)) were observed at sub-glass temperatures for polyimides based on 6HDA and 6FDA and only two (gamma and beta) relaxations were detected for polyimides based on BTDA. The cooperativity of the molecular motions associated with the relaxation processes was discussed. (C) 2011 Elsevier Ltd. All rights reserved.