Structural effects of pendant groups on thermal and electrical properties of polyimides

Two series of aromatic polyimides containing various-sized alkyl side groups were synthesized by thermal imidization of the poly(amic acid)s prepared from the polyaddition of benzophenonetetracarboxylic dianhydride and hexafluoro-isopropylidene bis(phthalic anhydride) with 4,4'-methylenedianiline, 4,4'-methylene-bis(2,6-dimethylaniline), 4,4'-methylene-bis(2,6-diethylaniline), and 4,4'-methylene-bis(2,6-diisopropylaniline). The extent to which alkyl substitutes affect the thermal properties of polyimides was examined by differential scanning calorimetry, thermomechanical analyzer, and thermogravimetric analysis techniques. The analytical results demonstrated that the incorporation of alkyl moieties causes a moderate increase in the coefficient of thermal expansion and. a slight decrease in thermal stability. Notably, all polymers had a decomposition temperature exceeding 500 degrees C. The glass transition temperature increases markedly when hydrogen atoms at ortho positions on aniline rings are replaced with methyl groups, but decreases with growing alkyl side group size. The dielectric measurements show that the polymer possessing a large alkyl side group would have the lower dielectric value. (c) 2006 Wiley Periodicals, Inc.