Synthesis and properties of novel sulfonated copolyimide-silica nanocomposite films

4,4'-Diaminodiphenyl ether-2,2'-disulfonic acid (ODADS) as a sulfonated diamine was fabricated by direct sulfonation of diamine, 4,4'-diaminodiphenyl ether in the presence of fuming sulfuric acid as the sulfonating agent. Additionally, a xanthene-containing diamine monomer was prepared in four steps, by the condensation of β-naphtol and 4-nitrobenzaldehyde in the presence of p-toluenesulphonic acid catalyst, reduction of the nitro intermediate, the nucleophilic substitution reaction of amine and 3,5-dinitrobenzoyl chloride, and subsequent reduction of the dinitro compound. The diamine monomer could be obtained in quantitative yield. Then, a series of new sulfonated polyimide/silica nanocomposites with different percentages of silica have been successfully synthesized through the in situ formation of silica within a polymer matrix via sol-gel process. Poly(amic acid) solution is prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride, ODADS and nonsulfonated diamine, 3,5-diamino benzoyl amino phenyl-14H-dibenzo[a,j]xanthene in N-methyl-2-pyrrolidinone solvent. Finally, the nanocomposites films were obtained by the hydrolysis-polycondensation of silica precursors (3-aminopropyl)triethoxysilane (APTES) and tetraethoxysilane (TEOS) in poly(amic acid) solution and then thermally imidized to form polyimide/silica nanocomposites. The chemical and morphological structure of the hybrid nanocomposites were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, and transmission electron microscopy. The electrical conduction of materials was measured by potentiometer. The results of TEM show that the silica particles are well dispersed in the copolyimide matrix with particle size between 50 and 70 nm in diameter. Thermogravimetric analysis results indicated that the addition of silica into the sulfonated polyimide matrix was increased the thermal stability of the resulted nanocomposites owing to the enhanced interaction and good dispersion the silica into the polyimide matrix. The prepared nanocomposites were exhibited good water stability as well as great electrical conduction.