Polyimides from isomeric diphenylthioether dianhydrides

3,3',4,4'-Diphenylthioether dianhydride (4,4'-TDPA), 2,3,3',4'-diphenylthioether dianhydride (3,4'-TDPA), and 2,2',3,3-diphenylthioether dianhydride (3,3'TDPA) were synthesized from 3-chlorophthalic anhydride and 4-chlorophthalic anhydride. A series of polyimides derived from the isomeric diphenylthioether dianhydrides with several diamines were prepared. The properties, such as the solubility, thermal and mechanical behavior, dynamic mechanical behavior, wide-angle X-ray diffraction, and permeability to some gases, were compared among the isomeric polyimides. Both 3,3'-TDPA- and 3,4-TDPA-based polyimides had good solubility in polar aprotic solvents and phenols. The 5% weight loss temperatures of all the obtained polyimides was near 500 degrees C in nitrogen. The glass-transition temperatures decreased according to the order of the polyimides based on 3,3'-TDPA, 3,4'-TDPA, and 4,4'-TDPA. The 3,4'-TDPA-based polyimides had the best permeability and lowest permselectivity, whereas the 4,4'-TDPA-based polyimides had the highest permselectivity and the lowest permeability of the three isomers. Furthermore, the rheological properties of thermoplastic polyimide resins based on the isomeric dipbenylthioether dianhydrides were investigated, and they showed that polyimide 3,4'-TDPA/4,4-oxydianiline had the lowest melt viscosity among the isomers; this indicated that the melt processibility had been greatly improved.