Synthesis and Properties of AB-Type Semicrystalline Polyimides Prepared from Polyamic Acid Ethyl Ester Precursors

Three AB-type polyimides with para-, meta- and ortho-linked main chain units based on hydroquinone, resorcinol and catechol, were synthesized and characterized. The polyimides were prepared in two steps by the direct polycondensation reactions of isomeric monomers, 4-(4-aminophenoxy)diphenyl ether-3‘,4‘-dicarboxylic acid monoethyl ester (I p ), 3-(4-aminophenoxy)diphenyl ether-3‘,4‘-dicarboxylic acid monoethyl ester (I m ) and 2-(4-aminophenoxy)diphenyl ether-3‘,4‘-dicarboxylic acid monoethyl ester (I o ), to form poly(amic acid ethyl ester)s followed by thermal or chemical imidization. The resultant polyimides were analyzed by tensile tests, thermogravimetry (TG), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and wide-angle X-ray diffraction measurements. It was shown that these AB-type polyimides were semicrystalline and the crystallinities were estimated to be 19−24%. These polyimides showed glass transition temperatures in the range of 178−198 °C and melting transition temperatures of 308−393 °C. DSC measurements suggested that the polyimide based on hydroquinone units crystallizes much faster than those based on resorcinol and catechol. The polyimide based on the fully para-ether linkages showed a single melting endotherm, whereas the polyimides containing meta- and ortho-ether linkages exhibited bimodal melting behavior.