Studies on organosoluble polyimides based on a series of new asymmetric and symmetric dianhydrides: Structure/solubility and thermal property relationships

Abstract

A series of new asymmetric biphenyl dianhydrides, 2-phenoxy(o-methylphenoxy, m-methylphenoxy, and p-methylphenoxy)-4,4,5,5-biphenyltetracarboxylic dianhydrides, was readily synthesized using a five-step route. A new symmetric biphenyl dianhydride, 2,2-di(o-methylphenoxy)-4,4,5,5-biphenyltetracarboxylic dianhydride, was also synthesized using the similar procedure. The polyimides were prepared from such new dianhydrides and commercial diamines by high-temperature one-step polymerization. The asymmetric and symmetric substituent solubility and thermal property relationships of the resulting polyimides were investigated. Interestingly, the thermally reversible sol-gel transitions were observed for the polymer solutions of the polyimides derived from those asymmetric dianhydrides and 1,4-phenylenediamine (p-PDA). Unexpectedly, the polyimides derived from asymmetric dianhydrides did not show better organosolubility than those of the homologous polyimides derived from symmetric dianhydrides. The structures of the substituents (phenoxy, o-methylphenoxy, m-methylphenoxy and p-methylphenoxy) have a significant effect on both the solubility and the thermal properties of these polyimides. The polyimides derived from asymmetric dianhydrides show enhanced thermal properties relative to the symmetric dianhydridesderived polyimides. These results can be attributed to their different degrees of molecular packing revealed by wideangle X-ray diffraction measurements.