Aromatic imide polymers for electrical insulation

Modified aromatic imide polymers have been prepared by the reaction of pyromellitic dianhydride with various aromatic diamines. The initial product is a polypyromellitamic acid, which is soluble in polar solvents, such as dimethylacetamide and dimethyl sulfoxide. When cured by heat, the polymer is converted to an insoluble polyimide. The cured resin is linear, consisting of a series of benzene rings alternating with various linking groups. In addition to the cyclic imide groups derived from the pyromellitic dianhydride, other connecting groups can be included as part of the diamine reactant. A variety of modified polyimide resins have been prepared in this way and evaluated for thermal stability by measuring weight losses of one mil films on aluminum at 325°C. Based on this test, an approximate order of thermal stability for the linking groups is: imide > ether, sulfide, direct phenyl-phenyl bond > amide, ester > methylene > isopropylidene.