Spontaneous degradation of aromatic polypromellitamic acids

The reaction of pyromellitic dianhydride with a solution of a primary diamine in a solvent such as dimethylacetamide gives a viscous solutionof a polypyromellitamic acid of high molecular weight. The viscosity of the solution decreases greatly when it is aged at 35°C. Higher temperatures or the presence of water, excess amine or excess anhydride accelerate the rate of decline. A rapid early drop in viscosity has been found to be due primarily to hydrolysis by adventitious water and to exchange reactions of amine or anhydride groups with the o-carboxyamide linkage. Slow decline over a long period of time results from hydrolysis by water produced in the slow conversion of polypyromellitamic acid to polypyromellitimide. Degradation by hydrolysis also occurs when polyimide films are cast by heating the solution. The effect can be minimized by casting thin films, from which water can diffuse rapidly, or by using a chemical method, such as treatment with acetic anhydride and pyridine, to remove the water produced in imidization.