Aromatic polyimides

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Abstract
Aromatic polyimides were prepared in two steps. Pyromellitic dianhy dride was added to a solution of an aromatic diamine in a highly polar solvent, such as dimethylformamide, dimethylacetamide, dimethyl sulfoxide, or N‐methyl‐2‐pyrrolidone, until a high molecular weight polypyromellitamic acid was obtained. The amount of pyromellitic dianhydride required was determined by the rate of viscosity increase with addition of the reagent, and it corresponded fairly closely to the stoichiometric quantity. Inherent viscosity of the polypyromellitamic acids in dimethylacetamide ranged from 0.54 to 3.22. Upon standing at room temperature, the viscosity of these solutions gradually declines. This viscosity decline is rapidly accelerated by an increase in temperature and is negligible at 0°C. Water and excess pyromellitic dianhydride result in a more rapid decline in viscosity. The polypyromellitimides were formed from the amide acids by heating. The solvent could be removed during the imidization step or earlier under vacuum at a lower temperature. Polypyromellitimides were prepared from m‐phenylenediamine, benzidine, 4,4′‐diaminodiphenyl ether, 3,4′‐diaminodiphenyl ether, 4,4′‐diaminodiphenyl sulfide, methylene dianiline, isopropylidene dianiline, several diaminobenzanilides, 4,4′‐diaminophenyl benzoate, and other amineterminated low molecular weight aromatic esters and amides. Polyimides cast in the form of thin films were for the most part clear, tough, flexible, insoluble, and infusible. Thick sections tended to be brittle and opaque. Several of the polymers in the form of films suffered less than 10% weight loss after aging 400 hr. at 325°C. in air. Most of these films were still intact and moderately flexible after this treatment.

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