Curing studies of a polyimide precursor. II. Polyamic acid

The cure of pyromellitic dianhydride–oxydianiline polyamic acid is compared with that of a model compound, diphenylcarbamido–dicarboxy–benzene, both having been synthesized in 1‐methyl‐2‐pyrrolidinone (NMP). Laser interferometry and thermogravimetric analysis showed that, analog to the diamic acid model compound, the polyamic acid forms complexes with NMP (repeat unit/NMP: 1/4 and 1/2). Thermal stability of these complexes was found to be similar to the ones of the diamic acid model compound as revealed by comparison of decomplexation temperature–heating rate curves. Activation energies of the decomplexation processes for the diamic acid are given. Anhydride formation, a reaction found in the cure of the model compound to compete with imidization, takes place also during the polyamic acid cure at fairly low temperatures. The molecular weight drop due to chain scission resulting from anhydride formation was followed by viscometry. Copyright © 1987 John Wiley & Sons, Inc.