Unexpected thermal conversion of hydroxy-containing polyimides to polybenzoxazoles

An aromatic polyimide containing pendent hydroxyl groups ortho to the heter ocyclic imide nitrogen was found to rearrange to a polybenzoxazole, with qu antitative loss of carbon dioxide, upon heating above 490 degrees C in an i nert atmosphere. A hydroxy-containing polyimide film based on 3,3',4,4'-bip henyltetracarboxylic dianhydride (BPDA) and 3,3'-dihydroxy-4,4'-diaminobiph enyl (HAB) was pre pared and converted to a fully aromatic polybenzoxazole by heating at 500 degrees C for 1 h under nitrogen. The resulting polybenzo xazole film was found to be amorphous by small angle X-ray scattering (SAXS ). The film also displayed excellent solvent resistance and good thermal st ability by thermogravimetric analysis with 5% weight loss in nitrogen occur ring at 625 degrees C. A hydroxy-containing imide model compound was also f ound to undergo thermal conversion to a bisbenzoxazole when heated above 40 0 degrees C under nitrogen. (C) 1999 Elsevier Science Ltd, All rights reser ved.