PMR Type Polyimides by Aqueous Cyclization Methods — Chemistry

PMR-15 has long been the leading resin system for high performance composite applications requiring long-term use temperatures up to 300°C. A mixture of carboxylic esters and acids, aromatic diamine and norbornyl end caps capable of crosslinking by a reverse Diels—Alder mechanism in methanol is pre-pregged into cloth and under careful processing conditions produces high quality composite parts. Some problems with handling and processing remain in this commercial system and in an attempt to overcome these problems we have synthesized a series of fully cyclized PMR type prepolymers under aqueous imidization conditions. Their solubility and cure behavior have been determined with a view to producing resins that will form good quality, high performance composites. While resins with these characteristics have been obtained, the best of them utilize significant proportions of relatively expensive co-monomers. Almost all of the materials presented in this study have very high temperature stability and high glass transition temperatures, although no one system out performs PMR-15 on a monomer cost basis, the advantages of water as the synthesis solvent includes both its low cost and low toxicity and its ability to fully react the monomers to cyclic imide structures.