Synthesis and properties of aromatic ether phosphine oxide containing oligomeric phthalonitrile resins with improved oxidative stability

A series of multiple aromatic ether-linked phthalonitrile resins containing an aromatic ether phosphine oxide group in the backbone have been synthesized and characterized. The oligomeric phthalonitrile monomers were prepared from the reaction of an excess amount of either bisphenol A or resorcinol with bis(4-fluorophenyl)phenylphosphine oxide in the presence of K₂CO₃ in a N,N-dimethylformamide/toluene solvent mixture, followed by reacting with 4-nitrophthalonitrile in a two-step, one-pot reaction. Rheometric measurements and thermogravimetric analysis of the cured resins showed that the oligomeric phthalonitrile resins maintained good structural integrity upon heating to elevated temperatures and exhibited excellent thermal properties. When subjected to long-term oxidative exposures, the phosphorus-containing phthalonitriles showed superior performance and less oxidative damage compared to other aromatic-containing phthalonitrile resins. Scanning electron spectroscopy studies showed the formation of microcracks during the oxidative aging at elevated temperatures.