Phenylnitrile and phthalonitrile-functional polybenzoxazines: Synthesis, characterization and thermal properties

Phenylnitrile and phthalonitrile functionalities are introduced into the molecular structure of a series of benzoxazine monomers in order to improve its thermal stability. Significant improvement in decomposition temperature (T_5%, T_10%) is achieved from 290°C for unfunctional aniline based polybenzoxazines to 390–490°C for phthalonitrile-functional polybenzoxazines. Additionally, these materials exhibit increase of char yield at 800°C in nitrogen from 30% by weight for aniline based polybenzoxazines to 80% by weight for plithalonitrile-functional resins. The char yield in air also exhibits significant values up to 30% by weight at 700°C. These materials show low flammability in part as a result of high char residue. The evolved gases analysis shows that predominantly carbon dioxide, ammonia and low concentration of substituted benzene species are present in the gas phase during thermal degradation of these resins.; Phthalonitrile-functional benzoxazines can be processed easier than any other phthaonitrile resins known to date, as a consequence of their low melt viscosity and moderate curing conditions. It is shown that polymerization of phthalonitrile-functional benzoxazine proceeds as a two step process, where in the first step oxazine ring opening polymerization occurs and this is followed by the reaction of a part of the phthalonitrile groups at higher temperature.; The polymerization and properties of phenylnitrile-functional polybenzoxazines depend solely on whether the nitrile group is in the ortho, meta or para position to the nitrogen atom of the oxazine ring. Ortho-nitrile and oxazine ring undergo an intramolecular cyclization, which results in the formation of a six-membered ring, leading to the previously unknown polymer.