Low-melting phthalonitrile monomers containing maleimide group: Synthesis, dual-curing behavior, thermal and mechanical properties

Abstract

Novel dual-functional phthalonitrile monomers containing a maleimide group were synthesized and characterized. Ortho, meta, and para isomers were obtained and only the meta-isomer can be considered as a low melting phthalonitrile monomer with a melting point of 107.4°С. Analysis of the dual-curing behavior of monomers showed radical homopolymerization of maleimides followed by phthalonitrile polymerization with the formation of isoindoline, triazine, and phthalocyanine ring structures. Two alternative copolymerization processes of maleimide groups via ene reaction and ring-opening amidation were investigated. The approach of dual-curing maleimide and phthalonitrile fragments in a single molecule allowed combining the excellent inherent mechanical properties of bismaleimide resins and the outstanding thermo-oxidative stability of phthalonitriles. After curing of both functional groups impact strength was as high as 11.81 kJ m−2, flexural strength was 108 MPa, Young's modulus was 4.38 GPa. On the contrary, Tg of this polymeric system was above 370°С and T5% was higher than 450°С in air.