A study of ortho-phthalimide functional benzoxazine resins with additional cross-linkable group

Aromatic polyimides have attracted wide attentions in fields of aerospace, microelectronics and defense applications due to their outstanding thermal and mechanical performances. However, the rigidity originating from the imide structure attached with an aromatic ring causes difficulties in processing. In this study, we synthesized two ortho-phthalimide functional benzoxazine monomers with additional cross-linkable groups (including both furan and alkynyl). An ortho-phthalimide functional phenol, 2-(2-hydroxyphenyl) isoindole-1,3-dione (oPP), was obtained by reacting o-aminophenol and phthalic anhydride in acetic acid. Two monofunctional ortho-phthalimide containing benzoxazine monomers (oPP-fa and oPP-pa) were then synthesized using the Mannich reaction based on raw materials of oPP, furfurylamine/propargylamine and paraformaldehyde in toluene. The structures of benzoxazine monomers were confirmed by FT-IR and ¹H NMR spectroscopies. We also investigated effects of cross-linkable functionalities on curing behaviors as well as their corresponding polymerization mechanisms. The activation energies of oPP-fa and oPP-pa were calculated by Kissinger and Ozawa equations based on DCS measurements with different heating rates. It demonstrates that the exothermic peak temperature of oPP-pa is located at 178.5 °C, which is much lower than that of oPP-fa (223.3 °C). The alkyne group in oPP-pa plays a catalytic effect role in reducing the ring-opening polymerization temperature of the oxazine ring. Moreover, the thermogravimetric analyzer (TGA) was used to evaluate thermal properties of the obtained polybenzoxazine thermosets. Both thermosets exhibited improved thermal stability compared with some other reported monobenzoxazine-derived polybenzoxazines.