Backbone-engineered bio-based monophenolic benzoxazine–phthalonitrile hybrids: Curing kinetics, thermal stability and fracture behavior

This study presents three partially bio-based benzoxazine-containing phthalonitrile monomers (EU-BZ-PN, GU-BZ-PN and 3MP-BZ-PN), synthesised via a facile solvent-less melt condensation method using eugenol, guaiacol and meta-methoxyphenol with 4-aminophenoxy phthalonitrile and paraformaldehyde. The chemical structures of the hybrids were validated by FTIR and 1 H NMR analyses. Their curing involves a sequential two-step self-promoted process: ring-opening polymerization of the oxazine groups followed by ring-forming cyclotrimerisation of the phthalonitrile units, yielding polybenzoxazine and N-heteroaromatic networks, respectively. Non-isothermal DSC combined with peak deconvolution and Starink isoconversional analysis reveals that the monophenolic backbone governs the curing kinetics, with apparent activation energies for the two stages spanning roughly 80–130 kJ mol − 1 ; the meta-methoxy 3MP-BZ-PN displays the lowest activation barriers and the lowest onset and peak curing temperatures. In-situ FTIR tracking of the benzoxazine (948 cm − 1 ) and nitrile (2228 cm − 1 ) bands corroborates the sequential mechanism and confirms the higher reactivity of 3MP-BZ-PN. After curing, all hybrids exhibit outstanding thermal robustness ( T 10%  > 460  ∘ C, char yield up to 71.6 wt% at 800 ∘ C) and high glass-transition temperatures ( T g between 325 and 373 ∘ C). DMA, flexural testing and SEM demonstrate a tunable balance between stiffness, T g and fracture toughness via simple backbone engineering, making these bio-based BZ–PN hybrids promising high-performance thermosetting matrices for advanced structural and extreme-environment composite applications.