Synthesis and Degradation Mechanisms of Schiff-base-containing Phthalonitrile Resins: Thermal Stability and Bond Reconfiguration

With the rapid development of the aerospace field, higher requirements have been put forward for the heat resistance of thermosetting resins. In this study, an asymmetrical phthalonitrile monomer containing Schiff base moiety (SchB-PN) was synthesized by a two-step method with N -4′-hydroxybenzylidene-4-hydroxyaniline (N-hy) and 4-nitro- o -phthalonitrile. The structure and properties of SchB-PN were characterized by ¹H NMR, 13 C NMR, 1 H- 1 H gCOSY, HRMS, FTIR, XRD, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and rheology. SchB-PN had a relatively low viscosity of 0.179 Pa·s when molten, and due to the presence of imine and cyano groups, SchB-PN resin exhibited self-curing characteristics. Furthermore, SchB-PN resins were cured with different curing agents and curing processes. It was found that SchB-PN-APn-400 resin cured by SchB-PN and p -aminobenzonitrile (APn) at 400 °C demonstrated excellent thermal stability and thermal oxidative stability, with the temperature of 5% weight loss ( T 5% ) of 530 °C in nitrogen and 534 °C in air, and the residual carbon rate was 74.9% at 800 °C in nitrogen atmosphere. The curing mechanism of SchB-PN-APn resin and the cooperative curing mechanism of imine bonds and cyano groups were studied and clarified through FTIR, in-situ infrared technology, SEM and EDS. In addition, the thermal stability and pyrolysis mechanism of SchB-PN were systematically investigated by TGA and TG-FTIR techniques, as well as the compensatory effect of the Schiff base bond breaking and reconnection during the degradation process. The relevant data and analysis results highlight the potential application prospects of SchB-PN-APn resin in aerospace and high-performance engineering fields.