Synthesis and Investigation of the Properties of a Branched Phthalonitrile Containing Cyclotriphosphazene

To study the properties of cyclotriphosphazene (CTP)-containing phthalonitriles, a branched phthalonitrile containing CTP (CTP-PN) with self-catalytic behavior was designed and synthesized. The structure of CTP-PN was characterized by FT-IR (Fourier transform infrared spectroscopy), MS (mass spectroscopy), ¹H-NMR (proton nuclear magnetic resonance spectroscopy), and ¹³C-NMR (carbon nuclear magnetic resonance spectroscopy). Then, the curing reaction of CTP-PN was studied using DSC (differential scanning calorimetry) and DRA (dynamic rheological analysis). The results show that the curing reaction of CTP-PN is initiated at 200 °C. Additionally, the change in the viscosity of CTP-PN as a function of the temperature was investigated. After curing at different temperatures, the generated structures were characterized by FT-IR. The fracture morphology and thermomechanical properties of cured CTP-PN were scanned and studied using SEM (scanning electron microscopy) and TMA (thermomechanical analysis), respectively. The results demonstrate that CTP-PN exhibits a smooth fracture surface and possesses a relatively low CTE (coefficient of linear thermal expansion) of approximately 25 ppm/°C at 285 °C. A <i>T</i>_d5% (temperature at which 5% weight loss occurs) of as high as 405 °C can be obtained for cured CTP-PN, and its char yield at 800 °C exceeds 70% in N₂. FT-IR and XPS (X-ray photoelectron spectroscopy) were used to study the thermal decomposition of cured CTP-PN, indicating that it remains stable below 350 °C. With an increasing temperature, there is decomposition first of CTP and P-NH-Ph and C-O-C bonds (>350 °C) and then nitrogen-containing aromatic heterocycles (>500 °C), ultimately resulting in the formation of P-containing residual char.