Phthalonitrile Containing Branched Cyanine: Structure, Curing, and Its Properties

In order to improve the reliability of phthalonitrile resins in extreme service environments, a high cyanodensity phthalonitrile monomer (BPN) containing branched cyano groups was synthesized from 3,4-dihydroxybenzonitrile and 4-nitrophthalonitrile and characterized by 1H NMR and FTIR. It was found that polymerized BPN (p-BPN) exhibited excellent thermal properties with a Td5 of 540 °C when tested in N2 in the thermogravimetric analysis (TGA), as well as a glass transition temperature (Tg) of p-BPN1 exceeded 480 °C when tested in the dynamic mechanical analysis (DMA). A comprehensive study was conducted to gain a detailed understanding of the diverse curing mechanisms in realistic operational settings. The results from TGA and in situ FTIR in different atmospheres demonstrated that oxygen plays a beneficial role in the thermal stability of the cured resin. The weight gain observed at lower temperatures is attributed to oxidation, which results in the formation of heat-resistant structures such as imide rings. The flexural strength of QF/p-BPN1 was 446.4 and 266.3 MPa, respectively, after treatment at room temperature and 500 °C, with high retention at 59.7%.