Abstract Phthalonitrile resins are promising matrices for high‐temperature composites owing to their outstanding thermal stability, flame resistance and chemical durability; however, achieving simultaneously high mechanical performance and controllable processability remains a key challenge. In this work, a designed and synthesized nitrile‐containing diamine curing agent (BADS) was incorporated into a biphenyl‐based phthalonitrile (BPH) system to prepare the prepolymers (BPDS), and the influence of BADS content on curing behavior, network evolution and composite performance was systematically investigated. The curing characteristics varied markedly with BADS content; moderate incorporation enables controlled curing, while excessive BADS results in premature gelation and limited molecular mobility. Among the investigated formulations, BPDS‐20 achieved a better balance between curing activity and chain mobility, promoting the formation of a dense and homogeneous aromatic network. The cured resin exhibited excellent thermal stability ( T 5% = 531.1 °C, CY 800°C = 77.84% (CY, char yield)). Quartz‐fiber‐(QF)‐reinforced composites based on BPDS matrices exhibited a pronounced dependence of mechanical performance on BADS content, with QF/BPDS‐20 achieving the highest flexural strength (769.3 MPa) and interlaminar shear strength (73.18 MPa). This study demonstrates that controlled incorporation of a nitrile‐containing diamine provides an effective molecular‐level strategy to enhance mechanical performance while maintaining acceptable processability in phthalonitrile‐based composite matrices. © 2026 Society of Chemical Industry.
Zhang et al. (2026) studied this question.