Synthesis of Magnolol‐Based Hyperbranched Cross‐Linkable Poly(Arylene Ether Nitrile) Sizing Agent to Intensify the Interfacial Properties of <scp>CF</scp> / <scp>PEEK</scp> Composites

ABSTRACT Carbon fiber (CF)/poly ether ether ketone (PEEK) composites are promising candidates for lightweight structural applications owing to their outstanding thermal stability, solvent resistance, and recyclability. However, the inherently high chemical inertness and crystallinity of CF, coupled with the high melt viscosity and low polarity of the PEEK matrix, fundamentally hinder the development of a strong and chemically compatible interface between them, thereby posing a critical challenge to achieving effective stress transfer and long‐term durability. Conventional sizing agents designed for thermosetting composites tend to be incompatible with the PEEK matrix and exhibit poor heat resistance, while highly cross‐linked thermoplastic sizing agents often cause excessive interfacial rigidity and brittleness. To address these issues, a bio‐based magnolol‐derived hyperbranched cross‐linkable poly(arylene ether nitrile) (M‐HPEN) sizing agent was synthesized using magnolol and a trifunctional fluorinated monomer (MBDT). The allyl groups in magnolol underwent thermal cross‐linking during composite processing, which enhanced interfacial stiffness and solvent resistance. Meanwhile, the hyperbranched molecular architecture effectively reduced intrinsic brittleness, improved interfacial energy dissipation capacity, and thereby contributed to enhanced interfacial toughness. Compared with uncoated CF (UCF) composites, the M‐HPEN‐sized CF/PEEK composites (CF‐1.5) exhibited 32%, 48%, and 64% improvements in flexural strength, interlaminar shear strength (ILSS), and interfacial shear strength (IFSS), respectively, along with excellent retention of ILSS after solvent exposure. This study presents a molecular design strategy for a bio‐based, cross‐linkable thermoplastic sizing agent, offering a novel approach for achieving strong and tough interface in CF/PEEK composites.