Constructing a Polarity–Compatibility Balanced Interphase Using Water‐Dispersible Polyarylether Sizing for Enhanced Interfacial Adhesion of <scp>CF</scp> / <scp>PEEK</scp> Composites

ABSTRACT Carbon fiber/poly(ether ether ketone) (CF/PEEK) composites often suffer from weak interfacial adhesion because of the inert carbon fiber surface and the high melt viscosity of PEEK. Here, carboxylated poly(aryl ether nitrile ketone) copolymers (C‐PENK) with different nitrile/ketone ratios were synthesized and converted into water‐dispersible sizing dispersions through neutralization of pendant carboxyl groups. After sizing, a continuous C‐PENK coating was formed on the carbon fiber surface, introducing tunable polar and aromatic structures, improving wettability, and partly smoothing surface grooves and defects, which enhanced the tensile reliability of single filaments. During hot pressing, the aromatic ether–ketone segments showed affinity with PEEK, while nitrile‐containing segments provided additional polar interaction sites. The sizing layer further regulated PEEK crystallization near the fiber surface and transformed the sharp modulus discontinuity into a modulus‐gradient interphase. Among the copolymers, C‐PENK‐7/3 offered the best balance between polarity and matrix compatibility, increasing the interlaminar shear strength from 78.89 to 94.46 MPa, a 19.7% improvement. Fracture morphology and dynamic mechanical analysis confirmed improved load transfer and reduced interfacial slippage.