Xiaoxu Wang

Interphase engineering of carbon‐fiber‐reinforced poly(ether ether ketone) (CF/PEEK) composites is hindered by the chemically inert fiber surface, the high melt viscosity of PEEK, and the poor thermal compatibility of conventional sizing agents. Herein, a hydroxyl‐functionalized cardo copoly(ether imide) containing PEEK‐mimetic ether–ketone segments was synthesized. To examine the effects of precursor state and deposition pathway, two sizing formulations derived from the same monomer composition were compared: a waterborne poly(amic acid) (PAA) precursor and a solvent‐based pre‐imidized polyetherimide (PEI). The waterborne PAA formed a rough particulate coating and underwent in situ imidization during composite consolidation, whereas the pre‐imidized PEI formed a smoother and more continuous layer. These differences in deposition and thermal evolution generated distinct interphase architectures, thereby regulating mechanical interlocking, PEEK transcrystallization, and the nanoscale modulus gradient across the interphase. The resulting W‐PEI‐CF/PEEK composite achieved an interlaminar shear strength of 88.81 MPa, representing a 40.4% increase over untreated CF/PEEK and exceeding the 78.95 MPa obtained for O‐PEI‐CF/PEEK. Meanwhile, the modulus‐transition width increased from approximately 55 nm in the untreated composite to 205 and 192 nm for the waterborne and solvent‐based routes, respectively. The superior performance of the precursor route originated from the synergistic effects of precursor‐derived polar functionalities, topographical anchoring, in situ formation of a thermally stable PEI interphase, and enhanced interfacial crystallization. These results demonstrate that the precursor state and deposition pathway are critical design parameters governing CF/PEEK interphase formation and establish waterborne PAA sizing as a scalable strategy for high performance thermoplastic composites.

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.

The effects of oxygen on the structure changes of PAN fibers during the rapid thermal stabilization are chronological, including thermal-driven dehydrogenation, oxidation and cyclization in the shorter, intermediate and longer periods, respectively.