Hydroxyl‐Functionalized Cardo Copoly(Ether Imide) Sizings With PEEK ‐Like Segments: Waterborne Precursor Versus Solvent‐Based Routes for CF / PEEK Interphase Engineering

ABSTRACT 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.