Triple-Coupled Organic-Nano-Transcrystalline Interphase Constructed via Eco-Friendly Crosslinkable Polyamide-Imide/Carboxylated CNT Waterborne Sizing for High Performance CF/PEEK Composites

Carbon fiber reinforced polyetheretherketone (CF/PEEK) composites suffer from severe fiber-matrix modulus mismatch and weak interfacial adhesion, limiting their mechanical performance. Herein, eco-friendly waterborne sizing agents consisting of self-crosslinkable polyamide-imide (CPI) and carboxylated carbon nanotubes (CCNTs) are developed to construct a triple-coupled organic-nano-transcrystalline interphase. Thermally robust CPI adapts to high processing temperature of PEEK, while uniformly dispersed CCNTs enrich surface active groups, increase fiber roughness and supply heterogeneous nucleation sites for thick transcrystalline (TC) layers. CCNT content governs interfacial morphology, TC thickness and composite mechanical properties. The optimized CF-CPI-25CCNT achieves tensile strength of 233.4 MPa, flexural strength of 348.0 MPa, impact strength of 41.0 MPa and IFSS of 84.4 MPa, rising by 70.4%, 55.1%, 54.1%, and 110.4% versus bare CF. CF-CPI-50CCNT delivers maximum tensile and flexural moduli, whereas CF-CPI-75CCNT obtains superior tensile strength at 200°C. AFM characterizations confirm the hybrid sizing builds a continuous gradient-modulus interphase to mitigate interfacial stress concentration. Moreover, CF-CPI-25CCNT exhibits balanced tow integrity and flexibility suitable for industrial processing. Synergistic reinforcement originates from improved wettability, mechanical interlocking, TC induction and rigid-flexible gradient architecture. This green sizing provides a scalable multi-scale interfacial design route for high-temperature-resistant CF/PEEK composites. Highlights The CPI/CCNT sizing agents build organic-nano-transcrystallinity interphase. CCNT serves nucleation sites to thicken transcrystallinity. The interfacial wettability of CF is significantly improved. Gradient interphase is formed and relieves modulus mismatch. CF-CPI-25CCNT achieves 110.4% higher IFSS.