A Multi‐Scale “Soft–Rigid” Structure Is Constructed on the Surface of Carbon Fibers to Enhance and Strengthen the Strength and Toughness of Epoxy Matrix Composite

ABSTRACT To enhance the mechanical performance of carbon fiber (CF)‐reinforced polymer composites, an elaborate multi‐scale “soft–rigid” interfacial architecture was constructed by the sequential self‐assembly of carboxymethyl cellulose (CMC), MXene (MX), and chitosan (CS) on the CF surface. This hierarchical structure markedly improves the wettability and surface roughness of CFs, thereby strengthening interfacial chemical bonding and mechanical interlocking with the epoxy (EP) matrix. Moreover, the multi‐scale soft–rigid interface functions as a modulus‐gradient transition layer, enabling more uniform stress distribution, suppressing stress concentration, buffering external loads, and inducing crack deflection and microcrack formation, which collectively enhance energy dissipation during fracture. As a result, the flexural modulus and interfacial shear strength of modified CF/EP composites are improved compared with unsized CF composites and commercial sized CFs, which are increased by 69.6%, 43.3% (to 48.0 GPa) and 165.7%, 73.1% (to 116.1 MPa), respectively. This bioinspired interfacial engineering strategy provides a promising and scalable route for the fabrication of high‐strength and high‐toughness CF/EP structural materials.