Enhanced Interfacial Performance of 3D-Printed CF/PLA Composites via a “Rigid–Flexible” Interface Constructed by PEG-PANI Copolymers with an Adjustable Rigidity–Flexibility Ratio

Abstract To enhance the interfacial performance of 3D-printed short carbon fiber-reinforced polylactic acid (CF/PLA) composites, this study designed and synthesized a series of methoxypolyethylene glycol-polyaniline (PEG-PANI) block copolymers with varying ratios of rigid PANI segments to flexible PEG segments to construct a modulus gradient interfacial layer between the CF and PLA. After introducing the PEG-PANI copolymer onto the surface of continuous carbon fibers (CCFs), modified CF/PLA filaments were prepared via twin-screw pelletization and single-screw extrusion and then formed using fused deposition modeling (FDM). Mechanical testing indicates that the mechanical properties of the composite are optimized when the molar ratio of rigid PANI to flexible PEG units is 1:1. Compared to pure CF/PLA, the interfacial shear strength (IFSS) and impact strength increased by 113.48 and 90.37%, respectively, while tensile strength and flexural strength increased by 46.88 and 32.44%, respectively. Atomic force microscopy peak-force quantitative nanomechanical mapping (AFM-PF-QNM) characterization revealed that the PEG-PANI block copolymer formed a continuous modulus gradient transition zone at the modified interface. This gradient structure facilitates effective stress transfer and uniform load distribution under external forces, thereby significantly improving mechanical compatibility and load-bearing efficiency. This study provides an effective interface control strategy for the development of high-performance FDM-printed composites.