Cross-scale optimization of interfacial adhesion and thermal-mechanical performance in carbon fiber-reinforced polyimide composites through sizing agent evolution

The modification of carbon fibers with sizing agents, as an alternative to incorporating nanoparticles , has emerged as a practical strategy to enhance the interfacial adhesion and improve the thermal and mechanical properties of carbon fiber-reinforced thermoplastic polyimide (CF/TPI) composites. However, the lack of comprehensive understanding of the behavior and transformation of sizing agents during composite processing limits the performance enhancement of the composites. In this study, the interfacial mechanisms of sizing agents are systematically analyzed, distinguishing between chemical bonding and physical interaction pathways, while addressing the stages of wetting, molecular diffusion, and interfacial crosslinking. By optimizing the balance between chemical and physical interfacial mechanisms, significant improvements in stress distribution and filler-matrix compatibility are achieved. A quantitative relationship between sizing agent concentration and interfacial evolution was established, enabling precise control of the interface formation stages, including diffusion and crosslinking. The thermal conductivity of the optimized CF/TPI composites is 490% of that of PI. When used as a heat sink, it reduces the LED center temperature by 26 °C, while maintaining a tensile strength of 73 MPa and a retention rate of 69% at 200 °C. These results indicate that the precise control of the sizing process improved stress transfer across the interface, reduced microstructural defects, and contributed to enhanced thermal management and structural durability. This work provides a novel perspective on the dynamic role of sizing agents in composite development and lays the groundwork for advanced design strategies to maximize the performance of polymer composites . A cross-scale interfacial structure was designed between the polyimide (PI) matrix, carbon fiber (CF), and graphene (GP) via a tailored sizing strategy, enhancing interfacial adhesion and optimizing phonon transport. This engineered interface facilitated uniform filler dispersion, reduced phonon scattering, and lowered interfacial thermal resistance, leading to an in-plane thermal conductivity of 11.13 W m −1 K −1 , which was 490 % improvement over pure PI. Meanwhile, the hierarchical reinforcement contributed to a tensile strength of 73 MPa, a Young's modulus of 2.7 GPa, and 69 % tensile strength retention at 200 °C, ensuring mechanical robustness under thermal stress. This study presents a scalable interfacial engineering strategy for high-performance polymer composites with superior thermal conductivity and mechanical integrity, offering promising applications in advanced thermal management.