Construction of “rigid-and-flexible” interphase by waterborne carboxylated polyimide sizing agent for interfacial enhancement of carbon fiber/poly ether ether ketone (CF/PEEK) composites

The application of polyimide (PI) as an effective sizing agent for carbon fiber (CF) in CF/poly ether ether ketone (PEEK) composites represents a promising approach. Here, a series of novel carboxylated waterborne PI sizing agents with different aliphatic ratios were designed to construct a rigid-flexible gradient interphase for the purpose of promoting interfacial adhesion between CF and PEEK. It was observed that the mechanical properties of the composites were optimal when the molar ratio of rigid diamine monomer to flexible diamine monomer reached 7:3. Significant augmentation was observed in various mechanical and interfacial properties, including tensile modulus (25.6 %), tensile strength (19.8 %), flexural modulus (28.5 %), flexural strength (15.8 %), impact strength (24.3 %), and IFSS (57.8 %). Furthermore, the tensile strength of the composites exhibited a greater increase (27.2 %) at elevated temperatures, thus indicating that the role of the interfacial layer was more pronounced at high temperatures. The interphase was analyzed by peak force quantitative nanomechanical imaging (PF-QNM) mode of atomic force microscopy (AFM), the results indicated the formation of a modulus transition zone between the CF and PEEK. This zone exhibited a gradient change of modulus, enabling the composite to transfer and distribute applied loads more effectively when subjected to external forces. Consequently, the findings of this study demonstrated the potential of waterborne PI sizing agents for application on CF, offering a promising avenue for surface modification of high-performance CF/PEEK composites.