Construction of an interphase with heat and solvent resistance by a waterborne self-crosslinking polyamide-imide sizing agent to enhance the interfacial and mechanical performance of CF/PEEK composites

The inherent inertness of carbon fiber (CF) and poly ether ether ketone (PEEK), coupled with the insufficient thermal stability of conventional sizing agents, limits the full realization of the exceptional properties of CF/PEEK composites. In this study, self-crosslinked aqueous polyamide-imide (PAI) sizing agents with varying rigid-to-flexible ratios are synthesized for interfacial modification of CF/PEEK composites. The results indicate that, when the ratio of rigid to flexible monomers is 5:5, the mechanical properties and interfacial performance of the composite reach their optimal values, with tensile modulus and strength, flexural modulus and strength, and impact strength increased by 20.2 %, 39.1 %, 32.1 %, 30.4 %, and 60.4 %, respectively. Additionally, the interfacial shear strength (IFSS) of the composite increases by 86.0 %. The interface between CF and PEEK is analyzed using atomic force microscopy (AFM), the results reveal the successful formation of a modulus transition layer with a gradual modulus decrease between them. This special structure facilitates more uniform load transfer within the composite and achieves higher transfer efficiency. Beyond these improvements, tensile strength of the composite at 200 °C increases by 63.7 %, and the sized CF demonstrates an IFSS retention rate of up to 97 % after immersion in organic solvents. This study proposes a novel and viable approach for the optimization of the interface of CF/PEEK composites.