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The weak interfacial bonding between fiber and matrix needs to be improved for obtaining good mechanical properties of carbon fiber/polyetheretherketone (CF/PEEK) composites due to the inert nature of both components. Organic sizing is a typical interface improvement method, but it is harmful to both the human and local environment. Here, a two-step process is proposed to modify PEEK for preparing an environmentally friendly water-based sizing agent, involving taking the ketone reduction via NaBH4 and then an esterification reaction with phosphoric acid. This PEEK derivative has good thermal stability, and its dispersion in water is stable for one month. It improves the interface between CF and PEEK by increasing the wettability and surface roughness of CFs, ultimately enhancing the mechanical properties of composites. The two-step process of fabricating the PEEK derivative may have potentials in the industrial application of the polyaryletherketone family.

The weak interfacial adhesion caused by the low wettability of carbon fiber (CF) and inert polyetheretherketone (PEEK) hinders the great application potential of CF/PEEK composites. In this study, the interface between CF and PEEK was modified by coating hydroxylated PEEK grafted onto multi-walled carbon nanotubes (M-HPEEK). Modified CFs by M-HPEEK sizing had rougher surfaces with more polar groups, showing decreased contact angle and increased surface energy. The wettability of these CFs was enhanced, beneficial to PEEK infiltration and physical adhesion improvement. As a result, the interlaminar shear strength, flexural strength, and flexural modulus of modified CF/PEEK composites were increased by 73.0% (84.7 MPa), 163.2% (906.2 MPa), and 84.8% (58.4 GPa), respectively. An improved interface with stress transferring was proved by the interface debonding failure changed to fiber crack and resin yield. M-HPEEK sizing is considered as a promising modification method to optimize the interface performance of composites.

The interfacial adhesion between carbon fiber (CF) and polyetheretherketone (PEEK) is poor due to their inert nature, limiting the mechanical properties of CF/PEEK composites. This study improved the interactions between CF and PEEK by sizing hydroxylated PEEK grafted multi-walled carbon nanotubes (HPEEK-g-MWCNT) synthesized via an esterification reaction. Molecular dynamics simulations and experiments were conducted to evaluate the microstructure and characterize the mechanical properties of CF/PEEK composites. A slightly decreased crystallinity of PEEK within modified composites resulted from the reduced nucleation effect of CF and inhibited PEEK mobility by HPEEK-g-MWCNT sizing. Hydrogen bonding and mechanical interlocking enhanced interface adhesion, and covalent bonds of HPEEK-g-MWCNT increased load transfer. These factors led to the flexural strength and interlaminar shear strength of HPEEK-g-MWCNT modified composites increased by 94.2% (669.7 MPa) and 55.2% (79.1 MPa) compared with control ones. The modification method of using chemically treated and grafted polymers like HPEEK-g-MWCNT would have great potential applications in thermoplastic composites.