The impact of crystalline PEEK sizing agent on the interfacial crystallization behavior and interfacial properties of carbon fiber reinforced PEEK composites

In recent years, numerous studies have reported the application of crystalline sizing agents for the interfacial modification of carbon fiber reinforced polyether ether ketone (PEEK/CF) composites. However, the underlying mechanism by which these agents improve the interfacial properties remains unclear. Therefore, the interfacial enhancement mechanism of PEEK/CF composites using crystalline PEEK sizing agents was explored from a microscopic perspective, specifically focusing on interfacial crystallization in this study. Firstly, the crystallization behavior of PEEK on the CF surfaces modified with varying concentrations of crystalline PEEK sizing agents was investigated using polarized optical microscope (POM). It was observed that the sizing agent could induce the formation of PEEK transcrystallinity (TC) on the CF surface, with the 1 wt% concentration of sizing agent resulting in the densest crystal nuclei. The lamellar morphology and molecular orientation of the TC layers were further analyzed. Correspondingly, higher interfacial shear strength (IFSS) was observed in PEEK/CF composites with denser TC structures. Furthermore, the film tensile tests and chemical solvent erosion experiments were conducted to investigate the distribution of weak interfacial phases. It was found that the TC layer did not introduce new "weak interfacial phases" into the composites and exhibited superior resistance to solvent erosion. Finally, the mechanism of action of crystalline PEEK sizing agents was discussed. It was proposed that PEEK molecules from the sizing agent could effectively fill the voids at the interface of the composites which were caused by the low mobility of high-viscosity PEEK resin matrix. This led to an increase in nucleation sites on the CF surface and the formation of a denser TC layer.