Simultaneous toughening and strengthening of CF/EP composites through bi-component thermoplastics with hybrid phases between composite layers

In this work, bi-component polyaryletherketone-cardo/polyethersulfone (PEK-C/PES) films with different areal densities were designed and fabricated by phase inversion approach, applied as interleaves to simultaneously toughen and strengthen the carbon fiber/epoxy resin (CF/EP) composites prepared by vacuum-assisted resin infusion process (VARI). Optical microscopy observations manifested that the PEK-C/PES films could maintain stability at the infusion temperature of epoxy resin. Nonetheless, PEK-C/PES films could dissolve into the epoxy resin at heating and initial stages of curing, by which the distribution of PEK-C and PES between the layers of CF/EP composites could be effectively controlled. Interestingly, both Mode I and Mode II fracture toughness of the CF/EP composites interleaved with PEK-C/PES films of optimal areal density (22.0 gsm) increased by 135.3% and 51.6%, respectively, compared with the untoughened composites. Furthermore, the PEK-C/PES films resulted in a 62.4% improvement in the compression-after-impact (CAI) strength CF/EP composites. Additionally, observation of the fracture surfaces of Mode I and Mode II specimens by scanning electron microscopy revealed that PEK-C had good compatibility with epoxy resin and formed a "scale-like" phase structure, which significantly improved the toughness of the CF/EP composites. While the "sea island-like" microspheres developed by PES between the CF/EP composite layers could cause the deflection and termination of cracks, thus enlarging the surface area of cracks. In addition, the tensile, flexural, and interlaminar shear properties of CF/EP composites were moderately enhanced, attributed to the incorporation of PEK-C/PES films. This cost-effective approach reconciles efficiently the conflict of the interlaminar toughness and mechanical properties of composites for fabricating high toughness CF/EP composites.