Synthesis of fractal crystallized organic microspheres together with constructing full covalent bonding at the interface to strengthen and toughen aramid fiber composites

Regulating the interface to simultaneously enhance the strength and toughness of aramid fiber composites remains a great challenge. Herein, a resilient, full covalent interface was constructed by introducing fractal crystallized organic microspheres into the interface of aramid fiber composites using the twice fluorination approach. Firstly, the aromatic polyamide with ether bonds was synthesized, and the fractal crystallized microspheres with a porous surface containing amino groups were prepared by thermally induced solution self-assembly. Subsequently, the twice fluorination method introduces C–F bonds to the surfaces of both fibers and microspheres, respectively. Then, the covalent bonds among fibers, microsphere, and resin were formed by the derivative reaction of C–F bonds. This interface boosts the interlaminar shear strength (ILSS) of composites by 100.5% to 36.7 MPa and the unnotched impact toughness of composites by 13.5%–1363 kJ/m2. Comparative analyses reveal the full covalent interface and the fractal structure of microspheres contribute 47.8% and 52.2% to the ILSS enhancement, respectively. In addition, the fractal structure of microspheres is the only factor for increased toughness. Finite element analysis reveals that the fractal crystallized microspheres combined with full covalent interface facilitate elastic stress transfer within the composites to enhance the performance of composites.