Construction of Polar Functional Groups on the Surface of a High-Modulus Carbon Fiber and Its Effect on the Interfacial Properties of Composites

[Image: see text] The interfacial bonding between carbon fibers and the resin matrix affects the mechanical properties of carbon fibers, and the increase of modulus brings a challenge to the interfacial properties of carbon fibers. The traditional anodic oxidation with ammonium bicarbonate as an electrolyte has a limited effect on the surface treatment for high-modulus carbon fibers. In this paper, anodic oxidation with an acidic electrolyte is used to treat high-modulus carbon fibers. The influence mechanism of a graphitized structure on the anodizing reaction of the carbon fiber surface was studied. Raman spectroscopy, XPS, scanning electron microscopy, dynamic contact angle, and micro-debonding were used to characterize the effect of surface treatment and its influence on interfacial properties. The results show that with a certain concentration of sulfuric acid as an electrolyte, the oxidation of the carbon fiber surface with high modulus occurs more on the graphite boundary defects. Carbonylation occurs mainly in carbon fibers with high modulus. The surface of the carbon fiber with a relatively low modulus is mainly hydroxylated and carboxylated. The surface energy and interfacial properties of high-modulus carbon fibers were improved effectively by anodic oxidation with sulfuric acid as an electrolyte. Under the condition that the mechanical properties of carbon fibers are not decreased, the surface energy of high-modulus carbon fibers with 352 GPa increases from 36.17 to 45.41 mN/m, and the interfacial shear strength (IFSS) with the epoxy resin increases by 80.8% from 34.9 to 63.1 MPa. When the fiber modulus is 455 GPa, the surface energy of the carbon fiber increases from 32.32 to 43.73 mN/m, and IFSS increases by 253.4% from 11.8 to 41.7 MPa.