Enhancing the interfacial properties of high-modulus carbon fiber reinforced polymer matrix composites via electrochemical surface oxidation and grafting

Surface modification of carbon fiber is a traditional research field and many techniques have been developed to improve the adhesion force between fiber and the polymer matrix. However, most studies were focused on the carbon fibers with standard modulus. The surface of high-modulus carbon fiber (HMCF) is difficult to be activated due to the highly inert structure after graphitization. A convenient and effective surface modification method that can simultaneously improve the content of function groups with containing oxygen and nitrogen on the surface of HMCF was developed in this paper. The surface of HMCF was first anodized to be activated by electrochemical oxidation, and then the diethylenetriamine (DETA) was grafted by electrochemical grafting. The effect of the type of electrolyte on the electrochemical grafting of DETA was studied by the addition of NH4HCO3 and (NH4)2SO4, respectively. The results indicated that the addition of NH4HCO3 electrolyte in DETA solution will promote the grafting and increase the grafted amount on HMCF surface. Moreover, the addition of (NH4)2SO4 electrolyte can not only promote the grafting, but also oxidize the surface of the fibers further to generate more oxygen-containing groups. In addition, the interfacial performance of HMCF composites was significantly improved after modification by this method. Especially, when (NH4)2SO4 was added to DETA solution, the inter-laminar shear strength (ILSS) of HMCF/epoxy composites reached 97.5 MPa, which was 257.1% higher than that of untreated HMCF.