Synergy of electrochemical grafting and crosslinkable crystalline sizing agent to enhance the interfacial strength of carbon fiber/PEEK composites

Carbon fiber/PEEK composites have gained significant attention in the field of continuous carbon fiber reinforced polymers (CFRPs). However, it remains challenging to improve the weak interfacial strength between carbon fiber (CF) and PEEK matrix. In this paper, strong interactions at the interface of sizing agent and carbon fiber were effectively achieved by introducing chemical bonds. Specifically, a soluble PEEK sizing agent where aniline served as pendant group and phenylacetylene served as blocking agent was synthesized. In the meantime, the phenylacetylene group was grafted on the carbon fiber surface through electroreduction. Then, the soluble sizing agent was used to size the electrochemical treated carbon fiber. After the heat treatment, chemical bonds were established by the crosslinking reaction of phenylacetylene groups existed in both the surface of carbon fiber and sizing agent. Meanwhile, the crystalline structure in the sizing layer was remained, which could improve the toughness of the interphase. As a result, the interfacial shear strength (IFSS) increased by 137.5% from 43.4 MPa of the untreated carbon fiber (U-CF) to 103.1 MPa of the reductive electrografted and coated carbon fiber (E-C-CF). The interlaminar shear strength (ILSS) also increased by 66.8% from 58.4 MPa of untreated CF/PEEK composites (U-CF/PEEK) to 97.4 MPa of reductive electrografted and coated CF/PEEK composites (E-C-CF/PEEK). The outstanding performances could be attributed to better compatibility of sizing agent and most importantly, the existence of chemical bonds between the carbon fiber and sizing agent.