Visualizing interface-specific chemical bonds in adhesive bonding of carbon fiber structural composites using soft X-ray microscopy

Adhesion is a technology for assembling carbon fiber (CF) reinforced polymer (CFRP), enabling them to maintain their lightweight and high-stiffness properties. Despite the importance of adhesion, the lack of a molecular-level understanding of the adhesion mechanisms has limited the reliability of adhesion for use in next-generation aircraft and automobiles. Here, we focused on the chemical-state distribution at a practical adhesive interface composed of an epoxy-based adhesive film bonded to an epoxy-based CF matrix. By fluorinating the OH group, we succeeded in visualizing the chemical state at the CF-matrix/adhesive interface using soft X-ray microscopy. The soft X-ray images exhibited a decrease in OH-related signals at the interface due to the local chemical interaction at the epoxy-epoxy adhesive interface. We also found that the N and O Kα signals were observable at the CF's surface, indicating the presence of nitrogen- and oxygen-containing functional groups. Based on these observations, we discuss the molecular-level adhesion mechanism at the CF-matrix/adhesive interface.