Chemical imaging of resin adhesive interface using soft X-ray microscopy

Adhesive bonding is an interfacial phenomenon that is critical for assembling polymer-matrix composite materials, enabling them to maintain their lightweight and high-stiffness properties. However, the lack of understanding of the adhesion mechanisms at the molecular level has limited the reliability of adhesive bonding for industrial uses. In this article, using soft X-ray microscopy, we demonstrate the visualization of one of the important factors of adhesion; that is, physical and chemical states at the adhesive interface. For this purpose, we prepared a model adhesive interface composed of a thermosetting epoxy resin adhered to a plasma-pretreated thermoplastic resin. We succeeded in observing multiscale phenomena in the adhesion mechanisms, including sub-mm complex interface structure, sub-μm distribution of the functional groups, and molecular-level covalent-bond formation. These results provide a benchmark for further research to examine how physical and chemical states correlate with adhesion, and demonstrate that soft X-ray imaging is a promising approach for visualizing the physical and chemical states at adhesive interfaces from the sub-mm level to the molecular level. (author)