Coarse-grained molecular dynamics simulations of nano-deformation behavior of epoxy adhesives’ interface during atomic force microscopy (AFM) mechanical testing

Adhesive interfaces with epoxy resin (adhesive joints and composites) play a critical role in bulk mechanical properties (e.g., load transfer), and it is important to elucidate the strength development mechanism near adhesive interfaces. However, the mechanical properties of epoxy resins on adhesive interfaces are known to be different from those of bulk resin and macroscopic adhesive interfaces because of a difference in the cross-linking ratio of epoxy resin. Since it is difficult to clarify the cross-linking ratio near the adhesive interface and to measure mechanical properties, it is important to establish nano-mechanical testing and simulation methods that can investigate mechanical properties and molecular structure around an adhesive interface. In this study, nanoindentation tests using atomic force microscopy (AFM) and molecular dynamics (MD) simulations that can reproduce the AFM tests were conducted on two-component epoxy resins to clarify the mechanical properties near the nanoscale adhesive interface and to investigate the mechanism of adhesive strength development. MD simulations employed a coarse-graining method that extends the spatial scale to cover the experimental scale of AFM nanoindentation. Furthermore, the mechanical properties of the adhesive interface were investigated from the viewpoint of deformation of molecular chains. It was found that at the interface, nanoindentation showed deformation in the adhesive due to reduction of cross-linking ratio. Coarse-grained (CG) MD simulations also simulated such deformation behavior in order to discuss molecular chain structure in deformation.