Molecular picture of curing and incomplete cross-linking of epoxy at a solid interface

Understanding the buried interface between silica and epoxy resin is crucial for improving the performance and reliability of epoxy composites and adhesives. Here, molecular dynamics simulations were used to reveal a heterogeneous molecular picture of aggregation and curing at the amorphous silica/epoxy interface. A density increase was observed within 2 nm of the interface, driven by the orientation and packing of epoxy and amine molecules. Smaller amines segregated near the substrate, accelerating interfacial curing in the early stages. However, subsequent reactions slowed due to restricted mobility, resulting in a ∼10% lower conversion than in the bulk. Unreacted monomers and isolated fragments accumulated near the interface, indicating adverse effects on adhesion. These findings provide molecular-level insights into buried interfaces and inform strategies for improving adhesion and reliability in epoxy-based composites and adhesives.