Insights into Mechanical Dynamics of Nanoscale Interfaces in Epoxy Composites Using Nanorheology Atomic Force Microscopy

Interfacial polymerlayers with nanoscale size play criticalrolesin dissipating the strain energy around cracks and defects in structuralnanocomposites, thereby enhancing the material's fracture toughness.However, understanding how the intrinsic mechanical dynamics of theinterfacial layer determine the toughening and reinforcement mechanismsin various polymer nanocomposites remains a major challenge. Here,by means of a recently developed nanorheology atomic force microscopymethod, also known as nanoscale dynamic mechanical analysis (nDMA),we report direct mapping of dynamic mechanical responses at the interfaceof a model epoxy nanocomposite under the transition from a glassyto a rubbery state. We demonstrate a significant deviation in thedynamic moduli of the interface from matrix behavior. Interestingly,the sign of the deviation is observed to be reversed when the polymerchanges from a glassy to a rubbery state, which provides an excellentexplanation for the difference in the modulus reinforcement betweenglassy and rubbery epoxy nanocomposites. More importantly, nDMA losstangent images unambiguously show an enhanced viscoelastic responseat the interface compared to the bulk matrix in the glassy state.This observation can therefore provide important insights into thenanoscale toughening mechanism that occurs in epoxy nanocompositesdue to viscoelastic energy dissipation at the interface.

Automated summary

Interfacial polymer layers with nanoscale size enhance the fracture toughness of structural nanocomposites. However, understanding their mechanical dynamics and toughening mechanisms remains a challenge.