Molecular Dynamics Simulations on Epoxy/Silica Interfaces Using Stable Atomic Models of Silica Surfaces

The adhesion between silica surfaces and epoxy resins was investigated via molecular dynamics (MD) simulations with stable atomic models of silica substrates prepared by density functional theory (DFT) calculations and reactive force field (ReaxFF) MD simulations. We aimed to develop reliable atomic models for evaluating the effect of nanoscale surface roughness on adhesion. Three consecutive simulations were performed: (i) stable atomic modeling of silica substrates; (ii) network modeling of epoxy resins by pseudo-reaction MD simulations; and (iii) virtual experiments via MD simulations with deformations. We prepared stable atomic models of OH- and H-terminated silica surfaces based on a dense surface model to consider the native thin oxidized layers on silicon substrates. Moreover, a stable silica surface grafted with epoxy molecules as well as nano-notched surface models were constructed. Cross-linked epoxy resin networks confined between frozen parallel graphite planes were prepared by pseudo-reaction MD simulations with three different conversion rates. Tensile tests using MD simulations indicated that the shape of the stress-strain curve was similar for all models up to near the yield point. This behavior indicated that the frictional force originated from chain-to-chain disentanglements when the adhesion between the epoxy network and silica surfaces was sufficiently strong. MD simulations for shear deformation indicated that the friction pressures in the steady state for the epoxy-grafted silica surface were higher than those for the OH- and H-terminated surfaces. The slope of the stress-displacement curve was steeper for surfaces with deeper notches (approximately 1 nm in depth), although the friction pressures for the examined notched surfaces were similar to those for the epoxy-grafted silica surface. Thus, nanometer-scale surface roughness is expected to have a large impact on the adhesion between polymeric materials and inorganic substrates.