Molecular dynamics simulations for interfacial structure and affinity between carboxylic acid-modified Al₂O₃ and polymer melts

Controlling the dispersion state of nanoparticles in a polymer matrix is necessary to produce polymer nanocomposites. The surface modification of nanoparticles is used to enable their dispersion in polymers. Moreover, molecular dynamics (MD) simulations are useful for revealing the interfacial properties between nanoparticles and polymers to aid in the design of materials. In this study, the effect of surface coverage, modifier length, and polymer species on the interfacial structure and affinity between surface-modified Al₂O₃ and polymer melts were investigated using all-atom MD simulations. Hexanoic, decanoic, and tetradecanoic acids were used as surface modifiers, and polypropylene (PP), polystyrene (PS), and poly (methyl methacrylate) (PMMA) were used as polymers. The work of adhesion W_adh and the work of immersion W_imm were selected as quantitative measures of affinity. W_adh was calculated using the phantom-wall approach, and W_imm was calculated by simply subtracting the surface tension of polymers γ_L from W_adh. The results showed that W_adh and W_imm were improved by surface modification with low coverage, owing to a good penetration of the polymer. The effect of modifier length on W_adh and W_imm was small. Whereas W_adh increased in the following order: PP < PS < PMMA, W_imm increased as follows: PMMA < PS < PP. Finally, the trend of W_adh and W_imm was organized using the Flory-Huggins interaction parameter χ between the modifier and the polymer. This study demonstrates that the interfacial affinity can be improved by tuning the surface coverage and modifier species depending on the polymer matrix.