Direct observation of nanoparticle embedding into the surface of a polymer melt

Direct embedding of metal nanoparticles (NPs) into the surface of a polymer melt is observed by TEM and a new embedding mechanism proposed. Upon annealing above the glass transition temperature of polystyrene (PS), NPs (20 nm gold) are rapidly covered by a thin PS wetting layer, h* approximately 1.3-1.8 nm (i.e., about two or three monomers). Because it creates capillary pressure on a NP, this "universal" wetting layer is proposed to be responsible for NP embedding. The value of h* is independent of the molecular weight of PS and constant during the embedding process. The value of h* is found to be similar to the equilibrium wetting layer thickness of a polymer melt spreading on a metal substrate. Using a model that includes the spreading coefficient, long-range van der Waals interactions, and a chain-stretching penalty, h* is shown to be independent of the molecular weight of the polymer. Using this model and the measured value of h*, the interfacial energy between Au NP and PS is estimated to be 8.7 J/m2.