Surface segregation and formation of silver nanoparticles created in situ in poly(methyl methacrylate) films

Nanocomposite films have been prepared by the thermal decomposition of (1,1,1,5,5,5-hexafluoroacetylacetonato)silver(I) (AgHFA) in PMMA films containing 5, 10, and 20 wt % silver nanoparticles (NPs). When Rutherford backscattering spectrometry is used, as-cast films display a relatively uniform distribution of precursor in the midregion and a small excess of Ag near the surface, Z(s)(*). Upon annealing at 185 degrees C, strong segregation of NPs to the surface and substrate is observed. The Z(s)(*) normalized by initial film thickness increases linearly with wt % Ag. For as-cast films, transmission electron microscopy (TEM) analysis shows that AgHFA forms small spherical domains (1-5 nm) prior to thermal decomposition. Upon preannealing at 107 degrees C, some aggregation of precursor is observed at the surface and substrate, consistent with the incompatibility of AgHFA with PMMA. Upon annealing at 185 degrees C, diffusion of the precursor to the surface and substrate occurs concurrently with NP formation. For 20 wt % Ag films, the NP diameters are 20-75 nm at the surface, 6 nm in the midregion, and 2-20 nm at the substrate. NP size and size distribution increase as the wt % Ag and annealing time increase. NPs near the surface are wet by a 1-5 nm thick polymer layer. Consistent with characterization studies, UV-visible spectroscopy shows a plasmon resonance attributed to Ag NPs. A model of the thermodynamic stability of submerged NPs is given and compared with experimental results. By understanding and controlling surface segregation and NP assembly, we can precisely control the surface conductivity and reflectivity in metal nanocomposite films.