Surface Segregation and Wetting of Nanoparticles in Polymer Nanocomposites*

The segregation and wetting of polymer grafted nanoparticles (NP) to the free surface of a polymer nanocomposite film is driven by both surface energy and thermodynamic forces. Here, we probe these two contributions in a model system of PMMA grafted silica NPs in a poly(styrene-ran-acrylonitrile) (SAN) matrix using Rutherford backscattering spectrometry (RBS) and atomic force microscopy (AFM) as a function of thermal annealing temperature and time. Studies are performed above and below the critical point of this LCST system to decouple the thermodynamic and interfacial energy contributions. With increasing time, a monotonic increase in surface excess of PMMA grafted NPs is observed in the miscible regime, which is attributed to the difference in surface energies between the PMMA brush and SAN matrix. Upon annealing above the critical point, a much stronger increase in PMMA NP surface coverage is observed because of the thermodynamic driving force, namely the Flory-Huggins interaction parameter between the PMMA grafts and SAN matrix. Using the measured surface excess values of PMMA NPs at multiple annealing times and temperatures, we then extract the apparent diffusion coefficients of the particles and compare them to the systems homopolymer analogue.