Nanoparticle aggregation behaviour in polymer nanocomposites: bulk vs. thin films

We report on the stability and cluster formation in C₆₀ fullerene-polystyrene mixtures, using a combination of small angle neutron scattering, optical and atomic force microscopy. Bulk nanocomposites are found to be stable for C₆₀ loadings up to 1-2% mass fraction in PS of M_w = 270 kg/mol and approximately 160°C. At larger fullerene concentration, the coherent scattering intensity grows with time, within 10s of min, and is well described by an asymptotic relationship with rate compatible with Arrhenius temperature dependence. The nanocomposite structure can be described by fractal scattering with a broad range of dimensions D_f ∼1.85-2.4 and polydisperse fractal sizes, up to hundreds of nm, depending on annealing temperature and time. Confinement in thin films of tens to hundreds of nm is shown to result in well-defined surface topographies, ranging from isolated clusters to percolated spinodal-like undulations with tuneable wavelength and amplitude. These results provide insight into the control of dispersion and self-assembly of nanoparticles in polymer matrices and thus their exploitation in functional materials, including organic photovoltaics and thin film coatings.