Mechanism and Kinetics of Fullerene Association in Polystyrene Thin Film Mixtures

We study the mechanism and kinetics of fullerene C-60 association in model homopolymer thin films upon isothermal annealing. Uniform polystyrene (PS) films loaded with 0-5% C-60 mass fraction, on both sides of the miscibility threshold, are annealed and monitored over time. A transition from heterogeneous, sparse, nucleation to a percolated spinodal morphology is observed with increasing C-60 loading and time. Fullerene association is followed by crystallization, which is quantified by Avrami kinetics in two dimensions. Key regimes of the process are identified as nucleation, percolation, saturation and coarsening stages, and defined in terms of the evolution of cluster average size < r >, number density N-d, area fraction A(f), and dominant wavenumber q*. The spinodal clustering process results in a percolated lateral morphology and undulating surface topography whose wavelength lambda and amplitude delta h can be tuned with temperature T, time t, film thickness h and polymer molecular mass M-w. These results provide insight into pattern formation in nanocomposites relevant to engineering functional materials, including organic photovoltaics and thin film coatings.