Influence of C60 Nanoparticles on the Stability and Morphology of Miscible Polymer Blend Films

We investigate the influence of fullerene (C-60) nanoparticle (NP) additives on a thermodynamically miscible polymer blend thin film of polystyrene (PS) and polybutadiene (PB). In this system both homopolymer components individually dewet from the commonly used silicon substrate. Three NP concentration regimes having distinct blend nanocomposite film morphologies are observed: (a) In the neat blend and low NP mass (0-1%) range, the blend films rapidly dewet, apparently due to fluctuations in the polymer surface tension arising from the composition fluctuations of a surface enrichment layer at the film air boundary. This behavior is in sharp contrast to the corresponding NP-filled homopolymer films where dewetting is progressively slowed by the segregation of NPs to the solid substrate in this same concentration range. (b) In the intermediate NP concentration range of 1-5 mass %, the C-60 additive acts as a "compatibilizing agent", progressively reducing the size of the dewetted droplets with increasing NP concentration. Dewetting is fully suppressed in the homopolymer films in this NP concentration range. We conclude that C-60 segregation to polymeric interfaces within blend film competes with the NP film stabilizing effect. (c) At higher NP concentrations between 5 and 10 mass %, the NPs enrich the substrate sufficiently to fully inhibit the blend film dewetting through a percolating blend-NP structure. At very high NP concentrations (10-15 mass %), the NPs form clusters within the blend film giving rise to a "spinodal clustering" NP morphology.