Suppression of dewetting in nanoparticle-filled polymer films

The perturbing influence of nanosize filler particles on the dewetting of spun-cast polymer films is investigated. Previous studies have shown that spun-cast films of unentangled polystyrene (PS) and polybutadiene (PB) dewet acid-cleaned silicon wafers when the films are annealed above their bulk glass transition temperature T-g. It is also known that "impurity" particles tend to promote dewetting in thin polymer films. Here we demonstrate that the addition of a small amount of C-60 fullerene nanoparticles to the spin-casting polymer solution leads to an inhibition of dewetting in thin (L < 100 nm) PS and PB films cast on both acid-cleaned and carbon-coated silicon wafer substrates. Neutron reflection measurements indicate that this effect is associated with the formation of a diffuse fullerene layer near the solid substrate. Evidently, the immobilized fullerene particles form an enrichment layer at the solid boundary and "pin" the contact line of the growing dewetted ("dry") regions so that hole growth in the filled films becomes arrested at a scale which diminishes with increasing filler concentration. Above a relative filler-polymer mass fraction of 1%, we no longer observe hole formation by atomic force microscopy on the time scales of our measurements (typically on the order of hours), suggesting the existence of a critical filler surface coverage for the suppression of dewetting. The roughness of the fullerene layer probably also contributes to the modification of the surface wetting properties through a change in the surface energy of the substrate, as suggested by recent measurements on substrates "roughened" by other means such as irradiation of polymer film substrates, grafting of polymer chains, etc.