Suppression of dewetting in nanoparticle-filled polymer films

The perturbing influence of nanosize filler particles on the dewetting of s pun-cast polymer films is investigated. Previous studies have shown that sp un-cast films of unentangled polystyrene (PS) and polybutadiene (PB) dewet acid-cleaned silicon wafers when the films are annealed above their bulk gl ass transition temperature T-g. It is also known that "impurity" particles tend to promote dewetting in thin polymer films. Here we demonstrate that t he addition of a small amount of C-60 fullerene nanoparticles to the spin-c asting polymer solution leads to an inhibition of dewetting in thin (L < 10 0 nm) PS and PB films cast on both acid-cleaned and carbon-coated silicon w afer substrates. Neutron reflection measurements indicate that this effect is associated with the formation of a diffuse fullerene layer near the soli d substrate. Evidently, the immobilized fullerene particles form an enrichm ent layer at the solid boundary and "pin" the contact line of the growing d ewetted ("dry") regions so that hole growth in the filled films becomes arr ested at a scale which diminishes with increasing filler concentration. Abo ve a relative filler-polymer mass fraction of 1%, we no longer observe hole formation by atomic force microscopy on the time scales of our measurement s (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 s urface wetting properties through a change in the surface energy of the sub strate, as suggested by recent measurements on substrates "roughened" by ot her means such as irradiation of polymer film substrates, grafting of polym er chains, etc.