Infululence of Polymer Nanoparticles on the Dewetting of Polymer Thin Films
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The influence of polystyrene star based nanoparticles on the dewetting of spun-cast linear polystyrene (PS) films on Si/SiOx surfaces is investigated as a function of temperature, concentration and type of the nanoparticles. The star polymers have polystyrene-benzocyclobutene copolymer arms which can undergo intra-molecular crosslinking to form relatively compact nanoparticles. The addition of a small amount of nanoparticles to linear PS thin films can suppress or enhance the dewetting of the films depending on the specifics of the star molecules. The suppression of dewetting in PS films by the nanoparticles is related to strong segregation of the nanoparticles to the polymer/silicon interface as observed by neutron reflection (NR) and atomic force microscopy (AFM). The stronger the segregation, the more effective the suppression of dewetting. Characterization of the hole morphology in the dewet films by AFM indicates there is a layer of polymer left behind on the Si substrate inside the hole which is consistent with the segregation layer observed by NR. Small angle neutron scattering showed that PS is miscible with the nanoparticles and no change in the surface energy was found by contact angle measurement at the polymer/air interface upon addition of nanoparticles. This suggests the segregation of nanoparticles to the Si surface is not due to immiscibility, or surface energy changes. The segregation of the star polymers at the polymer/Si interface is driven by both an attraction between star molecules and the Si/SiOx surface and possibly a relatively small enthalpic penalty resulting from the unfavorable interactions between the star and linear molecules.
Further investigation on the effects of nanoparticle concentration on the dewetting behavior showed that the lower the concentration, the less effective the dewetting suppression. This suggests a critical coverage of nanoparticles is essential for effective suppression of dewetting.