Control of nanoparticle dispersion in thin polymer films

Abstract

We have found larger (>5 nm) nanoparticles are uniformly distributed in a polymer film immediately after spin-coating. However, using the same thin film fabrication process leads to C₆₀nanoparticle segregation to the substrate, resulting in a kinetically trapped state that resists dispersion. The additional processing steps introduced here are found to yield uniform dispersion of these small nanoparticles in polymer films. Moreover, we show that although very small nanoparticles, such as C₆₀ fullerenes, can be homogeneously distributed in the film, larger organic nanoparticles subsequently phase separate to the solid substrate. Both processes, dispersion and segregation in thin films, are primarily driven through entropic terms dictated by the relative size of the nanoparticle and polymer statistical segment. This is in agreement with a simple theory and supported by density functional theory calculations that predict uniform dispersion in thin films when the nanoparticle diameter is less than ∼1.5 times the size of a polymer statistical segment. This is despite the fact that both the larger and C₆₀nanoparticles are miscible in the bulk, demonstrating the thermodynamic influence of a hard substrate. Morphologies are characterized vianeutron reflectivity measurements throughout the study to accurately characterize nanoparticle dispersion.