Chain Conformation near the Buried Interface in Nanoparticle-Stabilized Polymer Thin Films

It is known that when nanoparticles are added to polymer thin films, they often migrate to the film-substrate interface and form an "immobile interfacial layer", which is believed to be the mechanism behind dewetting suppression. We here report a new mechanism of dewetting suppression from the structural aspect of polymer chains accommodated at the film substrate interface. Dodecanethiol-functionalized gold (Au) nano-particles embedded in relatively low molecular weight PS thin films prepared on silicon (Si) substrates were used as a model. We mimicked the previously reported conditions, where the nano particles preferentially migrate to the substrate, and successfully stabilized the PS thin films via thermal annealing. A suite of, surface-sensitive techniques including atomic force microscopy, grazing incidence small-angle X-ray scattering, X-ray/neutron reflectivity, and sum frequency generation spectroscopy in conjunction with the established solvent leaching process enabled us to unveil the polymer chain conformation and the dispersion structure of the nanoparticles at the film substrate interface. The results evidenced that thermal annealing promotes irreversible polymer adsorption onto the substrate surface along with the migration of the nanoparticles. In addition, we found that the migration of the nanoparticles causes the changes in the conformations and interfacial orientations of the adsorbed polymer chains compared to those of the adsorbed polymer chains formed in the nanoparticle-free PS thin film. The resultant interfacial polymer structure allows for the interpenetration between free chains and the adsorbed chains, thereby stabilizing the thin film.