(396b) Substrate Effects on Polymer Thin Films as an Analogy for Polymer Nanocomposites

With the addition of a very small amount of nanofillers into the polymer matrix, these nanocomposites exhibit substantial increases in many physical properties. However, it is unclear how the nanofillers interact with the surrounding polymer chains and then determine the macroscopical properties of these nanocomposites. By using the well-defined systems, polymer thin films/substrate configurations, we resembled the interfaces of polymer-substrate and measured the polymer dynamics (i.e. glass transition temperature, T_g) in vicinity of the substrate using atomic force microscope with nanoparticles as the probe. Polystyrene (PS) thin films were spun-coat on two types of substrates, highly ordered pyrolytic graphite (HOPG) and silicon wafer with a native oxide layer. The film thickness was changed from 1 R_g (radius of gyration of PS) to a few R_g and the effects of both the polymer-air and the polymer-substrate interactions were investigated. The PS on these substrates shows different Tg profiles, depending on the polymer-substrate interactions. The influence of subcritical CO₂ on T_g of those polymer thin films was examined as well. The competing impacts of the entropic confinement (by substrate) and the free-volume increase (by CO₂) were evaluated. These results will provide valuable guides for designing and processing of new polymer nanocomposites.