(203d) Role of Co2 in Surface Tg Reduction of Polymers

Applying carbon dioxide to polymer processing remains a very attractive and innovative research area, for instance, in polymer-based nanodevices and nanofabrication technology. This poses a great challenge of understanding the effects of CO2 on the surface properties, in particular the surface glass transition temperature (Tg). It has been widely accepted that the Tg of nanoscale polymer thin film behaves anomalously relative to its bulk phase due to the presence of free surface and/or substrate. In this work, we addressed the surface Tg both experimentally and theoretically. An Atomic Force Microscopy (AFM)?based visualization method was developed, in which the gold nanoparticles were deposited onto surface and the embedding profile was used to probe the surface mobility at various annealing conditions of temperature and CO2 pressure. The results of monodisperse PS are consistent with literature reports, namely, a region with enhanced mobility occurring near the surface even at conditions below the bulk Tg. Moreover, CO2 further reduces the temperature where the dynamic enhancement is observed, and broadens the surface mobility layer. Meanwhile, we have been extending a previously developed entropy model that explains the origin of anomalous Tg for pure polymer to the binary system of CO2 and polymer. It is able to construct the mass density and entropy density profiles of both components across the surface region. As a result, the response of surface mobile layer to external CO2 pressure and treatment temperature, which is explicitly defined by the value of local entropy density, could be readily determined. The fundamental tools include the gradient theory for mixture, Gibbs-Di Marzio Tg criterion and Sanchez-Lacombe Equation of State (SLEOS). Moreover, the interfacial tension of polymer phase as a function of CO2 pressure could be evaluated in terms of a real density profile instead of a simple linear assumption, and compared to experimental observations to verify the model.