Sensitive Characterization of the Influence of Substrate Interfaces on Supported Thin Films

The perspective by Ediger and Forrest stated that, while we know that the dynamics of polymers in ultrathin films can be significantly altered by substrate interfaces, our understanding of how this depends on the polymer structure and the particular interfaces is rudimentary. Here, we show that fluorescence nonradiative energy transfer (NRET) is an extremely sensitive method for characterizing the interfacial adsorption of polystyrene onto silicon dioxide, even though their interaction is often suggested to be weak. We observed that tensile stress was generated in the supported film by substrate adsorption, which imposes constraints on molecular motion and prevents a reduction of the glass transition temperature (T_g). Furthermore, our investigation suggests that modifying the surface chemistry of the substrate can change the film conformation and dynamics when the film is thinner than 40 nm.