Probing interactions at the polymer thin film/substrate interface using Kelvin probe force microscopy*

Interfacial interactions between a thin polymer film and substrate influence a myriad of thickness-dependent physical properties of polymers, including glass transition temperature, dynamics, and phase transitions. Direct quantification of these interfacial interactions has been challenging because this interface is buried. Here, we utilized Kelvin probe force microscopy (KPFM) to better understand interactions at the polymer thin film/substrate interface. KPFM yields contact potential difference (CPD), determined by the polarities of the materials and processing conditions. We investigated different polymer thin film systems: polystyrene (PS), supported by gold (Au) and silicon (SiO_x/Si) substrates, and tetramethyl bisphenol-A polycarbonate (TMPC), supported by Au and SiO_x/Si substrates. We found CPDs of the polymer films were dependent on both thickness of the film and substrate. The CPD of TMPC on SiO_x/Si increased significantly with decreasing film thickness (<100 nm), whereas for PS on SiO_x/Si and for PS and TMPC on Au, the CPDs increased modestly as thicknesses increased. These observations are rationalized in terms of local interfacial the bonding environments between polymers and substrates and the relation to key thickness-dependent physical properties.