Investigation of the Surface Glass Transition Temperature by Embedding of Noble Metal Nanoclusters into Monodisperse Polystyrenes

Abstract

Chain mobility in a near surface region at a polystyrene/vacuum interface was investigated by embedding of noble metal nanosized clusters. The embedding process was monitored in situ by X-ray photoelectron spectroscopy. The embedding of nanosized clusters needs long-range chain mobility of the polymer. Therefore, the embedding process is a probe for the glass transition in a near surface region. The clusters used in this study are formed by the dewetting of evaporated noble metals onto the polymer surface. First, methodical influences on the embedding process were investigated. An onset embedding temperature T* was defined. T* increases with a heating rate comparable to Tg(bulk) determined with a differential scanning calorimeter. Furthermore, T* increases with nominal metal coverage which results in increasing average cluster radius. The cluster size distribution was investigated by transmission electron microscopy. It is shown that T* is an upper limit for Tg in a near surface region with a depth of a few nanometers. With optimized probe conditions, embedding processes were performed on monodisperse polystyrene (Mw = 3−1000 kg/mol). The T* values fit quite well with the Fox−Flory relation, but with a saturation temperature of approximately 8 K below the bulk value. ΔT = Tg(bulk) − T* increases with molecular weight. This molecular weight dependence of T* will be discussed in terms of several models. The chain end segregation model can be ruled out. To investigate the kinetics of the embedding process, isothermal experiments were performed. From these experiments surface viscosities were derived, which are well below bulk values.