Characterization of polymeric thin film by using positron annihilation spectroscopy

Nano-scale thin polymeric films supported on different substrates have been investigated by using positron annihilation spectroscopy (PAS). The object of this research was to characterize and to understand basic physical properties of thin films at different depths.; In this study, four positron techniques have been employed: Doppler Broadening Energy Spectroscopy (DBES) of positron annihilation, Positron Annihilation Lifetime Spectroscopy (PAL), Age Momentum Correlation (AMOC) and slow positron beam method. The data obtained from the DBES experiments on polystyrene films on Au, Si and polymeric substrates suggested that for both Au and Si substrates there is an interfacial layer between the film layer and the substrate. The interfacial layer has different properties from the film bulk values: lower density, shorter positron/positronium diffusion length and smaller S parameter. However for the polystyrene film on polymeric substrate there is no observable interfacial layer between the polystyrene film and the polymeric substrate. The slow positron beam based PAL experiments performed on the polystyrene film on Au and Si substrates provides us the variation of the size, concentration and distribution of the free volume in the polymer film as a function of depth. Near the surface of the film, the free volume is larger than in the bulk, and the distribution of the free volume is wider as well. It gradually narrows down to the bulk value, which is consistent to the conventional PAL result.; AMOC experiments conducted on a polystyrene sample not only provides the momentum shift and the lifetime of the positron/positronium annihilation but also offers an opportunity to explore the history of the positron/positronium in the polymer from the birth to the annihilation.; Glass transition temperature (Tg) is one of the most important properties of polymers. We selected experiments on 80 nm and 150 nm films supported on Si and Au substrates using DBES and PAL. We observed a significant Tg depression as a function of depth. We obtained the Tg variation as a function of depth from the surface, film and the interface in polystyrene films on Si and Au substrates. We interpret these results as a broadening of free-volume distribution in nano-scale thin films for the first time.; From this work, PAS is proved to be a powerful tool to measure thin film properties at the nano-scale based on free-volume data in polymeric systems.