Ultrathin polymer films probed by dielectric relaxation spectroscopy: interfacial effects

摘要

The aim of this dissertation is to investigate the influence of interfacial interactions on the dynamic properties of ultrathin polymer films. We used multilayer systems in conjunction with polymers of particular chemical structures, which confer them specific interfacial activity. The experimental investigation has been carried out by means of dielectric relaxation spectroscopy. The first part of the thesis has been devoted to the investigation of the segmental dynamics of freely-standing ultrathin films. The study comprised the implementation of a new experimental method based on three-dimensional electrode structures (Interdigitated comb electrodes, IDE). From the analysis of the dielectric spectra we found that films with no interaction with solid surfaces show large reductions of the glass transition temperatures upon confinement, which are associated to relaxations times much shorter than in the bulk. In the second part, we focused on the dynamic properties of thin films of neat and labelled-polystyrene (PS) in contact with solid surfaces. We observed that while the dynamics of the neat polymer is not affected by the thickness reduction, thin films of the labelled-PS display slower dynamics and a lower dielectric strength compared to the bulk. We then probed spatially resolved dynamics by inserting a very thin labelled-PS layer at different positions above the substrate. When the labelled layer has no contact with the substrate, the local glass transition temperature, Tg, and dielectric strength are bulk like. If the same layer is placed in contact with the solid surface, deviations from the bulk behaviour arise. These deviations are the result of the specific interaction of the labelled polymer with the solid surface. This experimental results are rationalized with a mathematical model that takes into account an immobile layer located in the vicinity of the solid surface.Furthermore, for strongly adsorbing polymer layers in contact with the surface, we observed that deviations from the bulk of Tg and the dielectric strength vary as a function of time. This effect is associated to the progressive immobilization of the labelling moieties on the solid surface, and to a change of the average molecular orientation of the labelling moieties. Both the dielectric strength and the mean orientation follow two comparable kinetics regimes, being the first faster than the second. These experimental evidences suggested the following interpretation based on adsorption. At the beginning, large bare spots are available for adsorption and the labelling moieties adsorb with a parallel orientation. At later times, the surface coverage is high and adsorption develops via an alignment of the dye moieties normal to the surface.