Free Volume Depth Profiling at Polymer Surfaces and Interfaces
Abstract: Pulsed low energy positron beams provide a unique and non-destructive tool to depth probe the free volume of polymers at surfaces and interfaces. Since the employment of low energy positron beams in polymer research is an emerging technique, some focus in this thesis has been on extracting methods and equations needed to interpret the experimental data. The polymers chosen for this study were selected on the basis of having previously well characterized free volume properties. This enabled interpretation of the positron beam data in more general quantities, thus permitting the results to be compared with other techniques and theoretical considerations of polymer surfaces and interfaces. The median positron implantation depth and the implantation depth distribution as a function of implantation energy have been studied in some detail. It was shown that the implantation characteristics are independent of the structure of the materials, and that the median implantation depth for the studied polymers, as well as those of some previously studied metals, can be described with the same equation. Furthermore, the free surfaces of polystyrene (PS), polymethyl methacrylate (PMMA), styrene-co-acrylonitrile (SAN) and styrene-co-maleic anhydride (SMA) were studied by measurements in the low implantation energy range. Linear density gradients could be fitted to the experimental data, yielding gradient widths in the order of a few nanometers. This was in good or reasonable agreement with theoretically derived density gradients. The discrepancy with the theory found for the copolymers SAN and SMA was suggested to arise from surface orientation driven by the difference in polarity between the molecular constituents. The free volume at a PMMA-PS interface was studied. By the preparation of samples with the interface located only 25 nm from the free surface, the sensitivity of the experimental data to properties of interface region was greatly increased in relation to previous investigations. A reduction of the free volume was found in the PMMA in the 20 nm region closest to the PS surface. This finding suggests that the PS surface constitutes a relatively attractive environment for the PMMA. By guiding UV-light into the positron beam, various measurements relevant for photo-responsive polymers were enabled. Free volume determination of an azobenzene-PMMA copolymer during isomerization was carried out at various temperatures, showing that the isomerization did not influence the free volume. The excited cis-azobenzene was however found to be an inhibitor for positronium annihilation, which enabled the isomerization kinetics of azobenzene to be probed with positrons.
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