Studies of Nanoparticle Marker Motion in Ultra Thin Polymer Films With X-ray Standing Waves

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Metal nanoparticles in polymeric matrices are of particular scientific interest due to their useful ability to self-assemble into complex nanocomposites. Recent examples involve using ultrathin diblock copolymers coupled with metal nanoparticles in the fabrication of novel electronic, magnetic and photonic devices. As this ordering process takes place far from equilibrium conditions, the controlled self-assembly of nanostructures in two-dimensions has to be guided by a thorough understanding of the ordering kinetics and nanoparticle dynamics of these composites. In order to understand these key parameters a technique was needed that could monitor the real-time evolution of the nanoparticles. In many cases these motions are small and require an extremely sensitive technique to accurately monitor them. X-ray standing waves (XSWs) generated by total external reflection above an x-ray mirror surface were used in this thesis to monitor the time evolution of a gold distribution as the polymer nanocomposites were heated above their polymer glass transition temperatures. To confirm the sensitivity of the XSW technique, a variety of simulations changing key parameters relative to the sample and the x-ray beam were performed. The parameters relating to the gold distribution (amount of Au present, the width of the distribution, and the height from the center to the mirror surface), were observed to be most critical in fitting experimental data. By understanding the effect these parameters have on the experimental data, better theoretical fits and future experiments can be performed. Two homopolymer systems were experimentally studied in this thesis, poly(tert-butyl acrylate) (PtBA) and poly(vinyl pyridine) (PVP). For comparison with existing data, diffusive broadening in PtBA/Au systems were studied. The particle diffusion coefficient was found to be ~10 <sup>-18</sup> cm <sup>2</sup>/s for a 100,000 g/mol symmetric system which agrees well with previous results found by Guico. In the case of the PVP/Au systems diffusive broadening was never observed over large relaxations times, unlike in the PtBA case. This effect was expected and attributed to hydrogen-bonding between the nitrogen atom in the pyridine ring and the metal particles and mirror surface. PVP samples were also created with a molecular weight asymmetry around the nanoparticle layer. In these cases the asymmetry in polymer mobility causes a net positive flux through the marker layer that moves the particles toward the layer with the higher mobility. When these samples were heated the lower molecular weight chains would swell into the higher molecular weight chains causing an observed motion of the gold layer. During initial relaxation of the system at short time scales, an initial swelling and reorganization of the system was observed. This motion occurred, equally, both away from and towards the mirror surface depending on which side of the marker layer the low molecular weight polymer was placed. A distinct plateau appeared signifying that the swelling had stopped, but after very long times more motion was observed as the system began experiencing segmental exchange of the polymer chains at the particle surfaces. Another plateau emerged once the old chains had been replaced with new chains on the particle surfaces. These results were of particular interest as metal nanoparticles in diblock copolymers initially organize into the block morphology, but upon longer relaxation times the particles begin to coalescence and destroy the templated morphology.

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  • 08/30/2018
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