Mobility in polymer thin films : diffusion and Marangoni driven patterning
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Polymer thin films are ubiquitous in a variety of everyday applications from cookware to packaging. Light can be used to both probe and manipulate the mobility of polymers in thin films. The first project involves the self-diffusion of poly(isobutyl methacrylate) (PiBMA) in thin films using fluorescence recovery after patterned photobleaching (FRAPP). PiBMA is an ideal polymer for this study because it exhibits a film thickness-independent glass transition temperature (Tg) on silicon oxide substrates in film thicknesses down to 14 nm. Since the diffusion coefficient of a polymer depends on the proximity of the experimental temperature to its Tg, nanoconfined diffusion can be measured without superimposed influence from Tg nanoconfinement effects. In this study, self-diffusion of PiBMA parallel to the confining interfaces was found to be film thickness independent to ~30 nm. The reason for the film thickness independence of the Tg of PiBMA is the balance between enhanced mobility at the free interface and hydrogen bonding with the substrate. However, when hydroxyls on the substrate are masked, the Tg of PiBMA decreases with decreasing film thickness. In this case, the diffusion coefficient increases with decreasing film thickness in a way consistent with additional distance from Tg. The second project involves a new approach for creating topographic patterns in thin films via the Marangoni effect, which describes how small variations in surface energy can promote dramatic movement of fluids. Topographic patterns created using this method are potentially useful in a variety of applications, such as the creation of soft lithography stamps. Using a photomask, surface energy gradients can be patterned into solid polymer films. Upon heating the polymer film to a liquid state the Marangoni effect causes the polymer to flow creating three-dimensional topography. This technique was first demonstrated in polystyrene, which undergoes a partial dehydrogenation of the polymer backbone upon photoexposure. However, as exposed and unexposed regions inter-diffuse the topographic features decay. A solution to this problem is to use two orthogonally acting photosensitizers in the polymer film, one for topography creation, and the other for cross-linking which stabilizes the topography at high temperature.