Browsing by Subject "Nanoconfinement"

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Effect of interfaces on thermophysical properties and block copolymer self-assembly in polymer thin films
(2016-12) Katsumata, Reika; Ellison, Christopher J.; Willson, Carlton G; Sanchez, Issac C; Lu, Nanshu
When materials are tailored for use at the nanoscale, thermophysical properties can deviate from their bulk values, and these phenomena are broadly referred to as nanoconfinement effects. In thin films, one of the critical factors of nanoconfinement effects is interfacial interactions; as the film thickness decreases, the interfacial area to volume ratio increases dramatically, often causing interfacial effects to dominate the properties of the entire film. As polymers continue to be leveraged in nanotechnology, from nanocomposites to lithography, understanding the effects of interfaces is highly desired. While numerous studies have revealed how thermophysical properties, (e.g., glass transition temperature (Tg), self-diffusion coefficient (D), and effective viscosity (η [subscript eff]) change with film thickness, correlations between these parameters are still unclear. Herein, the Tg, D, and η [subscript eff] are measured for a model system of unentangled poly (isobutyl methacrylate) (PiBMA, 16-300 nm thick) supported by SiOx. The non-bulk-like correlation between Tg, D, and η [subscript eff] is successfully explained using a three-layer model. To further investigate the effect of confining interfaces, the Tg and D of PiBMA are studied for four multilayer geometries, where the interfacial interactions are varied from strong to weak. The Tg-D relationship of thin films deviates from bulk behavior, and the magnitude of the deviations depends on the polymer-substrate interactions. A friction analysis reveals that this deviation originates from heterogeneous dynamics near the confining interfaces. Engineering interfaces between polymers and substrates is also crucial for BCP lithography, especially on non-traditional substrates (e.g. flexible or 2D materials). In particular, precise control of the surface energy of the underlying substrate is required to produce lithographically useful structures, such as lamellar domains oriented perpendicular to the substrate. In this study, polydopamine is first exploited as a universal adhesive to enable BCP nanopatterning on a variety of flexible materials. In addition, we developed a potentially scalable graphene nanoribbon fabrication method using wetting-transparency assisted BCP lithography. Lastly, inspired by the wetting transparency phenomenon, possible techniques to control the microdomain orientations of BCPs through thin layers are explored using a model bi-layer substrate made from homopolymers of each block, along with a theoretical model based on van der Waals forces.
Mobility in polymer thin films : diffusion and Marangoni driven patterning
(2013-05) Katzenstein, Joshua Max; Ellison, Christopher J.
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.