Self-assembly of block coplymer thin films in compressible fluids
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In recent years, rapid progress has been reported toward exploiting self-assembly in block copolymer thin films to create periodically ordered nano-patterned substrates for potential applications, including nanolithography and “bottom up” microelectronic device fabrication. Compressible fluids, such as supercritical carbon dioxide (CO2), have been widely used in many polymer related processes. Unlike conventional liquid solvents, the density and hence the “solvent strength” of supercritical CO2, can be tuned by small variations in pressure, temperature or both. This tunability, along with the low interfacial tension and high diffusion coefficient makes supercritical CO2 a strategic solvent to pattern block copolymer templates. This dissertation demonstrates how to effectively modify interfacial interactions and hence to control the self-assembly of asymmetric block copolymer thin films exposed to compressible fluids. We firstly investigated how interfacial forces determine the morphology of supported asymmetric polyethylene-b-poly(styrene-r-ethylene-r-butene) (E-b-SEB) viii diblock films under vacuum condition. It was shown that the relative strengths of intermolecular forces associated with crystallization, with block copolymer ordering and with long-range van der Waals forces influence the structure of the films. These interactions depend on temperature and on film thickness. In the second series of studies, supercritical CO2 was used as a selective solvent to modify interfacial interactions and to control the morphological structures of asymmetric block copolymer films. Specifically, supercritical CO2 annealing was found to (1) promote microphase segregation into spherical domains for poly(ethylene oxide)-b-poly(1,1’-dihydroperflurooctyl methacrylate) (PEO-b-PFOMA) thin films; (2) invert core-shell structure of the surface micelles formed by polystyrene-b-poly(1,1’,2,2’-tetrahydroperflurooctyl methacrylate) (PS-b-PFOMA) thin films. Finally, studies involving the swelling of polymer films and the wetting characteristics of liquids on polymer films under CO2 environments were performed in order to gain further insights into the thermodynamics of polymer-CO2 interfaces.