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A thermal approach to the generation of stable diblock copolymer templates using thin films
Thermally crosslinkable diblock copolymers were studied for their ability to self-assemble and microphase separate, creating polymeric domains on the order of nanometers that could be stabilized with the application of heat. Selective degradation of the uncrosslinked phase afforded a nanoporous template. The stability of these phase-separated structures is crucial for subsequent device fabrication, especially when one of the components is removed to create a nanoporous structure. Nanoporous structures can be used for the fabrication of nanoscopic materials, lithographic templates, and selective filtration on the nanometer size-scale. This dissertation focuses on the synthesis and characterization of novel diblock copolymers and their use in the creation of stable nanoporous templates via thermal crosslinking. Two methods were used to this end. The first involved the use of benzocyclobutene chemistry to create thermally crosslinkable styrenic polymers, namely poly(styrene- r-vinyl benzocyclobutene) (PSBCB). PSBCB was synthesized via living free radical polymerization using a hydroxy-functionalized alkoyamine initiator. This polymer was used to initiate D,L-lactide via ring-opening polymerization, to create a diblock copolymer with a base degradable poly(lactic acid) (PLA) block. Thin films (∼30 nm) of PSBCB-b-PLA polymer, on a gold-coated substrate, were shown to microphase separate with cylindrical domains of PLA oriented perpendicular to the substrate. Annealing the polymeric thin films, followed by subsequent thermal crosslinking, stabilized the microphase-separated structure. The PLA cylinders were removed using base to afford a nanoporous template. These nanoporous templates were shown to be thermally and solvent stable, unlike their uncrosslinked PS-b-PLA analogs. The second method to generate thermally stable nanoporous templates, used polystyrene (PS) as the uncrosslinked block and polyacrylonitfle (PAN) as the thermally crosslinkable block. Polystyrene macroinitiator with a bromine end-group (PS-Br) was synthesized using living anionic polymerization techniques. The PAN block was synthesized using atom transfer radical polymerization (ATRP), with the PS-Br as the initiator. These polymers were used to create carbonaceous replicas of the microphase separated structure using a completely thermal process. The PAN block, when heated to 250°C, undergoes stabilization and crosslinking to create a ladder-like polymeric network. Further heating to 600°C under nitrogen, leads to the pyrolysis of the microphase-separated structure, creating nanoporous carbon templates.
Leiston-Belanger, Julie M, "A thermal approach to the generation of stable diblock copolymer templates using thin films" (2006). Doctoral Dissertations Available from Proquest. AAI3216939.