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Enhanced mechanical performance of low dielectric constant thin films synthesized in supercritical CO2, and SANS studies of microemulsions induced or destabilized by compressed CO2
Block copolymer (BCP) phase segregation and self-assembly into two or more distinct domains are primarily dictated by two parameters: the block volume fraction, f, and the product of the segment-segment interaction parameter and the length of polymer chain, χN. The volume fraction determines a block copolymer's phase segregated morphology, whereas χN dictates its overall segregation strength, or phase stability. In order to achieve smaller domain sizes, the interaction parameter must be increased to compensate for the decrease in chain length. In the melt, PEO-b-PPO-b-PEO (Pluronic) triblock copolymer surfactants do not phase segregate primarily due to their low molecular weights and insufficient segregation strength, or low χN. Strong hydrogen bonding and selective interactions of PEO chains with homopolymers capable of hydrogen bonding, such as poly(acrylic acid) were shown to increase the effective segregation strength of the blend. Small angle X-ray scattering demonstrated highly ordered sub-10 nm domains resulting from phase segregation of the blends. ^ The strong hydrogen bonding interaction between PEO and H-bonding homopolymers was also utilized to incorporate polyhedral oligomeric silsesquioxanes (POSS) into silicate films. In order to improve the compatibility between hydrophobic POSS with hydrophilic Pluronic copolymers, POSS-decorated acrylate monomer was copolymerized with acrylic acid. This eliminated the macrophase segregation between the BCP templates and POSS molecules. The inclusion of POSS is shown to increase the mechanical performance of the low-k films. A supercritical CO2 synthesis route enables the transport of silica precursors into the polymer blends. An increase of hardness of up to 1.8 GPa at k = 2.4 and 1.2 GPa for k = 2.1 was observed for these mesoporous organosilicate films. ^ Finally, this work has also focused on the formation of ordered domains of the Pluronic surfactants into a ternary solvent system consisting of two liquid solvents and compressed CO2. Compressed CO2 can influence the compatibility of liquid solvents, inducing phase separation or phase mixing. CO2-induced phase separation of acetone and water and phase mixing of tetradecane and methanol were studied for the formation and breaking of nanoscale domains in the presence of Pluronic surfactants. Long-range ordered structures were observed using small angle neutron scattering. ^
Romang, Alvin Horatio, "Enhanced mechanical performance of low dielectric constant thin films synthesized in supercritical CO2, and SANS studies of microemulsions induced or destabilized by compressed CO2" (2012). Doctoral Dissertations Available from Proquest. AAI3518276.