Date of Award
Doctor of Philosophy (PhD)
Prof. Dvora Perahia, Committee Chair
Dr. Brian N. Dominy
Dr. Jason D. McNeill
Dr. Stevens J. Stuart
Interfacial structure and response of complex diblock copolymer thin films and polymer nanocomposites were studied using neutron reflectometry (NR), atomic force microscopy (AFM), and surface tension measurement. The interfacial structure and composition of the polymer at interfaces differ from those of bulk properties that underline their numerous applications such as smart coating, lubricants, and microelectronics. Here, I present the study of semifluorinated diblock copolymer poly trifluoro propyl methyl siloxane (PTFPMS-b-PS or SiF-b-PS) thin films and the dispersion of polyhedral oligomeric silsesquioxanes (POSS) nanoparticle (NP) in symmetric polystyrene-b-poly dimethyl siloxane (PS-b-PDMS) diblock copolymer. Semifluorinated polymers are of a particular interest because of their unique thermal and chemical stability induced by the fluorinated segments. Further, incorporation of fluorine affects the interfacial energy and consequently overall interactions of the polymers with their environment. Temperature response was first probed followed by the impact of exposure to hydrophilic (water) and hydrophobic (decane and toluene) solvents. To resolve the impact of fluorine in the diblock SiF-b-PS, a small fluorinated segment was introduced in the flexible siloxane backbone. The volume fraction of fluorinated segment (Î¦SiF) varied from 0.03 to 0.46. Temperature effects study showed that even small fraction of fluorinated segment altered interfacial behavior impacting surface energy as well as wetting behavior. Further, fluorinated segments enhanced the film stability. NR study showed that the incompatibility between fluorinated and protonated blocks drive layering structure where protonated diblocks are often fully mixed. In the layered structure, air interface was fluorine rich and substrate interface proton rich. The layered structure was maintained, even annealing at ~60°C higher than glass transition temperature (Tg) of block copolymer. The effect of quality of casting solvent and preparation pathway on surface properties was studied using AFM and contact angle measurement. Mix solvent of tetrahydrofuran and toluene, which is good for entire polymer and pure toluene, selective for PS block were used to dissolve the polymer and thin films were prepared using both spin cast and drop cast methods. The study found that even small segment of fluorine enhanced segregation and morphology formed depends on the solvent used and mode of preparation. Morphology developed during annealing found to depend on the structure formed at the pristine state. Exposure to water vapor resulted in water penetration throughout the film for the lowest SiF fraction whereas in the higher SiF fraction fluorinated and protonated segments rearranged allowing water to reside at the upper proton rich layers. In the presence of hydrophobic solvents, fluorinated and protonated blocks rearranged in a way solvents preferentially resided in proton rich layers. In both hydrophilic and hydrophobic solvents, layered structure was maintained even after extended exposure time. Effects of the relative size of POSS NPs with polymer layers and flexibility of the block in the distribution of NPs in PS-b-PDMS was also studied. It was found that the distribution of NPs depends on the size of the matrix used. In shorter diblock, NPs mostly distributed towards the interfaces whereas in longer segments NPs distributed isotropically. With annealing, NPs migrated toward PDMS rich layer and air interface due to the entropic influence.
Shrestha, Umesh M., "Interfacial Properties of Siloxane Containing Semifluorinated Diblock Copolymer and Nanocomposite Under Confinement" (2016). All Dissertations. 1675.