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Morphology and Morphological Anisotropy of Ion-Containing Pentablock Copolymers
Ion-containing block copolymers with phase-separated ionic and nonionic microdomains are promising ion and water transport materials. Understanding the morphology and orientation of ionic microphase is critical for designing and processing these polymers to create solid-state polymer films with advantageous transport and mechanical properties. This dissertation systematically investigates the morphology and morphological anisotropy of one kind of midblock-sulfonated polystyrene-based pentablock copolymers. The three used polymers, with sulfonated block volume fraction of 36~40%, are denoted as P-1.0, P-1.5 and P-2.0 corresponding to the increasing degrees of sulfonation. The microphase assembly is regulated by the electrostatic interaction among the ionic blocks. In nonpolar solvent blends of cyclohexane & heptane (C:H, dielectric constant ϵ=2.0), the electrostatic cohesion drives the assembly of spherical micellar cores composed of ionic blocks. These spherical cores are kinetically frozen until the polymer solidification, resulting in films with sphere-like ionic domains. In contrast, in an intermediate polar solvent of tetrahydrofuran (THF, ϵ=7.5), the electrostatic cohesion is reduced, but acts as ionic/nonionic block-segregating force to drive the formation of lamellar layers. The morphological anisotropy in the lamellar THF-cast films was clearly revealed by the different small angle X-ray scattering (SAXS) behaviors when the scattering vectors are perpendicular versus parallel to the film plane, and was also observed by the contrast between cross-sectional and plane-view transmission electron microscopy (TEM) images. The anisotropy intensifies with the degree of sulfonation. The lamellar layers within the THF-cast P-2.0 film are almost parallel to the film plane. The morphology and morphological anisotropy influence the ion and water transport. The longer lamellar ionic domains make the THF-cast P-1.0 film possess a double ion (proton) conductivity over the C:H-cast P-1.0 with sphere-like domains. But the increase of sulfonation compensates the less connectivity of sphere-like domains via increasing the ionic domain size and connecting chances. The C:H-cast P-1.5 and P-2.0 films have no lower in-plane conductivity than the THF-cast. And interestingly, their through-plane ion and water transport are much higher (around two and five times) compared to the later. This is because the preferentially parallel-to-plane alignment of the lamellar domains provides much more tortuous through-plane conducting pathways.
Zheng, Wenjian, "Morphology and Morphological Anisotropy of Ion-Containing Pentablock Copolymers" (2017). ETD collection for University of Nebraska - Lincoln. AAI10684837.