Controlling the Stimuli-Responsive Phase Behavior of Block Copolymer Solutions in Ionic Liquids

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Controlling the Stimuli-Responsive Phase Behavior of Block Copolymer Solutions in Ionic Liquids

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2022-10

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Stimuli-responsive polymers are found in many applications including actuators, sensors, and ion conducting or separation membranes. The path to an optimum material design requires a robust understanding of structure-morphology-property relationships. Polymer blends and solutions are well known to undergo macrophase separation upon cooling (i.e., upper critical solution temperature behavior, UCST) or heating (i.e., lower critical solution temperature, LCST). Block polymers, on the other hand, microphase separate into micelles that then organize into well-defined geometrical and ordered nanostructures. LCST behavior is under-explored due to challenges associated with solvent evaporation, and thus control over solution concentration upon heating, which is a critical parameter for the microphase separated structure. Ionic liquids (ILs) are ideal solvents for studying LCST behavior, as they have high thermostability and negligible vapor pressure, allowing them to be used under vacuum as well as over a wide range of temperatures. While temperature is a common stimulus, light can also be used to trigger phase changes in block polymers. Therefore, systems that exhibit both thermo- and photo-responsive behaviors are explored in the following chapters to best control the morphology changes.To study stimuli-based control over block polymer micelle formation and ordering, a series of block polymers of poly(methyl methacrylate)-block-poly(benzyl methacrylate) (PMMA-b-PBnMA) were synthesized with varying block volume fractions and placed in imidazolium-based ILs. PBnMA shows LCST behavior while PMMA remains soluble in the IL over the studied temperature range (25-175 °C), allowing the temperature-dependent behavior to solely impact the PBnMA block. We expanded the stimuli-responsive behavior of the system to photo-based response by incorporating a UV-responsive azobenzene-based monomer (AzoMA) into the PBnMA block. Control over order-disorder and order-order transitions with temperature and light was probed using small-amplitude oscillatory shear rheology in the dark and under UV exposure at variable temperatures. A second focus is addressed in Chapter 6, where the dependence of micelle formation kinetics on polymer concentration was studied using dynamic light scattering and small-angle X-ray scattering. Poly(n-butyl methacrylate), which also displays a LCST in imidazolium-based ionic liquids, replaced PBnMA for this study. Both LCST behavior and the unique properties of ionic liquid solvents are harnessed to study how external stimuli such as light and temperature control the nanoscale morphology.

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University of Minnesota Ph.D. dissertation. 2022. Major: Chemistry. Advisor: Timothy Lodge. 1 computer file (PDF); 228 pages.

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Seitzinger, Claire. (2022). Controlling the Stimuli-Responsive Phase Behavior of Block Copolymer Solutions in Ionic Liquids. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/250404.

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