Large-Strain Deformation Mechanics of Charge-Containing Polymer Membranes

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The use of polymeric membranes have become ubiquitous in our daily lives. From simple chip bags to million-dollar cockpits, these membranes find diverse applications in packaging, protective coating, electronic device, gas and liquid separation, and medicine. The 2-dimentional geometry of the membranes offered unique mechanical and interfacial properties that often surpass those in their bulk counterparts. While the diminishing dimensions of the membranes provide enhanced materials properties and superb versatility for wide-reaching applications, they also bring forth new challenges in the characterization and design of novel membrane systems.', 'The primary focus of this dissertation is the development of novel methods to robustly characterize the mechanical properties of polymer thin films relevant to the application context. This work begins with a brief discussion on the current research on charge-containing polymers. An examination on the development of a non-traditional biaxial inflation mechanical test is subsequently introduced, and marks the foundation of this work. A series of sulfonated pentablock copolymers used for water purification is thoroughly investigated using a combination of X-ray scattering, biaxial inflation, and transport experiments. It is found that the softening effect of polymer sulfonation can be mitigated by reducing the volume fraction of the charged domain without detrimentally affecting transport properties. Additionally, membrane compliance can be further reduced by increasing the packing density of the uncharged domains. Process-dependent membrane morphology of these pentablock copolymers is explored using x-ray scattering techniques. Water transport properties were found to be independent of membrane morphology, and guides our understanding of the processing-structure-property relationships in these block ionomers.', 'Next, a polyelectrolyte complex coacervate system was investigated to provide a better understanding of charge effects on the mechanical strength and relaxation behavior of complex network systems in low-salt environments. At these limiting conditions, the deformation behavior deviates from those predicted by the time-temperature and time-salt superposition methods. A fractional model used to describe power-law relaxation behavior is introduced, and the effect of counter ions on charge-pair interactions in a transient network system is discussed. ', 'Finally, the mechanics of polycarbonate thin films were investigated using a combined wrinkling-cracking methodology to provide insight into ductile fracture behavior of polymer bilayers that are geometrically relevant for flexible electronics. The supported bilayer structure enables the distribution of stress across the entire sample well after reaching the critical fracture strain, and minimizes the propagation of cracks. The findings here can better inform future design and optimization of materials for protective coatings, impact mitigation, and wearable electronics.

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  • 10/21/2019
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