Trigg, Edward Boynton
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PublicationControlled Folding In Precisely Functionalized Polyethylenes: Designing Nanoscale Lamellar Structures For Ion Transport(2018-01-01) Trigg, Edward BoyntonThere is great societal need for improved energy storage technology, in applications ranging from electric vehicles to grid storage to emergency power systems. New polymeric membranes with enhanced ion conductivities are needed in batteries and fuel cells to improve these technologies. Despite decades of research, these membrane materials are still not adequate for commercial use. The primary metric that requires improvement is ion or proton conductivity of the membrane under desirable operating conditions. Modifying linear polyethylene by the addition of precisely periodic functional groups leads to a rich array of intriguing morphologies and properties. Depending on the functional group chemistry and periodicity, functionalization can (1) increase or decrease the melting point; (2) produce amorphous, semicrystalline, or nearly fully crystalline morphologies; (3) cause hairpin chain folds within the crystalline regions; and (4) exhibit high proton conductivity when hydrated. In this dissertation, we focus on polyethylenes with long-spaced functional groups, which form layered crystallites. Herein, we show that incorporating backbone sulfone groups into polyethylene gives rise to nylon-like semicrystalline morphologies, where the sulfone groups form layers within the crystalline regions and hydrogen bond, increasing the melting temperature. The melting point is proportional to the sulfone concentration. Conversely, when the polyethylene contains pendant carboxylic acid groups, it exhibits hairpin folds at the position of each acid group within the crystalline regions. This produces multiple layers of acid groups whose normal vectors are oriented approximately perpendicular to the normal vector of the lamellar crystallite. Layers with this orientation could provide pathways through crystallites for selective transport of protons, ions, water, or other small molecules, allowing the use of a semicrystalline polymer for various membranes. Replacing carboxylic acid with sulfonic acid produces a nearly fully crystalline morphology containing hydrated acid layers, resulting in high proton conductivity. This is the first time, to our knowledge, that proton or ion conductivity has been reported in this type of morphology. Finally, we show that functionalized polyethylenes with a nearly precise placement of functional group – variable by one backbone methylene group – has a similarly well-ordered morphology as a truly precise polymer, and could provide a more practical route to the morphologies and properties discussed above.