Characterization of Self-Assembly and Charge Transport in Model Polymer Electrolyte Membranes

There is broad interest in creating polymer electrolyte membranes (PEMs) that have a charged hydrophilic nanophase, where the size and geometry of the phase can be precisely controlled. The applications for such materials range from portable power generating devices to water purification. There is a need to better characterize the self-assembly, thermodynamics, and performance of both current and future PEMs. To this end a series of chapters is presented, that explore the development of techniques, equipment, methods, and materials to enable further progress in the field./nThe interaction of PEMs with external ionic solutions can be used to determine fundamental thermodynamic properties of the ions that reside within the membrane itself. Traditional techniques used to probe ions in PEMs, such as conductivity, can be greatly enhanced by knowing the number of dissociated ions and their activity coefficients. A technique is presented that provides one of the first methods able to quantify such properties in PEMs./nThe ionic species in PEMs are believed to reside in nanoscale ionic aggregates. Only recently have researchers begun to focus on the properties of this aggregation in regards to PEM performance. A summary of this phenomenon, as well as speculation on its effect on transport and thermodynamic properties is presented. In addition, evidence that suggests block copolymers offer a method of inhibiting aggregate formation is discussed./nCharacterization of PEM morphology is critical to properly understand structure-function relationships. Due to a lack of proper equipment, the morphological characterization of PEMs has been mostly limited to the dry state. The design and operation of a novel sample stage, used to simultaneously measure morphology and conductivity in humid air as a function of temperature and relative humidity is presented. Precise control over humidity and accurate determination of morphology and conductivity over a wide range of temperatures is shown./nAt present there is an incomplete understanding of the thermodynamic interaction between PEMs and water of varying activity. The morphology, water uptake, and proton conductivity of sulfonated polystyrene-block-polyethylene (PSS-PE) was studied under controlled relative humidity (RH) and in liquid water. Extrapolation of the domain size, water uptake, and conductivity in humid vapor to RH = 100% allowed for an accurate comparison between the properties of PSS-PE hydrated in saturated vapor and in liquid water. Absent from this system was Schroeder's Paradox, which expects the properties in saturated water vapor to be less than those obtained in liquid water./nPolymers that are semi-crystalline are ubiquitous as commercial polymers because of their mechanical properties. Little is known about the effects of polymer crystallization on PEM structure and performance. The model system, PSS-PE, was synthesized at a variety of molecular weights to probe how crystallization affects performance for a variety of conducting domain sizes. Results are shown that indicate crystallization disrupts the self-assembly of low molecular weight PEMs, resulting in poor water uptake and proton conductivity in small domains. Increasing domain size results in less morphological disruption, leading to an improvement in performance at larger domain sizes./nThis work improves upon the ability of researchers to characterize and understand the relationship between the structure and performance of PEMs. The findings presented herein provide further understanding toward the goal of rational design of nanostructured membranes that show improved conductivity in a variety of conditions.

Advisor: Baslara, Nitash P