The focus of this dissertation was to investigate the influence of sidechain structure and sidechain content on the morphology and physical properties of perfluorosulfonic acid ionomer (PFSA) membranes. One of the primary objectives was to characterize the thermomechanical relaxations for short sidechain PFSAs developed by 3M and Solvay, as well as a new multi-acid sidechain perfluoroimide acid ionomer (PFIA) from 3M. Partial neutralization experiments played a key role in systematically manipulating the strength of the electrostatic interactions between proton exchange groups on each sidechain, leading to the elucidation of the molecular-level motions associated with multiple thermal relaxations observed by dynamic mechanical analysis (DMA). Particularly, 3M PFSA and Solvay Aquivion lack an observable β-relaxation in the sulfonic acid-form that is observed in the long sidechain PFSA, Nafion. By varying the strength of the physically-crosslinked network through exchanging the proton on the sulfonic acid groups for large counterions, we were able to conclude that the shorter sidechain length and increase in ion content in the 3M PFSA and Solvay Aquivion serves to restrict the mobility of the polymer backbone such that the onset of segmental motions of the main chains is not observed at temperatures below the α-relaxation temperature, where destabilization of the physically crosslinked network occurs. As a complementary technique to DMA for probing the relaxations in PFSAs, we introduced a new pretreatment method for differential scanning calorimetry (DSC) measurements that uncover a thermal transition in H+-form 3M PFSA, Aquivion, and Nafion membranes. This thermal transition was determined to be of the same molecular origin as the dynamic mechanical α-relaxation temperature in H+-form PFSAs, and the β-relaxation temperature in tetrabutylammonium (TBA+)-form PFSAs. The thermomechanical relaxations in multi-acid sidechain 3M PFIA were also investigated. Interestingly, the additional acidic site on PFIA led to unexpected differences in thermal and mechanical properties, including the appearance of a distinct glass transition temperature otherwise not seen in PFSA ionomers. We utilized small-angle X-ray scattering (SAXS) studies to probe the differences in aggregate structure between the PFIA and PFSA membranes in order to uncover the morphological origin of the anomalous thermomechanical behavior in PFIA membranes. Larger aggregate structures for PFIA, compared to PFSA, incorporate intervening fluorocarbon chains within the aggregate, resulting in increased spacing between ions that reduce the collective electrostatic interactions between ions such that the onset of chain mobility occurs at lower temperatures than the α-relaxation for PFSA. The SAXS profiles of PFSAs showed two scattering features resulting from scattering between crystalline domains and ionic domains distributed throughout the polymer matrix. In order to fit the "ionomer peak" to models used for the PFIA and PFSA aggregate structure determination, we presented a method of varying the electron density of the ionic domains by using different alkali metal counterions as a tool to make the intercrystalline feature indistinguishable. This allows for isolation of the ionomer peak for better fits to scattering models without any interference from the intercrystalline peak. Lastly, an investigation of annealing PFSAs of different sidechain structures in the tetramethylammonium (TMA+) counterion form above their α-relaxation showed a profound crystalline-like ordering of the TMA+ counterions within the ionic domains. This ordering is maintained after reacidification and leads to improved proton conductivity, which indicates that this method can be used as a simple processing method for obtaining improved morphologies in proton exchange membranes for fuel cell applications. / Doctor of Philosophy / Hydrogen fuel cells offer an environmentally friendly, high efficiency method for powering vehicles, buildings, and portable electronic devices. At the center of a hydrogen fuel cell is a polymer membrane that contains ionic functionalities, which conduct hydrogen ions (protons) from the anode to the cathode while preventing conduction of electrons. The electrons travel through an external circuit to produce electricity, while the protons travel through the polymer membrane and meet with oxygen on the other side to produce water, the only byproduct of a hydrogen fuel cell. The efficiency of this process relies on the ability of the polymer membrane to conduct protons, and the lifetime of a fuel cell depends on the mechanical stability of this membrane. Perfluorosulfonic acid ionomers are good candidates for use as polymer membranes in hydrogen fuel cells due to their Teflon backbone that provides mechanical stability and their sulfonic acid functionalities that form channels for proton conduction. In this work, we probe the structure-property relationships of different perfluorosulfonic acid ionomers for use as fuel cell membranes. We focus on thermal analysis techniques to develop a fundamental understanding of the effect of chemical structure and sulfonic acid content on the temperature-induced mobility of the polymer chains in these ionomers. This mobility at elevated temperatures can be utilized to rearrange the morphological structure of perfluorosulfonic acid ionomer membranes in order to enhance proton conductivity and mechanical integrity.

Sulfonated polyaromatic hydrocarbon membranes have emerged as desirable candidates for proton exchange membranes (PEMs) due to their excellent mechanical properties, high thermal and chemical stability, and low cost. Specifically, sulfonated multiblock copolymers are attractive because their phase-separated morphologies aide in facile proton transport. In this work, the functionalization of semicrystalline gels of poly(ether ether ketone) (PEEK) is explored as a novel post-polymerization method to prepared blocky copolymers, and the effect of copolymer architecture on membrane physical properties, structure, and performance is extensively investigated. First, the blocky sulfonation of PEEK was explored to prepare blocky copolymers (SPEEK) with densely sulfonated domains and unfunctionalized, crystallizable domains. Compared to random SPEEK ionomers at similar ion content, blocky SPEEK exhibited enhanced crystallizability, decreased melting point depression, and faster crystallization kinetics. Phase separation between the hydrophilic sulfonated blocks and hydrophobic PEEK blocks, aided by polymer crystallization, resulted in enhanced water uptake, superior proton conductivity, and more closely associated ionic domains than random SPEEK. Furthermore, the random and blocky bromination of PEEK was investigated to prepare PEEK derivatives (BrPEEK) with reactive aryl-bromides. Spectroscopic evidence revealed long domains of unfunctionalized homopolymer for blocky BrPEEK, and this translated to an increased degree of crystallinity, higher melting temperature, and more rapid crystallization kinetics than random BrPEEK at similar degrees of bromination. The subsequent sulfonation of blocky BrPEEK resulted in a hydrophilic-hydrophobic blocky copolymer with clear multi-phase behavior. The phase-separated morphology contributed to decreased water uptake and areal swelling compared to random SPEEK and resulted in considerably higher proton conductivity at much lower hydration levels. Moreover, Ullmann coupling introduced superacidic perfluorosulfonic acid side chains to the BrPEEK backbone, which yielded membranes with less water content and less dimensional swelling than random SPEEK. Superior proton transport than random SPEEK was observed due to the superacid side chain and wider hydrophilic channels within the membranes, resulting in more continuous pathways for proton transport. Overall, this work provided a novel platform for the preparation of functionalized PEEK membranes using a simple post-polymerization functionalization procedure. The established methods produced blocky-type copolymers with properties reminiscent of multiblock copolymers prepared by direct polymerization from monomers/oligomers. / PHD / Block copolymers are an important class of polymers that are composed of two or more blocks of distinct polymeric segments covalently tethered to one another. Dissimilarity in the chemical nature of the blocks leads to self-organization into well-defined structures, and this unique structural order imparts material properties that are different from (and often superior to) the properties of the individual blocks alone. Thus, block copolymers are advantageous for a diverse array of applications including membranes, gas separation, water purification, medical devices, etc. Although considerable synthetic progress has been made towards discovering novel methods to prepare block copolymers, their widespread use is somewhat limited by the complex, energy-intensive procedures necessary to precisely control the block sequencing during polymerization. In this dissertation, a straightforward, inexpensive physical procedure is explored to synthesize blocky copolymers with controlled sequencing from commercially available polymers. This process relies on performing reactions in the gel state, whereby segments of the polymer chain are effectively shielded from the functionalizing chemistry. In particular, the gel state sulfonation and bromination of poly(ether ether ketone), a high performance polymer, is investigated to develop novel, blocky materials for membrane applications. This work not only expands the methodology towards the synthesis of block copolymers, but alaso provides critical insight into the effect of copolymer architecture on membrane physical properties, structure, and performance. Furthermore, this work provides an economically feasible method to prepare blocky copolymers from commercially derived materials, thereby providing a means to progress the widespread use of block copolymers in industry.

Over the past three decades, the steady decrease in fossil energy resources, combined with a sustained increase in the demand for clean energy, has led the scientific community to develop new ways to produce energy. As is well known, one of the main challenges to overcome with fossil fuel-based energy sources is the reduction or even elimination of pollutant gases in the atmosphere. Although some advances have helped to slow the emission of greenhouse gases into the atmosphere (e.g., electric cars and more fuel-efficient gas-burning automobiles), most experts agree that it is not enough. Proton Exchange Membrane (PEM) fuel cells have been widely recognized as a potentially viable alternative for portable and stationary power generation, as well as for transportation. However, the widespread commercialization Proton Exchange Membrane Fuel Cells (PEMFCs) involves a thorough understanding of complex scientific and technological issues. This study investigated the various structure-property relationships and materials durability parameters associated with PEMFC development. First, the correlation between perfluorinated ionomer membranes and processing/performance issues in fuel cell systems was investigated. As confirmed by small-angle X-ray scattering data, impedance analysis, and dynamic mechanical analysis, solution processing with mixed organic-inorganic counterions was found to be effective in producing highly arranged perfluorinated sulfonic acid ionomer (PFSI) membranes with more favorable organization of the ionic domain. Moreover, thermal annealing was shown to enhance the proton mobility, thereby facilitating reorganization of the polymer backbone and the hydrophilic region for improved crystallinity and proton transport properties. This research also confirmed an increase in water uptake in the solution-processed membranes under investigation, which correlated to an increase in proton conductivity. Thus, annealing and solution-processing techniques were shown to be viable ways for controlling morphology and modulating the properties/performance of PFSI membranes. Second, this study investigated the role of the morphology on water and proton transport in perfluorinated ionomers. When annealed at high temperatures, a significant decrease in water uptake and an increase in crystallinity were observed, both of which are detrimental to fuel cell performance. Additionally, controlling the drying process was found to be crucial for optimizing the properties and performance of these membranes, since drying at temperatures close or above the α-relaxation temperature causes a major reorganization within the ionic domains. Third, although many investigations have looked at key PEMFC components, (e.g., the membrane, the catalyst, and the bipolar plates), there have been few studies of more "minor" components—namely, the performance and durability of seals, sealants, and adhesives, which are also exposed to harsh environmental conditions. When seals degrade or fail, reactant gases leak or are mixed, it can degrade the membrane electrode assembly (MEA), leading to a performance decrease in fuel cell stack performance. This portion of the research used degradation studies of certain proprietary elastomeric materials used as seals to investigate their overall stability and performance in fuel cell environments with applied mechanical stresses. Additionally, characterization of the mechanical and viscoelastic properties of these materials was conducted in order to predict the durability based on accelerated aging simulations as well. Continuous stress relaxation (CSR) characterization was performed on molded seals over a wide range of aging conditions using a customized CSR fixture. The effects of temperature, stress, and environmental conditions are reported in terms of changes in momentary and stress relaxations, chain scission and secondary crosslink formation. Aging studies provided insights on how anti-degradants or additives affect the performance and properties of sealing materials, as well as how a variety of environmental considerations might be improved to extend the lifetime of these elastomers. / Ph. D.