Over 30% of the world's population does not have access to safe drinking water, and the need for clean water spans further than just for human consumption. Currently, we use freshwater for growing agriculture, raising livestock, generating power, sanitizing waste, mining resources, and fabricating consumer goods. With that being said, the world is beginning to feel pressure from the excessive freshwater withdrawal compared to the current freshwater supply. This water stress is causing a water crisis. Places including Australia, South Africa, and California in the United States, just to name a few, are beginning to run out of fresh water to support daily societal demands. This is a phenomenon that is indiscriminately observed in all ranges of economically and politically developed countries and environments. However, it is important to note that less politically and economically developed countries especially those in arid climates, experience higher water stress than countries without such qualities. With only 2.5% of the world's water being freshwater and 30% of it being accessible as either ground or surface water, freshwater is a scarce resource, especially with the growing population and society's demand for water. Since the remaining 97.5% of water is composed of either brackish or seawater (saline water sources), one way to overcome the water stress would be to convert saline water into freshwater. As a result, various desalination techniques have been developed in the last 80 years that employ either membrane technology or temperature alterations to desalinate either brackish or seawater. One of the fastest growing methods for producing freshwater is reverse osmosis. Reverse osmosis uses an externally applied pressure, in the form of a cross flow back pressure, to overcome the osmotic pressure produced by the saline gradient across a semi-permeable membrane. The semi permeable membrane commercially consists of an interfacially polymerized aromatic polyamide thin film composite with a polysulfone porous backing that allows water to pass through while barring the transport of salt ions. This research focuses on the development of sulfonated poly(arylene ether sulfone) derivatives with differing amounts of sulfonation and with the ions placed at different structural positions. Previously, such materials were tested as potential high performance fuel cell membranes, but they are also of interest as potential high performance water desalination membranes, specifically for reverse osmosis. Two different methods were used to synthesize the sulfonated polysulfone derivatives: direct polymerization and post-modification of a non-sulfonated active polysulfone. The polysulfones from direct polymerization incorporated specialty sulfonated monomers, which were stoichiometrically controlled during the polymerization. Sulfonated polysulfones that were synthesized from post sulfonation incorporated biphenol and hydroquinone monomer units randomly throughout the polysufone backbones. These units could be sulfonated selectively because of their activation towards electrophilic aromatic substitution with sulfuric acid. Each of the polymers were cast into films ranging between 20-100 microns in thickness and tested for water uptake, hydrated uniaxial tensile properties, crossflow water and salt transport properties, and for crosslinked samples, gel fractions. The water uptakes from all the polysulfones were tuned by the degree of sulfonation or disulfonation present in the polymer. This was either controlled via the presence of a sulfonated monomer or a monomer that was active toward electrophilic aromatic substitution after polycondensation of the polysulfone. All polymers exhibited increases in their water uptake as the degree of sulfonation increased. We also observed a decreasing trend in the hydrated mechanical properties of the films for all the high molecular weight linear polymers as the water uptake was increased. The directly polymerized sulfonated polysulfones were found to have high hydrated elastic moduli ranging between 400 and 1000 MPa, while the post sulfonated counterparts (with either hydroquinone or biphenol incorporated in their structures) exhibited elastic moduli ranging between 1000 and 1500 MPa. It is important to note that the structures of the polymers were slightly different from one another because of the technique used to synthesize them. Thus, the increases in hydrated moduli among polymers synthesized via different routes may have influences from differences in chemical structures. Some of the polymers with higher degrees of sulfonation were synthesized as amine terminated oligomers with varying controlled molecular weights. The two targeted molecular weights were 5 and 10 kDa. Those oligomers were then crosslinked with a tetra-functional epoxide agent. The increases in sulfonation allowed for increases in water uptake and in theory, the water throughput through the sulfonated polysulfone membrane. Decreases in hydrated mechanical performance of the crosslinked networks with increasing degrees of sulfonation were also observed, similar to their high molecular weight linear counterparts. The directly polymerized crosslinked networks had salt permeabilities that plateaued at 70% disulfonation for both the 5 and 10 kDa polymers. Thus, we expect disulfonation content greater than 70% would lead to higher water throughput without significant increases in salt transport. / Doctor of Philosophy / A worldwide shortage of freshwater is becoming more problematic by each passing day. The World Health Organization and the United Nation's World Water Assessment Program predict that by 2025, 50-66% of the world's population will be living in a water-stressed area. This includes any area that experiences higher clean water withdrawals than are available. This includes but is not limited to areas that are politically unstable, technologically disadvantaged, resource deficient, located in arid climates, and highly populated. To put this further into perspective, only 2.5% of the available water on earth is freshwater. Freshwater typically has low concentrations of dissolved salts that are safe for human consumption and use. Of the available freshwater, only 30% of it is actually accessible for use through either surface or groundwater reservoirs, making the amount of clean water available for usage already a scarce resource. On the other hand, 97.5% of the world's water is composed of saline water reservoirs in the form of brackish and seawater. Through harnessing, seawater and removing the excess dissolved salt ions, the salt water can be converted to freshwater. Two major methods have been developed to remove the dissolved ions from water through either membrane filtration or thermal phase changes. One of the fastest growing membrane filtration techniques used worldwide is reverse osmosis. Reverse osmosis refers to the use of applied pressure across a semipermeable membrane to desalinate saline water. The semipermeable membrane prevents the migration of salt ions through the membrane while allowing transport of water. This work has focused on developing new polymers that can increase the overall efficiency of water desalination. Different types of high performance sulfonated polysulfone derivative polymers were synthesized and used to make membranes that were subsequently tested for performance. Relationships between the polymer structure, process, and properties were quantified through different analytical techniques. This study showed how the properties of sulfonated polysulfone membranes may be manipulated depending on structural modifications and processing to increase both the material's water throughput and salt rejection.

Every year, millions of people contract antibiotic-resistant bacterial infections, and tens of thousands die from infection-related complications in the United States alone. Bacterial infections are one of the leading causes of death worldwide, especially in healthcare institutes such as hospitals and nursing homes where people are more susceptible to infection and complications. Bacteria can cause infections in any part of the body and often interact with sugar molecules on the surface of cells; once bacteria are attached, the cells stop functioning properly. When a bacterial infection is suspected, samples from the patient's blood or urine are taken to confirm the diagnosis. If the bacterial infection is sever enough, patients are treated with broad-spectrum antibiotics before the type of bacteria is known, and once it has been identified they are given antibiotics that target the specific bacterial strain. The high death rate associated with bacterial infections is largely due to the emergence of antibiotic-resistant bacterial strains. Although antibiotic resistance is present in some naturally occurring bacterial strains, misuse and over-prescription of antibiotics have accelerated the process. To combat the ever-growing threat of antibiotic-resistant bacteria, antibacterial polymers have been developed. Antibacterial polymers prevent bacterial infections by either killing the bacteria themselves or by preventing them from interacting with the body altogether This dissertation primarily focuses on using sugar-containing polymers to prevent bacterial growth. These materials may potentially be used as a replacement for or supplement to traditional antibiotics. / Doctor of Philosophy / All living cells possess a coating of glycomaterials on, or as critical components of their cell walls. Bacteria, including invasive bacterial pathogens, are no exception and have cell walls comprised of peptidoglycans. Glycomaterials on cell surfaces play a role in critical biological processes such as molecular recognition, cellular interaction, infection, and inflammation. Traditional antibiotic remediations are becoming less effective in treating bacterial infections due to the emergence of antibiotic-resistant strains. The formation of biofilms, an extracellular coating composed of polysaccharides, contributes to the antibiotic resistance of bacteria. The development of novel antibiotics is extremely costly and often unsuccessful, with billions in investment often producing zero new drugs. As a result, antibacterial polymers have been investigated as they are comparatively less expensive and offer unique characteristics to combat bacterial infections. Polymers with inherently antibacterial properties, or those that can be conjugated with antibacterial compounds, offer a replacement for traditional antibiotic remediation. To investigate the role of glycomaterials in antibacterial activity, a series of sugar-containing norbornene homopolymers were prepared and evaluated for their antibacterial activity. Protected glycomonomers consisting of galactose, glucose, N-acetyl glucose, and mannose were prepared in a two- or three-step synthesis by first appending an acrylate to the anomeric carbon through Koenigs-Knorr-type chemistry. After generation of the -anomer, the norbornene carboxylate was prepared by the Diels-Alder reaction of the acrylate with cyclopentadiene. Homopolymers with molecular weights ranging from 25–250 KDa were synthesized using ring-opening metathesis polymerization (ROMP) catalyzed by Grubbs 3rd generation catalyst, and subsequently deprotected to reveal the sugar-norbornene. While the galactose polymers showed no bacterial inhibition, those composed of glucose, N-acetyl glucose, or mannose prevented the growth of Escherichia coli (E. coli) and were effective at concentrations as low as 1.25 mg mL-1. Some strains of pathogenic bacteria, such as Clostridioides difficile (formerly known as Clostridium difficile), interfere with the normal cell functions by indirect means, producing toxins that adversely interact with the surrounding tissue. To sequester the toxins produced by C. difficile before they cause damage to the gastrointestinal (GI) tract, polymers containing the -gal epitope, a naturally occurring trisaccharide, were also prepared. The -gal epitope possessing a propyl azide handle at the anomeric carbon was prepared in a 15-step reaction, followed by reaction with an alkyne-functionalized polymer resin using copper-catalyzed azide-alkyne Huisgen cycloaddition. After global deprotection and thorough washing to remove residual copper from the glycomaterial, cell viability studies showed >80% cell survival. While these materials showed good cell viability, the rigorous synthesis of -Gal and the affinity of the polymer scaffolding for copper was a deterrent to further toxin-binding studies. Non-biological surfaces are also often susceptible to bacterial colonization and fouling. Although such materials may be modified to impart antimicrobial properties, their modification may also be a detriment to other key physical properties. To investigate the tradeoffs between material properties and functionalization, we synthesized a series of poly(arylene ether)s from monomers that possessed a modifiable handle and differed only in the pattern of leaving group on the aromatic ring. These polymers were further modified using post-polymerization thiol-ene reactions to evaluate the effect of the side-chains on the material's properties. The regioisomer incorporated into the polymer was found to influence its thermal properties irrespective of the installed functional group, suggesting that new functionality can be incorporated into these polymers without adversely impacting their physical properties.

Sulfonated poly(arylene ether sulfone)s can exhibit several ideal features as potential desalination membranes for reverse osmosis applications, including chlorine resistance, low surface fouling, and high water flux. However, this class of polymer membranes has suffered from two major drawbacks that jeopardize effective levels of salt rejection in order to achieve high water flux. In mixed salt feed sources, monovalent salt rejection decreases when divalent cations such as Ca2+ bind with the anionic sulfonate groups to cause charge screening, and this can lead to too much salt passage for the membranes to be competitive with interfacially produced polyamides. Sulfonate fixed charge concentration must be high enough for sufficient membrane water uptake to obtain high membrane water flux, but if the water uptake is too high, this permits increased salt passage. The research described in this dissertation attempts to address both of these challenges through the design of a sulfonated monomer that strategically spaces the ionic groups along the polymer backbone chains to inhibit divalent ion binding. Free radical crosslinking further tunes the hydrated free volume in the RO membranes. A mono-sulfonated comonomer, sodium 3-sulfonate-4,4'-dichlorodiphenylsulfone (ms-DCDPS), was synthesized by stoichiometrically controlled electrophilic aromatic sulfonation of 4,4'-dichlorodiphenylsulfone (DCDPS). HPLC-UV revealed complete isolation of ms-DCDPS free of by-products after the 1st recrystallization and 1H NMR analysis confirmed the structure. A standard calibration curve was developed to accurately determine the leftover quantity of excess NaCl that was used for precipitation during the work-up procedures. A series of linear sulfonated poly(arylene ether sulfone)s with varying ms-DCDPS incorporation was synthesized. 1H NMR confirmed the structure of the polymers and size-exclusion chromatography confirmed that the intended molecular weights were achieved. The copolymers were cast into dense films and the mechanical and transport properties were measured in their fully hydrated states. Tensile tests revealed mechanically robust, tough membranes with glassy elastic moduli and high strains at break. The dense membrane prepared from sulfonated poly(arylene ether sulfone) with 51% of the repeat units sulfonated had NaCl rejection = 99.3% measured at 400 psi and 2000 ppm NaCl with a water permeability coefficient of 0.57 x 10-6 cm2/s. The salt rejection remained greater than 99% when a mixed salt feed source containing Ca2+ in the 0-200 ppm range together with the 2000 ppm NaCl was introduced. Crosslinked mono-sulfonated oligomers were synthesized with targeted molecular weights by utilizing stoichiometric quantities of monomers with the desired degrees of sulfonation, and the endgroups were functionalized with tetrafluorostryene. These end-functionalized sulfonated oligomers were crosslinked by both thermal and UV free radical methods in the presence of initiators without any additional crosslinking agents. Reaction conditions were thoroughly investigated and optimized to produce highly crosslinked membranes that yielded gel fractions greater than 87%, as measured by solvent extraction in dimethylacetamide. The hydrated crosslinked membranes were tested for both mechanical and transport properties, and the results were compared to their linear membrane counterparts. Crosslinking decreased the hydrated free volume and reduced water uptakes when compared to linear sulfonated membranes. Tensile tests of the fully hydrated crosslinked membranes showed good mechanical properties. The transport properties of a dense UV crosslinked membrane prepared with a 10,000 g/mol oligomer having 50% of the repeat units sulfonated was tested under RO cross-flow conditions at 400 psi and 2000 ppm NaCl in the feed. The membrane demonstrated a salt rejection = 98.4% with a water permeability coefficient of 0.49 x 10-6 cm2/s. / Doctor of Philosophy / Billions of individuals across the world lack clean, affordable drinking water, and the unavailability of fresh drinking water can be attributed to both physical and economic reasons. Several techniques have been utilized to produce potable water for human consumption that include both water desalination and recycling procedures. Water desalination is a process that allows for purifying salt contaminated water into drinking water. The two major desalination processes involve either distillation or passage through polymer membranes. Distillation separates water from salt by heating liquid water to form a gas, and collecting the vapor as condensate while impurities remain in the heated bulk material. Polymer membranes separate impurities through filtration where membranes allow water to pass through a physical barrier while rejecting the unwanted contaminants, including salt. Reverse osmosis desalination is the most common membrane separation process. Reverse osmosis membranes are comprised of either short-chain crosslinked oligomers or long-chain linear polymers. Commercial reverse osmosis membranes are largely poly(amide)s where a thin film is formed in an interfacial reaction. The membranes allow for almost quantitative salt rejection with high water fluxes. But, these membranes degrade over time from periodic cleaning with chlorine disinfectants. This dissertation primarily focuses on the implementation of an alternative polymer membrane material known as a mono-sulfonated polysulfone that strategically distributes the fixed sulfonate charged groups along the polymer backbone. Theses reverse osmosis mono-sulfonated polysulfones display comparable salt rejection with better chemical resistance than commercial poly(amide)-based membranes, and could potentially offer a replacement in the market.

Several types of novel proton exchange membranes which could be used for both direct methanol fuel cells (DMFCs) and hydrogen/air fuel cells were investigated in this work. One of the main challenges for DMFC membranes is high methanol crossover. Nafion, the current perfluorosulfonic acid copolymer benchmark membrane for both DMFCs and hydrogen/air fuel cells, shows very high methanol crossover. Directly copolymerized disulfonated poly(arylene ether sulfone)s copolymers doped with zirconium phosphates and phenyl phosphonates were synthesized and showed a significant reduction in methanol permeability. These copolymer/inorganic nanocomposite hybrid membranes show lower water uptake and conductivity than Nafion and neat poly(arylene ether sulfone)s copolymers, but in some cases have similar or even slightly improved DMFC performance due to the lower methanol permeability. These membranes also show advantages for high temperature applications because of the reinforcing effect of the filler, which helps to maintain the modulus of the membrane, allowing the membrane to maintain proton conductivity even above the hydrated glass transition temperature (Tg) of the copolymer. Sulfonated zirconium phenyl phosphonate additives were also synthesized, and membranes incorporating these materials and disulfonated poly(arylene ether sulfone)s showed promising proton conductivity over a wide range of relative humidities. Single-Tg polymer blend membranes were studied, which incorporated disulfonated poly(arylene ether sulfone) with varied amounts of polybenzimidazole. The polybenzimidazole served to decrease the water uptake and methanol permeability of the membranes, resulting in promising DMFC and hydrogen/air fuel cell performance. / Ph. D.

This thesis research focused on the synthesis and characterization of disulfonated poly(arylene ether sulfone) hydrophilic-hydrophobic segmented and multiblock copolymers for application as proton exchange membranes (PEMs) in fuel cells or as reverse osmosis (RO) membranes for water desalination. The first objective was to demonstrate that synthesizing blocky copolymers using a one oligomer, two monomer segmented copolymerization afforded copolymers with similar properties to those which used a previous approach of coupling two preformed oligomers. A 4,4′-biphenol based hydrophilic block of disulfonated poly(arylene ether sulfone) oligomer of controlled number average molecular weight (Mn) with phenoxide reactive end groups was first synthesized and isolated. It was then reacted with a calculated amount of hydrophobic monomers, forming that block in-situ. Copolymer and membrane properties, such as intrinsic viscosity, tensile strength, water uptake, and proton conductivity, were consistent with those of multiblock copolymers synthesized via the oligomer-oligomer approach. The segmented polymerization technique was then used to synthesize a variety of other copolymers for PEM applications. The well known bisphenol phenolphthalein was explored as a comonomer for either the hydrophilic and hydrophobic blocks of the copolymer. Membrane properties were explored as a function of block length for both series of copolymers. Both series showed that as block length increases, proton conductivity increases across the entire range of relative humidity (30-100%), as does, water uptake. This was consistent with earlier research which showed that the water self-diffusion coefficient scaled with block length. Copolymers produced with phenolphthalein had higher tensile strength, but lower ultimate elongation than the 4,4′-biphenol based copolymers. Multiblock copolymers were also synthesized and characterized to assess their feasibility as RO membranes. A new series of multiblock copolymers was synthesized by coupling hydrophilic disulfonated poly(arylene ether sulfone) (BisAS100) oligomer with hydrophobic unsulfonated poly(arylene ether sulfone) (BisAS0) oligomer. Both oligomers were derived using 4,´-isopropylidenediphenol (Bis-A) as the bisphenol. Phenoxide-terminated BisAS100 was end-capped with decafluorobiphenyl and reacted at relatively low temperatures (~ 100 oC) with phenoxide-terminated BisAS0. Basic properties were characterized as a function of block length. The initial membrane characterization suggested these copolymers may be suitable candidates for reverse osmosis applications, and water and salt permeability testing should be conducted to determine desalination properties. The latter measurements are being conducted at the University of Texas, Austin and will be reported separately. / Ph. D.

Segmented copolymers are important polymers with attractive properties and wide applications. In this dissertation, segmented polyurethanes containing triptycene units and multiblock copolymers containing poly(arylene ether sulfone) (PAES) and poly(1,4-cyclohexyldimethylene terephthalate) (PCT) segments were synthesized and systematically studied. Investigation of the influence of the bulky triptycene structure on the morphologies and properties of segmented polyurethanes was carried out by using triptycene-1,4-hydroquinone bis(2-hydroxyethyl)ether (TD) as the chain extender. Segmented polyurethanes based on poly(tetramethylene glycol) (PTMG) of 1000 g/mol were synthesized using a two-step polymerization procedure. Hydroquinone bis(2-hydroxyethyl)ether (HQEE) was used for the purpose of comparison. Hard segments with different bulkiness and flexibility were prepared with hexamethylene diisocyanate (HDI) and 4,4'-methylenebis(phenyl isocyanate) (MDI), and HQEE or TD as chain extenders. The incorporation of bulky TD and less flexible MDI significantly inhibited hydrogen bonding based on the Fourier transform infrared (FTIR) results. In addition, the microphase separation was also disturbed by the bulky and less flexible hard segments with confirmation from tapping mode atomic force microscopy (AFM) and small angle X-ray scattering (SAXS). The flexible HDI can be used to overcome the bulkiness of triptycene, promote microphase separation, and enhance mechanical properties. Novel PTMG based soft segments containing triptycene units were also prepared with number average molecular weight (Mn) around 2500 g/mol. Then this soft segment was reacted with MDI and HQEE to prepare segmented polyurethanes. Soft segments such as hydroquinone (HQ) containing PTMG (Mn = 2100 g/mol), and pure PTMGs (Mn = 1000 and 2000 g/mol) were used for comparison. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) results demonstrated that triptycene units led to an increased glass transition temperature (Tg) and an elimination of the crystallization of the soft segments. The absence of strain hardening for the triptycene-containing sample suggested a suppressed strain induced crystallization of soft segments, which was also confirmed by the analysis of wide-angle X-ray diffraction (WAXD) on the films strained to 370 %. Crystallizable PCT segments were copolymerized with PAESs to enhance solvent resistance and mechanical properties. PAES oligomers (Mn = 2000 g/mol) were first synthesized, and then reacted with dimethyl terephthalate (DMT) and 1,4-cyclohexanedimethanol (CHDM). Weight percentages of PCT segments were gradually changed from 20 wt% to 80 wt%. With PCT content greater than 50 wt%, crystallinity was observed by DSC, DMA, and WAXD. The extent of crystallinity of the copolymers was dependent on the wt% of PCT. Furthermore, crystallization behavior of copolymers based on two CHDMs with different isomer ratios (cis/trans 30/70 and all trans) were studied. Due to their more symmetric structure, copolymers based on all trans CHDM exhibited a higher extent of crystallization. / Ph. D.

Crosslinking is an effective technique for increasing the salt rejection of water purification membranes and the selectivity of gas separation membranes. An abundance of monomers, telechelic oligomers, and novel polymers were synthesized for use as separation membranes. These materials were often imbued with crosslinking functionalities to increase their performance during testing at the University of Texas-Austin. Crosslinking of sulfonated poly(arylene ether sulfone) oligomers was studied systematically with regard to end-group functionality, polymer composition, and polymer hydrophilicity. Sulfonated bisphenol A based poly(arylene ether sulfone) random copolymers were synthesized with reactive amine endgroups and further functionalized with a tetra-epoxy resin, acryloyl chloride, phenylethynyl phthalic anhydride, and maleic anhydride. The reaction between amine terminated oligomers and a tetra-epoxy produced large, ductile membranes with gel fractions approaching 99%, the highest reported for crosslinked sulfonated polysulfone oligomers. This crosslinking reaction was studied by synthesizing two series of oligomers, one based on a bisphenol A monomer and the other based on a 4,4’-biphenol monomer. Both series were synthesized with 40, 50 and 60% degrees of sulfonation, so that hydrophilicity and composition could be studied with regard to water purification properties. All six oligomers were produced with a gel fraction exceeding 90%, and the membranes were evaluated at the University of Texas-Austin. The crosslinked oligomers demonstrated relatively constant salt rejection across a range of hydrophilicity values, which proved that crosslinking restricted the large amount of swelling that non-crosslinked sulfonated polysulfones undergo. The crosslinked oligomers had the best water purification properties reported for sulfonated polysulfone, with similar water permeabilities and an order of magnitude higher selectivity (Pw/Ps = 1.69 ± 0.13 x 103) than analogous linear copolymers (Pw/Ps = 3.67 ± 0.53 x 102). An additional series of linear sulfonated copolymers were also synthesized based upon a hydroquinone bisphenol, which also had superior water purification properties (1.06 ± 0.06 L μm m-2 h-1 bar-1, Pw/Ps = 2.44 ± 0.15 x 103) compared to previously synthesized linear copolymers. Poly(arylene ether)s were also investigated for use as gas separation membranes. A poly(arylene ether ketone) and poly(arylene ether sulfone) were both synthesized with moieties capable of oxidation and/or photocrosslinking through benzylic hydrogen abstraction by an excited ketone. The polymers produced tough, ductile films. Gas transport properties of the linear polymers and crosslinked polymer were compared. The O2 permeability of one exemplary non-crosslinked poly(arylene ether) was 2.8 Barrer, with an O2/N2 selectivity of 5.4. Following UV crosslinking, the O2 permeability decreased to 1.8 Barrer, and the O2/N2 selectivity increased to 6.2. / Ph. D.

Hydrophilic/hydrophobic block copolymers as proton exchange membranes (PEMs) has become an emerging area of research in recent years. Three series of hydrophilic/hydrophobic, fluorinated/sulfonated multiblock copolymers were synthesized and characterized in this thesis. These copolymers were obtained through moderate temperature (~100°C) coupling reactions, which minimize the ether-ether interchanges between hydrophobic and hydrophilic telechelic oligomers via a nucleophilic aromatic substitution mechanism. The hydrophilic blocks were based on the nucleophilic step polymerization of 3,3′-disulfonated, 4,4′-dichlorodiphenyl sulfone with an excess 4,4′-biphenol to afford phenoxide endgroups. The hydrophobic (fluorinated) blocks were largely based on decafluoro biphenyl (excess) and various bisphenols. The copolymers were obtained in high molecular weights and were solvent cast into tough membranes, which had nanophase separated hydrophilic and hydrophobic regions. The performance and structure-property relationships of these materials were studied and compared to random copolymer systems. NMR results supported that the multiblock sequence had been achieved. They displayed superior proton conductivity, due to the ionic proton conducting channels formed through the self-assembly of the sulfonated blocks. The nano-phase separated morphologies of the copolymer membranes were studied and confirmed by atomic force microscopy. Through control of a variety of parameters, including ion exchange capacity and sequence lengths, performances as high, or even higher than those of the state-of-the-art PEM, Nafion, were achieved. / Ph. D.

The purity of the disulfonated monomer, such as 3,3"-disulfonated-4,4"-dichlorodiphenyl sulfone (SDCDPS), was very important for obtaining high molecular weight copolymers and accurate control of the oligomer's molecular weight. A novel method to characterize the purity of disulfonated monomer, SDCDPS, was developed by using UV-visible spectroscopy. This allowed for utiliziation of the crude SDCDPS directly in the copolymerization to save money, energy, and time. Three series of tert-butylphenyl terminated disulfonated poly(arylene ether sulfone) copolymers (BPSH35, 6FSH35, and 6FSH48) with controlled molecular weightsï¼ Mnï¼ , 20 to 50 kg·mol-1, were successfully prepared by the direct copolymerization method. The molecular weight of the copolymer was controlled by a monofunctional monomer tert-butylphenyl, and characterized by the combination of 1H NMR spectra and modified intrinsic viscosity measurements in NMP with 0.05 M LiBr, which was added to suppress the polyelectrolyte effect. The mechanical properties of the membranes, such as the modulus, strength and elongation at break, were improved by increasing the molecular weights, but water uptake and proton conductivities found insensitive to copolymers" molecular weights. Three series of disulfonated poly(arylene ether ketone) random copolymers have been synthesized and comparatively studied, according to their different chemical structures, for use as proton exchange membranes. The copolymers containing more flexible molecular structures had higher water uptake and proton conductivity than the rigid structures at the same ion exchange capacity. This may be due to the more flexible chemical structures being able to form better phase separated morphology and higher hydration levels. A new hydrophobic-hydrophilic multiblock copolymer has been successfully synthesized based on the careful coupling of a fluorine terminated poly(arylene ether ketone) (6FK) hydrophobic oligomer and a phenoxide terminated disulfonated poly(arylene ether sulfone) (BPSH) hydrophilic oligomer. AFM images and the water diffusion coefficient results confirmed that the multiblock copolymer formed better proton transport channels. This multiblock copolymer showed comparable proton conductivity and fuel cell performance to the Nafion® control and had much better proton transport properties than random ketone copolymers under partially hydrated conditions. This suggested that the multiblock copolymers are promising candidates for proton exchange membranes especially for applications at high temperatures and low relative humidity. / Ph. D.

The objectives of this research were to study the gas transport properties of proton exchange membranes (PEM), namely disulfonated poly(arylene ether sulfone) (BPSH-35), post sulfonated diels-alder poly(phenylene) (SDAPP), and poly(perfluoro sulfonic acid) (Nafion). The O2 gas permeabilities were found to be lower in BPSH and SDAPP as compared to poly(perfluoro sulfonic acid) because of difference in Tg (TgBSPH= 250 oC, TgSDAPP= 330 oC versus TgNafion=150 oC). Higher Tg polymers have a more rigid, inflexible polymer segments causing a reduction in gas permeability. In comparison to SDAPP, BPSH has a lower O2 gas permeability because of the bulky side groups in the SDAPP backbone. O2 sorption measurements were carried out both under non-humidified and humidified conditions as a function of relative humidity and temperature at a normal PEM operating pressure of 1 atm. Under non-humidified conditions, BPSH, SDAPP, and Nafion 112 exhibited Henry's Law sorption, consistent with dilute dissolution of O2 into the polymer matrix. The enthalpies of sorption were calculated to determine the interaction of O2 with each membrane. The sorption enthalpies in BPSH and SDAPP increased with increasing pressure indicating the formation of more O2-O2 interactions. The enthalpies in Nafion 112 were relatively constant with increasing pressure. In the presence of moisture, the sorption behavior changed from Henry's Law to Type IV sorption behavior, which is common in hydrophilic polymers. The SDAPP membrane was found to have the highest percent wet O2 mass uptake because of a higher number of sulfonic acid groups interacting with the water/O2 system. Finally the O2 sorption for various porous catalyst powders, consisting of platinum supported on carbon was measured in the non-humidified and humidified state. The catalysts were found to have Knudsen diffusion in the non-humidified state with 20 wt% Pt-C having the largest O2 sorption. In the humidified state, the highest O2 mass uptake was achieved with 40 wt% Pt-C. These results are explained in terms of the trade-off between catalyst dispersion and catalyst size. Furthermore, O2 sorption measurements were utilized for membrane electrode assemblies containing 40 wt% Pt-C and hot pressed at 210 oC for BPSH-35 (25 and 80K) and Nafion 112 membranes. The same sorption behavior occurred in the MEAs as in the neat membrane, but at a lower capacity. This is because the electrode introduces a more tortuous path to the gas molecules permeating across the membrane. / Ph. D.