This dissertation describes the past and recent progress in proton exchange membranes (PEM) for fuel cells. In particular the synthesis and characterization of materials for advanced alternative PEM were studied with an emphasis on structure-property and structure-property-performance relationships. The focus has included firstly a one-step synthesis and characterization of 3,3'-disulfonated 4,4'-dichlorodiphenyl sulfone comonomer. The procedure developed is adaptable for industrial-scale commercialization efforts. Secondly, the synthesis of aromatic nitrile containing poly (arylene ether sulfone) random copolymers was demonstrated. Various levels of disulfonation allowed the membrane characteristics to be investigated as a function of the membrane ion exchange capacity. The results favorably compare with the current state-of-the-art (Nafion™), particularly for direct methanol systems (DMFC). Thirdly, the mechanically and thermooxidatively stable copolymer membranes were blended with heteropolyacids producing nanocomposites which have potential in higher temperature fuel cell applications. Lastly, the basic PEM parameters such as water uptake, proton conductivity, and methanol permeabilities were controlled and presented as tunable properties as a function of molecular structure. This was achieved by in-situ control of chemical composition. The direct methanol fuel cell performance (DMFC) was much better than Nafion™. Hydrophobic surface properties of the membranes were improved by partial fluorination which made the Nafion™ bonded electrodes more compatible with the partially fluorinated copolymer membranes. The influence of surface enrichment had two important roles in increasing both initial and long term performance tests. The surface fluorine provided lower contact resistance and lower water uptake. The former was important for the initial tests and the latter provides for better long term performances. A delamination failure mechanism was proposed for the hydrocarbon membrane electrode assemblies (MEA) due to the large difference between water uptake of the catalyst layer and membrane and this was verified by a reduction in high frequency resistance (HFR) for the partially fluorinated systems. This thesis has generated the structure-property and structure-property-performance relationships which will provide direction for the development of next generation (PEM) materials. / Ph. D.

The potential success of a proton exchange membrane (PEM) fuel cell as an alternative energy source depends highly upon the development of high performance PEMs. Typically, state-of-the-art PEMs have been perfluorinated sulfonated ionomer membranes such as Nafion® by DuPont. Although these membranes demonstrate good mechanical and electrochemical properties under moderate operating conditions (e.g., < 80 ºC), their performance at high temperature (e.g., > 80 ºC) and low relative humidity (RH) drastically deteriorates. To overcome these problems, PEM materials with enhanced properties are essential. Recently, the McGrath group has shown that PEM materials with hydrophilic-hydrophobic segments can significantly improve proton conductivity under low RH by forming enhanced hydrophilic domain connectivity. In this dissultation, novel multiblock copolymers based on disulfonated hydrophilic-hydrophobic multiblocks were synthesized and investigated for their potential application as PEMs. The relationship between copolymer chemical composition and resulting properties was probed with a variety of hydrophilic and hydrophobic segments. Most multiblock copolymers in this research were developed with fully disulfonated poly(arylene ether sulfone) (BPS100) as the hydrophilic segment, and various high performance polymers including polyimides, poly(arylene ether sulfone)s, and poly(arylene ether ketone)s as the hydrophobic segment. Ionic groups on the hydrophilic blocks act as proton conducting sites, while the non-ionic hydrophobic segments provide mechanical and dimensional stability. The correlation between the fuel cell performances and the hydrophilic-hydrophobic sequences was also evaluated. The morphological structures of the multiblock copolymers were investigated using tapping mode atomic force microscopy (TM-AFM), transmission electron microscopy (TEM), and dynamic mechanical analysis (DMA). The experiments demonstrated a well-defined nanophase separated morphology. Moreover, changes in block length had a pronounced effect on the development of phase separated morphology of the system. Proton conductivity measurements elucidated the transport process in the system, with the multiblock copolymers demonstrating higher conductivities compared to Nafion and random copolymer systems with similar ion exchange capacity (IEC) values. The new materials are strong candidates for use in PEM systems. / Ph. D.

This work is concerned with the production of high performance polymer foams via a benign foaming process. The first goal of this project was to develop a process and the conditions necessary to produce a low density (>80% density reduction) foam from poly(arylene ether sulfone) (PAES). Water and supercritical carbon dioxide (scCO2) were used as the blowing agents in a one-step batch foaming process. Both water and scCO2 plasticize the PAES, allowing for precise control on both the foam morphology and the foam density. To optimize the foaming conditions, both thermogravimetric analysis and differential scanning calorimetery (DSC) were used to determine the solubility and the reduced glass transition temperature (Tg) due to plasticization of the polymer. It was determined that 2 hours was sufficient time to saturate the PAES with water and scCO2 when subjected to a temperature of 220 oC and 10.3 MPa of pressure. Under these conditions, a combination of 7.5% of water and scCO2 were able to diffuse into the PAES specimen, correlating to ~60 oC reduction in the Tg of the PAES. The combination of water and scCO2 produced foam with up to an 80% reduction in density. The compressive properties, tensile modulus, and impact strength of the foam were measured. The relative compressive properties were slightly lower than the commercially available structural foam made of poly(methacrylimide). The second objective of the dissertation was to enhance the compressive properties of the PAES foam, without concern for the foam density. Foam was produced over a range of density, by controlling the cell size, in order to optimize the compressive properties. Carbon nanofibers (CNFs) were also added to the PAES matrix prior to foaming to both induce heterogeneous nucleation, which leads to smaller cell size, and to reinforce the cell walls. Dynamic mechanical thermal analysis (DMTA), on saturated CNF-PAES, was used to determine the reduced Tg due to plasticization and establish the temperature for pressure release during foaming. DMTA proved to be more effective than DSC in establishing quantitative results on the reduction in the Tg. The CNF-PAES foam produced had compressive properties up to 1.5 times the compressive properties of the PAES foam. / Ph. D.

A designed series of directly copolymerized homo- and disulfonated copolymers containing controlled degrees of pendant sulfonic acid groups have been synthesized via nucleophilic step polymerization. Novel sulfonated poly (arylene ether sulfone) copolymers using 4,4'-bisphenol A, 4,4'-biphenol, hexafluorinated (6F) bisphenol AF, and hydroquinone, respectively, with dichlorodiphenyl sulfone (DCDPS) and 3,3'-disodiumsulfonyl-4,4'-dichlorodiphenylsulfone (SDCDPS) were investigated. Molar ratios of DCDPS and SDCDPS were systematically varied to produce copolymers of controlled compositions, which contained up to 70 mol% of disulfonic acid moiety. The goal is to identify thermally, hydrolytically, and oxidatively stable high molecular weight, film-forming, ductile ion conducting copolymers, which had properties desirable for proton exchange membranes (PEM) in fuel cells. Commercially available bisphenols were selected to produce cost effective alternative PEMs. Partially aliphatic bisphenol A and hexafluorinated (6F) bisphenol AF produced amorphous copolymers with different thermal oxidative and surface properties. Biphenol and hydroquinone was utilized to produce wholly aromatic copolymers. The sulfonated copolymers were prepared in the sodium-salt form and converted to the acid moiety via two different methodologies and subsequently investigated as proton exchange membranes for fuel cells. Hydrophilicity increased with the level of disulfonation, as expected. Moreover, water sorption increased with increasing mole percent incorporation of SDCDPS. The copolymers' water uptake was a function of both bisphenol structure and degree of disulfonation. Furthermore, the acidification procedures were shown to influence the Tg values, water uptake, and conductivity of the copolymers. Atomic force microscopy (AFM) in the tapping mode confirmed that the morphology of the copolymers could be designed to display nanophase separation in the hydrophobic and hydrophilic (sulfonated) regions. Morphology with either co-continuous hydrophobic or hydrophilic domains could be attained for all the sulfonated copolymers. The degree of disulfonation required for continuity of the hydrophilic phase varied with biphenol structure. Proton conductivity values for the sulfonated copolymers, under fully hydrated conditions, were a function of bisphenol and degree of sulfonation. However, at equivalent ion exchange capacities the proton conductivities were comparable. A careful balance of copolymer composition and acidification method was necessary to afford a morphology that produced ductile films, which were also sufficiently proton conductive. The copolymers of optimum design produced values of 0.1 S/cm or higher, which were comparable to the commercial polyperfluorosulfonic acid material Nafion™ control. / Ph. D.

Novel proton exchange (PEM) and reverse osmosis (RO) membranes for application in fuel cell and water purification respectively were developed by synthesis and crosslinking of disulfonated biphenol-based poly (arylene ether sulfone)s (BPS). Crosslinking is a prospective option to reduce the water swelling and improve the dimensional stability of hydrophilic BPS copolymers. Several series of controlled molecular weight, phenoxide-endcapped BPS copolymers were synthesized via direct copolymerization of disulfonated activated aromatic halide monomers. The degree of disulfonation was controlled by varying the molar ratio of sulfonated to non-sulfonated dihalide monomers. The molecular weights of the copolymers were controlled by offsetting the stoichiometry between biphenol and the dihalides. Biphenol was utilized in excess to endcap the copolymers with phenoxide groups, so that the phenoxide groups could be further reacted with a suitable crosslinker. Several crosslinking reagents such as methacrylate, multifunctional epoxy, phthalonitrile and phenylethynyls were investigated. A wide range of crosslinking chemistries i.e. free radical (methacrylate), step growth (epoxy), heterocyclic (phthalonitrile) and acetylenic (phenylethynyl) was explored. The effects of crosslinking on network properties as functions of molecular weight and degree of disulfonation of copolymers, crosslinking time and concentration of crosslinker were studied. The crosslinked membranes were characterized in terms of gel fraction, water uptake, swelling, self-diffusion coefficients of water, proton conductivity, methanol permeability, water permeability and salt rejection. In general, all of the crosslinked membranes had lower water uptake and swelling relative to their uncrosslinked counterparts, and less water uptake and volume swelling were correlated with increasing gel fractions. It was possible to shift the percolation threshold for water absorption of BPS copolymers to a higher ion exchange capacity (IEC) value compared to that of the uncrosslinked copolymers by means of crosslinking. This reduced water uptake increased the dimensional stability of higher IEC materials and extended their application for potential PEM or RO membranes. The reduction in water uptake and swelling also increased the effective proton concentration, resulting in no significant change in proton conductivity of the membranes after crosslinking. The self-diffusion coefficients of water and methanol permeability decreased with crosslinking, indicating restricted water and methanol transport. Therefore an improvement in the selectivity (ratio of proton conductivity to water swelling or methanol permeability) of PEMs for application in either H2/air or direct methanol fuel cells was achieved by crosslinking. The epoxy crosslinked BPS copolymers also had significantly enhanced salt rejection with high water permeability when tested in for RO applications. Reductions in salt permeability with increasing crosslinking density suggested that crosslinking inhibited salt transport through the membrane. In addition to the random copolymers, two series of multiblocks endcapped with either a phenoxide-terminated hydrophilic unit or a hydrophobic unit were synthesized and crosslinked with a multifunctional epoxy. Besides the crosslinking study, the effect of sequence distributions of the hydrophilic and hydrophobic blocks in the multiblock copolymers was also investigated. Similar to randoms, crosslinked multiblocks had lower water uptake and swelling but comparable proton conductivities relative to their uncrosslinked analogues. / Ph. D.

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.

Proton exchange membrane fuel cells (PEMFCs) have been extensively studied as clean, sustainable and efficient power sources for electric vehicles, and portable and residential power sources. As one of the key components in PEMFC system, proton exchange membranes (PEMs) act as the electrolyte that transfers protons from the anode to the cathode. The state-of-art commercial PEM materials are typically based on perfluorinated sulfonic acid containing ionomers (PFSAs), represented by DuPont's Nafion®. Despite their good chemical stability and proton conductivity at high relative humidity (RH) and low temperature, several major drawbacks have been observed on PFSAs, such as high cost, high fuel permeability, insufficient thermo-mechanical properties above 80°C, and low proton conductivity at low RH levels. Therefore the challenge lies in developing alternative PEMs which feature associated ionic domains at low hydration levels. Nanophase separated hydrophilic-hydrophobic block copolymer ionomers are believed to be desirable for this purpose Three series of hydrophobic/hydrophillic, partially fluorinated/sulfonated multiblock copolymers were synthesized and characterized in this thesis. The hydrophilic blocks were based upon the nucleophilic step polymerization of 3, 3′-disulfonated, 4, 4′-dichlorodiphenyl sulfone (SDCDPS) with an excess 4, 4′-biphenol (BP) to afford phenoxide endgroups. The partially fluorinated hydrophobic blocks were largely based on 4, 4′-hexafluoroisopropylidenediphenol (6F-BPA) and various difluoro monomers (excess). These copolymers were obtained through moderate temperature (~130-150°C) coupling reactions, which minimize the ether-ether interchanges between hydrophobic and hydrophilic telechelic oligomers via a nucleophilic aromatic substitution mechanism. 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 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 transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). 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. Another series of semi-crystalline hydrophobic poly(ether ether ketone)-hydrophilic sulfonated poly(arylene ether sulfone) (PEEK-BPSH100) multiblock copolymers was first synthesized and characterized. However due to their semi-crystalline structure, PEEK blocks are insoluble in most organic solvents at relatively low reaction temperatures, which prevents the coupling reaction between PEEK and BPS100. In order to facilitate the synthesis and processing, removable bulky ketimine was introduced to synthesize amorphous pre-oligomers poly(ether ether ketimine) (PEEKt). The synthetic procedure first involves the synthesis of hydrophobic poly(ether ether ketimine)-hydrophilic sulfonated poly(arylene ether sulfone) (PEEKt-BPS100) multiblock pre-copolymers via coupling reactions between phenoxide terminated hydrophilic BPS100 and fluorine terminated hydrophobic PEEKt blocks. The membranes cast from PEEKt-BPS100 were boiled in 0.5M sulfuric acid water solution to hydrolyze the amorphous PEEKt blocks to semi-crystalline PEEK blocks and acidify BPS100 blocks to BPSH100 blocks simultaneously. FT-IR spectra clearly showed the successful hydrolysis and acidification. The proton conductivity, water uptake and other membrane properties of the acidified semi-crystalline PEEK-BPSH100 membranes were then evaluated and compared with those of the state-of-the-art PEM, Nafion®. / 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.