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.

Sulfonated poly(arylene ether sulfone)s as ion conductive copolymers have numerous potential applications. Membranes cast from these copolymers are desirable due to their good chemical and thermal stability, excellent mechanical strength, satisfactory conductivity, and excellent transport properties of water and ions. These copolymers can be used in a variety of topologies. Structure-property-performance relationships of these membranes as candidates for electrolysis of water for hydrogen production and for purification of water from dissolved ions have been studied. Linear and multiblock sulfonated poly(arylene ether sulfone)s are potential alternative candidates to Nafion membranes for hydrogen gas production via electrolysis of water. In this investigation, these copolymers were prepared from the direct polymerization of di-sulfonated and non-sulfonated comonomers with bisphenol monomers. In systematic investigations, a series of copolymers with modified properties were synthesized and characterized by changing the ratio of the sulfonated/non-sulfonated comonomers in each reaction. These copolymers were investigated in terms of mechanical stability, proton conductivity and H2 gas permeability at a range of temperatures and under fully hydrated conditions. A multiblock copolymer was synthesized and evaluated for its potential as membranes for electrolysis of water and for fuel cell applications. The multiblock copolymer contained some fluorinated repeat units in the hydrophobic blocks, and these were coupled with a fully disulfonated hydrophilic block prepared from 3,3'-disulfonate-4,4'-dichlorodiphenyl sulfone and biphenol. After annealing, the multiblock copolymer showed enhanced proton conductivity and a more ordered morphology in comparison to the random copolymer counterparts. At 90 oC and under fully hydrated conditions, improved proton conductivity and controlled H2 gas permeability was observed. Finally, the performance of the multiblock copolymer, which was measured as the ratio of proton conductivity to H2 gas permeability, was improved when compared to the state-of-the-art membrane, Nafion 212, by a factor of 3. In another systematic study, two series of random copolymers were synthesized and characterized, and then cast into membranes to evaluate for electrolysis of water. One series contained solely hydroquinone as the phenolic monomer, while the second series contained a mixture of resorcinol and hydroquinone as phenolic comonomers. The polymers that contained only the hydroquinone monomer showed exceptionally good mechanical properties due to the para-substituted comonomer in the composition of the polymer. In the resorcinol-hydroquinone series, gas permeability was constrained due to the presence of 25% of the meta-substituted comonomer incorporated into its structure. Low gas permeability and high proton conductivity at elevated temperatures were obtained for both the linear random and multiblock copolymers. Performance of these copolymers was superior to Nafion at elevated temperatures (80-95°C). In order to enhance the durability of these materials in their hydrated states at elevated temperatures, the surfaces of these copolymer films were treated with fluorine gas. In comparison with pristine non-fluorinated membranes, the modified membranes showed decreased water uptake and longer durability in Fenton's reagent. A series of linear and crosslinked copolymers were investigated with respect to their potential for use as membranes for desalination of water by electrodialysis and reverse osmosis. The crosslinked membranes were prepared by reacting controlled molecular weight, disulfonated oligomers that were terminated with meta-aminophenol with an epoxy reagent. The oligomers had systematically varied degrees of disulfonation and either 5000 or 10,000 Da controlled molecular weights. Membrane casting conditions were established to fabricate highly crosslinked systems with greater than 90% gel fractions. At such a high gel fraction, the water uptake of the crosslinked membranes was lower than that of the linear biphenol-based, disulfonated random copolymer with a similar IEC. Among these series of copolymers, it was shown that the crosslinked membranes cast from the oligomers with 50% degree of disulfonation and a molecular weight of 10,000 Da had the lowest salt permeability of 10-8 cm2/sec. For desalination applications, a comonomer was synthesized with one sulfonate substituent on 4,4'-dichlorodiphenyl sulfone. This new monosulfonated comonomer allows for even distribution of the ions on the linear copolymer backbone, and this may be important for controlling ion transport. Mechanical tests were conducted on the membranes while they were submerged in a water bath. The ultimate strength of a fully hydrated copolymer with an IEC of 1.36 meq/g was approximately 60 MPa with an elongation at break of 160%. Moreover, in a monovalent/divalent mixed salt solution, the monosulfonated linear copolymer exhibited a constant Na+ passage of less than 1.0%. / Ph. D. / Purification systems have become an increasingly important scientific and technological need for millions around who face water shortages and/or impure sources of potable water. In response, water purification and hydrogen gas production have been widely used to produce pure products from a variety of water sources. In general, current state-of-the-art methods in separation technologies feature two major drawbacks: they are energy intensive and costly processes. In response to the growing need for purified water or pure hydrogen gas for energy generation, polymeric materials are increasingly used in the form of membranes to produce a purer product and overcome the hindrances associated with current energy intensive and inefficient methods. These membranes serve as a barrier for unwanted species, while at the same time allowing the desired species to pass through. Under proper conditions, these purification or chemical processes would generate pure materials that can be used on demand. The chemistry of candidate polymeric materials is extremely important to design a membrane with desired properties. Therefore, the principal goals of this investigation were to synthesize polymers for use as membranes in three areas: 1) Electrolysis of water for ultra-pure hydrogen gas generation 2) Fuel cells applications for electricity generation, and 3) Desalination of water to provide drinking water. For each technology, a series of sulfonated poly(arylene ether sulfone) copolymers were synthesized and characterized. By applying different monomers or chemistries, a range of appropriate copolymers were synthesized whose characteristics varied in topology and architecture, depending on the desired application. Once these copolymers were synthesized, they were cast into membranes under proper established conditions. In addition, the structure-property-performance relationship of these sulfonated polysulfone membranes were further investigated to provide a direction for future studies.