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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.

Aromatic polyimides have found widespread applicability which can be partially attributed to their thermal stability, chemical resistance, and high glass transition temperature. However, deficiencies in their processability, solubility, transparency, and relatively high dielectric constants do not always provide the optimum properties for many specialty microelectronics applications. The incorporation of aliphatic segments to form partially aliphatic polyimides, has been used to counteract these shortcomings. Many of the potential uses of partially aliphatic polyimides require them to adhere to ceramic substrates, a main topic of this research. Polyimides and copolyimides that varied in chemical composition by their aliphatic content were characterized by their molecular weight, glass transition temperature, thermal stability, coefficient of thermal expansion, refractive index, dielectric behavior, and mechanical properties. Structure-property relationships were established. The gamma and beta sub-Tg viscoelastic relaxations were investigated to understand their molecular origins. The adhesion performance of a selected series of partially aliphatic polyimides to SiO2/Si was examined using a shaft loaded blister test, which was designed and instrumented for use in a dynamic mechanical analysis instrument. The adhesion was studied at high and low percent relative humidities and for several temperatures to examine if adhesion strength is influenced by polymer chemical composition. The adhesion energy could not be quantified for the entire series of polyimides. It was possible to interpret the quantitative adhesive fracture energies along with the qualitative adhesion strength behaviors, the failure surface analyses, and to offer an understanding of the adhesive chemical structure-physical property relationships. These understandings provide a conclusion that the incorporation of aliphatic segments into the polyimide chemical structure improves the durability of the adhesive bond to SiO2/Si under high percent relative humidities. / Ph. D.

Novel proton exchange membranes (PEMs) were investigated that show potential for operating at higher temperatures in both direct methanol (DMFC) and H2/air PEM fuel cells. The need for thermally stable polymers immediately suggests the possibility of heterocyclic polymers bearing appropriate ion conducting sites. Accordingly, monomers and random disulfonated poly(arylene ether) copolymers containing either naphthalimide, benzoxazole or benzimidazole moieties were synthesized via direct copolymerization. The ion exchange capacity (IEC) was varied by simply changing the ratio of disulfonated monomer to nonsulfonated monomer in the copolymerization step. Water uptake and proton conductivity of cast membranes increased with IEC. The water uptake of these heterocyclic copolymers was lower than that of comparable disulfonated poly(arylene ether) systems, which is a desirable improvement for PEMs. Membrane electrode assemblies were prepared and the initial fuel cell performance of the disulfonated polyimide and polybenzoxazole (PBO) copolymers was very promising at 80 C compared to the state-of-the-art PEM (Nafion®); nevertheless these membranes became brittle under operating conditions. Several series of poly(arylene ether)s based on disodium-3,3′-disulfonate-4,4′-dichlorodiphenylsulfone (S-DCDPS) and a benzimidazole-containing bisphenol were synthesized and afforded copolymers with enhanced stability. Selected properties of these membranes were compared to separately prepared miscible blends of disulfonated poly(arylene ether sulfone) copolymers and polybenzimidazole (PBI). Complexation of the sulfonic acid groups with the PBI structure reduced water swelling and proton conductivity. The enhanced proton conductivity of Nafion® membranes has been proposed to be due to the aggregation of the highly acidic side-chain sulfonic acid sites to form ion channels. A series of side-chain sulfonated poly(arylene ether sulfone) copolymers based on methoxyhydroquinone was synthesized in order to investigate this possible advantage and to couple this with the excellent hydrolytic stability of poly(arylene ether)s. The methoxy groups were deprotected to afford reactive phenolic sites and nucleophilic substitution reactions with functional aryl sulfonates were used to prepare simple aryl or highly acidic fluorinated sulfonated copolymers. The proton conductivity and water sorption of the resulting copolymers increased with the ion exchange capacity, but changing the acidity of the sulfonic acid had no apparent effect. / Ph. D.

In this dissertation, two approaches were taken to design aqueous dispersible or soluble high performance ion containing polymers to be used as composite system interfacial modifiers and processing aids. In the first approach, thermally stable pyridine containing poly(ary/ene ether)s were designed which could be ionized by protonation in acidic aqueous media. A novel pyridine containing bisphenol monomer, 2,6-(p-hydroxyphenoxy)pyridine, was synthesized and utilized as a monomer for the synthesis of these pyridine moiety containing, high performance polymers containing sulfone, sulfoxide, phosphine oxide, ketimine, and ketone moieties. These pyridine containing poly(arylene ether)s can function as electrostatic stabilizers, but not as the more efficient steric stabilizers. ThE: second approach endeavored to form controlled molecular weight poly(ether-irTlides) via water soluble poly(amic acid) salt precursors. In this approach controlled molecular weight poly(amic acid)s were synthesized, and treated with stoichiometric quantities of tertiary or quaternary ammonium bases to form poly(amic acid) salts. The imidization conditions, and chemistry of the conversion of the poly(amic acid) salts to imide were studied, with the aim of maintaining the targeted molecular weight distribution and properties analogous to a control polyimide. For the above mentioned aqueous dispersion prepregging process, it is required that the matrix resin be in the form of small uniform particles capable of penetrating the interstices of a tight carbon fiber weave. Sub ~lm dimension poly(ether ether ketone) (PEEK) particles useful for aqueous dispersion prepregging were prepared on a large scale by precipitation from high temperature solvent, quantitatively purified, and shown to display properties analogous to the commercial precursor material. In the final chapter of this dissertation, the synthesis and characterization of a polyolefin macromer, and it's incorporation into a polyester is detailed. These macromers, and the graft polymers resulting, have applicability in the area of polymer blend compatibilization. / Ph. D.

The interaction of water vapor with the surface and bulk of poly(imidesiloxane) copolymers has been characterized in an attempt to determine the important factors in the copolymer's resistance to water ingression. The multi-block copolymers were synthesized from benzophenone tetracarboxylic dianhydride, bisaniline P and pre-formed amine-terminated poly(dimethylsiloxane) oligomers, with phthalic anhydride as an end-capping agent. Similar copolymers had been previously shown to have reduced water sorption, increased surface hydrophobicity, and increased adhesive durability in hot/wet environments. Inverse gas chromatography was used to conduct surface energetics studies on copolymers of different siloxane concentration and a polyirnide homopolymer. Free energies of specific interaction of water vapor, ΔG_sp°, with the polymer surfaces were found to decrease with the incorporation of siloxane into the polyirnide. The dispersive components of the solid surface free energy of the siloxane- containing copolymers were equal within error to that of pure poly(dimethylsiloxane), indicating a PDMS-rich, hydrophobic surface. The ΔG_sp° of the copolymers were not significantly different, suggesting that the copolymer surfaces were very similar. This indicated a minimum weight percent of siloxane incorporation required to maximize the copolymer's surface water resistance. The minimum amount for the studied system was at most ten percent. Diffusion coefficients of water vapor in the polyimide and copolymers were determined from gravimetric sorption experiments. Higher levels of siloxane incorporation caused a definite increase in the diffusion coefficient, indicating a decreased resistance to water ingression. The increase in diffusion was found to be influenced by siloxane block length and was interpreted in terms of morphological and free volume theories. The diffusion coefficient of a 10 weight percent PDMS copolymer, however, was found to be the same within error as the polyimide diffusion oefficient. The incorporation of siloxane into polyimides has been shown to increase water resistance due to the hydrophobicity of the siloxane-rich surface. However, high siloxane contents also increased the rate of water ingression in the bulk of the polymer. Increased water resistance of the surface may be achieved at lower siloxane concentrations without increasing diffusive (or decreasing mechanical) properties to undesirable levels. / Ph. D.