Synthesis and characterization of nanostructured sulfonated polyimides for proton exchange membrane fuel cells

Sulfonated polyimides (SPI) are considered to be good candidates for proton exchange membranes (PEMs) since they exhibit high strength, good film-forming ability, chemical resistance, thermal stability, and, in their hydrated state, relatively high proton conductivity. Despite intense research in the area of SPIs, fundamental investigations of hydrophilic/hydrophobic phase segregation and studies of humidity dependent morphologies are scarce. In an effort to influence the order and distribution of ionic groups in rigid-rod SPIs and to understand the interrelationships between morphology, hydration and proton conductivity, two novel model systems of SPI polymers containing hydrophobic polysiloxane (SPI-PSX) and hydrophilic silica nanoparticles (SPI-Si) were developed.;The first model system of sulfonated polyimide containing hydrophobic polysiloxane segmented copolymers was prepared by a one-pot synthesis. SPI-PSX materials were evaluated using 1H NMR, size-exclusion chromatography. The presence of ion-containing diamines in the reaction mixture was found to inhibit stoichiometric incorporation of hydrophobic siloxane segments. Siloxane segments were found to lower the thermal stability of the polyimide host. Equilibrium water sorption studies of free standing films of copolymers with and without siloxane segments show that the presence of siloxane segments does not interfere with water swelling, which suggests a microphase-segregated morphology may exist. TEM and SAXS analyses show evidence of phase-segregation in sulfonated polyimides and reveal that siloxane segments strongly affect ionic clustering. However, proton conductivity only changes slightly when polysiloxane segments are incorporated.;Sulfonated polyimides containing hydrophilic silica nanoparticles is our second model system developed for stabilizing the dispersed morphologies to promote proton conductivity. SPI-Si nanocomposites were prepared by a pre-polymer of anhydride-terminated sulfonated polyimides followed by introduction of reactive silanes. Casting, curing, and acidification routines lead to nanocomposites with significantly different properties compared to parent sulfonated polyimides. The presence of silica was qualitatively confirmed using energy dispersive X-ray spectrometry and solid-state 29Si NMR. Thermogravimetric analysis provided a quantitative assessment of the inorganic fraction. Electron microscopy, water vapor sorption, and impedance studies were conducted to understand how silica and ion content influences morphology and proton conductivity. Silica nanoparticles significantly reduce the solubility of prepared membranes, and promote membrane hydration. For nanocomposites prepared from high molecular weight pre-polymers, silica incorporation promotes conductivity at low relative humidity. However, for nanocomposites made from low molecular weight or high ionic content pre-polymers, silica dilutes the ion concentration and lowers proton conductivity.;In addition, sulfonated polyimide-poly(4-vinylimidazole) (SPI-P4VI) was also developed and introduced in this thesis. There is a strong need for PEMs capable of sustained operation at temperatures higher than 100°C, and we suspect that utilizing high-boiling, amphoteric molecules like imidazole is the key to achieving significantly higher proton conductivities at high temperatures and low humidities. Poly(4-vinylimidazole) (P4VI) has been successfully synthesized in our lab. SPI-P4VI complex films were fabricated by coating the acetic acid solution of P4VI onto a preformed protonated SPI film and they have been evaluated by ATR-IR. Initial proton conductivity measurements on the SPI26-P4VI600 polymers have shown SPI-P4VI materials could be a promising PEM system for high temperature and low humidity conditions though some system optimization is still required and future work was recommended.