Morphological and Mechanical Properties of Dispersion-Cast and Extruded Nafion Membranes Subjected to Thermal and Chemical Treatments
Osborn, Shawn James
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The focus of this research project was to investigate morphological and mechanical properties of both extruded and dispersion-cast NafionÂ® membranes. The project can be divided into three primary objectives; obtaining a fundamental understanding of the glass transition temperature of NafionÂ®, determining the effect of thermal annealing treatments on the morphology and mechanical properties of dispersion-cast NafionÂ®, and examination of dispersion-cast NafionÂ® subjected to an ex-situ, Fentonâ s chemical degradation test. NafionÂ®, a perfluorosulfonate ionomer, is considered a commercially successful semi-crystalline ionomer with primary applications in chlor-alkali cells and proton exchange membrane fuel cells. With the aid of dynamic mechanical analysis (DMA) and dielectric spectroscopy (DS), we were able to provide definitive evidence for a genuine glass transition in NafionÂ®. DMA of NafionÂ® samples that were partially neutralized with tetrabutylammonium counterions showed a strong compositional dependence suggesting that the Î²-relaxations of H+-form NafionÂ® and the neutralized ionomers have the same molecular origin with respect to backbone segmental motions. Building upon our previous studies of the molecular and morphological origins of the dynamic mechanical relaxations of NafionÂ® neutralized with a series of organic ions, the glass transition temperature of H+-form NafionÂ® is now confirmed to be the weak Î²-relaxation centered at -20 Â°C. Dielectric spectra also showed this transition from the perspective of dipole relaxation. The signature of cooperative long range segmental motions in dielectric spectra was seen here, as with other polymers, mainly through the excellent agreement of the Î²-relaxation time-temperature dependence with the Vogel-Fulcher-Tammann equation. We have also discovered that new dispersion-cast H+ form NafionÂ® membranes are susceptible to disintegration/dissolution when subjected to boiling methanol. In this work, we have achieved significant decreases in the percent solubility of H+-form NafionÂ® by either thermally annealing above 175 Â°C or solution-processing at 180 Â°C using a high boiling point solvent. Small Angle X ray Scattering (SAXS) displayed a change in the morphology of H+ form membranes with increasing annealing temperature by a shift in the crystalline scattering peak (q â 0.05 Ã 1) to lower q values. Counterion exchange of NafionÂ® from H+ to Na+ form had no influence on the membraneâ s susceptibility to disintegration in boiling methanol. In order to achieve mechanical stability in boiling methanol, Na+ form membranes had to be annealed at 275 Â°C for at least fifteen minutes. The SAXS data of annealed Na+ form membranes showed a dramatic decrease in crystalline order with annealing temperature, ultimately leading to the disappearance of the crystalline scattering peak after fifteen minutes at 275 Â°C. The onset of methanol stability with the melting of NafionÂ® crystallites suggests that chain entanglement is an important parameter in obtaining solvent stability. With respect to chemical stability, we performed studies aimed at examining the effects of Fentonâ s Reagent on the resistance to radical attack of new generation, dispersion-cast NafionÂ®. Changes in the 19F solid-state NMR spectra of dispersion-cast NafionÂ® before and after chemical degradation via Fentonâ s Reagent predicts a rather random attack by â ¢OH and â ¢OOH radicals. Several membranes were also thermally annealed between 100-250 Â°C in an attempt to correlate crystallinity with chemical degradation kinetics of NafionÂ® via Fentonâ s Reagent. The results indicate that the effect of counterion exchange into the Na+ form was minimal, but the degree of thermal degradation had a tremendous effect on the fluoride release rate and chemical degradation kinetics. By exchanging the membranes into the Na+ form, thermal degradation was avoided, allowing us to study the role of crystallinity as a function of fluoride release. Ultimately, NafionÂ® crystallinity was deemed an important factor in deterring peroxide radical attack. As the percent crystallinity decreased with annealing temperature, the fluoride concentration in the resulting Fentonâ s media increased accordingly, indicating that the amorphous regions of the polymer are more susceptible to chemical degradation via peroxide radical attack.
- Doctoral Dissertations