David E. (David Edward) Budil
Max Diem (1947-), Mary Jo Ondrechen, Eugene S. Smotkin
Date of Award
Doctor of Philosophy
Department or Academic Unit
College of Arts and Sciences. Department of Chemistry and Chemical Biology.
block copolymers, direct methanol fuel cells, electron spin resonance spectroscopy, nafion, permselective materials
Proton exchange membrane fuel cells, Ionomers
The physiochemical phenomena of liquids in restricted systems are an area of continuing interest. Electron spin resonance (ESR) methods that use spin probes, small molecules with stabilized free radicals, are used to probe the microenvironment of a sample and to determine aspects of the solvent motion in the probe environment including microscopic ordering, local ordering and microviscosity. The scope of this thesis focuses on the effects of water and methanol on proton exchange membranes (PEMS), such as Nafion, because of their potential use in direct methanol fuel cells (DMFCs). These inquiries are also extended to other applications of these ionomer membranes, particularly as protective barriers for warfare agents.
ESR was first used to monitor the local environment of 2,2,6,6-tetramethyl-4-piperidone N-oxide (TEMPONE) spin probe in water and methanol mixtures in solution and in Li+ ion exchanged Nafion 117 membranes. This study was extended to Li+, Ca2+, and Al3+ ion exchanged Nafion 117 membranes swollen with methanol and water at varying solvent contents. Investigation of cation exchanged Nafion offers the possibility for fundamental studies of morphological changes caused by the ions in the membrane. With Nafion available as a comparison for alternative proton exchange membranes, a comparison of methanol and water in sulfonated poly(ether ether ketone) (SPEEK) , sulfonated Poly (arylene ether sulfone) (SPES), and Nafion was undertaken followed by a detailed study of methanol and dimethylmethylphophonate, a nerve agent simulant, in H+, Mg2+, Zn2+, Al3+, Ba2+, and Cs+ exchanged sulfonated poly(styrene-isobutylene-styrene) triblock copolymers (sSIBS).
The results of these studies indicate that in Nafion the local polarity of the nitroxide probe decreases with increasing methanol concentration, the microviscosity of the membrane aqueous phase as reflected by the rotational correlation time (τc) of the probe, was nearly two orders of magnitude longer in the membrane than in solution, and varied by an order of magnitude over the composition range studied and the probe exhibits significant local ordering in the aqueous phase of Nafion membranes that is diminished with increasing methanol concentration. In studies of solvent content in Nafion membranes water-containing membranes, trends were fit using the Fujita free volume diffusion model, which indicated that multivalent ions tend to increase the free volume fraction of the polymer while decreasing that of the solvent phase. It was observed that the difference in the trends of swelling between water and methanol indicate different patterns of penetration for the two solvents.
In the aromatic alternative PEMs, both SPES and SPEEK membranes show dual phase segregation. One phase observed in both aromatic membranes shows high polarity and fast rotational rate. This phase correlates with the methanol/water trends collected in Nafion. The Tempone probe in the second phase shows lower mobility and as methanol concentration increases, local polarity of the probe increases. These results can be interpreted as one domain represents interactions between probe and the non-polar membrane backbone, while the other represents the polar domain of ion solvation. In the presence of water, rotational rate of the probe is low. Two separate components were also observed in the sSIBS membranes at lower DMMP and methanol solvation levels. ESR spectra of DMMP/water mixtures, methanol/water mixtures and methanol and DMMP swelling of ion exchanged sSIBS membranes have shown very different trends than what has previously been observed in Nafion membranes. Trends in local polarity and mobility of the probes in these two components were analyzed to draw a picture of the membrane morphologies of the ionomer block copolymers.
Jamie Shaw Lawton
Lawton, Jamie Shaw, "Electron spin resonance investigations of ionomer membranes" (2009). Chemistry Dissertations. Paper 16. http://hdl.handle.net/2047/d20000066
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