Title

Morphological Manipulation of Ionomers for Altered Transport Properties in Membrane Applications

Date of Award

2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Polymers and High Performance Materials

First Advisor

Robert B. Moore

Advisor Department

Polymers and High Performance Materials

Abstract

The focus of this research project was to investigate correlations between ionomer membrane morphology and swollen state transport properties. The project can be broken into two primary foci, the impact of processing and additives on the morphology of perfluorosulfonate ionomers and the impact of morphology on the performance of ionic actuators. Perfluorosulfonate ionomers, Nafion ® from DuPont being the most common example, are a class of commercially successful semi-crystalline ionomers that have been used in a number of membrane applications, the primary two being chlor-alkali cell and proton exchange membrane fuel cells. It has been shown that solution and melt processing of Nafion ® alters the membrane morphology and profoundly impacts the swelling characteristics and water transport properties of the membrane. The enhanced mobility present in both solution and melt facilitate increased crystallinity relative to the as-received state. Processing also causes a change in the spatial order of ionic aggregates, the average interaggregate spacing decreasing in both solution and melt processed membranes. Processing was also found to be an effective means to produce highly aggregated materials as the stated mobility during processing facilitates rearrangement and aggregation of the neutralized sulfonate functionalities. Of greatest interest, however, is the increase in water uptake seen in processed samples. Melt processing was found to increase water content by 60% and solution processing by 80% relative to as-received ionomer. As a result, water diffusion in processed materials was found to be much higher than as-received. Only solution processed membranes, however, were found to show high proton conductivity due to enrichment of surface water content through morphological changes which present a surface that is more readily hydrated. Manipulation of solution processed membrane morphology was further explored by producing membranes containing a mixture of alkali metal and alkylammonium counterions. Analysis of the mechanical properties and aggregation behaviors suggest the existence of mixed ionic aggregates containing both sodium and tetrabutylammonium counterions. Partitioning of counterions within the aggregates and alteration of the crystalline content are proposed to cause the appearance of a new correlation length intermediate to the lamellar long spacing and the ionomer peak. The unique morphology causes a minimum in water content and a corresponding maximum in proton conductivity that reinforces the link between membrane morphology and swollen state transport properties. (Abstract shortened by UMI.)