Exafs Analysis of Ionomers
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EXAFS ANALYSIS OF IONOMERS Y.S. DING, H.K. PAN*, R.A. REGISTER, AND S.L. COOPER Department of Chemical Engineering University of Wisconsin Madison, WI 53706 ABSTRACT Extended x-ray absorption fine structure (EXAFS) spectroscopy has been successfully applied to the analysis of the ionic domain microstructure in In dehydrated zinc neutralized zinc and rubidium neutralized ionomers. ionomers, the zinc cation was found to be tetrahedrally coordinated to oxygen atoms. For ionomers with hydrocarbon backbones, these oxygen atoms may For ionomers with originate from the sulfonate or carboxylate groups. perfluorinated backbones, some of the oxygen atoms may be ascribed to water In fully hydrated zinc neutralized ionomers, the coordination as well. zinc cation exists as Zn( H?0) 6 +. In contrast, the fully hydrated monovalent rubidium neutralized ionomers exhibited a highly disordered environment about the cation. 1.
INTRODUCTION
A small amount of ionic functionality, either as pendant groups or in the main chain, profoundly alters a polymer's physical properties.
Research on the microstructure of these polymers, termed ionomers, has been extensive over the past decade [1,2]. Some of the systems which have been investigated are the Nafionse (fluorocarbon/perfluorosulfonic acid copolymers), sulfonated polystyrene (styrene/p-sulfonatostyrene copolymer), It is generally and Surlyn* (ethylene/methacrylic acid copolymer). recognized that aggregation of the ionic species occurs in these polymers, which gives rise to their superior physical and unusual transport properties. A better understanding of the microstructure of ionomers is therefore desirable and is expected to lead to the development of new materials. Extended x-ray absorption fine structure (EXAFS) has proven very useful for microstructural investigations. Examination of the local ionic aggregate structure by EXAFS has indicated a number of differences among ionomer systems [3-6]. The systems investigated included monovalent and divalent cation neutralized ionomers of both the sulfonated and carboxylated types. 2.
EXPERIMENTAL
2.1. Samples The Nafione perfluorosulfonic acid materials have the structure: -+CF2 - CF2 -3ffCF - CF2--+-m
[0 - CF2 - CF-+-z 0 - CF2 - X CF3
X = COOH or CF 2 SO3 H
Current address: Xerox, Inc., Webster Research Center, Webster, NY 14580 * Trademark of E. I. DuPont de Nemours *
Mat. Res. Soc. Symp. Proc. Vol. 79. 1987 Materials Research Society
168
Membranes of Nafion® with equivalent were neutralized by soaking the membranes tate or rubidium hydroxide. Measurements and on samples which were boiled in water. Lightly sulfonated polysytrene (SPS)
÷-F-CH2 -C --
ECH2
weights of 1100, 1200, and 1500 in aqueous solutions of zinc acewere made on vacuum dried samples has the chemical structure:
- CH-
S03 H Four SPS samples with mole fractions of sulfonated units of 1.7%, 3.4%, 5.6% and 6.9% were studied. The solvent used for dissolving the acid SPS was tetrahydrofuran or a toluene/DMSO mixture. The polymers were neutralized with zinc acet
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