Effects of Hydrophilic and Hydrophobic Counterions on the Coulombic Interactions in Perfluorosulfonate Ionomers

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Polymers and High Performance Materials


Attenuated total reflectance FTIR spectroscopy was used to compare the influence of hydrophilic and hydrophobic counterions on the vibrational behavior of the sulfonate and perfluoroether groups in 1100 equivalent weight (EW) Nafion(R) and 808 EW Dow perfluorosulfonate ionomer membranes. For dry, Na+-form membranes, the infrared band attributed to the - SO3- symmetric stretching vibration was found to shift to higher frequencies with an increase in the degree of neutralization. In contrast, neutralization with tetrabutlyammonium ions caused the - SO3- vibrational band to shift to lower frequencies with increasing neutralization. This behavior is attributed to a diminished polarization of the - SO3- groups by the hydrophobic and diffusely charged TBA(+) cations, relative to the b strong polarization observed with Na+ ions. Vibrational bands attributed to perfluoroether groups in close proximity to the sulfonate groups were also found to be influenced by Coulombic interactions within the clusters. The frequency shifts of these bands followed a trend that was virtually identical to that observed for the - SO3- symmetric stretch. This behavior is attributed to a combination of through-space, dipolar field effects, and through-bond inductive effects, from the neighboring ionic groups. In Nafion(R), the ether groups directly attached to the backbone are shielded from the Coulombic interactions within the clusters, while the ether groups within three bonds of the terminal ionic groups are very sensitive to the state of polarization of the sulfonate anion. (C) 1995 John Wiley & Sons, Inc.

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Journal of Polymer Science Part B: Polymer Physics





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