EFFECTS OF HYDROPHILIC AND HYDROPHOBIC COUNTERIONS ON THE COULOMBIC INTERACTIONS IN PERFLUOROSULFONATE IONOMERS

Attenuated total reflectance FTIR spectroscopy was used to compare the influence of hydrophilic and hydrophobic counterions on the vibration al behavior of the sulfonate and perfluoroether groups in 1100 equival ent weight (EW) Nafion(R) and 808 EW Dow perfluorosulfonate ionomer me mbranes. 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 cont rast, neutralization with tetrabutlyammonium ions caused the - SO3- vi brational band to shift to lower frequencies with increasing neutraliz ation. 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. Vibrat ional bands attributed to perfluoroether groups in close proximity to the sulfonate groups were also found to be influenced by Coulombic int eractions within the clusters. The frequency shifts of these bands fol lowed a trend that was virtually identical to that observed for the - SO3- symmetric stretch. This behavior is attributed to a combination o f through-space, dipolar field effects, and through-bond inductive eff ects, from the neighboring ionic groups. In Nafion(R), the ether group s directly attached to the backbone are shielded from the Coulombic in teractions within the clusters, while the ether groups within three bo nds of the terminal ionic groups are very sensitive to the state of po larization of the sulfonate anion. (C) 1995 John Wiley & Sons, Inc.