Theoretical and Experimental X-Ray Photoelectron Spectroscopy Investigation of Ion-Implanted Nafion EUFROZINA A. HOFFMANN, 1,2 ZOLTAN A. FEKETE, 1,2 LJILJANA S. KORUGIC-KARASZ, 1 FRANK E. KARASZ, 1 EUGENE WILUSZ 3 1 Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003 2 Department of Physical Chemistry, University of Szeged, Szeged, Hungary H-6701 3 U.S. Army Natick Soldier Center, Natick, Massachusetts 01760 Received 18 March 2003; accepted 13 June 2003 ABSTRACT: The membrane properties of a Nafion surface can be modified by ion implantation with N + or F + . The results are presented of an X-ray photoelectron spectroscopy (XPS) study of implanted surfaces. For the interpretation of the XPS spectra, calculations using a semiempirical quantum chemical formalism (AM1) have been applied, in conjunction with a charge–potential model, to predict the C 1s core electron binding energies. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 551–556, 2004 Keywords: semiempirical calculations; quantum chemistry; fluoropolymers; ESCA/ XPS; theory INTRODUCTION Nafion is a copolymer of tetrafluoroethylene and perfluoro[2-(fluorosulfonylethoxy)-vinyl]ether, 1 and it is widely used in fuel cells as an electrolyte membrane. This polymer has been a subject of interest because of its membrane properties, such as proton transport combined with chemical in- ertness. 2–16 Ion implantation can further modify the properties by developing barrier layers that are selectively permeable to water vapor. In the ion-implantation process, the target sur- face is bombarded by a beam of ions. In this tech- nique, the ion energy is typically 100 keV or more, at normal incidence to the surface; this results in penetration depths of hundreds of nanometers. In contrast, in ion etching, a related technique, the removal of one or more layers of atoms is achieved by ion beams hitting the substrate at nonnormal angles with energies in the range of 1–5 keV. This leads to penetration depths of a few nanometers and the transfer of collisional energy to target atoms. X-ray photoelectron spectroscopy (XPS) is a widely used method for surface analysis that can be applied to the analysis of polymer materi- als. 17–19 Although X-ray exposure can damage polymers, this problem becomes significant in perfluorinated polymers only after exposures of over 30 min. 20,21 Insulators pose a problem in this technique because of charge buildup, and so peak positions are usually assigned with respect to a known peak in the spectrum. The assignment of the peaks can be greatly aided by parallel theo- retical considerations. It has been recognized that partial atomic charges play a decisive role in determining the shifts of the binding energy (E b ). 22 Calculated charges have been used to correlate chemical Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 551–556 (2004) © 2003 Wiley Periodicals, Inc. 551