AVS 46th International Symposium
    Applied Surface Science Division Thursday Sessions
       Session AS-ThA

Paper AS-ThA7
Determination of Helical Conformation Effects in PTFE NEXAFS Spectra with FEFF8 Calculations

Thursday, October 28, 1999, 4:00 pm, Room 6A

Session: Polymer Surfaces, Films, and Interfaces
Presenter: D.G. Castner, University of Washington
Authors: D.G. Castner, University of Washington
L. Gamble, University of Washington
D.A. Fischer, National Institute of Standards and Technology
B. Ravel, National Institute of Standards and Technology
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Near edge X-ray absorption fine structure (NEXAFS) experiments can provide information about the orientation of adsorbed small molecules, self-assembled monolayers and polymers. Long fluorocarbon chains are known to have a twisted (or helical) structure due to the steric interactions of the fluorine atoms. FEFF8 calculations of the C K-edge and F K-edge spectra of poly(tetrafluoroethylene) (PTFE) have been done to determine how changing the helical structure of the PTFE chains affects the NEXAFS spectra. Experimental NEXAFS spectra were obtained at beamline U7A of the NSLS on highly-oriented PTFE samples prepared by rubbing a block of PTFE across a gold-coated silicon wafer heated to 150°C. The helical chains in this well-ordered film are oriented parallel to the gold surface and along the rubbing direction. A large polarization dependence is observed at the F K-edge for the C-F @sigma@* peak and at the C K-edge for the C-F @sigma@* peaks (at 292.6eV and 299eV) and the C-C @sigma@* peak at (296.4eV). FEFF8 NEXAFS spectra calculated with self-consistent spherical muffin-tin potentials, a full multiple-scattering formalism, the structural coordinates for bulk PTFE, and no adjustable physical parameters are in good agreement with the experimental NEXAFS spectra. Variation of the F-C-F bond angle in the calcuated spectra showed that when this angle is commensurate with the helicity of the chains, F atoms from adjacent CF2 groups become aligned in the axial direction. This alignment results in a large change in the NEXAFS spectra. Calculations that varied the radial distance of the carbon atom from the molecular axis showed that a "straight" backbone produces significantly different NEXAFS spectra than a helical backbone. Further calculations show that "stretching" and "compressing" the helical chain can also effect the widths, positions, and intensities of peaks in the NEXAFS spectra.