| Abstract
| - Near-edge X-ray absorption fine structure (NEXAFS) experiments have provided information about theorientation of adsorbed small molecules, self-assembled monolayers, and polymers. Long fluorocarbonchains are known to have a twisted (or helical) structure due to the steric interactions of the fluorine atoms.Carbon K-edge and fluorine K-edge NEXAFS spectra of poly(tetrafluoroethylene) (PTFE) have beencalculated using FEFF8 to determine how changing the helical structure of the PTFE chains affects theNEXAFS spectra. Specific structural parameters varied in the calculations included unwinding the helix,changing bond angles, and theoretically “stretching” the PTFE chains. Changing these structural parametersresulted in changes in the calculated NEXAFS spectra. Experimental NEXAFS spectra were obtained atbeamline U7A of the NSLS on highly oriented, rubbed PTFE samples. A large polarization dependenceis observed at the fluorine K-edge for the C−F σ* peak and at the carbon K-edge for the C−F σ* peaks(at 292.3 and 299 eV) and the C−C σ* peak (at 295.7 eV), consistent with the fluorocarbon chains orientedparallel to the gold surface along the “rubbing” direction. FEFF8 NEXAFS spectra calculated with self-consistent spherical muffin-tin potentials, a full multiple-scattering formalism, the structural coordinatesfor bulk PTFE, and no adjustable physical parameters are in good agreement with the experimentalNEXAFS spectra, showing the fluorocarbon chains in the rubbed PTFE films have a helical structure.
|
