Surface structure and orientation of PTFE films determined by experimental and FEFF8-calculated NEXAFS spectra

Near-edge X-ray absorption fine structure (NEXAFS) experiments have provided 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. Carbon K-edge and fluorine K-edge NEXAFS spectra of poly(tetrafluoroethylene) (PTFE) have been calculated using FEFF8 to determine how changing the helical structure of the PTFE chains affects the NEXAFS spectra. Specific structural parameters varied in the calculations included unwinding the helix, changing bond angles, and theoretically "stretching" the PTFE chains. Changing these structural parameters resulted in changes in the calculated NEXAFS spectra. Experimental NEXAFS spectra were obtained at beamline U7A of the NSLS on highly oriented, rubbed PTFE samples. A large polarization dependence is observed at the fluorine K-edge for the C-F sigma* peak and at the carbon K-edge for the C-F sigma* peaks (at 292.3 and 299 eV) and the C-C sigma* peak (at 295.7 eV), consistent with the fluorocarbon chains oriented parallel 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 coordinates for bulk PTFE, and no adjustable physical parameters are in good agreement with the experimental NEXAFS spectra, showing the fluorocarbon chains in the rubbed PTFE films have a helical structure.