Surface Structure and Orientation of PTFE Films Determined by

the structural coordinates for bulk PTFE, and no adjustable physical parameters are in good agreement with the experimental NEXAFS spectra, showin...
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Langmuir 2002, 18, 2183-2189

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Surface Structure and Orientation of PTFE Films Determined by Experimental and FEFF8-Calculated NEXAFS Spectra Lara J. Gamble,†,‡ Bruce Ravel,§ Daniel A. Fischer,| and David G. Castner*,†,∧ National ESCA and Surface Analysis Center for Biomedical Problems, Departments of Bioengineering and Chemical Engineering, University of Washington, Box 3351750, Seattle, Washington 98195, U.S. Naval Research Laboratory, Code 6342, Washington, D.C. 20375, and National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received August 9, 2001. In Final Form: November 27, 2001 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 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 oriented parallel to the gold surface along the “rubbing” direction. FEFF8 NEXAFS spectra calculated with selfconsistent 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.

Introduction The orientation of surface-bound molecules (such as polymers, proteins, or DNA) can play an important role in the functionality of the surface-bound molecule and the properties of the surface. However, these types of molecules have complex three-dimensional structures. One technique that can determine the orientation of surface-bound molecules is near-edge X-ray absorption fine structure (NEXAFS). NEXAFS, also known as X-ray absorption near-edge structure (XANES), provides information about the bonding environment of surface atoms and can be used to determine the molecular orientation and ordering of surface species.1 In NEXAFS experiments polarized X-rays are directed onto a sample at various angles, and their energy is scanned through the atomic absorption edges. X-rays can be absorbed by the sample when the electric field vector overlaps (is parallel to) an unoccupied antibonding orbital, resulting in the transition of a core electron to that antibonding orbital. This absorption is observed as a peak in the spectrum specific for each * Address correspondence to the Department of Chemical Engineering, University of Washington, Box 3351750 Seattle, WA 98195. 206-543-8094 (phone). 206-543-3778 (fax). castner@nb. engr.washington.edu (e-mail). † Department of Bioengineering, University of Washington. ‡ Current address: Space Dynamics Laboratory, Utah State University, North Logan, UT 84341-1947. § U.S. Naval Research Laboratory. | National Institute of Standards and Technology. ∧ Department of Chemical Engineering, University of Washiongton. (1) Sto¨hr, J. NEXAFS Spectroscopy; Springer-Verlag: New York, 1992; Vol. 25.

antibonding orbital. NEXAFS has been used to determine the average orientation of molecular bonds and molecules themselves including self-assembled monolayers (SAMs) on gold,2,3 Langmuir-Blodgett monolayers,4 SAMs on oxides, and polymers such as (poly)tetrafluoroethylene (PTFE) and polyethylene.5-8 Models have been developed, by Stohr and Outka1,4,9 and Grunze et al.,2,10 to calculate the overall tilt (or orientation) of the molecular axis of all-trans hydrocarbon chains in Langmuir-Blodgett monolayers and SAMs. In such cases where the angle of the tilt is known, NEXAFS can also be used to calculate the amount of ordering of the SAMs molecules on the surface.11 The fluorocarbon polymer PTFE was chosen as a model helical structure to investigate the ability of NEXAFS to distinguish between three-dimensional structures. PTFE is known to have a helical conformation in its bulk form.12,13 (2) Ha¨hner, G.; Kinzler, M.; Thu¨mmler, C.; Wo¨ll, C.; Grunze, M. J. Vac. Sci. Technol., A 1992, 10, 2758. (3) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610. (4) Outka, D. A.; Sto¨hr, J.; Rabe, J. P.; Swalen, J. D. J. Chem. Phys 1988, 88, 4076-4087. (5) Castner, D. G.; K. B. Lewis, J.; Fischer, D. A.; Ratner, b. D.; Gland, J. L. Langmuir 1993, 9, 537-542. (6) Nagayama, K.; Sei, M.; Mitsumoto, R.; Ito, E.; Araki, T.; Ishii, H.; Ouchi, Y.; Seki, K.; Kondo, K. J. Electron Spectrosc. Relat. Phenom. 1996, 78, 375-378. (7) Ohta, T.; Seki, K.; Yokoyama, T.; Morisada, I.; Edamatsu, K. Phys. Scr. 1990, 41, 150-153. (8) Ziegler, C.; Schedel-Niedrig, T.; Beamson, G.; Clark, D. T.; Salaneck, W. R.; Sotobayashi, H.; Bradshaw, A. M. Langmuir 1994, 10, 4399-4402. (9) Sto¨hr, J.; Outka, D. A. Phys. Rev. B 1987, 36, 7891-7905. (10) Hahner, G.; Kinzler, M.; Woll, C.; Grunze, M.; Scheller, M. K.; Cederbaum, L. S. Phys. Rev. Lett. 1991, 67, 851-854. (11) Ågren, H.; Carravetta, V.; Vahtras, O.; Pettersson, L. G. M. Phys. Rev. B 1995, 51, 17848-17855.

10.1021/la011258l CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002

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The PTFE films investigated here were “rubbed” onto a gold-coated silicon wafer in the manner reported by Wittman et al.14 This has been shown to form highly orientated rows of PTFE. It is conceivable that this rubbing mechanism (which requires some melting of the PTFE) may stretch or deform the fluorocarbon chain from its reported bulk crystallographic structure. Theoretical calculations made with the FEFF8 modeling program15 were compared with experimental results to determine how NEXAFS spectra would vary with changes in the helical structure. The spatial parameters of PTFE were varied in the calculations to determine how changes in molecular conformation affect the NEXAFS spectra. The structural parameters varied included the number advances per rotation in the helix, the unit of advancement along the central axis between carbon atoms, the radial distance of the carbon and fluorine atoms from the central axis, and the F-C-F bond angle. A nonhelical (or zigzag) model was also compared to an all-trans alkanethiol chains on gold. Experimental Section Sample Preparation. The “rubbed” PTFE samples were prepared by heating Au-coated silicon wafers to ∼150 °C. A bulk PTFE block was drawn across the surface of the substrate under a mass of ∼1 kg and a speed of ∼1 mm/s.14 This produced a surface of long parallel (CF2)x chains with readily apparent grooves. X-ray Photoelectron Spectroscopy (XPS). The XPS experiments were carried out on a Surface Science Instruments S-probe spectrometer using a monochromatic Al KR X-ray source (hν ) 1486.6 eV). Binding energy (BE) scales for the monolayers on gold were referenced by setting the Au4f7/2 BE to 84.0 eV. The high-resolution C1s spectra were acquired with an analyzer pass energy of 50 eV. XPS elemental composition scans were acquired with 150 eV pass energy. At this pass energy the transmission function of the spectrometer was assumed to be constant.16 The peak areas were normalized by the number of scans, points/ electronvolt, Scofield’s photoionization cross sections,17 and sampling depth. The sampling depth was assumed to vary as KE0.7, where KE is the kinetic energy of the photoelectrons.16 High-resolution XPS C(1s) showed only CF2 carbon species with a BE of 292 eV were present. The F/C atomic ratio of 2.0 was determined from the areas of F(1s) and C(1s) peaks. Both of these results are consistent with the chemical composition of PTFE. Near-Edge X-ray Absorption Fine Structure (NEXAFS). The NEXAFS experiments were performed at the National Synchrotron Light Source (NSLS) U7A beamline located at Brookhaven National Laboratory, using an ∼80% polarized, highintensity beam. This beam line uses a monochromator with 600 l/mm grating for the carbon K-edge (C(1s)) providing a full-width at half-maximum resolution of ∼0.15 eV and a 1200 l/mm grating for the fluorine K-edge (F(1s)). The monochromator energy scale was calibrated by setting the C(1s) f π* transition in the graphite carbon K-edge NEXAFS spectrum to 285.35 eV.18 All NEXAFS spectra were normalized by the partial electron yield (I0) from an in situ gold-coated, 90% transmission grid placed in the incident X-ray beam. Partial electron yield (PEY) was monitored by a channeltron with the cone negatively biased (-100 eV). Samples were mounted to allow rotation about the vertical axis to change the angle between the sample surface and the incident X-ray beam. The NEXAFS angle is defined as the angle of the (12) Farmer, B. L.; Eby, R. K. Polymer 1985, 26, 1944. (13) Farmer, B. L.; Eby, R. K. Polymer 1981, 22, 1487-1495. (14) Wittmann, J. C.; Smith, P. Nature 1991, 352, 414. (15) Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Phys. Rev. B 1998, 58, 7565-7576. (16) Application Note; Surface Science Instruments: Mountain View, CA, 1987. (17) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129-137. (18) Morar, J. F.; Himpsel, F. J.; Hollinger, G.; Jordan, J. L.; Huges, G.; McFeely, R. Phys. Rev. B 1986, 33, 1346.

Gamble et al. Table 1. Structural Parameters Used in the FEFF8 Calculationsa param

15/7 helix

13/6 helix

planer “zigzag”

no. of advances no. of rotations unit of advance (Å) carbon R dist (Å) fluorine r dist (Å) fluorine z dist (Å) fluorine φ angle (deg)

15 7 1.3 0.397 1.564 0.076 43.1

13 6 1.3 0.428 1.647 0.076 43.1

2 1 1.3 0.417 1.3 0.076 43.1

a These parameters are taken from crystallographic data.12,13,19 See Figure 2 for a physical model representing some of these parameters.

incident X-ray beam with relation to the sample surface. The incident beam normal to the surface is defined as 90° while a glancing incident beam is generally 20° from the surface plane. The electric field vector (E) is perpendicular to the X-ray beam so when the beam is at normal incidence, the E vector lies parallel to the surface. The rubbing direction was maintained in the plane of the incident X-ray beam for the NEXAFS spectra reported here. NEXAFS spectra were also acquired with the rubbing direction perpendicular to plane of the incident X-ray beam (data not shown). The results from both orientations of the rubbing direction were consistent. FEFF8 Calculations. FEFF8 calculates NEXAFS spectra using self-consistent spherical muffin-tin potentials and a real space full multiple-scattering formalism. With minimal input consisting only of atomic species and coordinates within a cluster and no adjustable physical parameters, FEFF8 calculates the scattering contributions to the NEXAFS spectrum to all orders within the cluster. By projecting the full scattering matrix onto the direction of a polarization vector, FEFF8 calculates the orientational dependence of the NEXAFS spectrum, thus resolving the σ* and π* spectral resonances.15 In these experiments the “normal X-ray incidence”, or the E parallel to the backbone calculations, used theoretical light completely polarized along the helical axis. For the “glancing X-ray incidence” calculations the theoretical X-rays were circularly polarized about the helical axis. FEFF calculations used the structural coordinates of PTFE. Initial calculations used the structural coordinates for the 13/6 and 15/7 helical conformations of bulk PTFE.13,19 The 13/6 helical conformation was determined to be the stable structure below 19 °C while the 15/7 helical structure was found to be stable over 19 °C.19 These coordinates are listed in Table 1. Subsequently, the parameters were systematically varied to determine if a change in structure resulted in a change in the spectra. Coordinates for an “all-trans” fluorocarbon species were theorized on the basis of the coordinates for polyethylene.19 FEFF8 only approximates the absolute energy of the spectra, so the calculated spectra were shifted to 6 eV higher energy to more easily compare them to the experimental data. FEFF8 does, however, do a good job of calculating the relative energy of the peaks, and this has not been adjusted.15

Results and Discussion The experimental carbon K-edge NEXAFS spectra of “rubbed” PTFE on gold are shown in Figure 1 for incident photon angles of 90° (normal) and 20° (glancing). The sharp peak at 292.3 eV has been assigned to the C-F* absorption, and the peaks at 295.7 and 299 eV are assigned to the C-C* and another C-F* absorption, respectively, as has been previously reported for PTFE samples prepared with various treatments.5,6,8 FEFF8 simulations of the PTFE NEXAFS spectra used reported crystallographic data for bulk PTFE.12,13,20 These data are listed in Table 1 for two known helical structures of PTFE (the 15/7 and 13/6 helices). The 15/7 helix has (19) Tadokoro, H. Structure of Crystalline Polymers; John Wiley & Sons: New York, 1979. (20) Bunn, C. W.; Howells, E. R. Nature 1954, 18, 549.

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Figure 1. Carbon K-edge NEXAFS spectra of “rubbed” poly(tetrafluoroethylene) (PTFE) on a gold-coated silicon wafer at normal (90°) and glancing (20°) incident X-ray angles.

Figure 2. (a) Space-filling model showing a “zigzag” conformation of PTFE. (b) Space-filling model showing the 13/6 helical form of PTFE. (c) Structural representation of the spatial parameters used for the FEFF8 calculations. A dashed line depicts the central axis to which all parameters are referenced. The unit of advance (UOA) is the distance along a central axis between carbons. The carbon R and fluorine r parameters are the distances (radially) from the axis to the carbon and fluorine, respectively. (d) “Head-on” spatial model of the fluorocarbon molecules with the molecular axis depicted as the circle in the center. The fluorine Φ parameter is the angle of the fluorine atoms from the central axis (depicted by the dashed lines).

been reported to be the stable PTFE structure at room temperature (>19 °C), while the 13/6 helix is reported to be the stable form at lower temperatures.12,13 Also included in Table 1 are parameters for a “zigzag” form of PTFE similar to the structure seen for an all-trans hydrocarbon chain. While it has been reported that PTFE exists in a planer zigzag form at higher pressures, the parameters here were adopted from polyethylene crystallographic parameters.19 Space-filling models depicting the 13/6 helical form of PTFE and the “zigzag” structure are shown in Figure 2 a,b. The FEFF8 calculations use Cartesian coordinates to define the position of atoms in relation to each other. These coordinates, which are quantified for the different structures in Table 1, are referenced to an axis that passes through the center of the molecular chain. This axis is depicted by a dashed line in Figure 2c. Also shown in Figure 2c,d are the spatial representations of the parameters used in the FEFF8 calculations.

Figure 3. FEFF8 calculations for PTFE using the 13/6 helix parameters reported in Table 1. The number of CF2 groups used in the calculations is increased to determine the cluster sized required for convergence of the data. n ) 1 is one CF2 group and n ) 2 is one CF2 group on either side of the central group for a total of three CF2 groups. The n ) 3, 4, and 5 represent 2, 3, and 4 CF2 groups on either side of the central CF2 group making total cluster sizes of 5, 7, and 9 CF2 groups, respectively.

A series of FEFF8 calculations (using the 13/6 helix parameters) was performed to determine the minimum cluster size, or number of CF2 groups, required for the calculations to converge. Figure 3 shows carbon K-edge calculations at both normal and glancing X-ray incident angles for the series from just one central CF2 group (n ) 1) to four CF2 groups (n ) 5) on each side of the central atom being probed. Not surprisingly, there is no apparent structure with only one CF2 group. However, as the adjacent CF2 groups are added (n ) 2), structure is seen

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Figure 4. Experimental NEXAFS spectra of PTFE at normal and glancing incident X-ray angles compared to FEFF8 calculations for the 13/6 helix and the “zigzag” parameters found in Table 1. Experimental data for a methylhexadecanethiol (MHD) monolayer on gold is also included for comparison. The MHD data has been shifted to more readily compare it with the fluorocarbon data. Also, the MHD glancing and normal spectra have been switched to more easily compare with the rubbed PTFE data.

in the spectra, most noticeably in the normal incidence spectrum. The data appears to converge at n ) 4 to 5. As a result of this experiment, it was determined that clusters of five CF2 groups on each side of the central atom (or nine CF2 groups total) were sufficient to simulate NEXAFS spectra from PTFE chains. This calculation also indicates the distance that the photoelectron is probing the PTFE chain, which is related to the mean-free path of the photoelectron. Figure 4 shows the carbon K-edge results from calculations for the 13/6 helix and the zigzag fluorocarbon structures. The theoretical spectra were shifted by 6 eV, as mentioned in the Experimental Section. These simulated spectra are compared to experimental data for the rubbed PTFE and a self-assembled monolayer of methylhexadecanethiol (MHD) on gold. The MHD SAM is a monolayer of close packed all-trans (zigzag) hydrocarbon

Gamble et al.

chains oriented 35° from the surface normal. The MHD NEXAFS spectra are included for comparison to the alltrans (zigzag) theoretical calculations to determine if there are similarities due to similar structures. In Figure 4 the MHD data was shifted by ∼7 eV, and the normal and glancing MHD spectra have been exchanged in the figure to more readily compare to the PTFE data. There is a strong polarization dependence seen for the MHD spectra, similar to spectra previously reported for hydrocarbon SAMs2 and LB films.4 The lower energy peak (shown in Figure 4 at ∼295 eV but unshifted is at 287.9 eV) has been assigned to the transition from the C(1s) to the C-H*.2,4 This peak is usually enhanced when the X-ray beam is at normal incidence to the sample surface (θ ) 90°). The higher energy peak (shown at 300 eV but unshifted is at 293 eV) has been assigned to the transition to the C-C σ* orbital.2,4 This peak is normally enhanced when the X-rays are at glancing incidence. This indicates that the C-C σ* orbital has more of a perpendicular orientation. The zigzag FEFF8 calculation seen in Figure 4 does not compare well with the experimental PTFE data. However, the zigzag FEFF8 calculations show trends similar to the MHD experimental data. While the calculated peaks are much broader than the MHD experimental data, the parameters used for the zigzag calculations were adapted from fluorocarbon parameters and do not exactly correspond to the hydrocarbon atom positions. A sharper peak at ∼297 eV is seen to increase at glancing incidence calculations which parallels the trend expected for the C-F* peak of a “zigzag” PTFE, similar to the C-H or Rydberg transition peak for the MHD experimental data. In contrast, the 13/6 helix theoretical spectra closely reproduce the peaks and polarization dependence seen in the experimental PTFE data. A relatively sharp peak seen at ∼296 eV at normal incidence represents the C-C* absorbance. This peak loses intensity, and a peak at ∼293 eV (C-F*) is seen to increase at the “glancing incidence” calculations. A comparison of the FEFF8 calculations to the experimental data indicates that the calculations do predict trends seen in the NEXAFS experimental spectra such as relative peak positions and changes in peak intensities with changes in incident angle of the polarized X-rays. It appears, on the basis of these results, that the FEFF8 calculations can be used to identify and predict spectra for different three-dimensional structures on a surface. Specific parameters in the FEFF8 calculations were varied to predict how smaller structural changes in the fluorocarbon helix would affect the NEXAFS spectra. Parameters that were varied in these calculations included the units of advance versus rotation of the helix (helical periodicity), the angle of the F-C bond (Φ), and the distance of advancement of the CF2 units (UOA) (see Figure 2). The 13/6 helix (13 advances for 6 turns of the helix) was used as the starting point for changing these parameters. Some of the changes in the helical periodicity of the PTFE molecule showed dramatic differences in the resulting calculated NEXAFS spectra. The number of advances was changed from 13 down to 7 and up to 19 to give helix parameters of 7/6, 10/6, 13/6, and 19/6. This gives an effect of “unwinding” the helix. The 7/6, 10/6, 13/6, and 19/6 calculations are shown in Figure 5 for X-ray polarizations perpendicular and parallel to the fluorocarbon backbone. The 15/7 helix (the reported helical structure of PTFE at room temperature12,13) and the experimental data are also included for comparison. The almost completely “unwound” helix (7/6) shows little of the structural and polarizational dependence seen in the

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Figure 5. Experimental NEXAFS spectra of PTFE at normal and glancing incident X-ray angles compared to FEFF8 calculations for different helical conformations. See Table 1 for parameters of the 13/6 and 15/7 helices. For the other calculations all parameters were the same as for the 13/6 helix except for the number of advances versus rotations.

experimental data. As the helix is “coiled”, as seen in the 10/6 through 13/6 spectra, the C-F and C-C σ* peaks begin to sharpen and show the polarization dependence seen in the experimental PTFE spectra. As the helix begins to coil tighter (19/6), the calculations show peak positions once again moving to higher binding energies and peak widths broadening. The 13/6 and 15/7 show the best representation of the PTFE experimental data as expected, since they are the helical parameters determined from X-ray crystallography.20 In Figure 6 the PTFE molecule is “stretched” or “compressed” by changing the distance of advancement to the next CF2 species (not the C-C bond length). This is referred to as the unit of advancement (UOA) in Figure 6. The UOA ) 1.0 uses the parameters of the 13/6 helix reported in Table 1. A UOA of 1.1 “stretches” the molecule by increasing the unit of advance (1.3 Å) by a factor of 1.1, while a UOA of 0.9 “compresses” the helix by multiplying the UOA by 0.9. (See Figure 2c for UOA description.) Changing the unit of advancement does not appear to

Figure 6. Different unit of advances (UOA) compared with experimental NEXAFS spectra of PTFE at normal and glancing incident X-ray angles. All parameters except the unit of advance were the same as the 13/6 helix. The spectrum with a UOA of 1.0 in this figure uses the same parameters as the 13/6 helix spectra shown previously. See Table 1 for parameters of the 13/6 helix.

have a great affect on the glancing spectra. However, the normal calculations show a trend of the C-C* peak shifting to lower energies as the helix is stretched from a UOA of 0.9 to 1.1. Also there appears to be more of a separation between the C-F* and C-C* peaks in the normal angle calculations as the UOA is decreased. This indicates that C-C* peak position and C-C* to C-F* relative peak positions may provide information about the helical nature of the molecule. The fluorine Φ parameter (which is roughly half of the FCF bond angle) was varied initially without changing

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Figure 8. Experimental NEXAFS spectra of PTFE at normal and glancing X-ray incidence compared with FEFF8 calculations for varying carbon R distances. All parameters except the carbon R parameter were the same as the 13/6 helix. The spectra with the R ) 0.428 in the figure uses the same parameters as the 13/6 helix spectra shown previously. See Table 1 for parameters of the 13/6. As R gets smaller, the carbon chain becomes more collinear.

Figure 7. Phi (Φ) parameter varied and calculated spectra compared with experimental NEXAFS spectra of PTFE at normal and glancing incident X-ray angles. All parameters except the Φ angle were the same as the 13/6 helix structure. The spectra with Φ values of 43 in the figure use the same parameters as the 13/6 helix spectra shown previously. See Table 1 for parameters of the 13/6 helix.

any other structural parameter. (See Figure 2d for a physical representation of the Φ parameter.) Perhaps the most interesting result from this set of calculations is that when this angle is roughly commensurate with the helicity of the molecule (Φ ) 30), the F atoms from adjacent CF2 units line up in the axial direction. This has a very strong affect on the spectrum (see Figure 7). It may not be surprising that the C-F* peaks, strongest in the glancing incidence spectra, change with variation of the Φ parameter since the position of the fluorine atoms and C-F bonds are changing in the molecule. However, the C-C* peak,

which is most prominent in the normal incident spectra, also changes for different Φ parameters. The reason for this is not as clear since the C-C bonds do not change position in these calculations. Once again the parameters from the bulk crystallographic PTFE data produce the calculated spectra that best represent the experimental data. While most of the fluorine K-edge calculations did not show any noticeably different trends from the carbon K-edge data, the Φ parameter calculations did show an interesting feature. In the Φ ) 30 calculations there was a very strong π* peak in the fluorine spectrum at normal X-ray incidence. This peak is believed to be due to the lining up of CF2 units in the axial direction for this value of Φ, and the dominant contribution to this signal is believed to be the scattering back and forth between two of the fluorine atoms. It is, in effect, like a diatomic molecule that displays a strong π* shape resonance.

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In the final set of calculations reported here, the radial distance of the carbon from the molecular axis is changed (the carbon R distance). It is useful to note that at R ) 0 the carbon backbone would be completely linear. While this is physically unrealistic, it is interesting to note that a “straight” backbone shows a spectrum much different from that of the coiled helical backbone, as seen in Figure 8, at normal incidence where the C-C* peak is most prominent. However, there is minimal change in the glancing data where the CF* peaks are more prominent. As may be expected, the position and width of the C-C* peak appears dependent on the R value. As the R value goes toward 0, the carbon chain becomes nearly collinear and a high order multiple scattering contributes strongly to the spectra. Since the C-F* peaks are much more prominent than the C-C* peak in the glancing incidence spectra, this effect is not as noticeable. The FEFF8 calculations based on bulk PTFE structural parameters predict the NEXAFS spectra fairly well. As shown in Figure 4, a vastly different structure such as the “zigzag” fluorocarbon structure results in FEFF8-calculated NEXAFS spectra that are obviously different. The predicted spectra for the zigzag structure are more similar to those seen for the “all trans” MHD SAM. This gives confidence that the FEFF8 calculations are also properly predicting the spectral changes that occur when smaller changes in the helical nature of the PTFE are made. The FEFF8 calculations have indicated that peak positions, widths, and presence in NEXAFS spectra will change with changes in the helical nature of the PTFE. A comparison of the experimental data to the calculated spectra indicates the “rubbed” version of the PTFE still maintains most, if not all, of the same helical nature as bulk PTFE.

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Conclusions NEXAFS spectra of PTFE (both carbon and fluorine K-edges) were simulated using the FEFF8 calculations. Specific structural parameters were varied such as unwinding the helix, changing bond angles, and theoretically “stretching” the PTFE chains. Changing structural parameters showed, in some instances, significant changes in the NEXAFS spectra. These results help to better understand how changing the helical structure of a molecule will affect the NEXAFS spectra. Results indicate that the rubbed PTFE on gold has a helical structure similar to that reported for bulk PTFE at room temperature.12,13,19 The FEFF8 calculations showed that significantly changing the PTFE chains to a zigzag structure similar to that of hydrocarbon chains resulted in significant changes in the NEXAFS spectra. The effects of smaller changes in the helical structure of PTFE (e.g., F-C-F bond angle) on the NEXAFS spectra were also predicted from FEFF8 calculations. The combination of NEXAFS with FEFF8 calculations is a promising technique for determining the three-dimensional structure and orientation of molecules. Acknowledgment. L.J.G. and D.G.C. gratefully acknowledge support from UWEB (NSF Grant EEC9529161) and NESAC/BIO (NIH Grant No. RR01296 from the National Center for Research Resources). The NEXAFS studies were performed at the NSLS, Brookhaven National Laboratory, which is supported by the DOE, Division of Materials Science and Division of Chemical Sciences. LA011258L