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Determination of Surface Structure and Orientation of Polymerized Tetrafluoroethylene Films by Near-Edge X-ray Absorption Fine Structure, X-ray Photoelectron Spectroscopy, and Static Secondary Ion Mass Spectrometry David G.Castner,*J Kenneth B. Lewis, Jr.,t Daniel A. Fischer,t Buddy D. Ratner,tJ and John L. Gland1 National ESCA and Surface Analysis Center for Biomedical Problems, Department of Chemical Engineering, BF-10, University of Washington, Seattle, Washington 98195, Center for Bioengineering, BF-10, University of Washington, Seattle, Washington 98195, National Institute of Standards and Technology PRT, Building 510E, Brookhaven National Laboratory, Upton, New York 11973, and Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055 Received June 1,1992. In Final Form: October 26,1992 Films of conventional and radio frequency glow discharge (RFGD) polymerized tetrafluoroethylene (TFE)were examined by ultrasoft X-ray absorption spectroscopy(XAS),X-ray photoelectron spectroscopy (XPS),and staticsecondaryion mass spectrometry(SIMS). The polarization-dependent intensity changes of transitions to C-C and C-F u* orbitals in the carbon and fluorine near-edge X-ray absorption fine structure ( N E W S ) spectra revealed different CFZchain orientations. The surface region of skived poly(tetrafluoroethy1ene) (PTFE)was composed of CFZchains oriented along the surface striationspresent in PTFE. XPS confiied only CF2 groups were present in the PTFE surface region. Fluorocarbon (FC) films prepared by RFGD deposition of TFE onto substrates placed directly in the visible glow (TFEI) were randomly oriented. XPS showed the TFE-I films had CF, CF2, and CF3 groups in the surface region. Static SIMS indicated that the TFE-I film surface contained CF3 and C2F5 groups. XPS showed the FC films prepared by RFGD deposition of TFE onto substrates placed downstream from the visible glow (TFE-11) contained -90% CFZgroups. The strong polarization dependence of the C and F NEXAFS spectra of these films indicated the CF2 groups were aligned in vertical chains on the substrate. Static SIMS and XPS results suggested the outermost surface of the CF2 chains are terminated with CF3groups. For thin (SMOOA) FC RFGD films deposited onto polymeric substratessuch as poly(methy1methacrylate) or poly(ethy1eneterephthalate),fluorescence yield detection XAS could be used to examine the substrate, while XPS, static SIMS, and electron yield detection XAS could be used to examine the FC overlayer. These results demonstrate the complementarynature of ultrasoftXAS, XPS, and static SIMS for detailed surface structural characterization of polymers.
Introduction The surface properties of a polymeric biomaterial determine important biological responses such as blood compatibilityand tissue These responsesare mediated by the amount, binding strength, and conformation of adsorbed proteins. The surface orientation of molecules and chains in the surface layer can affect the adsorption and organization of species on that surface. For example, an ultrathin film of highly oriented poly(tetrafluoroethylene) (PTFE) has been used to produce a wide range of oriented materials (polymers, monomers, liquid crystals, small inorganic molecules, etc.) from solutions, melts, and vapor^.^ Such a surface may be expectedto organize protein molecules adsorbed to it. The organized protein molecules may, in turn, mediate unique biological reactions. To gain an understanding of the biological responses generated by a polymeric biomaterial, it is important to
obtain a detailed characterization of its surface stoichiometric composition (e.g., atomic percent fluorine), function group types (e.g., CFd, and orientation (e.g., alignment of CF2 groups in a specific direction). Ultrasoft X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and static secondary ion mass epectrometry (SIMS) are complementary techniques that can be used to obtain such a surface characterization. XAS provides information about the orientation, electronic structure, and short-range geometric structure of a materiaLs X P S furnishes information about the elemental and functional group composition of the outer -60 A of a material.&-8Static SIMS generates a mass spectrum of the outer -20 A of a materiaL9J0 In this study, the surface structure and orientation of PTFE and tetrafluoroethylene (TFE) films deposited by radio frequency glow discharge (RFGD) onto substrates placed either directly in the visible glow (TFEI) or
To whom correspondence should be addressed. Departmentof ChemicalEngineering,Universityof Washington. Center for Bioengineering, University of Washington. Brookhaven National Laboratory. * University of Michigan. (1) Andrnde, J. D.; Nagnoka, S.; Cooper, S.; Okano, T.; Kim, S. W. Tram. Am. Soc.Artif. Xntem. Organa 1987,33,76and referencestherein. (2) Ratner, B. D.;Johnston, A. B.; Lenk, T. J. J. Biomed. Mater. Res.: Appl. Biomater. 1987,21 (Al), 59 and references therein. (3) Ratner, B. D.; Caetner, D. C.; Horbett, T. A.; Lenk, T. J.; Lewis, K. B.; Rapoza, R. J. J. Vac. Sci. Technol., A 1990,8,2306 and references therein. (4) Wittmann, J. C.; Smith, P. Nature 1991,352, 414.
(5) X-ray Absorption: Principles,Applications, Techniques of EXAFS, SEXAFS,and XANES; Koningeberger,D.C.,Prins, R., Eds.;Wiley Interscience: New York, 1988. (6) Andrade, J. D. In Surface and Interfacial Aspects of Biomedical Polymers; Andrnde, J. D.; Ed.;Plenum Press: New York, 1986; p 106. (7) Rntner, B. D.; McElroy, B. J. In Spectroscopy in the Biomedical Sciences;Gendreau, R. M., Ed.;CRC Press: BocaRaton, FL, 1988, p !07. (8) Brigm,D., Seah,M. P., Eds. Practical Surface A ~ l y ~ iJohn s ; Wiley & Sona: Chicheater, 1983. (9) Henrn, M. J.: Briees, -- . D.:Yoon, S. C.: Ratner. B. D. Surf. Interface Anal. 1987,10,384. (10) Briggs, D.; Brown, A.; Vickennan, J. C. Handbook of Static Secondary Ion Mass SDectrometrv (SXMS): John Wiley & Sone: Chichest&, 1989.
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downstream from the visible glow (TFE-11)were examined. These filmshave different protein adsorption propertie8.l1 In particular, the strengthof albumin binding, as measured by the fraction retained after surfactant elution, increases as PTFE C TFE-I C TFE-11." The surface structure and orientation results obtained in this study offer an explanation for the different strengths of albumin binding on these materials.
Experimental Section X-ray Absorption Spectroscopy. The XAS experiments were done on beamline U1A at the National Synchrotron Light Source (NSLS)located at BrookhavenNational Laboratory. This beam line uses an extended range grasshopper monochromator that was set for a full width at half-maximum resolution of -0.5 eV at the carbon K edge and 1eV at the fluorine K edge. The monochromator energy scale was calibrated by setting the peak for the C1. r* transition in the graphite carbon K near-edge X-ray absorption fine structure (NEXAFS) spectrum to 285.35 eV.12 The positions of the intensity minima in the incident X-ray beam due to absorbed carbon on the beamline optics were then calibrated relative to the graphite r* energy and used as an internal energy reference for all carbon NEXAFS spectra. All NEXAFS spectra were normalized by the photocurrent from a gold-coated,90% transmission grid placed in the incident X-ray beam. The fluorescence yield (FY) NEXAFS spectra were also normalized by the signalfrom a Au control sample. Total electron yield (EY) spectra were acquired by placing a positive bias on the cone of a channeltron to collect all the secondary electrons, photoelectrons,and Auger electrons. The totalfluoreecenceyield spectra were acquired by using a specially designed proportional
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X-ray PhotoelectronSpectroscopy. The XPS experiments were done on a Surface Science Instruments X-probe spectrometer. This system has a monochromatic Al Ka X-ray source (hv = 1486.6 eV), hemispherical analyzer, and resistive strip, multichannel detector. A low-energy (- 5 eV) electron gun was used for charge neutralization on the nonconducting samples (e.g., PTFE). The binding energy (BE) scales for the fluorocarbon (FC) samples were referenced by setting the CFZpeak maxima in the C1, spectra to 292.0 eV. The high-resolution C1, spectra were acquired at an analyzer pass energy of 25 eV and an X-ray spot size of lo00 pm. XPS elemental compositions of samples were obtained using a pass energy of 150 eV. At this pass energy the transmission function of the spectrometer was assumed to be ~0nstant.l~ The peak areas were normalized by the number of scans, points per electronvolt, Scofield's photoionization crosssections,16andsamplingdepth. The samplingdepth was assumed to vary as KEF7 where KE is the kinetic energy of the photoelectron^.'^
Static Secondary Ion Mass Spectrometry. The static SIMS experiments were done using a Balzers 511 quadrupole and Leybold-Heraus ion gun mounted on the X-probe vacuum system. A 3.&keV, 0.5-nAbeamof Xe+ ions was used to generate the secondary ions. A 5 x 5 mm analysis area was obtained by a combination of defocusingand rastering the incident Xe+beam. Total analysis time per sample was
