Langmuir 1991, 7, 2266-2272
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Surface Analysis of Polycarbonate Thin Films by High-ResolutionElectron Energy Loss Spectroscopy: Negative Ion Resonances and Surface Vibrations G. Apai'Bt and W. P. McKenna*f* Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 Received January 2, 1991. In Final Form: May 1, 1991 High-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy are used to examine thin polycarbonate films. While the HREEL vibrational spectra for thin films exhibit bands that correlate well with known infrared vibrational modes of bulk bisphenol-A-polycarbonate,there are significant intensity differences that are attributed to different vibrational excitation mechanisms, surface sensitivity,and temporary negative-ion resonance effects. Severalvibrationalmodes showsignificant intensity variations with primary beam energy. Enhancement of carbonate vibrational features at -3.5 eV and CH stretching (aromaticand aliphatic) related vibrational modes at -4.5 and -5.0 eV, respectively, indicate electron interaction with the polymer surface via temporary negative-ion resonances. Annealing of the polymer thin film at the glass transition temperature enhances the intensity of the aromatic CH stretching mode with respect to the intensity of the aliphatic CH stretching mode. This enhancement is a reflection of increasing aromaticity at the polycarbonate surface. No evidence of radiation damage from the electron beam or decomposition due to annealing in ultrahigh vacuum is detected.
Introduction Many of the physical and chemical properties of polymeric materials depend on the structure, bonding, and morphology at the interface region. Thus, a fundamental understanding of this region is important. For example, polymer surfaces can influence interface properties important for adhesion, lubrication, wetting, and surface reactivity.' A knowledge of thin-film structures and long-range orientation of organic films is also of considerable importance. Recently, the high-resolution electron energy loss spectroscopy (HREELS) technique has provided a unique insight into the nature of polymer surfaces, demonstrating both surface sensitivity and spectroscopic vibrational differences between the surface and the The extreme surface sensitivity of the technique offers an opportunity to study polymer conformation and the nature of molecular groups existing preferentially at the surface. It should offer a powerful complementary technique to the vacuum techniques already used in polymer studies, such as ultraviolet photoelectron spectroscopy (UPS) and monochromatized X-ray photoelectron spectroscopy (XPS). Pireaux e t a1.2 have used HREELS to study vacuumevaporated hexatriacontane thin films and bulk slabs of polyethylene using charge neutralization. The majority of the observed loss features were assigned based upon comparisons with transmission infrared data from bulk samples. For polyethylene, they observed enhanced methyl group vibrational intensity indicating that the polymer surface differs from the bulk and that HREELS provides an extremely sensitive probe of surface morphology and + Corporate Research Laboratories. t Analytical Technology Division.
(1) Swalen, J. D.; et al. Langmuir 1987,3, 932,panel report for the Materiale Research Division of the Department of Energy. (2)Pireaux,J. J.;Thiry,P. A.; Caudano, R.; Pfluger, P. J.Chem. Phys. 1986,84, 6452. (3)Jenninga, W. D.; Chottiner, G. S.; Natarajan, C.; Melo, A. V.; Hoffman, R. W.; OGrady, W.E.; Lundstrom, I.; Salaneck, W. R. Appl. Surf. Sci. 1986.,~ 21., 80. -('4) Pireaux, J. J.; Gregoire,C.; Vermeerach, M.; Thiry, P. A.; Caudano, R. Surf. Sci. 1987,189/190,903. (5) Jones, T. S.; Ashton, M. R.; Richardson, N. V.; Unertl, W. N. J. Phys. Condena. Matter 1989,I , SB139. (6) Wandass, J. H.; Gardella, J. A., Jr. Langmuir 1987,3, 183.
chemical structure. Wandass and Gardella6 have also demonstrated extreme surface sensitivity for HREELS of Langmuir-Blodgett films. They observed an unusual energy dependence of the CH stretching vibrational mode and attributed it to possible resonance scattering effects. In this work, we analyze the HREELS vibrational losses of thin polycarbonate films on highly oriented pyrolytic graphite (HOPG) acquired at several primary beam energies. HREELS surface spectra acquired in the specular direction from thin films are compared with bulk transmission IR spectra of identical polycarbonate material. Analysis of the HREELS intensity variations is shown to provide information about the surface conformation and the energy loss mechanisms. Symmetryrelated temporary negative-ion resonance effects appear to be responsible for enhancements of specific group vibrational modes at particular electron beam energies.
Experimental Section The analysis of insulating polymer surfaces with low-energy electrons is possible if charging effects are eliminated. Two methods have been used: (1) thin films (e.g., 5200 A thickness) depositedon a conductingsubstrateor (2) flood gun neutralization of bulk polymer slabs. We have chosen the first technique with the idea that smoother surfaces may be obtained in this manner and there is no need to be concerned with surface decomposition from high-energy electrons used in the flood gun neutralization process. Thin films (-75-100 A) were dip cast onto highly oriented pyrolytic graphite (HOPG)surfaces from spectroscopicallypure methylene chloride or dioxane at solution concentrations of approximately 5 X 104 M. The choice of oriented pyrolytic graphitewas based upon several criteria: (1)ita excellent elastic scattering due to the smooth,clean surfacesresultingfrom freshly cleaved samples; (2) the inert nature of the material; and (3)the featureless HREELS spectrum permitting background subtraction for the thinnest films. Thin polymer films on HOPG were studied by HREELSusing the Vacuum Generators LEEL 400 spectrometer described pre~iously.~ Sample heating was carried out with a modified transfer assembly compatible with the VG system.7 Thus, the sample temperature was precisely monitored while maintaining the ability to transfer into and out of vacuum. Spectra were (7)Frederick, B.G.;Apai, G.;Rhodin, T. N. Surf. Sci. 1991,244,67.
0743-7463/91/2407-2266$02.50/0 0 1991 American Chemical Society
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Surface Analysis of Polycarbonate Thin Films
acquired in specular geometry with a scattering angle of 90° at room temperature. Beam energies were varied between 2and 12 eV kinetic energy with resolutions of 70-80 cm-'. The electron current at the sample was approximately A/cmZ and no chargingor degradation effects were detected. Experiments were carried out in an ultrahigh vacuum system at pressures mbar. X-ray photoemission measurements of the samples were made by using an A1 K a monochromatized source. Film thickness was determined at normal emission using the 0 1s and C 1s peak areas. Inelastic mean free paths were calculated from Seah's empirical analysis data for elements and organic compounds.s n VI As detailed previously? the total cross sections incorporated the c E photoionization cross section,1° the &asymmetry parameter," 3 and the analyzer transmission function ( The polycarbonate material used was from Scientific Polymer v 0 295 290 285 280 Products or Mobay Co. (Makrolon-5705). The polymer was purified by solubilization in methylene chloride followed by x c filtration and precipitation by acetone. This procedure removed VI E possible traces of excess bisphenol-A and lower molecular weight 9) c fractions. The resulting polymer material was analyzed by sizeE exclusion chromatography (SEC) in methylene chloride by using a refractive index detector. The measurements were calibrated with narrow molecular weight distribution polystyrene standards between 595 and 2 750 OOO g/mol. Before treatment with acetone the polycarbonate sample as received had a number average molecule weight of 20 500 with a molecular weight distribution of M w / M n= 2.71. Following treatment the number average molecular weight was 35 000 with Mw/M, = 1.64. This is indicative of a reasonably narrow molecular weight distribution of polycarbonate in the material used for the surface experiments. Clean polymer films were examined with both monochromatized XPS (- 100 A thickness) and transmiasion IR spectroscopy (microme ter thick films) and were found to be free of impurities within 540 535 530 525 detection limits. The transmission IR spectrum for the thick Binding energy (eV) film was used for comparison with HREELS spectra of thin films. The monomer unit of bisphenol-A-polycarbonate or poly[oxyFigure 1. C 1s and 0 1s core level spectra for a sample consisting carbonyloxy-1,4-phenylene(l-methylethylidene)-l,4-phenyl- of 100 A bisphenol-A-polycarbonate film on HOPG (highly ene] is oriented pyrolytic graphite). The peak assignments are listed according to the text.
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Rssults Initial characterization of the polymer was carried out by using X-ray photoemission and transmission infrared spectroscopies. For thin films XPS employing a monochromatized A1 K a source was used to confirm the identity of the polymer. Figure 1 shows the C 1s and 0 1s core levels for a typical olycarbonate thin film on HOPG, estimated to be 175 thick. Studies of model compounds and polymen12J3have provided characteristic peak binding energies for various functional groups in polymers. Therefore the peaks in the C 1s spectrum (Figure 1A) can be assigned as (a) neutral hydrocarbon (-CH,-, 284.6 eV), (b) carbon singly bonded to oxygen at 286.2 eV (4O(C4)O-C-), (c) carbonate carbon at 290.6 eV (40(C=O)OC-), and (d) at -292.0 eV the broad m*shakeup peak from the aromatic rings. The two peaks in the 0 1s spectrum (Figure 1)are from the carbonyl oxygen at 532.6 eV and from the oxygen singly bonded to carbon at
A:
(8) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979,1, 2. (9) Apai, G.; Frederick, B. G. Langmuir 1987,3,395. Frederick, B. G.; Apai, G.; Rhodin, T. N. J. Am. Chem. SOC. 1987,109,4797. (10) Scofield,J. H. J. Electron Spectrosc. Relat. Phenom. 1976,8,129. (11) Reilman, R. F.;Msezane,A.; Manson, S.T. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 389. (12) Clark, D. T.; Thomas, H. R. J. Polym. Sci. 1978,16, 791. (13) Dilks, A. Electron Spectroucopy Theory, Techniques and
Applicotiom Brundle, C. R., Baker, A. D., Eds.; Academic Press: New York, 1981; Vol. 4, p 277.
534.3 eV. From a determination of the areas of these peaks in a film thick enough not to be complicated by the underlying pyrolytic graphite signal, the internal stoichiometries among elements and functional groups are in good agreement with the known structure of bisphenolA-polycarbonate. The three carbon moieties are present in a 13:2:1 ratio and the two oxygen moieties have a 2:l ratio as expected based on the bisphenol-A-polycarbonate stoichiometry. The ratio of oxygen to carbon in the films was also as expected. Each stoichiometric ratio was within *t5 ?4 of the theoretical value. In Figure 2, we show the IR spectrum obtained from a thick film (several micrometers) of bisphenol-A-polycarbonatecoated out of methylene chloride. The spectrum was run at 4-cm-l resolution. All IR bands are indicative of pure polycarbonate samples. Thus, both thick and thin samples are representative of clean preparations. The HREELS spectra of polycarbonate on HOPG exhibit the same characteristic bands as produced in the IR spectrum with the exception that the resolution is degraded and the characteristic band intensities are significantly different. The raw data for thin film polycarbonate on a HOPG substrate, acquired at two different primary beam energies,are shown in Figure 3B. The films were cast from methylene chloride; however, use of dioxane solvent produced identical spectra. Using thin films on HOPG, it is possible to obtain adequate signal-to-noise ratios and sufficient resolution to correlate most of the major features in the IR spectrum. Our assignments of HREELS features are based on the data presented here and more extensive data not shown. The frequencies are characteristic of this polymer and correlate well with the
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2268 Langmuir, Vol. 7, No. 10,1991
reference IR spectrum. A number of the characteristic bands for bisphenol-A-polycarbonate have been assigned.+'Q The normal mode coordinate analysis of the
carbonate group in the polymer chain has also been reported.m Details of the assignments for major features are given below and in Table I. For comparison in Figure 3A, we have Gaussian broadened (fwhm = 80 cm-l) the infrared spectrum of Figure 2 to correspond to the instrumental resolution of the HREELS spectra. A comparison of spectra without this broadening would be unrealistic. A summary of the major spectral features for the HREELS spectra follows: (1) The lowest frequency features in the HREEL spectrum occur at -410,500,550, and 630 cm-l. All agree with IR frequencies; the 410- and 510-cm-l peaks can be attributed to deformation modes of the carbonate group while the 550-cm-l peak is a ring "out-of-plane" skeletal deformation mode of a para-substituted benzene ring. The -630-cm-1 feature is attributable to an in-plane ring deformation. (2) The aromatic CH out-of-plane deformation mode at 820 cm-I is more prominent in HREELS than in IR. (3) The features at 980-1020 cm-l, more intense at 5-eV than at 3-eV beam energy, are attributed to the symmetrical 0-C-0 stretching vibration of the carbonate group and to CH in-plane aromatic deformations. (4) The most intense feature in the HREELS spectrum is a broad peak which contains several discernibleshoulders assigned at -1150, -1190, and -1230 cm-I. These features are in excellentagreement with the IR frequencies. The two highest frequency features are attributed to the asymmetrical 040stretching vibrations of the carbonate group, while the low-frequency asymmetry is probably a C-(CH& skeletal mode. (5) Another feature in the HREELS spectrum that is enhanced significantly relative to the IR spectrum is the peak at -1460 cm-l. It appears to be composed of two features. The maximum intensity is at 1450 cm-l with an asymmetric shoulder to higher frequency, 1490 cm-l. These features match up very well with the IR assignments of 1465cm-' for the antisymmetric methyl CH deformation mode and 1505cm-l for the aromatic C-C ring mode. The HREELS intensity is skewed toward lower frequency, especially at 5-eV beam energy, because of the large enhancement of the methyl deformation. (6) The 1600-cm-I band is due to C-C aromatic skeletal ring modes of the bisphenol moiety. (7) The HREELS vibrational peak at 1780 cm-l, due to the C==Ostretching vibration of the carbonate group, is diminished in intensity with respect to ita intensity in IR spectra. Although this is a dominant feature in the highresolution IR spectrum, it is considerably reduced when Gaussian broadened due to ita narrow fwhm (see Figure 3A). (8) The CH stretching modes between 2800 and 3200 cm-I are composed of two primary bands, one at 2970 cm-I due to the antisymmetric methyl group C-H stretching modes and the other at 3060 cm-1 due to aromatic CH stretches. Distinct shoulders to the low-frequency side of the 2970-cm-I feature appear at -2850 and -2915 cm-1, which are attributable to the symmetric methyl group CH stretches. The most obvious difference for the HREELS spectrum compared with the IR spectrum is the large intensity of the aromatic CH stretch band versus the methyl group CH stretch intensity and the enhanced overall
(14) H u m e l , D. 0.Infrared Spectra ojPolymers; Wiley: New York, 1966; p 47. (15) Schnell, H. Chemistry and Physics of Polycarbonates; Wiley: New York, 1964; p 167. (16) Wielgosz, Z.; Boranowaka, Z.; Janicka, K. Plaste Kautsch. 1972, 19, 902.
(17) Politou, A. S.; Mortarra, C.; Low, M. J. D. Carbon 1990,28,629. (18) Nyquist, R. A.; Potts, W. J. Spectrochim. Acta 1961, 17,679. (19) Bellamy, L. J. The Infrared Spectra of ComplexMolecules; Chap man and Hall: London, 1976. Rao, C. N. R. Chemical Applicatiom of Infrared Spectroscopy; Academic Press: New York, 1963. (20) Kulczycki, A. Spectrochim. Acta 1985, 41A, 1427.
hi
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Figure 2. Transmission infrared spectrum of a bulk sample of bisphenol-A-polycarbonateacquired with a resolution of 4 cm-1. Major features are coded alphabetically and vibrational assignments are described in the text and in Table I.
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Figure 3. Vibrational spectra of bisphenol-A-polycarbonate: (A) transmission infrared spectrum convoluted with a Gaussian (fwhm = 80 cm-l) to provide resolution comparable to that for spectra in part B, alphabetic codes identical with those of Figure 2; (B)HREELS spectrum of 100 A film on HOPG substrate acquired at two different primary beam energies, 3 and 5 eV in specular geometry. Arrows denote regions of significantrelative intensity change.
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Langmuir, Vol. 7, No. 10, 1991 2269
Surface Analysis of Polycarbonate Thin Films
Table 1. Comparisons of the IR and HREELS Vibrational Frequencies (cm-')of Polycarbonate Films peak code FTIR HREEL band assignmentdcm 410,435,510 400, -510 deformation modes of 040-0carbonate group a 560. . 550 ring "out-of-plane" bending vibration of para-substituted benzene ring 630 -630 in-plane ring deformation "out-of-plane" vibrations of carbonate group 770 b 830 820 aromatic CH deformation C 890 d "in-plane" carbonate group vibration 1015 symmetrical 0-C-0 stretching vibration of carbonate group e 1080 f aromatic C-H and ring vibrations 1000-1225 aromatic CH in-phase deformation modes -1150 C-(CH& skeletal mode 1165 B 1195 h -1190 asymmetrical 04-0stretching vibrations of carbonate group i 1235 1230 1290 aromatic C-H in plane bend j 13800 1365,1410 methyl CH bending modes k, 1 m 1460 1465 antisymmetric methyl CH deformation modes -1490 1505 n aromatic C--C stretching aromatic C-C skeletal ring modes 1600 1600 0 C = O stretching vibration of carbonate group 1780 1775 P 2850,2915 2874 symmetric methyl group C-H stretch q r 2970 2970 antisymmetric methyl group C-H stretch 8 3060 3040,3060 aromatic C-H stretches a Confirmed by deconvolution of spectra as discussed in text. ~
CH intensity at EO= 5 eV in comparison with the remaining part of the spectrum, especiallywith respect to the 0-C-0 features at -1230 cm-l. We have not observed any definitive bands in the region 3485-3535 cm-l, which have been ascribed to the OH stretching vibration of a phenol end-group of the p01ymer.l~ This band is recognized to be present or absent in bulk samples depending on the method of preparation. The spectral feature at -2460 cm-l is a multiple vibrational loss from the intense -1230-cm-l feature. Energy Dependence Effects. One can see that there are relative intensity changes occurring for some of the loss features at various primary beam energies. This effect is clearly observed for the two spectra shown in Figure 3B with arrows denoting regions of significant relative intensity change. In a comparison of spectra run at several different primary beam energies, two vibrational regions exhibit dramatic intensity changes over a relatively small change in beam energy. At 3 eV the CH stretch modes for both aliphatic and aromatic modes (2900-3100 cm-') have low intensity relative to the carbonate related modes at 12W1250 cm-l. In contrast, the intensity of the CH stretch region is more comparableto that of the carbonate group at beam energies 1 5 eV. For 3-eV primary beam energy, the intensity of the CH stretch region above 3000 cm-l, attributable to aromatic character, is significantly greater than that of the region below 3000 cm-l, attributable to aliphatic character. However, at 5 eV primary beam energy the two regions are more nearly equal in intensity. Between 5- and 12-eV beam energy, the two regions are more nearly comparable. Several other energydependent changes are also apparent. The aromatic C-C skeletal ring modes at 14% cm-l exhibit enhanced intensity with respect to the carbonate modes at 5 eV versus 3 eV. Several modes at 9OG1100 cm-l, that cannot be specifically identified, are also enhanced relatively at 5 eV. Other subtle changes can be discerned in Figure 3B;however, the mode assignments are not easily made due to the inherent resolution. Two spectral features in the HREELS spectra are of sufficient intensity and are separated from other features enough that their intensities relative to the elastic peak can be analyzed as a function of primary beam energy. The particular lose features that are selected are the CH stretch region (2900-3100 cm-*) and the carbonate group
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Primary b e a m energy (eV) Figure 4. Primary beam energy dependence of the ratio of lose features to elastic peak intensity for a n unannealed polycarbonate film on HOPG open squares represent the ratio for the carbonate vibrational mode at 1230 cm-'; solid dots represent the ratio for the CH(aromatic) stretching modes above -3000 cm-l; the open dots represent the CH(aliphatic1stretching modes below 3000 cm-l. The inset shows the ratio Z & * e / Z m as a function of beam energy.
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vibrations near 1230 cm-I. As discussed above, the CH stretch region is composed of aliphatic and aromatic regions. The graph of these three intensity ratios over the energy range 2-12 eV primary beam energy is shown in Figure 4. One can see in this figure that the carbonate group modes peak around 3.5 eV primary beam energy while the C-H stretch modes peak at higher beam energy. Separate intensities for the aliphatic and aromatic regions were determined at each beam energy by using standard curve fitting techniques. The aromatic CH stretching modes show the most intensity at about 4.5 eV, while the
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2270 Langmuir, Vol. 7, No. 10,1991
n M c C
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Figure 5. HREELS spectra of the CH stretch region of a thin polycarbonate film for increasing thermal treatment: (A) 15 min at 348 K; (B) additional 5 min a t 423 K; (C) 20 min total time at 423 K;(D) additional 15 min at 453 K.
methyl group CH stretching modes show the greatest intensity at about 5.0 eV. These variations of intensity over a small energy range suggest a resonant enhancement mechanism which will be discussed later. Thermal Annealing Effects. Annealing of the polymer near the glass transition temperature (Tg= 423 K) produced a general improvement in the sharpness of the vibrational bands. The assignment of bands were independent of annealing; however, several spectral changes did occur. For a fixed primary beam energy of 5 eV, the aliphatic CH stretching mode intensity progressively decreased with respect to the aromatic CH stretching mode as a function of annealing. Figure 5 shows the relative peak intensity changes at Eo = 5 eV for the two stretching modes for various periods of annealing. Spectrum 5A was recorded for a film undergoing minimal annealing (348 K for 15 min). The other spectrashow the changes occurring with more thorough annealing: (B) 423 K for an additional 5 min; (C) 423 K for 20 min total time; (D) 453 K for an additional 15 min. For a number of heat treatments of several different samples, this behavior was always observed. We observed no indications of thermal degradation in ultrahigh vacuum from either the vibrational spectra or XPS core level changes.
Discussion The HREEL surface spectra and IR bulk spectra show consistently similar group vibrational frequencies, making the assignment of peaks rather straightforward. However, the very different band intensities in the two spectroscopies and the resonance enhancement in HREELS are noteworthy. The spectrum for a primary beam energy at 3 eV is distinctly different from that of a 5-eV beam. Several peaks show large intensity changes. Multiple losses are more intense than the corresponding combination modes in IR, however, this appears not to be problematic. The most prominent multiple loss feature is at -2460 cm-’. An overall observation is that, a t 5 3 eV primary beam energy, where dipole scattering would be most favorable,
the general vibrational fingerprints for HREELS in specular geometry and IR are similar. At higher primary beam energies, the vibrational modes are influenced by several other scattering mechanisms such as resonant scattering and impact scattering. Resonant Scattering Features. For the primary beam energies studied (2 eV
