Langmuir 1998, 14, 4827-4835
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Plasma Treatment of Polymers: The Effects of Energy Transfer from an Argon Plasma on the Surface Chemistry of Polystyrene, and Polypropylene. A High-Energy Resolution X-ray Photoelectron Spectroscopy Study Richard M. France and Robert D. Short* Laboratory for Surface and Interface Analysis, Department of Engineering Materials, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, U.K. Received December 1, 1997. In Final Form: May 13, 1998 Argon plasma treatment and subsequent atmospheric exposure have been used to incorporate new oxygen functionalities at the surface of polystyrene (PS) and polypropylene (PP). High-energy resolution X-ray photoelectron spectroscopy (XPS) has yielded molecular information regarding the site of modification in both polymers. Core level and valence band spectra have been interpreted. We have adopted a simple subtraction process to highlight the changes occurring in the valence band spectra upon treatment. In the case of PS, modification is found to be occurring at ring sites. In PP, modification is thought to be occurring at two sites. We propose that modification at the tertiary carbon site leads to chain scission and subsequent etching, while modification at the methyl side group results in functionalities being incorporated at the polymer surface. These data help to substantiate mechanisms that were previously proposed, based on the accepted mechanisms of cross-linking vs chain scission for these polymers.
Introduction Radio frequency (rf) plasma treatments may be employed to improve the surface properties of commodity polymers, increasing the range of potential applications for these materials.1 For certain applications the required treatment need not be specific, for example, to overcome the adhesion problems associated with hydrocarbon polymers. In other cases, highly specific treatments are required, for example, in the modification of polymer surface properties for potential use as biomaterials.2 The reactive species in a plasma, resulting from ionization, fragmentation, and excitation processes, include positive and negative ions, neutral species, atoms, metastables, and free radicals. Inductively coupled inert gas plasmas, of the type used in this study, may be considered as an energy source with a broad range of energies (all under 50 eV). Plasma components include ions (10-30 eV), electrons (0-10 eV), and UV (200 > λ < 400 nm) and vacuum UV (λ < 200 nm) radiation (3-40 eV).1 In reactive gas plasmas ablation or etching occur, resulting in the formation of small volatile degradation products.3 In inert gas plasmas, the dominant process is hydrogen abstraction.3 In the former, functionalization occurs within the plasma;4 in the latter, exposure to the laboratory atmosphere generally leads to functionalization,5 although oxygen-containing plasma species (arising from residual water) are also thought to be responsible for some functionalization.6 For oxygen plasma treatment, it has been shown that some functionalization still occurs after plasma treatment.7 (1) Liston, E. M. Plasma Surface Modification of Polymers: Relevance to Adhesion; VSP Publishing: Utrecht, 1994; Chapter 1. (2) Lee, H. B.; Lee S. S.; Khang, G. The Biomedical Engineering Handbook; Bronzino, J. D., Ed., IEE/CRC Press: Boca Ratan, FL, 1994; Chapter 42. (3) Clouet, F.; Shi, M. K. J. Appl. Polym. Sci. 1992, 46, 1955. (4) Occhiello, E.; Morra, M.; Garbassi, F.; Humphrey, P.; Vickerman, J. C. SIMS VII-Proceedings of International Conference; Wiley: Chichester, 1990; p 789. (5) Momose, Y.; Tamura, Y.; Ogino, M.; Okazaki, S.; Hirayama, M. J. Vac. Sci. Technol. 1992, A10, 229.
A number of different plasma treatments have been used to incorporate oxygen into the surfaces of polymeric materials. Oxygen plasma treatments have been the most widely employed. In a comparison of polystyrene (PS), polypropylene (PP), and polyethylene (PE), higher levels of oxygen incorporation were achieved in PS and PP than in PE.8 When there is an oxygen functionality present in the polymer backbone, etching, rather than oxygen incorporation, is observed.3 The same is true with argon plasma treatments.3 However, etching may be offset by aromaticity within the polymer backbone. For example, in a study of the plasma treatment of poly(methyl methacrylate) (PMMA) and poly(ethylene terephthalate) (PET),9 the carboxylate moiety was found to be the prime site of attack, but the PET showed the lower rate of decomposition. This was attributed to stabilization arising from the aromatic ring,9 although the unzipping mechanism of PMMA may have also accounted for the increased rate of degradation of this polymer. Pendant aromatic groups also provide stability to etching: in a comparison between PS and PMMA, PS was shown to have an increased resistance to oxygen plasma treatment. In this polymer it is thought that oxygen is incorporated at ring sites, reducing etching/chain scission.10 In a number of studies, a maximum in the level of plasma treatment has been identified. Beyond this maximum, surfaces become unstable to washing with a polymer nonsolvent and measurable amounts of material are removed. In a previous study, we determined the saturation and “stable” levels in argon-plasma-treated PS, low(6) Callen, B.; Ridge, M. L.; Lahooti, S.; Neumann, A. W.; Sodhi, R. N. S. J. Vac. Sci. Technol. 1995, A13, 2023. (7) Clouet, F.; Shi, M.; Prat, R.; Holl, Y.; Marie, P.; Leonard, D.; DePuydt, Y.; Bertrand, P.; Dewez, J. L.; Doren, A. Plasma Surface Modification of Polymers: Relevance to Adhesion; VSP Publishing: Utrecht, 1994; p 65. (8) Clark, D. T.; Dilks, A. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 957. (9) Groning, P.; Collaud, M.; Dieter, G.; Schlapbach, L. Vide-Couches Minces 1994, 272, 140. (10) Moss, S. J.; Jolly, A. M.; Tighe, B. J. Plasma Chem., Plasma Proc. 1986, 6, 401.
S0743-7463(97)01305-X CCC: $15.00 © 1998 American Chemical Society Published on Web 07/23/1998
4828 Langmuir, Vol. 14, No. 17, 1998
France and Short Table 2. Peak Fit Results for π f π* Shake-up Satellite in Untreated PS position (eV)
fwhm
G/L (%)
area (%)
291.10 291.99 293.08 295.58
1.40 1.14 1.62 1.77
1.00 1.00 1.00 0.76
23.2 43.9 20.4 12.3
Table 3. Binding Energy Shifts, Relative to Aliphatic Hydrocarbon, for Carbon-Oxygen Functionalities Employed in the Peak Fitting of the C 1s Core Levels
Figure 1. Peak fits for untreated PS; (a) C 1s core level; (b) π f π* shake-up satellite. Table 1. C 1s Peak Fit Results for Untreated PS position (eV)
environment
fwhm
asym
G/L (%)
area (%)
284.76 285.00
aromatic aliphatic
0.82 1.02
0.10 0.10
0.82 1.00
74.9 25.0
density polyethylene (LDPE), PP, and PET.11 Saturation is the point at which no more oxygen is incorporated with further treatment, and “stable” is the point below which no measurable (by XPS) amounts of material are removed by washing. Callen et al.6 have also found similar treatment levels for argon-plasma-treated PS. Foerch et al.12 have established the optimum treatment level for LDPE in an oxygen plasma, although some loss of modification was always observed upon rinsing. This apparent inability of oxygen plasmas to effect a “stable” modification presents a serious potential problem, namely, that interfacial material can be readily removed. For example, loss in adhesion in PE/PE and PE/PET laminates has been correlated with the presence of low molecular weight material (LMWM).13 In a study of the attachment of Chinese hamster ovary cells to oxygen-plasma-treated PE, cellular attachment reached a maximum below the maximum treatment level; this observation is consistent with the production of LMWM at the surface of the polymer when above the “stable” treatment level.2 The presence of this material probably has a detrimental effect upon cellular attachment. In this study we present high-energy-resolution X-ray photoelectron spectroscopy (XPS) data obtained from argon-plasma-treated PS and PP. All analyses were exsitu; that is, samples are removed from the plasma to atmosphere prior to insertion in the XPS. High-energyresolution XPS provides clues to the site of modification in these two polymers. Previously we observed large differences in the modification depths of PS, PP, and LDPE (11) France, R. M.; Short, R. D. J. Chem. Soc., Faraday Trans. 1997, 93, 3173. (12) Foerch, R.; Kill, G.; Walzak, M. J. Plasma Surface Modification of Polymers: Relevance to Adhesion; VSP Publishing: Utrecht, 1994; p 99. (13) Sapiela, S.; Cerny, J.; Klemberg-Sapiela, J. E.; Martinu, L. J. Adhes. 1993, 42, 91.
environment
shift (eV) relative to 285 eV
aliphatic aromatic C-OR cyclic C-O-C CdO COOR/H C-COOR/H CO32π f π* 1 π f π* 2 π f π* 3 π f π* 4
0 -0.20 T -0.24 1.10 T 1.90 2.00 T 2.30 2.70 T 2.90 3.80 T 4.50 0.70 5.00 T 5.50 6.10 6.99 8.08 10.58
after argon plasma treatment. These results were rationalized on the basis of the accepted mechanisms of crosslinking vs chain scission for the polymers.11 The data presented in this study go some way in helping to substantiate the validity of these mechanisms. Experimental Section PS was obtained from the Aldrich Chemical Co. (U.K.) and had an average molecular weight (M h w) of 280 000 g mol-1 and a polydispersity of 3.0. Free-standing films were solution cast from 0.3 g of PS in a minimum of toluene. PP was obtained from Courtaulds Chemicals, U.K., and was used as received. Gel permeation chromatography (GPC) measurements carried out by RAPRA Technology (Shawbury, U.K.) gave a M h w of 346 000 g mol-1 and a polydispersity of 10.6. All glassware used in this study, for the casting of PS and the washing of treated polymer surfaces, was subject to rigorous cleaning involving a detergent wash, nitric acid (20%) soak (1 h minimum), and final rinse with methanol. All solvents and chemicals were of HPLC grade and obtained from Aldrich Chemical Co. (U.K.). Plasma treatments were carried out in an inductively coupled rf reactor. The reactor design is described fully elsewhere.14 The base pressure obtained in the reactor was better than 10-3 mbar. The reactor was flooded with argon to a pressure of approximately 1 mbar for 5 min prior to plasma treatment at a pressure of 2.5 × 10-2 mbar and a plasma power of 10 W. Argon gas was obtained from BOC (U.K.) and was of industrial grade. No effort was made to dry the gas, because of the amount of residual water adsorbed to the surfaces of the reactor vessel, which could not be readily removed. After plasma treatment, samples were exposed to the laboratory atmosphere, prior to insertion into the spectrometer. Argon plasma treatments were undertaken the day before analysis. Acquisition of XP spectra was carried out within 48 h of treatment. The stability of samples over the first 48 h has been reported upon in ref 11. High-energy-resolution XPS analyses were performed on the Scienta ESCA300 instrument situated at the RUSTI facility at Daresbury Laboratory (U.K.). The performance and characteristics of this spectrometer have been described by Beamson and Briggs.15 The monochromated Al KR X-ray source was operated at 2.7 kW; charge compensation was achieved through the use of a low-energy electron flood gun. Survey spectra were acquired from 0 to 1100 eV, with a pass energy of 300 eV and slit widths on entrance to the analyzer of 1.9 mm. A step size of 1 eV and (14) O’Toole, L.; Short, R. D.; Ameen, A. P.; Jones, F. R. J. Chem. Soc., Faraday Trans. 1995, 91, 1363. (15) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; John Wiley and Sons: Chichester, England, 1992.
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Table 4. C 1s Peak Fit Results for Argon-Plasma-Treated PSa percentage of functionalities in the C 1s core level
untreated below “stable’ “stable” saturation washed a
C (aliphatic)
C (aromatic)
22.9 (1.01) 40.1 (1.58) 42.7 (1.47) 33.4 (1.49) 31.6 (1.52)
69.3 (0.82) 45.0 (1.15) 31.1 (1.07) 19.9 (1.11) 32.8 (1.17)
C-OH/R
6.0 (1.48) 9.7 (1.48) 12.4 (1.74) 13.4 (1.85)
cyclic C-O-C
2.3 (1.48) 5.8 (1.48) 6.5 (1.74) 3.5 (1.85)
CdO
1.1 (1.65) 4.5 (1.65) 8.7 (1.83) 4.6 (1.55)
COOH/R [+β shift]
0.2 [0.2] (1.30) 1.5 [1.5] (1.30) 7.7 [7.7] (1.36) 4.8 [4.8] (1.35)
CO32-
0 0.1 (1.30) 2.0 (1.36) 1.4 (1.35)
π f π* (1)
π f π* (2)
π f π* (3)
π f π* (4)
3.4 (1.20) 2.1 (1.34) 1.8 (1.52) 1.0 (1.75) 2.1 (1.86)
3.2 (1.18) 1.8 (1.37) 1.1 (1.39) 0.8 (1.30) 0.2 (1.09)
0.4 (0.63) 0.2 (0.86) 0.2 (1.17) 0.1 (3.52) 0.8 (3.15)
1.1 (2.52) 0.7 (2.50) 0 0 0
Figures in parentheses are values of fwhm (eV) for the various components.
Figure 2. C 1s core level peak fits of argon-plasma-treated PS: (a) “stable” level (O/C ratio ) 0.19); (b) saturation level (O/C ratio ) 0.33). a dwell time of 0.1 s were employed. Two scans were added together. Valence band spectra were acquired with a step size of 0.1 eV and a dwell time of 3 s. Each region was scanned twice. Core level spectra were acquired with a pass energy of 150 eV and slit widths of 0.5 mm. A step size of 0.05 eV and dwell time of 0.1 s were employed, and each region was scanned 10 times. An electron take off angle (R) of 15° was employed for all analyses.
Results Polystyrene. Untreated PS was examined by XPS and found to be free from any contaminants (e.g., oxygen arising from surface oxidation or silicon from siloxane contamination). A peak fit of the C 1s core level is shown in Figure 1a and the results of the fit in Table 1. In the
4830 Langmuir, Vol. 14, No. 17, 1998
France and Short Table 5. N 1s Peak Fit Results for Argon-Plasma-Treated PS (Saturation Level) position (eV)
fwhm
G/L ratio
area (%)
400.90 402.98
2.28 3.07
1.00 1.00
73.8 23.7
Table 6. F 1s Peak Fit Results for Argon-Plasma-Treated PS (Saturation Level)
Figure 3. Ratios of (a) Caromatic/Caliphatic and (b) π f π* shakeup satellite intensity (expressed as a percentage of the whole C 1s core level) vs O/C ratio for argon-plasma-treated PS.
C 1s core level peak fit, the ratio of the two peaks reflects the ratio of aromatic to aliphatic carbon environments15 that is expected from the polymer molecular structure. The π f π* shake-up satellite was peak-fitted with four components15 and is displayed in Figure 1b. The results of the peak fit are shown in Table 2. Argon plasma treatment and subsequent atmospheric exposure resulted in oxygen incorporation at the PS surfaces (Figure 2). Four different levels of treatment were examined, based upon previous work.11 These treatment levels were (1) below the “stable” level, (2) at the “stable” level, (3) at the “saturation” level, and (4) “washed” with methanol from “saturation”. The O/C ratios at these levels were measured as 0.10, 0.19, 0.33, and 0.26, respectively. The confidence interval has been calculated for argon-plasma-treated PS as 0.03.11 This is a measure of the sample to sample variability and has been applied to data obtained here. Nitrogen and fluorine (approximately 2% and 1%, respectively) were observed on the sample at the “saturation” level (10 min treatment time). The appearance of these elements at longer treatment times has been attributed to residual gas/small undetected leaks and the use of PTFE gaskets, respectively.11 The C 1s core levels of the plasma-treated PS were peakfitted for various new carbon-oxygen functionalities. The fitted functionalities and their associated shifts (relative to the aliphatic hydrocarbon peak charge corrected to 285 eV) are displayed in Table 3.15 Optimization of each fit was achieved using the following procedure. A linear background was placed over the C 1s core level. Initially
position (eV)
fwhm
G/L ratio
area (%)
686.84 689.78
2.37 1.96
1.00 1.00
15.8 77.5
a single hydrocarbon component was employed. The new carbon-oxygen functional group components were tied to this peak in terms of their position, full width at halfmaximum (fwhm), and G/L ratio. Component peaks shifts were initially fixed at nominal values (e.g., +1.5 eV for C-O).11 Once an initial fit was complete, the hydrocarbon peak was then fitted for two environments (aliphatic and aromatic). The two components were initially set at the same height and allowed to find their own respective areas in the core level. With this second fit complete, the carbon-oxygen components were then allowed to achieve their optimum values (peak position, fwhm, and G/L ratio). During this optimization, the “chemical sense” of the peak fit was maintained, this being the most important aspect of any peak fit. If any unreasonable values (for position, fwhm, and G/L ratio) were obtained, peak fitting was halted and constraints were placed on that particular component prior to restarting the fit. The position of the components in the π f π* shake-up satellite remained fixed to the values obtained in the peak fit of untreated PS (see Table 2). The β-shift associated with the carboxylate component was held at a shift of +0.7 eV from the aliphatic carbon environment.15 The results of peakfitting are shown in Table 4. The peak-fitting of the C 1s core levels revealed similar trends in the concentrations of carbon-oxygen functionalities, with treatment level, to those previously described.11 Surfaces at or below the “stable” level, displayed a high selectivity toward alcohol/ether functionalities. At the “saturation” level, more highly oxidized carbon environments were observed in significant quantities. Most of these were removed upon “washing” of the surface. While these trends agree with previously reported data, two important differences were observed. First, when the peak positions of the carboxylate and carbonate components were allowed to reach their optimum values in the C 1s core level, it became apparent that the majority of these functionalities were carboxylates (+3.8-4.5 eV from the aliphatic hydrocarbon) rather than carbonates (+5.0 eV from the aliphatic hydrocarbon). The latter are present, although in smaller quantities than previously reported.11 Second, to achieve a good fit for argon-plasmatreated PS, a peak at a shift of ca. 2.2 eV had to be introduced, suggesting epoxies or cyclic ethers. The reported peak position for the epoxide functionality is 287.02 eV in poly(glycidyl methacrylate).15 The presence of one or both of these chemical environments was not noted in earlier peak fits. With plasma treatment the fwhm of all the components employed in the C 1s core level peak fit were observed to increase (Tables 1, 2, and 4), indicating that these functionalities are not introduced into the surface in one “unique” chemical environment. The fwhm of particular peaks are large (alcohol/ether and aldehyde/ketone). The fwhm of single component peaks when measured on the ESCA300 are generally of the order of 0.3 eV lower than those measured on medium-energy resolution XPS spec-
Plasma Treatment of Polymers
Figure 4. Valence band spectrum of untreated PS.
trometers.15 Therefore, the large fwhm of certain components appears to be inherent to the samples. The placement of a linear background over the entire C 1s core level of untreated PS has an effect on the relative contributions of each of the component peaks. This is
Langmuir, Vol. 14, No. 17, 1998 4831
because a small portion of the spectrum to the high binding energy side of the main peak is excluded. This effect can be seen comparing Tables 1 and 2 to Table 4. The data in Tables 1 and 2 were obtained when the C 1s core level and the π f π* shake-up satellite were fitted separately. To obtain Table 4, a linear background was placed over the entire C 1s core level. The effect of this has been to increase the relative contribution of the first two components at 291.10 and 291.99 eV in the π f p* shake-up satellite at the expense of those at 293.08 and 295.58 eV. The changes occurring in the ratio of C(aromatic)/C(aliphatic) with increasing treatment level (O/C ratio) are shown in Figure 3a. This ratio decreases with treatment level indicating the primary site of modification in PS is at the aromatic ring. Modification at aliphatic carbon environments would have resulted in an increase in this ratio. Washing results in some restoration of the ratio. The change in the π f π* shake-up satellite intensity with treatment level is shown in Figure 3b. The data show a
Figure 5. Difference valence band spectra of PS for (a) below “stable” level, untreated, (b) “stable” level, untreated, and (c) saturation level, untreated.
4832 Langmuir, Vol. 14, No. 17, 1998
decrease in the shake-up intensity with increasing O/C ratio, again indicating the aromatic ring to be the primary site of modification in the argon plasma. The intensity of the shake-up would have remained unaffected were substantial modification at aliphatic sites to have occurred. Washing the sample from “saturation” again resulted in some restoration of the original shake-up satellite intensity. As previously noted, nitrogen and fluorine were incorporated into the sample at long treatment time. The N 1s and F 1s core levels were peak-fitted. The results of the peak-fitting are shown in Tables 5 and 6, respectively. The data show both elements are present in more than one chemical environment. Examining first the N 1s peak fit, the data suggest two environments at 400.90 and 402.98 eV. The binding energy of the lower (dominant) component is consistent with imide- and urethane-type functionalities.15 The component at higher binding energy is most likely representative of -NH3+ and -N[CH3]3+ type functionalities.15 The positions of the components in the F 1s core level suggest two fluorine environments at the surface. The lower binding energy component is consistent with poly(vinyl fluoride) (PVF)-type functionalities, while the higher binding energy component is indicative of poly(tetrafluoroethylene) (PTFE)-type functionalities.15 Displayed in Figure 4 is the valence band spectrum of PS. This valence band is very similar to the valence band spectrum of solid benzene.16,17 The close similarity arises from the fact that most of the molecular orbitals residing on the pendant ring group of PS have much more pσ or pπ character and therefore a greater cross section to photoionization. Spectral features arising from the C 2sσ orbitals in benzene are seen at 13, 17, and 20 eV. The features at approximately 8 and 11 eV correspond to C 2pσ orbitals and those at approximately 4 and 6 eV to C 2pπ orbitals in benzene.16 There are no well-established fitting routines for the valence band region, and it is difficult to monitor changes in spectra simply from comparison. For this reason, it was decided to adopt a simple subtraction procedure and examine the difference spectra between treatment levels. Difference spectra were obtained using the following procedure: The spectra were normalized in height using the background at approximately 40 eV. Normalization on the height of the background, rather than a feature in the spectrum, should allow differences in feature intensities to be pulled out. On completion of normalization, one spectrum is then subtracted from the other to obtain the difference spectrum. The method draws attention to changes that cannot be observed through visual comparison of the raw spectra alone. The difference spectrum obtained when the untreated spectrum of PS has been subtracted from the spectrum of PS below the “stable” level (1) is shown in Figure 5a. The difference spectrum obtained when the untreated spectrum of PS has been subtracted from the spectrum of PS at the “stable” level (2) is shown in Figure 5b. The difference spectrum obtained when the untreated spectrum of PS has been subtracted from the spectrum of PS at the “saturation” level (3) is shown in Figure 5c. The difference spectra in Figure 5 show a reduction in intensity of the C 2s and C 2p features, seen prominently in untreated PS (marked *). There has also been a redistribution in intensity from C 2p to C 2s. Features are apparent that have been previously attributed to C-O (16) Riga, J.; Pireaux, J. J.; Verbist, J. J. Mol. Phys. 1977, 34, 131. (17) Sherwood, P. M. A. J. Vac. Sci. Technol. 1992, A10, 2783.
France and Short
Figure 6. C 1s core level peak fit of untreated PP. Table 7. C 1s Peak Fit Results for Untreated PP position (eV)
environment
fwhm
asym
G/L (%)
area (%)
285.00 285.16
C1 C2
0.85 1.04
0.11 0.12
0.97 0.80
33.4 66.8
Table 8. C 1s Core Level Peak Fit Results for Argon-Plasma-Treated PPa
below “stable” “stable” saturation washed
C1
C2
C-O
CdO
38.7 (1.06) 38.3 (1.38) 37.6 (1.27) 39.0 (1.21)
59.7 (1.39) 46.5 (1.56) 45.1 (1.43) 48.5 (1.40)
1.5 (1.36) 8.8 (1.58) 7.0 (1.52) 6.9 (1.54)
0.0 3.6 (1.55) 4.3 (1.56) 2.8 (1.47)
COOH/R [+β shift] 0.0 1.3 [1.3] (1.42) 3.0 [3.0] (1.48) 1.4 [1.4] (1.12)
a Figures in parentheses are values of fwhm (eV) for the various components.
functionalities.17 C 2s orbitals associated with C-O are observed at 18 eV. The small features at approximately 5 and 8 eV correspond to π and σ orbitals associated with the oxygen lone pair in C-O. Other features emerging in the region below 10 eV may correlate to O 2p-H 1s bonding orbitals. The features at approximately 11 and 15 eV have been attributed to the oxygen lone pair in CdO functionalities.17 Broadening of the O 2s peak has been attributed to carboxylate functionalities17 and is observed at the saturation level (Figure 5c). Also apparent are small shoulders in the O 2s peak that can be attributed to contributions from N 2s and F 2s.15 These changes provide confirmation of the conclusions drawn from the core level data; namely, there is a reduction in aromaticity and the introduction of new elements and functionalities. They do not provide much further insight into the nature of the modified surface. Polypropylene. Untreated PP was examined by XPS and was found to be free from contamination. The C 1s core level was peak-fitted for two environments,15 as shown in Figure 6. The peak-fitting results are shown in Table 7. The area ratio of the two environments C2/C1 is 2:1, as expected from the polymer molecular structure.15 It is important to note that the PP C 1s core level could have been fitted for vibrational fine structure, as also described in ref 15. PP samples were argon-plasma-treated. Four different levels of treatment were examined. These were (1) below the “stable” level, (2) at the “stable” level, (3) at the “saturation” level, and (4) “washed” from “saturation”. The O/C ratios for these levels were 0.01, 0.09, 0.09, and 0.06 ( 0.01, respectively. The O/C ratio of the sample at “saturation” is down on that previously reported. The
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Langmuir, Vol. 14, No. 17, 1998 4833
Figure 7. C 1s core level peak fit of argon-plasma-treated PP at “stable” level of treatment (O/C ratio ) 0.09)
Figure 9. Valence band spectrum of untreated PP.
Figure 8. Ratio of C2/C1 environments vs O/C ratio for argonplasma-treated PP.
reason for this is unknown. Clear differences were observed between the peak fit of the sample at “saturation” (O/C ratio ) 0.09) and the sample at the “stable” level. Calculation of the amount of oxygen based upon the C 1s peak fit suggests a much greater amount of oxygen in the saturated sample. This indicates the presence of more ether and/or ester in the plasma-treated PP surface at “saturation”, where the contribution of an oxygen is effectively counted twice. The C 1s core levels were peak-fitted for new carbonoxygen functionalities using the procedure described for PS. The results of peak fitting are displayed in Table 8. An example peak fit for PP at the “stable” level of treatment is shown in Figure 7. From this peak fit it can be seen that the most significant new peak arises from alcohol/ ether functionalities. Increasing contributions from carbonyl and carboxylate are observed at higher treatment levels (at and above the “stable” level). These results agree well with those we have previously reported for PP, with one important exception: here, no carbonates were observed on the treated PP surface. The effect of increasing treatment level (O/C ratio) on the C2/C1 ratio is shown in Figure 8. The reduction in this ratio suggests that the primary site of modification is at the C2 position. (Modification at C1 would have resulted in an increase in this ratio.) In this fit the C2 position
corresponds to the pendant methyl group and the secondary carbon on the main chain. Caution must, however, be excerised as there is some ambiguity as to the correct fitting pocedure for PP. The valence band spectrum of untreated PP is shown in Figure 9. Features at approximately 19 and 13 eV have been attributed to C 2s-C 2s bonding and antibonding orbitals, respectively, in the backbone.18 The feature at 16 eV has been assigned to the C 2s-C 2s bonding orbital in the methyl side group. Features in the broad envelope below 10 eV correspond to C 2p methyl orbitals (9 eV) and C 2p-H 1s orbitals (5 eV). The valence band spectrum of untreated PP was subtracted from the spectra of the treated PPs to obtain difference spectra. The difference spectrum obtained when the spectrum of untreated PP has been subtracted from the spectrum of PP below the “stable” level (1) of treatment is shown in Figure 10a. The difference spectrum obtained when the spectrum of untreated PP has been subtracted from the spectrum of PP at the “stable” level (2) of treatment is shown in Figure 10b. The difference spectrum obtained when the spectrum of untreated PP has been subtracted from the spectrum of PP at the “saturation” level (3) of treatment is shown in Figure 10c. The changes in the valence band spectra are similar to those seen for PS. Namely, in the appearance of the O 2s peak and a reduction in intensity of peaks diagnostic of the untreated polymer (marked * in Figure 10a). New (18) Gross, Th.; Lippitz, A.; Unger, W. E. S.; Friedrich, J.; Woll, Ch. Polymer 1994, 35, 5590.
4834 Langmuir, Vol. 14, No. 17, 1998
France and Short
Figure 10. Difference valence band spectra of PP for (a) below “stable” level, untreated, (b) “stable” level, untreated, and (c) saturation level, untreated.
features appear corresponding to C-O and CdO functionalities. The major reduction in intensity displayed by the C 2s feature at 16 eV, which is representative of the methyl side group, may indicate more significant modification at the C2 methyl position. Hence, the valence band difference spectra support the core level data. Discussion Argon-plasma-treated surfaces are observed to pick up oxygen. The incorporation of oxygen is thought to occur: first, by the reaction of surface free radicals with atmospheric oxygen;5,20 second, by reaction with oxygencontaining species (e.g., H2O) within the plasma.6,20 The treatment levels examined agree well with those that we have previously reported for these polymers.11 (19) Petrat, F. M.; Wolany, D.; Schwede, B. C.; Wiedman, L.; Benninghoven, A. Surf. Interface Anal. 1994, 21, 402. (20) France, R. M.; Short, R. D. Unpublished data.
The high-energy resolution of the XP spectrometer provides information on the site of modification in both PS and PP. In PS, the site of modification has been found to be at the aromatic ring. This is deduced from the reductions in the Caromatic/Caliphatic ratio and π f π* shakeup satellite intensity, as shown in parts a and b of Figures 3, respectively. This conclusion is supported by the valence band spectra presented, where a reduction in the intensity of spectral features arising from the aromatic ring site is seen. Confirmatory secondary ion mass spectrometry (SIMS) data have been obtained in the study of PS with reactive gas plasmas19 and inert gas plasmas.20 The slight recovery in the Caromatic/Caliphatic ratio upon “washing” indicates there to be more aliphatic-like material in the LMWM, which is removed from the surface during “washing”. The spectroscopic data suggest that in PP, modification is occurring predominantly at the C2 site (methyl side groups and the secondary carbon on the main
Plasma Treatment of Polymers
chain). However, this conclusion is subject to some uncertainty as to the best fitting procedure for the C 1s core line of PP. Previously we reported dramatic differences in the modification depth in PS and PP surfaces with nominally the same amount of oxygen incorporation: PP having a much shallower depth of modification. An explanation for this result was put forward based on the accepted mechanisms of UV and high-energy degradation of these polymers.11,21 It was argued that PP had a greater propensity to undergo chain scission, leading to etching of surface material. Here, we show in PP modification is predominately occurring at the pendant methyl carbon. In UV degradation, radicals are produced at two sites: the tertiary carbon and the methyl carbon. The former is a precursor to chain scission, while the latter remains stable and is available for cross-linking or, in the case of plasma treatment, functionalization. Functionalization at the methyl manifests in (1) the valence band, where there is a large change in the intensity of the C 2s feature (assigned to the methyl side group) and (2) in the C 1s core level, where there is a reduction of the C2/C1 ratio. At the tertiary carbon, radicals produce chain scission, leading to either etching of the surface (resulting in the low depth of modification) or LMWM. Thus plasma attack is most probably taking place at both sites, abstraction of hydrogen at the tertiary carbon being the most favorable of the two, resulting in etching. The remaining (unetched) material may be functionalized at the methyl carbon. In PS, the low cross-linking and chain scission probabilities result in a greater depth of modification, because of reduced amounts of etching at the surface.11 The demonstrated modification at ring sites should not lead to chain scission and etching within the plasma. The availability of high-energy-resolution XPS data also allow for some correction with regards to the functionalities we have previously reported to be present at the two polymer surfaces after treatment. First, in our earlier studies there was an overestimation of the amount of carbonates. Second (here) we find in PS a previously undetected carbon singly bonded to oxygen environment. This has been assigned to an epoxy or cyclic ether.15 The appearance of these functionalities in the argon-plasmatreated PS and not in the argon-plasma-treated PP suggests these to be located at ring sites. Oxygen incorporation through the reaction of oxygen with radical sites at the polymer surface will proceed through the (21) Grassie, N. Polymer Degradation and Stabilisation; Cambridge University Press: Cambridge, 1985.
Langmuir, Vol. 14, No. 17, 1998 4835
formation and degradation of hydroperoxides to produce alkoxy and peroxy radicals as observed in photooxidation.22 An epoxy type functionality could form from the reaction between adjacent alkoxy and peroxy radicals.22 The stability of this species was not studied; it would be interesting to know if it was present in samples after 48 h. The measured peak positions in the N 1s and F 1s peakfitted core levels (corrected to aliphatic carbon at 285 eV) provide information on the nature of these functionalities. Nitrogen is thought to be incorporated in imide- and urethane-type functionalities. These assignments are supported by SIMS, where species corresponding to CxHyNO+ and CxHyNO2+ fragments have been observed.20 Fluorine is thought to be incorporated in PVF- and PTFEtype functionalities. This would not be implausible as the fluorine in the plasma is thought to originate from the PTFE gaskets in the reactor. This is probably being etched by the plasma and redeposited in small quantities at the surface of the polymer undergoing treatment. The deposition of PTFE-type polymers from the sputtering of a PTFE target is an effect which is well documented in the literature.23 Conclusions Core level and valence band spectra have been obtained for untreated and argon-plasma-treated PS and PP. The XPS data have confirmed the site of modification in PS and provided new insight into the site of modification for PP. In PS, new functionalities are introduced at ring sites. In PP, new functionalities are introduced at the methyl side group. New functionalities previously undetected have been shown to be present in PS, and carbonates thought to be present in PP have been shown not to be. These data have helped to substantiate the previously proposed mechanisms for modifications of the two polymers based upon a knowledge of UV and high-energy degradation. Acknowledgment. R.D.S. gratefully acknowledges the University of Sheffield for the scholarship for R.M.F. and EPSRC for access to the ESCA300. LA9713053 (22) Rånby, B.; Rabek, J. F. In Photodegradation, Photooxidation and Photostabalisation of Polymers; Wiley: Chichester, 1975. (23) Blanchet, G. B.; Fincher, C. R.; Jackson, C. L.; Shah, S. I.; Gardner, K. H. Science 1993, 262, 719.
