Langmuir 1994,10,3629-3634
3629
X-ray Photoelectron Spectroscopy Study of ArgonPlasma-TreatedFluoropolymers Morton A. Golub,*??Eugene S. Lopata,* and Lorie S. Finneys Ames Research Center, NASA, MoffettField, California 94035-1000, Himont /Plasma Science, 353 Hatch Drive, Foster City, California 94404, and Surface Science Laboratories, 1206 Charleston Road, Mountain View, California 94043 Received January 24, 1994. In Final Form: July 14, 1994@ Films of poly(tetrafluoroethy1ene) (PTFE) and of a tetrafluoroethylene-perfluoroalkyl vinyl ether (x 49:l) copolymer (PFA) were exposed to a radio-frequency argon plasma and then examined by X-ray
photoelectron spectroscopy (XPS).The use of fluoropolymer films nearly free of surface hydrocarbon contamination as well as the use of a monochromatizedX-raysourcefor XPS removed two factors contributing to conflicting reports on the effect of exposure time on the fluorine-to-carbon (F/C) and oxygen-to-carbon (OK) ratios for several Ar-plasma-treated fluoropolymers. Contrary to literature indications, a common pattern was found for PTFE and PFA a moderate decrease in F/C ratio (from 1.99 to 1.40,and from 1.97 to 1.57, respectively),together with a moderate increase in O/C ratio (from negligible to ~ 0 . 1 0and , from 0.012 to ~ 0 . 1 0 ,respectively) at very short exposures, after which the F/C ratios remained essentially constant on prolonged exposures, while the O/C ratios for PTFE and PFA leveled off at 0.11 and 0.15, respectively. The XPS ClS spectra for these polymers exposed to the Ar plasma for 20 min were similar and presented, besides a prominent peak at 292.0 eV (CF2) and a minor peak at 294.0 or 294.1 eV (CF3), a composite band of four curve-resolvedpeaks (x285-290 eV) representing various CH, CC, CO, CN, and CF functionalities.
Introduction Recently, a number of have investigated for a variety of reasons the argon plasma treatment of several perfluorinated polymers, using X-ray photoelectron spectroscopy (XPS)for polymer surface characterization. A survey of their published XPS results reveals, however, a conflicting picture concerning the effect of exposure time on both the fluorine-to-carbon (F/C) and oxygen-to-carbon (OK)ratios of the Ar-plasma-treated fluoropolymers. For poly(tetrafluoroethy1ene)(PTFE), Zhou et al.' and Tan et a1.2showed a precipitous and continuous decrease in F/C with exposure time, the F/C dropping from a n initial value 8 0.17 after 20 min in the former study' and of ~ 1 . 5 to from 1.61 (or 1.58)to 0.35 (or 0.28) after only 1 min in the latter,2 the ratios for Tan et al. having been obtained a t a n XPS take-off angle of 20" (or 75"). In contrast, Morra et ale3found F/C to decrease sharply from 1.52 to 0.49 after only 1 min, but then to reverse direction and increase continuously to 1.43 after 15 min, not quite reaching the initial F/C ratio for their PTFE. On the other hand, for Teflon PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer; symbolic structure -(CF~CFZ)~-(CF(OC3F7)CF2)y-,x/y x 491, which was shown by Momose et aL4 to behave like PTFE, F/C dropped sharply from x1.74 to ~ 1 . 0 at 0 the 0.5-min Ar plasma exposure, after which F/C decreased very slowly to 0.86 a t the 20-min e x p ~ s u r e . ~For a perfluorinated ethylene-propylene
* To whom correspondence should be addressed. t Ames Research Center.
*
Himonfllasma Science. Present address: Airco Coating Technology, 4020 Pike Lane, Concord, CA 94524. 8 Surface Science Laboratories. Abstract published in Advance A C S Abstracts, September 1, @
1994. (1)Zhou, M.; Wang, S.; Chen, J.YingyongHumue 1990,7,82[Chin. J.Appl. Chem.];Chem. Abstr. 1991, 114, 63394. (2)Tan, K. L.;Woon, L. L.; Wong, H. K.; Kang, E. T.; Neoh, K. G. Macromolecules 1993,26,2832. (3) Morra, M.; Occhiello, E.; Garbassi, F. Su$. Interface Anal. 1990, 16,412. (4)Momose, Y.; Tamura, Y.; Ogino, M.; Okazaki, S.; Hirayama, M. J . Vac. Sei. Technol. A 1992,10, 229. ( 5 ) Griesser, H. J.; Youxian, D.; Hughes, A. E.; Gengenbach, T. R.; Mau, A.W. H. Langmuir 1991, 7,2484.
0743-7463/94/2410-3629$04.50/0
copolymer (FEP; symbolic structure -(CFZCF~)~-(CF(CF3)CFdY-, xly x 41, Griesser e t al.5 reported data indicating F/C to decrease dramatically from 1.82 to 0.98 after only 1 min, a rate of change falling between the corresponding rates for PTFE indicated in the work of Zhou et al. and Tan et al. The conflicting results noted here concerning changes in the F/C ratio of fluoropolymers with time of exposure to a n Ar plasma are reflected in the accompanying changes in the O/C ratios. Thus, for PTFE, Zhou et a1.l found O/C to increase continuously with exposure time, going from ~ 0 . 0 to 1 0.40 after 20 min; Tan et al.2 found O/C to increase sharply from 0.01 to ~ 0 . 3 (or 1 0.37) after just 1 min, the ratio again referring to a n XPS take-off angle of 20"(or 75");Morra et aL3reported O/C to increase sharply from 0.02 to ~ 0 . 4 8after only 1 min, then to reverse direction and level off at xO.21-0.24 after 5-15 min. For PFA, Momose et al.4 found O/C to increase from 0.01 to ~ 0 . 1 after 4 a 2-min exposure but then to level off a t ~ 0 . 1 5 after 20 min. For FEP, Griesser et aL5 found O/C to , just 1 min, a rate ofchange increase from XO to ~ 0 . 1 6after again falling between corresponding rates for PTFE indicated in the work of Zhou et al. and Tan et al. All five studies cited above have shortcomings with respect to XPS analysis: apart from the fluoropolymers having initial F/C ratios well below the theoretical value of 2.00 for pristine material-due to the presence of adventitious hydrocarbon c ~ n t a m i n a t i o n ~ ~ ~ ~ ~are -there complications from nonmonochromatized X-ray ex~ i t a t i o n lgiving -~ rise to satellite peaks and in some cases, a t least, sample degradation.6 The work of Morra et al.3 presents a special case in that the reversals observed in the F/C and O/C ratios with time of exposure of PTFE t o a n Ar plasma are reminiscent of the "spikes" seen previously in their F/C- and O/C-exposure time plots for PTFE subjected to a n oxygen plasma7 and which we showeda were due entirely to surface hydrocarbon contamination. Although hydrocarbon contamination is not apparent in the X P S spectrum of the untreated PTFE in Figure 2 of Morra et a1.k Ar plasma paper? its presence in their polymer must be presumed since Table 1 of that paper reported a n initial F/C of only 1.52! That pre0 1994 American Chemical Society
Golub et al.
3630 Langmuir, Vol. 10, No. 10,1994
sumption is reinforced by the fact that the latter paper3 also presents new data for 0 2 plasma treatment of PTFE that essentially duplicates their prior data7 which first revealed spikes in the F/C- and O/C-exposure time plots. The existence of those spikes in the 0 2 plasma case was explained8 by concurrent oxidation and etching away of the hydrocarbon contamination to yield eventually a PTFE surface that was more or less free of hydrocarbon while possessing an oxidized as well as cross-linked,fluorocarbon structure. The spikes in Morra et ale’s Ar plasma work-but not in the other studies considered heresuggest strongly that oxygen must have entered their reactor during plasma treatment to serve as a highly effective etcher of the hydrocarbon contamination.8 Having called attention also to misinterpretations of XPS results for PTFE exposed to a nitrogen plasma when the interfering role of surface hydrocarbon contamination was not taken into a c c ~ u n twe , ~ elected to perform our own Ar plasma treatment of near-pristine PTFE and PFA films, and using a monochromatized X-ray source for X P S , with the aim of trying to resolve the conflict noted above. Although PFA was shown by Momose et ala4to behave qualitatively the same as PTFE, we included the former polymer in this work because their reported F/C- and OKexposure time plots for PFA were unlike those indicated above for PTFE1-3 or FEP;5 hence, the possibility arose that PTFE and PFA might actually respond differently t o the Ar plasma.
Experimental Section Specimens cut out of 50 p m thick PTFE film (Chemfab, Merrimack, NH) and 25 p m thick Teflon PFA 100 LP film (Du Pont Electronics, Wilmington, DE)-films used as received-were individually exposed at ambient temperature on an rf-driven electrode for periods of 0.5-20 min to an argon plasma generated in a 13.56-MHz radio-frequency Plasma Science PS 0500 plasma reactor, operated at 160 W, a pressure of 385 mTorr, and with a gas flow rate of 650 mL (STP)/min. XPS analysis of the fluoropolymer specimens, before and after Ar plasma exposure, was performed on a Surface Science Instruments SSX-101 spectrometer, using monochromatized Al Ka X-rays and with a ~~
(6)(a)Griesser et a1.,6in anote added in proof, stated that their FEP, when analyzed on a monochromatic Surface Science Instruments XPS spectrometer, actually had an F/C ratio of 1.99,instead of the 1.82 given in the body of their paper and mentioned in this report. Although the C1,spectrum of their cleaned, untreated FEP presented a satellite peak about 10 eV below the main C F z peak at 292.2eV, they concluded “that there [was] no contribution by surface contaminants ... and the lower [F/Cl values using [their XPSI unit relaterdl both to a different calibration procedure and to unavoidable sample degradation ... due to nonmonochromatic excitation,[andl this degradation reduce[dl the F/C ratio ... [thus] underestimat[ingl the fluorine content in a systematic way but [leaving]the relative values and ...trends shown [as] correct.” Hence, it was considered appropriate to cite here the changes in F/C and O/C ratios for Ar plasma-treated FEP reported by those workers, since no other comparable data on FEP exist in the literature. We took the same approach with respect to the changes in F/C and O/C ratios indicatedby Tan et aL2in their Table 1,althoughthose workers provided a single “calculated F/C ratio of 1.91 for the untreated PTFE which differed from the initial 1.61(or 1.58)cited in this paper.That calculation was “based on the C1. peak area at ... 291.4eV ... attributable to - C F 2 species, [and disregarding the] broad minor component at about 8 eV lower in [bindingenergy andl attributable to the combined contribution of ... satellite peaks ... and ... adventitious hydrocarbon CH species [amountingto ca. 9% and 6%ofthe mainx-ray component,respectively].” Also, Tan et a1.,2 aware of possible degradation occurring in the XPS spectrometer, “[ran their] X-ray source [so as] to minimize radiation damage to the polymer films.” Apropos, Yasuda et aLBbreported timedependentdegradationofuntreated PTFE within an XPS spectrometer, where the intensities of the CF2 FlSand CF2 C1, peaks each decayed by % 25% in a 5-min exposure to a Mg Ka source,accompanied by increased intensity of a “hydrocarbon”CI, peak. (b) Yasuda, H.; Marsh, H. C.; Brandt, E. S.; Reilley, C. N. J.Polym. Sci., Polym. Chem. Ed.1977,15, 991. (7)Morra, M.;Occhiello, E.; Garbassi, F. Langmuir 1989,5, 872. ( 8 ) Golub, M.A.; Wydeven, T.; Cormia, R. D. Langmuir 1991,7, 1026. (9)Golub,M.A.; Lopata, E. S.; Finney, L. S.Langmuir 1993,9,2240.
2.0T
I
I
I
5
10
15
I
0
20
Time of exposure (min)
Figure 1. Effect of time of exposure to an Ar plasma on t h e F/C ratio of FTFE, as determined by XPS analysis: 0 , this work; 0, Zhou et al.;l V, Tan et a1.;2 A, Morra e t aL3 take-off angle of 38” (measured with respect to the sample surface). The binding energies ofthe XPS peaks were referenced by setting the CF2 peak in the C1, spectra to 292.0 eV. The full width at half maximum (fwhm) for the latter peak, while not maintained during curve fitting, varied only slightly for Ar plasma-exposed and unexposed polymers: The fwhm was 1.521.54 eV for the initial FTFE or PFA and 1.68-1.72 eV for these polymers after the 20-min exposure. The initial PTFE had an FIC ratio of 1.99 and showed no detectable surface-bound oxygen. The initial PFA had an F/C ratio of 1.97 and an OIC ratio of 0.012, the latter value consistent with a vinyl ether monomer (molar) content of about 2%,o r d y % 49 in the symbolic structure for PFA given above. Based on the ratio of intensities of the C1, peaks at a 2 8 5 eV (>CH- and/or -CHz-; “hydrocarbon”)and at 292 eV(-CF2-), the surface hydrocarbon content in the untreated PTFE was negligible and that in t h e untreated PFA was slight, or ~ 0 . and 3 1.5 >CH- and/or -CH2- units per 100 carbon atoms, respectively. Because the different plasma reactors and conditions for plasma treatment of polymers used by the various workers’-5 represent a factor possibly contributing to the conflicting results noted above, we have assembled in Table 1 the essential parameters for their plasma operations as well as those used in this work.
Results Figures 1 and 2 present composite plots of F/C and O/C ratios, respectively, as a function of time of exposure to a n Ar plasma for the “new” PTFE film as well as for the PTFE films/plaques used by the other workers. The data for Zhou et al.’ were derived from a photoenlargement of their Figure 4 using a variable scale for accurate interpolations, while the data for Tan et a1.2were taken from Table 1 of their paper but for a take-off angle of 20”instead of 75”,since the former angle was closer to that (38”)used to obtain our XPS data. The data for Morra et al.3 were taken directly from Table 1 of their paper. As may be seen in Figures 1 and 2, the F/C- and O/C-exposure time plots for the near-pristine PTFE used in this work are in marked contrast to the corresponding plots for PTFE used by the other workers. Thus, contrary to either a precipitous and steady decrease in F/C ratio to very low values (0.17;0.36),1,2 or to a reversal or “spike”inthe F/C-exposure time plot,3 the F/C ratio for the “new” PTFE dropped immediately from 1.99 to 1.40 a t the 0.5-min exposure, and then it remained substantially constant up to the
Argon-Plasma-Treated Fluoropolymers
0-
5 10 15 Time of exposure (min)
Langmuir, Vol. 10, No. 20, 1994 3631
20
Figure 2. Effect of time of exposure t o an Ar plasma on the O/C ratio of PTFE, as determined by XPS analysis: 0 , this work; 0,Zhou et aL;l V, Tan et a1.;2A, Morra et al.3
1.6
1).4
y7j1
.4
0
5 10 15 Time of exposure (min)
0 20
Figure 3. Effect of time of exposure to an Ar plasma on the F/C and O/C ratios of PFA, as determined by XPS analysis: 0 , F/C; A O/C (this work); 0 F/C; A, O/C (data from Momose et
a1.4).
20-min exposure. Again, contrary either to a pronounced increase in O/C ratio to rather high values (0.40; 0.31)’~~ or to a reversal or “spike”in the OK-exposure time plot,3 the O/C ratio for the “new” PTFE increased to only 0.10 a t the 2-min exposure, and then it increased very slightly to ~ 0 . 1 at 1 the 20-min exposure. The pattern just described for the “new” PTFE, i.e., a moderate decrease in F/C ratio accompanied by a moderate increase in O/C ratio, at quite short Ar plasma exposures, followed by near-constancy of those ratios a t relatively long exposures, was also displayed by the “new”PFA film (Figure 31, although the F/C and O/C ratios for the latter polymer leveled off a t values ( X 1.57 and x 0.15, respectively) that differed somewhat from those for the “new” PTFE (1.40andO.11, respectively). Also showninFigure 3 are the F/C- and O/C-exposure time plots for the PFA film used by Momose et the ratios for which were
obtained by photoenlargement of Figure 5 in their paper. Interestingly, a similar pattern of changes in O/C ratio with Ar plasma exposure time is exhibited by Momose et al.’s PFA, with that ratio leveling off a t ~ 0 . 1 as 5 with the “new” PFA. On the other hand, the F/C ratio of Momose et al.’s PFA undergoes a n immediate drop from ~ 1 . 7 to 4 ~ 1 . 0 a0t the 1-min exposure (“paralleling”a smaller drop in F/C of ~ 0 . for 4 the “new” PFA at the same 1-min exposure), after which it continues to decrease slowly, reaching a value of ~ 0 . 8 6at the 20-min exposure. But for the hydrocarbon contamination present in Momose et al.’s untreated PFA film (amounting to some 13 >CHand/or -CH2- units per 100 carbon atoms), and the likely degradatioddefluorination of their polymer arising from the use of a nonmonochromatized X-ray source, we could envisage the F/C-exposure time plots for the “new” PFA and Momose et al.’s PFA being quite comparable. Indeed, based on the present work we suggest that the pattern (of F/C or O/C changes with exposure time) observed for both the “new” PFA and the “new” PTFE might apply also to other pristine perfluoropolymers exposed to a n Ar plasma, although the particular F/C and O/C ratios reached on prolonged exposures could differ somewhat for the individual polymers. This notion would extend to FEP despite the fact that the data for Griesser et al.’s FEP5(not plotted here for convenience but would fall between the plots for Zhou et a1.l and Tan et a1.2 in Figures 1 and 2) showed precipitous decline in F/C, along with a sharp rise in O K , a t exposures of 0.25-1.0 min. In common with Zhou et al. and with Griesser et al., we also observed small amounts of nitrogen incorporated in the A r plasma-treated fluoropolymersurfaces, presumably as a result of plasma-generated, trapped free radicals subsequently reacting with nitrogen in the air prior to the XPS analysis.l0 For the “new” PTFE and the “new” PFA films, the N/C ratios were 0.015 and 0.011, respectively, after exposure for 20 min to the Ar plasma. That the uptake of nitrogen is much less than that of oxygen in the postplasma exposure of PTFE or PFA to air reflects the fact that N2 is much less reactive with free radicals than 02.11 A similar finding regarding the relative 0 and N uptakes by Ar-plasma-treated PTFE is seen in the very recent work of Wells et a1.,12 who reported for a 20-min exposure (no other times mentioned) the following ratios: N/C = 0.024; O/C = 0.15; F/C = 0.69 (also obtained with a nonmonochromatized Mg K a X-ray source). The N/O ratio in the latter study (0.16) is remarkably close to the N/O ratio for the “new” PTFE after the 20-min exposure (0.141, despite the more pronounced defluorination ofWells et al.’s PTFE sample. Figure 4 presents typical XPS C1, spectra of the “new” PTFE film before and after Ar plasma exposure for 0.5 and 20 min. The spectrum for the untreated film displayed the expected prominent peak a t 292.0 eV (-CF2-) plus a n extremely weak peak at 284.5 eV (>CH- and/or -CH2-) due to a trace of surface hydrocarbon. After only (10) (a) Indeed, those same free radicals are considered responsible for the even larger amounts of oxygen incorporated in the surfaces of Ar-plasma-treatedfluoropolymers when those surfaces are exposed to the atmosphere during transfer from the plasma reactor to the XPS spectrometer. This view is consistent with Gerenser’s observationlob that “inert gas plasma treatments [performed in situ, i.e., in the preparation chamber of the XPS spectrometer] introduce no new detectable chemical species onto the polymer surface, but can induce degradation and rearrangement of the polymer surface.”(b) Gerenser, L. J. J.Adhes. Sci. Technol. 1993, 7, 1019. (11)Yasuda et al.,6bin what was perhaps the first qualitative XPS analysis of Ar-plasma-treated PTFE, also observed that polymer to undergo a small N uptake, accompanied by a much larger 0 uptake together with a pronounced defluorination. (12) Wells, R. K.; Ryan, M. E.; Badyal, J. P. S. J.Phys. Chem. 1993, 97, 12879.
3632 Langmuir, Vol.10,No.10,1994
Gblub et al.
1
I 693
I
I
I
689
I
I
685
Binding energy (eV)
Figure 5. XPS F1,spectra of the “new”F’TFE film before and after exposure t o an Ar plasma for 20 min. J
I 296
1
1 292
I
I 288
I
I
1
284
Binding energy (eV)
Figure 4. XPS C1,spectra of the “new”F’TFE film before and after exposure t o an Ar plasma for 0.5 and 20 min.
a 0.5-min exposure, PTFE yielded a spectrum in which the 284.5-eV peak was replaced by a composite band comprising four curve-resolved peaks in the range 285.5289.8 eV, after a 20-min exposure, that band became somewhat more intense relative to the 292.0-eV peak, and a n additional peak appeared a t 294.0 eV, assigned to -CF3 groups.13 Apart from the hydrocarbon peak a t 285.2-285.5 eV, assignments for the other three peaks in the composite band must represent overlapping contributions from various CF, CO, and CN functionalities. One plausible interpretation for those three peaks is as follows: 289.8 eV (>CF-; -CF=), 288.4-288.5 eV (>CF-0-; >C=O), and 286.7-286.9 eV (+C-CF< ; X’-O-); another interpretation derived from Momose et al.4 for essentially these same peaks found in their XPS CI, spectra ofAr-plasma-treated PFAis >CF-, > C < ,and +COO’, respectively. Plasma-induced rupture of C-F bonds and chain scission of the PTFE macromolecule, followed by capture of floating F atoms by the -CF2* chain ends, can account for the formation of CF3 groups, while the other functionalities considered here can arise from polymer degradation, free radical formation, cross-linking and postplasma reactions with 02 and NZ in the air. Because the various peaks in Figure 4 cannot be uniquely assigned, it appeared impractical to plot their relative intensities as a function of exposure time, a s was done for “-CFz, -CH, -C-0, -C=O, and - C F by Zhou et a1.l for PTFE, and for -CF3, -CFz-, >CF-, > C < , +COO and contaminants by Momose et al.4 for PFA. Figure 5 depicts typical XPS F1, spectra of the “new” PTFE before and after Ar plasma exposure for 20 min. (13)Golub, M. A.; Wydeven, T.;Cormia, R. D.J.Polym. Sci., Polym. Chem. Ed. 1992,30,2683.
The spectrum of the untreated film displays a single, essentially symmetric peak at 689.0 eV (-CFz-) which has become broadened in the spectrum of the plasmatreated film. Except for a slight component peak at 686.8 eV, the broadened peak at 689.1 eV could not be further deconvoluted into separate peaks corresponding to the various CF, (and related) moieties resulting from the plasma treatment, despite a n F/C ratio of 1.38 indicating substantial surface chemical modification. The 686.8-eV peak (XF- in a highly cross-linked network) became barely perceptible after an exposure of only 0.5 min, but it grew in intensity until it constituted ~ 2 . 4 % of the total F1, spectrum for the 20-min exposure shown in Figure 5. There was no need to show here the 01, spectra of PTFE before and after a n exposure for 0.5 or 20 min since they exhibited similar broad envelopes containing but two deconvoluted peaks, one a t =533 eV (> C=O) and another a t 535 eV (CF,-0-1. Figure 6 shows the XPS C1, spectra of the “new” PFA before and after exposure to a n Ar plasma for 0.5 and 20 min. Given PFA’s symbolic structure, -(CFzCFzX-(CF(OC~FT)CFZ),-,where x/y ~ 4 9it, is not surprising that these spectra closely resemble the corresponding spectra for PTFE [-(CFzCFz),-] shown in Figure 4. Evidently, the perfluoroalkyl vinyl ether content in PFA is too small for the -CFO- and - 0 ’ 3 functionalities to appear as distinct peaks to the right or left, respectively, of the prominent peak at 292.0 eV (-CFz-) in the spectrum of the untreated PFA. The very minor peak a t 284.9 eV in the latter spectrum indicates, again, the presence of some surface hydrocarbon ( ~ 1 . 5>CH- and/or -CHz- units per 100carbon atoms). As with PTFE, the spectra of PFA exposed for 0.5 or 20 min both show a composite band of four peaks a t 285.2-289.7 eV to the right of the - C F z peak a t 292.0 eV, while the spectrum for PFA exposed for 20 min shows the same higher binding energy peak (294.1 eV, due to -CF3) seen previously for PTFE exposed for 20 min (Figure 4). Since the F1, spectra of PFA before and after A r plasma exposure for 20 min showed only a single CF,, peak at x 689 eV, there was no need to present them
Argon-Plasma-Treated Fluoropolymers
Langmuir, Vol. 10, No. 10,1994 3633 of the correspondingspectra for the “new”PFA (Figure 6), but to a lesser extent than that noted here for the various studies on PTFE. In fact, of the prior studies presenting kinetic data for Ar plasma treatment of flu~ropolymers,’-~ that of Momose et al.4 comes closest to matching the F/Cor OK-exposure time plots obtained for the “new”PTFE or the “new” PFA.
Discussion
J
I 296
1
1
I
I
288 Bindlng energy (eV) 292
1
1
L
284
Figure 6. XPS ClS spectra of the “new” PFA film before and after exposure to an Ar plasma for 0.5 and 20 min.
here. Likewise, there was no need t o show the corresponding 01, spectra: the spectrum of the untreated PFA showed a very weak symmetric peak at x 535 eV (CF,-O-) while the spectrum of PFA exposed for 20 min showed a broad envelope comprising two deconvoluted peaks at ~ 5 3 2 . 9and 534.8 eV. Although the Ar plasma surface modifications of PFA and PTFE are consideredto yield very similar structures, the processes apparently are not identical. This is demonstrated by the different leveling-off values for the F/C and OK ratios for the two polymers (Figures 1-3) and by the presence of a peak a t x 686.8 eV in the F1, spectrum of PTFE after exposure for 20 min (Figure 5) but not in that of PFA. Evidently, the very small perfluoroalkyl vinyl ether content in PFA causes this otherwisePTFE-likepolymer to behave slightly differently from PTFE itself when subjected to an Ar plasma. Given the contrasting plots for F/C and O/C ratios as a function of exposure time indicated in Figures 1and 2, it is not surprising that the XPS C1, spectra reported by Zhou et al.,l Tan et al.,2and Morra et al.3 for PTFE exposed to an Ar plasma for 2-20 min, 40-60 s, or 0.5-5 min, respectively, have shapes or appearances that are distinctly different from those of the corresponding spectra for the “new”PTFE exposed for 0.5 or 20 min (Figure 4). And the X P S C1, spectrum reported by Wells et a1.12 for PTFE exposed to an Ar plasma for 20 min presents still another shape which is a t once unlike those in Figure 4 and those of the studies just m e n t i ~ n e d . l - ~Again, reflecting the differences in F/C ratios indicated in Figure 3, the X P S C1, spectra reported by Momose et aL4 for Ar plasma-treated PFA have shapes that differ from those
The use of PTFE and PFA virtually free of surface hydrocarbon contamination as well as the use of a monochromatized X-ray source for XPS analysis has removed two factors that likely contributed to the conflicting data from refs 1-5 regarding the F/C- and OKexposure time plots shown in Figures 1-3. Thus, in trying to account for the very sharp decreases in F/C ratio and accompanying sharp increases in O/C ratio for the PTFE films examined by Zhou et al.’ and by Tan et al.2-changes which are in marked contrast to the much smaller changes in F/C and O/C ratios observed for the “new”PTFE-we contend that the use of nonmonochromatized X-ray excitation by those workers caused their PTFE films to undergo, over and above the Ar plasma-induced degradation, an additional degradation during the XPS analysis. (In fact, Yasuda et a1.6bstated that “PTFE in particular and fluorocarbons in general are particularly susceptible to ... [XPS-induced1 damage.”) Moreover, hydrocarbon present in the surface of PTFE films would be even more susceptible to free-radical production in the Ar plasma than the base fluoropolymer, such that on subsequent contact with air prior to XPS analysis, the hydrocarboncontaminated PTFE films would experience a higher oxygen uptake than the plasma-treated, near-pristine “new” PTFE films.’* It is also likely that hydrocarbon contamination will lead to enhanced defluorination by virtue of the relative stabilities of H-F, C-F, C-H, and even F-F bonds that are broken or formed in the plasmainduced surface reactions. Such a scenario could explain in part the different F/C- and OK-exposure time plots for PTFE (Figures 1and 2) obtained in this work and by Zhou et al.’ and Tan et al. As to the corresponding plots of Morra et al.3exhibiting spikes, they have to be dismissed as artifactual, being the consequence-as noted above-of extensive surface hydrocarbon contamination in their untreated PTFE (as much as 24 >CH- andor -CH2units per 100 carbon atoms) that is largely etched away to leave an Ar plasma-modifiedsurface with an F/C ratio similar to that obtained in this work (see Figure 1). To account for the spikes in the plots of Morra et al.3but not in those ofZhou et al.‘ and Tan et aL2(or those ofMomose et al.4 for PFA), even though these other s t u d i e ~ l , ~ . ~ involved similarly hydrocarbon-contaminatedfluoropolymers, we have to presume that Morra et al.’s Ar plasma operation was compromised by the presence of unintended oxygen. This presumption is reinforced by the fact that the O/C ratio for their 15-min exposure of PTFE is somewhat higher than the corresponding ratio obtained in this work (see Figure 21, while the respective F/C ratios (14)(a)Asevidence for this, Suzuki et al.14bfound O/C andN/C ratios for polyethylene-which can serve as a model for hydrocarbon-to attain steady-state values of 0.26 and 0.086, respectively, after an exposure for only 5 min to an Ar plasma operated at 24 W. Significantly, this O/C ratio is larger than the O/C ratios of 0.11 and 0.15 for near-pristine PTFE exposed for 20 min to an Ar plasma reported in this work and by Wells et al.,l* respectively; on the other hand, that ratio is not as large as the O/Cratios of 0.40and 0.31 for the hydrocarbon-contaminated PTFE samples used by Zhou et al.’ and Tan et a1.,2 respectively. Note again that nitrogen uptake (this time by polyethylene) is a minor sidereaction associated with the “non-in situ” Ar plasma treatment of a polymer. (b)Suzuki,M.; Kishida, A.; Iwata, H.; Ikada,Y.MmromolecuZes 1986,19, 1804.
3634 Langmuir, Vol. 10, No. 10, 1994
Golub et al.
Table 1. Reactor and XPS Details Regarding Argon Plasma Treatment of Various Fluoropolymers reactor pressure, flow rate, XPS X-ray source; polymer
reference
electrodes
PTFE PTFE PTFE PFA PFA PTFE; PFA
Zhou et a1.l Tan et a1.2 Morra et aL3 Momose et al.4 Griesser et aL5 this work
externaln internalb internalC internale internap internalh
rf freq, MHz
power, W
mTorr
13.56 13.56 13.56 0.700 13.56
100 28 100 2-6@ 10 160
50 150 15 500 600 385
ml/min 50 8
40 8 650
monochromatized Mg Ka;no Mg Ka;no Mg Ka;nod Mg Ka;no Al Ka, no Al Ka;yes
a Capacitively coupled tubular reactor. b Parallel-plate reactor, with samples placed on the powered electrode. System like that of Tan et a1.2 except that samples were placed on the grounded electrode. Not specified, but presumed to be nonchromatized. e Samples mounted horizontally between parallel-plate electrodes. f The XPS data of ref 4 cited in the present paper were obtained at 40 W. g Capacitively coupled, parallel electrodes. e Box-shapedreactor equipped with three horizontaland equally spaced,internal shelves (orelectrodes)arranged in the order, powered-grounded-powered, with samples placed on the lowest, powered shelf, while the box itself is grounded.
are alike. Turning now to the somewhat different F/Cexposure time plots for PFA (Figure 3)) we can ascribe them, as suggested earlier, to surface hydrocarbon contamination and possible XPS-induced degradation in the work of Momose et al.4 but not in this work. Again, the precipitous decrease in F/C ratio for Ar plasma-treated FEP5 was previously accounted for by unavoidable defluorination caused by the use of a nonmonochromatized X-ray source.6 Another factor that might contribute to the conflicting data shown in Figures 1-3 is, of course, the variety of plasma reactors and operating conditions employed in refs 1-5 and in this work, as summarized in Table 1. Differences in reactor geometry and/or configuration, sample location in the plasma, and basic plasma parameters (which affect electron density, electron energy distribution, gas density, and residence time in the plasma as well as the concentrations and energies of electrons, ions, metastables, and ultraviolet photons) could be imagined to lead to different rates and extents ofpolymer surface modification. Thus, we can visualize those parameters and reactor configurations affecting the rates a t which the F/C and O/C ratios of the fluoropolymers level off as well as their ultimate values, but we do not regard them as a major factor causing the contrasting results described in this paper. Of the various plasma constituents-electrons, ions, metastables, UV/visible photons15-we consider the potential effect of differences in ion energy and/or ion flux in the various A r plasma reactors to be less important than the role of energetic electrons or metastables or vacuum ultraviolet photons. Although the maximum energy of ions that may be encountered in a plasma exposure can range from tens to hundreds of electron such energetic ions, if actually ~ here, are present in the Ar plasma ~ t u d i e s l -discussed not believed to have a significant impact on the different rates of change in the F/C ratios observed in those studies. This follows from the recent report16 indicating that scarcely any changes occurred in the F/C ratio of PFA exposed for 1-10 min to a n Ar+ ion beam of 500 eV, but definite decreases in F/C ratio were observed for PFA exposed to Ar+ions in the range of 1.0-3.0 keV. Although vacuum ultraviolet photons generated in Ar plasmas with energies equal to or greater than the first ionization potential of PTFE or PFA (9.93-11.28 eV), or Ar metastables (typical excitation energy of 11.5 eV), are known to promote defluorination and hence surface modification of fluorocarbon polymers,17we have no means to assess the relative contributions of such photons or Ar metastables or energetic electrons to the conflicting results (15) Morosoff, N. In Plasma Deposition, Treatment and Etching of Polymers; d'Agostino,R., Ed.;Academic Press: Boston, MA, 1990, p 20. (16)Tan, B. J.; Fessehaie, M.; Suib, S. L. Langmuir 1993, 9, 740. (17)(a)Egitto, F. D.; Matienzo, L. J. Polym. Degrad. Stab. 1990,30, 293. (b) Takacs, G . A.;Vukanovic, V.; Tracy, D.; Chen, J.X.;Egitto, F. D.; Matienzo, L.J.; Emmi, F. Polym. Degrad. Stab. 1993, 40, 73.
discussed above. Nor do we find a simple correlation between the plasma operating parameters given in Table 1 and the nature or shape of the F/C- or O/C-exposure time plots, the one common denominator being the use of a nonmonochromatized X-ray source for XPS in refs 1-5 as opposed to the monochromatized source in this work. Despite the apparently similar plasma operations of Tan et aL2and Momose et the plots obtained from their data for PTFE and PFA, respectively, are evidently very different.18 On the other hand, despite the different plasma operations of Momose et al.4 and our work, the respective plots seen in Figure 6 are qualitatively rather similar, as previously noted. At any rate, it is necessary to challenge the conflicting literature results discussed above lest the informed reader is left with the unsatisfying impression that the effect of exposure time on the surface composition of Ar-plasma-treated fluoropolymers is an uncertain or undefinable function of the particular plasma reactor employed and how it is operated. Summary
Through the use of essentially hydrocarbon-free PTFE and PFA films as well as the use of a monochromatized X-ray source for surface characterization, this work has offered a new, probably more realistic, picture concerning the effect of exposure time on both the F/C and O/C ratios of several Ar-plasma-treated fluoropolymers than that suggested by the recent literature. In this picture, there is a nearly instantaneous drop in F/C ratio, and rise in O/C ratio, followed by a more or less steady-state surface composition on prolonged exposure of PTFE or PFA to a n Ar plasma. A plausible mechanism for the levelling off of F/C and O/C ratios is a dynamic competition between Ar plasma-induced degradation (chain scission, C-F bond rupture, cross-linking, CF3 formation) and etching (or regeneration) of the fluoropolymer surface. The extent of this competition, which would determine the values a t which the F/C and O/C ratios eventually level off, could be expected to depend on the particular reactor configuration and plasma parameters selected.
Acknowledgment. The authors thank the anonymous reviewers for constructive criticism of the original manuscript. (18)We disregard the fact that different polymers were used by those workers, since the F/C-and OK-exposuretime plots for PTFE and PFA obtained in this study were obviously quite similar, as were the XPS spectra obtained by Momose et aL4for those two polymers before and after Ar plasma treatment. Likewise, we disregard the question of placementof samples on the powered electrode in one casezand between parallel plate electrodes in the other: since the PS 0500 reactor used in this study gave uniformly treated materials whether the samples were on the grounded or powered electrodes.
