2240
Langmuir 1993,9, 2240-2242
X-ray Photoelectron Spectroscopy Study of the Effect of Hydrocarbon Contamination on Poly(tetrafluoroethy1ene) Exposed to a Nitrogen Plasma Morton A. Golub,’i+ Eugene 5. Lopata) and Lorie S . Finneyc Ames Research Center, NASA, Moffett Field, Califomia 94035-1O00, HimontlPlasma Science, 353 Hatch Drive, Foster City, California 94404, and Surface Science Laboratories, 1206 Charleston Road, Mountain View, California 94043 Received March 1, 1993
In a prior note,’ it was shown that unless the surface of poly(tetrafluoroethy1ene)(PTFE) is free of hydrocarbon contamination, anomalous changes in the oxygen and fluorine contents, as measured by X-ray photoelectron spectroscopy(XPS),and hence also the surfaceproperties (e.g., contact angles, surface tension, wetting behavior), may be improperly ascribed to a PTFE film exposed to an oxygen plasma. Thus, for example, a PTFE film with fluorine-to-carbon ratio (F/C) of 1.73 (well below the theoretical F/C = 2.00 of pristine PTFE), and showing definite XPS evidence for considerable hydrocarbon contamination, was reported by Morra et al.2 to exhibit a sharp increase in surface oxygen-to-carbon ratio (OK, increasing from 0.014 to 0.129), and a correspondingly sharp decrease in F/C ratio (to 1.26),upon exposure to an 0 2 plasma for -1 min. However, at longer exposure times (-1-5 min),the changes in the O/C and F/C ratios reversed directions, and these ratios approached values similar to those of the unexposed PTFE. Another PTFE film(initial F/C = 1.96, O/C = 0.01),having less hydrocarbon contamination than that in Morra’s PTFE film, was also found’ to yield “spikes” in the O/C- and F/C-exposure time plots, but they were much less pronounced than the spikes observed with the latter, more hydrocarboncontaminatedfilm. In contrast, avery clean,hydrocarbonfree PTFE film (F/C = 2.0) yielded no spikes at all, undergoing instead a very small, monotonic increase of surface oxidation with time of exposure to an 0 2 plasma and, simultaneously,a very small, monotonic decrease in fluorinecontent.’ The existence of spikes in the O/C- and F/C-exposure time plots-evidently associated with the surface hydrocarbon content-was explained by the oxidation and concomitant etching away of the hydrocarbon contamination by the 0 2 plasma so as to yield a PTFE surface that was more or less free of hydrocarbon while possessing a slightly oxidized, and perhaps cross-linked, fluorocarbon structure. In this note, we show that the XPS data and the changes in surface properties attending exposure of PTFE films to a nitrogen plasma can likewise be misinterpreted when the interfering role of minor surface hydrocarbon contamination is not taken into account. Thus, Kusabiraki3i4 reported that N2 rf plasma treatment of PTFE (initial F/C = 1.86;O/C= 0.065;N/C = 0.019,values determined as indicated below) “reduced FldC1, to 0.95 for the initial t Ames Reeearch Center. t Himont/Plaema Science. f
Surface Science Laboratories.
(1) Golub, M . A.; Wydeven,T.; Cormia,R.D.hngmuir 1991,7,1026. (2) Morra, M.; Occhiello, E.;Garbassi, F.Langmuir 1989,6, 872. (3) Kusabiraki, M . J. Appl. Polym. Sci., Appl. Polym. Symp. 1990,46, 473. (4) Kusabuaki, M . Mem. Fac. EM., Osaka City Uniu. 1989,30, 39; Chem. Ab&. 1991,114,103391.
1 min, and simultaneously increased OIJC~,to 0.22 and N1JC1~to0.15.Thereafter,theFldC1,wasincreasedYwhile the OldC1, and NlJC1, were decreased.” These changes in elementalratios were reflected in correspondingchanges in surface tension and in contact angles of water on PTFE films with plasma treatment time. Kusabiraki explained his XPS results, in part, by defluorination of the PTFE surface at very short exposure times, followed at longer exposure times by refluorination with “cleaved fluorine atoms ... present in the plasma.” However, the reversals in values of the F/C, O/C, and N/C ratios with time of exposure to an N2 plasma for Kusabiraki’s PTFE film, the XPS spectrum of which revealed substantial hydrocarbon content,u were reminiscent of the reversals previously noted for F/C and O/C ratios for hydrocarbon-contaminated PTFE film exposed to an 0 2 plasma.’ We therefore surmised that Kusabiraki’s unusual XPS data were caused by preferential N2 plasma-induced reaction with, and concomitantetching away of, the hydrocarbon present on his film and not by his suggested defluorination-refluorination of PTFE. That view is strengthened by the fact that Kusabiraki’s PTFE f i b (F/C = 1.86),upon exposure to an 0 2 plasma, yielded O/C- and F/C-exposure time plots3p4which likewise exhibited spikes-spikes having amplitudesthat were intermediatebetween the amplitudes of the corresponding spikes for Morra’s PTFE (F/C = 1.73) and for our prior PTFE (F/C = 1.96) depicted in Figures 1 and 2 of ref 1. To confirm that view, we performed our own N2 plasma exposure of the cleanest PTFE film available to us, a 75 pm thick film from Chemfab,Merrimack, NH (initialF/C = 1.97,nodetectable surface-bound oxygen or nitrogen, and with a surface hydrocarbon content corresponding to =1.4 >CH- and/ or 4 H 2 - units per 100 carbon atoms). Figures 1 and 2 show plots of O/C, N/C, and F/C ratios as a function of time of exposure to an N2 rf plasma for the “new” PTFE film as well as for Kusabiraki’s PTFE, the ratios for the latter polymer having been obtained from a photoenlargement of Figure 8 in ref 3 using a variable scale for accurate interpolations. Figure 3 shows the C1, XPS spectra of this ”new”PTFE before and after exposure to an N2 plasma for 2 and 20 min in the same PS0500 plasma reactor (and obtained with the same SSX101 spectrometer and the same data acquisition parameters) used in the prior study.’ A rough measure of the hydrocarboncontamination in the unexposed “new”PTFE is given by the ratio of areas under the peaks at =285 eV (>CH- and/or -CHp) and 292 eV ( - C F r ) , which ratio has the value of 0.014. The corresponding ratio for the unexposed PTFE used by Kusabiraki is very much larger: 0.29 as determined by planimetering the areas under the two ClSpeaks in a photoenlargement of Figure 5 in ref 5 (published also as Figure 7, ref 3, and Figure 8,ref 4). Taking into account the nitrogen and oxygen contents in Kusabiraki’s initial PTFE (withassociated carbons falling within the broad hydrocarbon peak centered at -284 eV), we find that the XPS spectrum of that polymer corresponds to -14.1 CH,, 6.5 CO, 1.9 CN, and 77.5 CF2 units per 100 carbon atoms-for a hydrocarbon content -10 times that of the “new” PTFE. [Actually, a discrepancy exists between this analysis and the XPS data reported by Kusabiraki: the above-indicated 77.5 CF2 units translate to an F/C ratio of 1.55,not the 1.86 read off from his F/C-exposuretime plot. Nor have we been able to obtain internal consistency in elemental analysis from the XPS spectra= for Kusabiraki’sPTFE exposed to an N2 plasma ( 5 ) Kusabiraki, M . Jpn. J . Appl. Phys. 1990,29,2809.
0743-7463/93/2409-2240$04.00/00 1993 American Chemical Society
Notes
Langmuir, Vol. 9, No. 8,1993 2241
.20
.I 5
.-
0
1
B
8 b .10
Y 2
.05
0
0
5
10
15
20
Time of plasma exposure (min)
25 296
Figure 1. Effect of time of exposure to an Nz plasma on the O/C and N/C ratios of PTFE, as determined by XPS analysis: 0, O/C;A, N/C (‘new” PTFE); 0,O/C; A, N/C (data from
KusabirakW).
292 288 Blnding energy (eV)
284
Figure 3. C1,XPS spectra of “new”PTFE (initial F/Cratio of 1.97)before and after exposure to an Nz plasma for 2 and 20 min.
the N/C and O/C ratios, we arrive at the following distribution of functionalitiesfor the 2-minexposure: 87.4 C F 2 , 4.5 CF, 3.9 CO, 3.1 CH,and 1.1 CN. The correspondingdistribution for the 20-min exposure is 87.7 CF2, 3.4 CF,2.8 CO, 4.7 CH, and 1.4 CN. These XPS data yield a‘calculated” F/C ratio of 1.79 for both the 2- and 20-min exposures-aratio which is consistentwith the “measured” F/C ratio of 1.75 obtained directly from the XPS survey scans for both of these N2 plasma-treated PTFE films as seen in Figure 2. Returning to Figure 1, we first observe that Kusabiraki’s PTFE exhibits much greater oxygen and nitrogen uptakes when exposed to an N2 plasma than does the “new”PTFE. Moreover,in common with the former polymer, the latter also exhibits maximal 0 and N uptakes at relatively short exposuretimes, followed by progressivelysmaller uptakes with increasing exposure time. Significantly, the amplitudes of the “spikes”in the O/C-and N/C-exposure time plots for Kusabiraki’s PTFE, with a hydrocarbon content -10 times that of the “new” PTFE, are 40.208 - 0.065)/ 0.040 or 3.6, and (0.135 - 0.019)/0.022 or 5.3, times the corresponding amplitudes, respectively, for the “new” PTFE. The very large maximal O/C and N/C ratios indicated for Kusabiraki’s PTFE clearly must be due to that polymer’ssubstantial hydrocarbon content inasmuch as Foerch et ale6obtained N/C and O/C ratios as high as -0.37 and 0.25, respectively, for polyethylene (PE)-a typical hydrocarbon polymer-exposed to an N2 microwave plasma for 1 min. Polypropylene, another common hydrocarbon polymer, was reported by Poncin-Epaillard et al.’ to yield maximum N/C and O/C ratios of -0.24 and 0.15, respectively, after exposure to a similar plasma for 5 min. On the other hand, Marchant et al.8obtained N/C ratios of 0.062 and 0.087, and O/Cratios of 0.089 and 0.069,
.
0
5
10
15
20
25
Time of plasma exposure (min)
Figure 2. Effect of time of exposure to an NOplasma on the F/C ratio of PTFE,as determined by X P S analysis: 0 (‘new” PTFE); 0
(data from Ku~abiraki~*~).
for 1 or 20 min. Both those spectra show a very broad, intense band extending from -281 to 288 eV (functionalities other than (3’2) in addition to the prominent C F 2 band centered at -291 eV, the areas of these two bands being in the ratios of =1.3:1 and 0.851, respectively!] At any rate, the XPS spectrum for Kusabiraki’s untreated PTFE is simply not representative of a clean fluorocarbon polymer surface, in contrast to the rather clean initial PTFE depicted in Figure 3. As indicated in that figure, exposure of the “new”PTFE to an N2 plasma for 2 (or 20) min has led to the replacement of the very weak CI, XPS peak at 285.1 eV by the composite band containing curve-resolved peaks at 284.9 (or 285.1),286.7 (or 287.31, and 289.2 (or 289.4) eV, these peaks together represent contributions from various CH, CN, CO, and CF functionalities. If, as a rough approximation, we associate the peak at 284.9-285.1 eV with CH and the peak at 292.0 eV with C F 2 , and we taken CN and CO to be determined from
(6)Foerch,R.;Izawa,J.;McIntyre,N.S.;Hunter,D.H.J.Appl.Polym. Sei., Appl. Polym. Symp. 1990,46,415. (7) Poncin-Epaillard,F.; Chevet, B.; Broeae, J. C. Makromol. Chem. 1991,192,1589.
2242 Langmuir, Vol. 9, No. 8,1993
Notes
followed by progressive etching away of the hydrocarbon, for polypropylene exposed to an N2 rf plasma for 1 and 20 min, respectively-ratios that are higher than the as the dominant plasma-induced process. As in the case corresponding maximum ratios (N/C = 0.022; O/C = 0.041) of the O/C and N/C plots, the amplitude of the spike in observed in Figure 1for the “new” PTFE exposed to an the F/C exposure time plot for Kusabiraki’s PTFE (AN2 rf plasma. The incorporation of CO along with CN (F/C) = -0.91) is greater than that (-0.33) for the “new” functionalitiesin these N2 plasma treatments (evenwhere PTFE. It appears from Figures 1and 2 that both PTFE the highest purityN2 was used) is a common o b s e r v a t i ~ n ~ ~ films ~ upon prolonged exposure to an N2 plasma tend that is explained by the reaction of free radical sites toward a similar fluorocarbon surface compositionhaving generatedin the polymer surfaceswith atmosphericoxygen relatively low 0 and N contents, once the hydrocarbon when those surfaces are removed from the plasma for contamination is fully etched away. This is in keeping subsequent XPS analysis. Gerenser,Io who obtained a with the expectation, based on prior 0 2 plasma treatment saturation-level N/C ratio of -0.25 in his N2 rf plasma of a pristine PTFE film (initial F/C = 2.00),1 that PTFE treatment of PE, was able to avoid 0 uptake by having per se would undergo a very small monotonic increase in that operation conducted “in the preparation chamber of O/C and N/C ratios with time of exposure to an Nz plasma, the [XPS]spectrometer,”thereby preventing any exposure eventually attaining steady-state F, 0, and N contents as of the treated film to the atmosphere prior to surface a result of a dynamic competition between oxidation/ analysis. For our N2 plasma treatment of PTFE, that was nitrification and etching (or surface regeneration) of the neither possible nor desirable, since we wished to obtain PTFE. Unfortunately, a totally hydrocarbon-free PTFE 0 and N uptakes for a PTFE with as little hydrocarbon was not available to test that expectation. However, the contamination as possible for comparison with Kusabi“new”PTFE discussed in this note, apart from displaying raki’s results. another example of the effect of hydrocarbon contamiThe changes in the O/C and N/C ratios for the N2 nation on the XPS of plasma-treated PTFE, apparently plasma-exposed PTFE (Figure 1) are, of necessity, acconstitutes the “cleanest” sample reported to date that companied by corresponding changes in the F/C ratios has been subjected to an N2 plasma. To conclude, we (Figure 2). Acting in a manner opposite to that of the O/C suggest that in future studies on plasma-induced effects and N/C ratios, the F/C ratios first decrease to minimum in fluorocarbon polymers in particular, and in all polymers values at short exposure times, after which they increase in general, researchers should carefully examine by XPS to higher values at longer exposure times. This again their initial, untreated polymer films for potential surface points to 0 and N uptake by the hydrocarbon surface contaminationso as to avoid reporting spuriousdata which (and to apparent loss of F by the underlying PTFE), do not represent real effects for the polymers investigated. (8) Marchant, R. E.;Chou, C. J.; Khoo, C. J. Appl. Polym. Sei., Appl. Polym. Symp. 1988,42,126. (9) Yaeuda, H.; Marsh, H. C.; Brandt, E.S.;W i e y , C . N . J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 991. (10)Gerenser, L.J. J. Adhee. Sci. Technol. 1987,1, 303.
Acknowledgment. The authors thank Robert D. Cormia of Surface Science Laboratories for helpful discussions concerning XPS data for plasma-treated PTFE.
