3360
Langmuir 1996, 12, 3360-3361
Concerning Apparent Similarity of Structures of Fluoropolymer Surfaces Exposed to an Argon Plasma or Argon Ion Beam Morton A. Golub Ames Research Center, NASA, Moffett Field, California 94035-1000 Received September 22, 1995. In Final Form: April 3, 1996
Recently, Wells et al.1 noted a strong chemical similarity between the argon plasma modification of poly(tetrafluoroethylene) (PTFE) and the low-energy argon ion beam irradiation of PTFE, and they stated that “[the latter treatment] serves as a good model study for [the former] (where ion-surface interactions are regarded as being a predominant process).” That statement was evidently based on the finding that the C1s X-ray photoelectron spectrum (XPS) of PTFE exposed to an Ar glow discharge at 50 W and 200 mTorr for 20 min resembled the C1s XPS spectrum of PTFE subjected to a prolonged Ar+ ion beam of 0.8 keV energy. The apparent parallelism between Ar plasma- and Ar+ ion-beam-induced changes in PTFE was explained in terms of similar physicochemical phenomena associated with the respective polymer surface modifications. In particular, the ion-beam treatment was stated “to result in the emission of electrons, neutrals, ions and photons at comparable energies to those typically found in non-isothermal glow discharges, [the latter processes involving ions] that can range from tens to hundreds of electronvolts.” Another example of the parallelism between Ar plasma and Ar+ ion beam treatment of a fluoropolymer may be seen in the very similar C1s XPS spectra of Teflon PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer; symbolic structure -(CF2CF2)x-(CF(OC3F7)CF2)y-, x/y ≈ 49), subjected to an Ar plasma2 at 160 W and 385 mTorr for 20 min or to an Ar+ ion beam3 of 3.0 keV energy for 10 min. Since PFA was foundsnot unexpectedlysto yield C1s XPS spectra before and after exposure to an Ar plasma that were, respectively, nearly identical to the corresponding spectra obtained for PTFE2,4 (structure -(CF2CF2)n-), these two polymers may be regarded as virtually alike5 from the standpoint of their response to an Ar plasma and presumably also to an Ar+ ion beam. Thus, it was surprising to observe that the pair of similar C1s spectra for PTFE subjected to an Ar plasma or an Ar+ ion beam (ref 1) differ markedly in appearance from another set of very similar C1s spectra for PFA or PTFE exposed to an Ar plasma (ref 2) or for PFA exposed to an Ar+ ion beam (ref 3). Regrettably, no C1s spectrum for Ar+ ion beam-irradiated PTFE was obtained by Tan et al.3 that would have provided a more direct comparison with the C1s spectrum for the Ar plasma-treated PTFE shown in ref 2. This note aims (1) to extend and reinforce the observation made by Wells et al.1 that there is a remarkable similarity between the microstructural changes produced in PTFE when subjected to either an Ar plasma or an Ar+ (1) Wells, R. K.; Ryan, M. E.; Badyal, J. P. S. J. Phys. Chem. 1993, 97, 12879. (2) Golub, M. A.; Lopata, E. S.; Finney, L. S. Langmuir 1994, 10, 3629. (3) Tan, B. J.; Fessehaie, M.; Suib, S. L. Langmuir 1993, 9, 740. (4) Momose, Y.; Tamura, Y.; Ogino, M.; Okazaki, S.; Hirayama, M. J. Vac. Sci. Technol. A 1992, 10, 229. (5) The perfluoroalkyl vinyl ether component in PFA is too small (≈2 mol % in the polymer) for the -CFO- and -CF3 functionalities to appear as distinct peaks flanking the dominant -CF2- peak in the C1s spectrum of untreated PFA. The spectrum is thus practically identical to the C1s spectrum of untreated PTFE.2
Figure 1. XPS C1s spectra of PFA films drawn from different sources: (A) film exposed to an Ar plasma2 at 160 W and 385 mTorr for 20 min; (B) another film exposed to an Ar+ ion beam3 of 3.0 keV energy for 10 min.
ion beam by including the closely related fluoropolymer PFA and (2) to offer a plausible explanation for the fact that the very similar C1s XPS spectra for PFA or PTFE subjected to an Ar+ ion beam3 and/or an Ar plasma2 have markedly different shapes from those of the other similar pair of C1s XPS spectra for PTFE subjected to the same kinds of surface modification.1 Figure 1 is a composite of tracings of C1s XPS spectra of Ar plasma- (A) and Ar+ ion-beam-treated (B) PFA found in refs 2 and 3, respectively. Clearly, these spectra are remarkably similar, each possessing five minor curveresolved peaks in addition to the prominent peak at 291.8 or 292.0 eV (-CF2 -). Apart from a hydrocarbon peak at ≈285.2 or 286.1 eV and a peak at 293.9 or 294.1 eV (-CF3), the other three peaks correspond to overlapping contributions from various CF functionalities as well as extraneous CO and CN groups, with comparable assignments made by several groups,2-4 of which one set is as follows:2 289.7 or 289.8 eV (>CF-; -CFd), 287.9 or 288.7 eV (>CF-O-; >CdO), and 286.5 or 287.3 eV (mC-CF
