J. Phys. Chem. 1994, 98, 12442-12446
12442
Surface Defluorination of PTFE by Sodium Atoms S. Tasker, R. D. Chambers, and J. P. S. Badyal' Department of Chemistry, Science Laboratories, University of Durham, Durham DHl 3LE, England, U.K. Received: June 22, 1994; In Final Form: August 30, 1994@
Reaction of sodium atoms with poly(tetrafluoroethy1ene) (PTFE) under ultrahigh vacuum conditions has been followed by X-ray photoelectron spectroscopy (XPS). Surface defluorination and formation of NaF are found to occur. Initial stages of reaction result in desorption of NaF molecules; this is followed by permeation of Na adatoms into the polymer subsurface region to yield trapped NaF moieties.
Introduction
100
Poly(tetrafluoroethy1ene) (PTFE or Teflon) consists of a fluorinated straight chain polymer, (-CFzCFz-),; the stable C-F bond gives rise to its chemical inertness. Metallization of poly(tetrafluoroethy1ene) has been the subject of numerous studies; this is important for electronic device manufacture,' for the improvement of polymer wear characteristic^,^-^ and packaging t e ~ h n o l o g y . ~A, ~variety of methods are available for the metallization of PTFE;these include hot wall chemical vapor deposition (CVD),7electron beam evaporation,8 resistive evaporation: mechanical transfer: and catalytic or thermal reduction of mineral salt supported on a PTFE ~ubstrate.~ Electron beam,7 X-ray irradiation?3l0 ion beam sputtering,' and wet ~ h e m i c a l ~ J ~treatments 9'~ of the PTFE substrate prior to deposition, and X-ray irradiati~n,'~ ion b~mbardment,'~ and thermal treatments following metallization can also be employed to achieve the desired physicochemical characteristics at the metdpolymer interface. Despite the relative chemical inertness of PTFE, most metal fluxes do adhere to the PTFE surface, usually forming a boundary layer with a mixed metaumetal fluorideldefluorinatedFWEPTFE composition.l5 The adhesion strength of the metal layers bonded to PTFE is reported to be primarily dependent upon the reactivity and size of the metal adatom.' Less reactive metals such as gold tend to form metallic 0verlayers.8-l~ In the case of more electropositive metals, a breakdown in the polymer substructure becomes likely. An important example of the latter category is Na/NH3 wet chemical defluorination of PTFE;this treatment is used industrially to improve the bonding characteristics of PTFE.16 Although surface science studies of alkali metal adsorption onto and s e m i c ~ n d u c t i n g ~substrates ~ - ~ ~ are widespread, until now there have been no well-defined investigations published which address the mechanistic aspects of alkali metal deposition onto polymeric surfaces. In this article, sodium metallization of PTFE under ultrahigh vacuum conditions is used to model the wet chemical Na/NH3 defluorination of PTFE. Current understanding of this chemical etch reaction is fairly limited due to indirect methods having being previously employed which suffered from the major drawback of surface oxidation of the treated material prior to characterization.1°
-0- %Na
90
g
.*
80
-f %F-
Z 70
8
3
60
6 50 Q:
%
40
d
2
30
8 20 10
0 0
50
100
150
Time /mins Figure 1. Elemental composition as a function of sodium metal exposure.
a Vacuum Generators ESCALAB spectrometer (base pressure 2x mbar). The sodium source was thoroughly outgassed prior to deposition. Metal dosage was controlled by using a rotatable shutter arrangement. X-ray photoelectron spectroscopy (XPS) studies were performed using an unmonochromatized magnesium K a X-ray photoexcitation source and a hemispherical analyzer operating in the constant analyzer energy mode (CAE, 50 eV pass energy), with an electron take-off angle of 30" from the substrate normal. A PC computer was used for XPS data accumulation and component peak analysis, assuming linear background subtraction and Gaussian fits with fixed full-width-at-half-maximum (fwhm).26 All binding energies are referenced to the C(1s) core level at 291.2 eV for the (-CF2-) linkage contained in PTFE.27,28Instrumentally determined sensitivity factors for unit stoichiometry were taken as C( ls):F( ls):Na( 1s) equals 1.OO: 0.20:O. 18.
Results Experimental Section Additive-free FTFE film (Goodfellows) was washed in a 50: 50 isopropyl alcohoyhexane mixture and then dried in air. A sodium metal vapor source (SAES Getters) was mounted onto
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, October 1, 1994.
0022-365419412098-12442$04.50/0
The variation in elemental concentration at the polymer surface with increasing sodium dosage is summarized in Figure 1. No oxygenated species were observed during metal uptake. The overall attenuation in C( 1s) intensity is associated with the surface build-up of sodium species; however, it is of interest to note that there is an initial increase in C(1s) peak area with sodium exposure. 0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 47, 1994 12443
Surface Defluorination of PTFE by Sodium Atoms
3 Mins V
0 Mins 280
284
288
292
296
Binding Energy /eV Figure 2. XPS spectra of PTFE as a function of sodium metal dosage: (a) C(1s); (b) F(1s); and (c) Na(1s).
Clean PTFE displays a main C(1s) peak at 291.2 eV and a weak Mg Ka3,4 satellite at lower binding energy, Figure 2a. Sodium metallization of PTFE resulted in a marked change in the appearance of the C(1s) envelope. This can be fitted to six Mg Kat,, component peaks, Figure 3a, corresponding to4,8,27,28 (284.3 eV), =C-CF,(286.6 eV), =FC-C= (288.3 eV), =CF-CF,(289.5 eV), -CF*- (291.2 ev), and -cF3 (293.6 eV). Additional Mg Ka3; satellites (with a different fixed fwhm) were also taken into consideration. The relative variation of these component peaks is consistent with sodium atoms defluorinating the F'TFE surface to form predominantly cross-linked/graphitic/olefiniccarbon species,* partially fluorinated carbon moieties, and -CF3 groups at the expense of -CFzcenters, Figure 4. Two components are discemible in the F(1s) peak which correspond to covalently bonded fluorine atoms in the 688689 eV range and fluoride ions at approximately 685 eV,29 Figure 3b. The latter species increases in concentration with sodium exposure, Figure 1. There is a shift in the Na( 1s) core level binding energy from 1070.3 eV (8 min deposition) to 1071.5 eV (95 min deposition), Figure 5. This compares with quoted Na(1s) binding energies of 1071.8 and 1071.2 eV for sodium metal and sodium fluoride, r e s p e c t i ~ e l y . It ~ ~is interesting to note that the formation of the plateau in the elemental composition plot and the reduction in the rate of change of the Na( 1s) shift coincide at around 55 min total deposition time. Further deposition leads to an increasingly metalized surface layer and a Na( 1s) binding energy approaching that of metallic sodium at 1071.8 eV.29
-cx-
Discussion The sticking probability of sodium atoms to the PTFE surface is primarily govemed by the thermodynamic driving force for the formation of NaF: -(CF,-CF,),-(s)
+ 2nNa(g)
-
-(CF=CF),-(s)
AHF x -1400 kJ mol-'
+ 2nNaF(s)
(where n = 1)
Therefore, the heat of formation for the reaction of Na(g) with PTFE to yield NaF is extremely favorable. The initial stages
of reaction of sodium atoms with the PTFE surface can be expected to produce NaF molecules and free radical centers along the polymer backbone, Scheme 1. At the polymer surface, the NaF molecules experience low attractive forces with the underlying substrate and therefore will desorb under ultrahigh vacuum conditions to leave behind a carbon-rich surface. This description is consistent with the observed rise in elemental carbon concentration at the PTFE surface for short sodium exposures, in conjunction with the loss of covalent fluorine centers, and the absence of fluoride ions, Figure 1.
SCHEME 1: Initial Defluorination Na
c
-
-CF---CF,--CF,-
t
NaF
It is important to realize that one of the major physicochemical differences between polymers and bulk metals or inorganics is that the former class of materials are much less rigid and therefore allow permeation of species into the bulk. The core level electron escape depths30 (Na(1s) = 5 A, F(1s) = 10 A, and C( 1s) = 20 A) are sufficiently large that there will also be some sampling of the polymer subsurface during XPS analysis. Larger doses of sodium can result in the permeation of metal atoms through the defluorinated surface and into the subsurface region, where they undergo reaction with C-F bonds to form buried NaF molecules; the relatively larger size of this product species will attenuate its mobility, thereby causing it to become trapped in the subsurface. As a consequence, these NaF molecules will now be detected by XPS. On this basis, one can also account for the observed trend in Na(1s) core level binding energies, Figure 5 : isolated NaF molecules will possess a degree of covalent character;' thereby yielding a lower Na(1s) core level binding energy. As the subsurface concentration of NaF molecules builds up, (NaF), clusters32will start to form and eventually NaF crystallites; this will in tum cause an increase in Na( 1s) core level binding energy, approaching the literature value for bulk NaF, Figure 5. Prolonged exposure of Na atoms will eventually lead to a metallic layer being formed on top of the carbon-rich surface layer; hence, the Na( 1s) core level binding energy will finally reach the reported value for bulk sodium Growth of the sodium metal overlayer
12444 J. Phys. Chem., Vol. 98, No. 47, 1994
Tasker et al. 100 90
g
.* 42
$
s
m Y G
80
70
60 SO
u
10
0 0 280
284
288
292
100
150
Time /mins
296
Figure 4. Variation of C(1s) environments with increasing sodium
Binding Energy /eV
..
SO
metal dosage.
2
F(ls:
1
3
3 2
0.4
-
0.2
:
0.0
-
$
-0.2
-i2
-0.4
+
.r(
cd
3cn n VI 4
s
v
-0.6
-0.8 -1.0
0 680
684
688
692
50
100
150
Time /mins
696
C(1s)
Figure 5. Variation of Na(1s) core level binding energy with increasing sodium exposure (referenced with respect to the Na(1s) binding energy of 1071.2 eV for NaF solid), f0.05eV.
begins at approximately 50 min, where an attenuation in the rate of change of Na(1s) signal intensity and Na(1s) binding energy is seen, Figures 1 and 5 , respectively. The observed defluorination of FTFE by sodium atoms is in agreement with previous metallization studies: Pb,2,3 Zn,2s3 Cr,1-33 Sn,22 Cd,' In$ Sn$ cu,1S97,11Ti,1,5 4,132 Au,1SS14Ag,4 and Ni.4333 In all of the aforementioned cases, except for Au, a mixed metal fluoride/polymeric interfacial region is reported. Carbides with a C(1s) XPS binding energy of 282.0 eV5 tend to form for the more reactive multivalent metals, e.g. Cr and Ti d e p o s i t i ~ n . ~ItJ ~is interesting to note that carbide species are not generated during the exposure of sodium atoms to PTFE polymer; instead the carbonaceous species formed are more akin to those observed following radiation-induced cross-linking of PTFE." In the case of a monovalent atom such as sodium, there will be only one valence electron available for transfer to the fluorine atom in the C-F bond, thereby leaving a free radical site behind on the polymer backbone. Whereas for multivalent
metal adatoms, there exists the possibility for any surplus valence electrons participating in carbide formation. Mechanistic theories have emphasized the importance of electron transfer processes during the reaction of Na/NH3 with Also there are a wealth of studies concerned with surface-induced cross-linking of PTFE by various methods which include electron beam irradiation,' 1,34-37 X-ray irradiation,11,L3J5,39-41 RF-induced chemical treatm e n t ~ ? ~ electrochemical ,~~,~~ and ion beam b~mbardment.~ The ~ ,resultant ~~~~~ chemical ~~ functionalities are similar in nature to those observed in this study (although in some cases to differing degrees). Generally, reaction is considered in terms of radiation-induced free radical chemistry,11 the first step of which has been suggested to be the cleavage of the C-F bond due to low-energy valence excitation^,^^ forming -CF F radical species. The complexity of the subsequent reactions has prevented a thorough understanding of the chain reaction, but the free radical centers on the polymer surface are believed to undergo branching and cross-linking to yield C,.
Binding Energy /eV Figure 3. Core level peak fit of metalized PTFE (130 min):
(a)
and (b) F(1s).
+
Surface Defluorination of PTFE by Sodium Atoms
J. Phys. Chem., Vol. 98, No. 47, 1994 12445
fluoride ion52)on the PTFE chain, leading to the formation of an unsaturated fluorinated chain and -CF3 species, Scheme 3A. It is worth emphasizing that molecular NaF would offer a fluoride ion in an unsolvated form, which is devoid of a bulk crystal lattice, hence an inherent reactivity that could not be realized under conventional synthetic organic chemistry conditions, thus giving rise to extremely strong nucleophilic behavior. An alternative mechanistic explanation for -CF3 formation could encompass fluorine atom migration during the rearrangement of a -CS* end group, Scheme 3B. Clearly, the present study serves as a good model system for the wet chemical3 " treatment of FTFE16 since bulk NaF and extensive defluorination of the polymer substrate are observed in both cases, although the etching rate is much greater for the latter, due to the higher concentration of Na atoms in the vicinity of the PTFE surface.
SCHEME 2: Cross-linking
P
+F-cF~-
OR
F
Conclusions -CF-CF2-
SCHEME 3: CF3 Formation (A)
-CF,
r:
+ ?F,-CF-CF,-
+
Na+
cf:
I I
-CF,
+ CF,+F-F-
Sodium deposition onto PTFE polymer leads to surface defluorination and NaF formation. Initial reaction of sodium atoms with PTFE results in an increase in carbon concentration and loss of covalent fluorine at the surface, which may be taken as being indicative of desorption of NaF product molecules. Longer metal deposition times give rise to sodium atoms diffusing into the polymer subsurface, where trapped NaF moieties can eventually coalesce to form NaF crystallites. The absence of carbide species is indicative of the single valence electron donation mechanism for defluorination to yield polymer radical species. Secondary reactions of these radical centers cause polymer backbone cleavage, rearrangement, and crosslinking. Prolonged exposure of sodium atoms causes the buildup of a metallized surface layer.
References and Notes
+ NaF
4 etc
(B)
W F , > C S , and -CF3 groups, as identified by xPS.39 Some examples of such cross-linking mechanisms are illustrated in Scheme 2. In the present study, clearly there is a significant amount of -CF3 group generation during sodium exposure. The likely absence of F species due to the energetically favorable formation of NaF, Scheme 1, means that -CF3 groups can not form via the conventional r e c ~ m b i n a t i o n of ~ ~-CF$ , ~ ~ radical species with F.A more plausible reaction pathway may involve nucleophilic attack by the NaF molecule (or effectively the
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