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SSIMS (FABMS) studies on plasma treated polymers
218
Surface
SSIMS (FABMS) STUDIES E. OCCHIELLO, Isriruro
Donegmi 18 June
ON PLASMA
F. GARBASSI @A, accepted
Vu Fouser, for
Science 211/212 (19X9) 21X-226 North-Holland, Amsterdam
TREATED
POLYMERS
and M. MORRA Norwrrr. It& 23 September
The use SSIMS in plasma treated surfaces obtained information the site attack of species on ing. loss aromaticity and are described. and is assessed.
is
Examples polymer surface, the complementarity
of XPS
1. Introduction Static secondary ion mass spectroscopy [l] is by now an established surface analysis technique, with a wide range of applications. The importance of acquiring mass spectra by sputtering fragments out of polymeric surfaces without a remarkable damage was originally recognized by the groups of Vickerman [2,3], Briggs [4,5], Hercules [6] and Benninghoven [7]. Plasma surface treatments of polymers are known since more than twenty years with the general aim to modify the surface in order to improve wettability, adhesion, friction, etc. [8,9]. Only very recently SUMS started to be applied to plasma [lO,ll] and flame [12] treatments of polymeric surfaces.
2. Experimental To obtain SSIMS-FABMS (fast atom bombardment mass spectroscopy) spectra a VG SIMSLAB instrument was used [3,12214]. Static SIMS spectra were collected using 2 keV Ar atoms with a flux density of 3 x 10’ atoms/ cm*. s. With a combined set-up and spectra acquisition time of 600 s the total dose for positive and negative ion spectra was approximately 2 x 10” atoms/cm* per sample. This falls well within the established regime for static SIMS spectra of “undamaged” polymer surfaces [15]. The base pressure in the Pa, but the working pressure during operation of the system was lo-’ ion/atom gun was in the range of lo-’ Pa of Argon. An electron flood gun (VG LEG 31, 500 eV energy, 0.1 nA, 110 PA/cm’ current density) was used to stabilize the surface potential when sample charging occurred. 0039-6028/89/$03.50 c Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
E. Occhiello et al. / SSIMS
studies on plusma treated polymers
219
The plasma treatments were performed at the University of Bari in a small home-made parallel plate reactor. The experimental apparatus is described in detail elsewhere [16]. In the present work the conditions for all the experiments were the following: pressure 0.2 Torr ( - 18 Pa), gas flow 30 cm3 (STP)/min. RF power 150 W (13.56 MHz generator), treatment time 5 min.
3. Results SSIMS is a very good complement to XPS in describing the effect of polymer surface treatments. XPS provides semiquantitative information about the composition of a < 5 nm thick surface layer, together with partial information about the bonding of carbon and other elements in this layer. SSIMS is restricted to a < 1 nm thick layer and gives useful qualitative information on bonding. 3.1. Site of modifications A good example of the ability of SSIMS to provide information about the site of attack of active species present in a plasma is a polypropylene (repetitive unit: -CH(CH,)-CH,-). Three possible sites are available for attack: CH,, CH, and CH. On the basis of conventional chemical knowledge the energy requirement sequence for reaction on these sites is -CH- < -CHZ- < -CH,. In fig. 1 the positive SSIMS spectra of untreated, oxygen and argon plasma treated polypropylene are presented. In both treated samples a remarkable decrease in the relative intensity of the peaks at 15 (methyl cation) and 69 (dimethylcyclopropyl cation) is observed, pointing to loss or modification of pendant methyl groups, as already observed in the case of flame treatments [12] and suggested for reactive gas treatments with F,/O, mixtures
P71. 3.2. Crosslinking Surface crosslinking as a consequence of plasma treatment with inert gases (CASING: cross-linking by activated species of inert gases) has been suggested in the past by Shonhorn et al. [18]. It is a very good example to show how XPS and SSIMS complement each other. For polypropylene samples treated with oxygen and argon plasmas XPS provides information about the amount of oxygen introduced by the treatment. But no evidence can be obtained about crosslinking. In figs. 2a and 2b the C(ls) peaks of Ar and 0, plasma treated polypropylene are presented. Clearly the amount and chemistry of oxygen functionalities introduced at the sample surface are very similar. But, nothing can be said about crosslinking.
220
E. Occhiello et al. / SSIMS studies on pfusnm treated pol)~~rs
n
25
Mass
Fig, 1. P&tive
ion SSIMS (FAMBS) spectra relative to untreated plasma treated (c) polypropylene.
(Daltons)
(a). Ar pkma
treated
(b). 0,
221
E. Occhiello et al. / SSIMS studres on plasma treated polymers
1
I
282
I
284
1
I
I
288
1
I
288 Binding
Fig. 2. XPS C(ls)
peaks
relative
to Ar plasma treated propylene.
I
1
290
(a) and
energy
Oz plasma
treated
(b) poly-
On the other hand SSIMS is very sensitive to alterations in the backbone of the polymer. It is interesting to observe the difference between the spectra of 0, and Ar plasma treated polypropylene (figs. lc and lb). In both cases there is evidence of reaction on methyl groups, as shown by the reduction in the
222
CH3
C CH3
CH,-
C -
\ Ar”
Av(2keV) \‘\i
I CH
CH,--
-.
. A?(2
-
CH, -
+
~-
-
CH,-
-
CH,
CH
(2keVi
-
I -
CH
--
CH2
keV)
CH I CH2 I
Fig. 3. Possible
CH-
rearrangement routes leading. under rnergrtlc partlcle bombardment, emission of methylcyclopropyl iotls from crosslinked polypropylene.
to the
intensity of the peaks at 15 and 69 amu (methyl and dimethylcyclopropyl cations). The difference lies mainly in the relative intensity of the 55 daltons (methylcyclopropyl cation) peak. which becomes markedly higher after the Ar treatment. The scheme in fig. 3 shows how links between polymer chains can lead, under bombardment by energetic Ar atoms, to methylcyclopropyl fragments. So crosslinking seems rather important after the Ar plasma treatment. but not so in the 0, plasma treatment case. 3.3. Aromaticit~,~ To study the aromaticity of polymer surfaces by XPS it is necessary to take into consideration the shake-up features due to the aromatic ring and their variations upon plasma treatment [19]. Unfortunately. shake-up features are broad and weak. furthermore they overlap with components due to carboxyl acids and CF, groups. On the other hand the fragmentation of aromatic and aliphatic groups is quite different, so the examination of the relative intensities of typical fragments can give information about the damage to aromatic rings. To do this we chose to plot the normalized intensities of some of the peaks versus the composition of the gas feed to obtain information about the mechanism of the plasma-surface interaction. The normalization of intensities has been obtained using the peaks at 41 daltons (positive ions spectrum) and at 25 daltons (negative ions spectrum) as internal standards.
E. Occhiello er al. / SSIMS
n.t.
10
studes
30
on plusmu treated po(vmer.s
50
70
223
90
% 0, Fig. 4. C,Hl/C,Hc
intensity ratio as a function of O2 percentage in the gas feed of CF,/Oz plasmas: (a) polycarbonate, (b) polyethyleneterephtalate.
Interesting information was obtained in the case of polycarbonate and polyethyleneterephtalate treated with CF,/O, plasmas. In fig. 4 the ratio of the C,H: ion (typical of aromatic groups) to the C,Hc ion (typical of aliphatic groups) is plotted versus the 0, percentage in the gas feed. We observed that the oxygen enrichment in the plasma led to extensive damage of the aromatic ring for both polycarbonate and polyethyleneterephtalate. 3.4. Functionalization In 0%-20% 0, CF,/O, plasmas both fluorine atoms and fluorocarbon radicals are present. XPS showed that treating polypropylene with these plasmas a large amount of fluorine was introduced at the surface, giving a very broad C(ls) spectrum, due to the presence of all sorts of components. In fig. 5 the negative ion spectra of untreated and 10% CFJO, treated PP are shown. In the spectrum of plain polypropylene only peaks relative to C,HJ fragments are present, apart from a small quantity of O- and OH-. After treatment intense peaks due to F- and Fzp (19 and 38 amu) appear. Furthermore fragmentation patterns similar to those of PTFE are present, for instance there are C,F- (x = l-5) and C&F,- (x = 1, 2, 3, 5) fragmentations, together with
CHC2H-
a
OH
ci-i- *-
cg-
I
CF&F2”
I CFi A
~ 0
25
50
c3F;b I
i 100
75
Mass Fig. 5. Negative
ion SSIMS
(FABMS) spectra relative to untreated plastna treated (b) PP.
(a) and
(Dal tons)
10% O,CF,/O,
an intense CF3- fragment (85 daltons). This evidence points to the formation of a fluoropolymer-like surface as a consequence of the treatment, probably due to fluorocarbon radical grafting to the polypropylene surface.
E. Occhrello et ul. / SSIMS
studies on plasma
treated po!vmers
225
4. Conclusion XPS and SSIMS together prove very valuable in assessing the effect of plasma treatments on polymeric surfaces. While XPS reveals the surface composition and some information about the bonding of carbon, SSIMS is very informative from a qualitative point of view, since it is very sensitive to the alterations of the polymer backbone. We used quadrupole SSIMS, but time-of-flight SSIMS will prove even more informative since higher mass fragments become available. To study the plasma modification of surfaces it is important to know how much oxygen or fluorine has been introduced at the surface. This can be examined by XPS. SSIMS allows one to understand where the alien functional group has been introduced and whether the treatment had side effects such as crosslinking or loss of aromaticity of the surface. Furthermore, negative ion SSIMS is very sensitive to fluorine containing fragments and provides information about the bonding of fluorine on the polymer surface.
Acknowledgments We are grateful to Mr. D. Thompson and Mr. P. Humphrey of UMIST for recording SSIMS spectra and helping in their interpretation. We wish also to thank Professor d’Agostino (University of Bari and Centro CNR di Chimica dei Plasmi) for access to plasma equipment and helpful discussions.
References [l] A. Benninghoven, F.G. Ruedenauer and H.W. Werner, Secondary Ion Mass Spectroscopy (Wiley, New York, 1987). [2] A. Brown and J.C. Vickerman, Surface Interface Anal. 6 (1984) 1. [3] A. Brown, J.A. Van den Berg and J.C. Vickerman, Spectrochim. Acta B 40 (1985) 871. [4] D. Briggs, Polymer 25 (1984) 1379. [5] D. Briggs, Surface Interface Anal. 6 (1986) 391. [6] J.A. Gardella, Jr. and D.M. Hercules, Anal. Chem. 52 (1980) 226. [7] I.V. Bletsos, D.M.Hercules, D. Greifendorf and A. Benninghoven, Anal. Chem. 57 (1985) 2384. [8] D.T. Clark. A. Dilks and D. Shuttleworth, in: Polymer Surfaces, Eds. D.T. Clark and W.J. Feat (Wiley, New York. 1978) p. 185. [9] S. Wu, Polymer Interface and Adhesion (Dekker, New York. 1982). [lo] M.A. Khan. M.C. Davies, A. Brown, J. Watts and S.S. Davis, Presented at ECASIA 87. 19-23 October 1987, Fellbach, Fed. Rep. of Germany. [ll] W.J. van OoiJ, R.H.G. Brinkhuis and J. Newman, Presented at 6th Intern. Conf. on SIMS, 14-18 September 1987, Versailles, France. (121 F. Garbassi, E. Occhiello, F. Polato and A. Brown, J. Mater. Sci. 22 (1987) 1450. [13] K. Wittmaack, J. Maul and F. Schulz, Intern. J. Mass. Spectrom. Ion Phys. 11 (1973) 23.
[14] [15] [16] [17] [1X] [I91
D. Brig+. M.J. Hewn and B.D. Ratnrr. Surface Interface Anal. 6 (19X4) 184. D. Brigs and A.B. Wootton. Surface Interface Anal. 4 (1982) 109. R. d’Agostino. F. Cramarossa and F. Illuzzi, J. Appl. Phys. 61 (19X7) 2754. J.L. Adcock. S. lnoue and R.J. Lagow, J. Am. Chem. Sot. 100 (1978) 194X. H. Shonhorn and R.H. Hansen. J. Appl. Polym. Sci. 11 (1971) 1461. J.A. Gardella. Jr.. S.A. Ferguaon and R.L. Chin. Appl. Spectrosc. 40 (1986) 224
Surface
SSIMS (FABMS) STUDIES E. OCCHIELLO, Isriruro
Donegmi 18 June
ON PLASMA
F. GARBASSI @A, accepted
Vu Fouser, for
Science 211/212 (19X9) 21X-226 North-Holland, Amsterdam
TREATED
POLYMERS
and M. MORRA Norwrrr. It& 23 September
The use SSIMS in plasma treated surfaces obtained information the site attack of species on ing. loss aromaticity and are described. and is assessed.
is
Examples polymer surface, the complementarity
of XPS
1. Introduction Static secondary ion mass spectroscopy [l] is by now an established surface analysis technique, with a wide range of applications. The importance of acquiring mass spectra by sputtering fragments out of polymeric surfaces without a remarkable damage was originally recognized by the groups of Vickerman [2,3], Briggs [4,5], Hercules [6] and Benninghoven [7]. Plasma surface treatments of polymers are known since more than twenty years with the general aim to modify the surface in order to improve wettability, adhesion, friction, etc. [8,9]. Only very recently SUMS started to be applied to plasma [lO,ll] and flame [12] treatments of polymeric surfaces.
2. Experimental To obtain SSIMS-FABMS (fast atom bombardment mass spectroscopy) spectra a VG SIMSLAB instrument was used [3,12214]. Static SIMS spectra were collected using 2 keV Ar atoms with a flux density of 3 x 10’ atoms/ cm*. s. With a combined set-up and spectra acquisition time of 600 s the total dose for positive and negative ion spectra was approximately 2 x 10” atoms/cm* per sample. This falls well within the established regime for static SIMS spectra of “undamaged” polymer surfaces [15]. The base pressure in the Pa, but the working pressure during operation of the system was lo-’ ion/atom gun was in the range of lo-’ Pa of Argon. An electron flood gun (VG LEG 31, 500 eV energy, 0.1 nA, 110 PA/cm’ current density) was used to stabilize the surface potential when sample charging occurred. 0039-6028/89/$03.50 c Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
E. Occhiello et al. / SSIMS
studies on plusma treated polymers
219
The plasma treatments were performed at the University of Bari in a small home-made parallel plate reactor. The experimental apparatus is described in detail elsewhere [16]. In the present work the conditions for all the experiments were the following: pressure 0.2 Torr ( - 18 Pa), gas flow 30 cm3 (STP)/min. RF power 150 W (13.56 MHz generator), treatment time 5 min.
3. Results SSIMS is a very good complement to XPS in describing the effect of polymer surface treatments. XPS provides semiquantitative information about the composition of a < 5 nm thick surface layer, together with partial information about the bonding of carbon and other elements in this layer. SSIMS is restricted to a < 1 nm thick layer and gives useful qualitative information on bonding. 3.1. Site of modifications A good example of the ability of SSIMS to provide information about the site of attack of active species present in a plasma is a polypropylene (repetitive unit: -CH(CH,)-CH,-). Three possible sites are available for attack: CH,, CH, and CH. On the basis of conventional chemical knowledge the energy requirement sequence for reaction on these sites is -CH- < -CHZ- < -CH,. In fig. 1 the positive SSIMS spectra of untreated, oxygen and argon plasma treated polypropylene are presented. In both treated samples a remarkable decrease in the relative intensity of the peaks at 15 (methyl cation) and 69 (dimethylcyclopropyl cation) is observed, pointing to loss or modification of pendant methyl groups, as already observed in the case of flame treatments [12] and suggested for reactive gas treatments with F,/O, mixtures
P71. 3.2. Crosslinking Surface crosslinking as a consequence of plasma treatment with inert gases (CASING: cross-linking by activated species of inert gases) has been suggested in the past by Shonhorn et al. [18]. It is a very good example to show how XPS and SSIMS complement each other. For polypropylene samples treated with oxygen and argon plasmas XPS provides information about the amount of oxygen introduced by the treatment. But no evidence can be obtained about crosslinking. In figs. 2a and 2b the C(ls) peaks of Ar and 0, plasma treated polypropylene are presented. Clearly the amount and chemistry of oxygen functionalities introduced at the sample surface are very similar. But, nothing can be said about crosslinking.
220
E. Occhiello et al. / SSIMS studies on pfusnm treated pol)~~rs
n
25
Mass
Fig, 1. P&tive
ion SSIMS (FAMBS) spectra relative to untreated plasma treated (c) polypropylene.
(Daltons)
(a). Ar pkma
treated
(b). 0,
221
E. Occhiello et al. / SSIMS studres on plasma treated polymers
1
I
282
I
284
1
I
I
288
1
I
288 Binding
Fig. 2. XPS C(ls)
peaks
relative
to Ar plasma treated propylene.
I
1
290
(a) and
energy
Oz plasma
treated
(b) poly-
On the other hand SSIMS is very sensitive to alterations in the backbone of the polymer. It is interesting to observe the difference between the spectra of 0, and Ar plasma treated polypropylene (figs. lc and lb). In both cases there is evidence of reaction on methyl groups, as shown by the reduction in the
222
CH3
C CH3
CH,-
C -
\ Ar”
Av(2keV) \‘\i
I CH
CH,--
-.
. A?(2
-
CH, -
+
~-
-
CH,-
-
CH,
CH
(2keVi
-
I -
CH
--
CH2
keV)
CH I CH2 I
Fig. 3. Possible
CH-
rearrangement routes leading. under rnergrtlc partlcle bombardment, emission of methylcyclopropyl iotls from crosslinked polypropylene.
to the
intensity of the peaks at 15 and 69 amu (methyl and dimethylcyclopropyl cations). The difference lies mainly in the relative intensity of the 55 daltons (methylcyclopropyl cation) peak. which becomes markedly higher after the Ar treatment. The scheme in fig. 3 shows how links between polymer chains can lead, under bombardment by energetic Ar atoms, to methylcyclopropyl fragments. So crosslinking seems rather important after the Ar plasma treatment. but not so in the 0, plasma treatment case. 3.3. Aromaticit~,~ To study the aromaticity of polymer surfaces by XPS it is necessary to take into consideration the shake-up features due to the aromatic ring and their variations upon plasma treatment [19]. Unfortunately. shake-up features are broad and weak. furthermore they overlap with components due to carboxyl acids and CF, groups. On the other hand the fragmentation of aromatic and aliphatic groups is quite different, so the examination of the relative intensities of typical fragments can give information about the damage to aromatic rings. To do this we chose to plot the normalized intensities of some of the peaks versus the composition of the gas feed to obtain information about the mechanism of the plasma-surface interaction. The normalization of intensities has been obtained using the peaks at 41 daltons (positive ions spectrum) and at 25 daltons (negative ions spectrum) as internal standards.
E. Occhiello er al. / SSIMS
n.t.
10
studes
30
on plusmu treated po(vmer.s
50
70
223
90
% 0, Fig. 4. C,Hl/C,Hc
intensity ratio as a function of O2 percentage in the gas feed of CF,/Oz plasmas: (a) polycarbonate, (b) polyethyleneterephtalate.
Interesting information was obtained in the case of polycarbonate and polyethyleneterephtalate treated with CF,/O, plasmas. In fig. 4 the ratio of the C,H: ion (typical of aromatic groups) to the C,Hc ion (typical of aliphatic groups) is plotted versus the 0, percentage in the gas feed. We observed that the oxygen enrichment in the plasma led to extensive damage of the aromatic ring for both polycarbonate and polyethyleneterephtalate. 3.4. Functionalization In 0%-20% 0, CF,/O, plasmas both fluorine atoms and fluorocarbon radicals are present. XPS showed that treating polypropylene with these plasmas a large amount of fluorine was introduced at the surface, giving a very broad C(ls) spectrum, due to the presence of all sorts of components. In fig. 5 the negative ion spectra of untreated and 10% CFJO, treated PP are shown. In the spectrum of plain polypropylene only peaks relative to C,HJ fragments are present, apart from a small quantity of O- and OH-. After treatment intense peaks due to F- and Fzp (19 and 38 amu) appear. Furthermore fragmentation patterns similar to those of PTFE are present, for instance there are C,F- (x = l-5) and C&F,- (x = 1, 2, 3, 5) fragmentations, together with
CHC2H-
a
OH
ci-i- *-
cg-
I
CF&F2”
I CFi A
~ 0
25
50
c3F;b I
i 100
75
Mass Fig. 5. Negative
ion SSIMS
(FABMS) spectra relative to untreated plastna treated (b) PP.
(a) and
(Dal tons)
10% O,CF,/O,
an intense CF3- fragment (85 daltons). This evidence points to the formation of a fluoropolymer-like surface as a consequence of the treatment, probably due to fluorocarbon radical grafting to the polypropylene surface.
E. Occhrello et ul. / SSIMS
studies on plasma
treated po!vmers
225
4. Conclusion XPS and SSIMS together prove very valuable in assessing the effect of plasma treatments on polymeric surfaces. While XPS reveals the surface composition and some information about the bonding of carbon, SSIMS is very informative from a qualitative point of view, since it is very sensitive to the alterations of the polymer backbone. We used quadrupole SSIMS, but time-of-flight SSIMS will prove even more informative since higher mass fragments become available. To study the plasma modification of surfaces it is important to know how much oxygen or fluorine has been introduced at the surface. This can be examined by XPS. SSIMS allows one to understand where the alien functional group has been introduced and whether the treatment had side effects such as crosslinking or loss of aromaticity of the surface. Furthermore, negative ion SSIMS is very sensitive to fluorine containing fragments and provides information about the bonding of fluorine on the polymer surface.
Acknowledgments We are grateful to Mr. D. Thompson and Mr. P. Humphrey of UMIST for recording SSIMS spectra and helping in their interpretation. We wish also to thank Professor d’Agostino (University of Bari and Centro CNR di Chimica dei Plasmi) for access to plasma equipment and helpful discussions.
References [l] A. Benninghoven, F.G. Ruedenauer and H.W. Werner, Secondary Ion Mass Spectroscopy (Wiley, New York, 1987). [2] A. Brown and J.C. Vickerman, Surface Interface Anal. 6 (1984) 1. [3] A. Brown, J.A. Van den Berg and J.C. Vickerman, Spectrochim. Acta B 40 (1985) 871. [4] D. Briggs, Polymer 25 (1984) 1379. [5] D. Briggs, Surface Interface Anal. 6 (1986) 391. [6] J.A. Gardella, Jr. and D.M. Hercules, Anal. Chem. 52 (1980) 226. [7] I.V. Bletsos, D.M.Hercules, D. Greifendorf and A. Benninghoven, Anal. Chem. 57 (1985) 2384. [8] D.T. Clark. A. Dilks and D. Shuttleworth, in: Polymer Surfaces, Eds. D.T. Clark and W.J. Feat (Wiley, New York. 1978) p. 185. [9] S. Wu, Polymer Interface and Adhesion (Dekker, New York. 1982). [lo] M.A. Khan. M.C. Davies, A. Brown, J. Watts and S.S. Davis, Presented at ECASIA 87. 19-23 October 1987, Fellbach, Fed. Rep. of Germany. [ll] W.J. van OoiJ, R.H.G. Brinkhuis and J. Newman, Presented at 6th Intern. Conf. on SIMS, 14-18 September 1987, Versailles, France. (121 F. Garbassi, E. Occhiello, F. Polato and A. Brown, J. Mater. Sci. 22 (1987) 1450. [13] K. Wittmaack, J. Maul and F. Schulz, Intern. J. Mass. Spectrom. Ion Phys. 11 (1973) 23.
[14] [15] [16] [17] [1X] [I91
D. Brig+. M.J. Hewn and B.D. Ratnrr. Surface Interface Anal. 6 (19X4) 184. D. Brigs and A.B. Wootton. Surface Interface Anal. 4 (1982) 109. R. d’Agostino. F. Cramarossa and F. Illuzzi, J. Appl. Phys. 61 (19X7) 2754. J.L. Adcock. S. lnoue and R.J. Lagow, J. Am. Chem. Sot. 100 (1978) 194X. H. Shonhorn and R.H. Hansen. J. Appl. Polym. Sci. 11 (1971) 1461. J.A. Gardella. Jr.. S.A. Ferguaon and R.L. Chin. Appl. Spectrosc. 40 (1986) 224