- Email: [email protected]
Chemical selectivity and energy transfer mechanisms in the radiation-induced modification of polyethersulphone
__ _!iB
As
Nuclear Instruments and Methods in Physics Research B 116 (19%) 246-252
NOMB
Beam Interactions with Materials&Atoms
ELSEVIER
Chemical selectivity and energy transfer mechanisms in the radiation-induced modification of polyethersulphone
*,F. Iacona
G. Marletta a,
b
a,Dipartimento di Chimica, lJniversit6 della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy b CNR4.Me.Te.M..
Stradale Primosole 50, 95121 Catania, Italy
Abstract The mechanism of radiation-induced degradation of PES has been studied in a great detail by using XPS technique. PES films have been irradiated both with 6 keV Ar and 3 keV e- beams. The Ar irradiation is assumed to be representative of the processes which strictly depend on collisional energy loss, while the electron irradiation puts in evidence mainly the processes related to electronic energy loss. In particular, three basic chemical reactions have been followed by XPS: 1) the reduction of the sulphonyl groups (-SO,-) to sulphidic-like groups (-S-); 2) the elimination of sulphur-containing groups; 3) the formation of new oxygen-containing groups, as ether, hydroxyl or carbonyl groups. Differences are observed when irradiating with Ar and with e- projectiles both for the rate of evolution of the different species and for the quantitative trends of modification. The rates of the reactions I) and 2) are found to depend essentially on the total deposited energy. The trend and the rate of reaction 3) depend dramatically on the energy transfer mechanism. The key factor determining the sensitivity to the energy deposition mechanisms seems to be the inherent chemical selectivity of the involved reactions. In particular, this means that some types of reactions are sensitive to the energy transfer mechanism due to their selectivity (as it is the case of the “oxygen attachment” reactions), while others will depend only on the total deposited energy (as the sulphur loss or the sulphonyl reduction) due to the availability of many concurrent pathways producing a random succession of chemical events yielding an unique product.
1. Introduction The understanding of the irradiation-induced chemical modifications of polysulphones is an outstanding topic, due to the many different applications of these polymeric materials in various fields involving the interaction with energetic particles. Thus, for instance, aliphatic poly(olefin sulphone)s are important for their growing applications due to the unusually high lithographic sensitivity to ionising radiations, while aromatic polysulphones have wide applications in the aerospace industry as thermo- and radiation-resistant materials [ 1,2]. Classically, electron beams and y-rays have been used to study the radiolysis of polysulphones by several research groups to gain a better understanding of the degradation process [2-51. More recently also a number of studies reported about the nature and the mechanisms of chemical events induced in such polymers by W photons as well as keV ions, electrons and soft X-rays. These last
* Corresponding author.
topics have been addressed in particular in recent papers from this group [6-81. Until recently the debate on the decomposition mechanism of polysulphones has mainly been centred on the elimination of the sulphonyl group from the polymeric chain. Several studies pointed to demonstrate that SO, is indeed the main decomposition product of the irradiation of polysulphones [3-51. In our papers we showed that, in the case of polyethersulphone (PES) irradiation with keV ions, electrons and soft X-rays, such a process is quantitatively exceeded by another leading mechanism involving the complete reduction of the sulphonyl groups to sulphide-like groups, without loss of sulphur 16-81. This peculiar mechanism has been observed also for W irradiation [93. An important result of our previous studies is that different chemical reactions may depend critically on either the total deposited energy or the energy deposition mechanisms. Up to now there is not a defined criterion for deciding which type of reaction will be sensitive to the energy deposition mechanism, unless we do a few basic considerations about the chemistry of the different targets.
0168-583X/%/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SOl68-583X(96)00041-9
G. Marletta, F. Iacona/Nucl.
Ins@. and Meth. in Phys. Res. B 116 (1996) 246-252
Thus, we could justify the fact that the depletion of N from polyimide does not correlate with a specific energy deposition mechanism because this process essentially reflects the collisional events, randomly directed and randomly effective in producing the decomposition [lo]. The aim of the present paper is to report on the role of energy deposition mechanisms versus that of the total deposited energy or the related linear energy transfer (LET) for the relatively complex and interdependent patterns of chemical reactions induced by irradiation in PES, chosen as the model compound. These mechanistic arguments will be used to try to derive a criterion for deciding if, for a given set of reactions, one should expect sensitivity to the energy deposition mechanism or to the LET. The experiment has been performed by using keV Ar atoms and electron beams, chosen to optimise respectively the collisional and the electronic energy deposition terms. The chemical modifications have been followed by in situ XPS, owing to the need to obtain reliable information on the oxygen-related reactions.
2. Experimental Poly- 1,4-phenylene-ether-sulphone chased from Aldrich. The monomeric following chemical structure:
247
peak fitting, using an inelastic Shirley-type background. The chemical composition of the samples was obtained by correcting the XPS peak areas for the calculated photoionisation cross sections [ll] and the experimentally determined transmission function of the spectrometer. Both electron and fast atom bombardments were performed in situ, by using sources fitted to the XPS apparatus. After each irradiation step, the samples were transferred to the chamber of analysis without any interruption of the vacuum, minimising the possible occurrence of reactions between residual gases and the polymer surface. Fast atom bombardments were performed using a 1lNF FAB source (from Ion Tech Ltd.), operating at l-10 keV of noble gas atom beams. The particle current was estimated by measuring the steady-state current due to the emission of secondary electrons and charged particles on a Cu target of known area, considering an ionisation cross section of about 0.5. For the energy used in the present work (6 keV) the estimated current was 5 p,A/cm2 with a residual Ar pressure of 6 X lop5 Ton: The electron bombardment was performed using a Kratos electron gun, for REELS spectroscopy, operating with 50-5000 eV electron beams at variable currents (between 100 nA and 100 PA). In the present work we used 3 keV electron beams, with a current of 2 pA/cm2.
(PES) was purunit of PES has the
The polymer was studied in the form of thin films (about 1 Frn of thickness), obtained by spinning a solution in chloroform at 5000 rpm for 4.5 s onto 5 in. Si wafers. The wafers were heated for 5 min at 90°C in air to drive off the residual solvent. The complete removal of the solvent as well as the integrity of the polymer after the heating step was confirmed by the XPS analysis. The thickness of the films was measured by means of a stylus profilometer (Alpha Step 100, by Tencor Instruments). XPS analysis was performed using a Kratos ES 300 electron spectrometer, operating at a base pressure of 1 X lo-’ Ton; equipped with a dual anode (Al-Mg) and a preparation chamber held at the same pressure of the main chamber. The XPS take-off angle was in all cases about 1.5”, measured with respect to the sample normal. The energy scale of the XPS spectra was calibrated assuming the low binding energy component of the C 1s peak of unbombarded PES at 285.0 eV of B.E. For the bombarded films, due to the difficulties to use the same procedure, the calibration was performed by putting a gold sheet in electrical contact with the samples, assuming at 83.8 eV the B.E. of the Au 4f,,, peak. The XPS peaks were analysed by means of a computer program of Gaussian
3. Results and discussion PES films have been irradiated with 6 keV Ar atoms and 3 keV electrons. The characteristic patterns of irradiation-induced modification of S 2p and 0 Is XPS peaks have been thoroughly investigated in previous papers from this Laboratory [6-81. The irradiation of PES with energetic radiation beams (ions, electrons and photons) has been found to induce three basic chemical modifications: 1) the reduction of the sulphonyl groups (-SO,-) to sulphidic-like groups (-S-); 2) the elimination of sulphur either in the form of SO, (as in the thermal case) or as sputtered S-containing species; 3) the formation-and-destruction of the ether or hydroxyl groups, due to recoiling-induced reactions or to transfer reactions of oxygen (Fries-like mechanism, see below) or to elimination of O-containing gaseous molecules
181. A fourth reaction involves the destruction of the backbone aromatic rings. This reaction has been followed mainly by monitoring the relative intensity of the P * + IT transitions by using reflection electron energy loss spectroscopy (REELS) [6,10,.12,13] and it will not be further discussed in the present paper. The chemical mechanisms of reduction of -SO,groups as well as the sulphur elimination turn to be qualitatively very similar for both Ar and e- irradiation. Here we report the detailed quantitative comparison for the two cases as a function of fluence. The fluence ranges
248
F. Iacona / Nucl. Instr. and Meth. in Phys. Res. B I16 (I 996) 246-252
G. Marlerta.
100
been determined according to the reaching of a steady state of the PES chemical modification.
have
Fig. I shows the quantitative modification trends for the total sulphur as well as the three components found respectively at 168.0 eV (-SO,- groups), 165.7 eV (-SOgroups) and 163.5 eV (-Slinkages) in S 2p peaks, respectively for Ar-and e--irradiated PES. Let us consider as a first observable the loss of sulphur. In particular, we draw the attention to the fact that both the projectiles produce a surface concentration, at the steady state, of about 70% of residual sulphur, mostly consisting in reduced -S- groups. Clearly, this effect depends on a
. .s.-
, -.
. . . . . ..I O .
n
. 1. . . ...
0 (al
90 -
0 .
.
0
%S *O70
. o .
1
i
=
A?
0
e-
“‘,,,,I
601E22
0
“‘.,,,’
1E23
.
......,I
I E24
.
I
IE25
0 x s,o, (d/m?) 100r
.
.= . . .
..I
3.
.
.
O n
.
t
.
03 0
.
w
0
m
-.-7:
*O-
.
0 .
70
0
s,ot
-o--so*n
_so_
v...
_S_
.
.
90 -
%S
.1....,
0
_
60
I E22
.
,,,,’
.,,,’
IE23
I E24 0 x
’
..,.I
I E2.5
s, (eV/cm3)
Fig. 2. Residual sulphur concentration versus the total deposited energy (a), and the energy deposited by the electronic term alone (b), for PES irradiated with 6 keV Ar and 3 keV e-. The contributions are normalised to the total initial sulphur content.
fluence (Af’km2)
set ofdi&mt
80 -
60 -
IEl5
IEl6
lE17
nuence (e-lcm2) Fig. 1. Quantitative modification trends versus fluence for the total sulphur as well as for the three components found respectively at 168.0 eV (-SO, - groups), 165.7 eV (-SO- groups) and 163.5 eV C-S- linkages) in the XPS S 2p peaks for PES irradiated with 6 keV Ar (a) and 3 keV e- (b). All the contributions are normalised to the total initial sulphur content.
reactions having the same result, i.e., the sulphur loss. We can analyse this first set of reactions in terms of the energy deposition parameters by plotting the residual sulphur percentage as a function respectively of the total deposited energy and the electronic term alone. Fig. 2 shows the obtained trends. An unique correlation is obtained only by plotting the S residual concentration versus the total deposited energy, while the effect of the simple electronic stopping is not sufficient to give a unique trace. A second crucial set of reactions includes those producing the reduction of -SO,to -S- groups. In this case, the rates for both the Ar- and e--induced reductions, including the rate of formation of transient -SO- groups, are quite different in terms of their fluence dependence. In particular, this set of reactions can be analysed by using as a working concept that of “inversion point” of the relative abundances of S oxidation states, which defines the fluence for which the relative concentrations of -SO,and -S- become equivalent. Such an “inversion point” shows a clear dependence on the type of the projectile, falling respectively at about 1 X 1014 cme2 for Ar and at 4 X 10” cmv2 for e- projectiles (see Fig. 1).
G. Marletta, F. Iacona/Nucl.
instr. and Meth. in Phys. Res. 3 116 (1996) 246-252
80
60
.
0 .
m 60 % -s-
m
. 0
.
0
. 0 0
0
40
0
L
PE22
.
‘A.,,,.’
IE23
,.,,.I
IE24
‘,..,,I
1E25
.
1
@ x s, (&&)
Fig. 3. Sulphide (4%) groups concentration versus the total deposited energy (a), and the energy deposited by the electronic term alone (b), for PES irradiated with 6 keV Ar and 3 keV e-. The contributions are nortnalised to the total initial sulphur content.
Again, this behaviour can be analysed in terms of the energy deposition parameters. Fig. 3 shows that an unified trace is obtained only by plotting the rate of formation of -S- versus the total deposited energy, while the simple electronic stopping gives scattered data. Therefore, the rate of formation of -S- groups can be accounted for in terms of the total deposited energy. The very same result is obtained by plotting the residual -SO,- component versus the relevant energy deposition data (not shown here). Interestingly, the energy density seemingly has no effect on the two sets of considered reactions, as far as 3 keV electrons (depositing typically 1 eV/$ and 6 keV Ar (typically depositing about 70 eV/& induce almost identical effects with the same rate in terms of the total deposited energy. We can explain such a dependence on the total deposited energy for the two above described sets of reactions as due to the fact that the reaction mechanisms are so complex that the selectivity is completely lost; i.e., the kinetic of the two reactions depends on a random succession of events loosing memory of the way the energy has been provided to the molecular system. Thus, we can say
249
that our observables consist in a sum of many elementary events, which can be equally produced by mere electronic as well as collisional events, the final results being a “convolution” of such steps. More in detail, in the case of sulphur loss, we can propose at least the following three mechanisms: a) S can be lost as SO, molecules, as in the thermal case; b) S can be lost after the “mild” reduction step, via a chemical pathway involving either the formation of H,S or the formation-and-elimination of S-containing gaseous molecules, deriving from the degradation of the chain; c) S can be lost as a result of a collisional sputtering, alone or in S-containing fragments. It is clear that the mechanisms a) and b) can be defined as the product of a “mild” chemistry, i.e., according to the related concept described in Ref. [13], these mechanisms show a certain degree of selectivity. At variance of this, the process c) is totally random in nature and clearly it does not depend on the thermodynamic of the process, the S atoms or S-containing species being eliminated on the base of statistical bond-breaking or rearrangement events. A similar analysis can also be done in the case of the reduction reaction. For this reaction we have already proposed three possible mechanisms mainly based on “mild” chemical events. These consist respectively in the primary attack of a radical H. onto a sulphonic 0, evolving to the elimination of OH. radicals, or in the electron attachment in an O-S orbital by forming a -SO;moiety, evolving towards the capture of a H+ and successive elimination of OH. , or finally through a very peculiar transfer scheme, analogous to the Fries mechanism for polycarbonates, based on the formation of a radicalic site on a phenyl ring adjacent to the -SO,group which evolves with the transfer a sulphonyl 0 to the phenyl ring radicalic site and the simultaneous reduction of the sulphonyl group to sulphoxide and finally (after repetition of the process) to -S-. These mechanisms have been extensively discussed in Ref. [8]. Furthermore, it is possible to propose also a random-like mechanism based on the collision-induced rupture of the S-O bonds. Such a last mechanism is characteristic of the very low energy range (a few keV), while for high energy (hundreds of keV) a correlation of the -SO,- reduction with the electronic term has been observed [7], suggesting that the previously mentioned mechanisms, and particularly the Fries-like one, are predominant in the electronic regime. However, it is interesting to note that the leading factors driving the Fries-like mechanism (and the other “mild” reactions above summarized) are the concentration and the formation rate of the radical sites close to the sulphonyl groups, prompting the “mild” oxygen transfer reaction. Hence, the unified dependence observed in the present paper for the -SO,- reduction is explained if we
250
G. Marletta, F. Iacona/Nucl.
Instr. and Meth. in Phys. Res. B I16 (1996) 246-252
take into account that also the collisional mechanism produces a very high and statistically distributed concentration of radical sites, thus increasing in an overwhelmingly way the effectiveness of the oxygen transfer from the sulphonyl groups. Therefore, a very similar dependence on the energy transfer mode is expected both for e- and Ar projectiles, due to the fact that the crucial intermediates are radical sites and that their concentration will be simply proportional to the total deposited energy. At variance of this, a marked dependence on the nature of the irradiating particle is instead observed for sets of reaction affecting the O-containing functional groups.
20 -\-. IO t I
YE22
.
i .
I
1E23
.
. . . . .I
I E24
.
,
..,,,I
1
I E25
@ x S, (eV/cm3) Fig. 5. Ether (-O-1 and hydroxyl (-OH) groups concentration versus the total deposited energy for PES irradiated with 6 keV Ar and 3 keV e-. The contributions are normalised to the total initial oxygen content. 80
60 -y
‘.. 0,
%0
,.A
‘\ 40
-
_.__._.. ....’
Q
L
20
.__._.
e-4..
,k.,’
-
.\ ‘0..
-m-
0,
--o--
OSulr
I 4.. ...
&..Y,
“(I
Ya
O----
El
o’.....l
..--A-.--%r
IEl5
.
~~~.~~~I
1El6 fluence (ehl~)
Fig. 4. Quantitative
,....J
iEl7
modification trends versus fluence for the total oxygen as well as for the two components found respectively at 533.8 eV C-O- and -OH groups) and 532.2 eV (0 in -SO, groups) in the XPS 0 1s peaks for PES irradiated with 6 keV Ar (a) and 3 keV e- (b). All the contributions are normalised to the total initial oxygen content.
Fig. 4 illustrates the characteristic behaviour of the 0 1s components, i.e., the ether oxygen (B.E. of 533.8 eV> and the sulphonyl groups (B.E. of 532.2 eV) 181, for Ar and e- irradiated PES. As it is expected, the sulphonyl component in the 0 Is band decreases dramatically both for Ar- and e--irradiation, this process being obviously related to the reduction process of the -SO,- groups. Accordingly, the rate of the O(sulphony1) decrease is well accounted for in terms of the total deposited energy, as that of the formation of reduced -s- groups. An interesting point comes from the observation that, also when almost no residual -SO,- groups are present within the irradiated region, we still have an oxygen species having a B.E. very similar to that of the sulphonyl group. The relative abundance of such a component is very similar for both the types of projectiles, i.e., about 15% of the total 0 1s band at the steady state, while the intensity of the -SO,- component in the S 2p band at the very same fluence is between 0 (for Ar) and 5% (for e-1. Thus, the component is assigned to oxygen belonging to new carbonyl groups, having the same B.E. with the sulphonyl groups, created by reaction of the recoiling oxygen with suitable chain sites. Evidence for the formation of such groups has been obtained for this and analogous aromatic system by FT-IR analysis (not reported here). Fig. 4 also shows that a remarkable difference can instead be observed for the modification of the ether groups. Let us discuss the main experimental features: in the case of Ar irradiation we have essentially an initial increase of ether or -OH groups (indistinguishable by XPS) concentrations, followed by the fast and almost complete depletion of these species. In the case of eirradiation the increase of the “ether-like” species concentration is occurring only after a given total energy threshold has been exceeded, and at very high fluence the
G. Marlena, F. Iacona /Nucl. Instr. and Meth. in Phys. Res. B 116 (1996) 246-252
concentration of such species level off at the initial value. Such a dramatic difference is also well reflected in Fig. 5, reporting the intensity of the component at 533.8 eV in the 0 1s band (i.e., the concerned ether-like or -OH component) versus the total deposited energy. Fig. 5 shows clearly that the rate of the reactions leading to the formation and destruction of the ether-like groups does not correlate with the total deposited energy, i.e., it is sensitive to the energy deposition mechanisms. Actually, we may consider the reactions leading to the formation of carbonyl groups and those related to the formation of ether or -OH groups, as “hot atom” reaction of “oxygen attachment” to carbon sites in the chain. Let us discuss in some detail the basic mechanistic differences between Ar and e--irradiation. In the case of Ar irradiation, the “oxygen attachment” reaction is prompted by the formation of energetic recoiling oxygen, produced in the primary Ar 3 -SO,- impact. The subsequent fast depletion of such ether-like groups is due to the predominant collisional sputtering effect, possibly involving the formation of volatile molecules as CO and related species. In the case of electron irradiation, there is essentially no possibility of formation of energetic recoiling oxygen and the collisional sputtering effect is completely absent. These two features suggest that a different mechanism is responsible for the “oxygen attachment” and for the depletion reactions in the case of electron irradiation. In particular, the experimentally observed reactions in the case of the electronic stopping may be better described as an “oxygen-transfer” process in the framework of the Frieslike mechanism above discussed. Indeed, the production of -OH groups or also ether linkages is the main result of this reaction, together with the reduction of the -SO,groups. As to the depletion after the first increase step, presently we have no experimental evidence of the kind of species which are produced and more experiments have to be performed. The above arguments justify that for the ether-like reactions no correlation is found with the total deposited energy. Indeed, in this case the primary events are intrinsically different on going from the collisional to the electronic term, aa we could summarize the two processes as a random “hot atom” process inducing an “oxygen attachment” in the case of collisional stopping and as a mild and selective “oxygen transfer” in the case of electronic stopping regime.
4. Conclusions In the present paper we report the experimental data on in situ irradiated PES showing that: a) a single chemical modification can be the result of several different and concurrent reactions. Accordingly, we
2.51
introduce the view of speaking about sets of reactions producing a unique modification. b) As in previous papers [ 14,151, we may find that (for a given polymer) sets of reactions depending on a specific energy deposition mechanism can be identified, while others depend only on the total deposited energy. Furthermore, the key factor determining the sensitivity to the energy deposition mechanisms seems to be the chemical selectivity of the involved reaction pathways. c) We discussed the experimental rates in terms of detailed mechanistic chemical processes, showing that some reactions have to be selective (as it is the case of the “oxygen attachment” reactions) due to their “mild” nature, while others cannot be selective (as the sulphur loss or the sulphonyl reduction), due to the availability of many concurrent pathways producing an unique product by a random succession of different chemical events. More future efforts will be dedicated to the further clarification of the complex link between the characteristics of chemical reactivity (i.e., the “allowed reactivity”) and the peculiarity of the energy deposition conditions.
Acknowledgement The authors acknowledge the financial support to this research by CNR, by the Strategic Project “Microtecnologie” of the CNR (Rome) and by MURST (40%).
References [I] E. Reichmanis and J.H. O’Donnell (eds.), The effects of radiations on high technology polymers, Am. Chem. Sot., Washington D.C. (1989). [2] T. Yamashita, H. Tomitaka, T. Kudo, K. Horie and I. Mita, Polym. Dcgrad. Stabil. 39 (1993) 47. [3] A.R. Lyons, M.C.R. Symons and J.K. Yandell, Makrom. Chem. 157 (1972) 103. [4] J.R. Brown and J.H. O’Donnell, J. Appl. Polym. Sci. 19 (1975) 405. [5] J.R. Brown and J.H. O’Donnell, J. Appl. Polym. Sci. 23 (1979) 2763. [6] G. Marletta, S.M. Catalano and S. Pignataro, Surf. Interf. Anal. 16 (1990) 407. [7] G. Marletta, Nucl. Instr. and Meth. B 46 (1990) 295. [S] G. Marletta, S. Pignataro, A. Toth, I. Bertoti, T. Szekely and B. Keszler, Macromolecules 24 (1991) 99. 191 H.S. Munro and D.T. Clark, Polym. Degrad. Stabil. 17 (19871319. [lo] G. Marletta and F. Iacona, in: Materials and Processes for Surface and Interface Engineering, NATO-AS1 Series, Series E: Applied Sciences, ed. Y. Pauleau, vol. 290 (Kluwer Academic Publishers, Dordnxht, The Netherlands, 1995) p. 597. [l 11 J.H. Scofield, J. Electron. Spectrosc. Relat. Phenom. 8 (1976) 129.
252
G. Marletta, F. lacona/Nucl.
Instr. and Meth. in Phys. Res. B I16 (1996) 246-252
[12] G. Marletta, S. Pignataro and C. Oliveri, Nucl. Instr. and Me& B 39 (1989) 773. [13] S. Pignataro and G. Marletta, in: Metallized Plastics 2 Fundamentals and Applied Aspects, ed. K.L. Mittal (Plenum, New York, 1991) p. 269.
[14] F. Iacona and G. Marletta, Nucl. Instr. and Meth. B 65 (1992) 50. [15] G. Marletta and F. Iacona, Nucl. Instr. and Meth. B 80/81 (1993) 1405.
As
Nuclear Instruments and Methods in Physics Research B 116 (19%) 246-252
NOMB
Beam Interactions with Materials&Atoms
ELSEVIER
Chemical selectivity and energy transfer mechanisms in the radiation-induced modification of polyethersulphone
*,F. Iacona
G. Marletta a,
b
a,Dipartimento di Chimica, lJniversit6 della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy b CNR4.Me.Te.M..
Stradale Primosole 50, 95121 Catania, Italy
Abstract The mechanism of radiation-induced degradation of PES has been studied in a great detail by using XPS technique. PES films have been irradiated both with 6 keV Ar and 3 keV e- beams. The Ar irradiation is assumed to be representative of the processes which strictly depend on collisional energy loss, while the electron irradiation puts in evidence mainly the processes related to electronic energy loss. In particular, three basic chemical reactions have been followed by XPS: 1) the reduction of the sulphonyl groups (-SO,-) to sulphidic-like groups (-S-); 2) the elimination of sulphur-containing groups; 3) the formation of new oxygen-containing groups, as ether, hydroxyl or carbonyl groups. Differences are observed when irradiating with Ar and with e- projectiles both for the rate of evolution of the different species and for the quantitative trends of modification. The rates of the reactions I) and 2) are found to depend essentially on the total deposited energy. The trend and the rate of reaction 3) depend dramatically on the energy transfer mechanism. The key factor determining the sensitivity to the energy deposition mechanisms seems to be the inherent chemical selectivity of the involved reactions. In particular, this means that some types of reactions are sensitive to the energy transfer mechanism due to their selectivity (as it is the case of the “oxygen attachment” reactions), while others will depend only on the total deposited energy (as the sulphur loss or the sulphonyl reduction) due to the availability of many concurrent pathways producing a random succession of chemical events yielding an unique product.
1. Introduction The understanding of the irradiation-induced chemical modifications of polysulphones is an outstanding topic, due to the many different applications of these polymeric materials in various fields involving the interaction with energetic particles. Thus, for instance, aliphatic poly(olefin sulphone)s are important for their growing applications due to the unusually high lithographic sensitivity to ionising radiations, while aromatic polysulphones have wide applications in the aerospace industry as thermo- and radiation-resistant materials [ 1,2]. Classically, electron beams and y-rays have been used to study the radiolysis of polysulphones by several research groups to gain a better understanding of the degradation process [2-51. More recently also a number of studies reported about the nature and the mechanisms of chemical events induced in such polymers by W photons as well as keV ions, electrons and soft X-rays. These last
* Corresponding author.
topics have been addressed in particular in recent papers from this group [6-81. Until recently the debate on the decomposition mechanism of polysulphones has mainly been centred on the elimination of the sulphonyl group from the polymeric chain. Several studies pointed to demonstrate that SO, is indeed the main decomposition product of the irradiation of polysulphones [3-51. In our papers we showed that, in the case of polyethersulphone (PES) irradiation with keV ions, electrons and soft X-rays, such a process is quantitatively exceeded by another leading mechanism involving the complete reduction of the sulphonyl groups to sulphide-like groups, without loss of sulphur 16-81. This peculiar mechanism has been observed also for W irradiation [93. An important result of our previous studies is that different chemical reactions may depend critically on either the total deposited energy or the energy deposition mechanisms. Up to now there is not a defined criterion for deciding which type of reaction will be sensitive to the energy deposition mechanism, unless we do a few basic considerations about the chemistry of the different targets.
0168-583X/%/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SOl68-583X(96)00041-9
G. Marletta, F. Iacona/Nucl.
Ins@. and Meth. in Phys. Res. B 116 (1996) 246-252
Thus, we could justify the fact that the depletion of N from polyimide does not correlate with a specific energy deposition mechanism because this process essentially reflects the collisional events, randomly directed and randomly effective in producing the decomposition [lo]. The aim of the present paper is to report on the role of energy deposition mechanisms versus that of the total deposited energy or the related linear energy transfer (LET) for the relatively complex and interdependent patterns of chemical reactions induced by irradiation in PES, chosen as the model compound. These mechanistic arguments will be used to try to derive a criterion for deciding if, for a given set of reactions, one should expect sensitivity to the energy deposition mechanism or to the LET. The experiment has been performed by using keV Ar atoms and electron beams, chosen to optimise respectively the collisional and the electronic energy deposition terms. The chemical modifications have been followed by in situ XPS, owing to the need to obtain reliable information on the oxygen-related reactions.
2. Experimental Poly- 1,4-phenylene-ether-sulphone chased from Aldrich. The monomeric following chemical structure:
247
peak fitting, using an inelastic Shirley-type background. The chemical composition of the samples was obtained by correcting the XPS peak areas for the calculated photoionisation cross sections [ll] and the experimentally determined transmission function of the spectrometer. Both electron and fast atom bombardments were performed in situ, by using sources fitted to the XPS apparatus. After each irradiation step, the samples were transferred to the chamber of analysis without any interruption of the vacuum, minimising the possible occurrence of reactions between residual gases and the polymer surface. Fast atom bombardments were performed using a 1lNF FAB source (from Ion Tech Ltd.), operating at l-10 keV of noble gas atom beams. The particle current was estimated by measuring the steady-state current due to the emission of secondary electrons and charged particles on a Cu target of known area, considering an ionisation cross section of about 0.5. For the energy used in the present work (6 keV) the estimated current was 5 p,A/cm2 with a residual Ar pressure of 6 X lop5 Ton: The electron bombardment was performed using a Kratos electron gun, for REELS spectroscopy, operating with 50-5000 eV electron beams at variable currents (between 100 nA and 100 PA). In the present work we used 3 keV electron beams, with a current of 2 pA/cm2.
(PES) was purunit of PES has the
The polymer was studied in the form of thin films (about 1 Frn of thickness), obtained by spinning a solution in chloroform at 5000 rpm for 4.5 s onto 5 in. Si wafers. The wafers were heated for 5 min at 90°C in air to drive off the residual solvent. The complete removal of the solvent as well as the integrity of the polymer after the heating step was confirmed by the XPS analysis. The thickness of the films was measured by means of a stylus profilometer (Alpha Step 100, by Tencor Instruments). XPS analysis was performed using a Kratos ES 300 electron spectrometer, operating at a base pressure of 1 X lo-’ Ton; equipped with a dual anode (Al-Mg) and a preparation chamber held at the same pressure of the main chamber. The XPS take-off angle was in all cases about 1.5”, measured with respect to the sample normal. The energy scale of the XPS spectra was calibrated assuming the low binding energy component of the C 1s peak of unbombarded PES at 285.0 eV of B.E. For the bombarded films, due to the difficulties to use the same procedure, the calibration was performed by putting a gold sheet in electrical contact with the samples, assuming at 83.8 eV the B.E. of the Au 4f,,, peak. The XPS peaks were analysed by means of a computer program of Gaussian
3. Results and discussion PES films have been irradiated with 6 keV Ar atoms and 3 keV electrons. The characteristic patterns of irradiation-induced modification of S 2p and 0 Is XPS peaks have been thoroughly investigated in previous papers from this Laboratory [6-81. The irradiation of PES with energetic radiation beams (ions, electrons and photons) has been found to induce three basic chemical modifications: 1) the reduction of the sulphonyl groups (-SO,-) to sulphidic-like groups (-S-); 2) the elimination of sulphur either in the form of SO, (as in the thermal case) or as sputtered S-containing species; 3) the formation-and-destruction of the ether or hydroxyl groups, due to recoiling-induced reactions or to transfer reactions of oxygen (Fries-like mechanism, see below) or to elimination of O-containing gaseous molecules
181. A fourth reaction involves the destruction of the backbone aromatic rings. This reaction has been followed mainly by monitoring the relative intensity of the P * + IT transitions by using reflection electron energy loss spectroscopy (REELS) [6,10,.12,13] and it will not be further discussed in the present paper. The chemical mechanisms of reduction of -SO,groups as well as the sulphur elimination turn to be qualitatively very similar for both Ar and e- irradiation. Here we report the detailed quantitative comparison for the two cases as a function of fluence. The fluence ranges
248
F. Iacona / Nucl. Instr. and Meth. in Phys. Res. B I16 (I 996) 246-252
G. Marlerta.
100
been determined according to the reaching of a steady state of the PES chemical modification.
have
Fig. I shows the quantitative modification trends for the total sulphur as well as the three components found respectively at 168.0 eV (-SO,- groups), 165.7 eV (-SOgroups) and 163.5 eV (-Slinkages) in S 2p peaks, respectively for Ar-and e--irradiated PES. Let us consider as a first observable the loss of sulphur. In particular, we draw the attention to the fact that both the projectiles produce a surface concentration, at the steady state, of about 70% of residual sulphur, mostly consisting in reduced -S- groups. Clearly, this effect depends on a
. .s.-
, -.
. . . . . ..I O .
n
. 1. . . ...
0 (al
90 -
0 .
.
0
%S *O70
. o .
1
i
=
A?
0
e-
“‘,,,,I
601E22
0
“‘.,,,’
1E23
.
......,I
I E24
.
I
IE25
0 x s,o, (d/m?) 100r
.
.= . . .
..I
3.
.
.
O n
.
t
.
03 0
.
w
0
m
-.-7:
*O-
.
0 .
70
0
s,ot
-o--so*n
_so_
v...
_S_
.
.
90 -
%S
.1....,
0
_
60
I E22
.
,,,,’
.,,,’
IE23
I E24 0 x
’
..,.I
I E2.5
s, (eV/cm3)
Fig. 2. Residual sulphur concentration versus the total deposited energy (a), and the energy deposited by the electronic term alone (b), for PES irradiated with 6 keV Ar and 3 keV e-. The contributions are normalised to the total initial sulphur content.
fluence (Af’km2)
set ofdi&mt
80 -
60 -
IEl5
IEl6
lE17
nuence (e-lcm2) Fig. 1. Quantitative modification trends versus fluence for the total sulphur as well as for the three components found respectively at 168.0 eV (-SO, - groups), 165.7 eV (-SO- groups) and 163.5 eV C-S- linkages) in the XPS S 2p peaks for PES irradiated with 6 keV Ar (a) and 3 keV e- (b). All the contributions are normalised to the total initial sulphur content.
reactions having the same result, i.e., the sulphur loss. We can analyse this first set of reactions in terms of the energy deposition parameters by plotting the residual sulphur percentage as a function respectively of the total deposited energy and the electronic term alone. Fig. 2 shows the obtained trends. An unique correlation is obtained only by plotting the S residual concentration versus the total deposited energy, while the effect of the simple electronic stopping is not sufficient to give a unique trace. A second crucial set of reactions includes those producing the reduction of -SO,to -S- groups. In this case, the rates for both the Ar- and e--induced reductions, including the rate of formation of transient -SO- groups, are quite different in terms of their fluence dependence. In particular, this set of reactions can be analysed by using as a working concept that of “inversion point” of the relative abundances of S oxidation states, which defines the fluence for which the relative concentrations of -SO,and -S- become equivalent. Such an “inversion point” shows a clear dependence on the type of the projectile, falling respectively at about 1 X 1014 cme2 for Ar and at 4 X 10” cmv2 for e- projectiles (see Fig. 1).
G. Marletta, F. Iacona/Nucl.
instr. and Meth. in Phys. Res. 3 116 (1996) 246-252
80
60
.
0 .
m 60 % -s-
m
. 0
.
0
. 0 0
0
40
0
L
PE22
.
‘A.,,,.’
IE23
,.,,.I
IE24
‘,..,,I
1E25
.
1
@ x s, (&&)
Fig. 3. Sulphide (4%) groups concentration versus the total deposited energy (a), and the energy deposited by the electronic term alone (b), for PES irradiated with 6 keV Ar and 3 keV e-. The contributions are nortnalised to the total initial sulphur content.
Again, this behaviour can be analysed in terms of the energy deposition parameters. Fig. 3 shows that an unified trace is obtained only by plotting the rate of formation of -S- versus the total deposited energy, while the simple electronic stopping gives scattered data. Therefore, the rate of formation of -S- groups can be accounted for in terms of the total deposited energy. The very same result is obtained by plotting the residual -SO,- component versus the relevant energy deposition data (not shown here). Interestingly, the energy density seemingly has no effect on the two sets of considered reactions, as far as 3 keV electrons (depositing typically 1 eV/$ and 6 keV Ar (typically depositing about 70 eV/& induce almost identical effects with the same rate in terms of the total deposited energy. We can explain such a dependence on the total deposited energy for the two above described sets of reactions as due to the fact that the reaction mechanisms are so complex that the selectivity is completely lost; i.e., the kinetic of the two reactions depends on a random succession of events loosing memory of the way the energy has been provided to the molecular system. Thus, we can say
249
that our observables consist in a sum of many elementary events, which can be equally produced by mere electronic as well as collisional events, the final results being a “convolution” of such steps. More in detail, in the case of sulphur loss, we can propose at least the following three mechanisms: a) S can be lost as SO, molecules, as in the thermal case; b) S can be lost after the “mild” reduction step, via a chemical pathway involving either the formation of H,S or the formation-and-elimination of S-containing gaseous molecules, deriving from the degradation of the chain; c) S can be lost as a result of a collisional sputtering, alone or in S-containing fragments. It is clear that the mechanisms a) and b) can be defined as the product of a “mild” chemistry, i.e., according to the related concept described in Ref. [13], these mechanisms show a certain degree of selectivity. At variance of this, the process c) is totally random in nature and clearly it does not depend on the thermodynamic of the process, the S atoms or S-containing species being eliminated on the base of statistical bond-breaking or rearrangement events. A similar analysis can also be done in the case of the reduction reaction. For this reaction we have already proposed three possible mechanisms mainly based on “mild” chemical events. These consist respectively in the primary attack of a radical H. onto a sulphonic 0, evolving to the elimination of OH. radicals, or in the electron attachment in an O-S orbital by forming a -SO;moiety, evolving towards the capture of a H+ and successive elimination of OH. , or finally through a very peculiar transfer scheme, analogous to the Fries mechanism for polycarbonates, based on the formation of a radicalic site on a phenyl ring adjacent to the -SO,group which evolves with the transfer a sulphonyl 0 to the phenyl ring radicalic site and the simultaneous reduction of the sulphonyl group to sulphoxide and finally (after repetition of the process) to -S-. These mechanisms have been extensively discussed in Ref. [8]. Furthermore, it is possible to propose also a random-like mechanism based on the collision-induced rupture of the S-O bonds. Such a last mechanism is characteristic of the very low energy range (a few keV), while for high energy (hundreds of keV) a correlation of the -SO,- reduction with the electronic term has been observed [7], suggesting that the previously mentioned mechanisms, and particularly the Fries-like one, are predominant in the electronic regime. However, it is interesting to note that the leading factors driving the Fries-like mechanism (and the other “mild” reactions above summarized) are the concentration and the formation rate of the radical sites close to the sulphonyl groups, prompting the “mild” oxygen transfer reaction. Hence, the unified dependence observed in the present paper for the -SO,- reduction is explained if we
250
G. Marletta, F. Iacona/Nucl.
Instr. and Meth. in Phys. Res. B I16 (1996) 246-252
take into account that also the collisional mechanism produces a very high and statistically distributed concentration of radical sites, thus increasing in an overwhelmingly way the effectiveness of the oxygen transfer from the sulphonyl groups. Therefore, a very similar dependence on the energy transfer mode is expected both for e- and Ar projectiles, due to the fact that the crucial intermediates are radical sites and that their concentration will be simply proportional to the total deposited energy. At variance of this, a marked dependence on the nature of the irradiating particle is instead observed for sets of reaction affecting the O-containing functional groups.
20 -\-. IO t I
YE22
.
i .
I
1E23
.
. . . . .I
I E24
.
,
..,,,I
1
I E25
@ x S, (eV/cm3) Fig. 5. Ether (-O-1 and hydroxyl (-OH) groups concentration versus the total deposited energy for PES irradiated with 6 keV Ar and 3 keV e-. The contributions are normalised to the total initial oxygen content. 80
60 -y
‘.. 0,
%0
,.A
‘\ 40
-
_.__._.. ....’
Q
L
20
.__._.
e-4..
,k.,’
-
.\ ‘0..
-m-
0,
--o--
OSulr
I 4.. ...
&..Y,
“(I
Ya
O----
El
o’.....l
..--A-.--%r
IEl5
.
~~~.~~~I
1El6 fluence (ehl~)
Fig. 4. Quantitative
,....J
iEl7
modification trends versus fluence for the total oxygen as well as for the two components found respectively at 533.8 eV C-O- and -OH groups) and 532.2 eV (0 in -SO, groups) in the XPS 0 1s peaks for PES irradiated with 6 keV Ar (a) and 3 keV e- (b). All the contributions are normalised to the total initial oxygen content.
Fig. 4 illustrates the characteristic behaviour of the 0 1s components, i.e., the ether oxygen (B.E. of 533.8 eV> and the sulphonyl groups (B.E. of 532.2 eV) 181, for Ar and e- irradiated PES. As it is expected, the sulphonyl component in the 0 Is band decreases dramatically both for Ar- and e--irradiation, this process being obviously related to the reduction process of the -SO,- groups. Accordingly, the rate of the O(sulphony1) decrease is well accounted for in terms of the total deposited energy, as that of the formation of reduced -s- groups. An interesting point comes from the observation that, also when almost no residual -SO,- groups are present within the irradiated region, we still have an oxygen species having a B.E. very similar to that of the sulphonyl group. The relative abundance of such a component is very similar for both the types of projectiles, i.e., about 15% of the total 0 1s band at the steady state, while the intensity of the -SO,- component in the S 2p band at the very same fluence is between 0 (for Ar) and 5% (for e-1. Thus, the component is assigned to oxygen belonging to new carbonyl groups, having the same B.E. with the sulphonyl groups, created by reaction of the recoiling oxygen with suitable chain sites. Evidence for the formation of such groups has been obtained for this and analogous aromatic system by FT-IR analysis (not reported here). Fig. 4 also shows that a remarkable difference can instead be observed for the modification of the ether groups. Let us discuss the main experimental features: in the case of Ar irradiation we have essentially an initial increase of ether or -OH groups (indistinguishable by XPS) concentrations, followed by the fast and almost complete depletion of these species. In the case of eirradiation the increase of the “ether-like” species concentration is occurring only after a given total energy threshold has been exceeded, and at very high fluence the
G. Marlena, F. Iacona /Nucl. Instr. and Meth. in Phys. Res. B 116 (1996) 246-252
concentration of such species level off at the initial value. Such a dramatic difference is also well reflected in Fig. 5, reporting the intensity of the component at 533.8 eV in the 0 1s band (i.e., the concerned ether-like or -OH component) versus the total deposited energy. Fig. 5 shows clearly that the rate of the reactions leading to the formation and destruction of the ether-like groups does not correlate with the total deposited energy, i.e., it is sensitive to the energy deposition mechanisms. Actually, we may consider the reactions leading to the formation of carbonyl groups and those related to the formation of ether or -OH groups, as “hot atom” reaction of “oxygen attachment” to carbon sites in the chain. Let us discuss in some detail the basic mechanistic differences between Ar and e--irradiation. In the case of Ar irradiation, the “oxygen attachment” reaction is prompted by the formation of energetic recoiling oxygen, produced in the primary Ar 3 -SO,- impact. The subsequent fast depletion of such ether-like groups is due to the predominant collisional sputtering effect, possibly involving the formation of volatile molecules as CO and related species. In the case of electron irradiation, there is essentially no possibility of formation of energetic recoiling oxygen and the collisional sputtering effect is completely absent. These two features suggest that a different mechanism is responsible for the “oxygen attachment” and for the depletion reactions in the case of electron irradiation. In particular, the experimentally observed reactions in the case of the electronic stopping may be better described as an “oxygen-transfer” process in the framework of the Frieslike mechanism above discussed. Indeed, the production of -OH groups or also ether linkages is the main result of this reaction, together with the reduction of the -SO,groups. As to the depletion after the first increase step, presently we have no experimental evidence of the kind of species which are produced and more experiments have to be performed. The above arguments justify that for the ether-like reactions no correlation is found with the total deposited energy. Indeed, in this case the primary events are intrinsically different on going from the collisional to the electronic term, aa we could summarize the two processes as a random “hot atom” process inducing an “oxygen attachment” in the case of collisional stopping and as a mild and selective “oxygen transfer” in the case of electronic stopping regime.
4. Conclusions In the present paper we report the experimental data on in situ irradiated PES showing that: a) a single chemical modification can be the result of several different and concurrent reactions. Accordingly, we
2.51
introduce the view of speaking about sets of reactions producing a unique modification. b) As in previous papers [ 14,151, we may find that (for a given polymer) sets of reactions depending on a specific energy deposition mechanism can be identified, while others depend only on the total deposited energy. Furthermore, the key factor determining the sensitivity to the energy deposition mechanisms seems to be the chemical selectivity of the involved reaction pathways. c) We discussed the experimental rates in terms of detailed mechanistic chemical processes, showing that some reactions have to be selective (as it is the case of the “oxygen attachment” reactions) due to their “mild” nature, while others cannot be selective (as the sulphur loss or the sulphonyl reduction), due to the availability of many concurrent pathways producing an unique product by a random succession of different chemical events. More future efforts will be dedicated to the further clarification of the complex link between the characteristics of chemical reactivity (i.e., the “allowed reactivity”) and the peculiarity of the energy deposition conditions.
Acknowledgement The authors acknowledge the financial support to this research by CNR, by the Strategic Project “Microtecnologie” of the CNR (Rome) and by MURST (40%).
References [I] E. Reichmanis and J.H. O’Donnell (eds.), The effects of radiations on high technology polymers, Am. Chem. Sot., Washington D.C. (1989). [2] T. Yamashita, H. Tomitaka, T. Kudo, K. Horie and I. Mita, Polym. Dcgrad. Stabil. 39 (1993) 47. [3] A.R. Lyons, M.C.R. Symons and J.K. Yandell, Makrom. Chem. 157 (1972) 103. [4] J.R. Brown and J.H. O’Donnell, J. Appl. Polym. Sci. 19 (1975) 405. [5] J.R. Brown and J.H. O’Donnell, J. Appl. Polym. Sci. 23 (1979) 2763. [6] G. Marletta, S.M. Catalano and S. Pignataro, Surf. Interf. Anal. 16 (1990) 407. [7] G. Marletta, Nucl. Instr. and Meth. B 46 (1990) 295. [S] G. Marletta, S. Pignataro, A. Toth, I. Bertoti, T. Szekely and B. Keszler, Macromolecules 24 (1991) 99. 191 H.S. Munro and D.T. Clark, Polym. Degrad. Stabil. 17 (19871319. [lo] G. Marletta and F. Iacona, in: Materials and Processes for Surface and Interface Engineering, NATO-AS1 Series, Series E: Applied Sciences, ed. Y. Pauleau, vol. 290 (Kluwer Academic Publishers, Dordnxht, The Netherlands, 1995) p. 597. [l 11 J.H. Scofield, J. Electron. Spectrosc. Relat. Phenom. 8 (1976) 129.
252
G. Marletta, F. lacona/Nucl.
Instr. and Meth. in Phys. Res. B I16 (1996) 246-252
[12] G. Marletta, S. Pignataro and C. Oliveri, Nucl. Instr. and Me& B 39 (1989) 773. [13] S. Pignataro and G. Marletta, in: Metallized Plastics 2 Fundamentals and Applied Aspects, ed. K.L. Mittal (Plenum, New York, 1991) p. 269.
[14] F. Iacona and G. Marletta, Nucl. Instr. and Meth. B 65 (1992) 50. [15] G. Marletta and F. Iacona, Nucl. Instr. and Meth. B 80/81 (1993) 1405.