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Bonding of dopants to irradiated polymers
2&J . . __
Nuclear Instruments and Methods in Physics Research B 116 (1996) 434-439 KIOIMI
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B
Beam Interactions with Materials&Atoms
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Bonding of dopants to irradiated polymers D. Fink a,*, R. Klett a, V. Hnatowicz b, J. Vacik b, C. Mathis ‘, H. Omichi d, F. Hosoi d, L.T. Chadderton e, L. Wang f a Hahn-Meitner-Institut GmbH, Dept. FD, Glienickerstr. 100, D-14109 Berlin, Germany b Institute of Nuclear Physics, Czech Academy of Sciences, 25068 Rez near Prague, Czech Republic ’ ZnstitutCharles Sadron, CNRS, 6 rue Boussingault, F-67083 Strassbourg Cedex. France ’ Takasaki Radiation Chemistry Research Establishment, JAERI, 1233 Watanuki-machi, Takosaki, Gunma 370-12, Japan e Division of Applied Physics, CSIRO, and Research School for Physical Sciences, Institute ofAdvanced Studies, Australian National University, GPO Box 4, Canberra, ACT 2601, Australia ’ Gesellschafrfuer Schwerionenforschung, Planckstr. I, D-64291 Darmstadt, Germany Abstract Latent tracks are formed in polyimide, polypropylene and polyethylene by irradiation with 0.5 to 3 GeV heavy ions. They are then doped with aqueous LiCl, organo-Li/THF, or C,,/THF solutions for well-defined periods after the irradiation. Subsequently, the excess of non-trapped Li dopant fraction is washed out. The remaining lithium-doped polymer zones along the latent ion tracks are studied here. The depth distribution of the bonding efficiency, as determined by neutron depth profiling, points out an electronic nature of the bonding process.
1. Introduction
It is known for long that mobile atoms, ions or molecules in polymers frequently interact with the host matrix via sorption processes. This leads either to a retarded dopant mobility if the dopant sorption is weak (i.e. if the dopant in shallow traps is associated with easy release), or to dopant immobilization if the trapping centers are deep, i.e. if the release probability approaches to zero. Such deep trapping centers - if not yet existing intrinsically - can also be introduced by irradiation, e.g. by y-rays, electrons, or energetic ions. In all these cases chemical bonds are broken by electronic (or seldomly collisional) energy transfer. The result of these processes is the formation of radicals, to which any reactive dopant may be bound chemically. Specifically, the bonding of monomers to reactive sites at polymeric chains (‘grafting’) is of technological interest. As many of these reactive sites are metastable with lifetimes in the order of some psec to several months, the bonding efficiency is expected to depend on the time interval between irradiation - hence radical formation - and doping. Whereas bonding by energetic y or electron irradiation is a well-established and commercially used technique, bonding or grafting of organic monomers - such as styrene
* Corresponding author. 0168-583X/96/$15.00
[l], methylene blue [2], Cs-Cocarborane [3], fullerene [4], butyl-Li [4] and butyl-Cs (still unpublished) - to polymers by means of energetic ion irradiation has been studied recently, as shown in Table la. There have been made additionally quite a number of experiments where simple inorganic ions or molecules such as iodine [5,6], lithium [4,7], cesium [3], and lead [8] have been bonded to ion-irradiated polymers (Table lb). In this work we shall expand the knowledge on ion-induced bonding by studying its dependence on some more parameters.
2. Experimental Stacks of 1.5 pm thin polypropylene (PP) and of 8 pm thin polyimide (PI) foils, and 1 mm thick polyethylene (PE) samples have been irradiated with 0.5 GeV Xe7+ or 3 GeV U**+ ions at fluences of lo* to lo’* ions cm-*. The irradiated polymers (and, for comparison, pristine material) were doped either with 5 mole/l aqueous LiCl solution at temperatures between room temperature and 100°C for 1 s to some 300 h, with a saturated butyl-Li solution in THF at room temperature for 1 day, or with a saturated (3.5 X 10e3 mole/l) C,, solution in toluene at room temperature for 1 day. After doping, the sample surface was briefly cleaned by rapidly dipping the sample into pure solvent and subsequent drying. We have shown in a recent study [lo] that the dopant depth profiles in irradiated polymers are not affected by the eventual dopant precipitation at the surface during the drying process.
Copyright 0 1996 Elsevier Science B.V. All rights reserved
PII SO168-583X(96)00084-5
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D. Fink et al,/Nucl. Instr. and Meth. in Phys. Rex B 116 (1996) 434-439
To account for the change of the metastable radical concentration with time, the doping was performed at different waiting times after the ion irradiation (we denote these times below as the ‘track age’), ranging up to half a year. Unfortunately, we could not yet reduce the track age to less than about 10 min (= time necessary for dismounting the samples from the irradiation chamber) before bonding took place, so that we could not yet study the bonding efficiency of short-living defects. The samples were stored during this waiting time at ambient air (22 f 4”C, 40 + 10% humidity). To check additionally whether eventual photochemical processes might affect the bonding efficiencies, a part of the samples was exposed to bright sunlight during ageing, and the other part was stored in the dark. The depth distributions of the dopants were measured by means of non-destructive techniques such as neutron depth profiling (NDP) and fullerene tracer profiling @I?), respectively. Both these techniques have been described in detail earlier [4,11]. The total dopant uptake in a given polymer is a measure for the empty volume in this material, which is distributed intrinsically in between the polymer chains and - in case of ion-irradiated material - additionally along the latent ion tracks. It affects both the freely mobile dopant
Table 1 (a) Overview work
of some recent experiments
on bonding
component and the trapped one. In order to determine the trapped dopant component alone, we have washed out the mobile dopant fractions by rinsing the doped samples in the appropriate solvents up to 10 times, and then remeasured the remaining dopant depth distributions in the leached samples. The remaining dopants are either chemically bound to the polymer by sharing of valence electrons, or they can be bound to other types of traps, e.g. at geometrically favored sites, voids, bubbles, etc. To distinguish between the different types of traps, we compare doping of irradiated and non-irradiated polymers. The difference of the dopant concentrations should give us some hint for the type of bonding.
3. Results and discussion 3.1. Bonding ofLi + ions to irradiated polypropylene Let us first consider the bonding of thin pristine PP foils with Li+ ions from LiCl solution. These thin foils (1.5 pm thickness) have been produced by stretching of thicker material, by which the crystalline fraction was
of organic monomers
to ion-irradiated
polymers.
Comparison
with results from this
Target
Ion irradiation
Dopant, solvent
Doping conditions
Bonding at
Ref.
PVDF
470 MeV Xe 5.35 GeV Xe 132 MeV 0 200 MeV 0 10W’4 cm-*
styrene
60-6l”C, 2h
electronic defects
ill
PI
300 keV H lo’* crnm2 500 MeV I lop-10” cm-’ 1.6 GeV MO IO” cm-* 150 keV F, As <5X 10L4cm-* i5 GeV I log-10” crnm2 0.5 GeV I log-lo” cm-* 200 keV He
aqueous methylene blue dye solution
85”C, 1 h
electronic defects
La
aqueous cs-cocarborane butyl-Li in THF &ill toluene Li grafted to PE Butyl-Li
RT-85°C. 5-65 h
electronic defects
[31
RT 24h
[41
RT
electronic defects electronic defects electronic defects electronic defects
Fullercne
RT
PE
PI PI PE-(Li) PE
PE
lo”-10’5
m-2
0.5 GeV Xe 10” crnm2 0.5 GeV Xe 10” cm-*
RT, 24h RT
electronic defects
[41 071 this work, Tables 3and4 this work, Tables 3and4
436
D. Fink et al./Nucl.
Table 1 (b) Overview of some recent experiments from this work
Instr. and Meth. in Phys. Res. B 116 (1996) 434-439
on bonding
of inorganic
ions and molecules
to ion-irradiated
polymers.
Comparison
Target
Ion irradiation
Dopant, solvent
Doping conditions
Bonding at
Ref.
PI
150 keV1.5 MeV He, Ne, Ar, Kr, Xe 2 x 10’4-2 x lOI cm-’ 150 keV F, -> 1OL4 cm-*
I, vapor doping
45”C, 60h
collisional defects
t51
I, vapor
9O”C, 4h
70% electronic + 30% collisional defects
[61
150 keV As,
I, vapor
PI
PE
doping
k51
90°C 4h
doping -c - lOI* cm-*,
lOO”C, lh
lOI cm-’ 150 keV F,
aqueous LiCl solution aqueous
electronic defects collisional defects collisional defects
RT
electronic
As,
Pb acetate
20h
defects
-> 5 x lOI cm-2 150 keV Li,
Li ions
RT
electronic
t91
10’3-10’4 cm-* 0.5 GeV Xe, 10” cm-* 3 GeV U, lo*-lo’* cmm2
implanted aqueous LiCl aqueous LiCl
RT Id 85”C, 3h
defects electronic defects electronic defects
this work, Fig. 1 this work, Table 2
> 1OL3cm-* _ PI
100 keV Ne,
PE
PP PI PP
increased up to 90%. We have recently shown [12] that doping of these highly crystalline PP foils is a bimodal process with initial rapid dopant migration through the amorphous phase, and subsequent slow dopant diffusion Table 2 Lif dopant uptake of highly crystalline
PP, irradiated
[71
@I
into the crystalline zones. This dopant uptake with increasing doping time is counteracted by the polymeric swelling which leads to drastic changes of the polymeric chain conformation and macroscopic morphology. The overall
with 3 GeV U **+ ions. All values are given with an accuracy
of 10%
Donine time
pristine PP
total uptake after
10’ cm-* irrad.
leaching total uptake after
10” cmm2 irrad.
leaching total uptake after
1Ol2 cm-* irrad.
leaching total uptake after leaching
with results
lh
3h
27 h
312h
1.3 x 10’5
1.6 x lOI
5.7 x 10’5
5.3 x 10’6
8.3 X lOI
8.9 X lOI
9.1 x lOI
6.5 X 1015
1.2 x 10’6
1.6 X 1Om
4.0 x 10’6
7.9 x lOI
7.2 X 10”
2.2 x 10’6
1.2 x 10’6
2.6 X 10m
1.7 x 1om
4.2 X lOI
1.5 x 10’6
2.8 x 10’5
4.0 x lOI
7.6 X 1Om
1.8 X lOI
2.4 X 1Om
D. Fink et al./Nucl. Instr. and Meth. in Phys. Res. B II6 (1996) 434-439 change in dopant concentration with doping time is small, see Table 2. Even at very low ion irradiation fluences the dopant uptake capability increased already by about an order of magnitude or more, as compared with pristine material (Table 2). Just as in the pristine material, the dopant penetrates rapidly into the irradiated polymer within the first minutes, whereas further dopant uptake proceeds very slowly. This indicates that the ion tracks act as some kind of ‘irrigation pipes’ of the polymeric material - similarly as the amorphous phase of PP does. Leaching leaves unaffected as well the dopants trapped at deep traps in the surviving pristine polymer (with both amorphous and crystalline phases), as the dopant fraction bound to radiation defects. From the available dam of Table 2 it is seen that, for fluences below 10” cm-*, the immobile dopant fraction in irradiated polymers is around (50 k 101% of the total dopant uptake, with no recognizable dependence on doping time. This indicates that only a limited supply of radicals is available to which Li+ can bond, and that these traps are saturable. It was estimated that after low fluence ion irradiation one can fmd up to lo4 radicals per A track length in each ion track. This high number unambiguously points at some spatial expansion of the tracks; tracks cannot at all be regarded as one-dimensional objects only. Assuming ‘close packing’ of these radicals along the tracks, one derives track diameters of 100 to 500 A. This is in acordance with the general experience from other techniques. With increasing fluence the radical density decreases from lo4 down to 10’ radicals per ion track and per A track length. This points at the beginning of the overlapping of outer track zones, whereby radicals can recombine. Furthermore supersonic shockwaves originating from the impinging heavy ions during the ion track formation are expected to compress the surrounding material including older ion tracks, thereby reducing the radical-to-radical distances in the older tracks. As a consequence, many radicals might bound to each other and thus annihilate. The fraction bound to irradiated material exceeds the fraction bound in pristine material by about one order of magnitude. Taking into consideration that about 5 to 10% of the immobile fraction stems from trapping at pristine PP, the bound fraction can be estimated to be in the order of (40 & 10) at.% of the total dopant uptake. In other words, ion track bonding to PP is a quite efficient process. 3.2. Bonding of Li + ions to irradiated polyimide Now let us turn to the doping of PI with Lif ions. We have shown in another paper [ 131 that ion track doping in PI strongly depends on the track age. The dopant uptake capability is roughly constant for ages up to a few days, and then increases gradually until it comes to saturation after roughly half a year. This had been explained by the
437
increase of track volume due to gradual diffusion of oxygen into irradiated polymers [ 141, whereby a part of the reactive ion track components might be converted to volatile CO and CO,. Another, possibly better explanation might be to correlate the increase of dopant uptake with the increase of reacted 0 centers, due to the progressive in depth oxidation, as it is well-known for the oxidation of y-irradiated polymers [15]. These extrinsic centers might act as traps for the dopants [15]. Thus there exist two different extrinsic trap populations in aged irradiated polymers with different bonding strengths to dopants, namely the ion-induced defects and the 0 centers, the latter ones increasing in number with track age. Indeed, first preliminary leaching experiments of dopants from aged tracks point at the existence of two different dopant components - a leachable, easily mobile fraction which increases with track age, and another immobile one which is rather independent of the track age. We tend to assume that the mobile dopant component along the tracks relates to the above mentioned extrinsic 0 centers. On one hand, they provide the necessary free volume for additional dopant uptake, and on the other hand they act as shallow traps from which the dopants can readily be released upon leaching. For contrast, the immobile dopant component is understood to stem from dopants trapped in stable or metastable deep traps such as long-living ion-induced defects. The ageing process does not affect the number of available deep traps, as there exists only a limited amount of such defects. We deal here with defects of very long lifetimes f B- 1 month, as all other relevant short-living radiochemical reaction products have already decayed in the initial phase of the ageing process. Assuming that each such long-living deep trap along an ion track is capable to bond only one dopant ion or molecule, the density of these traps can be estimated to be in the order of some 10 traps per ion track and per 1 pm track length. Recent EPR measurements [16] on aged tracks of 135 MeV Ar7+ ions in PI yield similar densities of unpaired spins per track. In comparison with the above discussed bonding of Li to PP, bonding to tracks in PI appears to be a rather inefficient process. This again points out that the lifetimes t are much less than 10 min for most of the radicals in PI, and it indicates the necessity to develop more rapid bonding techniques if the bonding yield of PI is to be enhanced markedly. Fig. 1 shows that both the depth profiles of the total Li uptake and of the bound Li only follow the electronic energy transfer, which means that both free volume and radical formation along tracks are consequences of the projectile’s electronic energy loss. 3.3. Doping of C,, polyethylene
molecules and orgam-Li
to irradiated
In this section we study the uptake of C,, molecules from toluene solution by pristine PE and by ion tracks
D. Fink et al./Nucl.
438
Instr. and Meth. in Phys. Res. B 116 (1996) 434-439
Table 3 The structure of the analysis project for PE bound with C, and organo-Li. Left: preparation procedure of the samples, right: spectrum of traps to which bonding of organo-Li is expected. Abbreviations for preparation procedure: PEO = pristine PE, @ = irradiation with 0.5 GeV Xe, 10” ions cm-*, I = track age between irradiation and doping. Crosses indicate that the corresponding step is included in the preparation procedure, or that the corresponding type of trap is expected to be found No.
1 2 3 4 5 6 7 8 9 10 11 12
Preparation
steps before doping with organo-Li
PEO
@
+ + + +
+
+
+
+
+
t
10 min 5h 37 h 10 min 5h 37 h 10 min 5h 37 h
Specific type of formed trap population
add
leach
C,
C Ln
+ +
+
+
+
+
lP
Rp
+ + +
MCI
MCR
CI
+ +
+
+
+
+
+
+
+
+
CT
+
+
+
+
+
Abbreviations for trap specification: IP = intrinsic traps in PE, RP = radiation induced traps (radicals) in PE, MCI = mobile C, in pristine PE, MCR = mobile C, in tracks in PE, CI = C,,, trapped at intrinsic defects in PE, CT = C6a, trapped at radiation induced traps (radicals) in PE.
therein. The existence of C,, in solids is verified by marking the buckymolecules with 6Li+ [4] or Cs’ (still unpublished result) ions from organic solution and subsequent determination of the amount of Li or Cs ions bonded after leaching out the excess of non-bonded to C,, organo-Li or organ0Cs molecules. Of course, some organo-Li (or -Cs> will be bound not only to the fullerene molecules under consideration here, but also to intrinsic traps or to ion-produced radicals along the tracks in PE. This complication makes a more refined analysis necessary. For this sake please consider Table 3. A number of carefully selected working steps leads to different combinations of trapping centers for the immobilized organo-Li, resulting in specific Li concentrations.
-
7 6
4L
_
I 2 0
2-
z h z 2 z
8: 6=----, 4
d
Depth profiles of dopnnt uptake SOOM~V Xe-Pl,10"cm-2,r=3.10's.LiCL,ld.R.T.
-doped
1
---leached L----L____
-Ss.IX) -----
?.10'5; 0
Their differences give us the required information for the efficiencies of the individual trapping processes, Table 4. We conclude that (a) organo-Li is bound as well to fullerene as to the polymer. In the polymer, intrinsic traps and radiation-induced traps bond organo-Li. The fullerenes reacting with the organo-Li may be either freely mobile, or bound to intrinsic or radiation induced traps. The total Li trapping efficiency is larger for irradiated PE than for the pristine polymer. Whereas mobile C,, in pristine PE (abbreviation ‘MCI’ in Tables 3 and 4) and in fresh tracks (‘MCR’) and C,, trapped in intrinsic traps (‘CI’) appear to play a minor role, the reactions of organo-Li with radiation induced traps in PE (‘RP’), with freely mobile fullerene along aged ion tracks (‘MCR’), and with fullerene trapped at tracks (‘CT’) appear to be the dominant processes. Interestingly, the trapping efficiency for organo-Li to fullerene doped tracks in PE increases with track age, in
, 10
I 30
I 20
I LO
Oepth[pml
Fig. 1. Depth profiles of Li in latent tracks in a stack of PI foils, after aging time r = 3 X lo4 s. Total Li concentmtion distribution (‘doped’) and bonded fraction (‘leached’) in comparison with theory (S,( x1).
Table 4 The order of magnitude of different trapping efficiencies for organo-Li in ion-irradiated C, doped PE. For explanation of the abbreviations, see Table 3a Type of trap
Trapped Li atoms [lOi
IP RP MCI MCR
7f4 12+5 O&6 O&5-30+6 age 10 min-37 13+5 056
CT CI
h
cm-‘]
D. Fink et al./Nucl.
Instr. and Meth. in Phys. Res. B 116 (1996) 434-439
contrast to our findings for PI, where the grafting efficiency remained independent of age. This increase is correlated with the increasing C,, uptake capability of aged tracks in this case, due to the increasing free volume within them. Up to 1000 C,, molecules are bound to each of the 0.5 GeV Xe tracks in PE, which means that in average one C,, molecule is bound every 300 A along an individual track. This signifies that the capability of fullerene to bond (graft) to latent ion tracks in PE is quite poor.
4. Conclusions Energetic ions impinging into polymers create ion tracks rich in free volume and radicals. The free track volume enables a certain amount of dopants to be accomodated. A part of them are strongly bound by the radicals which act as deep traps. The non-bound fraction can be released from the track by leaching. Polymer oxidation leads to the progressive formation of extrinsic defect centers which act as additional shallow traps for the dopants. This is verified here for the doping of PP, PI and PE foils with Lif ions from aqueous and organo-liquid phases, and with fullerene from liquid phase. Strong bonding of dopants to tracks - as verified by the study of the nonleachable dopant components - is observed in all cases. Great system-specific peculiarities show up: whereas bonding to PP is highly efficient, it is less efficient to PE and PI under the given conditions (track age 2 10 min). The bonding takes place at polymeric defects of electronic origin. The bonding efficiency is independent of the track age.
Acknowledgement One of the authors (D.F.) acknowledges the support from JAERI, Takasaki, Japan and from the CSIRO Canberra, Australia. Financial support of the Czech Grant Agency (V.H. and J.V.) under contract No. 202/93/0121 is also acknowledged. This work would not have been possible without the help of our accelerator and reactor
staffs in Berlin, Prague and Darmstadt. Thanks referee for careful discussion and valuable hints.
439
to the
References [ll N. Betz, C. Ducouret, A. LeMoel and E. Balanzat, Nucl. Instr. and Meth. B 91 (1994) 151. [2] D. Fink, W.H. Chung, R. Klett, M. Dijbeli, H.A. Synal, L.T. Chadderton and L. Wang, Nucl. Instr. and Meth. B 108 (1996) 377. [3] V. Hnatowicz, J. Vacik, V. Svorcik, V. Rybka, V. Popok, 0. Jankovskij, D. Fink and R. Klett, Nucl. Instr. and Meth. B 105 (1995) 241. [4] D. Fink, R. Klett, C. Mathis, J. Vacik, V. Hnatowicz and L.T. Chadderton, Nucl. Instr. and Meth. B 100 (1995) 69. [5] J. Davenas and X.L. Xu, Nucl. Instr. and Meth. B 71 (1992) 33. [6] V. Hnatowicz, J. Vacik, V. Svorcik, V. Rybka, V. Popok, 0. Jankovskij, D. Fink and R. Klett, to be submitted to Nucl. Instr. Meth. B. [7] D. Fink, V. Hnatowicz, J. Vacik and L.T. Chadderton, Radiat. Eff. Def. Solids 132 (1994) 1. [S] V. Hnatowicz, J. Kvitek, V. Perina, V. Svorcik, V. Rybka and V. Popok, Nucl. Instr. Meth. B 93 (1994) 282. [9] D. Fink, M. Behar, J. Kaschny, R. Klett, L.T. Chadderton, V. Hnatowicz, J. Vacik and L. Wang, submitted to Appl. Phys. [lo] D. Fink, R. Klett, C. Mathis, J. Vacik, V. Hnatowicz and L.T. Chadderton, submitted to Appl. Phys. [Ill D. Fink, Neutron Depth Profiling, Hahn-Meitner-Institute Berlin, Special Report (1995). This report can be obtained directly from the author. [12] R. Klett and D. Fink, in preparation (1995/96). [13] D. Fink, R. Klett, X. Hu, M. Miller, G. Schiwietz, G. Xiao, L.T. Chadderton, C. Mathis, V. Hnatowicz and J. Vacik, presented at the Int. Conf. on Radiation Effects in Insulators, Catania, Italy, 10.9.-16.9. 1995. [14] V. Hnatowicz, J. Vacik, V. Perina, V. Svorcik, V. Rybka, V. Popok, 0. Jankovskij D. Fink and R. Klett, submitted to Nucl. Instr. Meth. B. 1151 See for instance H.S. Munro, Polym. Degrad. Stab. 12 (1985) 249. [16] D. Fink, R. Klett, L.T. Chadderton, J. Cardoso, R. Montiel, H. Vazquez and A.A. Karanovich, Nucl. Instr. and Metb. B 111 (1996) 303. [17] D. Fink, R. Klett, H. Omichi, F. Hosoi, V. Hnatowicz, J. Vacik and L.T. Chadderton, work in preparation, to be published in 1996.
Nuclear Instruments and Methods in Physics Research B 116 (1996) 434-439 KIOIMI
__
B
Beam Interactions with Materials&Atoms
@ ELSEVIER
Bonding of dopants to irradiated polymers D. Fink a,*, R. Klett a, V. Hnatowicz b, J. Vacik b, C. Mathis ‘, H. Omichi d, F. Hosoi d, L.T. Chadderton e, L. Wang f a Hahn-Meitner-Institut GmbH, Dept. FD, Glienickerstr. 100, D-14109 Berlin, Germany b Institute of Nuclear Physics, Czech Academy of Sciences, 25068 Rez near Prague, Czech Republic ’ ZnstitutCharles Sadron, CNRS, 6 rue Boussingault, F-67083 Strassbourg Cedex. France ’ Takasaki Radiation Chemistry Research Establishment, JAERI, 1233 Watanuki-machi, Takosaki, Gunma 370-12, Japan e Division of Applied Physics, CSIRO, and Research School for Physical Sciences, Institute ofAdvanced Studies, Australian National University, GPO Box 4, Canberra, ACT 2601, Australia ’ Gesellschafrfuer Schwerionenforschung, Planckstr. I, D-64291 Darmstadt, Germany Abstract Latent tracks are formed in polyimide, polypropylene and polyethylene by irradiation with 0.5 to 3 GeV heavy ions. They are then doped with aqueous LiCl, organo-Li/THF, or C,,/THF solutions for well-defined periods after the irradiation. Subsequently, the excess of non-trapped Li dopant fraction is washed out. The remaining lithium-doped polymer zones along the latent ion tracks are studied here. The depth distribution of the bonding efficiency, as determined by neutron depth profiling, points out an electronic nature of the bonding process.
1. Introduction
It is known for long that mobile atoms, ions or molecules in polymers frequently interact with the host matrix via sorption processes. This leads either to a retarded dopant mobility if the dopant sorption is weak (i.e. if the dopant in shallow traps is associated with easy release), or to dopant immobilization if the trapping centers are deep, i.e. if the release probability approaches to zero. Such deep trapping centers - if not yet existing intrinsically - can also be introduced by irradiation, e.g. by y-rays, electrons, or energetic ions. In all these cases chemical bonds are broken by electronic (or seldomly collisional) energy transfer. The result of these processes is the formation of radicals, to which any reactive dopant may be bound chemically. Specifically, the bonding of monomers to reactive sites at polymeric chains (‘grafting’) is of technological interest. As many of these reactive sites are metastable with lifetimes in the order of some psec to several months, the bonding efficiency is expected to depend on the time interval between irradiation - hence radical formation - and doping. Whereas bonding by energetic y or electron irradiation is a well-established and commercially used technique, bonding or grafting of organic monomers - such as styrene
* Corresponding author. 0168-583X/96/$15.00
[l], methylene blue [2], Cs-Cocarborane [3], fullerene [4], butyl-Li [4] and butyl-Cs (still unpublished) - to polymers by means of energetic ion irradiation has been studied recently, as shown in Table la. There have been made additionally quite a number of experiments where simple inorganic ions or molecules such as iodine [5,6], lithium [4,7], cesium [3], and lead [8] have been bonded to ion-irradiated polymers (Table lb). In this work we shall expand the knowledge on ion-induced bonding by studying its dependence on some more parameters.
2. Experimental Stacks of 1.5 pm thin polypropylene (PP) and of 8 pm thin polyimide (PI) foils, and 1 mm thick polyethylene (PE) samples have been irradiated with 0.5 GeV Xe7+ or 3 GeV U**+ ions at fluences of lo* to lo’* ions cm-*. The irradiated polymers (and, for comparison, pristine material) were doped either with 5 mole/l aqueous LiCl solution at temperatures between room temperature and 100°C for 1 s to some 300 h, with a saturated butyl-Li solution in THF at room temperature for 1 day, or with a saturated (3.5 X 10e3 mole/l) C,, solution in toluene at room temperature for 1 day. After doping, the sample surface was briefly cleaned by rapidly dipping the sample into pure solvent and subsequent drying. We have shown in a recent study [lo] that the dopant depth profiles in irradiated polymers are not affected by the eventual dopant precipitation at the surface during the drying process.
Copyright 0 1996 Elsevier Science B.V. All rights reserved
PII SO168-583X(96)00084-5
435
D. Fink et al,/Nucl. Instr. and Meth. in Phys. Rex B 116 (1996) 434-439
To account for the change of the metastable radical concentration with time, the doping was performed at different waiting times after the ion irradiation (we denote these times below as the ‘track age’), ranging up to half a year. Unfortunately, we could not yet reduce the track age to less than about 10 min (= time necessary for dismounting the samples from the irradiation chamber) before bonding took place, so that we could not yet study the bonding efficiency of short-living defects. The samples were stored during this waiting time at ambient air (22 f 4”C, 40 + 10% humidity). To check additionally whether eventual photochemical processes might affect the bonding efficiencies, a part of the samples was exposed to bright sunlight during ageing, and the other part was stored in the dark. The depth distributions of the dopants were measured by means of non-destructive techniques such as neutron depth profiling (NDP) and fullerene tracer profiling @I?), respectively. Both these techniques have been described in detail earlier [4,11]. The total dopant uptake in a given polymer is a measure for the empty volume in this material, which is distributed intrinsically in between the polymer chains and - in case of ion-irradiated material - additionally along the latent ion tracks. It affects both the freely mobile dopant
Table 1 (a) Overview work
of some recent experiments
on bonding
component and the trapped one. In order to determine the trapped dopant component alone, we have washed out the mobile dopant fractions by rinsing the doped samples in the appropriate solvents up to 10 times, and then remeasured the remaining dopant depth distributions in the leached samples. The remaining dopants are either chemically bound to the polymer by sharing of valence electrons, or they can be bound to other types of traps, e.g. at geometrically favored sites, voids, bubbles, etc. To distinguish between the different types of traps, we compare doping of irradiated and non-irradiated polymers. The difference of the dopant concentrations should give us some hint for the type of bonding.
3. Results and discussion 3.1. Bonding ofLi + ions to irradiated polypropylene Let us first consider the bonding of thin pristine PP foils with Li+ ions from LiCl solution. These thin foils (1.5 pm thickness) have been produced by stretching of thicker material, by which the crystalline fraction was
of organic monomers
to ion-irradiated
polymers.
Comparison
with results from this
Target
Ion irradiation
Dopant, solvent
Doping conditions
Bonding at
Ref.
PVDF
470 MeV Xe 5.35 GeV Xe 132 MeV 0 200 MeV 0 10W’4 cm-*
styrene
60-6l”C, 2h
electronic defects
ill
PI
300 keV H lo’* crnm2 500 MeV I lop-10” cm-’ 1.6 GeV MO IO” cm-* 150 keV F, As <5X 10L4cm-* i5 GeV I log-10” crnm2 0.5 GeV I log-lo” cm-* 200 keV He
aqueous methylene blue dye solution
85”C, 1 h
electronic defects
La
aqueous cs-cocarborane butyl-Li in THF &ill toluene Li grafted to PE Butyl-Li
RT-85°C. 5-65 h
electronic defects
[31
RT 24h
[41
RT
electronic defects electronic defects electronic defects electronic defects
Fullercne
RT
PE
PI PI PE-(Li) PE
PE
lo”-10’5
m-2
0.5 GeV Xe 10” crnm2 0.5 GeV Xe 10” cm-*
RT, 24h RT
electronic defects
[41 071 this work, Tables 3and4 this work, Tables 3and4
436
D. Fink et al./Nucl.
Table 1 (b) Overview of some recent experiments from this work
Instr. and Meth. in Phys. Res. B 116 (1996) 434-439
on bonding
of inorganic
ions and molecules
to ion-irradiated
polymers.
Comparison
Target
Ion irradiation
Dopant, solvent
Doping conditions
Bonding at
Ref.
PI
150 keV1.5 MeV He, Ne, Ar, Kr, Xe 2 x 10’4-2 x lOI cm-’ 150 keV F, -> 1OL4 cm-*
I, vapor doping
45”C, 60h
collisional defects
t51
I, vapor
9O”C, 4h
70% electronic + 30% collisional defects
[61
150 keV As,
I, vapor
PI
PE
doping
k51
90°C 4h
doping -c - lOI* cm-*,
lOO”C, lh
lOI cm-’ 150 keV F,
aqueous LiCl solution aqueous
electronic defects collisional defects collisional defects
RT
electronic
As,
Pb acetate
20h
defects
-> 5 x lOI cm-2 150 keV Li,
Li ions
RT
electronic
t91
10’3-10’4 cm-* 0.5 GeV Xe, 10” cm-* 3 GeV U, lo*-lo’* cmm2
implanted aqueous LiCl aqueous LiCl
RT Id 85”C, 3h
defects electronic defects electronic defects
this work, Fig. 1 this work, Table 2
> 1OL3cm-* _ PI
100 keV Ne,
PE
PP PI PP
increased up to 90%. We have recently shown [12] that doping of these highly crystalline PP foils is a bimodal process with initial rapid dopant migration through the amorphous phase, and subsequent slow dopant diffusion Table 2 Lif dopant uptake of highly crystalline
PP, irradiated
[71
@I
into the crystalline zones. This dopant uptake with increasing doping time is counteracted by the polymeric swelling which leads to drastic changes of the polymeric chain conformation and macroscopic morphology. The overall
with 3 GeV U **+ ions. All values are given with an accuracy
of 10%
Donine time
pristine PP
total uptake after
10’ cm-* irrad.
leaching total uptake after
10” cmm2 irrad.
leaching total uptake after
1Ol2 cm-* irrad.
leaching total uptake after leaching
with results
lh
3h
27 h
312h
1.3 x 10’5
1.6 x lOI
5.7 x 10’5
5.3 x 10’6
8.3 X lOI
8.9 X lOI
9.1 x lOI
6.5 X 1015
1.2 x 10’6
1.6 X 1Om
4.0 x 10’6
7.9 x lOI
7.2 X 10”
2.2 x 10’6
1.2 x 10’6
2.6 X 10m
1.7 x 1om
4.2 X lOI
1.5 x 10’6
2.8 x 10’5
4.0 x lOI
7.6 X 1Om
1.8 X lOI
2.4 X 1Om
D. Fink et al./Nucl. Instr. and Meth. in Phys. Res. B II6 (1996) 434-439 change in dopant concentration with doping time is small, see Table 2. Even at very low ion irradiation fluences the dopant uptake capability increased already by about an order of magnitude or more, as compared with pristine material (Table 2). Just as in the pristine material, the dopant penetrates rapidly into the irradiated polymer within the first minutes, whereas further dopant uptake proceeds very slowly. This indicates that the ion tracks act as some kind of ‘irrigation pipes’ of the polymeric material - similarly as the amorphous phase of PP does. Leaching leaves unaffected as well the dopants trapped at deep traps in the surviving pristine polymer (with both amorphous and crystalline phases), as the dopant fraction bound to radiation defects. From the available dam of Table 2 it is seen that, for fluences below 10” cm-*, the immobile dopant fraction in irradiated polymers is around (50 k 101% of the total dopant uptake, with no recognizable dependence on doping time. This indicates that only a limited supply of radicals is available to which Li+ can bond, and that these traps are saturable. It was estimated that after low fluence ion irradiation one can fmd up to lo4 radicals per A track length in each ion track. This high number unambiguously points at some spatial expansion of the tracks; tracks cannot at all be regarded as one-dimensional objects only. Assuming ‘close packing’ of these radicals along the tracks, one derives track diameters of 100 to 500 A. This is in acordance with the general experience from other techniques. With increasing fluence the radical density decreases from lo4 down to 10’ radicals per ion track and per A track length. This points at the beginning of the overlapping of outer track zones, whereby radicals can recombine. Furthermore supersonic shockwaves originating from the impinging heavy ions during the ion track formation are expected to compress the surrounding material including older ion tracks, thereby reducing the radical-to-radical distances in the older tracks. As a consequence, many radicals might bound to each other and thus annihilate. The fraction bound to irradiated material exceeds the fraction bound in pristine material by about one order of magnitude. Taking into consideration that about 5 to 10% of the immobile fraction stems from trapping at pristine PP, the bound fraction can be estimated to be in the order of (40 & 10) at.% of the total dopant uptake. In other words, ion track bonding to PP is a quite efficient process. 3.2. Bonding of Li + ions to irradiated polyimide Now let us turn to the doping of PI with Lif ions. We have shown in another paper [ 131 that ion track doping in PI strongly depends on the track age. The dopant uptake capability is roughly constant for ages up to a few days, and then increases gradually until it comes to saturation after roughly half a year. This had been explained by the
437
increase of track volume due to gradual diffusion of oxygen into irradiated polymers [ 141, whereby a part of the reactive ion track components might be converted to volatile CO and CO,. Another, possibly better explanation might be to correlate the increase of dopant uptake with the increase of reacted 0 centers, due to the progressive in depth oxidation, as it is well-known for the oxidation of y-irradiated polymers [15]. These extrinsic centers might act as traps for the dopants [15]. Thus there exist two different extrinsic trap populations in aged irradiated polymers with different bonding strengths to dopants, namely the ion-induced defects and the 0 centers, the latter ones increasing in number with track age. Indeed, first preliminary leaching experiments of dopants from aged tracks point at the existence of two different dopant components - a leachable, easily mobile fraction which increases with track age, and another immobile one which is rather independent of the track age. We tend to assume that the mobile dopant component along the tracks relates to the above mentioned extrinsic 0 centers. On one hand, they provide the necessary free volume for additional dopant uptake, and on the other hand they act as shallow traps from which the dopants can readily be released upon leaching. For contrast, the immobile dopant component is understood to stem from dopants trapped in stable or metastable deep traps such as long-living ion-induced defects. The ageing process does not affect the number of available deep traps, as there exists only a limited amount of such defects. We deal here with defects of very long lifetimes f B- 1 month, as all other relevant short-living radiochemical reaction products have already decayed in the initial phase of the ageing process. Assuming that each such long-living deep trap along an ion track is capable to bond only one dopant ion or molecule, the density of these traps can be estimated to be in the order of some 10 traps per ion track and per 1 pm track length. Recent EPR measurements [16] on aged tracks of 135 MeV Ar7+ ions in PI yield similar densities of unpaired spins per track. In comparison with the above discussed bonding of Li to PP, bonding to tracks in PI appears to be a rather inefficient process. This again points out that the lifetimes t are much less than 10 min for most of the radicals in PI, and it indicates the necessity to develop more rapid bonding techniques if the bonding yield of PI is to be enhanced markedly. Fig. 1 shows that both the depth profiles of the total Li uptake and of the bound Li only follow the electronic energy transfer, which means that both free volume and radical formation along tracks are consequences of the projectile’s electronic energy loss. 3.3. Doping of C,, polyethylene
molecules and orgam-Li
to irradiated
In this section we study the uptake of C,, molecules from toluene solution by pristine PE and by ion tracks
D. Fink et al./Nucl.
438
Instr. and Meth. in Phys. Res. B 116 (1996) 434-439
Table 3 The structure of the analysis project for PE bound with C, and organo-Li. Left: preparation procedure of the samples, right: spectrum of traps to which bonding of organo-Li is expected. Abbreviations for preparation procedure: PEO = pristine PE, @ = irradiation with 0.5 GeV Xe, 10” ions cm-*, I = track age between irradiation and doping. Crosses indicate that the corresponding step is included in the preparation procedure, or that the corresponding type of trap is expected to be found No.
1 2 3 4 5 6 7 8 9 10 11 12
Preparation
steps before doping with organo-Li
PEO
@
+ + + +
+
+
+
+
+
t
10 min 5h 37 h 10 min 5h 37 h 10 min 5h 37 h
Specific type of formed trap population
add
leach
C,
C Ln
+ +
+
+
+
+
lP
Rp
+ + +
MCI
MCR
CI
+ +
+
+
+
+
+
+
+
+
CT
+
+
+
+
+
Abbreviations for trap specification: IP = intrinsic traps in PE, RP = radiation induced traps (radicals) in PE, MCI = mobile C, in pristine PE, MCR = mobile C, in tracks in PE, CI = C,,, trapped at intrinsic defects in PE, CT = C6a, trapped at radiation induced traps (radicals) in PE.
therein. The existence of C,, in solids is verified by marking the buckymolecules with 6Li+ [4] or Cs’ (still unpublished result) ions from organic solution and subsequent determination of the amount of Li or Cs ions bonded after leaching out the excess of non-bonded to C,, organo-Li or organ0Cs molecules. Of course, some organo-Li (or -Cs> will be bound not only to the fullerene molecules under consideration here, but also to intrinsic traps or to ion-produced radicals along the tracks in PE. This complication makes a more refined analysis necessary. For this sake please consider Table 3. A number of carefully selected working steps leads to different combinations of trapping centers for the immobilized organo-Li, resulting in specific Li concentrations.
-
7 6
4L
_
I 2 0
2-
z h z 2 z
8: 6=----, 4
d
Depth profiles of dopnnt uptake SOOM~V Xe-Pl,10"cm-2,r=3.10's.LiCL,ld.R.T.
-doped
1
---leached L----L____
-Ss.IX) -----
?.10'5; 0
Their differences give us the required information for the efficiencies of the individual trapping processes, Table 4. We conclude that (a) organo-Li is bound as well to fullerene as to the polymer. In the polymer, intrinsic traps and radiation-induced traps bond organo-Li. The fullerenes reacting with the organo-Li may be either freely mobile, or bound to intrinsic or radiation induced traps. The total Li trapping efficiency is larger for irradiated PE than for the pristine polymer. Whereas mobile C,, in pristine PE (abbreviation ‘MCI’ in Tables 3 and 4) and in fresh tracks (‘MCR’) and C,, trapped in intrinsic traps (‘CI’) appear to play a minor role, the reactions of organo-Li with radiation induced traps in PE (‘RP’), with freely mobile fullerene along aged ion tracks (‘MCR’), and with fullerene trapped at tracks (‘CT’) appear to be the dominant processes. Interestingly, the trapping efficiency for organo-Li to fullerene doped tracks in PE increases with track age, in
, 10
I 30
I 20
I LO
Oepth[pml
Fig. 1. Depth profiles of Li in latent tracks in a stack of PI foils, after aging time r = 3 X lo4 s. Total Li concentmtion distribution (‘doped’) and bonded fraction (‘leached’) in comparison with theory (S,( x1).
Table 4 The order of magnitude of different trapping efficiencies for organo-Li in ion-irradiated C, doped PE. For explanation of the abbreviations, see Table 3a Type of trap
Trapped Li atoms [lOi
IP RP MCI MCR
7f4 12+5 O&6 O&5-30+6 age 10 min-37 13+5 056
CT CI
h
cm-‘]
D. Fink et al./Nucl.
Instr. and Meth. in Phys. Res. B 116 (1996) 434-439
contrast to our findings for PI, where the grafting efficiency remained independent of age. This increase is correlated with the increasing C,, uptake capability of aged tracks in this case, due to the increasing free volume within them. Up to 1000 C,, molecules are bound to each of the 0.5 GeV Xe tracks in PE, which means that in average one C,, molecule is bound every 300 A along an individual track. This signifies that the capability of fullerene to bond (graft) to latent ion tracks in PE is quite poor.
4. Conclusions Energetic ions impinging into polymers create ion tracks rich in free volume and radicals. The free track volume enables a certain amount of dopants to be accomodated. A part of them are strongly bound by the radicals which act as deep traps. The non-bound fraction can be released from the track by leaching. Polymer oxidation leads to the progressive formation of extrinsic defect centers which act as additional shallow traps for the dopants. This is verified here for the doping of PP, PI and PE foils with Lif ions from aqueous and organo-liquid phases, and with fullerene from liquid phase. Strong bonding of dopants to tracks - as verified by the study of the nonleachable dopant components - is observed in all cases. Great system-specific peculiarities show up: whereas bonding to PP is highly efficient, it is less efficient to PE and PI under the given conditions (track age 2 10 min). The bonding takes place at polymeric defects of electronic origin. The bonding efficiency is independent of the track age.
Acknowledgement One of the authors (D.F.) acknowledges the support from JAERI, Takasaki, Japan and from the CSIRO Canberra, Australia. Financial support of the Czech Grant Agency (V.H. and J.V.) under contract No. 202/93/0121 is also acknowledged. This work would not have been possible without the help of our accelerator and reactor
staffs in Berlin, Prague and Darmstadt. Thanks referee for careful discussion and valuable hints.
439
to the
References [ll N. Betz, C. Ducouret, A. LeMoel and E. Balanzat, Nucl. Instr. and Meth. B 91 (1994) 151. [2] D. Fink, W.H. Chung, R. Klett, M. Dijbeli, H.A. Synal, L.T. Chadderton and L. Wang, Nucl. Instr. and Meth. B 108 (1996) 377. [3] V. Hnatowicz, J. Vacik, V. Svorcik, V. Rybka, V. Popok, 0. Jankovskij, D. Fink and R. Klett, Nucl. Instr. and Meth. B 105 (1995) 241. [4] D. Fink, R. Klett, C. Mathis, J. Vacik, V. Hnatowicz and L.T. Chadderton, Nucl. Instr. and Meth. B 100 (1995) 69. [5] J. Davenas and X.L. Xu, Nucl. Instr. and Meth. B 71 (1992) 33. [6] V. Hnatowicz, J. Vacik, V. Svorcik, V. Rybka, V. Popok, 0. Jankovskij, D. Fink and R. Klett, to be submitted to Nucl. Instr. Meth. B. [7] D. Fink, V. Hnatowicz, J. Vacik and L.T. Chadderton, Radiat. Eff. Def. Solids 132 (1994) 1. [S] V. Hnatowicz, J. Kvitek, V. Perina, V. Svorcik, V. Rybka and V. Popok, Nucl. Instr. Meth. B 93 (1994) 282. [9] D. Fink, M. Behar, J. Kaschny, R. Klett, L.T. Chadderton, V. Hnatowicz, J. Vacik and L. Wang, submitted to Appl. Phys. [lo] D. Fink, R. Klett, C. Mathis, J. Vacik, V. Hnatowicz and L.T. Chadderton, submitted to Appl. Phys. [Ill D. Fink, Neutron Depth Profiling, Hahn-Meitner-Institute Berlin, Special Report (1995). This report can be obtained directly from the author. [12] R. Klett and D. Fink, in preparation (1995/96). [13] D. Fink, R. Klett, X. Hu, M. Miller, G. Schiwietz, G. Xiao, L.T. Chadderton, C. Mathis, V. Hnatowicz and J. Vacik, presented at the Int. Conf. on Radiation Effects in Insulators, Catania, Italy, 10.9.-16.9. 1995. [14] V. Hnatowicz, J. Vacik, V. Perina, V. Svorcik, V. Rybka, V. Popok, 0. Jankovskij D. Fink and R. Klett, submitted to Nucl. Instr. Meth. B. 1151 See for instance H.S. Munro, Polym. Degrad. Stab. 12 (1985) 249. [16] D. Fink, R. Klett, L.T. Chadderton, J. Cardoso, R. Montiel, H. Vazquez and A.A. Karanovich, Nucl. Instr. and Metb. B 111 (1996) 303. [17] D. Fink, R. Klett, H. Omichi, F. Hosoi, V. Hnatowicz, J. Vacik and L.T. Chadderton, work in preparation, to be published in 1996.