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A study on radiation grafting of styrene induced by swift heavy-ions in poly(vinylidene fluoride)
NIUMI B
Nuclear Instruments and Methods in Physics Research B 91 (1994) 1.5-156 North-Holland
Beam Interactions with Materials&Atoms
A study on radiation grafting of styrene induced by swift heavy-ions in poly(vinylidene
fluoride) * *
N. Betz *, C. Ducouret
and A. Le Mocl
CEA-CE Saclay, DSM/DRECAM/SRSIM,
91 191 Gif sur Yvette Cedq
France
E. Balanzat CEA-CNRS,
CIRIL, BP 51 33, 14 040 Caen Cedex, France
Radiation grafting of polymers is nowadays a rather “classical” way of modifying the physicochemical properties of polymers: y-rays or electrons are used to induce reactive sites on the polymer chains from which can be initiated the polymerisation of monomers different from the initial irradiated polymer. Taking into account the homogeneous distribution of these sites, the final copolymer is homogeneously grafted. Swift heavy ions are another type of ionising particles. These high energy particles create on their wake through the solid a high density of excitations and ionisations which induce a cylindrical damage zone called the latent track. The radicals formed in the latent track can be used to initiate the grafting. We present a study on the post-irradiation grafting of styrene in poly(vinylidene fluoride) (PVDF) induced by swift heavy 0 and Xe ions (energies > 1 MeV amu-‘). The evolution of the grafting yield and the grafting rate with the absorbed dose or fluence shows differences depending on the type of ion used. Higher yields are obtained when the grafting is induced by swift heavy ions rather than by y-rays.
1. Introduction Radiation grafting of polymers is now a well known means of modifying polymers to change their physicochemical properties [1,2]. Due to the interaction of these particles with the polymer, the final compound is a homogeneously grafted copolymer. Swift heavy ions induce, along their wake through the solid, a continuous trail of excitations and ionizations giving rise to the creation of a latent track [3-51. As with other ionising radiation such as y- or X-photons, or electrons, radicals are created when polymers are submitted to swift heavy ion irradiation. They can initiate the grafting of molecules or monomers to produce a graft copolymer [6]. In a previous paper [7], we studied the post-radiation grafting of the styrene in a cw-poly(vinylidene fluoride) (PVDF) film. The structure of the graft film was investigated. It was shown that the grafting occurs in the bulk of the sample but with a thickness gradient. The results were compared to those obtained with a y-ray initiation, but no significant differences in the structure of the films were observed between a 44 MeV amu- ’ Xe ion grafting and a y-ray grafting at similar absorbed dose (30 kGy). Moreover,
* Corresponding author. ** Experiment performed (Caen, France). 0168-583X/94/$07.00
at GANIL heavy ion accelerator
some very preliminary results concerning the influence of irradiation parameters were given [8]. Three different particle beams were used (3.6 MeV amu-’ Xe, 40.7 MeV amu- ’ Xe and 8.3 MeV amu-t 0 ions) within a dose-range varying from 0.023 to 488 kGy. In this paper, we present new results on the radiation grafting of polystyrene (PS) in PVDF films. New data concerning the 3.6 MeV amu-’ Xe and 8.3 MeV amu- i 0 ions are given. The dose-range was enlarged (from 0.023 to 697 kGy) and another 0 energy was used. The results show that the radiation grafting induced by swift heavy ions is different from the one initiated by y-rays. Moreover, a dependence with the type of ions exists.
2. Experimental
A commercial PVDF 25 ym thick was provided by Solvay, Belgium. All the films were extracted with toluene in a Soxhlet apparatus for 24 h and dried to constant weight prior to use. The samples, fixed on a plate, were irradiated in an oxygen atmosphere at GANIL, Caen, France. The ion beam was diaphragmed to a 5 mm X 40 mm size. The plate, perpendicular to the ion beam, was vertically scanned at a speed of 0.5 cm s-i in order to get long 40 mm wide bands of irradiated polymer. Two types of
0 1994 - Elsevier Science B.V. All rights reserved
SSDZ 0168-583X(93)E0988-S
section
II. ENERGETIC HEAVY IONS
1.52
N. Betz et al. /Nucl. Instr. and Meth. in Phys. Rex B 91 (1994) 151-156
Table 1 Heavy-ion irradiation conditions. Ei and Ef are, respectively, the energy at the entrance and the exit of the film, @t is the fluence (number of ions per cmm2), D is the mean absorbed dose, (dE/dx), is the electronic stopping power, @ is the incident particle flux, and d is the distance between two tracks (calculated by l/a). Ion
Xe
Xe
0
0
E; (MeV amu-‘) E; (MeV amu-‘) @t range (cm-‘)
3.6 0.96 5.5 x 107 6.0 X lOlo 0.64 697 72.6 8.2 x 106 4.8 x 10s 1348 41
40.7 39.6 6.7 x 10’ 9.0 x 10’0 37.9 488 31.7 2.4 x 10’
8.3 7.5 4.9 x 107 3.3 x 1011 0.023 158 3.0 1.1 x 107 2.8 x 109 1429 17
12.5 11.9 1.4 x 2.8 x 5.0 97.9 2.3 5.7x 2.7 x 84.5 18.9
min. max. min. max.
D range (kGy) (dE/dx), (MeV cm2 mg-I) @ range (ions cm-’ s-l)
min. max. min. max.
d range (nm)
122 33
swift heavy ions with different energies were used to initiate the grafting: Xe ions (3.6 and 40.7 MeV amu-‘) and 0 ions (8.3 and 12.5 MeV amu-‘>. Irradiation
conditions are listed in Table 1. The experimental process of grafting is described in a previous paper [7]. The grafting yield Y is given by: Y = (Wf - Wi>/Wi, where Wi and W, are respectively the weights of the samples before and after grafting. It i8 expressed as a percentage. The grafting temperature was between 60 and 61°C in all the experiments.
3. Results In Fig. la are presented the grafting kinetics (grafting yield versus grafting time) of styrene in PVDF obtained for the 3.6 MeV amu-’ Xe initiation. The grafting yield increases when the grafting time increases. Two shapes of curves can be distinguished. Within a dose-range extending from 7.9 to 67.5 kGy, a saturation level appears after 16 h, after an initial
10’0 10’1
10s 10’
rapid rise of the grafting yield with increasing time. Outside this dose-range, the grafting yield does not saturate at 16 h. At low doses (< 1 kGy), the 0.64 kGy kinetic is similar to the one observed on an unirradiated sample (except in the first hour of grafting where the grafting is not observed in this latter case): this kind of curve shape can be related to the retention of the homopolymer. The initial grafting rate (ui> and the saturation grafting yield (S) increase when the absorbed dose increases until the dose of 232 kGy is reached. Above this dose (the case of the 697 kGy grafting kinetic), ui decreases. Moreover, in this latter case all the grafting yields, whatever the grafting time is, are smaller than those corresponding to the 232 kGy kinetic. In Fig. lb are shown the post-heavy-ion-grafting kinetics obtained after a 12.5 MeV amu-’ 0 initiation within a dose-range varying from 5.0 to 97.9 kGy. All the kinetic curves exhibit the same shape: the grafting yield rises with increasing grafting time and finally reaches a saturation level at about 16 h. v, and S
160
140
140
120
b
0 0
4
8 Grafting time Ih)
12
16
0
1
2
3
4
Grafting time thl
Fig. 1. Influence of the absorbed dose on the swift heavy ion grafting kinetics. Grafting temperature: 61°C; sample thickness: 25 pm, the values of the absorbed dose are indicated on each curve, (a) 3.6 MeV amu -’ Xe initiation, (b) 12.5 MeV amu-’ 0 initiation.
N. Betz et al. /Nucl.
Instr. and Meth. in Phys. Res. B 91 (1994) 151-156
1.53
a 100 ;; a\ -
80
s ‘s
60
E ._ r
40
-0 0
100
200
20
300
40
400
60
80
500
100
600
700
800
Dose (kGy) Fig. 2. Evolution of the grafting yield as a function of the absorbed dose. Grafting temperature:
MeV amu-’ 0, (+) 8.3 MeV amu-’ 0, (0) 3.6 MeV amu-’ Xe, (w) 40.7 MeV amu-’ Xe, (a) O-700 kGy scale; (b) zoom on the O-100 kGy zone.
160
Tb
140
140
120
120
100
100
80
80
60
60
40
40
20
20
0
0 0
4
8
12
Grafting time lhl
16
61°C; grafting time: 2 h; (0): 12.5 y-rays, 6oCo source, 5 kGy h-l;
initiation [8] are reported for the comparison. In this latter case, the grafting yield rises rapidly between 0 and 10 kGy (ui = 15% kGy_‘) then a saturation level is reached at around 50% (2 h of grafting time). Different curves are needed to describe the evolution of the grafting yield with the increasing absorbed dose when the grafting is initiated by means of swift heavy ions. None of the heavy ions used was able to give an initial slope higher than the one observed with the y-ray initiation. The slowest increase (1.6% kGy-‘> is seen in the case of an initiation with the 3.6 MeV amu-’ Xe ions. The data corresponding to the 8.3 MeV arm-’ and 12.5 MeV amu- ’ 0 ions initiation can be fitted by the same curve. The initial slope is close to the y-ray one (9.1% kGy_’ compared to 15 % kGy_‘). Above 10 kGy, and unlike the case of the y-ray-induced grafting, the saturation of the grafting yield as the absorbed
increase when the absorbed dose increases. It should be noticed that at a comparable absorbed dose when swift 0 ions are used, the kinetic parameters (ui and S) are larger than in the case of the 3.6 MeV amu-’ Xe irradiations The 40.7 MeV amu-r Xe, 8.3 MeV amu-’ 0 and y-rays kinetics have already been published [7,8]. The increase of the grafting yields with grafting time was observed. A saturation level at 16 h was seen whenever the absorbed dose was above 1 kGy. vi and S increased when the absorbed dose increased. Fig. 2 shows the evolution of the grafting yields as a function of the absorbed dose for the different ions used. The grafting time is 2 h but similar curves are obtained at 30 min, 1 h, 4 h, 7 h or 16 h. 0 and Xe, with in both cases two different energies, were used to initiate the grafting. Data corresponding to a y-ray
160
(0)
Ob 0
4
8
12
Grafting time (h)
16
0
4
8
12
16
Grafting time lhl
Fig. 3. Comparison of the grafting kinetics induced by 8.3 MeV amu -’ 0 ions and 3.6 MeV amu-’ Xe ions at similar fluences. Grafting temperature: 61°C; the irradiation conditions, given respectively for the 8.3 MeV amu-’ 0 and 3.6 MeV amu-’ Xe ions, are the following: (a) (4.9 X lo7 cm- ‘, 0.023 kGy) and (5.5X107 cm-‘, 0.64 kGy); (b) (1.1X109 cm-‘, 0.53 kGy) and (1.0~10~ cm-‘, 11.6 kGy); (c)(6.4X101’ cm-*, 31 kGy) and (6.0 1Oro cm-‘, 697 kGy).
II. ENERGETIC HEAVY IONS
N. Betz et al. /Nucl.
154
Ins@. and Meth. in Phys. Res. B 91 (1994) 151-156
100 75 50 25 0 O.OOE+OO
2,50E+lO
5.00E+
7.50E+lO
10
1 .OOE + 11
I
Ol O.OOE+OO
5.00E+lO
1.50E+ll
l.OOE+ll
2.00E
+ 11
2.50Ecll
3.00E+
11
Fluence (ions/cm*)
Fig. 4. Evolution of the grafting yields as a function of the fluence. Grafting temperature: 61°C; grafting time: 2 h, (0): 12.5 MeV amu- ’ 0, (+): 8.3 MeV amu-’ 0, (0) 3.6 MeV amu-’ Xe ions; (w>: 40.7 MeV amu-’ Xe ions. (a) 0-3.0~10~~ cm-* scale; (b) zoom on the O-l.0 X 10” cm-* zone.
dose increases was never observed in the case of a heavy ion grafting. The 3.6 MeV amu-’ Xe curve reaches a maximum, around 70% of grafting yield, between 200-500 kGy. This maximum is not due to an experimental artefact since it was observed whatever the grafting time (see Fig. la). In the case of the 40.7 MeV amu-’ Xe ions initiation, the grafting yield shows a monotonic increase with increasing dose. Data do not fit well on the 3.6 MeV amu-’ Xe curve; no maximum is seen. The 8.3 MeV amu-i and 12.5 MeV amu-’ 0 ions curves are merged: the grafting yield continuously increases when the absorbed dose increases, but no tendency to saturation is observed in the dose range here studied. Moreover, the grafting yields are higher than those obtained with either the y-ray or the Xe initiations. It was previous shown [Xl that, due to the radial distribution of the absorbed dose in the case of a heavy ion irradiation, the dose might not be the only irradiation parameter to be considered. Fig. 3 presents a comparison of the grafting kinetics, obtained after a 8.3 MeV amu-’ 0 irradiation and a 3.6 MeV amu-’ Xe irradiation at similar fluences: 5 X 107, 1 X lOlo and 6 X lOlo ions cm-‘. In the first case (Fig. 3a), the kinetics are comparable in oi and in the grafting yield magnitude (16% at 16 h). The grafting yields are small and no saturation of the grafting yield is observed because the corresponding doses are lower than 1 kGy (0.023 kGy and 0.64 kGy for the 0 and Xe irradiation respectively). In the second case (Fig. 3b), the 3.6 MeV amu- ’ Xe kinetic curve is placed above the 8.3 MeV amu-’ 0: ui and S are larger (0.14 h-l versus 0.11 h-l and 50% versus 30%, respectively). Fig. 3c compares the grafting kinetics initiated by the 3.6 MeV amu-’ Xe ions and the 8.3 MeV amu-’ 0 ions for fluences that are 6.0 x lOlo and 6.4 X lOlo ions cmP2 respectively. Between 0 and 8 h of grafting time, the grafting yields when the grafting is initiated after the 0 irradiation are larger than when the initiation is
performed after the Xe irradiation. The 0 kinetic has a higher u, than Xe (0.89 h-l against 0.42 h-i). The 0 to Xe initial grafting rate ratio is 2.1 when the fluence ratio is 1.1: the difference is significant. Fig. 4 shows the evolution of the grafting yield at 2 h as a function of the fluence for the different initiations. Similar curves are obtained whatever the grafting time is (30 min, 1 h, 4 h, 7 h, 16 h). In the case of the 3.6 MeV amu- ’ Xe irradiation, the grafting yield shows a rapid increase when the fluence increases from 0 to 6.0 X 10’ cm-2. Above, a maximum is observed at 2.0 x lOi cmF2. In the case of the 0 irradiations (8.3 MeV amu-’ and 12.5 MeV amu-‘), the curves are merged. The grafting yield rises continuously as the fluence increases, but with a decreasing rate: no maximum was observed in the fluence range used here. The 0 curves and 3.6 MeV amu-’ Xe curve cross around 3 x lOlo cm-2. Below this fluence, the grafting yields obtained after the Xe irradiation are higher than those obtained with the 0 irradiation. Above, the opposite prevails. The points corresponding to the 40.7 MeV amu-’ Xe initiation fit on the 0 curves. The 3.6 MeV amu- ’ Xe curve has a quite different shape. In this case, there is a significant energy gradient in the film (see Table 1) and one may think that it is responsible for the distinct behaviour of the 3.6 MeV amu-’ Xe curve. This energy gradient is expected to cause the lowering of the grafting yields rather than their increase. It neither explains the higher yields observed between 0 and 3 x lOlo cm-’ with the 3.6 MeV amu-r ions nor the decrease in the grafting yield above 2 X lOlo cm-2.
4. Discussion The grafting kinetics show that the grafting yield can be controlled by the grafting time (the yield increases with increasing time) and by the fluence, hence
155
N. Betz et al. /Nucl. Instr. and Meth. in Phys. Rex B 91 (1994) 151-156
the dose (the yield increases with increasing dose) (see Fig. 1). The saturation of the grafting yield with the grafting time corresponds to an increase in the termination of the styrene polymerisation during the grafting process. The initial rapid rise of the grafting yield with the dose is due to an increase in number of peroxides in the irradiated film. In the case of a y-ray initiation, it seems that we cannot get grafting yields superior to 100% as saturation is observed with the grafting time (16 h) and with the absorbed dose (from 10 kGy). This is not the case for initiation, with swift heavy ions. Higher grafting yields can be obtained, whatever the grafting time is, with all kinds of heavy ions when doses are higher than 10 kGy for the 8.3 MeV amu-’ and 12.5 MeV amu-i 0 and in the 200-700 kGy range for the 3.6 MeV amu-i and 40.7 MeV amu-’ Xe ions (Fig. 2). Moreover, the data (grafting yields versus absorbed dose) concerning heavy ions cannot be fitted with one master curve. This means that other parameters than the mean absorbed dose control the grafting yield. At similar absorbed doses, higher grafting yields are always obtained with the 0 initiation, whereas the opposite occurs if the comparison is made at similar fluences in the peculiar case of the 3.6 MeV amu-’ Xe initiation and under 3 X lOlo cm-’ (Fig. 4). The two parameters (the mean absorbed dose D and the fluence @t) are related to each other by the following linear relationship: D[kGy]
= (dE/dx),[MeV x
mg cm-‘]
X @t[cm-‘1
1.6 x 10-l’,
where (dE/dx), is the electronic stopping power and 1.6 X 1O-1o is a constant resulting from conversion units. The value of (dE/dx), is very different in the case of the irradiations used here: it is 72.6 MeV mg cm-’ (3.6 MeV amu-’ Xe), 31.7 MeV mg cn-’ (40.7 MeV amu- ’ Xe>, 3.0 MeV mg cm-’ (8.3 MeV amum 0) and 2.3 MeV mg cmd2 (12.5 MeV amu-’ 0) (Table 1). This means that higher values of fluence are needed for, for example, the 40.7 MeV amu-’ Xe irradiation to obtain the same mean absorbed dose as for the 3.6 MeV amu- ’ Xe irradiation. It is clearly seen that the role of (dE/dx), has to be considered. In addition, if the question of the overlapping of tracks (resulting from high fluences) stands, we have to take the radius of the latent track into account. In the case of the ion irradiation, S-electrons are ejected mostly perpendicularly to the ion trajectory, and a radial dose distribution exists. Thus, in the case of ion irradiation, the absorbed dose is a mean absorbed dose that takes into account the high values of the dose in the track core and the smaller values of the doses in the track halo. Moreover, the diameter of the latent track depends on the atomic number of the ion and its
velocity. The higher the ion velocity is, the higher the S-electrons energy is and the further the energy will be deposited [9]. When the ion energy increases (at same atomic number), the dose deposited in the core track is lowered but the distribution is enlarged. Therefore two definitions of the track radius may be considered depending on the criteria used: (1) the radius of the cylinder in which a fixed fraction of the energy is deposited or (2), the radius where a fixed dose is reached. We will consider the second definition and take the value of approximately 1 kGy for the deposited dose (under this limit, the kinetic is closer to an homopolymerisation one than to a grafting one). Based on Waligorski calculations [9], values of radii of 20 nm (0 8.3 MeV amu-’ and 12.5 MeV amu-’ ions), 60 nm (40.7 MeV amu-’ Xe ions) and 100 nm (3.6 MeV amu Xe ions) were calculated. Comparing these results with the values of the mean distance between two tracks given in Table 1, it is seen that a fluence range exists, in all cases except for the 40.7 MeV amu-’ Xe irradiation, where an overlapping of latent tracks during the irradiation does not occur. This fluence limit is around 6 X lOlo cm-’ (20 kGy) and 6 x 10’ cme2 (67.5 kGy) for, respectively, the two 0 irradiations and the 3.6 MeV amu-’ Xe irradiation. In the O-20 kGy range, as the deposited energy is higher in the case of the Xe ions, the grafting yields are then expected to be higher than in the case of an O-induced grafting. The opposite is observed (Fig. 2). In fact, this indicates that considering the overlapping of the tracks is not sufficient to explain the experimental data. A different evolution is seen when the grafting yields are observed as a function of the fluence (Fig. 4). An exception made for the 3.6 MeV amu-’ Xe ions case, the grafting yield shows an identical evolution as the fluence increases. It is surprising that whatever the kind of ion used and whatever its energy is, the grafting yield seems to depend only on the number of ions, and thus seems to be independent, to a first approximation, of parameters such as the (dE/dx),, the local dose or the radial extension of the track.
5. Conclusion Our results shows that the post-radiation grafting, of styrene into o-PVDF, initiated by swift heavy ions (0 and Xe of different energies) is different from the one initiated with a y-ray irradiation. The grafting yields obtained can be higher. Very different behaviours are obtained whether the grafting yield is analysed relative to the absorbed dose or the fluence. It seems that the efficiency of grafting depends on the fluence rather than on the deposited dose. This is a clear indication that the formation of radicals proceeds in a different way when the energy is deposited by II. ENERGETIC
HEAVY
IONS
156
N. Betz et al. /Nucl. Instr. and Meth. in Phys. Rex B 91 (1994) 151-156
means of swift heavy ions rather
than by y-rays or
electrons. The recombination of radicals may be favoured in the track core as a consequence of both an enhanced mobility due to the space left by the degradation of the macromolecules and a clustering effect. As the radial distance increases from the ion track core, both the number of radicals created and their mobility could then be decreased.
References [l] A. Chapiro, in: High Polymers, Vol. XV (Interscience, 1962).
[2] M. Dole, in: The Radiation Chemistry of Macromolecules (Academic Press, 1972). [3] L. Chadderton, in: Radiation Damage in Crystals (Methuen, London, 1965). [4] R.L. Fleisher, P.B. Price and R.M. Walker, in: Nuclear Tracks in Solids (University of California Press, 1975). [S] E. Balanzat, J.C. Jousset and M. Toulemonde, Nucl. Instr. and Meth. B 32 (1988) 36. [6] N. Betz, J.P. Duraud A. Le Moe1 and E. Balanzat, Radiat. Eff. and Defects in Solids 110 (1989) 81. [7] N. Betz, A. Le Moe1 J.P. Duraud and E. Balanzat, Macromolecules 25 (1992) 213. [8] N. Betz, J.P. Duraud, A. Le Moe1 and E. Balanzat, Radiat. Eff. and Defects in Solids 110 (1993) 221. [9] M.P.R. Waligorski, R.N. Hamm and R. Katz, Nucl. Tracks Radiat. Meas. 11 (1986) 309.
Nuclear Instruments and Methods in Physics Research B 91 (1994) 1.5-156 North-Holland
Beam Interactions with Materials&Atoms
A study on radiation grafting of styrene induced by swift heavy-ions in poly(vinylidene
fluoride) * *
N. Betz *, C. Ducouret
and A. Le Mocl
CEA-CE Saclay, DSM/DRECAM/SRSIM,
91 191 Gif sur Yvette Cedq
France
E. Balanzat CEA-CNRS,
CIRIL, BP 51 33, 14 040 Caen Cedex, France
Radiation grafting of polymers is nowadays a rather “classical” way of modifying the physicochemical properties of polymers: y-rays or electrons are used to induce reactive sites on the polymer chains from which can be initiated the polymerisation of monomers different from the initial irradiated polymer. Taking into account the homogeneous distribution of these sites, the final copolymer is homogeneously grafted. Swift heavy ions are another type of ionising particles. These high energy particles create on their wake through the solid a high density of excitations and ionisations which induce a cylindrical damage zone called the latent track. The radicals formed in the latent track can be used to initiate the grafting. We present a study on the post-irradiation grafting of styrene in poly(vinylidene fluoride) (PVDF) induced by swift heavy 0 and Xe ions (energies > 1 MeV amu-‘). The evolution of the grafting yield and the grafting rate with the absorbed dose or fluence shows differences depending on the type of ion used. Higher yields are obtained when the grafting is induced by swift heavy ions rather than by y-rays.
1. Introduction Radiation grafting of polymers is now a well known means of modifying polymers to change their physicochemical properties [1,2]. Due to the interaction of these particles with the polymer, the final compound is a homogeneously grafted copolymer. Swift heavy ions induce, along their wake through the solid, a continuous trail of excitations and ionizations giving rise to the creation of a latent track [3-51. As with other ionising radiation such as y- or X-photons, or electrons, radicals are created when polymers are submitted to swift heavy ion irradiation. They can initiate the grafting of molecules or monomers to produce a graft copolymer [6]. In a previous paper [7], we studied the post-radiation grafting of the styrene in a cw-poly(vinylidene fluoride) (PVDF) film. The structure of the graft film was investigated. It was shown that the grafting occurs in the bulk of the sample but with a thickness gradient. The results were compared to those obtained with a y-ray initiation, but no significant differences in the structure of the films were observed between a 44 MeV amu- ’ Xe ion grafting and a y-ray grafting at similar absorbed dose (30 kGy). Moreover,
* Corresponding author. ** Experiment performed (Caen, France). 0168-583X/94/$07.00
at GANIL heavy ion accelerator
some very preliminary results concerning the influence of irradiation parameters were given [8]. Three different particle beams were used (3.6 MeV amu-’ Xe, 40.7 MeV amu- ’ Xe and 8.3 MeV amu-t 0 ions) within a dose-range varying from 0.023 to 488 kGy. In this paper, we present new results on the radiation grafting of polystyrene (PS) in PVDF films. New data concerning the 3.6 MeV amu-’ Xe and 8.3 MeV amu- i 0 ions are given. The dose-range was enlarged (from 0.023 to 697 kGy) and another 0 energy was used. The results show that the radiation grafting induced by swift heavy ions is different from the one initiated by y-rays. Moreover, a dependence with the type of ions exists.
2. Experimental
A commercial PVDF 25 ym thick was provided by Solvay, Belgium. All the films were extracted with toluene in a Soxhlet apparatus for 24 h and dried to constant weight prior to use. The samples, fixed on a plate, were irradiated in an oxygen atmosphere at GANIL, Caen, France. The ion beam was diaphragmed to a 5 mm X 40 mm size. The plate, perpendicular to the ion beam, was vertically scanned at a speed of 0.5 cm s-i in order to get long 40 mm wide bands of irradiated polymer. Two types of
0 1994 - Elsevier Science B.V. All rights reserved
SSDZ 0168-583X(93)E0988-S
section
II. ENERGETIC HEAVY IONS
1.52
N. Betz et al. /Nucl. Instr. and Meth. in Phys. Rex B 91 (1994) 151-156
Table 1 Heavy-ion irradiation conditions. Ei and Ef are, respectively, the energy at the entrance and the exit of the film, @t is the fluence (number of ions per cmm2), D is the mean absorbed dose, (dE/dx), is the electronic stopping power, @ is the incident particle flux, and d is the distance between two tracks (calculated by l/a). Ion
Xe
Xe
0
0
E; (MeV amu-‘) E; (MeV amu-‘) @t range (cm-‘)
3.6 0.96 5.5 x 107 6.0 X lOlo 0.64 697 72.6 8.2 x 106 4.8 x 10s 1348 41
40.7 39.6 6.7 x 10’ 9.0 x 10’0 37.9 488 31.7 2.4 x 10’
8.3 7.5 4.9 x 107 3.3 x 1011 0.023 158 3.0 1.1 x 107 2.8 x 109 1429 17
12.5 11.9 1.4 x 2.8 x 5.0 97.9 2.3 5.7x 2.7 x 84.5 18.9
min. max. min. max.
D range (kGy) (dE/dx), (MeV cm2 mg-I) @ range (ions cm-’ s-l)
min. max. min. max.
d range (nm)
122 33
swift heavy ions with different energies were used to initiate the grafting: Xe ions (3.6 and 40.7 MeV amu-‘) and 0 ions (8.3 and 12.5 MeV amu-‘>. Irradiation
conditions are listed in Table 1. The experimental process of grafting is described in a previous paper [7]. The grafting yield Y is given by: Y = (Wf - Wi>/Wi, where Wi and W, are respectively the weights of the samples before and after grafting. It i8 expressed as a percentage. The grafting temperature was between 60 and 61°C in all the experiments.
3. Results In Fig. la are presented the grafting kinetics (grafting yield versus grafting time) of styrene in PVDF obtained for the 3.6 MeV amu-’ Xe initiation. The grafting yield increases when the grafting time increases. Two shapes of curves can be distinguished. Within a dose-range extending from 7.9 to 67.5 kGy, a saturation level appears after 16 h, after an initial
10’0 10’1
10s 10’
rapid rise of the grafting yield with increasing time. Outside this dose-range, the grafting yield does not saturate at 16 h. At low doses (< 1 kGy), the 0.64 kGy kinetic is similar to the one observed on an unirradiated sample (except in the first hour of grafting where the grafting is not observed in this latter case): this kind of curve shape can be related to the retention of the homopolymer. The initial grafting rate (ui> and the saturation grafting yield (S) increase when the absorbed dose increases until the dose of 232 kGy is reached. Above this dose (the case of the 697 kGy grafting kinetic), ui decreases. Moreover, in this latter case all the grafting yields, whatever the grafting time is, are smaller than those corresponding to the 232 kGy kinetic. In Fig. lb are shown the post-heavy-ion-grafting kinetics obtained after a 12.5 MeV amu-’ 0 initiation within a dose-range varying from 5.0 to 97.9 kGy. All the kinetic curves exhibit the same shape: the grafting yield rises with increasing grafting time and finally reaches a saturation level at about 16 h. v, and S
160
140
140
120
b
0 0
4
8 Grafting time Ih)
12
16
0
1
2
3
4
Grafting time thl
Fig. 1. Influence of the absorbed dose on the swift heavy ion grafting kinetics. Grafting temperature: 61°C; sample thickness: 25 pm, the values of the absorbed dose are indicated on each curve, (a) 3.6 MeV amu -’ Xe initiation, (b) 12.5 MeV amu-’ 0 initiation.
N. Betz et al. /Nucl.
Instr. and Meth. in Phys. Res. B 91 (1994) 151-156
1.53
a 100 ;; a\ -
80
s ‘s
60
E ._ r
40
-0 0
100
200
20
300
40
400
60
80
500
100
600
700
800
Dose (kGy) Fig. 2. Evolution of the grafting yield as a function of the absorbed dose. Grafting temperature:
MeV amu-’ 0, (+) 8.3 MeV amu-’ 0, (0) 3.6 MeV amu-’ Xe, (w) 40.7 MeV amu-’ Xe, (a) O-700 kGy scale; (b) zoom on the O-100 kGy zone.
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61°C; grafting time: 2 h; (0): 12.5 y-rays, 6oCo source, 5 kGy h-l;
initiation [8] are reported for the comparison. In this latter case, the grafting yield rises rapidly between 0 and 10 kGy (ui = 15% kGy_‘) then a saturation level is reached at around 50% (2 h of grafting time). Different curves are needed to describe the evolution of the grafting yield with the increasing absorbed dose when the grafting is initiated by means of swift heavy ions. None of the heavy ions used was able to give an initial slope higher than the one observed with the y-ray initiation. The slowest increase (1.6% kGy-‘> is seen in the case of an initiation with the 3.6 MeV amu-’ Xe ions. The data corresponding to the 8.3 MeV arm-’ and 12.5 MeV amu- ’ 0 ions initiation can be fitted by the same curve. The initial slope is close to the y-ray one (9.1% kGy_’ compared to 15 % kGy_‘). Above 10 kGy, and unlike the case of the y-ray-induced grafting, the saturation of the grafting yield as the absorbed
increase when the absorbed dose increases. It should be noticed that at a comparable absorbed dose when swift 0 ions are used, the kinetic parameters (ui and S) are larger than in the case of the 3.6 MeV amu-’ Xe irradiations The 40.7 MeV amu-r Xe, 8.3 MeV amu-’ 0 and y-rays kinetics have already been published [7,8]. The increase of the grafting yields with grafting time was observed. A saturation level at 16 h was seen whenever the absorbed dose was above 1 kGy. vi and S increased when the absorbed dose increased. Fig. 2 shows the evolution of the grafting yields as a function of the absorbed dose for the different ions used. The grafting time is 2 h but similar curves are obtained at 30 min, 1 h, 4 h, 7 h or 16 h. 0 and Xe, with in both cases two different energies, were used to initiate the grafting. Data corresponding to a y-ray
160
(0)
Ob 0
4
8
12
Grafting time (h)
16
0
4
8
12
16
Grafting time lhl
Fig. 3. Comparison of the grafting kinetics induced by 8.3 MeV amu -’ 0 ions and 3.6 MeV amu-’ Xe ions at similar fluences. Grafting temperature: 61°C; the irradiation conditions, given respectively for the 8.3 MeV amu-’ 0 and 3.6 MeV amu-’ Xe ions, are the following: (a) (4.9 X lo7 cm- ‘, 0.023 kGy) and (5.5X107 cm-‘, 0.64 kGy); (b) (1.1X109 cm-‘, 0.53 kGy) and (1.0~10~ cm-‘, 11.6 kGy); (c)(6.4X101’ cm-*, 31 kGy) and (6.0 1Oro cm-‘, 697 kGy).
II. ENERGETIC HEAVY IONS
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100 75 50 25 0 O.OOE+OO
2,50E+lO
5.00E+
7.50E+lO
10
1 .OOE + 11
I
Ol O.OOE+OO
5.00E+lO
1.50E+ll
l.OOE+ll
2.00E
+ 11
2.50Ecll
3.00E+
11
Fluence (ions/cm*)
Fig. 4. Evolution of the grafting yields as a function of the fluence. Grafting temperature: 61°C; grafting time: 2 h, (0): 12.5 MeV amu- ’ 0, (+): 8.3 MeV amu-’ 0, (0) 3.6 MeV amu-’ Xe ions; (w>: 40.7 MeV amu-’ Xe ions. (a) 0-3.0~10~~ cm-* scale; (b) zoom on the O-l.0 X 10” cm-* zone.
dose increases was never observed in the case of a heavy ion grafting. The 3.6 MeV amu-’ Xe curve reaches a maximum, around 70% of grafting yield, between 200-500 kGy. This maximum is not due to an experimental artefact since it was observed whatever the grafting time (see Fig. la). In the case of the 40.7 MeV amu-’ Xe ions initiation, the grafting yield shows a monotonic increase with increasing dose. Data do not fit well on the 3.6 MeV amu-’ Xe curve; no maximum is seen. The 8.3 MeV amu-i and 12.5 MeV amu-’ 0 ions curves are merged: the grafting yield continuously increases when the absorbed dose increases, but no tendency to saturation is observed in the dose range here studied. Moreover, the grafting yields are higher than those obtained with either the y-ray or the Xe initiations. It was previous shown [Xl that, due to the radial distribution of the absorbed dose in the case of a heavy ion irradiation, the dose might not be the only irradiation parameter to be considered. Fig. 3 presents a comparison of the grafting kinetics, obtained after a 8.3 MeV amu-’ 0 irradiation and a 3.6 MeV amu-’ Xe irradiation at similar fluences: 5 X 107, 1 X lOlo and 6 X lOlo ions cm-‘. In the first case (Fig. 3a), the kinetics are comparable in oi and in the grafting yield magnitude (16% at 16 h). The grafting yields are small and no saturation of the grafting yield is observed because the corresponding doses are lower than 1 kGy (0.023 kGy and 0.64 kGy for the 0 and Xe irradiation respectively). In the second case (Fig. 3b), the 3.6 MeV amu- ’ Xe kinetic curve is placed above the 8.3 MeV amu-’ 0: ui and S are larger (0.14 h-l versus 0.11 h-l and 50% versus 30%, respectively). Fig. 3c compares the grafting kinetics initiated by the 3.6 MeV amu-’ Xe ions and the 8.3 MeV amu-’ 0 ions for fluences that are 6.0 x lOlo and 6.4 X lOlo ions cmP2 respectively. Between 0 and 8 h of grafting time, the grafting yields when the grafting is initiated after the 0 irradiation are larger than when the initiation is
performed after the Xe irradiation. The 0 kinetic has a higher u, than Xe (0.89 h-l against 0.42 h-i). The 0 to Xe initial grafting rate ratio is 2.1 when the fluence ratio is 1.1: the difference is significant. Fig. 4 shows the evolution of the grafting yield at 2 h as a function of the fluence for the different initiations. Similar curves are obtained whatever the grafting time is (30 min, 1 h, 4 h, 7 h, 16 h). In the case of the 3.6 MeV amu- ’ Xe irradiation, the grafting yield shows a rapid increase when the fluence increases from 0 to 6.0 X 10’ cm-2. Above, a maximum is observed at 2.0 x lOi cmF2. In the case of the 0 irradiations (8.3 MeV amu-’ and 12.5 MeV amu-‘), the curves are merged. The grafting yield rises continuously as the fluence increases, but with a decreasing rate: no maximum was observed in the fluence range used here. The 0 curves and 3.6 MeV amu-’ Xe curve cross around 3 x lOlo cm-2. Below this fluence, the grafting yields obtained after the Xe irradiation are higher than those obtained with the 0 irradiation. Above, the opposite prevails. The points corresponding to the 40.7 MeV amu-’ Xe initiation fit on the 0 curves. The 3.6 MeV amu- ’ Xe curve has a quite different shape. In this case, there is a significant energy gradient in the film (see Table 1) and one may think that it is responsible for the distinct behaviour of the 3.6 MeV amu-’ Xe curve. This energy gradient is expected to cause the lowering of the grafting yields rather than their increase. It neither explains the higher yields observed between 0 and 3 x lOlo cm-’ with the 3.6 MeV amu-r ions nor the decrease in the grafting yield above 2 X lOlo cm-2.
4. Discussion The grafting kinetics show that the grafting yield can be controlled by the grafting time (the yield increases with increasing time) and by the fluence, hence
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the dose (the yield increases with increasing dose) (see Fig. 1). The saturation of the grafting yield with the grafting time corresponds to an increase in the termination of the styrene polymerisation during the grafting process. The initial rapid rise of the grafting yield with the dose is due to an increase in number of peroxides in the irradiated film. In the case of a y-ray initiation, it seems that we cannot get grafting yields superior to 100% as saturation is observed with the grafting time (16 h) and with the absorbed dose (from 10 kGy). This is not the case for initiation, with swift heavy ions. Higher grafting yields can be obtained, whatever the grafting time is, with all kinds of heavy ions when doses are higher than 10 kGy for the 8.3 MeV amu-’ and 12.5 MeV amu-i 0 and in the 200-700 kGy range for the 3.6 MeV amu-i and 40.7 MeV amu-’ Xe ions (Fig. 2). Moreover, the data (grafting yields versus absorbed dose) concerning heavy ions cannot be fitted with one master curve. This means that other parameters than the mean absorbed dose control the grafting yield. At similar absorbed doses, higher grafting yields are always obtained with the 0 initiation, whereas the opposite occurs if the comparison is made at similar fluences in the peculiar case of the 3.6 MeV amu-’ Xe initiation and under 3 X lOlo cm-’ (Fig. 4). The two parameters (the mean absorbed dose D and the fluence @t) are related to each other by the following linear relationship: D[kGy]
= (dE/dx),[MeV x
mg cm-‘]
X @t[cm-‘1
1.6 x 10-l’,
where (dE/dx), is the electronic stopping power and 1.6 X 1O-1o is a constant resulting from conversion units. The value of (dE/dx), is very different in the case of the irradiations used here: it is 72.6 MeV mg cm-’ (3.6 MeV amu-’ Xe), 31.7 MeV mg cn-’ (40.7 MeV amu- ’ Xe>, 3.0 MeV mg cm-’ (8.3 MeV amum 0) and 2.3 MeV mg cmd2 (12.5 MeV amu-’ 0) (Table 1). This means that higher values of fluence are needed for, for example, the 40.7 MeV amu-’ Xe irradiation to obtain the same mean absorbed dose as for the 3.6 MeV amu- ’ Xe irradiation. It is clearly seen that the role of (dE/dx), has to be considered. In addition, if the question of the overlapping of tracks (resulting from high fluences) stands, we have to take the radius of the latent track into account. In the case of the ion irradiation, S-electrons are ejected mostly perpendicularly to the ion trajectory, and a radial dose distribution exists. Thus, in the case of ion irradiation, the absorbed dose is a mean absorbed dose that takes into account the high values of the dose in the track core and the smaller values of the doses in the track halo. Moreover, the diameter of the latent track depends on the atomic number of the ion and its
velocity. The higher the ion velocity is, the higher the S-electrons energy is and the further the energy will be deposited [9]. When the ion energy increases (at same atomic number), the dose deposited in the core track is lowered but the distribution is enlarged. Therefore two definitions of the track radius may be considered depending on the criteria used: (1) the radius of the cylinder in which a fixed fraction of the energy is deposited or (2), the radius where a fixed dose is reached. We will consider the second definition and take the value of approximately 1 kGy for the deposited dose (under this limit, the kinetic is closer to an homopolymerisation one than to a grafting one). Based on Waligorski calculations [9], values of radii of 20 nm (0 8.3 MeV amu-’ and 12.5 MeV amu-’ ions), 60 nm (40.7 MeV amu-’ Xe ions) and 100 nm (3.6 MeV amu Xe ions) were calculated. Comparing these results with the values of the mean distance between two tracks given in Table 1, it is seen that a fluence range exists, in all cases except for the 40.7 MeV amu-’ Xe irradiation, where an overlapping of latent tracks during the irradiation does not occur. This fluence limit is around 6 X lOlo cm-’ (20 kGy) and 6 x 10’ cme2 (67.5 kGy) for, respectively, the two 0 irradiations and the 3.6 MeV amu-’ Xe irradiation. In the O-20 kGy range, as the deposited energy is higher in the case of the Xe ions, the grafting yields are then expected to be higher than in the case of an O-induced grafting. The opposite is observed (Fig. 2). In fact, this indicates that considering the overlapping of the tracks is not sufficient to explain the experimental data. A different evolution is seen when the grafting yields are observed as a function of the fluence (Fig. 4). An exception made for the 3.6 MeV amu-’ Xe ions case, the grafting yield shows an identical evolution as the fluence increases. It is surprising that whatever the kind of ion used and whatever its energy is, the grafting yield seems to depend only on the number of ions, and thus seems to be independent, to a first approximation, of parameters such as the (dE/dx),, the local dose or the radial extension of the track.
5. Conclusion Our results shows that the post-radiation grafting, of styrene into o-PVDF, initiated by swift heavy ions (0 and Xe of different energies) is different from the one initiated with a y-ray irradiation. The grafting yields obtained can be higher. Very different behaviours are obtained whether the grafting yield is analysed relative to the absorbed dose or the fluence. It seems that the efficiency of grafting depends on the fluence rather than on the deposited dose. This is a clear indication that the formation of radicals proceeds in a different way when the energy is deposited by II. ENERGETIC
HEAVY
IONS
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means of swift heavy ions rather
than by y-rays or
electrons. The recombination of radicals may be favoured in the track core as a consequence of both an enhanced mobility due to the space left by the degradation of the macromolecules and a clustering effect. As the radial distance increases from the ion track core, both the number of radicals created and their mobility could then be decreased.
References [l] A. Chapiro, in: High Polymers, Vol. XV (Interscience, 1962).
[2] M. Dole, in: The Radiation Chemistry of Macromolecules (Academic Press, 1972). [3] L. Chadderton, in: Radiation Damage in Crystals (Methuen, London, 1965). [4] R.L. Fleisher, P.B. Price and R.M. Walker, in: Nuclear Tracks in Solids (University of California Press, 1975). [S] E. Balanzat, J.C. Jousset and M. Toulemonde, Nucl. Instr. and Meth. B 32 (1988) 36. [6] N. Betz, J.P. Duraud A. Le Moe1 and E. Balanzat, Radiat. Eff. and Defects in Solids 110 (1989) 81. [7] N. Betz, A. Le Moe1 J.P. Duraud and E. Balanzat, Macromolecules 25 (1992) 213. [8] N. Betz, J.P. Duraud, A. Le Moe1 and E. Balanzat, Radiat. Eff. and Defects in Solids 110 (1993) 221. [9] M.P.R. Waligorski, R.N. Hamm and R. Katz, Nucl. Tracks Radiat. Meas. 11 (1986) 309.