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Radiation-induced grafting of styrene onto polyimide ion track membranes
Nuclear Instruments and Methods in Physics Research B 105 (1995) 139-144
Beam Interactions with Materials & Atoms HSEVIER
Radiation-induced
grafting of styrene onto polyimide ion track membranes
K. Friese a, V. Placek a, R. Mehnert a, N. Angert b, R. Spohrb, Ch. Trautmannb aInstitutjTir OberflichenmodifizierungLeipzig, Permoserstr. 15, D-04303 Leipzig, Germany b GSI Darmstadt, Germany
Abstract
The radiation induced grafting of styrene onto polyimide was carried out by applying the method of preirradiation as well as the simultaneous irradiation of polymer and monomer. The polymerization was initiated by gamma irradiation and by swift heavy ions. The result of these experiments was that no grafting occurs by applying the preirradiation method but grafting (up to 30%) is obtained by using the simultaneous method. For an explanation some ESR-experiments were carried out. We found that even the non-irradiated polyimide has a clear ESR signal with two different lines, probably as a result of absorbed oxygen and thermally formed radicals. The preirradiation method was not successful, because no peroxy-radicals are formed after irradiation. Grafting by initiation with heavy ions results in small grafting yields, but etching of the membranes is markedly reduced. -_ 3. Experimental
part
1. The properties of polyimide
It has been known for many years that polyimide is one of the most stable polymers. This is due to its thermal stability from -270°C up to + 400°C and a very good resistivity towards solvents and chemicals. Another property is the good radiation stability. Up to a high irradiation dose of lo6 kGy we note no changes in the mechanical properties [l]. Therefore polyimide is a valuable polymer for the preparation of ion track membranes.
Our aim was to modify ion track membranes from polyimide by radiation grafting with styrene. Both the method of preirradiation of the polymer in air and the sumultaneous irradiation of polymer and monomer were applied. All experiments were carried out with a cobalt-60 irradiation plant. The activity of this plant was 516 Tbq and the dose rate normally was 2 kGy/h. Besides this, we tested an on-line grafting of styrene during the heavy ion irradiation of a polyimide membrane swollen with monomer. 3.1. Materials
2. Grafiing of polyimide
Perhaps because of the excellent properties of polyimide - nearly all grafted polymers decrease these properties - only few papers are known dealing with radiation grafting of polyimide; Kabanov [2] published results about the grafting of polyimide with acrylic acid using the preirradiation method. In a large number of experiments investigating the dependence on several parameters, the grafting yield was between 3 and 4% by weight. The number of radicals at - 196°C at an irradiation dose of 1.2 kGy was measured to be 4.7 x 10” spins/g. The authors assume that this amount is high enough to initiate the grafting reaction but the low grafting yield of 4% by weight was explained by steric hindrance.
The polyimide foil was KAPTON HN from Du Pont with a thickness between 25 and 125 pm. Styrene was purified by shaking with 10% NaOH and water, drying and distilling under vacuum. 3.2. The preirradiation
method
In a large number of experiments we varied the irradiation dose, the time between the end of irradiation and the start of polymerization, the temperature of grafting, and the grafting time (Table 1). In agreement with the literature, all experiments resulted in a grafting yield between 3 and 4% by weight. The grafting yield was determined by measur-
0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)00824-1
II. SURFACE/BULK
MODIFICATIONS
140
K Friese et al. / Nucf. Instr. and Meth. in Phys. Res. B 105 (1995) I39-
144
Table 1 Experimental conditions for grafting applying the preirradiation method Irradiation dose Temperature of grafting Grafting time Time between end of irradiation and start of grafting Type of polyimide
ing the weight increase after the extraction of the homopolymer with toluene or ethyl acetate and drying the membranes at 80°C under vacuum for up to 24 days. Because it was surprising that all experiments resulted in the same grafting yield and because steric hindrance seemed an insufficient explanation for a limitation of the grafting, we carried out an additional ectraction with methanol and found that in every case the weight was due to the swelling of solvent, acting as a plasticizer for the polyimide.
Fig. 1. Apparatus for grafting applying the simultaneous method. 1 - polyimide foils, 2 - glass flask, A, B, C - vacuum cock, 4 - glass flask with styrene, 5 - dewar with liquid nitrogen.
10,39,100,140,320, and 480 kGy 43,60,75, and 90°C 2,5,8,10 and 25 h lo,30 min, 1.5,5,21 h, 45 days Kapton HN 50,75,125 pm
This means that in all experiments no grafting occurs when the preirradiation method was used. 3.3. The simultaneous method The simultaneous method of grafting was applied in the absence of oxygen. Therefore the oxygen dissolved in the polymer and the monomer was removed by a process of freezing and thawing under vacuum in a special glass appartus (Fig. 1). After that, the monomer was poured onto the polymer until the polymer was saturated with styrene (contact time up to 120 h) and afterwards poured back into the glass flask. The swollen foils were irradiated at room temperature under vacuum. In order to increase the grafting yield some foils were heated to 160°C for 20 min after stopping the irradiation. The grafted foils extracted with ethylacetate in a Soxhlet apparatus for 48 hours, then the solvent was removed by boiling in methanol and the polymer was dried. The grafting yield was determined by weighing. The grafting yield as a function of the absorbed dose (irradiation time) is shown in Fig. 2. Because preirradiation grafting proved to be unsuccessful, it is assumed that the grafting time corresponds to the irradiation time (e.g. 50 h for 100 kGy). It can be seen that the grafting yield increases up to 15% by weight and after heating to 160°C up to 30% by weight. The result of these experiments was: No grajkg by
1
Fig. 2. Grafting yield as a function of irradiation dose. Some samples were heated to 160°C for 20 min after stopping the irradiation. Irradiation time: Dose/2 kGy/h
I2 Friese et al. / Nucl. Instr and Meth. in Phys. Res. B 105 (1995) 139 - 144
141
JQ$+*
J-g&_
A+
*
#z?EE$Ld
b il JQQ+ 0 fJ aadefc.!ma&m
Fig. 3. Two main processes at the particle bombardment of polyimide (Marietta et al.). applying the preirradiation method and grafting by using the simultaneous irradiation of polymer and monomer. To explain this unusual behavior, some ESR-experiments were carried out.
4. Radical formation in polyimide Results of ESR-investigations of polyimides in the literature are not very concordant, but all papers agree that irradiation attacks at first the imide ring (Figs. 3 and 4) and only at higher dose the aromatic ring [2-41. Our ESR-measurements were carried out by using a Bruker ESR-spectrometer ESP 300e. We found the unusual result that even the non-irradiated polyimide has a clear ESR-signal. The spectrum shows two different lines: a narrow one with a width of 0.6 mT (5 x lOI spins/g), and a second line of about 130 mT (spin concentration of 2.4 X 1017 spins/g (Fig. 5). An explanation of this fact is not clear. It is highly improbable that the origin of these ESR signals is due to additives resulting from the polyimide processing. Laboratory synthesized polyimide showed the same signals. The spin concentration was determined by double integration of the narrow singlet indicated in Fig. 5.
0
Fig. 4.
al.).
Radicals after y-irradiation in Kapton (Pasal’skii et
using a ‘strong pitch’ standard. We suggest that the signal of the broad line is formed by absorbed oxygen. Oxygen is a paramagnetic molecule because of the parallel orientation of its valence electrons. Normally it is not possible to detect these ESR signals (they are very broad and weak) but at low pressure or in the case of absorption, the reorientation of oxygen is hindered. This orientation will be the reason for the measured ‘stable’ ESR-signals. Evidence for this hypothesis was found by evacuating and heating the foils within the ESR tube for 16 hours to remove the dissolved oxygen: The result was that the ESR-signal decreased. After passing air into the glass the ESR signal increased again. An explanation for the narrow line can be the thermal formation of stable radicals during the synthesis of polyimide. The imidization step is carried out at 200 to 300°C. Therefore we heated the polyimide foils within the ESR-resonator in air for 10 min up to 400°C and noted an increase of the narrow line by 200% during a heating period of 3 hours. In contrast to this result no increase was measured under vacuum, but after that in air the same concentration of radicals was formed (Fig. 6). These radicals are not able to initiate the grafting of a monomer. The gamma-irradiation of polyimide was done under air at a dose rate of 3.3 kGy/h up to a dose of 600 kGy. The ESR measurements were started 10 min after irradiation. Compared to the non-irradiated polymer we measured only an increase of stable radicals of 20%. Only at higher doses (more than 400 kGy) another type of radical was measured with a very small concentration (probably phenyl-radicals). The most important result was that no peroxy-radicals were detected! This is probably the reason why grafting is unsuccessful by applying the method of preirradiation of the polymer under air. II. SURFACE/BULK MODIFICATIONS
K Friese et al. / Nucl. Instr and Meth. in Phys. Res. B 105 (1995) 139 - 144
142
-
Moo
2500
3500
-
-
to1
Fig. 5. ESR-spectrum of non-irradiated Kapton HN.
Experiments of irradiating the polyimide under vacuum (0.01 Pa) with a dose up to 308 kGy lead to an increase of the spin concentration to 1.9 x 1017spins/g.
5 -
3
10 I
I
10
lal
I lcK?l
-._..L_ ._ __ 1W min
(This was detected only for the narrow line, the broad line remained unchanged). After pouring stryene (free of oxygen) onto these samples, the signal intensity of the narrow line increased by 67%. However, the radical concentration should remain unchanged after styrene addition. The increase of the signal intensity points to the evolution of a new radical type, exhibiting other microwave saturation behavior. This means that radicals present on the polyimide surface are capable to react with styrene. Probably grafting takes place at the interface polyimide/styrene.
I -... ‘wo
I
5. Grafting onto nuclear track membranes
lccnl
Tmm (mm)
Fig. 6. Increase of the narrow line radical concentration in unirradiated polyimide after heating.
The irradiation of the polyimide foils with heavy ions was carried out at GSI Darmstadt. The conditions
Table 2 Reduction of the pore diameter of particle track membranes by grafting with styrene, using the simultaneous method Grafting yield
diameter before
diameter after graft-
reduction of the di-
(%Io)
grafting
ing in nm
ameter
7.3 9.9 12.7 13.7 15.5 29.9 32.9
180 390 500 600 600 160 180
130 310 400 480 460 130 120
27 21 20 20 23 19 33
in nm
in %
-
K Friese et al. / Nucl. Instr and Meth. in Phys. Res. B 105 (1995) 139- 144
35
orrrvlow
143
06) .
.
30-
2..._
r
.--
. ~~_..
__,__
.,_
.,,__
._~..
_ .
.._.
_....
&.
.--
...-._--....--.--.--_
~~~hedhg20mhat166*C
2!5-
Fig. 7. Grafting yield as a function of number of pores (direct ly-raygrafting).
of irradiation were: Au- and Bi-ions with an energy of 11.6 and 12.5 MeV/u. Dose = 1.1 X 106-5.9X 10’ ions/cm*. Etching of the irradiated foils, was carried out with sodium hypochloride at pH 10 and 58°C. The diameter of the pores was between 160 and 2700 nm. The grafting was carried out by applying the simultaneous method in absence of oxygen. The time for the saturation with monomer was between 40 and 60 hours, the irradiation dose by gamma irradiation was 140 kGy. Some samples were heated for 20 minutes to 160°C. The grafting yield is related to the total surface of the nuclear track membrane which can come into contact with styrene. It is shown that the grafting yield rises with the number of tracks (Fig. 7). In this picture the area of the foils indicates the sum of the area of the foil surface plus the inner area of all tracks. If the grafting takes place also in the inner part of the tracks, their diameter should be reduced. The diameter of the pores was determined by scanning electron microscopy. By using a Kapton foil with a thick-
ness of SOpm, the diameter of the porse was reduced by 25% (Table 2).
6. Grafting with heavy ion irradiation Irradiaton with heavy ions may result in great differences compared with gamma irradiation, because of the strong LET effects. Besides this, the grafting should occur mainly in or near the spurs, thus reducing the overall grafting yield. In our experiments the Kapton HN foils were saturated for many hours with styrene up to 12.5% by weight under air as well as under vacuum. For the irradiation experiment the foils were fixed on a support (contact for this operation in air 5 min) or the foils were welded into a compound foil before starting the irradiation to prevent a reaction with oxygen. Irradiation was carried out with swift heavy ions of ‘29Xe with a specific energy of 11.4 MeV/u, and 238U with an energy of 11.6 MeV/u. The doses were between 4 x 10’ and 1 X 10” ions/cm2. The grafting yields were inde-
Table 3 Grafting with swift heavy ion irradiation. Grafting yield as 0.06
function of the ion type and the fluence Ion
Uran Uran Uran Uran Uran Uran Xenon Xenon
fluence (ions/cm’)
grafting yield
4x107 8X108 I5 x 108 35 x 108 70 x 10s 70 x 108 6~10~ 1 x 1010
0.4 0.7 0.8 0.8 0.8 1.1 1.2 1.5
g
ii::
f
0.05 -
.p
0.04 -
% 0.03 2 0.02 “n (L 0.01 0.00 0.0
f
I 0.2
I 0.4
I 0.6
I 0.6
I 1.0
r I 1.2
E I 1.4
1.6
Gr&ingyield(%) Fig. 8. Radial etching rate as function of the grafting yield. II. SURFACE/BULK MODIFICATIONS
K Friese et al. / Nucl. Instr. and Meth. in Phys. Res. B 105 (1995) 139 - 144
144
pendent on the preparation technique between 0.3 and 1.5% by weight. This is shown in Table 3. In relation to the real volume of the spurs the grafting yield is very much higher (more than 100%) but also the limit of error is very high because of the very small weight increase (maximum 1.5 mg). The grafting yield was determined as usual after extraction of the monomer and homopolymer with pentane and ethyl acetate. Also in the case of heavy ion-induced grafting track etching was possible. However, the etching rate decreases with growing grafting yield. This points to a modification of polyimide within the ion tracks. In comparison to ungrafted polyimide the pore diameter is markedly smaller for grafted polyimide. This is seen in Fig. 8. The
markedly
reduced
etching
rate
is a further
indication of grafting of styrene into polyimide.
References 111D.W. Clegg, A.A. Collyer, Irradiation Effects on Polymers, Elsevier Sci. Publ. Ltd 1991.
121T.I. Reschetilova, L.P. Sidorova and V.Ya. Kabanov, Plast. Massy 10 (1983) 9.
131G. Marletta, S. Pignataro and C. Olivieri, Nucl. Instr. Meth. Phys. Res. B. 39 (1989) 792. G. Marletta and F. Iacona, Nucl. Instr. Meth. Phys. Res. B 80/81 (1993) 1045. G. Marletta, S. Pignatoro, C. Oliveri and G. Ferla, Surf. Interf. Anal. 12 (1988) 447. F. Iacona and G. Marietta, Nucl. Instr. Meth. Phys. Res. B 65 (1992) 50. G. Marletta, F. Iacona and A. Toth, Macromolecules 25 (121 (1992) 3190. G. Marletta, Nucl. Instr. and Meth. B 46 (19901 295. [4] B.K. Pasal’skii, D.I. Shvets, L.P. Rusin, and Ya.1. Lavrentovich, Teor. Eksp. Khim., 15 (11, (1987) 91.
151M.A. George, B.L. Ramakrishna and W.S. Glausinger, J. Phys. Chem. 94 (1990) 5159.
Beam Interactions with Materials & Atoms HSEVIER
Radiation-induced
grafting of styrene onto polyimide ion track membranes
K. Friese a, V. Placek a, R. Mehnert a, N. Angert b, R. Spohrb, Ch. Trautmannb aInstitutjTir OberflichenmodifizierungLeipzig, Permoserstr. 15, D-04303 Leipzig, Germany b GSI Darmstadt, Germany
Abstract
The radiation induced grafting of styrene onto polyimide was carried out by applying the method of preirradiation as well as the simultaneous irradiation of polymer and monomer. The polymerization was initiated by gamma irradiation and by swift heavy ions. The result of these experiments was that no grafting occurs by applying the preirradiation method but grafting (up to 30%) is obtained by using the simultaneous method. For an explanation some ESR-experiments were carried out. We found that even the non-irradiated polyimide has a clear ESR signal with two different lines, probably as a result of absorbed oxygen and thermally formed radicals. The preirradiation method was not successful, because no peroxy-radicals are formed after irradiation. Grafting by initiation with heavy ions results in small grafting yields, but etching of the membranes is markedly reduced. -_ 3. Experimental
part
1. The properties of polyimide
It has been known for many years that polyimide is one of the most stable polymers. This is due to its thermal stability from -270°C up to + 400°C and a very good resistivity towards solvents and chemicals. Another property is the good radiation stability. Up to a high irradiation dose of lo6 kGy we note no changes in the mechanical properties [l]. Therefore polyimide is a valuable polymer for the preparation of ion track membranes.
Our aim was to modify ion track membranes from polyimide by radiation grafting with styrene. Both the method of preirradiation of the polymer in air and the sumultaneous irradiation of polymer and monomer were applied. All experiments were carried out with a cobalt-60 irradiation plant. The activity of this plant was 516 Tbq and the dose rate normally was 2 kGy/h. Besides this, we tested an on-line grafting of styrene during the heavy ion irradiation of a polyimide membrane swollen with monomer. 3.1. Materials
2. Grafiing of polyimide
Perhaps because of the excellent properties of polyimide - nearly all grafted polymers decrease these properties - only few papers are known dealing with radiation grafting of polyimide; Kabanov [2] published results about the grafting of polyimide with acrylic acid using the preirradiation method. In a large number of experiments investigating the dependence on several parameters, the grafting yield was between 3 and 4% by weight. The number of radicals at - 196°C at an irradiation dose of 1.2 kGy was measured to be 4.7 x 10” spins/g. The authors assume that this amount is high enough to initiate the grafting reaction but the low grafting yield of 4% by weight was explained by steric hindrance.
The polyimide foil was KAPTON HN from Du Pont with a thickness between 25 and 125 pm. Styrene was purified by shaking with 10% NaOH and water, drying and distilling under vacuum. 3.2. The preirradiation
method
In a large number of experiments we varied the irradiation dose, the time between the end of irradiation and the start of polymerization, the temperature of grafting, and the grafting time (Table 1). In agreement with the literature, all experiments resulted in a grafting yield between 3 and 4% by weight. The grafting yield was determined by measur-
0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)00824-1
II. SURFACE/BULK
MODIFICATIONS
140
K Friese et al. / Nucf. Instr. and Meth. in Phys. Res. B 105 (1995) I39-
144
Table 1 Experimental conditions for grafting applying the preirradiation method Irradiation dose Temperature of grafting Grafting time Time between end of irradiation and start of grafting Type of polyimide
ing the weight increase after the extraction of the homopolymer with toluene or ethyl acetate and drying the membranes at 80°C under vacuum for up to 24 days. Because it was surprising that all experiments resulted in the same grafting yield and because steric hindrance seemed an insufficient explanation for a limitation of the grafting, we carried out an additional ectraction with methanol and found that in every case the weight was due to the swelling of solvent, acting as a plasticizer for the polyimide.
Fig. 1. Apparatus for grafting applying the simultaneous method. 1 - polyimide foils, 2 - glass flask, A, B, C - vacuum cock, 4 - glass flask with styrene, 5 - dewar with liquid nitrogen.
10,39,100,140,320, and 480 kGy 43,60,75, and 90°C 2,5,8,10 and 25 h lo,30 min, 1.5,5,21 h, 45 days Kapton HN 50,75,125 pm
This means that in all experiments no grafting occurs when the preirradiation method was used. 3.3. The simultaneous method The simultaneous method of grafting was applied in the absence of oxygen. Therefore the oxygen dissolved in the polymer and the monomer was removed by a process of freezing and thawing under vacuum in a special glass appartus (Fig. 1). After that, the monomer was poured onto the polymer until the polymer was saturated with styrene (contact time up to 120 h) and afterwards poured back into the glass flask. The swollen foils were irradiated at room temperature under vacuum. In order to increase the grafting yield some foils were heated to 160°C for 20 min after stopping the irradiation. The grafted foils extracted with ethylacetate in a Soxhlet apparatus for 48 hours, then the solvent was removed by boiling in methanol and the polymer was dried. The grafting yield was determined by weighing. The grafting yield as a function of the absorbed dose (irradiation time) is shown in Fig. 2. Because preirradiation grafting proved to be unsuccessful, it is assumed that the grafting time corresponds to the irradiation time (e.g. 50 h for 100 kGy). It can be seen that the grafting yield increases up to 15% by weight and after heating to 160°C up to 30% by weight. The result of these experiments was: No grajkg by
1
Fig. 2. Grafting yield as a function of irradiation dose. Some samples were heated to 160°C for 20 min after stopping the irradiation. Irradiation time: Dose/2 kGy/h
I2 Friese et al. / Nucl. Instr and Meth. in Phys. Res. B 105 (1995) 139 - 144
141
JQ$+*
J-g&_
A+
*
#z?EE$Ld
b il JQQ+ 0 fJ aadefc.!ma&m
Fig. 3. Two main processes at the particle bombardment of polyimide (Marietta et al.). applying the preirradiation method and grafting by using the simultaneous irradiation of polymer and monomer. To explain this unusual behavior, some ESR-experiments were carried out.
4. Radical formation in polyimide Results of ESR-investigations of polyimides in the literature are not very concordant, but all papers agree that irradiation attacks at first the imide ring (Figs. 3 and 4) and only at higher dose the aromatic ring [2-41. Our ESR-measurements were carried out by using a Bruker ESR-spectrometer ESP 300e. We found the unusual result that even the non-irradiated polyimide has a clear ESR-signal. The spectrum shows two different lines: a narrow one with a width of 0.6 mT (5 x lOI spins/g), and a second line of about 130 mT (spin concentration of 2.4 X 1017 spins/g (Fig. 5). An explanation of this fact is not clear. It is highly improbable that the origin of these ESR signals is due to additives resulting from the polyimide processing. Laboratory synthesized polyimide showed the same signals. The spin concentration was determined by double integration of the narrow singlet indicated in Fig. 5.
0
Fig. 4.
al.).
Radicals after y-irradiation in Kapton (Pasal’skii et
using a ‘strong pitch’ standard. We suggest that the signal of the broad line is formed by absorbed oxygen. Oxygen is a paramagnetic molecule because of the parallel orientation of its valence electrons. Normally it is not possible to detect these ESR signals (they are very broad and weak) but at low pressure or in the case of absorption, the reorientation of oxygen is hindered. This orientation will be the reason for the measured ‘stable’ ESR-signals. Evidence for this hypothesis was found by evacuating and heating the foils within the ESR tube for 16 hours to remove the dissolved oxygen: The result was that the ESR-signal decreased. After passing air into the glass the ESR signal increased again. An explanation for the narrow line can be the thermal formation of stable radicals during the synthesis of polyimide. The imidization step is carried out at 200 to 300°C. Therefore we heated the polyimide foils within the ESR-resonator in air for 10 min up to 400°C and noted an increase of the narrow line by 200% during a heating period of 3 hours. In contrast to this result no increase was measured under vacuum, but after that in air the same concentration of radicals was formed (Fig. 6). These radicals are not able to initiate the grafting of a monomer. The gamma-irradiation of polyimide was done under air at a dose rate of 3.3 kGy/h up to a dose of 600 kGy. The ESR measurements were started 10 min after irradiation. Compared to the non-irradiated polymer we measured only an increase of stable radicals of 20%. Only at higher doses (more than 400 kGy) another type of radical was measured with a very small concentration (probably phenyl-radicals). The most important result was that no peroxy-radicals were detected! This is probably the reason why grafting is unsuccessful by applying the method of preirradiation of the polymer under air. II. SURFACE/BULK MODIFICATIONS
K Friese et al. / Nucl. Instr and Meth. in Phys. Res. B 105 (1995) 139 - 144
142
-
Moo
2500
3500
-
-
to1
Fig. 5. ESR-spectrum of non-irradiated Kapton HN.
Experiments of irradiating the polyimide under vacuum (0.01 Pa) with a dose up to 308 kGy lead to an increase of the spin concentration to 1.9 x 1017spins/g.
5 -
3
10 I
I
10
lal
I lcK?l
-._..L_ ._ __ 1W min
(This was detected only for the narrow line, the broad line remained unchanged). After pouring stryene (free of oxygen) onto these samples, the signal intensity of the narrow line increased by 67%. However, the radical concentration should remain unchanged after styrene addition. The increase of the signal intensity points to the evolution of a new radical type, exhibiting other microwave saturation behavior. This means that radicals present on the polyimide surface are capable to react with styrene. Probably grafting takes place at the interface polyimide/styrene.
I -... ‘wo
I
5. Grafting onto nuclear track membranes
lccnl
Tmm (mm)
Fig. 6. Increase of the narrow line radical concentration in unirradiated polyimide after heating.
The irradiation of the polyimide foils with heavy ions was carried out at GSI Darmstadt. The conditions
Table 2 Reduction of the pore diameter of particle track membranes by grafting with styrene, using the simultaneous method Grafting yield
diameter before
diameter after graft-
reduction of the di-
(%Io)
grafting
ing in nm
ameter
7.3 9.9 12.7 13.7 15.5 29.9 32.9
180 390 500 600 600 160 180
130 310 400 480 460 130 120
27 21 20 20 23 19 33
in nm
in %
-
K Friese et al. / Nucl. Instr and Meth. in Phys. Res. B 105 (1995) 139- 144
35
orrrvlow
143
06) .
.
30-
2..._
r
.--
. ~~_..
__,__
.,_
.,,__
._~..
_ .
.._.
_....
&.
.--
...-._--....--.--.--_
~~~hedhg20mhat166*C
2!5-
Fig. 7. Grafting yield as a function of number of pores (direct ly-raygrafting).
of irradiation were: Au- and Bi-ions with an energy of 11.6 and 12.5 MeV/u. Dose = 1.1 X 106-5.9X 10’ ions/cm*. Etching of the irradiated foils, was carried out with sodium hypochloride at pH 10 and 58°C. The diameter of the pores was between 160 and 2700 nm. The grafting was carried out by applying the simultaneous method in absence of oxygen. The time for the saturation with monomer was between 40 and 60 hours, the irradiation dose by gamma irradiation was 140 kGy. Some samples were heated for 20 minutes to 160°C. The grafting yield is related to the total surface of the nuclear track membrane which can come into contact with styrene. It is shown that the grafting yield rises with the number of tracks (Fig. 7). In this picture the area of the foils indicates the sum of the area of the foil surface plus the inner area of all tracks. If the grafting takes place also in the inner part of the tracks, their diameter should be reduced. The diameter of the pores was determined by scanning electron microscopy. By using a Kapton foil with a thick-
ness of SOpm, the diameter of the porse was reduced by 25% (Table 2).
6. Grafting with heavy ion irradiation Irradiaton with heavy ions may result in great differences compared with gamma irradiation, because of the strong LET effects. Besides this, the grafting should occur mainly in or near the spurs, thus reducing the overall grafting yield. In our experiments the Kapton HN foils were saturated for many hours with styrene up to 12.5% by weight under air as well as under vacuum. For the irradiation experiment the foils were fixed on a support (contact for this operation in air 5 min) or the foils were welded into a compound foil before starting the irradiation to prevent a reaction with oxygen. Irradiation was carried out with swift heavy ions of ‘29Xe with a specific energy of 11.4 MeV/u, and 238U with an energy of 11.6 MeV/u. The doses were between 4 x 10’ and 1 X 10” ions/cm2. The grafting yields were inde-
Table 3 Grafting with swift heavy ion irradiation. Grafting yield as 0.06
function of the ion type and the fluence Ion
Uran Uran Uran Uran Uran Uran Xenon Xenon
fluence (ions/cm’)
grafting yield
4x107 8X108 I5 x 108 35 x 108 70 x 10s 70 x 108 6~10~ 1 x 1010
0.4 0.7 0.8 0.8 0.8 1.1 1.2 1.5
g
ii::
f
0.05 -
.p
0.04 -
% 0.03 2 0.02 “n (L 0.01 0.00 0.0
f
I 0.2
I 0.4
I 0.6
I 0.6
I 1.0
r I 1.2
E I 1.4
1.6
Gr&ingyield(%) Fig. 8. Radial etching rate as function of the grafting yield. II. SURFACE/BULK MODIFICATIONS
K Friese et al. / Nucl. Instr. and Meth. in Phys. Res. B 105 (1995) 139 - 144
144
pendent on the preparation technique between 0.3 and 1.5% by weight. This is shown in Table 3. In relation to the real volume of the spurs the grafting yield is very much higher (more than 100%) but also the limit of error is very high because of the very small weight increase (maximum 1.5 mg). The grafting yield was determined as usual after extraction of the monomer and homopolymer with pentane and ethyl acetate. Also in the case of heavy ion-induced grafting track etching was possible. However, the etching rate decreases with growing grafting yield. This points to a modification of polyimide within the ion tracks. In comparison to ungrafted polyimide the pore diameter is markedly smaller for grafted polyimide. This is seen in Fig. 8. The
markedly
reduced
etching
rate
is a further
indication of grafting of styrene into polyimide.
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