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Volatile evolution induced by energetic He++ ions in a polyurethane and the effects of previous gamma irradiation
Nuclear Instruments and Methods in Physics Research B 236 (2005) 223–228 www.elsevier.com/locate/nimb
Volatile evolution induced by energetic He++ ions in a polyurethane and the effects of previous gamma irradiation J.J. Murphy a
a,*
, C.J. Wetteland
b
AWE, Aldermaston, Organic Materials Ageing Section, Reading, Berkshire, RG7 4PR, UK b Ion Beam Materials Laboratory, Los Alamos National Laboratory, NM 87545, USA Available online 31 May 2005
Abstract Irradiation of polymer samples using an accelerated beam of He++ ions passed through a 10 lm thick window of havar foil, has been performed. Such an irradiation simulates the effects of large a radiation doses, on a vastly reduced time-scale. The experimental set up was designed to allow analysis of volatiles evolved from the irradiated samples by means of a residual gas analyser (RGA). This was located in close proximity to the sample chamber. A radiation study on a poly(urethane) materials using an RGA to analyse volatiles indicated the dominant degradation products to be H2, CO and CO2. A series of polyurethane samples previously conditioned by c irradiation to between 1 and 5 MGy were irradiated in the ion beam. Identification of differences in trends in the rates of volatile evolution between these samples indicated the precise vacuum conditions at the time of irradiation had a major influence. There was also an indication that the surface of the sample had a small effect on rates of volatile evolution. Comparative plots of CO and CO2 evolution for series of 5 · 1 MGy irradiations indicated variations in behaviour between samples with different c doses. Evolution during the first 1 MGy was inhibited for the unirradiated sample, the extent of inhibition diminished with increasing c dose and was no longer evident in a sample with 1.5 MGy c dose. H2 does not show an equivalent inhibition. Evidence for a low dose crosslinking reaction is put forward as the reason for the inhibition. Chemical reaction mechanisms are postulated and used to explain the differences in behaviour observed between CO/CO2 and H2. 2005 Elsevier B.V. All rights reserved. Keywords: Alpha radiation; Poly(urethane); Volatile evolution; RGA
1. Introduction
*
Corresponding author. Tel.: +44 1189 825811; fax: +44 1189 824739. E-mail address: [email protected] (J.J. Murphy).
Performing investigations into long term ageing effects caused by exposure to high doses of alpha radiation is very difficult from and experimental perspective. Irradiating a material with large
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.04.035
224
J.J. Murphy, C.J. Wetteland / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 223–228
radiation doses is extremely difficult in a short time scale. Furthermore, alpha radiation effects are localised near the surface, so only a small amount of material is actually exposed. The exposed portion of material in any sample will therefore only constitute a small percentage of the whole. The unexposed material dominates measurements performed upon such samples making identification of changes brought about as a consequence of exposure to alpha radiation extremely difficult. Accelerated ion beams allow a stream of He++ ions to be implanted into the surface of a material. The energy of the He++ ions can be controlled allowing the major components of a radiation from chosen sources to be simulated. The vastly increased dose rates available in such a procedure allows exposure to high doses to be performed in a limited time. The energy of the He++ particles can also be increased so that they have enough to pass through a metal foil window before impacting on the sample. An extremely strong foil can act as an interface between a chamber housing the irradiated sample and the vacuum chamber of the ion beam. The atmosphere in the small sample holder is isolated and gaseous products given off by irradiated samples are trapped in a small volume. This small volume can be sensitively monitored allowing accurate analysis of any volatiles evolved as a result of irradiation to be performed. Measurement of volatile species evolved from gamma and e-beam irradiated samples has been used to investigate degradation mechanisms in polymers [1]. Studies of volatiles evolved as a result of bombardment with highly energetic ions have been reported [2]. The linear energy transfer and doses employed in such studies are far in excess of the region of interest to simulated alpha radiation studies though. Data from experiments using a mass spectrometer to analyse volatiles evolved from unirradiated samples of the polyurethane discussed here under He++ ion bombardment have been reported [3]. Recently, comparative pyrolysis studies looking for possible thermal effects have also been published [4]. Such work has primarily concentrated on identifying specific degradation species as the mass spectrometer is highly accurate and yields mass data over a large mass range. The data obtained by mass spec-
tometric analysis is, however, amenable to investigations into possible rate effects. In the following paper experiments and analysis of volatiles evolved from a polyurethane polymer subjected to He++ irradiation are described and discussed. Volatiles are analysed by means of a residual gas analyser (RGA) positioned adjacent to the irradiation chamber. Relative rates of evolution for H2, CO and CO2 from a series of samples previously exposed to gamma radiation are analysed. Variations are ascribed to the effect of a competing crosslinking reaction. Different trends in the relative rates of H2 and CO/CO2 are explained by reference to the chemical reaction mechanisms most likely to result in the production of the relevant volatile product. The effect of radiation type and irradiation dose rate on radiation degradation mechanisms are outlined and the implications for performing accelerated radiation ageing procedures are discussed.
2. Materials and sample preparation The polyurethane is a three component system consisting of a polyester polyol (Baycoll AS1160), a diphenylmethane diisocyanate (Desmodur VK10) both supplied by Bayer chemical company and an amorphous fused silica filler (cab-O-Sil) supplied by Cabot Corporation. Schematic structures and the cure mechanism for the polymer are given in Fig. 1. The polyurethane was produced by thoroughly mixing the three constituents together in an glass beaker can with a nickel spatula. The resulting mixture was degassed in a vacuum chamber
HO
O C
O
H2 C
OCN
4
O O
H2 C
4
NCO
n
O C
CH2
OH O
C O
n
H N
H N CH2
C
O
O
m
Fig. 1. Schematic structures and the cure mechanism for the polyurethane polymer.
J.J. Murphy, C.J. Wetteland / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 223–228
and then cured in the beaker for 24 h at room temperature. Afterwards it was baked at 75 C under vacuum for 1 week. 0.5 mm thick disk samples were cut from the resulting disk using a Wells diamond wire cutter equipped with a 30 lm thick diamond coated wire. After preparation, the disk samples were dried at 75 C under vacuum for 1 day then stored under dry nitrogen prior to exposure.
3. Equipment and procedures The irradiations were performed on the 3 MV tandem accelerator housed at the Ion Beam Materials Laboratory within the Centre for Materials Science at Los Alamos National Laboratory. The irradiation fixture was attached to the outside of a general purpose work station and connected to the RGA in the configuration given schematically in Fig. 2. The accelerator delivered 7.5 MeV He++, such ions lose 3.25 MeV on their passage through the 10 lm thick Havar foil window, which thus emits 4.25 MeV He++ ions. The dose rate employed for all irradiations was 0.5 MGy min 1. Before commencing irradiation, the fixture and sample were evacuated until the pressure and the signal from the RGA stabilised. The RGA was set up to constantly monitor peaks at 2, 28 and 44 amu, corresponding to H2, CO and CO2. During some irradiations higher mass peaks were also monitored.
225
The samples for gamma exposure were sealed in stainless steel containers under nitrogen and exposed in the Co60 gamma irradiation facility run by Raditec (formerly AEA Technology Harwell) at a dose rate of approximately 1.2 kGy hr 1. All radiation doses are quoted as absorbed doses in MGy. For the samples exposed in an accelerated ion beam the doses quoted are for the irradiated layer. The calculated dose is based upon a penetration depth of 30 lm for 4.25 MeV a particles into the material (from a TRIM calculation using a polymer with a similar density, approximately 1 gcm 3).
4. Results The plots in Fig. 3 show changes in the relative ion intensity for ions of mass 2, 28 and 44 amu over an exposure resulting in a 1 MGy dose to the irradiated material. The ions have been ascribed to H2, CO and CO2. Note the differences in the rise and fall times, H2 being far faster because of high diffusion rate. CO and CO2 appear similar with CO2 being slightly more abundant. Peaks resulting from fragments with masses of 58, 69 and 85 amu were also investigated. These were chosen because they had been identified in a previous study [3,4]. However, analysis with the RGA showed no identifiable fragments at the dose rate employed.
1.0x10-6
Ion Count
8.0x10-7
CO CO2
-7
6.0x10
H2 -7
4.0x10
2.0x10-7 0.0 250
300
350
400
450
500
Time (sec) Fig. 2. Schematic representation showing the configuration of the irradiation fixture, ion beam and RGA.
Fig. 3. Plots of relative ion intensity for a 1 MGy irradiation. Note the rise and fall times for H2 is far faster because of high diffusion rate. CO and CO2.
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H2 Evolution No γ Irradiation
CO2 Evolution Profiles for 1st 1 MGy Irradiations -6
4.0x10-7
0
1.4x10-6
0.25 MGyγ
1.2x10-6
0.50 MGyγ
1.0x10-6
0.75 MGyγ
8.0x10-7
1.50 MGyγ
Relative Ion Intensity
Relative Ion Intensity
1.6x10
5.00 MGyγ
-7
6.0x10
4.0x10-7 2.0x10-7
2.0x10-7 1 MGy 2 MGy 3 MGy 4 MGy 5 Mgy
-7
1.0x10
0.0
0.0 -2.0x10-7
3.0x10-7
0.0
0.2
0.4
0.6
0.8
1.0
0.0
Alpha Dose (MGy)
CO2 Evolution No γ Irradiation
Relative Ion Intensity
1 MGy 2 MGy 3 MGy 4 MGy 5 MGy
2.0x10-7
0.6
0.8
1.0
Fig. 6. Comparative plots showing the evolution of the H2 relative ion intensity over a series of 5 · 1 MGy irradiations performed sequentially upon the same sample. Note different trend for this molecule, plot for first irradiation is in trend with the rest of the data set.
There is a trend in the plots where the ion intensity decreases for subsequent irradiations. The exception is for the first irradiation, which has the lowest intensity. Equivalent plots for CO indicate an identical trend. The graph in Fig. 6 shows the equivalent plots for H2 where a different trend is displayed. In this graph the plot relating to the initial irradiation is not out of trend with the rest of the data set, it shows the highest intensity.
4.0x10-7
3.0x10
0.4
Alpha Dose (MGy)
Fig. 4. Comparative plots showing the evolution of the CO2 relative ion intensity for a series of 1 MGy irradiations on an unirradiated and previously gamma irradiated series of samples. No discernible trend in volatile evolution with gamma radiation dose is identifiable.
-7
0.2
1.0x10-7
0.0 0.0
0.2
0.4
0.6
0.8
1.0
5. Discussion
Alpha Dose (MGy) Fig. 5. Comparative plots showing the evolution of the CO2 relative ion intensity over a series of 5 · 1 MGy irradiations performed sequentially upon the same sample. Note trend where ion intensity decreases for subsequent irradiations except for first irradiation, which has the lowest peak height.
The plots in Fig. 4 show the evolution of CO2 peak intensity for a series of 1 MGy irradiations. The samples had been pre-irradiated with gamma radiation to doses between 0 and 5 MGy. Comparison of the raw data yields no discernible trend in the plots. In Fig. 5 a series of plots showing the evolution of the CO2 relative ion intensity for a series of 5 · 1 MGy irradiations performed sequentially upon the same sample are shown.
The three main volatiles species evolved from HR2177 upon irradiation in the ion beam are H2, CO and CO2. Using an RGA to analyse volatiles it was not possible to identify higher mass species at dose rates below 25 MGy min 1. Previous work on ion beam induced volatile evolution indicated some subtle effects in the relative rates of evolution for some volatile species. These were ascribed to an inhibition effect caused by a competing cross linking reaction [3]. Volatile evolution induced by a series of 1 MGy irradiations on a series of previously gamma irradiated samples was measured to test this hypothesis by seeing whether the inhibition was affected by previous radiation exposure. There is, however, no dis-
J.J. Murphy, C.J. Wetteland / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 223–228
1 Performed using the simple extrapolation facility in Origin graph plotting package.
Relative Ion Intensity
1.5x10-6
1.0x10-6 nd
rd
Averaged Line for 2 , 3 th th 4 and 5 irradiations 5.0x10-7 Calculated area
0.0 0.0
0.2
0.4
0.6
0.8
1.0
Alpha Dose (MGy) Fig. 7. Graph containing comparative plots of CO2 evolution for a series of 5 · 1 MGy irradiations performed sequentially upon the same sample previously gamma irradiated to 0.25 MGy. Lined area between averaged line and line for initial irradiation used to generate data plotted in Fig. 8.
25
20
Area between average and initial curve
cernible trend in the raw data. Small differences in the vacuum during irradiation appeared to have a large effect on volatile evolution. It is also possible that the surface of the samples subtly effects the evolution of volatiles thus causing variation between samples that masks underlying chemical effects. This means that direct comparison between samples is not possible. Irradiations performed sequentially upon the same samples do not suffer from variations in vacuum and surface morphology. A comparison between the evolution of volatiles for a series of such irradiations does indicate a qualitative difference in the behaviour of samples previously exposed to a gamma radiation dose. As the gamma dose increases the suppressing effect on the ion intensity value seen during the initial irradiation decreases. The plots for the first irradiation appear to move upwards and to the left, in relation to the plots arising from subsequent irradiations, with increasing gamma dose. To overcome the discrepancies introduced by vacuum changes in the fixture, differences in surfaces and to obtain a more quantitative measure of the effect of previous gamma irradiation on volatile evolution during the initial ion beam exposure the following procedure was developed. The relative ion intensity plots for irradiationÕs 2–5 were averaged and the areas under the averaged curves and above the initial irradiation curves were measured. This is shown schematically in Fig. 7 where the hashed region between the averaged plot and initial irradiation plot shows the area that is measured. The plot the Fig. 8 displays data obtained by this procedure. It can be seen that the inhibition effect observed during the initial 1 MGy irradiation is directly proportional to previous gamma radiation dose experienced by the sample. The correlation is considered to be of critical importance for a number of reasons. The inhibition area is directly proportional to the gamma irradiation dose and an extrapolation predicts a point of zero inhibition at a dose of 0.96 Mgy.1 All previous work using both gamma and simulated alpha radiation has
227
15 Data extropolated to 0 area indicates gamma dose of 0.95 MGy
10
5
0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Previous gamma radiation dose Fig. 8. Graph displaying the relative values for inhibition area. The values are obtained by averaging the plots for irradiations two to five and then determining the area under the averaged plot and the plot resulting from the initial 1 MGy irradiation.
indicated a cross linking reaction that occurs between 0 and 1 MGy [3–5]. The strong indication is therefore that irradiation in an accelerated ion beam induces the same chemical changes observed during gamma irradiation studies, which are performed at far lower dose rates. The high dose rates associated with accelerated ion beam irradiation are therefore unlikely to induce different types of chemical degradations. Comparisons between the trends observed for CO2/CO and H2 (Figs. 5 and 6), indicate additional
228
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γ O
γ O C
O O
CH2 CH2 CH2 CH2 C
O
C
O O
CH2 CH2 CH2 CH2 C
H
O
CH2 CH2 CH
CH2 C
O
CH2 CH2 CH2 CH2 C
O
O
CH2 CH2 CH2 CH2
CH2 CH2 CH2 CH2
Hydrogen abstraction -H2
- CO2 O C
O
O C
C
CH2 CH2 CH2 CH2
CH2 CH2 CH2 CH2 O
O
O
O
O
CH2 CH2 CH2 CH2
CH2 CH2 CH2 CH2
C
O O
CH2 CH2 CH
(a)
CH
C
O
CH2 CH2 CH2 CH2
(b)
Fig. 9. Schematics of a likely dominant reaction scheme resulting in the production of CO2/CO and H2.
subtle differences in the chemical reactions that affects the rate of volatile evolution. The inhibition of CO2/CO evolution is associated with a competing cross linking reaction involving residual double bonds within the polyester sections of the polyurethane polymer used to formulate the polyurethane [3–6]. This reaction is thought to takes place at doses between 0 and 1 MGy. After 1 MGy the double bonds are exhausted and any scission reactions resulting in volatile production can then take place without competition. The most likely degradation mechanism resulting in the production of CO2/ CO is shown schematically in Fig. 9(a). From this it can be seen that there must be an actual scission of the polymer backbone resulting in a long lived and mobile carboxyl radical. This radical in the absence of reactive double bonds can degrade to yield CO2/CO. In the presence of such double bonds some will, however, undergo a cross linking reaction causing the suppression in the ion intensity value. A different degradation mechanism is likely to dominate production of H2, which is shown schematically in Fig. 9(b). This reaction mechanism does not require chain scission of the polymer backbone so the resulting radical is far less mobile. In addition, the resulting chain centred radical is not stabilised and so is relatively short lived. These factors mean that any radicals, which result in the production of H2 are far less likely to react with the residual double bonds. The evolution of H2 would not therefore be expected to show the same behaviour as that seen for CO2/CO.
6. Conclusions An irradiation fixture and RGA attached to an accelerated ion beam allows analysis of low mass volatiles evolved upon irradiation. The rates of evolution can be related to chemical changes within the polyurethane polymer and yield information on fundamental chemical changes. A series of previously gamma irradiated samples showed trends in volatile evolution that could be related back to chemical changes. The exact correlation between gamma and accelerated He++ ion induced changes indicated no rate effects associated with the large acceleration factors associated with accelerated ion beams.
References [1] T. Bowmer, M. Bowden, J. Macromol. Sci. Chem. 43 (1983) 171. [2] V. Picq, E. Balanzat, Nucl. Instr. and Meth. B 151 (1999) 76. [3] J.J. Murphy, M. Patel, S.J. Powell, P.F. Smith, Radiat. Phys. Chem. 63 (2002) 101. [4] J.J. Murphy, M. Patel, A.R. Skinner, I.M. Horn, S.J. Powell, P.F. Smith, Nucl. Instr. and Meth. B 215 (2004) 423. [5] J. Murphy, M. Patel, A. Skinner, P. Smith, Proceedings of the Conference on Ageing Studies and Lifetime Prediction, Kluwer Academic/Plenum Publishers, New York, 1999. [6] A. Charlesby, V. Wycherley, T. Greenwood, Proc. Roy. Soc. A 242 (1957) 54.
Volatile evolution induced by energetic He++ ions in a polyurethane and the effects of previous gamma irradiation J.J. Murphy a
a,*
, C.J. Wetteland
b
AWE, Aldermaston, Organic Materials Ageing Section, Reading, Berkshire, RG7 4PR, UK b Ion Beam Materials Laboratory, Los Alamos National Laboratory, NM 87545, USA Available online 31 May 2005
Abstract Irradiation of polymer samples using an accelerated beam of He++ ions passed through a 10 lm thick window of havar foil, has been performed. Such an irradiation simulates the effects of large a radiation doses, on a vastly reduced time-scale. The experimental set up was designed to allow analysis of volatiles evolved from the irradiated samples by means of a residual gas analyser (RGA). This was located in close proximity to the sample chamber. A radiation study on a poly(urethane) materials using an RGA to analyse volatiles indicated the dominant degradation products to be H2, CO and CO2. A series of polyurethane samples previously conditioned by c irradiation to between 1 and 5 MGy were irradiated in the ion beam. Identification of differences in trends in the rates of volatile evolution between these samples indicated the precise vacuum conditions at the time of irradiation had a major influence. There was also an indication that the surface of the sample had a small effect on rates of volatile evolution. Comparative plots of CO and CO2 evolution for series of 5 · 1 MGy irradiations indicated variations in behaviour between samples with different c doses. Evolution during the first 1 MGy was inhibited for the unirradiated sample, the extent of inhibition diminished with increasing c dose and was no longer evident in a sample with 1.5 MGy c dose. H2 does not show an equivalent inhibition. Evidence for a low dose crosslinking reaction is put forward as the reason for the inhibition. Chemical reaction mechanisms are postulated and used to explain the differences in behaviour observed between CO/CO2 and H2. 2005 Elsevier B.V. All rights reserved. Keywords: Alpha radiation; Poly(urethane); Volatile evolution; RGA
1. Introduction
*
Corresponding author. Tel.: +44 1189 825811; fax: +44 1189 824739. E-mail address: [email protected] (J.J. Murphy).
Performing investigations into long term ageing effects caused by exposure to high doses of alpha radiation is very difficult from and experimental perspective. Irradiating a material with large
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.04.035
224
J.J. Murphy, C.J. Wetteland / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 223–228
radiation doses is extremely difficult in a short time scale. Furthermore, alpha radiation effects are localised near the surface, so only a small amount of material is actually exposed. The exposed portion of material in any sample will therefore only constitute a small percentage of the whole. The unexposed material dominates measurements performed upon such samples making identification of changes brought about as a consequence of exposure to alpha radiation extremely difficult. Accelerated ion beams allow a stream of He++ ions to be implanted into the surface of a material. The energy of the He++ ions can be controlled allowing the major components of a radiation from chosen sources to be simulated. The vastly increased dose rates available in such a procedure allows exposure to high doses to be performed in a limited time. The energy of the He++ particles can also be increased so that they have enough to pass through a metal foil window before impacting on the sample. An extremely strong foil can act as an interface between a chamber housing the irradiated sample and the vacuum chamber of the ion beam. The atmosphere in the small sample holder is isolated and gaseous products given off by irradiated samples are trapped in a small volume. This small volume can be sensitively monitored allowing accurate analysis of any volatiles evolved as a result of irradiation to be performed. Measurement of volatile species evolved from gamma and e-beam irradiated samples has been used to investigate degradation mechanisms in polymers [1]. Studies of volatiles evolved as a result of bombardment with highly energetic ions have been reported [2]. The linear energy transfer and doses employed in such studies are far in excess of the region of interest to simulated alpha radiation studies though. Data from experiments using a mass spectrometer to analyse volatiles evolved from unirradiated samples of the polyurethane discussed here under He++ ion bombardment have been reported [3]. Recently, comparative pyrolysis studies looking for possible thermal effects have also been published [4]. Such work has primarily concentrated on identifying specific degradation species as the mass spectrometer is highly accurate and yields mass data over a large mass range. The data obtained by mass spec-
tometric analysis is, however, amenable to investigations into possible rate effects. In the following paper experiments and analysis of volatiles evolved from a polyurethane polymer subjected to He++ irradiation are described and discussed. Volatiles are analysed by means of a residual gas analyser (RGA) positioned adjacent to the irradiation chamber. Relative rates of evolution for H2, CO and CO2 from a series of samples previously exposed to gamma radiation are analysed. Variations are ascribed to the effect of a competing crosslinking reaction. Different trends in the relative rates of H2 and CO/CO2 are explained by reference to the chemical reaction mechanisms most likely to result in the production of the relevant volatile product. The effect of radiation type and irradiation dose rate on radiation degradation mechanisms are outlined and the implications for performing accelerated radiation ageing procedures are discussed.
2. Materials and sample preparation The polyurethane is a three component system consisting of a polyester polyol (Baycoll AS1160), a diphenylmethane diisocyanate (Desmodur VK10) both supplied by Bayer chemical company and an amorphous fused silica filler (cab-O-Sil) supplied by Cabot Corporation. Schematic structures and the cure mechanism for the polymer are given in Fig. 1. The polyurethane was produced by thoroughly mixing the three constituents together in an glass beaker can with a nickel spatula. The resulting mixture was degassed in a vacuum chamber
HO
O C
O
H2 C
OCN
4
O O
H2 C
4
NCO
n
O C
CH2
OH O
C O
n
H N
H N CH2
C
O
O
m
Fig. 1. Schematic structures and the cure mechanism for the polyurethane polymer.
J.J. Murphy, C.J. Wetteland / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 223–228
and then cured in the beaker for 24 h at room temperature. Afterwards it was baked at 75 C under vacuum for 1 week. 0.5 mm thick disk samples were cut from the resulting disk using a Wells diamond wire cutter equipped with a 30 lm thick diamond coated wire. After preparation, the disk samples were dried at 75 C under vacuum for 1 day then stored under dry nitrogen prior to exposure.
3. Equipment and procedures The irradiations were performed on the 3 MV tandem accelerator housed at the Ion Beam Materials Laboratory within the Centre for Materials Science at Los Alamos National Laboratory. The irradiation fixture was attached to the outside of a general purpose work station and connected to the RGA in the configuration given schematically in Fig. 2. The accelerator delivered 7.5 MeV He++, such ions lose 3.25 MeV on their passage through the 10 lm thick Havar foil window, which thus emits 4.25 MeV He++ ions. The dose rate employed for all irradiations was 0.5 MGy min 1. Before commencing irradiation, the fixture and sample were evacuated until the pressure and the signal from the RGA stabilised. The RGA was set up to constantly monitor peaks at 2, 28 and 44 amu, corresponding to H2, CO and CO2. During some irradiations higher mass peaks were also monitored.
225
The samples for gamma exposure were sealed in stainless steel containers under nitrogen and exposed in the Co60 gamma irradiation facility run by Raditec (formerly AEA Technology Harwell) at a dose rate of approximately 1.2 kGy hr 1. All radiation doses are quoted as absorbed doses in MGy. For the samples exposed in an accelerated ion beam the doses quoted are for the irradiated layer. The calculated dose is based upon a penetration depth of 30 lm for 4.25 MeV a particles into the material (from a TRIM calculation using a polymer with a similar density, approximately 1 gcm 3).
4. Results The plots in Fig. 3 show changes in the relative ion intensity for ions of mass 2, 28 and 44 amu over an exposure resulting in a 1 MGy dose to the irradiated material. The ions have been ascribed to H2, CO and CO2. Note the differences in the rise and fall times, H2 being far faster because of high diffusion rate. CO and CO2 appear similar with CO2 being slightly more abundant. Peaks resulting from fragments with masses of 58, 69 and 85 amu were also investigated. These were chosen because they had been identified in a previous study [3,4]. However, analysis with the RGA showed no identifiable fragments at the dose rate employed.
1.0x10-6
Ion Count
8.0x10-7
CO CO2
-7
6.0x10
H2 -7
4.0x10
2.0x10-7 0.0 250
300
350
400
450
500
Time (sec) Fig. 2. Schematic representation showing the configuration of the irradiation fixture, ion beam and RGA.
Fig. 3. Plots of relative ion intensity for a 1 MGy irradiation. Note the rise and fall times for H2 is far faster because of high diffusion rate. CO and CO2.
226
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H2 Evolution No γ Irradiation
CO2 Evolution Profiles for 1st 1 MGy Irradiations -6
4.0x10-7
0
1.4x10-6
0.25 MGyγ
1.2x10-6
0.50 MGyγ
1.0x10-6
0.75 MGyγ
8.0x10-7
1.50 MGyγ
Relative Ion Intensity
Relative Ion Intensity
1.6x10
5.00 MGyγ
-7
6.0x10
4.0x10-7 2.0x10-7
2.0x10-7 1 MGy 2 MGy 3 MGy 4 MGy 5 Mgy
-7
1.0x10
0.0
0.0 -2.0x10-7
3.0x10-7
0.0
0.2
0.4
0.6
0.8
1.0
0.0
Alpha Dose (MGy)
CO2 Evolution No γ Irradiation
Relative Ion Intensity
1 MGy 2 MGy 3 MGy 4 MGy 5 MGy
2.0x10-7
0.6
0.8
1.0
Fig. 6. Comparative plots showing the evolution of the H2 relative ion intensity over a series of 5 · 1 MGy irradiations performed sequentially upon the same sample. Note different trend for this molecule, plot for first irradiation is in trend with the rest of the data set.
There is a trend in the plots where the ion intensity decreases for subsequent irradiations. The exception is for the first irradiation, which has the lowest intensity. Equivalent plots for CO indicate an identical trend. The graph in Fig. 6 shows the equivalent plots for H2 where a different trend is displayed. In this graph the plot relating to the initial irradiation is not out of trend with the rest of the data set, it shows the highest intensity.
4.0x10-7
3.0x10
0.4
Alpha Dose (MGy)
Fig. 4. Comparative plots showing the evolution of the CO2 relative ion intensity for a series of 1 MGy irradiations on an unirradiated and previously gamma irradiated series of samples. No discernible trend in volatile evolution with gamma radiation dose is identifiable.
-7
0.2
1.0x10-7
0.0 0.0
0.2
0.4
0.6
0.8
1.0
5. Discussion
Alpha Dose (MGy) Fig. 5. Comparative plots showing the evolution of the CO2 relative ion intensity over a series of 5 · 1 MGy irradiations performed sequentially upon the same sample. Note trend where ion intensity decreases for subsequent irradiations except for first irradiation, which has the lowest peak height.
The plots in Fig. 4 show the evolution of CO2 peak intensity for a series of 1 MGy irradiations. The samples had been pre-irradiated with gamma radiation to doses between 0 and 5 MGy. Comparison of the raw data yields no discernible trend in the plots. In Fig. 5 a series of plots showing the evolution of the CO2 relative ion intensity for a series of 5 · 1 MGy irradiations performed sequentially upon the same sample are shown.
The three main volatiles species evolved from HR2177 upon irradiation in the ion beam are H2, CO and CO2. Using an RGA to analyse volatiles it was not possible to identify higher mass species at dose rates below 25 MGy min 1. Previous work on ion beam induced volatile evolution indicated some subtle effects in the relative rates of evolution for some volatile species. These were ascribed to an inhibition effect caused by a competing cross linking reaction [3]. Volatile evolution induced by a series of 1 MGy irradiations on a series of previously gamma irradiated samples was measured to test this hypothesis by seeing whether the inhibition was affected by previous radiation exposure. There is, however, no dis-
J.J. Murphy, C.J. Wetteland / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 223–228
1 Performed using the simple extrapolation facility in Origin graph plotting package.
Relative Ion Intensity
1.5x10-6
1.0x10-6 nd
rd
Averaged Line for 2 , 3 th th 4 and 5 irradiations 5.0x10-7 Calculated area
0.0 0.0
0.2
0.4
0.6
0.8
1.0
Alpha Dose (MGy) Fig. 7. Graph containing comparative plots of CO2 evolution for a series of 5 · 1 MGy irradiations performed sequentially upon the same sample previously gamma irradiated to 0.25 MGy. Lined area between averaged line and line for initial irradiation used to generate data plotted in Fig. 8.
25
20
Area between average and initial curve
cernible trend in the raw data. Small differences in the vacuum during irradiation appeared to have a large effect on volatile evolution. It is also possible that the surface of the samples subtly effects the evolution of volatiles thus causing variation between samples that masks underlying chemical effects. This means that direct comparison between samples is not possible. Irradiations performed sequentially upon the same samples do not suffer from variations in vacuum and surface morphology. A comparison between the evolution of volatiles for a series of such irradiations does indicate a qualitative difference in the behaviour of samples previously exposed to a gamma radiation dose. As the gamma dose increases the suppressing effect on the ion intensity value seen during the initial irradiation decreases. The plots for the first irradiation appear to move upwards and to the left, in relation to the plots arising from subsequent irradiations, with increasing gamma dose. To overcome the discrepancies introduced by vacuum changes in the fixture, differences in surfaces and to obtain a more quantitative measure of the effect of previous gamma irradiation on volatile evolution during the initial ion beam exposure the following procedure was developed. The relative ion intensity plots for irradiationÕs 2–5 were averaged and the areas under the averaged curves and above the initial irradiation curves were measured. This is shown schematically in Fig. 7 where the hashed region between the averaged plot and initial irradiation plot shows the area that is measured. The plot the Fig. 8 displays data obtained by this procedure. It can be seen that the inhibition effect observed during the initial 1 MGy irradiation is directly proportional to previous gamma radiation dose experienced by the sample. The correlation is considered to be of critical importance for a number of reasons. The inhibition area is directly proportional to the gamma irradiation dose and an extrapolation predicts a point of zero inhibition at a dose of 0.96 Mgy.1 All previous work using both gamma and simulated alpha radiation has
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15 Data extropolated to 0 area indicates gamma dose of 0.95 MGy
10
5
0 0.0
0.2
0.4
0.6
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Previous gamma radiation dose Fig. 8. Graph displaying the relative values for inhibition area. The values are obtained by averaging the plots for irradiations two to five and then determining the area under the averaged plot and the plot resulting from the initial 1 MGy irradiation.
indicated a cross linking reaction that occurs between 0 and 1 MGy [3–5]. The strong indication is therefore that irradiation in an accelerated ion beam induces the same chemical changes observed during gamma irradiation studies, which are performed at far lower dose rates. The high dose rates associated with accelerated ion beam irradiation are therefore unlikely to induce different types of chemical degradations. Comparisons between the trends observed for CO2/CO and H2 (Figs. 5 and 6), indicate additional
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γ O
γ O C
O O
CH2 CH2 CH2 CH2 C
O
C
O O
CH2 CH2 CH2 CH2 C
H
O
CH2 CH2 CH
CH2 C
O
CH2 CH2 CH2 CH2 C
O
O
CH2 CH2 CH2 CH2
CH2 CH2 CH2 CH2
Hydrogen abstraction -H2
- CO2 O C
O
O C
C
CH2 CH2 CH2 CH2
CH2 CH2 CH2 CH2 O
O
O
O
O
CH2 CH2 CH2 CH2
CH2 CH2 CH2 CH2
C
O O
CH2 CH2 CH
(a)
CH
C
O
CH2 CH2 CH2 CH2
(b)
Fig. 9. Schematics of a likely dominant reaction scheme resulting in the production of CO2/CO and H2.
subtle differences in the chemical reactions that affects the rate of volatile evolution. The inhibition of CO2/CO evolution is associated with a competing cross linking reaction involving residual double bonds within the polyester sections of the polyurethane polymer used to formulate the polyurethane [3–6]. This reaction is thought to takes place at doses between 0 and 1 MGy. After 1 MGy the double bonds are exhausted and any scission reactions resulting in volatile production can then take place without competition. The most likely degradation mechanism resulting in the production of CO2/ CO is shown schematically in Fig. 9(a). From this it can be seen that there must be an actual scission of the polymer backbone resulting in a long lived and mobile carboxyl radical. This radical in the absence of reactive double bonds can degrade to yield CO2/CO. In the presence of such double bonds some will, however, undergo a cross linking reaction causing the suppression in the ion intensity value. A different degradation mechanism is likely to dominate production of H2, which is shown schematically in Fig. 9(b). This reaction mechanism does not require chain scission of the polymer backbone so the resulting radical is far less mobile. In addition, the resulting chain centred radical is not stabilised and so is relatively short lived. These factors mean that any radicals, which result in the production of H2 are far less likely to react with the residual double bonds. The evolution of H2 would not therefore be expected to show the same behaviour as that seen for CO2/CO.
6. Conclusions An irradiation fixture and RGA attached to an accelerated ion beam allows analysis of low mass volatiles evolved upon irradiation. The rates of evolution can be related to chemical changes within the polyurethane polymer and yield information on fundamental chemical changes. A series of previously gamma irradiated samples showed trends in volatile evolution that could be related back to chemical changes. The exact correlation between gamma and accelerated He++ ion induced changes indicated no rate effects associated with the large acceleration factors associated with accelerated ion beams.
References [1] T. Bowmer, M. Bowden, J. Macromol. Sci. Chem. 43 (1983) 171. [2] V. Picq, E. Balanzat, Nucl. Instr. and Meth. B 151 (1999) 76. [3] J.J. Murphy, M. Patel, S.J. Powell, P.F. Smith, Radiat. Phys. Chem. 63 (2002) 101. [4] J.J. Murphy, M. Patel, A.R. Skinner, I.M. Horn, S.J. Powell, P.F. Smith, Nucl. Instr. and Meth. B 215 (2004) 423. [5] J. Murphy, M. Patel, A. Skinner, P. Smith, Proceedings of the Conference on Ageing Studies and Lifetime Prediction, Kluwer Academic/Plenum Publishers, New York, 1999. [6] A. Charlesby, V. Wycherley, T. Greenwood, Proc. Roy. Soc. A 242 (1957) 54.