Volatile evolution induced by energetic He++ ions in a poly(ester) based polyurethane

Radiation Physics and Chemistry 63 (2002) 101–108

Volatile evolution induced by energetic He++ ions in a poly(ester) based polyurethane J.J. Murphya,*, M. Patela, S.J. Powellb, P.F. Smithb a

Atomic Weapons Establishment, Aldermaston, Reading, Berkshire RG7 4PR, UK b Los Alamos National Laboratory, New Mexico, USA

Abstract Irradiation of polymer samples using an accelerated beam of He++ ions passed through a 10 mm thick window of havar foil has been performed. Such an irradiation simulates the effects of large a radiation doses. The experimental set up was designed so that the irradiated material was contained within a small sample chamber, which was isolated from the main vacuum chamber of the ion beam by means of the foil window. A mass spectrometer linked directly to the sample chamber facilitated analysis of gaseous products evolved from the materials as a consequence of irradiation. Samples of a poly(ester) based poly(urethane) polymer evolved mainly CO2 along with a number of higher mass volatile species. Assignment of chemical structures to the main molecular ions has allowed deductions about the chemical processes underlying radiation induced change to be made. Furthermore, identification of trends in volatile production affords information about radiation induced crosslinking reactions, which do not directly result in the production of volatile species to be deduced. Crown Copyright r 2001 Published by Elsevier Science Ltd. All rights reserved. Keywords: Alpha radiation; Poly(urethane); Poly(ester); Scissioning and crosslinking

1. Introduction Investigating long term ageing effects caused by exposure to high doses of a radiation poses a number of experimental difficulties. Irradiating a material with large radiation doses is extremely difficult in an experimental time scale. Furthermore, a radiation effects are localised near the surface, so only a small amount of material is actually exposed. If the materials under investigation are not amenable to thin film production, then the exposed material in any sample will only constitute a small percentage of the whole. The unexposed material thus dominates measurements performed upon such samples. This makes the identification of changes brought about as a consequence of exposure to a 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, thus allowing the major components of a radiation from a *Corresponding author. Fax: +44-1189-824739.

chosen source to be simulated. The vastly increased dose rates available in such a procedure allows exposure to high doses in a convenient time scale. In addition, the energy of the He++ particles can be increased, to the extent that they have enough to pass through a metal foil window before impacting on the sample. If the foil is extremely strong, then it 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 thus isolated, alleviating issues of contamination from the large ion beam chamber. Moreover, 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 (Bowmer and Bowden, 1983). However, the equivalent experiments for a radiation have, to the best knowledge of the authors, not been performed. Studies of volatiles

0969-806X/01/$ - see front matter Crown Copyright r 2001 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 4 8 9 - 3

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from the syringe cylinders using a Wells diamond wire cutter equipped with a 30 mm thick diamond coated wire. After preparation, the disk samples were dried at 751C under vacuum for 1 day then stored under dry nitrogen prior to exposure.

evolved as a result of bombardment with highly energetic ions have been reported (Picq and Balanzat, 1999; Picq, et al., 1998). The linear energy transfer and doses employed in such studies are far in excess of the region of interest to simulated a radiation studies though. In the following paper experiments and analysis of volatiles evolved from a poly(ester)/polyurethane polymer subjected to He++ irradiation are described and discussed. Volatiles are analysed by mass spectrometry. A description of the underlying chemical mechanisms of radiation induced change is proposed. Rates of volatile evolution are analysed and used to probe a crosslinking reaction that does not result in volatile products, but does suppress the evolution of volatile species while it takes place.

2.2. 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, a schematic of which is shown in Fig. 2, was attached to the outside of a general purpose work station. A HP 5972A mass selective detector (MSD) was connected to the back of the irradiation fixture by means of a fused silica transfer line. The accelerator delivered 7.5 MeV He++ ions at a beam current of 50 and 12.5 nA to an area of approximately 2 cm2. Such ions lose 3.25 MeV on their passage through the 10 mm thick Havar foil window, which thus emits 4.25 MeV He++ ions (based on a Transport of Ions in Mater (TRIM) calculation). Before commencing irradiation, the fixture and sample were evacuated until the pressure stabilised at approximately 5.0  10 6 and the total ion chromatogram (TIC) displayed a relatively flat baseline. The MSD was set up to sample every 0.42 s, and was restarted approximately 10 min before the first irradiation to obtain sufficient baseline data. The data from the MSD was obtained both as a TIC, which shows how the total peak count changes, and as individual mass spectra which show the relative distribution of peaks at any one time. All radiation doses are quoted as absorbed doses in MGy for the irradiated layer. The calculated dose is based upon a penetration depth of 30 mm for 4.25 MeV a

2. Experimental 2.1. 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 shown in Fig. 1. The polyurethane was produced by thoroughly mixing the three constituents together in an aluminium can with a nickel with a spatula. The resulting mixture was degassed in a vacuum chamber and then drawn into a 50 ml plastic syringe. The polyurethane was allowed to cure for 24 h at room temperature and was then baked at 751C under vacuum for 1 week. Two millimetre thick disk samples were cut

HO

O C

O

O

4

n

O C

CH2

OH H2 C

O O

H2 C

4

+

C O

OCN

NCO

H N

H N CH2

C

O

O

n

Fig. 1. Schematic structures and cure mechanism for the polyurethane polymer.

m

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Helium ion beam Havar foil window

Fixing bolt Washer

O Ring

Aluminium button

Fixing bolt

Sample

Gas leak path

Vacuum couple

Fig. 2. Schematic of the polymer irradiation fixture.

Abundance 180000 160000 140000 120000 100000

Time (min)--> 10.00

20.00

30.00

40.00

50.00

60.00

Fig. 3. TIC for first and second 1.25 MGy irradiation.

particles into the material (from a TRIM calculation using a polymer with a similar density, approximately 1 g cm 3).

3. Results 3.1. Irradiation of sample 1 Irradiation details: Total dose 5 MGy in 4  1.25 MGy segments, dose rate 0.18 MGy min 1. The changes in the TIC for the first two 1.25 MGy irradiations are shown in Fig. 3. Irradiation induced an observable increase in the TIC. The spectrum shown in

Fig. 4 was taken 2 min into the analysis, before irradiation. The spectrum is typical of that seen for the empty fixture indicating some ingress of air, but little or no outgassing of volatiles from the sample. The exception is the 18 amu peak, which is slightly higher than for the empty fixture. This may indicate the release of some water from the sample. The second spectrum shown in Fig. 5 was taken after 13.5 min, the time corresponding to the highest point of the first peak in the TIC, Fig. 3. The most striking difference between the spectra is the dramatic increase in the peak at 44 amu, which arises from CO2. In addition, a number of very small but distinct peaks corresponding to high mass species are also evident. There is little

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Abundance

28

42000

34000

26000

14

18000

32 10000 16 0

6

m/z-->

8

10

12

14

16

18

20

22

24

26 27 28

30

32

34

36

38

40 4142

44

46

48

50

52

Fig. 4. Background mass spectrum showing peaks indicative of a small air leak.

Abundance 28

65000

45000

25000 14 44 32 5000 m/z-->

0

20

40

60

80

100

120

140

160

180

200

Fig. 5. Mass spectrum obtained during irradiation displaying large CO2 peak and small higher mass peaks.

change in the peak at 18 amu upon irradiation at this dose rate.

follows a similar trend to the irradiation at a lower dose rate. However, at the higher dose rate the intensity of the high mass peaks (Fig. 7) is great enough to allow accurate integration of the peak heights.

3.2. Sample 2 Irradiation details: Total dose 50 MGy in 10  5 MGy segments, dose rate 1.0 MGy min 1. The changes in the TIC for all of the irradiations are displayed in Fig. 6. Irradiation at such a dose rate causes a dramatic increase in the TIC. The evolution of spectral features

4. Discussion Both of the samples indicated water in the background spectra. This may be associated with insufficient

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Abundance 1.2e+07 1e+07 8000000 6000000 4000000 2000000

Time(min)--> 20.00

40.00

60.00

80.00

100.00

Fig. 6. TIC showing peaks for 10 5 MGy irradiations.

Abundance 58 5000

3500 55 2000

69 85

500

m/z-->

0 50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

1

Fig. 7. Expanded view of the high mass region of the mass spectrum obtained during irradiation at 1 MGy min .

drying of the samples, which are exposed to large amounts of water while being cut. Alternatively, the samples might have adsorbed small quantities of water from the air to which they are exposed while the sample fixture is being assembled, water is considered unlikely to result from radiation induced degradation of the material. At a dose rate of 0.18 MGy min 1 it is possible to observe enhancement of volatiles evolved from the polyurethane. The data used to plot Fig. 8 is taken from the first peak of the TIC in Fig. 3, which corresponds to a total dose of 1.25 MGy. The graph shows the increase in the ion counts for three peaks at

14, 18 and 44 amu. The peak at 14 is due to the presence of nitrogen and is chosen as it is relatively constant in the background spectra, in addition, it is of the right order of magnitude to compare with the peaks at 18 and 44, which result from H2O and CO2. The nitrogen peak increases slightly during irradiation indicating that the rate of diffusion of nitrogen, trapped and/or dissolved in the polymer sample is increased by exposure to radiation. The comparatively large increase in the H2O peak possibly indicates a relatively high initial concentration of water within the material. Both of these plots show an initial increase upon irradiation, which is followed by a plateau region. The peak height

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J.J. Murphy et al. / Radiation Physics and Chemistry 63 (2002) 101–108 6

1.2

6

1.1

3.0x10

6

2.0x10

6

1.5x10

6

1.0x10

5

5.0x10

0.0 5

10

15

10 MGray count

Ion count normalised to

Ion count

2.5x10

1.0 0.9 0.8 0.7 0.6 0.5

20

10

Time/minutes

30

40

Dose MGray

Fig. 8. Plots showing the peak height evolution for peaks at 14’, 18K, and 44m, amu.

Fig. 10. Normalised plots comparing the variation in the 18’, 44K, 58m, 69., and 85E peak heights over 35 MGy.

Table 1

2.5x106

Dose MGy 18 amu 2.0x106

Ion count

20

5 10 15 20 25 30 35 40

1.5x106

1.0x106

0

10

20

30

1.108  106 1.218  106 1.047  106 980000 928000 832000 758000 633000

44 amu

58 amu 69 amu 85 amu

1.686  106 2.407  106 2.185  106 2.143  106 2.217  106 1.88  106 1.751  106 1.515  106

3946 5334 5234 5257 5303 4834 4358 3819

1267 1163 1024 1002 1056 960 853 739

604 614 609 624 726 674 632 519

40

Dose MGray Fig. 9. Plots showing the variation in the 18’, and 44K, amu peak heights over 40 MGy.

then diminishes rapidly when irradiation ceases. The greatest degree of change is seen in the CO2 peak, which increases dramatically upon irradiation. The CO2 plot displays a different behaviour to the other two plots. The increase upon irradiation is extremely large, the peak height during irradiation is four or five orders of magnitude greater than before irradiation. The shape of the plots also differs. The peak height increases throughout the irradiation; there is no plateau region, possibly indicating that the increases in CO2 evolution is not solely due to an increase in the rate of diffusion. The peak intensity values given in Table 1 were obtained by averaging the spectra over the middle portions of the peaks in the TIC displayed in Fig. 6 resulting from ten 5 MGy irradiations. Fig. 9 displays plots showing how the intensities of the H2O and CO2 peaks vary with dose. The two features of particular note are the apparent anomalous positions of the first points, which correspond to the first 5 MGy irradiation.

These points appear to be lower than the trend predicted by the other points in the data sets, with the CO2 point being particularly low. A difference in the shape of the plots is also apparent. If the data points corresponding to a dose of 5 MGy are discounted, then the H2O plot shows a relatively linear decrease with subsequent 5 MGy doses, while the CO2 plot appears to plateau before dropping linearly after a dose of 25 MGy. The data contained in Table 1 also gives peak intensity values for the dominant peaks in the groupings observed in the high mass region of the spectra obtained during irradiation. The data set for the 58 amu peak again indicates that the point corresponding to the first 5 MGy irradiation is out of trend, being lower than predicted by the rest of the data. If the data points corresponding to the first 5 MGy dose are discarded and the data is normalised to the peak intensity at 10 MGy, then the data sets in Table 1 can be plotted on the same graph to allow a comparison of trends, Fig. 10. All of the higher mass peaks indicate similar behaviour, a linear drop after a 25 MGy dose following a plateau region, only the H2O peak displays a different trend. The higher molecular mass peak are only evident

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J.J. Murphy et al. / Radiation Physics and Chemistry 63 (2002) 101–108 3.0x106

O

44 amu ion count

2.5x106

H3C

C

O

m/z = 58

H2 C

H2 C

H C

CH2

m/z = 69

O

H2 C

H C

CH2

m/z = 85

2.0x106 1.5x106 1.0x106

O 5.0x105 0.0

O 0

1

2

3

4

5

Dose MGray Fig. 11. Plots showing the evolution of the CO2 peak height for 0–5’, 5–10K, 10–15m, and 15–20. MGy irradiations.

in the spectra obtained during irradiation, in addition, the peaks intensities only become significant at high dose rates. Volatile precursors to such species are not contained within the material before irradiation, it is therefore highly probable that they result directly from radiation induced degradation of the polymer. The CO2 peak follows the same trend as the higher mass peaks, it is thus concluded that the observed CO2 also results from radiation induced degradation of the polymer. The H2O peak displays a linear decrease over the entire dose range. This behaviour is possibly to be expected if the volatile species does not arise, as a result radiation induced degradation of the material. As water is removed from the system the concentration is lowered and subsequent irradiations will cause less to be evolved from the material. The anomalous nature of the peak corresponding to the initial 5 MGy irradiation can be further demonstrated by the plots displayed in Fig. 11. These plots shows how the CO2 peak evolves during the irradiations, the data points were obtained by averaging the spectra over 10–15 s periods of the first four peaks in the TIC displayed in Fig. 6. The difference in the plots appears to arise from an apparent inhibition of CO2 evolution and/ or production over the initial section of the curve arising from the first 5 MGy dose. The curves arising from the 10, 15 and 20 MGy doses in contrast display a striking similarity. This difference can be explained by considering what chemical processes are likely to be caused by irradiation at doses below 5 MGy. Gamma irradiation experiments have indicated that at low doses (up to 1 MGy) the dominant reaction is crosslinking of a finite supply of double bonds initially present within the polyester regions of the system (Murphy et al., 1999). This observation is in accordance with previously published studies investigating radiation effects in unsaturated polyesters (Charlesby et al., 1957).

C

Fig. 12. Structures of degradation products possibly responsible for some of dominant peaks in mass spectrum obtained during irradiation.

At higher doses the double bonds are exhausted and the dominant reaction becomes chain scissioning of the ester (Roberts, 1974). The inhibition of the 5 MGy data point is therefore caused by the competing crosslinking reaction, which occurs at low doses. It lowers the rate of diffusion of gaseous molecules by increasing the crosslink density of the polymer (Amorongen, 1964), thus lowering their rate of evolution from the system. The CO2 peak height for the first 5 MGy irradiation also appears to be lowered to a greater extent than the corresponding H2O point, Fig. 9. This may indicate that reactive species, which breakdown to give CO2 in the absence of reactive double bonds act as radical initiators for the crosslinking reaction at low doses. At higher doses the original source of double bonds is exhausted and the evolution of CO2 is governed by energy deposition and the radiation quantum yield of chain scissioning, which appears relatively consistent up to a dose of 20 MGy.

4.1. Peak assignments It is not possible to sensibly assign chemical structures to the main peaks observed in the spectra obtained during irradiation (58, 69 and 85 amu) by considering degradation of the methylenediphenyldiisocyanate derived sections of the system. Radiation induced degradation of the polyester structure would, however, yield species of the type pictured in Fig. 12, which may be responsible for some of the main ions observed in the mass spectra. It is possible to derive sensible degradation mechanisms, which lead from the basic chemical structure of the poly(ester) to the postulated degradation products,

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O

O H2 C

C

O

H2 C

H2 C

H2 C

H2 C

O

C

H2 C

O

H2 C

H2 C

H2 C

C

O

Radiation Induced Bond Scission

C

O

O

O H2 C

O

H2 C

H2 C

CH2 H2C

C

O

H2 C

H2 C

H2 C

H2 C

H2 C

H2 C

H2 C

C

O

Hydrogen Abstraction

H2 C

C

O

O

O O

H2 C

H C

CH2 H3C

C

O

H2 C

C

O

Secondary Bond Scission O C

O O

H2 C

H C

CH2

O

H2 C

H C

CH2

H3C

C

O

Fig. 13. Possible degradation mechanism leading from basic polyester repeat unit to degradation products shown in Fig. 12.

Fig. 13. The primary process is scissioning of the C–C bond adjacent to the ester followed by hydrogen abstraction leading to the allyl double bond formation and the methyl end group. Secondary scissioning processes of the indicated bonds yield the radical species shown in Fig. 12.

Acknowledgements The authors would like to acknowledge the help of the staff of the Ion Beam Materials Laboratory at Los Alamos National Laboratory. References

5. Conclusions Analysis of volatile species evolved from samples subjected to simulated a radiation, using the described equipment, affords an excellent analysis technique for assessing a radiation induced degradation in polymers. The dominant degradation reaction, which leads to the production of volatile species in the poly(urethane) studied is chain scissioning of the poly(ester) constituent. Degradation mechanisms leading from the poly(ester) to the identified volatile species have been proposed. The most abundant volatile species is CO2, which results directly from radiation induced degradation of the polymer. The rate of CO2 evolution is inhibited during the initial stages of the first 5 MGy irradiation. This has been explained by considering a competing crosslinking reaction of a small concentration of reactive groups, which takes place at low doses.

Amorongen, G., 1964. Diffusion in elastomers. Rubber Chem. Technol. 37 (5), 1065–1152. Bowmer, T., Bowden, M., 1983. The radiation degradation of poly(2-methyl-1-pentene sulphone) II. Radiolysis products. J. Macromol. Sci. Chem. 43, 171–177. Charlesby, A., Wycherley, V., Greenwood, T., 1957. Radiation reactions in unsaturated polyesters. Proc. Roy. Soc. A 242, 54–71. Murphy, J., Patel, M., Skinner, A., Smith, P., 1999. Modelling radiation damage in polyurethane materials based upon polyester poly(ol)s and methylendiphenyldiisocyanate formulations, Proceedings of the Ageing Conference, Oxford, UK, to be published. Picq, V., Balanzat, E., 1999. Ion induced molecular emission of polymers: analytical potentialities of FTIR and mass spectroscopy. Nucl. Instrum. Methods B 151, 76–83. Picq, V., Ramillon, J., Balanzat, E., 1998. Swift heavy ions on polymers: hydrocarbon gas release. Nucl. Instrum. Methods B 496–503. Roberts, E., 1974. Applied Polymer Symposium No. 23, Wiley, New York.