Measurement of Radical Yields To Assess Radiation Resistance in

38 Measurement of Radical Yields To Assess Radiation Resistance in Engineering Thermoplastics Kirstin Heiland , David J. T. Hill *, Jefferson L. Hopewell , David A. Lewis , James H. O'Donnell , Peter J. Pomery , and Andrew K. Whittaker 1

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Downloaded by MONASH UNIV on May 23, 2013 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch038

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1Faculty of Science, Griffith University, Nathan, Queensland 4111, Australia Polymer Materials and Radiation Group, Department of Chemistry, The University of Queensland, Brisbane, Queensland 4072, Australia Watson Research Center, IBM, Yorktown Heights, NY 10598 The Centre for Magnetic Resonance, The University of Queensland, Brisbane, Queensland 4072, Australia

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3 4

Radiation

chemical yields for radicals were assessed for a variety of

engineering thermoplastics following

γ-irradiation

under vacuum at 77

Κ and at a low dose rate. On the basis of these radical yields the radiation

resistance of the polymers

poly(phenylene poly(arylene

oxide),

polyamide,

increased in the following poly(arylene

ether phosphine oxide), polyimide,

ether

order: sulfone),

and poly(arylene

ether

ketone). This order was similar to that found by other workers

based

on measurements tron-beam

of the tensile strength of the polymers following elec

irradiation

of a high dose at a high dose rate.

INCORPORATION O F AROMATIC UNITS into a polymer chain imparts a resis tance to degradation by high-energy radiation to the polymer (J). The aromatic units are capable of degrading absorbed energy to heat through their manifold of vibrational energy states, and these units can also scavenge small radical species such as hydrogen atoms or methyl radicals to prevent them for par ticipating in abstraction reactions. These reactions may lead to further break down of the polymer chain. Highly aromatic polymers with phenylene groups in the backbone of the polymer chain would thus be expected to exhibit a resistance to degradation by high-energy radiation such as 7-radiation and electron beams. These poly mers generally have high glass-transition and melting temperatures (2). These * Corresponding author

0065-2393/96/0249-0637$12.00/0 © 1996 American Chemical Society In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


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POLYMER DURABILITY

characteristics make the polymers suitable for use in high temperature envi ronments, particularly in environments that are beyond the useful range for most other thermoplastics. The phenylene groups also give the chains greater rigidity; this class of polymers is characterized by having a high modulus and a high tensile strength (2-4). Polymers that are strong and resistant to high thermal and ionizing ra diation environments can find uses in a wide variety of applications, such as in nuclear facilities, satellites, and other aerospace structures. Thus, there is considerable interest in assessment of the radiation resistance of these poly mers both in air and in vacuum. In many of the applications for these mate rials, an appropriate balance must be reached between the desirable polymer properties and the requirement that the materials must be processible. This need for processibility has resulted in the incorporation of heteroatoms in the polymer chains, and particularly in the incorporation of oxygen in the form of arylene ether groups, such as those found in the engineering thermoplastics Kapton and polyetheretherketone ( P E E K ) . Comparison of the radiation sensitivity of various polymers that exhibit a resistance to high-energy radiation poses a problem. In their practical appli cations the materials are generally subjected to irradiation at relatively low dose rates; to study the behavior of these polymers under the conditions of their use, very long irradiation times are required to induce significant and observable changes in many of their properties (3, 4). Some of these prop erties, such as tensile strength, chemical composition, and molecular weight, can often be difficult to measure for these materials. To overcome this prob lem, some researchers have resorted to accelerated degradation studies using high dose rates, such as those available through the use of electron beams (4). However, radiation chemistry may be dose-rate dependent (5), and at very high dose rates significant changes in the temperature of the irradiated ma terial occur, in some cases >25 °C. We found (6) that electron spin resonance (ESR) spectroscopy can pro vide a valuable means of assessing the radiation resistance of materials, even when the radiation dose rate is low and the materials are relatively insensitive to radiation damage. When high-energy radiation is absorbed by a polymer, radical anions and cations are formed along with excited states and neutral radical species (6, 7). The primary radical cations that are formed undergo further reactions and are not usually observed even at liquid-nitrogen tem peratures. However, the radical anions and neutral radical intermediates are more stable and can be thermally trapped at 77 K. Previous studies demon strated (6, 8) that there is usually a good correlation among the trapped radical yields measured at low temperature and many of the other property changes that take place in a polymer on irradiation. Thus, studies of radiation chemical yields for radicals can provide a measure of the radiation damage to a material and provide a means of assessing the relative radiation resistance of a family of polymers.

In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

38.

HEILAND ET A L .

Radiation Resistance in Thermoplastics

639

Because of the improved sensitivity of modern E S R spectrometers, it is possible to measure, with acceptable accuracy, radical concentrations as low as 10 radicals per gram of polymer. Therefore, the radiation resistance of a polymer can be assessed at low absorbed dose (typically 10~ to 10~ times that required for tensile strength measurements) and hence at low dose rates. In this chapter we review our results from studies of the radiation chem istry of a range of radiation-resistant, highly aromatic thermoplastics. We as sess the radiation resistance of the materials by E S R studies at low temperature by using low dose rates and relatively short irradiation times. We also compare our results with those obtained by other techniques that require much higher dose rates or much longer irradiation times. 14


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Experimental Polymer samples of approximately 50 mg were placed in Spectrasil quartz ESR tubes and evacuated for approximately 24 h at >10~ Pa. The temperature of the samples was then raised gradually to approximately 20 Κ above their glasstransition temperatures to ensure that any absorbed oxygen or residual solvent was completely removed. The tubes were then sealed under vacuum before radiolysis. Irradiations were carried out by using an Atomic Energy of Canada Limited (AECL) Gammacell at the University of Queensland or the ^Co facility at the Australian Nuclear Science and Technology Organization. The irradiation dose rates for each source were determined by Fricke dosimetry at room temperature and ranged from 1 to 5 kGy/h. Irradiations at 77 Κ were carried out in a Dewar flask, and appropriate allowances were made for attenuation of the radiation dose rate by the liquid nitrogen. Absorbed doses for the polymers were obtained by correcting for mass-energy absorption coefficients. The ESR spectra were obtained by using a Bruker ER200D X-band spec trometer that was fitted with a provision for improvement of the spectrum signalto-noise ratio by data accumulation. The radical concentrations were obtained by use of a Varian pitch standard reference. Care was taken to avoid microwave power saturation of the sample during spectral acquisition, and spectra were routinely obtained at approximately 2 μλΥ. 2

Polymers The range of engineering thermoplastics studied in this work include the proprietary polyimides Kapton (DuPont) and Ultem (General Electric), polyamides Kevlar and Nomex (DuPont), poly(phenylene oxide) (PPO) (Gen eral Electric), poly(arylene ether ether ketone)s, P E E K , P E E K ketone ( P E E K K ) (Hoechst), poly(arylene ether sulfone)s, Udel (ICI), synthetic poly mers (9, 10), and synthetic poly(arylene ether phosphine oxide)s (9, 11). The chemical structures of these polymers are given in Tables I and II.

Results and Discussion The E S R spectra observed after radiolysis of the polymers at 77 Κ are characterized by singlets centered in the region g = 2.003-2.004, where g is


POLYMER DURABILITY


640

the spectroscopic splitting factor (Figure 1). A n exception to this general rule was P P O , which was characterized by a singlet with superimposed fine struc ture. This fine structure was accounted for by the formation of benzyl-type radicals arising from the loss of a hydrogen atom from the methyl groups in the polymer (12). The sharp peaks observable near the center in these spectra arise from the formation of paramagnetic species in the Spectrasil quartz tubes during radiolysis. The contribution to the area of the spectra arising from these


38.

HEILAND ET A L .


Table II. Chemical Structures of Synthetic Polymers Polymer

Structure

Bis-A PSO

HQ

PSO


BP PSO

Bis-A PEPO

H Q PEPO

BP PEPO

quartz signals were subtracted when the polymer-radical concentrations were calculated. The observed singlet spectra are composite spectra made up of contri butions from three or more components (3, 10, 11, 13). This conclusion was based on the use of microwave power saturation of the E S R spectra, photobleaching, and thermal annealing studies of the irradiated samples. For the polymers, the major components of the spectra were assigned to anion radicals and neutral phenoxyl, phenyl, and cyclohexadienyl radicals in varying propor tions. Importandy, no evidence for the formation of main-chain scission rad icals, methyl radicals, or hydrogen abstraction radicals was determined by


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POLYMER DURABILITY

Figure 1. ESR spectra obtained at 77 Κ after radiolysis at 77 Κ of A, Udel (20 kGy); B, PEEK (10 kGy); C, poly(bisphenyl-A phosphine oxide) (8 kGy); and D, PPO (15 kGy). radiation chemistry at the isopropylidine linkage in those polymers containing bisphenyl-A as a component (3, 13). Plots of the radical concentration versus absorbed dose for several rep resentative polymers irradiated at 77 Κ are shown in Figure 2. In every case the plots show evidence of dose saturation, even at relatively low doses. This saturation effect was attributed (11) to saturation of the anion radicals, but dose saturation is also observed for the polymers if they are irradiated at ambient temperature (Figure 3). This observation suggests that dose satura tion of the neutral radical intermediates may also play a role, because the anion radicals are thermally unstable at temperatures above about 150-200 Κ (10-12), and these anion radicals will not be present following radiolysis at ambient temperature. At 77 Κ the motions of the polymer chains will be very restricted, so that radical recombination reactions will be limited to those that can occur within the cage. Thus, the radical concentration versus dose plots in Figure 2 provide a reliable means by which to assess the relative radiation resistance of the polymers. Those polymers with the greatest resistance to radiation damage


38.


16

643


HEILAND ET A L .

j

1

1

!

0

5

10

15

•

1

1

Γ

20

25

30

35

Dose ( k G y ) Figure 2. Flots of the radical yield versus dose for γ-radiolysis of polymers at a dose rate of IS kGy/h at 77 Κ will have the lowest radical concentrations following irradiation at 77 K. How ever, at higher temperatures some chain motions can take place, and these motions can lead to radical decay reactions. Evidence for this phenomenon is demonstrated by the lower radical yields observed following annealing from 77 Κ to higher temperatures or following radiolysis of poly(bisphenyl-A sulfone) and poly(bisphenyl-A phosphine oxide) at ambient temperature (Figure 3). On the basis of the information provided in Figure 2, the polymers were ranked in order of their resistance to high-energy radiation under vacuum. The ranking is presented in Table III. Also given in Table III are values of the radical concentrations for each polymer obtained for an absorbed dose of 10 kGy. Irradiation of the polymers to this dose, which can be achieved with reasonably short irradiation times at a low dose rate, is sufficient to establish the relative ranking of a polymer in the series.



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POLYMER DURABILITY

Dose ( k G y ) Figure 3. Plots of the radical yield versus dose for γ-radiolysis of poly(hisphenylA sulfone) (Π) and poly(bisphenyl-A phosphine oxide) (Ύ) at a dose rate of' I 5 kGy/h at 303 Κ This ranking can be compared with that reported by Sasuga et al. (4), who studied the tensile properties of a range of engineering thermoplastics following electron-beam irradiation at ambient temperature, dose rate of 5 X 10 Gy/s, and doses up to 120 M G y for Kapton. The ranking proposed by Sasuga et al. (4) was polyimide > polyetherketone > polyamide > polyetherimide > polyarylate > polysulfone > P P O . The ranking obtained on the basis of our E S R studies differs slighdy from this order in that the polyamide was found to produce significantly more radicals at 77 Κ than the polysulfone and polyetherimide, but the polyimides and polyetherketones remain ranked higher than the polysulfones and P P O . The higher ranking of the polyamide by Sasuga et al. could result from secondary radiation chemistry of these poly mers, such as cross-finking, at the higher temperature used in the electron beam study. 3


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HEILAND ET A L .


645

Table III. Ranking of Polymers by Radical Yields Measured after Irradiation at 77 Κ Radical Yield Polymer Ranking (10 spin/kg)


m

PEEK PEEKK Kapton Ultem Bis-A PEPO Udel Nomex Kevlar PPO

1.0 1.1 1.7 2.2 2.5 2.8 5.6 5.6 5.7

NOTE: Radical yields are those for an ab sorbed dose of 10 kGy and a dose rate of 15 kGy/h. Experimental error in radical yields is approximately 5%.

Sasuga and Hagiwara (14) studied the radiolysis of engineering thermo plastics in air and in oxygen. They irradiated the polymers at ambient tem perature to high dose, up to 30 M G y in air and 12 M G y in oxygen, and at a dose rate of 10 kGy/h. In general, irradiation in air or oxygen increases the yield of scission compared to that observed for irradiation in vacuum and thus increases the yield of radicals. O n the basis of tensile measurements, they reported (14) that the relative radiation resistance of the polymers in an ox ygen atmosphere was similar to that observed in their previous electron-beam study: radiation resistance of the aromatic polymers increased from poly(sulfone) to polyester to polyamide to P E E K . This order of radiation resistance is similar to that observed in our present study, except that the polyamide is ranked higher than the polysulfone. Thus, although a relatively high concentration of radicals are formed on the 7-radiolysis of the polyamides at 77 K, the tensile properties observed after 7-irradiation in air at ambient temperature do not reflect this feature of their radiation chemistry. This observation suggests that the secondary radiation chemistry of the polyamides is different from that of the other polymers, as indicated previously. The E S R measurement of radical yields at 77 Κ can also provide useful information about the radiation resistance of a group of closely related poly mers, such as the poly(arylene ether sulfone)s or the poly(arylene ether phosphine oxide)s. The radiation chemical yields at 77 K, called G-values, for formation of radicals in the biphenyl, bisphenyl-A, and hydroquinone poly(sulfone)s at low dose are given in Table IV. These data indicate that the G-values are similar for the three copolymers but that the biphenyl polymer is slightly more radiation resistant than the bisphenyl-A and hydroquinone


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POLYMER DURABILITY

Table IV. Comparison of Low-Dose Radiation Chemical Yields for Radicals at 77 Κ and Sulfur Dioxide at 303 Κ for a Series of Poly(arylene ether sulfone)s Polymer

G(R')

BP PSO Bis-A PSO HQ PSO

0.51 0.57 0.56

0.06 0.15 0.14


NOTE: G values are numbers of events per 16 aj of absorbed energy. Experimental errors in G values are approximately 5%.

polymers. Interestingly, this order is similar to that observed for the formation of gaseous sulfur dioxide assessed by gas chromatography. Sulfur dioxide re quires a much higher absorbed dose to allow accurate quantitation of the product yield (Table IV). This observation is also consistent with that of Sasuga and co-workers (4, 14). The radical yields for the three poly(phosphine oxide)s were the same at low dose [G(R*) 0.58]. These chemical yields for radical formation are similar in magnitude to that observed for the corresponding poly(sulfone)s at this temperature. However, the radical yields for P E E K at 77 Κ were significantly lower than those for poly(hydroquinone sulfone) and poly(hydroquinone phosphine oxide) (Figure 4). In comparing the relative radiation resistance of the poly(sulfone)s and the poly(phosphine oxide)s, our studies indicated that the nature of the radi ation chemistry in the two polymers is different, even though the radiation chemical yields for radical formation are very similar for the two polymers. In the sulfone polymers, the radiation chemistry occurred at both the sulfone and ether groups in the polymer chain (3); whereas in the phosphine oxide polymers the bonds to phosphorus are stable toward radiation degradation, and scission occurs at the ether unit of the polymer chain (13). Thus, subtle differences in the radiation chemistry of these two polymer families exist. The higher radiation resistance of P E E K compared with the poly(sulfone)s and poly(phosphine oxide)s could be due to the semicrystalline nature of P E E K . However, the studies carried out by Sasuga and co-workers (4, 14), in which the effect of crystallinity on the radiation resistance of P E E K was in vestigated, showed that the semierystalline nature of the polymer, while play ing an important role, does not completely account for the higher radiation resistance observed for this polymer. The carbonyl group seems to confer a radiation resistance greater than that for the corresponding sulfone and phos phine oxide units.



38.

HEILAND ET A L .


647

Dose ( k G y ) Figure 4. Flots of the radical yield versus dose for y-radiolysis of poly(hydroquinone sulfone) (A), poly(hydroquinone phosphine oxide) (M), and PEEK (A) at a dose rate of 1-5 kGy/h at 77 Κ

Summary The radiation resistance of a series of engineering thermoplastics were inves tigated at low dose by using 7-radiation at 77 K. The radical yields measured by E S R spectroscopy indicated that the radiation resistance of the polymers increased in the order P P O , polyamide, poly(arylene ether phosphine oxide) and poly(arylene ether sulfone), polyimide, poly(arylene ether ketone). The radiation chemical yields for the three arylene ethers in the poly(arylene ether sulfone)s and poly(arylene ether phosphine oxide)s were very similar in mag nitude, but the biphenyl group in the polysulfone series yielded a slightly smaller value of G(R') than that observed for the hydroquinone and bisphenylA polymers. This order of relative radiation resistance differs from that ob served by other workers based on tensile strength measurements following electron-beam irradiation at high dose rates: The radiation resistance of the


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POLYMER DURABILITY

polyamide was ranked lower than that reported from the electron-beam study. This difference could be due to • • •

thermal heating of the samples subjected to electron-beam irradiation at high dose rates the high absorbed doses used in the electron-beam study differences in the secondary radiation chemistry of the different poly mers.


Acknowledgments We thank the Australian Research Council and the Australian Institute of Nuclear Science and Engineering for financial support for this research.

References 1. O'Donnell, J. H . ; Sangster, D. F. Principles of Radiation Chemistry; Edward Ar nold: London, 1970. 2. Smith, C. D.; Gunyor, Α.; Keister, K. M . ; Marand, H. A.; McGrath, J. E. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1991, 32, 93-94. 3. Lewis, D. L . O'Donnell, J. H.; Hedrick, J. L . Ward, Τ C.; McGrath, J. E. In The Effects of Radiation on High-Technology Polymers; Reichmanis, E.; O'Donnell, J. H., Eds.; ACS Symposium Series 381; American Chemical Society: Washington, DC, 1989; pp 252-261. 4. Sasuga, T.; Hayakawa, N . ; Yoshida, K.; Hagiwara, M . Polymer 1985, 26, 10391045. 5. O'Donnell, J. H . In The Effects of Radiation on High-Technology Polymers; Reich manis, E.; O'Donnell, J. H., Eds.; ACS Symposium Series 381; American Chemical Society: Washington, DC, 1989; pp 1-13. 6. Hill, D. J. T.; O'Donnell, J. H . ; Pomery, P. J. In Materials for Microlithography; Thompson, L. F.; Willson, C. G.; Frechet, J. M . J., Eds.; ACS Symposium Series 266; American Chemical Society: Washington, DC, 1984; pp 125-149. 7. Campbell, K. T. Hill, D. J. T. O'Donnell, J. H . ; Pomery, P. J.; Winzor, C. L. In The Effects of Radiation on High-Technology Polymers; Reichmanis, E.; O'Donnell, J. H., Eds.; ACS Symposium Series Series 381; American Chemical Society: Wash ington, DC, 1989; pp 80-94. 8. Tenney, D. R.; Slemp, W. S. In The Effects of Radiation on High-Technology Polymers; Reichmanis, E.; O'Donnell, J. H . , Eds.; ACS Symposium Series 381; American Chemical Society: Washington, DC, 1989; pp 224-251. 9. These polymers were synthesised by Professor J. E. McGrath and Co-workers, Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA. 10. Hill, D. J. T.; Lewis, D. Α.; O'Donnell, J. H . Pomery, P. J.; Hedrick, J. L.; McGrath, J. E. Polym. Int. 1992, 28, 233-237. 11. Hill, D.J.T.; Hopewell, J.L.; O'Donnell, J.H.; Pomery, P.J.; McGrath, J. E . Priddy, D. B.; Smith, C. D. Polym. Degrad. Stab. 1994, in press. 12. Hill, D. J. T.; Hunter, D. S.; Lewis, D. Α.; O'Donnell, J. H.; Pomery, P. J. Radiat. Phys. Chem. 1990, 36, 559-563. ;

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13. Heiland, Κ; Hill, D. J. T.; O'Donnell, J. H.; Pomery, P. J. Polym. Adv. Technol. 1994, 5, 116-121. 14. Sasuga, T.; Hagiwara, M . Polymer 1987, 28, 1915-1921. RECEIVED

for review January 26, 1994.

ACCEPTED

revised manuscript September 28,


1994.


Measurement of Radical Yields To Assess Radiation Resistance in

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