Irradiation of Polymers - American Chemical Society

Chapter 23

Effects of Ionizing Radiation on the Optical Properties of Polymers Julie P. Harmon , Emmanuel Biagtan , Gregory T. Schueneman , and E. P. Goldberg

Downloaded by UNIV MASSACHUSETTS AMHERST on October 11, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0620.ch023

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Department of Chemistry, University of South Florida, Tampa, FL 33620-5250 Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003

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The UV/Visible transmission spectra of glassy, optical polymers are greatly affected by ionizing radiation. The effect of gamma radiation on the transparency of styrene polymers is studied. Modifications on the styrene ring can reduce or enhance the radiation induced discoloration. The color center population is monitored in a fundamental study via UV/VIS spectroscopy. This fundamental study is used to help interpret the behavior of polymeric scintillator exposed to gamma radiation. The effect of ionizing radiation on polymer properties has been extensively studied for many years. Most of the attention has focused on mechanical properties and on the chemical reactions responsible for radiation induced changes in these properties. Reference 1 is an excellent review of this subject matter. More recently, radiation effects on optical properties of polymers have commanded attention. This is due to applications involving optical polymers that are used in high radiation environments. When transparent biomedical devices are sterilized via gamma irradiation, doses as low as 2.5 Mrads induce discoloration of the articles. Particle accelerators use polymeric scintillator to detect radioactive species formed during collisions. Polymeric scintillator is made up of polystyrene or poly(4-methyl styrene) doped with fluorescent dyes. In the new generation of high energy accelerators, scintillator will see doses of radiation in the Mrad range. These doses are sufficient to induce color center formation which diminishes the sensitivity of the scintillator. These applications triggered an interest in understanding the nature of the radiation induced color center formation and in designing polymers which resist color center formation. Radiation effects on polymer structure vary with the energy or type of source (alpha, gamma, neutron, X-ray or electron beam) and with the dose. In addition, radiation induced reactions in the presence of oxygen differ significantly from those which occur in an oxygen-free environment. Irradiation induces a number of complicated effects on polymer optical properties ranging from decreases in refractive index(2) to the production of color centers. Earlier literature sites many examples of the effects of ionizing radiation on the production of permanent and transient color centers in polymeric materials (3-8). Recent work of Clough and Wallace defines two types of color centers, "annealable" and "non-annealable"(9). "Annealable" color centers are associated with reactive species which disappear during or after 0097-6156/96/0620-0302$12.00/0 © 1996 American Chemical Society In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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Optical Properties of Polymers

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irradiation. Non-annealable color centers are associated with a permanent change in chemical structure. Wallace, Sinclair, Gillen and Clough(70) define the annealable color centers as free radicals that anneal in the presence of and in the absence of air. While air annealing effects are attributed to oxygen quenching of free radicals, annealing in the absence of oxygen is the result of radical recombination. Rates for both types of annealing increase with annealing temperature. The "colored" nature of free radicals has been well documented in non-polymeric organic molecules. Phenoxy radicals and their derivatives prepared by flash photolysis in gaseous and liquid systems and by photolysis in glassy solutions at low temperatures have absorption maxima between 270 and 615 nm(77). Similarly, gamma irradiation induces the formation of free radicals in 3-methylpentane glasses at 77°K, and these absorb in the U V region of the electromagnetic spectrum(72). Again, certain enzymes form stable, delocalized free radicals that absorb in the UV/VIS region(73). Non-annealable color centers resulting from radical-radical reactions are thought to be due to the formation of conjugated double bonds( 70). Possible permanent, colored oxidation products for gamma-irradiated polystyrene are suggested (conjugated ketones, aromatic ketones and quinones). It has also been noted that when polystyrene is irradiated in air, diffusion limited oxidation bands appear on the outsides of the samples(74). Analysis of bleaching rates in samples allows the quantitative determination of oxygen consumption rates. We conducted a series of studies aimed at identifying ways of altering the chemistry of macromolecules to make them more resistant to radiation induced discoloration (72-27). We investigated increasing the flexibility and permeability of macromolecules in order to increase annealing and radical recombination rates. A flexible polymer matrix enhances the probability of radical recombination and a more flexible matrix is likely to exhibit enhanced oxygen diffusion. The nature of the group or additive used to enhance flexibility may be chemically prone to or resistant to irradiation, or the enhancer may alter oxygen solubility. Furthermore, oxygen permeability may be increased via the incorporation of stiff bulky groups which deter bulk flexibility i.e.. t-butyl groups. Finally, attempts to altet permeability or recombination rates may result in the incorporation of moieties which exhibit different degrees of radical stability. The first part of this study addresses this issue by focusing on the effect of styrene substituents on radiation induced color center formation. The second part of this study focuses on dye doped polystyrene and poly(4-methyl styrene) scintillator as they respond optically to radiation. Scintillators emit light when exposed to ionizing radiation. The light is detected by photomultiplier tubes and compared to light production by a standard. This provides a comparative measure of photons is termed light output, L O . Ionizing radiation diminishes L O due to the formation of permanent and transient color centers which absorb the light emitted by the scintillator. Additional loss in L O occurs when radiation alters the dye chromophore. The effect of dose and dose rate on scintillator light output is reviewed.

Experimental Styrene and p-methyl styrene monomers were obtained from Scientific Polymer Products, Inc. in Ontario, N Y . T-butyl styrene was obtained from Monomer-Polymer Laboratories in Windham, N H . P-ethyl styrene and p-propyl styrene were synthesized as described in reference 22. Inhibitor, t-butyl catechol, was removed from the commercial monomers via an activated alumina column. Styrene, methyl styrene and t-butyl styrene were initiated with 0.2 wt % (l,l-di(t-butylperoxy)-333trimethylcyclohexane). Ethyl and propyl styrene were initiated with 0.2% A I B N (23). Samples were polymerized under nitrogen for 12 hrs. at 85 ° C The polymers are coded: polystyrene (PS), poly (4-methyl styrene) (PMS), poly (4-ethyl styrene) (PES), poly(4-n-propyl styrene) (PPS) and poly (4-t-butyi styrene) (PtBS). Six tenths

In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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IRRADIATION OF POLYMERS

cm thick discs were compression molded between plates with optical surfaces. Glass transition temperatures were characterized with a 2910 Du Pont Instruments D S C . Samples were scanned at 10 ° C under nitrogen. Tgs were determined from the second scan. No further characterization was undertaken. Oxygen permeability was determined on discs machined to 0.25-0.35 mm thick. A Createch Model 201T Permeometer was used following the method of Fatt (24). Permeability is characterized by the Dk value which is in units of (cnAsec) (ml 02/ml x mm Hg). Densities were determined via water displacement or with a Quantachrome Corp. MPY-1 micropyncnometer. UV/VIS spectra were recorded on a Hewlett Packard 8452A Diode Array Spectrophotometer. Samples were irradiated with a ^ o source to a total dose of 10 Mrads at a dose rate of 0.039 Mrads/hr in air. Scintillators, SCSN-38, SCSN-81 and Bicron BG499-35, were obtained from Kurarey Company Ltd. and Bicron Corporation. Four mm thick samples were machined into 1 inch O D disks. Compositions and pre irradiation relative light outputs are in Table I. A l l light outputs are relative to an unirradiated sample of SCSN-38.

Table L Commercial Polymer Scintillators Scintillator

Base polymer

Primary Dye

Secondary Dye

Pre irradiation Light Output ±5% a

SCSN-38

Cross-linked Polystyrene

l%b-PBD

0.02% B D B

100%

SCSN-81

Cross-linked Polystyrene

1%PTP

0.02% B B O T

95%

Bicron

Uncross-linked Poly(methylstyrene)

PIP

BBOT

105%

BC^9^35 a

Light outputs are relative to unirradiated sample of SCSN-38.

Each sample was irradiated with a *°Co at a constant dose rate in air. Dose rates were 1.5,035,0.14,0.04,0.013,0.0069,0.0045 and 0.0023 Mrads/hr. LOs were measured immediately after irradiation, and at intervals afterwards. Measurements were made by a T H O R N EMI type 9124B PMT (11 stages, 1100V) located in a light-tight box. Excitation occurred via a 1.0 micro Curie Am-241 alpha-source mounted on the scintillator. Further details are described in reference 25. U V / V I S transmission spectra were recorded as above. ESR spectra were recorded on bars of the Bicron sample, 76 mm x 10 mm x 3mm which were gamma irradiated to 10 Mrads at 1.5, 0.14, and 0.04 Mrads/hr. Radical intensities were measured with a Bruker ER 200D SRC Electron Spin Resonance Detector operating at 9500 M H z and scanning from i000 to 4000 Gauss.

Results and Discussion Polystyrene and alkylated Polystyrene study. Figure 1 shows the U V / V I S transmission spectra for the polystyrenes before and at intervals after irradiation at 0.039 Mradshr to a total dose of 10 Mrads. In figure 2, the wavelength at 50% transmission before and after irradiation is summarized for the styrene polymers. At 0.039 Mrads/hr, PS and PMS exhibit a significant amount of post irradiation


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Figure 1: UV/VIS spectra for the polystyrenes before and after gammairradiation


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recovery, while PES, PPS and PtBS do not. PES and PPS have the shortest wavelengths at 50% transmission at all times after irradiation. The 50% transmission wavelength is used for the comparative purpose of relating structure to color center formation. It must be noted, however, that all samples exhibited a decrease in transmission that extends to much longer wavelengths at transmission percentages greater that 80%. Table II, Glass Tr Polymer

Density Dk (gm/cm?) aO^xfcmVsecXrnl Q x i 1.050 3.10 1.020 5.70 1.008 10.20 0.997 9.10 0.963 15.80


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PS PMS PES PPS PtBS

94 111 76 48 140

Table II shows the measured T s, densities and oxygen permeabilities for the polystyrenes. There is a trend showing an increase in permeability as density decreases, except for PPS which has a lower permeability than expected. Para substituted ethyl and propyl groups decrease the glass transition temperature, while methyl and t-butyl groups increase die glass transition temperature. One might reason that the flexibility of the side group could effect radical recombination, but the situation is actually too complex to sort out such effects. We anticipated that increasing the permeability would result in an increase in recovery rate. Indeed this is the case. PPS, PES and PtBS (Figure 2) show little or no recovery after irradiation, an indication that recovery occurred during irradiation. PtBS showed a slight, unexplained increase in the 50% transmission wavelength between 7 and 60 days after irradiation. If PtBS is not considered, there is a decrease in permanent damage as determined by the 50% transmission point that accompanies an increase in the Dk value. This indicates that increases in free volume which enhance permeability may also increase radical recombination rates. We are currently exploring the use of ESR to investigate this issue. PtBS exhibits the largest permanent bathochromic shift. This may be the result of preferential cleavage of the phenyl-t-butyl bond during irradiation due to the stability of the resulting t-butyl radical. This may result in the formation of a higher population of free radicals in PtBS as compared to the other polymers. g

Scintillator study. Alpha particles excite the samples near the surface and light produces by this surface scintillation is transmitted through the 4 mm thick sample to the photomultiplier tube. Therefore, decreases in IX) are due to degradation of the scintillating and emitting centers in the penetrated surface and to color center formation throughout the entire sample thickness. Figure 3 is a plot of the light outputs for the Bicron BC-499-35 scintillator during the seven day period immediately after irradiation. There is less recovery in the light output as the dose rate for irradiation decreased. The Bicron samples in particular displayed a wide range of recovery behavior depending on the dose rate. For all scintillators, LOs measured 2 to 3 months after irradiation showed no additional increases over readings that were taken within three to seven days after irradiation. Figure 4 is a representative semilogarithmic plot of the light outputs for the Bicron scintillator immediately after irradiation and after full recovery versus the irradiation dose rate. As the dose rate for irradiation decreased logarithmically, the immediate LO either decreased (Bicron BC-499-35), stayed constant (SCSN-81), or increased (SCSN-38), while the final LO consistently decreased. In all cases, the


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0.039 Mrads/hr

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Initial

H 7 Days Recovery

Hi 10 Mrads 0

60 Days Recovery

Figure 2: Summary of UV/VIS data showing 50% transmission versus polymer. 100

Bicron-499-35

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I o

3 •J

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4

ft A

A 20± 1

0

+

• o

A A +

Dose Rate (Mrads/hr)

• •

o

O •

A

o • A +

• • • O

1.5 035 0.14 0.04 A 0.013 A 0.0069 0.0045 + 0.0023

i

0-1

r-

Days after gamma irradiation

Figure 3: Bicron-499-35 light output after gamma-irradiation to 10 Mrads at different dose rates. Values relative to unirradiated SCSN-38. Reproduced with permission from ref. 25.



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post-irradiation recovery, which is the difference between the immediate and final LOs, decreased as the dose rate decreased. The immediate and final LOs converged until below a certain dose rate, no recovery was observed. The dose rate where the two LOs converged was different for each scintillator. More importantly though, the final LOs below the convergence dose rate still follow the semilogarithmic relationship established above the convergence dose rate. In figure 5, just the final LOs for the three scintillators are plotted versus the dose rate. The respective semilogarithmic equations based on regression analysis are presented in Table III. This suggests that one can estimate the decrease in final LO at low dose rates of irradiation by irradiating samples at high dose rates, fitting a semilogarithmic equation to the data, and then extrapolating the equations to the lower dose rates. Table m, Semilogarithmk Equations Relating Final LOs to Dose Rate Scintillator Equation of line Correlation Coefficient SCSN-38 ftnal L O = &.6iS + li.$33*Log(dose rate) .988 SCSN-81 Final LO = 74.095 + 15398*Log(dose rate) .975 Bicron Final LO = 87.251 + 26.247*Log(dose rate) .994 BG499-35 Doseratein Mrads/hr. light outputs relative to unirradiated sample of SCSN-38. 9

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Bicron BC-499-35 had the highest log constant of the three scintillators, indicating that it is more dependent on the irradiation doseratethan the other two. Furthermore, note that at high doserates,the Bicron scintillator had the highest final LO, while at low doseratesit had die lowest. Thus the order ofradiationstabilities between different scintillators at high doseratesmay not be the same at lower dose rates. Radiation damage data determined at high dose rates may be incorrect in estimating scintillator stabilities and useful lifetimes under real operating conditions. Figure 6 is the UV/VIS transmission spectra for the Bicron scintillators before irradiation and after recovery from irradiation to 10 Mrads at selected constant dose rates. There is a gradual red shifting of the transmission spectra with decreasing dose rate of irradiation. This suggests that more permanent color centers are being created at the lower dose rates. The decrease in LO is more dramatic than that seen in transmission spectra shifts. This is because polymer and fluor degradation diminished the initial light produced by scintillation before it travels through the remainder of the damaged matrix. Figure 7 is a plot of the ESR derivative curves for Bicron-499-35 taken immediately after they were irradiated to 10 Mrads at the selected constant dose rates. The ordinate is in arbitrary units of intensity, while the abscissa is in Gauss. The curves best represent the most stableradicalin poly(4-methyl styrene), the p-methylbenzylradical(26). The primary benzylradicalis not ruled out, but its presence may be obscured by the more prevalent radical species or it may not dominate at these irradiation conditions (27). The radical intensity decreased as the dose rate for irradiation decreased. Thus, irradiation at high dose rates creates a large concentration of radicals within the scintillator, while irradiation at lower dose rates creates a smaller concentration. It is important to note that the final light output, the recovery in light output and theradicalconcentration all decreased as the constant doseratefor irradiation decreased.



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Gamma Dose Rate (Mrads/hour) Figure 4: Bicron-499-35 immediate and final light outputs versus dose rate after gamma-irradiation to 10 Mrads. Values relative to unirradiated SCSN38. Reproduced with permission from ref. 25.

Gamma Dose Rate (Mrads/hour) Figure 5: Final light outputs versus gamma dose rate for all three scintillators. Values relative to unirradiated SCSN-38, Reproduced with permission from ref. 25.


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Figure 6: Bicron-499-35 UV/VIS transmission spectra before and after gamma irradiation to 10 Mrads at different dose rates. Normalized to 93% transmittance at 800 nm. Reproduced with permission from ref. 25.

1.5Mrads/hr

3400

3425

3450

3475

3500

3525

3550

Gauss Figure 7: Bicron-499-35 ESR spectra immediately after gamma irradiation to 10 Mrads at the indicated dose rates.


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Summary The results for the study on PS and the alkylated PS samples indicate that controlling the population Of color centers in irradiated samples is a complex endeavor. Increasing the oxygen permeability does increase the annealing rate. However, the substituents used to increase the oxygen permeability effect color center formation in an additional way in that they may enhance or decrease permanent color center formation. The highly permeable PtBS sample annealed rapidly, but exhibited the largest permanent bathochromic shift, presumable due to the susceptibility of the tbutyl group to radiation induce cleavage. The ethyl and propyl substituted samples exhibited the smallest permanent bathochromic shifts. This may be due to the effect of these flexible groups on radical recombination. Future research on these polymers will correlate ESR results with optical results at different radiation doses and dose rates. The scintillator observations can be explained by assuming that radiationinduced oxidation occurs during irradiation to create effective light absorbing centers and that some of the unreacted radicals after irradiation convert into non-light absorbing or less absorbing centers. High radiation dose rates create large radical concentrations in the scintillator within a short time period. Before the irradiation is completed, radicals react with ail of the dissolved oxygen to create effective light absorbing centers. The concentration of dissolved oxygen is quickly depleted and not readily replenished by oxygen diffusion, so many radicals are left unreacted. Since the permeability of dissolved oxygen in polystyrene and poly(4-methy1 styrene) is low, very few permanent light absorbing centers are created. Immediately after irradiation, there are many unreacted radicals, hence there is a high ESR intensity, and eventually high LO recovery and high final LOs. Lower radiation dose rates create fewer radicals within a given time period. The absorbed oxygen is never depleted and may be replenished by diffusion. Oxidation degradation during irradiation is sustained so more light absorbing centers are created. Immediately after irradiation, mere is a lower concentration of radicals, hence there is a lower ESR intensity and eventually less light output recovery. Acknowledgments

The authors would like to thank Dr. K. Williams, Dr. H. Hanrahan, Dr. Talham and Dr. H. Byrd for their help in mis research and for giving us access to their equipment;. The authors would also like to thank the Bicron Corporation and Fermilab for providing scintillators. Support was provided by the U.S. Department of Energy grant DE-FG05-86ER40272 and by the Texas National Research Commission grant RGFY93-281. References

1. Clough, R. L., In Encyclopedia of Polymer Science and Engineering, vol. 1 Radiation Resistant Polymers ; Bikales, N., Ed; John Wiley and Sons, New York, 1983, 667. 2. Darraud, C., Bannamane, B., Gagnadre, C., Decossas, J. L., Vareille, J. C., Polymer; 1994,11,2447. 3. Day., M. J., Stein, G., Nature, 1951, 168, 644. 4. Day., M. J., Stein, G., Nature, 1951, 168, 645. 5. Charlesby, A., Nucleonics, 1954, 12, 18. 6. Fowler, J. F., Day, M. J., Nucleonics, 1955, 13, 52. 7. Boag, J.W., Dolphin, G. W., Rotblat, J., Radiation Res., 1958, 9, 589. 8. Barker, R. E., J. Polym., Sci., 1962, 58, 553.



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9. Clough, R. L., , Wallace, J. S., In Symposium on Detector Research and Development for the Superconducting Super Collider: Radiation Effects o Organic Scintillators: Studies of Color Center Annealing. T. Dombeck, V. Ke and G. Yost Eds., World Scientific, 1990, 661. 10. Wallace, J. S., Sinclair, M. B., Gillen, K. T., Clough, R. L., Radiat. Phys. Chem, 1993, 41, 85. 11. Land, E. J., Porter, G., Strachan, E., Trans. Faraday Soc., 1962, 57, 1885. 12. Neiss, M. A., Willard, J. E., J. Phys. Chem, 1975, 79, 783. 13. Atkin, C. L., Thelander, L., Reichard, P., Lang, G., J. Biol. Chem., 1973, 248. 14. Gillen, K. T., Wallace, J. S., Clough, R. L., Radiat. Phys. Chem, 1993,41, 101. 15. Harmon, J. P., Jhaveri, T., Gaynor, J., Walker, J. K., Chen, Z., J. Apply. Polym. Sci., 1992, 44, 1695. 16. Harmon, J., Gaynor, G., Feygelman, V., Walker, J., Nucl. Instrum. and Meth. in Phys. Res., 1991, B53, 309. 17. Harmon, J. P., Taylor, A. G., Schueneman. G. T., Goldberg, E. P., Polym. Deg. and Stab., 1993, 319. 18. Harmon, J. P. Gaynor, J. F., J. Polym. Sci. Part B: Polym. Phys., 1993, 31. 235. 19. Taylor, A. G., Harmon, J. P., Polym. Deg. and Stab., 1993,41, 9. 20. Gaynor, J., Fischer, V., Walker, J., Harmon, J. P., Nucl. Instrum. and Meth. in Phys. Res., 1992, B69, 332. 21. Harmon, J. P., Gaynor, J. F., Taylor, A. G., Radiat. Phys. Chem, 1993, 41, 153. 22. Schueneman, G. T., "Radiation Stability of Polymers for High Energy Radiation Detectors" Ph. D. Thesis, University of Florida, 1994, 14. 23. Davies, T. E., BritishPlast.,1959, 19, 283. 24. Fatt,I.,Int. Cont. Lens Clinic, 1975, 11, 179. 25. Biagtan, E., Goldberg, E. P., Stephens, R., Harmon, J. P., Nucl. Instrum. and Meth. in Phys. Res., 1994, B 93, 296. 26. Parkinson, W., Keyser, R., The Radiation Chemistry of Macromolecules, Edito M. Dole, Academic Press, 1973,, II, 57. 27. Herod, T., Johnson, K., Schlenoff, J., Radiat. Phys. Chem., 1993, 41, 1/2, 65. RECEIVED

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Irradiation of Polymers - American Chemical Society

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