High-Energy Ion Irradiation Effects on Polymer Materials - ACS

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High-Energy Ion Irradiation Effects on Polymer Materials H. Kudoh, T. Sasuga, and T. Seguchi

Downloaded by 71.242.243.104 on October 23, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0620.ch001

Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, Takasaki, Gunma 370-12, Japan

The changes in mechanical properties of several polymers induced by high energy protons (10, 30 and 45 MeV protons) were studied and compared with those induced by 2 MeV electrons and Co-60 gamma rays. Changes in elongation at break showed the same trends for 10 MeV proton and electron irradiations in the case of both polyethylene (PE) and polytetrafluoroethylene (PTFE). The flexural strength at break of polymethylmethacrylate (PMMA) and glass fiber reinforced plastic (GFRP) also showed no difference between 30 or 45 MeV proton and Co-60 gamma ray irradiations.

The evaluation of radiation resistance of polymer materials to high energy ion irradiation is necessary for the selection of materials applied to space environments. The radiation resistance of polymer materials has been studied extensively with gamma rays and electron beams. Radiation effects may be imagined to be different among radiation sources such as gamma rays, electrons and ions, because the linear energy transfer (LET), which is the energy deposited along the track of ionizing particles, is different for ions, electrons and gamma rays. In this case, the evaluation of radiation resistance to ions would be necessary. However, reports on this subject are very limited because there are few facilities for uniform dose irradiation of sufficiently wide areas to allow reasonable measurements of mechanical property changes such as tensile elongation and flexural strength. Based on this, the Japan Atomic Energy Research Institute, Takasaki Radiation Chemistry Research Establishment, constructed ion accelerators (Takasaki Ion Accelerators for Advanced Radiation Application; TIARA) and irradiation chambers for the irradiation of polymer films or sheets uniformly over wide areas. In this paper, 0097-6156/96/0620-0002$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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High-Energy Ion Irradiation Effects

the changes in mechanical properties induced by proton irradiation were studied for several polymers such as polyethylene, polytetrafluoroethylene, polymethylmethacrylate and glass fiber reinforced plastic and compared with those by 2 MeV electron or Co-60 gamma rays irradiations. EXPERIMENTAL


Materials. Polymers used in this work are polyethylene (PE, medium density), polytetrafluoroeythylene (PTEE), polymethylmethacrylate (PMMA), and bis phenol A type epoxy resin matrix glassfiberreinforced plastic (GFRP). PE and PTFE in the form of 0.5 and 0.1 mm thick film were cut into JIS No. 4 type dumbbell specimens for tensile tests. PMMA and GFRP were 3 mm and 2 mm thick specimens forflexuraltests, respectively. Irradiation. Proton beams of 10 mm diameter spot with 10, 30 and 45 MeV from a cyclotron accelerator were used for uniform irradiation over a 100 mm x 100 mm wide area by beam scanning. Thefrequencyof beam scanning was 50 Hz in the horizontal and 0.5 Hz in the vertical direction. Specimens were placed on a graphite plate cooled by water in the chamber. Irradiations were carried out under high vacuum of around 10"? torr at room temperature. 10 MeV protons were used for irradiation of PE and PTFE, and 30 MeV protons were used in the case of PMMA and GFRP. 45 MeV protons were also applied to the PMMA. The proton energy for irradiation was selected as shown in Table I so that ions can pass through the materials entirely for all irradiation conditions. The mass stopping power and the range of proton penetration in the polymers were calculated by Bethe's formula and Seltzer and Berger's method as shown also in Table I. The mass stopping power depends on the proton accelerating energy and is higher than that of 2 MeV electrons. The stopping power defines the LET. The theoretical absorbed dose (D) is calculated as the product of mass stopping power (S) and thefluence(Q); 2

D(kGy)-S(MeVcm2g-l)xQ(M Ccnr ). 12

Proton current was 500 nA (3. l x l O p/ ) for PE, 50 nA for PTFE, and 200-300 nA for PMMA and GFRP. The current is equivalent to the average dose rate of 0.2 kGy/s for PE, 0.02 kGy/s for PTFE and 0.03 kGy/s for PMMA and GFRP. A cellulose iri-acetate (CTA) film dosimeter, which was developed for electron beam dosimetry, was applied to dosimetry. The observed dose determined from optical density at 280 nm showed good agreement with the calculated dose by the above formula. For comparison with proton irradiation effects, 2 MeV electron beam irradiation was carried out for PE and PTFE under vacuum at room temperature, and for PMMA and GFRP Co-60 gamma ray irradiation was conducted in nitrogen gas atmosphere at RT. s

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

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IRRADIATION OF POLYMERS

Measurements. Tensile tests were carried out for PE and PTFE, and threepoint flexural tests for PMMA and GFRP were carried out at room temperature. By the analysis of differential scanning calorimeter (DSC) measurements, the molecular weight changes in PTFE were determined. Also the glass transition temperature (Tg) of GFRP was measured. For PMMA, the molecular weight and its distribution were measured by gel permeation chromatography (GPC).

Table I Stopping Powers and Ranges of Ions in Polymer (CTA*) 2


Ion(Energy, MeV) S(MeVcm /g) R(mm) Irradiated Polymer e-(2) H+XIO) H+(30) H+(45)

I. 77 40.8 16.1 II. 3

1.1 7.8 16.6

PE,PTFE GFRP, PMMA PMMA

•CTA; Cellulose tri-acetate RESULTS and DISCUSSION PE and PTFE. Figures 1 and 2 show the change in elongation at break for PE and PTFE as a function of dose. Tensile tests for PE and PTFE showed the same degradation behavior with 10 MeV proton and 2 MeV electron irradiations at equivalent absorbed dose. Figure 3 shows the change in molecular weight of PTFE determined by DSC and Suwa's method (/). The number average molecular weight (Mn) decreases with dose, suggesting that PTFE undergoes chain scission. Changes in Mn were similar for both irradiations, suggesting that the probability of chain scission for PTFE is not sensitive to LET in spite of the large differences in LET (2 MeV electron; 1.77 MeVcm2/g, 10 MeV proton; 40.8 MeVcm /g for CTA). 2

PMMA. Figure 4 shows the change in flexural strength of PMMA measured at RT as a function of absorbed dose using protons (30 and 45 MeV) and Co-60 gamma rays (H. Kudoh et al., Polymer, submitted, 1995). The flexural strength of PMMA decreases with dose over 0.1 MGy, and the difference between Co60 gamma rays and protons was very small. The molecular weight of PMMA decreased similarly for both protons and Co-60 gamma rays. Figure 5 shows the change in number and weight average molecular weight as a function of dose. The reciprocal of Mn increased linearly with dose, showing only chain scission. The G value of chain scission is 1.7fromthe slope of the line in Fig. 5, which agrees well with values in literature (2). Figure 6 shows the change in


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1000-

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0.5 1 Dose (MGy)

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Fig. 1 Elongation at break of PE ( A:10 MeV H+ 0:2 MeV e")

5 10 Dose (kGy) Fig. 2 Elongation at break of PTFE (A:10 MeV IT*", Q:2 MeV e")


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Dose (kGy) Fig. 3 Molecular weight of PTFE (A: 10 MeV H ", 0:2 MeV e~) 4

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[ 0

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Dose (MGy) Fig. 4 Flexural strength of PMMA (•:30 MeV H+ «:45 MeV H+ O:gamma ray)




KUDOH ET AL.

Fig. 5 Molecular weight of PMMA, (a)number average, (b)weight average (•:30MeVH+ •:45MeVH+, Orgammaray)



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Dose (MGy) Fig. 6 Molecular weight distribution, the ratio of weight average to number average molecular weight ofPMMA (•:30MeVH+, B : 4 5 M e V H , 0:gammaray) +

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Dose (MGy) Fig. 7 Flexural strength of GFRP (•:30MeVH+ 0:gammaray)


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the ratio of weight average to number average molecular weight. Mw/Mn is about 3 before irradiation, and becomes around 2 after irradiation even with a small dose of around 50 kGy. This means that the molecular weight distribution is random; that is, die chain scission occurs randomly. The decrease of flexural strength is related to the decrease of the molecular weight. The probability of main chain scission occurs in the same way in number and spatial distribution for Co-60 gamma rays, and for both 45 MeV and 30 MeV protons. Thus, LET effects on PMMA degradation are not observed in the LET range of this work: that is, 1.8-16 MeVg~*cm (ca. 0.02 - 0.15 eV/A). On die other hand, Yates et al.,(2) reported that the G value of scission for PMMA decreases with large LET. In Yates'work also, G (scission) seems to be almost constant below 0.2 eV/A. Downloaded by 71.242.243.104 on October 23, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0620.ch001

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GFRP. Figure 7 shows the change inflexuralstrength of GFRP measured at RT as a function of dose (H. Kudoh et al., Polymer, submitted, 1995.). The strength increases by about 20% at around 0.5 MGy (probably because dangling molecular chains allow larger displacement after undergoing chain scission by radiation), then decreases with dose above 1 MGy. This behavior is the same for 30 MeV proton and gamma-ray irradiation. The radiation degradation of GFRP is caused mainly by the degradation of matrix resin. Figure 8 shows the change in Tg of epoxy resin determined by DSC. The decrease of Tg indicates the destruction of network structure of epoxy resin with main chain scission. The scission probability deduced by DSC is the same for gamma rays and 30 MeV proton irradiation, and therefore the degradation of mechanical properties induced by chain scission is the same for the two radiation types. LET effects. It has been assumed that a high LET radiation forms active species in high density in organic materials, that can result in different chemical reactions when compared to low LET radiation. For example (3), dimer formation in benzene increases with LET, whereas this is not observed in cyclohexane. This phenomenon is interpreted as being due to the recombination of excited molecules formed in the aromatic compound Studies {4,5) of changes in mechanical properties induced by 8 MeV proton and 2 MeV electron irradiations for several polymers indicated that LET effects were scarcely observed in aliphatic polymers such as PE and polypropylene, whereas the radiation deterioration of the aromatic polymers such as polyethersulphone and bisphenol A type polysulphone was reduced in 8 MeV proton irradiation. These results indicate that the LET effect appears in aromatic compounds. In our experiments, there seem to be little or no LET effects in four different polymers in terms of mechanical properties and chemical reactions such as crosslinking and chain scission. However, studies using much higher LET radiations and higher ion masses are necessary to more fully evaluate LET effects.


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Dose (MGy) Fig. 8 Glass transition temperature(Tg) of GFRP (•:30MeVH+ Orgammaray) CONCLUSION Cyclotron and ion-irradiation-chamber facilities were prepared at JAERI, Takasaki for evaluation of the radiation resistance of polymers to ion irradiation. By using them, high energy ion irradiation effects on polymer materials such as PE, PTFE, PMMA and GFRP were studied by changes in mechanical properties, molecular weight, and glass transition temperature. The elongation at break for both PE and PTFE showed the same degradation behavior with 10 MeV proton and 2 MeV electron irradiatioa The molecular weight of PTFE also decreased in a similar way upon irradiation with either protons or electrons. The degradation of flexural strength at break of PMMA and GFRP showed no difference between Co-60 gamma rays and 30 MeV protons. The scission probability was the same with both radiation types. We concluded that in the degradation of these polymers, LET effects are very small for 10-45 MeV proton, electron and gamma ray irradiation. Literature Cited 1. Suwa T., Takehisa M. and Machi S., J. ofAppl.Polym. Sci., 1973, 17, 3253 2. Yates B. W. and Shinozaki D. M.,J.Polym.Sci.B,1993, 31, 179 3. Burns W. G., Trans. Faraday Soc., 1962, 58, 961 4. Sasuga T., Kawanishi S., Nishii M., Seguchi T. and Kohno I., Polymer, 1989, 30, 2054 5. Sasuga T., Kawanishi S., Nishii M., Seguchi T. and Kohno I., Radiat. Phys. Chem., 1991, 37, 135 RECEIVED August 26, 1995


High-Energy Ion Irradiation Effects on Polymer Materials - ACS

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