investigation of poly(vinylidene fluoride) copolymers ...

2007 International Nuclear Atlantic Conference - INAC 2007 Santos, SP, Brazil, September 30 to October 5, 2007 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-02-1

INVESTIGATION OF POLY(VINYLIDENE FLUORIDE) COPOLYMERS APPLIED TO HIGH GAMMA DOSE DOSIMETRY Adriana S. Medeiros1 and Luiz O. Faria2 1

2

Depto. de Engenharia Nuclear (DEN / UFMG - MG) Av. Antônio Carlos 6627 31270-970 Belo Horizonte, MG [email protected]

Centro de Desenvolvimento da Tecnologia Nuclear (CDTN / CNEN – MG) Rua Mário Werneck s/n, Campus Pampulha, C.P. 941, 30123-970 Belo Horizonte, MG [email protected]

ABSTRACT Poly(vinylidene fluoride) [PVDF] is a semicrystalline linear homopolymer worldwide known by its good chemical, mechanical and electromechanical properties. Its polymeric chain is composed by the repetition of CH2-CF2 monomers. PVDF and some of its copolymers has demonstrated to be sensitive to ionizing radiation. Their piezoelectric properties are highly enhanced after ultraviolet, electrons or gamma irradiation. The sensibility of PVDF and its copolymers to ionizing radiation has encouraged us to investigate their use for high dose gamma dosimetry. In this work it is reported the relationship between the delivered gamma doses and the optical absorption (OA) at wavelengths raging from 190 to 900 nm. Particularly, the study is focused in the optical absorption peaks at 185 nm, 223 nm and 274 nm, once it has been demonstrated that they are related to the appearing of C=C conjugated bonds after X-ray and UV irradiation. It is demonstrated that there is a linear correlation between the gamma dose and the peak intensity at 223.5 nm for gamma doses ranging from 0.3 kGy to 13 kGy. Based on these results, it is concluded that PVDF fluorinated copolymer is a good candidate for use in high dose gamma dosimetry applications.

1. INTRODUCTION High dose dosimetry is an essential tool in the fields of food irradiation, surgery equipment sterilization and radiotherapy treatments. For example, the success of radiation processing of food depends to a large extent on the ability of the processor to measure the absorbed dose delivered to the food product, to determine the dose distribution patterns in the product package and to control the routine radiation process. In all these tasks, a reliable high dose dosimetry is required. Calorimeter, Alanine and also Ceric-cerous sulphate, ECB, Ferrous Sulphate and dichromate solutions are some examples of standard reference dosimetry systems commercially available elsewhere [1]. Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) dosimetry and also polymer based dosimetry have been alternatively used for high gamma dose dosimetry [2-5].

Among the polymeric materials, poly(methyl methacrylate) [PMMA] and poly(vinyl chloride) [PVC] have been successfully employed [1,5]. On these dosemeters, ionizing radiation induces a change in the optical absorbance at specific wavelengths. The changes are proportional to the delivered dose. In this work it is investigated the radiation induced optical absorbance behavior in fluorinated poly(vinylidene fluoride) [PVDF] copolymers. PVDF is a semicrystalline linear homopolymer worldwide known by its good chemical, mechanical and electromechanical properties. Its polymeric chain is composed by the repetition of CH2-CF2 monomers and the random introduction of fluorinated monomers such as CHF-CF2 in its main chain may change the mechanical and electrical properties. These copolymers has demonstrated to be sensitive to ionizing radiation. Their piezoelectric properties can be highly enhanced after ultraviolet, electrons or gamma irradiation [6-8]. The sensibility of PVDF and its copolymers to ionizing radiation has encouraged us to investigate their use for high dose gamma dosimetry.

2. EXPERIMENTAL The polymeric film samples were produced by melting at 200oC under 300 bar and subsequent air-cooling to room temperature. The PVDF copolymers resins were supplied by ATOCHEM (France). This process produces transparent films of about 170 µm. The samples were irradiated with a Co-60 source at constant dose rate (12 kGy/h), with doses ranging from 0.1 kGy to 13.0 kGy. Optical absorption measurements were taken in a Shimadzu UV240 PC spectrometer at wavelengths ranging from 190 to 900 nm. All measurements were taken immediately after the irradiation process.

3. RESULTS AND DISCUSSION

In order to verify the effect of gamma radiation in the PVDF copolymers, the film samples were exposed to doses of 0.1, 0.3, 0.6, 1.2, 5.0 and 13.0 kGy. For each dose two different samples were used. After each irradiation process the UV-VIS absorption spectrum was collected. The time between the end of the irradiation and the beginning of the optical absorption measurement was less than one hour. The results are shown in Fig. 1, for wavelengths ranging from 190 to 400 nm. The remaining part of the spectra, i.e., from 400 to 900 nm, were omitted for clarity purposes, once they have, in this range, the same value as they have at 400 nm. As one can be seen in Fig. 1, the optical absorption starts to increase for wavelengths between 190 and 300 nm, when the gamma dose increases from 0.3 to 0.6 kGy. The absorption spectra for samples exposed to 0.1 (not shown here) and 0.3 kGy were identical. It is also can see in Fig.1 that as the gamma dose increases, the optical absorption around 196, 220 and 273 nm also increases. Actually, the large peak at 196 nm seems to saturate for dose higher than 1.2 kGy. These absorption bands were also observed in PVDF copolymers irradiated with X rays and are attributed to the formation of C=C conjugated bonds (185 nm), doublets (223 nm) and triplet (274 nm) [6], and also for UV irradiation, with absorption bands at 194, 221 and 266 nm [8].

INAC 2007, Santos, SP, Brazil.

Optical Absorption (a.u.)

1,4

Virgin 0.301 0.606 1.262 5.00 13.00

P(VDF-TrFE)

1,2 1,0

kGy kGy kGy kGy kGy

0,8 0,6 0,4 0,2 0,0 200

250

300

350

400

Wavelength (nm)

Figure 1. Optical absorption spectra for PVDF copolymers exposed to 0.3, 0.6, 1.2, 5.0 and 13.0 kGy of gamma radiation. The optical absorbance spectra were peak fitted in order to identify the absorption peaks related to the C=C conjugated bonds, and also to check if the any of these peaks shows a linear correspondence with the delivered dose. In Fig. 2 it is shown the absorption spectrum for the sample irradiated with 1.2 kGy and 4 individual peak fitted peaks. The sum of these 4 peaks can also well fit the absorption spectra for the samples irradiated with 0.3, 0.6, 5.0 and 13.0 kGy. In Table 1 it is reported the peak fitting data using Lorentzian lines. The peak fitting data is in agreement with other authors [6], showing optical absorption peaks at 223.5 and 273.5 nm, that may be related to radiation induced conjugated of C=C bonds (doublet and triplet, respectively). The peak at 197.2 nm is shifted compared to the reported 185 nm for X rays [6]. This can be attributed to the amount of TrFE in the PVDF copolymer used.

1,6


D = 1.261 kGy 1,2

0,8

0,4

0,0 200

250

300

350

400

Wavelength (nm)

Figure 2. Optical absorption spectrum for PVDF copolymers exposed to 1.261 kGy (solid line) and the absorption peaks obtained after peak fitting.


Peak 1

Peak 2

Peak 3

Peak 4

Wavelength (nm)

197.5

223.5

273.5

321.0

Amplitude (a. u.)

1.16

0.51

0.142

0.063

Table 1. Data for the absorbance spectrum peak fitted with Lorentzian lines. The fit is for the 1.262 kGy irradiated sample. Finally, based on the peak fitting data, it was investigated if there is one or more peaks which could be used for dosimetry purposes. In Figure 3 it is shown the linear behavior of the peak 2 intensities with the exposed doses. It should be noted that this linear relationship have been well fitted just for the peak at 223.5 nm, i.e, the peak attributed to doublet of C=C bonds. It is well known that, in most of the polymers exposed to high doses of ionizing radiation, the radiation induced chemical process continues at least on month after the end of the irradiation. This behavior puts a limitation in the use of polymers as high dose dosimeters: the reading process must be undertaken soon after the irradiation process. In fact, in the case of PVDF copolymers, the optical absorption fading in all irradiated samples was evaluated for a 2-month period. All samples present a non regular decrease in the absorbance signal. 0,9


0,8

224 nm

0,7 0,6 0,5 0,4 0,3 1

10

Dose (kGy)

Figure 3. The optical absorption intensities at 223.5 nm for PVDF copolymers exposed to gamma doses ranging from 0.3 to 13 kGy (open circles). The dotted line is a linear fitting (cc = 0.9998).


4. CONCLUSIONS

PVDF fluorinated copolymers have been investigated from the view point of high dose dosimetry. In this context, transparent films of 170 µm prepared from melting were exposed to gamma doses ranging from 0.1 to 13 kGy, from a Co-60 source. The correspondent optical absorption spectra were peak fitted with Lorentzian lines and the peak centered at 223.5 nm was found to behave linearly with the delivered gamma dose. Radiation induced chemical process provokes a irregular fading in the overall optical absorption signal in a 2month period. The optical absorption at 223.5 nm is attributed to doublet of conjugated C=C bonds. The results of this investigation points out that the PVDF fluorinated copolymer is a good candidate for use in high dose gamma dosimetry applications, in the 0.3 to 13 kGy dose range. ACKNOWLEDGMENTS The authors acknowledge the financial support from Brazilian government agencies CNPq and FAPEMIG. REFERENCES 1. Technical Report Series, “Dosimetry for Food Irradiation”, IAEA, 409, (2002). 2. A.H. Ranjbar, M.W. Charles, S.A. Durrani, and K. Randle, “Electron Spin Resonance and Thermoluminescence Dosimetry of Clear Fused Quartz: Its Possible Use for Personal, High Dose and High Temperature Dosimetry”, Rad. Prot. Dosiemtry 65, 351 (1996). 3. 3. S.D. Miller, “High Dose Dosimetry Using Optically Stimulated Luminescence”, Rad. Prot. Dosiemtry 66, 201 (1996). 4. 4. I. Milman, V. Putyrsky, M. Naimark, and V. Popov, “PTFE in High Dose ESR-NMR Gamma Dosimetry”, Rad. Prot. Dosiemtry 47, 271 (1993). 5. K. H. Chadwick, “The choice of measurement wavelength for clear HXPerspexdosimetry”, Dosimetry in Agriculture, Industry, Biology and Medicine (Proc. Symp.Vienna, 1972), IAEA, Vienna 563–576, (1973). 6. B. Daudin, J. F. Legrand and F. Macchi, “Microscopic and macroscopic effects of electron irradiation on ferroelectric poly(vinylidene fluoride-TrFe) copolymers”, J. Appl. Phys. 70, 4037 (1991). 7. C. Welter, L. O. Faria and R. L. Moreira, “Relaxor Behavior of gamma-irradiated poly(vinylidene fluoride-trifluorethylene) copolymers”, Phys. Review B 67, 144103 (2003). 8. L. O. Faria, C. Welter and R. L. Moreira, “Relaxor Ferroelectric Behavior of Poly(Vinylidene Fluoride-Trifluorethylene) Copolymer Modified by Low Energy Irradiation” Applied Physics Letters, EUA, 88, n. 19, 192903, (2006).
