High gamma dose response of poly(vinylidene fluoride) copolymers

ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 587 (2008) 315–318 www.elsevier.com/locate/nima

High gamma dose response of poly(vinylidene fluoride) copolymers A.S. Medeirosa, L.O. Fariab, a

Depto. de Engenharia Nuclear (DEN/UFMG), Av. Antoˆnio Carlos 6627, 31270-970 Belo Horizonte, MG, Brazil Centro de Desenvolvimento da Tecnologia Nuclear, Rua Ma´rio Werneck s/n, C.P. 941, 30123-970 Belo Horizonte, MG, Brazil

b

Received 21 June 2007; received in revised form 14 November 2007; accepted 10 January 2008 Available online 30 January 2008

Abstract Poly(vinylidene fluoride) [PVdF] is a semicrystalline linear homopolymer known worldwide 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 have demonstrated to be sensitive to ionizing radiation. In this work, we investigate the changes in the ultraviolet–visible (UV–vis) and infrared (FTIR) optical absorbance spectra of poly(vinylidene fluoride–trifluorethylene) [P(VdF–TrFE)] copolymers exposed to high gamma doses, in order to evaluate the possibility of using them as high-dose dosimeters. We have found out a strong linear correlation between the gamma dose and the absorption peak intensities in the UV region of the spectrum, i.e., at 223 and 274 nm. The absorption peak at 223 nm is the most sensitive to gamma rays and can be used for detecting gamma doses ranging from 0.3 to 75 kGy. Simultaneously, the absorption peak at 274 nm can be used for doses ranging from 1 to 100 kGy. Thus, for gamma doses ranging from 1 to 75 kGy, the great advantage of this new high gamma dosimeter is concerned to the ability of evaluating exposed doses at two different wavelengths. FTIR spectrometry data were also used to complement the characterization of the radiation-induced chemical bonds. On the basis of these results we conclude that the P(VdF–TrFE) copolymer is a good candidate for use in high-dose gamma dosimetry applications. r 2008 Elsevier B.V. All rights reserved. PACS: 07.85.Fv; 29.40.Wk Keywords: High-dose dosimetry; Gamma dosimetry; P(VdF–TrFE) copolymers

1. Introduction High-dose gamma dosimetry is an essential tool in fields such as food irradiation and surgery equipment sterilization. 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, reliable high-dose dosimetry is required. Calorimetry, alanine and also ceric–cerous sulfate, ECB, ferrous sulfate and dichromate solutions are some examples of standard reference dosimetry systems commercially available elsewhere [1,2]. Thermoluminescence (TL) and optically stimulated luminescence (OSL) Corresponding author. Tel.: +55 31 3069 3128; fax: +55 31 3069 3164.

E-mail address: [email protected] (L.O. Faria). 0168-9002/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.01.081

dosimetry and also polymer-based dosimeters have been alternatively explored for high gamma dose dosimetry [3–6]. However, because of the intrinsic limitations of each dosimetric system, such as small measuring dose range, the development of new high-dose dosimeters is still an interesting and remarkable field of investigation. 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 [7]. On the other hand, the poly (vinylidene fluoride–trifluorethylene) [P(VdF–TrFE)] copolymer is obtained with the random introduction of fluorinated CHF–CF2 monomers in the PVdF main chain. These copolymers have demonstrated to have sensitiveness to high doses of ionizing radiation. For instances, their piezoelectric properties can be highly enhanced after 0.5 MGy of electrons or gamma irradiation [8–10].

ARTICLE IN PRESS A.S. Medeiros, L.O. Faria / Nuclear Instruments and Methods in Physics Research A 587 (2008) 315–318

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Also, the piezoelectric properties can be improved with low-energy (UV) irradiation. It seems that there is a linear correlation between the UV exposure and the number of radiation-induced double bonds, especially the formation of CQC conjugated bonds [11]. All the above-mentioned facts have encouraged us to investigate the possibility of using P(VdF–TrFE) copolymers as high-dose dosimeters. In this work, we propose to investigate the radiationinduced optical absorbance changes in P(VdF–TrFE) copolymers, exploring the ultraviolet–visible (UV–vis) and infrared (FTIR) region of the electromagnetic spectrum, for purposes of high-dose gamma dosimetry. 2. Experimental P(VdF–TrFE) copolymers resins with 50% of TrFE contents were supplied by ATOCHEM (France). The polymeric film samples were produced by melting at 200 1C under 300 bar, and subsequent air-cooling to room temperature. This process produced transparent films of about 170 mm. The samples were irradiated with a 60Co source at a constant dose rate (12 kGy/h), with doses ranging from 0.1 to 300.0 kGy. Optical absorption measurements were taken in a Shimadzu UV-240 PC spectrometer at wavelengths ranging from 190 to 900 nm. The FTIR spectra were measured at a Perkin Elmer Spectrum 100 spectrometer for wavelengths ranging from 200 to 4000 cm1. Measurements were taken immediately after the irradiation process. 3. Results and discussion First of all, let us investigate the behavior of the absorption intensities of gamma-irradiated P(VdF–TrFE) films in the UV–vis range. In Fig. 1 we show the spectrograms for wavelengths ranging from 190 to 900 nm of virgin and gamma-irradiated samples. As it

can be seen, the optical absorption increases for wavelengths between 190 and 450 nm, when the gamma dose increases from 0.3 to 300 kGy. We can also see in Fig. 1 that for increasing gamma dose, the optical absorption around 196, 220, 273 and 410 nm also increases in a gradual manner, suggesting a correlation between the gamma doses and the absorption intensities. The large peak at 196 nm seems to saturate for doses higher than 1.2 kGy while the peak around 410 nm seems to be measurable only for doses higher than 75 kGy. These absorption bands are very similar to those reported in literature for P(VdF–TrFE) copolymer with 30% of TrFE content, irradiated with X-rays or 3 MeV electrons [8,9]. They have attributed these bands to the formation of individual CQC conjugated bonds (185 nm), doublet (223 nm) and triplet (274 nm). In a recent work, we have also reported the appearance of these absorption bands for P(VdF–TrFE) copolymers with 50% of TrFE content exposed to a commercial UV lamp (8 W, 254 nm), during 70 h at 100 1C [11]. In order to identify the absorption peaks related to the CQC conjugated bonds, and also to check if at least one of these peaks shows a linear correspondence with the delivered dose, the optical absorbance spectra of Fig. 1 were peak fitted. In Fig. 2, we show the absorption spectrum for the sample irradiated with 1.2 kGy, resolved into four individual peaks. If only the absorption intensities are adjusted, the sum of these four peaks can also well fit the absorption spectra for the samples irradiated with doses ranging from 0.1 to 300 kGy. In Table 1, we report the peak fitting data using the Lorentzian lines. The data are in agreement with other authors, showing optical absorption peaks at 223 and 274 nm, that may be related to radiation-induced conjugated CQC bonds (doublet and triplet, respectively). The peak at 197 nm is shifted compared to the reported 185 nm for X-rays. We think that it can be explained by the

3

1.6 D = 1.2 kGy 1.2

Absorbance

2

Absorbance (a.u.)

300 kGy

1

0.8

0.4

virgin

0.0 200

0 200

300 Wavelength (nm)

400

Fig. 1. Optical absorption spectra for PVdF copolymers exposed to 0.0, 0.3, 0.6, 1.2, 5, 13, 25, 37, 49, 75, 100 and 300 kGy of gamma radiation.

250

300 350 Wavelength (nm)

400

450

Fig. 2. Optical absorption spectrum for PVdF copolymers exposed to 1.2 kGy (empty circles) and the absorption peaks obtained after peak fitting (solid line).

ARTICLE IN PRESS A.S. Medeiros, L.O. Faria / Nuclear Instruments and Methods in Physics Research A 587 (2008) 315–318

amount of TrFE present in the PVdF copolymer used (30% for Ref. [9] and 50% for this work). The peak fitting data were then used to check if one or more absorption peaks could be used for dosimetric purposes. In Fig. 3(a), we show the linear behavior of the peak absorption intensities at 223 nm for gamma doses ranging from 0.1 to 13 kGy. In Fig. 3(b), we show the linear behavior for the absorption peaks at 223 and 274 nm, respectively, for gamma doses ranging from 1.0 to 100 kGy. The useful dose range for the peak at 223 nm is 0.3 to 75 kGy, once for doses higher than 75 kGy the absorption signal appears to saturate, as it can be seen in Fig. 3(b). On the other hand, the peak absorption intensity at 274 nm can be used to detect gamma doses ranging from 1.0 to 100 kGy. For doses higher than 100 kGy, the optical absorption starts to decrease. Thus, for doses ranging from 1.0 to 75 kGy, it is possible to infer gamma doses at two distinct wavelengths. It should be noted that all the absorption intensities decrease in a regular manner for long periods of time after irradiation. The absorption intensities of the two dosimetric peaks, evaluated for the samples irradiated with 13, 37 and 75 kGy, decrease 50% and 95% for periods of 15 and 30 days after irradiation, respectively. We remark that this fading behavior is very common in most of the commercially available polymer-based dosimetric systems. In PMMA dosimeters, for example, because of their dye content, the manufacturers recommend that the measurement must be done within 2 h following the irradiation process. We may now discuss about the nature of the radiationinduced double bonds. According to the model proposed in

Table 1 Data of the absorbance spectrum peak fitted with Lorentzian lines. The fit is for the 1.2 kGy irradiated sample

Wavelength (nm) Amplitude (arb. units)

Peak 1

Peak 2

Peak 3

Peak 4

197 1.16

223 0.51

274 0.142

321 0.063

Ref. [8], when the ionizing radiation interacts with the copolymer, it is possible to have the following chemical reactions that create the appearance of CQC bonds: 2CH2 2CF2 2 þ photons ! 2CHQCF2 þ HF 2CF2 2CHF2 þ photons ! 2CFQCF2 þ HF 2CF2 2CHF2 þ photons ! 2CFQCH2 þ F2 : By this process, the number of CQC bonds increases with the increase of the irradiation doses, as indicated by the above UV–vis spectroscopy. It is worth noting in Fig. 1 that the double-bond concentration (223 nm) in the virgin sample is non-zero, because of the double bonds present after polymerization. We remark that it is well known that the presence of a double bond along the chain molecule decreases the binding energy of the hydrogen atoms in a position, favoring the production of another radiationinduced conjugated CQC bond, close to the first one. In this way, doublets and triplets are produced. It should be noted that the number of CQC bonds, doublets and triplets are correlated. For instance, the appearing of a doublet is necessarily linked to the disappearing of a singlet one. This can explain why the saturation in the optical absorption intensities comes first to the singlet (195 nm), then to doublets (223 nm) and then to triplets (274 nm). We should remark that the absorption peak around 410 nm, active for doses higher than 75 kGy, may be linked to the appearing of four neighboring CQC bonds and, as the others, could be used for dosimetric purposes. Additional research is currently in course to elucidate this point. Finally, we have measured the FTIR spectra for the irradiated samples in order to check the existence of absorption peaks that could be relevant for gamma dosimetry. The spectrograms are shown in Fig. 4. The only relevant absorption bands that have appeared after gamma irradiation are the 3526 and 3585 cm1 ones. We attribute these absorption bands to the stretching vibrations of NH and OH pendant groups, once all the irradiations were performed free in the air. In this case, these pendant groups could be participating in the

0.9

2.0 223 nm

0.6 0.5 0.4

Absorbance

Absorbance

0.8 0.7

317

223 nm

1.6 1.2 0.8

274 nm

0.4

0.3 1 Dose (kGy)

10

0

25

50 75 Dose (kGy)

100

Fig. 3. The optical absorption intensities at 223 nm (full squares) for doses ranging from 0.1 to 13 kGy (a) and at 223 and 274 nm (open circles) for doses ranging from 1.0 to 100 kGy (b), for P(VdF–TrFE) copolymers.

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A.S. Medeiros, L.O. Faria / Nuclear Instruments and Methods in Physics Research A 587 (2008) 315–318

70 65

60 55

3894.21

50 45

%T

3525

2753.12

3853.77

3585

2504.33

2020.50 2307.42 2205.87

40

1855

35

30 25 297804 30.317

20

1752.61

15

10

4000

3600

3200

2800

2400

2000

1800

1600

cm-1 Fig. 4. FTIR transmission spectra ranging from 1500 to 4000 cm1 for P(VdF–TrFE) samples irradiated with gamma doses ranging from 1.0 kGy (top) to 300 kGy (bottom).

radiation-induced cross-linking bonds. On the other hand, the only possible evidence of CQC bonds in the whole spectra is the increase observed in the 1855 cm1 band, which can be assigned to CQCQC and also to CQO bonds. Once there are strong evidences pointed out by the UV–vis spectrogram, we believe that the increase in this absorption band must be assigned to conjugated CQC bonds. Although FTIR can be used to following the transvinylene content in polyethylenes, which corresponds to irradiation dose, because of the very small changes provoked by gamma irradiation on the absorption peaks it is not possible to use FTIR for dosimetric purposes in gamma irradiated P(VdF–TrFE) copolymer with 50% of TrFE content. 4. Conclusion This investigation has been performed to determine the changes in the optical absorbance spectrum of P(VdF–TrFE) copolymers exposed to high gamma doses, in order to evaluate the possibility of using films from these copolymers as high-dose dosimeters. In the UV–vis spectrum we have found out a strong linear correlation between the gamma dose and the peak intensities at 223 and 274 nm. The absorption peak at 223 nm is the most sensitive to gamma rays and can be used for detecting gamma doses ranging from 0.3 to 75 kGy. Simultaneously, the absorption peak at 274 nm can be used for doses ranging from 1 to 100 kGy. Thus, for gamma doses ranging from 1 to 75 kGy, the great advantage of this new high gamma dosimeter is concerned to the ability in evaluating the exposed dose at two different wavelengths. The dosimetric UV absorption bands are attributed to the appearing of conjugated CQC bonds (doublet and triplet, respectively). FTIR analysis revealed the appearing of OH,

NH and CQC bonds after irradiation. However, their assigned absorption bands are not suitable for dosimetric purposes. The large range of high-dose evaluation and the possibility of measuring doses ranging from 1 to 75 kGy at two different wavelengths lead us to conclude that P(VdF–TrFE) copolymer is a good candidate for use in high gamma dose dosimetry. Acknowledgments The authors acknowledge the financial support from the Brazilian government agencies Conselho Nacional de Pesquisa e Desenvolvimento (CNPq) and Fundac- a˜o de Amparo a` Pesquisa do Estado de Minas gerais (FAPEMIG). References [1] IAEA, Dosimetry for food irradiation, Technical Report Series 409, 2002. [2] ISO, 2002. Guide for Selection and Calibration of Dosimetry Systems for Radiation Processing, ISO/ASTM 51261:2002. [3] K.H. Chadwick, The Choice of Measurement Wavelength for Clear HX-Perspex Dosimetry. Biology and Medicine (Proceedings of the Symposium, Vienna, 1972), IAEA, Vienna, 1973 (pp. 563–576). [4] S.D. Miller, Radiat. Prot. Dosimetry 66 (1996) 201. [5] I. Milman, V. Putyrsky, M. Naimark, V. Popov, Radiat. Prot. Dosimetry 47 (1993) 271. [6] A.H. Ranjbar, M.W. Charles, S.A. Durrani, K. Randle, Radiat. Prot. Dosimetry 65 (1996) 351. [7] A.J. Lovinger, Science 220 (1983) 1115. [8] B. Daudin, J.F. Legrand, F. Macchi, J. Appl. Phys. 70 (1991) 4037. [9] F. Macchi, B. Daudin, A. Ermolieff, S. Marthon, J.F. Legrand, Radiat. Effects Defects Solids 118 (2) (1991) 117. [10] C. Welter, L.O. Faria, R.L. Moreira, Phys. Rev. B 67 (2003) 144103. [11] L.O. Faria, C. Welter, R.L. Moreira, Appl. Phys. Lett. 88 (19) (2006) 192903.