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Study of effects in polyethylene terephthalate films induced by high energy Ar ion irradiation
Nuclear Instruments and Methods in Physics Research B 169 (2000) 78±82
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Study of eects in polyethylene terephthalate ®lms induced by high energy Ar ion irradiation q Changlong Liu *, Zhiyong Zhu, Yunfan Jin, Youmei Sun, Mingdong Hou, Zhiguang Wang, Yanbin Wang, Chonghong Zhang, Xiaoxi Chen, Jie Liu, Baoquan Li Institute of Modern Physics, The Chinese Academy of Sciences, P.O. Box 31, Lanzhou 730000, People's Republic of China
Abstract Semicrystalline polyethylene terephthalate (PET) foil stacks were irradiated with 1.373 GeV Ar ions to dierent ¯uences ranging from 1:0 1010 to 5:0 1012 ions/cm2 . The induced eects were investigated by means of the Fourier transform infrared spectroscopy (FTIR), ultraviolet±visible absorption spectroscopy (UV/VIS) and electron spin resonance spectroscopy (ESR). FTIR measurements show that bond breaking processes are mainly observed at the ethylene glycol residue of trans con®guration and at the para position of benzene rings above a critical dose of about 4.0 MGy. Damage cross-section has been extracted for the band at 973 cmÿ1 from the dependence of the absorbance on ¯uence and it shows a linear dependence on the mean electronic energy loss. UV/VIS measurements show a strong increase in absorbance in the ultraviolet and visible regions. It is found that for the same absorbed dose, more increase in absorbance is induced at higher electronic energy loss. ESR measurements indicate the creation of free radicals. The radical concentration is found to increase rapidly with the increasing absorbed dose above 4.0 MGy. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 79.20.Rf; 61.41.+e Keywords: High energy Ar ion irradiation; Polyethylene terephthalate; Bond breaking; Free radical; Absorbed dose
1. Introduction Recently, high energy ion bombardment induced physico-chemical modi®cations in polymeric materials have been attracting more and q Project is supported by the Foundation of the Chinese Academy of Sciences (KJ952-S1-423), the Foundation of National and Natural Sciences (19775058) and of the Xibuzhiguang. * Corresponding author. Fax: +86-931-888-1100. E-mail address: [email protected] (C. Liu).
more interesting [1,2]. High energy ions in matter lose their energies mainly via electronic processes. By electromagnetic interaction, high concentration of excited and ionized target atoms can be produced around the ion path in a very short time. Dense breaking of atomic bonds and the rearrangements of molecular structures around the ion path result in a heavily modi®ed cylindrical area which is referred to as latent track [3]. Since in the track core the energy deposition by high energy heavy ions is very high and can reach several thousands of eV/nm3 , it is therefore expected that
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 0 2 0 - 3
C. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 78±82
more complex eects and new phenomena can be created in polymeric materials by high energy heavy ion irradiations [4]. Although lots of work have been done in investigating eects in polymeric materials induced by high energy ion irradiation, the dependence of the eect parameters related to ions and materials is still incompletely understood. In this paper, we report on the characterization of structure changes in polyethylene terephthalate (PET) ®lms resulting from irradiation with 1.373 GeV Ar ions. 2. Experimental The PET ®lm used in this study is a commercial semicrystalline ®lm of about 15 lm in thickness. The ®lms were cut to an area of approximately 18 mm 18 mm and piled up in order to make a stack of thickness slightly larger than the ion projected range. PET samples were irradiated under vacuum at room temperature with 35 MeV/u Ar ions at our accelerator HIRFL in Lanzhou. During the irradiation, the beam was defocused so that an area of about 10 mm 10 mm of the sample can be uniformly irradiated. The ¯uence was monitored continuously by measuring the emission of secondary electrons induced when the ions were passed through a stack of Al/Al/Al foils (the thickness of each piece of Al foil is about 8 lm) in front of the samples. The calibrations were carried out by means of a Faraday cup. The ion ¯uence was chosen in the range from 1:0 1010 to 5:0 1012 ions/cm2 . Taking into account the energy loss in the Al foils, the actual energy of Ar ions is about 1.373 GeV. In order to suppress the thermal decomposition of the irradiated samples, the current density was controlled below 1.0 nA. The irradiated samples as well as the pristine PET ®lm were studied by three techniques: 1. Fourier transform infrared absorption spectroscopy (FTIR), 2. Ultraviolet±visible absorption spectroscopy (UV/VIS), and 3. Electron spin resonance spectroscopy (ESR). FTIR measurements were performed on a Perkin± Elmer FTIR-2000 system in transmission mode. Bands which modi®ed after irradiation were ana-
79
lyzed with base line method determining the absorbance of the centroid height. UV/VIS measurements were carried out on a Lambda 9 Perkin± Elmer UV±VIS-NIR spectrometer in transmission mode. ESR measurements were taken using a standard X-band spectrometer of Varian.
3. Results and discussion FTIR measurements of the irradiated samples show that the modi®cations of the chemical structure of PET depend on both the ion ¯uence and the electronic energy loss averaged over the whole sample volume. As an example, Fig. 1(a) and (b) gives two selected areas of the absorption spectra of PET samples before and after irradia-
Fig. 1. Two selected areas of absorption spectra for PET samples before and after irradiation with Ar ions to dierent ¯uences at the mean electronic energy loss of 3.08 keV/nm.
80
C. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 78±82
tion with Ar ions to dierent ¯uences for the same mean electronic energy loss of about 3.08 keV/nm. An overall degradation is evidenced by the decrease of the intensities of most of the original IR bands, which is similar to what was observed in PET ®lms irradiated with keV ions [5]. The bands corresponding to the ethylene glycol residue of the trans con®guration, including 1472, 1387, 973, 849 cmÿ1 , etc., are more susceptible to the ion beam interaction. Bands related to the para-disubstituted benzene rings of PET (e.g., 1507, 1411, 873 cmÿ1 , etc.), show a slower decay as a function of ion ¯uence. The intensity of the band at 1579 cmÿ1 (which is assigned to the normal vibration of benzene rings) remains almost constant for the whole irradiation. These results indicate that the induced bond breaking processes mainly occur at the ethylene glycol residue of trans con®guration and at the para position of benzene rings, and the aromatic system is stable even under irradiation with high-energy heavy ions. The stability of benzene ring under high energy heavy ion irradiation has also been demonstrated by the work of Steckenreiter et al. [6], in which more heavier ions like Kr and Mo were used, and is attributed to their ability in delocalizing the excited energies [7]. In Fig. 2(a), the normalized absorbance A=A0 (absorbance of the irradiated sample/absorbance of the pristine sample) for 973 cmÿ1 is plotted as a function of ion ¯uence for irradiations at dierent mean electronic energy losses. The results clearly indicate that the decay of the band depends on both the ion ¯uence and the mean electronic energy loss. For the same ion ¯uence, the higher the mean electronic energy loss is, the larger the damage induced in chemical structure of PET. Fig. 2(b) shows the relation between the normalized absorbance A=A0 and the absorbed dose D (Gy). The absorbed dose represents an average quantity over the whole irradiated volume. It can be determined by using the following relation: D Gy 1:6 10ÿ10 Utqÿ1 DE=Dx;
1
where Ut (cmÿ2 ) is the ion ¯uence, q (1.397 g/cm3 ) the density of the PET sample, and DE/Dx (MeV/ cm) is the mean energy loss in the ®lm, which can be calculated by using the TRIM code [8]. From
Fig. 2. The normalized absorbance A=A0 of the band at 973 cmÿ1 versus (a) ion ¯uence at various mean electronic energy loss and (b) the absorbed dose D.
Fig. 2(b), one can see that all data points in Fig. 2(a) collapse into one curve when plotted against the absorbed dose. This means that for the same absorbed dose, the eects induced by irradiations at dierent electronic energy loss are the same. Signi®cant decay of the absorption band at 973 cmÿ1 is found to occur only above a critical dose, D0 . D0 can be evaluated by linear extrapolation of the steepest middle part of the decay curve A=A0 versus absorbed dose to A=A0 1, which is found to be about 4.0 MGy. Since the intensity of the band at 973 cmÿ1 decays exponentially with increasing ion ¯uence, the damage cross-section rb for disruption of the corresponding bonds can be obtained by ®tting the experimental data points in Fig. 2(a) to the equation [9] A=A0 exp ÿrb Ut. In Fig. 3 the obtained rb values for 973 cmÿ1 band are plotted
C. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 78±82
Fig. 3. Plot of the damage cross-section rb for disruption of the band at 973 cmÿ1 versus the mean electronic energy loss hSe i. The solid line is a linear ®t to the experimental points.
against the mean electronic energy loss. It can be seen that rb is proportional to the mean electronic energy loss for the studied electronic energy loss ranging from 0.7 to 3.08 keV/nm. As analyzed by Papaleo et al. [10], a linear dependence of rb on electronic energy loss would mean that the tracks are continuous or the damage saturates inside the ion track. Since the decay of the band at 973 cmÿ1 reveals the amorphization of PET under irradiation [11], we therefore concluded that the amorphization occurs continuously along the ion path. Ion beam induced modi®cations in optical absorption of PET samples were further investigated using UV/VIS technique. The typical UV/VIS spectra for PET before and after irradiation with Ar ions to dierent ¯uences for the same electronic energy loss of 3.08 keV/nm, are shown in Fig. 4. One can see that there exists a pronounced short wavelength cut-o in its optical absorption for each of the PET samples. Meanwhile, a strong increase in absorbance in the ultraviolet and visible regions for the irradiated samples is also clearly seen. The increase in absorption may be attributed to the formation of a conjugated system of carbon bonds (chromophore groups) as a consequence of bond breaking and reconstruction induced by ion beam [12]. The dramatic modi®cations in absorption are found to be in agreement with the change of PET sample in color, which varies from transparent for the pristine ®lm to yellowish at the
81
Fig. 4. Ultraviolet±visible absorption spectra of PET ®lms before and after irradiation with 1.373 GeV Ar ion at the same electronic energy loss of 3.08 keV/nm to dierent ¯uences.
middle ion ¯uence, and eventually to dark brown for the highest dose used. The radiation-induced absorbance A ÿ A0 (absorbance of the irradiated ®lm ) absorbance of the pristine ®lm) at wavelength of 350 nm is plotted versus the absorbed dose in Fig. 5. From the ®gure, one can see that for the same absorbed dose, larger changes in optical absorption were induced by irradiations with higher electronic energy losses. It implies that the radiation-induced absorbance is not linear in electronic energy loss. Higher quadratic dependences of absorbances in the ultraviolet and visible ranges on electronic energy loss have been reported for PPS [10] and PS [13].
Fig. 5. The dierence in the UV/VIS absorbance A ÿ A0 versus the absorbed dose for PET samples irradiated with 1.373 GeV Ar ions at various electronic energy losses.
82
C. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 78±82
the decay of the band at 973 cmÿ1 . The linear dependence of the damage cross-section on electronic energy loss implies that the amorphization occurs continuously along the ion path. 4. Formation of free radicals in the irradiated samples has been observed. The concentration of the free radicals increases rapidly with increase of the absorbed dose above 4.0 MGy. Acknowledgements Fig. 6. The free radicals concentration as a function of the absorbed dose for PET samples irradiated with 1.373 GeV Ar ions.
ESR can give information on free radicals produced by ion irradiation, such as chemical species, concentration, aggregation condition and also mobility of radicals [14]. In this paper, ESR technique is used to investigate the creation of free radicals in PET ®lm under 1.373 GeV Ar ion irradiation. High concentration of free radicals has been observed in the irradiated samples, which depends both on ion ¯uence and the mean electronic energy loss. In Fig. 6, we present the dose dependence of the radical concentration. It is found that the concentration of the free radicals increases rapidly with increase of the absorbed dose above 4.0 MGy. 4. Summary Modi®cations of PET ®lms under 1.373 GeV Ar ion irradiation have been studied by means of FTIR, UV±VIS and ESR techniques. The following conclusion can be drawn: 1. High energy Ar ion bombardment can induce dramatic modi®cations in the structure of PET ®lm, such as bond breaking, formation of free radicals and rearrangement of chemical bonds. 2. The bond breaking is mainly at the ethylene glycol residue of the trans con®guration and at the para position of benzene rings. Signi®cant bond breaking occurs at the absorbed doses higher than 4.0 MGy. 3. Damage cross-section for the amorphization of PET under irradiation has been extracted from
The authors wish to thank all the stas for their help in the irradiation of PET samples. The authors also thank Engineer Du Junli, Prof. Tao Yejian and Prof. Feng Liangbo for their help in the FTIR, UV±VIS and ESR measurements respectively, and also for their useful discussions. References [1] D. Fink, F. Hosoi, H. Omichi, Radiat. E. Def. Sol. 132 (1994) 313. [2] E. Balanzat, S. Buard, A. La Moel, N. Betz, Nucl. Instr. and Meth. B 91 (1994) 140. [3] T. Steckenreiter, H. Fuess, D. Stamm, C. Trautmann, Nucl. Instr. and Meth. B 105 (1995) 200. [4] E. Balanzat, N. Betz, S. Buard, Nucl. Instr. and Meth. B 105 (1995) 46. [5] R.M. Papaleo, M.A. de Araujo, R.P. Livi, Nucl. Instr. and Meth. B 65 (1992) 442. [6] T. Steckenreiter, E. Balanzat, H. Fuess, Nucl. Instr. and Meth. B 131 (1995) 159. [7] A. Chapiro, Nucl. Instr. and Meth. B 32 (1988) 111. [8] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in Solids, Pergamon, New York, 1985. [9] M. Salehpour, P. Hakansson, B.U.R. Sundquist, Nucl. Instr. and Meth. B 2 (1984) 752. [10] R.M. Papaleo, A. Hallen, B. Sundqvist, L. Farenzena, R. Livi, M.A. de Araujo, R.E. Johnson, Phys. Rev. B 53 (1996) 2303. [11] C. Liu, Y. Jin, Z. Zhu, Y. Sun, M. Hou, Z. Wang, Y. Wang, C. Zhang, X. Chen, J. Liu, B. Li, Nucl. Instr. and Meth. B 169 (2000) 72. [12] C.N. Rao, Ultarviolet and Visible Spectrocopy ± Chemical Applications, 3rd ed., Butterworth, London, 1975. [13] Z. Zhu, Y. Jin, C. Liu, Y. Sun, M. Hou, C. Zhang, Z. Wang, J. Liu, X. Chen, B. Li, Y. Wang, Nucl. Instr. and Meth. B 169 (2000) 83. [14] B. Wasserman, M.S. Presselhaus, G. Braunstein, J. Electron. Mater. 14 (1985) 157.
www.elsevier.nl/locate/nimb
Study of eects in polyethylene terephthalate ®lms induced by high energy Ar ion irradiation q Changlong Liu *, Zhiyong Zhu, Yunfan Jin, Youmei Sun, Mingdong Hou, Zhiguang Wang, Yanbin Wang, Chonghong Zhang, Xiaoxi Chen, Jie Liu, Baoquan Li Institute of Modern Physics, The Chinese Academy of Sciences, P.O. Box 31, Lanzhou 730000, People's Republic of China
Abstract Semicrystalline polyethylene terephthalate (PET) foil stacks were irradiated with 1.373 GeV Ar ions to dierent ¯uences ranging from 1:0 1010 to 5:0 1012 ions/cm2 . The induced eects were investigated by means of the Fourier transform infrared spectroscopy (FTIR), ultraviolet±visible absorption spectroscopy (UV/VIS) and electron spin resonance spectroscopy (ESR). FTIR measurements show that bond breaking processes are mainly observed at the ethylene glycol residue of trans con®guration and at the para position of benzene rings above a critical dose of about 4.0 MGy. Damage cross-section has been extracted for the band at 973 cmÿ1 from the dependence of the absorbance on ¯uence and it shows a linear dependence on the mean electronic energy loss. UV/VIS measurements show a strong increase in absorbance in the ultraviolet and visible regions. It is found that for the same absorbed dose, more increase in absorbance is induced at higher electronic energy loss. ESR measurements indicate the creation of free radicals. The radical concentration is found to increase rapidly with the increasing absorbed dose above 4.0 MGy. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 79.20.Rf; 61.41.+e Keywords: High energy Ar ion irradiation; Polyethylene terephthalate; Bond breaking; Free radical; Absorbed dose
1. Introduction Recently, high energy ion bombardment induced physico-chemical modi®cations in polymeric materials have been attracting more and q Project is supported by the Foundation of the Chinese Academy of Sciences (KJ952-S1-423), the Foundation of National and Natural Sciences (19775058) and of the Xibuzhiguang. * Corresponding author. Fax: +86-931-888-1100. E-mail address: [email protected] (C. Liu).
more interesting [1,2]. High energy ions in matter lose their energies mainly via electronic processes. By electromagnetic interaction, high concentration of excited and ionized target atoms can be produced around the ion path in a very short time. Dense breaking of atomic bonds and the rearrangements of molecular structures around the ion path result in a heavily modi®ed cylindrical area which is referred to as latent track [3]. Since in the track core the energy deposition by high energy heavy ions is very high and can reach several thousands of eV/nm3 , it is therefore expected that
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 0 2 0 - 3
C. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 78±82
more complex eects and new phenomena can be created in polymeric materials by high energy heavy ion irradiations [4]. Although lots of work have been done in investigating eects in polymeric materials induced by high energy ion irradiation, the dependence of the eect parameters related to ions and materials is still incompletely understood. In this paper, we report on the characterization of structure changes in polyethylene terephthalate (PET) ®lms resulting from irradiation with 1.373 GeV Ar ions. 2. Experimental The PET ®lm used in this study is a commercial semicrystalline ®lm of about 15 lm in thickness. The ®lms were cut to an area of approximately 18 mm 18 mm and piled up in order to make a stack of thickness slightly larger than the ion projected range. PET samples were irradiated under vacuum at room temperature with 35 MeV/u Ar ions at our accelerator HIRFL in Lanzhou. During the irradiation, the beam was defocused so that an area of about 10 mm 10 mm of the sample can be uniformly irradiated. The ¯uence was monitored continuously by measuring the emission of secondary electrons induced when the ions were passed through a stack of Al/Al/Al foils (the thickness of each piece of Al foil is about 8 lm) in front of the samples. The calibrations were carried out by means of a Faraday cup. The ion ¯uence was chosen in the range from 1:0 1010 to 5:0 1012 ions/cm2 . Taking into account the energy loss in the Al foils, the actual energy of Ar ions is about 1.373 GeV. In order to suppress the thermal decomposition of the irradiated samples, the current density was controlled below 1.0 nA. The irradiated samples as well as the pristine PET ®lm were studied by three techniques: 1. Fourier transform infrared absorption spectroscopy (FTIR), 2. Ultraviolet±visible absorption spectroscopy (UV/VIS), and 3. Electron spin resonance spectroscopy (ESR). FTIR measurements were performed on a Perkin± Elmer FTIR-2000 system in transmission mode. Bands which modi®ed after irradiation were ana-
79
lyzed with base line method determining the absorbance of the centroid height. UV/VIS measurements were carried out on a Lambda 9 Perkin± Elmer UV±VIS-NIR spectrometer in transmission mode. ESR measurements were taken using a standard X-band spectrometer of Varian.
3. Results and discussion FTIR measurements of the irradiated samples show that the modi®cations of the chemical structure of PET depend on both the ion ¯uence and the electronic energy loss averaged over the whole sample volume. As an example, Fig. 1(a) and (b) gives two selected areas of the absorption spectra of PET samples before and after irradia-
Fig. 1. Two selected areas of absorption spectra for PET samples before and after irradiation with Ar ions to dierent ¯uences at the mean electronic energy loss of 3.08 keV/nm.
80
C. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 78±82
tion with Ar ions to dierent ¯uences for the same mean electronic energy loss of about 3.08 keV/nm. An overall degradation is evidenced by the decrease of the intensities of most of the original IR bands, which is similar to what was observed in PET ®lms irradiated with keV ions [5]. The bands corresponding to the ethylene glycol residue of the trans con®guration, including 1472, 1387, 973, 849 cmÿ1 , etc., are more susceptible to the ion beam interaction. Bands related to the para-disubstituted benzene rings of PET (e.g., 1507, 1411, 873 cmÿ1 , etc.), show a slower decay as a function of ion ¯uence. The intensity of the band at 1579 cmÿ1 (which is assigned to the normal vibration of benzene rings) remains almost constant for the whole irradiation. These results indicate that the induced bond breaking processes mainly occur at the ethylene glycol residue of trans con®guration and at the para position of benzene rings, and the aromatic system is stable even under irradiation with high-energy heavy ions. The stability of benzene ring under high energy heavy ion irradiation has also been demonstrated by the work of Steckenreiter et al. [6], in which more heavier ions like Kr and Mo were used, and is attributed to their ability in delocalizing the excited energies [7]. In Fig. 2(a), the normalized absorbance A=A0 (absorbance of the irradiated sample/absorbance of the pristine sample) for 973 cmÿ1 is plotted as a function of ion ¯uence for irradiations at dierent mean electronic energy losses. The results clearly indicate that the decay of the band depends on both the ion ¯uence and the mean electronic energy loss. For the same ion ¯uence, the higher the mean electronic energy loss is, the larger the damage induced in chemical structure of PET. Fig. 2(b) shows the relation between the normalized absorbance A=A0 and the absorbed dose D (Gy). The absorbed dose represents an average quantity over the whole irradiated volume. It can be determined by using the following relation: D Gy 1:6 10ÿ10 Utqÿ1 DE=Dx;
1
where Ut (cmÿ2 ) is the ion ¯uence, q (1.397 g/cm3 ) the density of the PET sample, and DE/Dx (MeV/ cm) is the mean energy loss in the ®lm, which can be calculated by using the TRIM code [8]. From
Fig. 2. The normalized absorbance A=A0 of the band at 973 cmÿ1 versus (a) ion ¯uence at various mean electronic energy loss and (b) the absorbed dose D.
Fig. 2(b), one can see that all data points in Fig. 2(a) collapse into one curve when plotted against the absorbed dose. This means that for the same absorbed dose, the eects induced by irradiations at dierent electronic energy loss are the same. Signi®cant decay of the absorption band at 973 cmÿ1 is found to occur only above a critical dose, D0 . D0 can be evaluated by linear extrapolation of the steepest middle part of the decay curve A=A0 versus absorbed dose to A=A0 1, which is found to be about 4.0 MGy. Since the intensity of the band at 973 cmÿ1 decays exponentially with increasing ion ¯uence, the damage cross-section rb for disruption of the corresponding bonds can be obtained by ®tting the experimental data points in Fig. 2(a) to the equation [9] A=A0 exp ÿrb Ut. In Fig. 3 the obtained rb values for 973 cmÿ1 band are plotted
C. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 78±82
Fig. 3. Plot of the damage cross-section rb for disruption of the band at 973 cmÿ1 versus the mean electronic energy loss hSe i. The solid line is a linear ®t to the experimental points.
against the mean electronic energy loss. It can be seen that rb is proportional to the mean electronic energy loss for the studied electronic energy loss ranging from 0.7 to 3.08 keV/nm. As analyzed by Papaleo et al. [10], a linear dependence of rb on electronic energy loss would mean that the tracks are continuous or the damage saturates inside the ion track. Since the decay of the band at 973 cmÿ1 reveals the amorphization of PET under irradiation [11], we therefore concluded that the amorphization occurs continuously along the ion path. Ion beam induced modi®cations in optical absorption of PET samples were further investigated using UV/VIS technique. The typical UV/VIS spectra for PET before and after irradiation with Ar ions to dierent ¯uences for the same electronic energy loss of 3.08 keV/nm, are shown in Fig. 4. One can see that there exists a pronounced short wavelength cut-o in its optical absorption for each of the PET samples. Meanwhile, a strong increase in absorbance in the ultraviolet and visible regions for the irradiated samples is also clearly seen. The increase in absorption may be attributed to the formation of a conjugated system of carbon bonds (chromophore groups) as a consequence of bond breaking and reconstruction induced by ion beam [12]. The dramatic modi®cations in absorption are found to be in agreement with the change of PET sample in color, which varies from transparent for the pristine ®lm to yellowish at the
81
Fig. 4. Ultraviolet±visible absorption spectra of PET ®lms before and after irradiation with 1.373 GeV Ar ion at the same electronic energy loss of 3.08 keV/nm to dierent ¯uences.
middle ion ¯uence, and eventually to dark brown for the highest dose used. The radiation-induced absorbance A ÿ A0 (absorbance of the irradiated ®lm ) absorbance of the pristine ®lm) at wavelength of 350 nm is plotted versus the absorbed dose in Fig. 5. From the ®gure, one can see that for the same absorbed dose, larger changes in optical absorption were induced by irradiations with higher electronic energy losses. It implies that the radiation-induced absorbance is not linear in electronic energy loss. Higher quadratic dependences of absorbances in the ultraviolet and visible ranges on electronic energy loss have been reported for PPS [10] and PS [13].
Fig. 5. The dierence in the UV/VIS absorbance A ÿ A0 versus the absorbed dose for PET samples irradiated with 1.373 GeV Ar ions at various electronic energy losses.
82
C. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 78±82
the decay of the band at 973 cmÿ1 . The linear dependence of the damage cross-section on electronic energy loss implies that the amorphization occurs continuously along the ion path. 4. Formation of free radicals in the irradiated samples has been observed. The concentration of the free radicals increases rapidly with increase of the absorbed dose above 4.0 MGy. Acknowledgements Fig. 6. The free radicals concentration as a function of the absorbed dose for PET samples irradiated with 1.373 GeV Ar ions.
ESR can give information on free radicals produced by ion irradiation, such as chemical species, concentration, aggregation condition and also mobility of radicals [14]. In this paper, ESR technique is used to investigate the creation of free radicals in PET ®lm under 1.373 GeV Ar ion irradiation. High concentration of free radicals has been observed in the irradiated samples, which depends both on ion ¯uence and the mean electronic energy loss. In Fig. 6, we present the dose dependence of the radical concentration. It is found that the concentration of the free radicals increases rapidly with increase of the absorbed dose above 4.0 MGy. 4. Summary Modi®cations of PET ®lms under 1.373 GeV Ar ion irradiation have been studied by means of FTIR, UV±VIS and ESR techniques. The following conclusion can be drawn: 1. High energy Ar ion bombardment can induce dramatic modi®cations in the structure of PET ®lm, such as bond breaking, formation of free radicals and rearrangement of chemical bonds. 2. The bond breaking is mainly at the ethylene glycol residue of the trans con®guration and at the para position of benzene rings. Signi®cant bond breaking occurs at the absorbed doses higher than 4.0 MGy. 3. Damage cross-section for the amorphization of PET under irradiation has been extracted from
The authors wish to thank all the stas for their help in the irradiation of PET samples. The authors also thank Engineer Du Junli, Prof. Tao Yejian and Prof. Feng Liangbo for their help in the FTIR, UV±VIS and ESR measurements respectively, and also for their useful discussions. References [1] D. Fink, F. Hosoi, H. Omichi, Radiat. E. Def. Sol. 132 (1994) 313. [2] E. Balanzat, S. Buard, A. La Moel, N. Betz, Nucl. Instr. and Meth. B 91 (1994) 140. [3] T. Steckenreiter, H. Fuess, D. Stamm, C. Trautmann, Nucl. Instr. and Meth. B 105 (1995) 200. [4] E. Balanzat, N. Betz, S. Buard, Nucl. Instr. and Meth. B 105 (1995) 46. [5] R.M. Papaleo, M.A. de Araujo, R.P. Livi, Nucl. Instr. and Meth. B 65 (1992) 442. [6] T. Steckenreiter, E. Balanzat, H. Fuess, Nucl. Instr. and Meth. B 131 (1995) 159. [7] A. Chapiro, Nucl. Instr. and Meth. B 32 (1988) 111. [8] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in Solids, Pergamon, New York, 1985. [9] M. Salehpour, P. Hakansson, B.U.R. Sundquist, Nucl. Instr. and Meth. B 2 (1984) 752. [10] R.M. Papaleo, A. Hallen, B. Sundqvist, L. Farenzena, R. Livi, M.A. de Araujo, R.E. Johnson, Phys. Rev. B 53 (1996) 2303. [11] C. Liu, Y. Jin, Z. Zhu, Y. Sun, M. Hou, Z. Wang, Y. Wang, C. Zhang, X. Chen, J. Liu, B. Li, Nucl. Instr. and Meth. B 169 (2000) 72. [12] C.N. Rao, Ultarviolet and Visible Spectrocopy ± Chemical Applications, 3rd ed., Butterworth, London, 1975. [13] Z. Zhu, Y. Jin, C. Liu, Y. Sun, M. Hou, C. Zhang, Z. Wang, J. Liu, X. Chen, B. Li, Y. Wang, Nucl. Instr. and Meth. B 169 (2000) 83. [14] B. Wasserman, M.S. Presselhaus, G. Braunstein, J. Electron. Mater. 14 (1985) 157.