Method to measure composition modifications in polyethylene terephthalate during ion beam irradiation

Nuclear Instruments and Methods in Physics Research B 267 (2009) 108–112

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Method to measure composition modifications in polyethylene terephthalate during ion beam irradiation M. Abdesselam a, J.P. Stoquert b,*, S. Chami a, M. Djebara a, A.C. Chami a, M. Siad c a

Faculté de Physique, Université des Sciences et de la Technologie d’Alger, BP 32, El Alia, 16111 Bab Ezzouar, Algeria Institut d’Electronique du Solide et des Systèmes, UMR 7163, 23 rue du Loess – BP20, F-67037 Strasbourg Cedex 02, France c Centre de Recherche Nucléaires d’Alger – COMENA, 02 Blvd Frantz Fanon, BP Alger-gare, Algeria b

a r t i c l e

i n f o

Article history: Received 4 April 2008 Received in revised form 13 October 2008 Available online 1 November 2008 PACS: 61.80.Az 61.80.Jh 61.82.Pv Keywords: Polyethylene terephthalate Polymer irradiation Radiolysis Forward elastic scattering Nuclear reaction analysis

a b s t r a c t Matter losses of polyethylene terephthalate (PET, Mylar) films induced by 1600 keV deuteron beams have been investigated in situ simultaneously by nuclear reaction analysis (NRA), deuteron forward elastic scattering (DFES) and hydrogen elastic recoil detection (HERD) in the fluence range from 1  1014 to 9  1016 cm2. Volatile degradation products escape from the polymeric film, mostly as hydrogen-, oxygen- and carbon-containing molecules. Appropriate experimental conditions for observing the composition and thickness changes during irradiation are determined. 16O(d,p0)17O, 16O(d,p1)17O and 12C(d,p0)13C nuclear reactions were used to monitor the oxygen and carbon content as a function of deuteron fluence. Hydrogen release was determined simultaneously by H(d,d)H DFES and H(d,H)d HERD. Comparisons between NRA, DFES and HERD measurements show that the polymer carbonizes at high fluences because most of the oxygen and hydrogen depletion has already occured below a fluence of 3  1016 cm2. Release curves for each element are determined. Experimental results are consistent with the bulk molecular recombination (BMR) model. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Physical and chemical changes in polymeric materials induced by ion beam irradiation [1,2] and their low production cost make many of them suitable for commercial applications. However, many fundamental questions related to the degradation processes during ion beam irradiation and basic ion-collision phenomenons remain unsolved [3–6]. When striking a polymer, accelerated ions induce irreversible structural and chemical damage within a small volume surrounding the ion trajectory [7] where the deposited energy density can reach several keV/nm3. All studies confirm a gradual microstructure modification related to cross-linking, bond and chain scission, production of volatile species and morphology changes in polymers with increasing irradiation fluence [1,3–7]. The degradation process is related to the stopping cross section [8]. In the MeV range, deuteron projectiles deposit their energy mainly via electronic interactions and the transferred energies largely exceed the primary ionization and excitation energies required to cleave any polymer bond. After bond rearrangements, or returns to original configurations, some radicals remain free, and depending on * Corresponding author. Tel.: +33 3 88106252; fax: +33 3 88106548. E-mail address: [email protected] (J.P. Stoquert). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.10.048

their density can combine to form molecules which escape from the polymer. Here we report results obtained on polyethylene terephthalate (PET) films irradiated with 1600 keV deuterons at fluences from 1  1014 to 9  1016 cm2 in order to study the evolution of hydrogen, carbon and oxygen contents during irradiation. The incident energy and ion have been chosen because: – the range of the deuterons is much greater than the polymer film thickness and their stopping power can be considered constant in the film, – the nuclear reaction analysis (NRA) differential cross sections remain almost constant, – measurements by three different ion beam analysis (IBA) techniques: NRA [9], deuteron forward elastic scattering (DFES) [10] and hydrogen elastic recoil detection (HERD) [10] are possible to determine simultaneously changes in atomic hydrogen, carbon and oxygen composition, – the deuteron beam can be used at the same time for irradiation and analysis, some information being redundant. DFES permits both monitoring of the hydrogen loss and thickness decrease (H(d,d)H) and also gives data on the evolution of the sum of carbon and oxygen contents (12C(d,d)12C + 16O(d,d)16O).

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2. Experimental setup The PET irradiations using deuteron beams were performed at the 4 MV Van de Graaff accelerator of the InESS laboratory in Strasbourg. The accuracy of the current measurement was better than 5%. At the entrance of the target chamber 15 cm from the impact point on the target was placed a 1 mm diameter collimator. A Faraday cup behind the target monitored the transmitted beam current. The polymer films were irradiated at room temperature with the beam current kept at approximately 10 nA to avoid thermal degradation of the polymers. The total fluence was in the range 1  1014–9  1016 cm2. The PET (C10H8O4) foils used in the present study are commercially available from Somar International, Inc. and their 3.6 lm thickness was verified by energy loss measurements of 2000–2600 keV He+ ions backscattered from an Au foil [12]. Their homogeneity and uniformity within 5% was checked by scanning their surface using NRA. A defocalized ion beam was used to ensure beam uniformity of the irradiated spots. All measurements were performed several times. The irradiated films were inspected by optical microscopy and data from foils with artifacts were not retained. For 1600 keV incident deuterons the energy loss in the film is 136 keV, calculated using the SRIM08 code [13]. The approximately constant differential cross sections for the 12C(d,p0)13C and 16 O(d,p1)17O* reactions are 35 and 8 mb/sr, respectively [14–16]. The experimental errors on the mass loss quantification depend on the precision of the charge measurement and on statistical errors of peak areas. The cumulated uncertainty is estimated to be ±5%. A large 314 mm2 silicon surface barrier detector with an energy resolution of 25 keV was positioned in the backward direction at a scattering angle hNR = 150° to collect NRA products. To reduce geometrical straggling, a rectangular slit (6  18 mm2) was mounted in front of the detector which was covered with a 12 lm thick Mylar foil to stop backscattered deuterons and a-particles from the 16 O(d,a)14N nuclear reaction [15] and avoid overlapping NRA peaks. Protons from the 16O(d,p0)17O and 16O(d,p1)17O* reactions are emitted at 2851 and 2067 keV, respectively, and protons from the 12C(d,p)13C reaction with energy 3372 keV are simultaneously collected by this Si detector. A 25 mm2 silicon surface barrier detector with an energy resolution of 16 keV collected recoils and forward scattered deuterons at hFS = 24° with a reduced acceptance angle to limit counting rates. The hydrogen recoils were separated from H(d,d)H scattered deuterons by a 3.6 lm thick Mylar foil mounted in front of this detector which gathered also NRA complementary information on the oxygen and carbon loss. All peaks were normalized to the beam fluence and to the original hydrogen, carbon or oxygen content of the PET. 3. Results and discussion 3.1. NRA of oxygen and carbon Fig. 1 shows typical NRA spectra for 1600 keV deuterons on a 3.6 lm PET film at several fluences. Assuming a homogeneous

PET composition of the irradiated region at a given fluence / and constant nuclear differential cross sections over the film thickness, the evolution of the ratio a(/) between the areal density n(/)X(/) at fluence / relative to the initial areal density n(0)X(0) is given by

að/Þ ¼

nð/ÞXð/Þ Að/Þ ¼ ; nð0ÞXð0Þ Að0Þ

ð1Þ

where A(/), n(/) and X(/) are peak area, atomic density and film thickness, respectively. The measured initial oxygen to carbon atomic ratio c(0) in PET is given by

cO=C ð0Þ ¼

nO ð0Þ AO ð0Þ ¼ nC ð0Þ AC ð0Þ

rNR C ðE0 Þ ¼ 0:392; rNR O ðE0 Þ

ð2Þ

where rNR i ðE0 Þ is the differential cross section for 1600 keV deuterons on element i. No oxygen and carbon release was measurable on fresh Mylar irradiated at low ion fluences (0.2, 0.3 and 0.4  1015 cm2) because differences in the NRA peaks were not observed and the initial experimental cO/C(0) = 4/10 remained in agreement with PET stoichiometry within uncertainties. The oxygen and carbon release with increasing fluence is seen in Fig. 1 by changes in both nuclear reaction peak areas and peak width. The ratios aC(d,p)(/) and aOðd;p0;1 Þ ð/Þ giving the magnitude of the release are plotted in Fig. 2 as a function of fluence. Similar trends for oxygen and carbon release occur, but a larger oxygen loss is observed. At the highest ion fluences the carbon loss is of the order of 15–20% of its initial value in the original PET and the carbon concentration begins to remain constant at around 2  1016 cm2 where a stable state is reached. This low rate of carbon release is attributable to the strong bond energies of aromatic rings. The material depletion of PET is dominated high oxygen loss, which exceeds 90% at a fluence of 9  1016 cm2 and continues to diminish at higher fluences, although with a lower depletion rate. Modifications in carbon and oxygen contents are accompanied by visual changes of beam spots on the PET samples that show darkening, shrinkage and brittleness after irradiation at a fluence greater than 1016 cm2. The bulk molecular recombination model (BMR), proposed by Adel et al. [11] to explain hydrogen loss under irradiation, describes the release dynamics in terms of statistical processes. This

Fresh

1200

15

-

16

-

C(d,p)

4 x 10 cm 2

C(d,p)

2 x 10 cm 2 16

-

9,5 x 10 cm 2

900 0(d,p1)

Fresh End of irradiation

8

X

Counts

The hydrogen content is also determined by HERD (H(d,H)d), whereas oxygen and carbon are measured separately by NRA (16O(d,p0)17O, 16O(d,p1)17O and 12C(d,p0)13C). Hydrogen, carbon and oxygen contents are followed as a function of fluence. It can be mentioned that release measurements of one element only (for example H by HERD) do not provide enough information to deduce compositional changes. Finally, all concentrations measured simultaneously are analysed in the framework of the bulk molecular recombination (BMR) model [11].

740

600

760

800

Fresh 4 x 10 2 x 10

300

0

780

15

cm 2

16

cm 2

9.5 x 10

400

500

-

16

-

0(d,p0) X8

cm 2

600

700

800

Channel Fig. 1. NRA spectra for 1600 keV deuterons on PET film at various fluences. In the inset, PET thickness decrease as a function of fluence is shown by the enlarged proton peak from 12C(d,p0)13C reaction.

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Typical spectra at several fluences are represented in Fig. 3. The DFES peak located around channel 750 corresponds to incident deuterons scattered by carbon and oxygen. These two contributions are not separated because their kinematic factors are too close, but their sum can be compared to the sum of corresponding NRA peaks for consistency. The other two peaks contain detailed information on the hydrogen in the irradiated film. The collision kinematics indicate that the HERD peak (H(d,H)d) is located near channel 580 whereas the DFES peak (H(d,d)H) is observed around channel 475. We have measured, step by step, simultaneously with the NRA spectra during the irradiation:

1.0

0.8

O(d,p ) 0

0.9

O(d,p ) 1

α

C(d,p ) 0 BMR fit

0.4

0.6

α

0.6

0.3 30

0.0

1

10

– the DFES peak area for oxygen and carbon AOCðd;dÞ ð/Þ ¼ ACðd;dÞ ð/Þ þ AOðd;dÞ ð/Þ, – the DFES peak area for hydrogen AH(d,d)H(/), – the HERD peak area AH(d,H)d(/).

40 60 80

0.2

0

20

40

60

80

100

15

φ (10 cm-2 ) Fig. 2. Carbon (aC) and oxygen (aO) release versus ion fluence. The solid curves are least squares fits to Eq. (3) for the bulk molecular recombination (BMR) model.

model which assumes that atoms leave the irradiated material in molecular form has been extended and improved by Marée et al. [17] and de Jong et al. [18]. We assume the general functional dependence for oxygen and carbon release described by the following BMR equation:





að/Þ ¼ 1= 1=af  ð1=a0  1=af ÞeK/ ;

ð3Þ

where af is the carbon or oxygen ratio at the highest measured fluence and K is an effective molecular release cross section. Fig. 2 shows fits of the experimental data to Eq. (3) with af = 0.059 and 0.867 for oxygen and carbon, respectively. The corresponding cross sections are K = 6.7  1018 and 1.1  1016 cm2, respectively. The assumption that molecular release disappears [11] when the oxygen atomic density in the material falls below some critical value is not borne out in our experiments. Liberated free radicals can diffuse over long distances before being trapped or secondary ionizations may occur outside the ion track, so the probability of finding partners to form releasable oxygen-containing molecules is increased. These recombination possibilities can explain the quasi-complete oxygen release at a fluence of 9.5  1016 cm2 where the oxygen loss is much reduced but not completely stopped. A more complete understanding of oxygen depletion mechanisms will be necessary to offer an explanation for the oxygen density drop. 3.2. DFES and HERD measurements of H The degradation processes during deuteron irradiation lead to hydrogen release from the target at the same time as the oxygen and carbon impoverishment. HERD measurements in a reflection geometry [10] using a deuteron beam are not appropriate to study the hydrogen content of irradiated films, for only the near surface region can be explored and quantitative analysis is inaccurate. A transmission geometry is better adapted because both scattering and recoil events can be recorded (DFES and HERD) and quantitative analysis over the entire thickness can be deduced from peak areas similarly as for NRA. All elements seen in DFES spectra in the transmission geometry were registered simultaneously with NRA as a function of ion fluence to correlate the evolution of all PET volatile species.

Assuming again a homogenous PET composition of the irradiated region at a given fluence /, the evolution of the ratio a(/) between the areal density n(/)X(/) and the initial areal density is given by

að/Þ ¼ ½nð/ÞXð/Þ=½nð0ÞXð0Þ ¼ ½Að/Þ=Að0Þ½Em ð0Þ=Em ð/Þ;

ð4Þ

where Em(/) is the mean energy of the incident beam in the sample. The hydrogen evolutions measured by the HERD and DFES methods are reported in Fig. 4. The two results are in quite good agreement and the front edge shifts of the peaks confirm the film thickness decrease with increasing fluence. From the shape of the hydrogen ratio curve aH(/) one can deduce that the hydrogen loss process might be similar to that of oxygen and carbon. The hydrogen concentration falls to approximately 65% of its initial value at a fluence of 9.5  1016 cm2. Several authors have reported for a variety of materials [19–23] the same behavior of the hydrogen content versus ion fluence. As for the oxygen loss, we note that: – up to a fluence of 2  1016 cm2, hydrogen release accompanies the carbon loss, – above a fluence of 2  1016 cm2, when the carbon release stops, hydrogen release is reduced because it continues to leave the film as H2 only, – the hydrogen content decrease does not completely stop, even at a fluence of 9  1016 cm2.

4000

Fresh

C(d,d)C + O(d,d)O

Fresh

15

2000

700

725

750

Channel

775

-

6 x 10 cm 2 16

-

9.5 x 10 cm 2

3000

Counts

110

C(d,d) + O(d,d)

HERD x8

DFES x8 1000

0

400

500

600

700

800

Channel Fig. 3. DFES and HERD spectra for 1600 keV deuterons on PET film at various fluences. In the insert, detail of the PET thickness decrease as a function of fluence is shown by the enlarged (C + O) sum peak shifts during irradiation.

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1.0

H(d,d)H H(d,H)d

1.0

1

0.8

αH

α (φ )

0.9

0.6 0.1

0.8

60

80

12 From Eq. (5).

20

40

60

80

1

100

φ (1015 cm-2 ) Fig. 4. Hydrogen release (aH) versus ion fluence. The solid curves are least squares fits to Eq. (3) for the bulk molecular recombination (BMR) model.

Fig. 4 shows the fit of the experimental hydrogen data to Eq. (3) with af = 0.656 and are K = 45.4  1018 cm2. However, we notice that carbon loss can be due either to the formation of a light hydrocarbon [24] or to the formation of CO and CO2 molecules [8,24–27]. This suggests that oxygen and hydrogen play a significant role in the release of carbon. When the carbon loss ceases, hydrogen and oxygen leave the foil as H2 and O2 only. Several mechanisms may therefore be involved in the hydrogen and hydrocarbon release. The BMR model used here, which supposes an independent release of oxygen, carbon and hydrogen represents a simplified description of the mechanisms involved during irradiation. The good quality of fit probably indicates that the statistical nature of the release processes is correctly described, but a more sophisticated description of all mechanisms involved in the releases will be necessary to deduce physically relevant parameters.

C+

16

16

0.2

0

C H

0.6

0.4

0.7

0.6

12

0.8

αH

H(d,d)H H(d,H)d BMR fit

1.0

20 40 15 -2 φ (10 cm )

60

O

O

80

Fig. 5. Recapitulation of the normalized areal density evolution of hydrogen, carbon and oxygen in PET films irradiated with 1600 keV deuterons measured by NRA, HERD and DFES as a function of irradiation fluence /. Comparison of hydrogen between NRA and DFES using Eq.(5) shows good agreement between the two measurements.

– no measurable material loss at the beginning of the irradiation below 5  1014 cm2, – similar shapes of a(/) curves, but with different effective molecular release cross sections. These trends are clearly seen for irradiations in the fluence range from 1 to 6  1016 cm2, – at high fluences (over 6  1016 cm2) a stable state is approached, but further small releases cannot be excluded. The physics governing the release mechanisms is known to depend on specific bond breaking and recombination probabilities, and different trapping and detrapping efficiencies modify the gas loss. With increasing fluence the increase in damage enhances the number of trapping centers and therefore the molecular release is partially inhibited and the release rate diminishes with long irradiation time. 4. Conclusion

3.3. Carbon and oxygen measurements and comparison to NRA results In order to compare NRA and DFES oxygen and carbon measurements, we consider the unresolved peak of the forward elastic scattering on oxygen and carbon. The area ratio AOC(d,d)(/)/AOC(d,d)(0) can also be written as a combination of oxygen and carbon contributions deduced from NRA measurements:

 2 Z0 AOCðd;dÞ ð/Þ Em ð0Þ 1 þ c0=C ð/Þ ZC ¼ aC ðf Þ  2 ; Em ð/Þ AOCðd;dÞ ð0Þ 1 þ c0=C ð0Þ ZZC0

ð5Þ

where we assume that: – the atom release along the ion beam track is uniform, – the forward elastic differential cross sections are Coulombic. The DFES ratio AOC(d,d)(/)/AOC(d,d)(0) measured as a function of ion fluence is presented in Fig. 5 and compared to the calculations according to Eq. (5). The two curves are in good agreement over the entire fluence range, confirming the consistency of the experimental data. Fig. 5 shows several common characteristics and a similar trend:

To understand how molecular release occurs during ion-polymer interactions, we have irradiated PET films with 1600 keV deuteron beams at fluences ranging from 1  1014 to 9  1016 cm2 at a beam current of 10 nA. Using appropriate geometrical conditions in the experimental setup, hydrogen, carbon and oxygen depletion have been measured simultaneously by hydrogen elastic recoil, deuteron forward scattering and nuclear reactions 16O(d,p0,1)17O and 12C(d,p)13C. The passage of deuterons through the polymer causes electronic excitation and ionization, which results in the breaking of various bonds liberating hydrogen, carbon and oxygen atoms. Nascent oxygen and hydrogen combine with other atoms to produce different gas releases (H2, O2, CO, CO2 and hydrocarbons). These gaseous molecules having high diffusivity escape from the polymer, causing reduction in atomic content. Similar trends for hydrogen, oxygen and carbon loss occur, but a greater oxygen loss is observed. At fluences between 1  1015 and 1  1016 cm2 the deuteron beam induces a large loss of oxygen and hydrogen. Carbon loss stops around a fluence of 2  1016 cm2 and the PET is progressively transformed into an amorphous carbon layer. Most oxygen and hydrogen mass loss has already occurred at a fluence of 2  1016 cm2, but it continues to diminish at higher fluences, even if the depletion rate is lower. At the highest fluences, the

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PET films have lost approximately 15% of the carbon, 90% of the oxygen and 35% of the hydrogen of their initial content. These compositional changes induce a decrease in the PET film thickness. Good fits of the release curves have been obtained in the framework of the BMR model. We have seen however that the loss of a particular species is correlated with the behavior of the other species, and a description of the overall concentration evolution will necessitate an improved BMR description. We can mention that the IBA method with a 1600 keV deuteron beam as described here could be generalized to derive hydrogen, carbon and oxygen concentration modifications for all types of polymers and all types of irradiation. Acknowledgements The technical assistance of Y. Legall and A. Midouni during the experiments is greatly appreciated. References [1] Irene T.S. Garcia, F.C. Zawislakb, D. Samiosc, Appl. Surf. Sci. 228 (2004) 63; Y.Q. Wang, Nucl. Instr. and Meth. B 161/163 (2000) 1027. [2] J.W. Park, C.W. Sohn, B.H. Choi, Curr. Appl. Phys. 6 (2006) 188. [3] E.H. Lee, G.R. Rao, M.B. Lewis, L.K. Mansur, J. Mater. Res. 9 (1994) 1043. [4] O. Puglisi, A. Licciardello, L. Calcagno, G. Foti, Nucl. Instr. and Meth. B 19–20 (1987) 865. [5] Y.Q. Wang, R.E. Giedd, L.B. Bridwell, Nucl. Instr. and Meth. B 79 (1993) 659; O. Fageeha, J. Howard, F.L.C. Block, J. Appl. Phys. 75 (1994) 2317; E.H. Lee, Nucl. Instr. and Meth. B 151 (1999) 29. [6] J. Davenas, X.L. Xu, G. Boiteux, D. Sage, Nucl. Instr. and Meth. B 39 (1989) 754; A. Chapiro, Nucl. Instr. and Meth. B 105 (1995) 5.

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