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ESR investigations on ion beam irradiated polycarbonate
Nuclear Instruments and Methods in Physics Research B 88 (1994) 418-422 North-Holland
ESR investigations
on ion beam irradiated
M.I. ChiparS a, V.V. Grecu b, P.V. Notingher
Beam Interactions with Materials 8 Atoms
polycarbonate
‘T*, J. Reyes Romero d and M.D. Chiparg e
a Institute for Physics and Technology of Materials, P.O. Box MG-7, Magurele, Bucharest, R-76900, Romania b University of Bucharest, Faculty of Physics, P.O. Box MG-II, Magurele, Bucharest, R-76900, Romania ’ University Politehnica of Bucharest, Electrotechnical Faculty, 313, Splaiul Independentei, Str., 77206 Bucharest, Romania d Universidad Central de Venezuela, Fact&ad de Ingineria, Dpto de fuica Aplicada, Ciudad Universitaria, Chaguaramos, Caracas, Venezuela e Research Institute for Electrotechnics, 45- 47 Tudor Vladimirescu, Bd., Bucharest, R-79423, Romania
Received 27 July 1993 and in revised form 14 February 1994
Electron spin resonance (ESR) investigations with a polycarbonate solid state nuclear detector, irradiated with oxygen ions, are reported. The nature of the paramagnetic defects induced by irradiation is discussed. The temperature dependence of resonance line parameters is studied. From the experimental data, obtained by ESR, spectroscopy, the activation energy for defect recombination, the average isotropic exchange integral between paramagnetic defects as well as the average distance between defects, are estimated. Correlations with latent tracks structure are discussed.
1. Introduction
Electron spin resonance spectroscopy (ESR) is a powerful tool in the investigation of radiation induced paramagnetic defects (free radicals). From the resonance line parameters (g value, resonance line width, line intensity, shape, anisotropy, resonance line structure, etc.) information concerning the nature of free radicals [1,2], their concentrations [3], the interactions among paramagnetic defects [4], may be derived. The temperature dependence of ESR, spectra allows the estimation of the activation energy for free radicals [5] as well as the evaluation of the Curie temperature, whereas from the time dependence of resonance line parameters the overall reaction order [5] for free radicals generation and/or decay and the associated reaction rates [5] may be obtained. Polycarbonate (lexan) is widely used as a solid state nuclear tracks detector [6]. Under the influence of ionizing radiation, this polymer degradates [7], even after a storing time of about 50 h [8]. Ion beam bombardment of lexan leads to microscopical collective defects, along the incident particle trajectory, known as latent tracks [6], which may be developed using various methods (chemically [6,8], electrochemically [9], etc. Defects generation, as noticed from ESR, data, may be described within the thermal spikes model [lo] or supposing that the main contribution is due to secondary electrons [ll]. * Corresponding author.
This contribution concerns the effects of ion beam bombardment on lexan, as they are revealed by electron spin resonance spectroscopy, in connection with the structure and stability of latent tracks.
2. Experimental
methods
Thin foils of polycarbonate (bisphenol-A polycarbonate, lexan from General Electric, with the chemical formula [(OC,H,)C(CH,),(C,H,O)CO],) with a thickness of about 25 pm, have been irradiated in vacuum, at 10-5-10-6 bar, at room temperature, with oxygen ions accelerated up to 25 MeV, at a fluence of about 5 x 10” cm-2s-1 during 5 min. The sample surface has been about lo2 cm2. The ESR, spectra have been measured, using a JES-ME3X spectrometer, operating in the X-band (9GHz). The samples have been stored in air, at room temperature. The temperature dependence of ESR spectra, in the range - 150°C to 250°C has been investigated using a JES-VT3X, variable temperature accessory. Freshly recrystallized DPPH has been used as field and intensity marker. The unirradiated sample gives no resonance signal.
3. Experimental
results and discussions
The resonance spectrum of polycarbonate, irradiated with accelerated oxygen ions, at room tempera-
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0168-583X(94)00149-P
M.I. Chipara’et al. /Nucl. Instr. and Meth. in Phys. Res. B 88 (1994) 418-422
I
&
250
Fig. 3. The temperature
Fig. 1. The resonance spectra of polycarbonate irradiated with oxygen ions, at various temperatures; (a) T = - llO”C, (b) T = - 6o”C, (c) T = - 2O”C,(d) T = 9o”C, (e) T = 230°C.
ture, is a slightly asymmetric singlet, located at g = 2.020 & 0.015 with a peak to peak linewidth of about 9.00 f 0.25 Gs (1 Gs = lop4 T). Some resonance spectra of oxygen irradiated lexan, at various, temperatures, are represented in Fig. 1. The same resonance spectra have been recorded in gamma irradiated polycarbonate, at long irradiation times [1,12-151 neutron [I], electron beam irradiated [13] samples and heavy ion irradiated [26,27] polycarbonate. The small differences noticed in the peak to peak line width of the resonance spectra (for example in gamma irradiated lexan, the reported peak to peak linewidth is about 7 Gs [1,12-H], may be ascribed to the competition between dipolar and exchange interactions [2,4]. Consequently, in agreement with Grosescu et al. [14], we have ascribed the resonance spectrum to a convolution of two resonance lines, assigned to . R, and .R, free radicals (Fig. 2).
“-y-0.
R,
-\=
-<->.
R, D
Fig. 2. The structure of the free radicals induced by oxygen bombardment of lexan.
419
I
350
,
TIKI
LSO
dependence of the peak to peak line width, Hpp.
The time stability of these radicals, at room temperature, is very high, as only a 10% decrease of defects concentration has been noticed after few months, for samples stored in air, at room temperature. The temperature dependence of the peak to peak linewidth HPP, in gauss, is given in Fig. 3. The resonance linewidth of irradiated lexan decreases as the sample temperature is increased due to the motional narrowing of the resonance line [4] and to defects decay. As may be noticed from Fig. 3, the slope of the peak to peak linewidth dependence on the sample temperature, changes at about 50°C and 15O”C, respectively. This behaviour may be related to ESR data on irradiated polycarbonate [12-151 according to which around 90°C the . R, free radicals are converted in *R, free radicals and at about 16O”C, the . R, free radicals disappear. The double integral of the ESR spectrum, S, has been estimated using the relation [l-3]. S =KIH&
(1)
where K is the resonance line shape factor, and I is the resonance line amplitude. The temperature dependence of S, in relative units, given in Fig. 4, is analogous to the temperature dependence of Hpp, confirming that the main free radicals, induced by oxygen irradiation in lexan, are . R, and .R,. The parameter S is proportional to the sample static susceptibility [16] and consequently to the free radical concentration c [l-4]. As the free radicals lifetime, at room temperature, is very high, it is possible to neglect the free radicals recombination, below room
temperature.
Accordingly,
a Curie-Weiss-like
M.I. Chipara’ et al. / Nucl. Instr. and Meth. in Phys. Res. B 88 (1994) 418-422
420
From T,, the isotropic exchange integral J, may be extracted [l], J=
%A zS(S + 1) ’
(3)
where k, is the Boltzmann constant, z is the number of the nearest neighbours, and S is the spin (i in our case). Supposing that z = 2, the maximum value of the isotropic exchange integral is about lO-*l J (10 Gs). Due to the dipolar nature of the exchange interaction, it is possible to estimate the minimum average distance (r > between the free radicals, coupled by antiferromagnetic (T, < 0 K) exchange interactions, using the relation [ 181.
Fig. 4. The temperature dependence of the resonance line double integral S, in relative units.
dependence of S on the sample temperature pected [16],
is ex-
where C is a constant, and T, is the Curie temperature. As may be noticed from Fig. 5, the temperature dependence of S-‘, in the range - 120°C to -20°C is linear. From the experimental data, using a linear regression program written within the least squares approximation, we have obtained T, = -76 f 8 K. The correlation coefficient of this dependence is high (99.79%), supporting a Curie-Weiss dependence of S, on the sample temperature, in the low temperature range.
S=S,
t
I
150
where J is the exchange interaction expressed in gausse and (r) is the average distance between free radicals, expressed in Angstroms. Consequently, our experimental data are consistent with a value of the average distance between free radicals of about 30 A suggesting that almost all paramagnetic defects are located within the latent tracks [19] and strongly coupled by antiferromagnetic exchange interactions. As (r) corresponds to the electron range in plastics [19], we suppose that the main contribution to free radicals generation is due to secondary electrons. This hypothesis is supported by the resonance line features, which allow us to identify the same free radicals *R, and *R, as in electron beam irradiated lexan. In the high temperature range (SO’C to 230”(Z), the main contribution to the decrease of resonance line double integral is related to free radicals recombination. Consequently, we have supposed that in this temperature range, S obeys an Arrhenius-like dependence,
1
I
250
350
_
T[Kl
Fig. 5. The temperature dependence of S-‘, temperature range.
in the low
exp-
$,
where So is a constant, and EA is the activation energy for free radicals recombination. The dependence of In S versus the reciprocal temperature is almost linear, as may be noticed from Fig. 6. The correlation coefficient, estimated using the least squares method, by linear regression, is high (97.99%), supporting an Arrhenius-like dependence of S on the sample temperature, in the high temperature range, and confirming that in this temperature range the Curie-Weiss contribution is negligible. The experimental data are consistent with an activation energy, for free radicals recombination, of 6 f 1 kcal/mole. This value represents about 30% from the activation energy
M.1. Chip& et al. / Nucl. Instr. and Meth. in Phys. Res. B 88 (1994) 418-422
0.6
421
1151 on polycarbonate irradiated with gamma, 252Cf and 241Am supports our data unarming the change in the free radicals ~ncentration at about 160°C. The polymer pyrolysis leads to the enhancement of the electrical conductivity [25]. For a degenerate semiconductor, the spin susceptibility is almost independent of temperature [16] whereas for a conducting system the resonance line has a Dysonian shape f4], due to the skin effect. No evidence for these effects has been noticed in our case.
4. Conclusions
Fig. 6. The dependence of the logarithm of S on the reciprocal temperature.
calculated from the temperature dependence of etching velocity (E, = 17.3 f 0.8 kcai/mole [20]), and may be associated to diffusion processes 1211. The glass transition temperature, in polycarbonate, is located around 145°C [22]. It is therefore possible to suppose that below the glass transition temperature, the free radicals are frozen and consequently their recombination is controlled by the diffusion process, which is the slowest step. Above 145”C, free radicals recombination is suddenly increased, explaining the reported disappearance of free radicals above 160°C [12-151. In the case of polycarbonate irradiated with oxygen ions, the resonance spectrum ascribed to .R, and *R2 free radicals disappears around 230°C. The higher thermal stability of free radicals in the case of our experiment may be due to a cage effect, as the free radicals are coupled by strong exchange interactions, suggesting a cluster of defects along the incident ion trajectory, within the polymer. However, some differences in the sensitivity of ESR spectrometers may also play an jmportant role. Around 220°C a new resonance line develops, as may be noticed from Fig. le. This line, located close to g = 2.0 is a narrow singlet which may be ascribed to polymer thermo-oxidative degradation. This result gives a further support to the hypothesis that the free radicals induced by ion beam bombardment of polymers are not p~olysis-like free radicals, generated by thermal spikes. All experimental data are consistent with the presence of -R, and *R, free radicals, suggesting that the nature of free radicals generated by irradiation does not depend on the incident particle nature, in agreement with the h~thesis of Chipara and Georgescu [11] but at variance with the descriptions suggested by Chambaudet et al. [23] and Marietta [24]. The thermoluminescence data reported by Edmond and Durrani
From the time and temperature dependence of resonance spectra, as well as from the resonance spectra parameters, we may conclude that the nature of free radicals induced by oxygen bombardment of polycarbonate coincides to the nature of free radicals generated by gamma, neutrons, electrons irradiation. We failed to notice the presence of p~ol~is-like active centers or of any other contribution susceptible to indicate their contribution. The free radicals are located within the latent track:, with an average distance between them of about 30 A, and strongly coupled by antiferromagnetic interactions. The close relation between the free radicats and the latent tracks is further supported by the thermal fading of free radicals and latent tracks. From the temperature dependence of the resonance line, Curie temperature, the average isotropic exchange integral, the average distance between free radicais, the activation energy for defects recombination, have been estimated. The activation energy for free radicals recombination is lower than the activation energy estimated from the temperature dependence of etching velocity. This result is expected as chemical etching does not affect, usually, the free radicals. The experimental data suggest that the free radical recombjnation is dominated by the dif~sion step. From the ESR data, obtained by us, it is possible to conclude that the free radicals are produced mainly by &electrons 1111rather than by thermal spikes 1231and that the free radicals are preferentially trapped within the latent tracks.
References [l] B. Ranby and J.F. Rabek, ESR Spectroscopy in Polymer Research (Springer, Berlin, 1977) p. 227. 121P.B. Aysgouch, Electron Spin Resonance in Chemistry, (Methue & Co, London, 1967) p. 105-168, p. 361-398. [3] C.P. Poole, Electron Spin Resonance - A Comprehensive treatise on Experimental Techniques, (Interscience, New York,19671 p. 550-560.
422
M.I. Chipara’ et al. / Nucl. Instr. and Meth. in Phys. Res. B 88 (1994) 418-422
[4] C.P. Poole and H.A. Farach, The Theory of Magnetic
Resonance (Wiley, New York, 1972) p. 364-395. [5] MI. Chipara, D. Barb, M. Cojocaru, D. Tudor, M.D. Chipara and M. Rogalski, Polym. Degrad. Stab. 37 (1992) p. 61. [6] R.L. Fleisher, P.B. Price and R.M. Walker, Nuclear Tracks in Solids: Principles and Applications (University of California Press, Berkeley, 1975) p. 51-113. [7] W. Schabel, Polymer Degradation - Principles and Application, (Akademie Verlag, Berlin, 1981) p. 130-150. [8] M.A. Fade], A.A. Abdalla and M.A. Hamied, Nucl. Instr. and Meth. 187 (1981) 505. [9] M. Monnin, J. Gauray, G. Somogy and D. Dayko, Radiat. Phys. Chem. 15 (1980) 473. [lo] S. Bouffard and M. Toulemonde, Le Vide, Ies Couches Minces, Suppl. 260 (1992) p. 204. [Ill MI. Chipara and L. Georgescu, Nucl. Instr. and Meth. 209/210 (1983) 395. [12] M.I. Chipara, D. Hasegan and M. Velter-Stefanescu, Proc. 10th Int. Conf. on SSNTDs, Lyon (Pergamon, Oxford, 1981) p. 75-80. [13] 0. Hama and S. Shinohara, J. Polym. Sci. 8 (1970) 651. [14] R. Grosescu, 0. Constantinescu and D. Balla, Rev. Roum. Phys. 14 (1968) p. 1271. [15] E.A. Edmond and S.A. Durrani, Nucl. Tracks 3 (1979) 3.
[16] Ch. Kittel, Introduction to Solid State Physics (Wiley, New York, 1980) p. 543-600. [17] A.H. Morrish, The Physical Principles of Magnetism (Wiley, New York, 1966) p. 265-287. [18] W. Gordy, Theory and Applications of Electron Spin Resonance (Wiley, New York, 1980) p. 543-600. [19] H. Paretzke, Rad. Eff. 34 (1977) p. 3. [20] T.A. Gruhn, W.K. Li, E.V. Benton, R.M. Casson and C.S. Johnson, Proc. 10th Int. Conf. on SSNTDs, Lyon (Pergamon, Oxford, 1979) p. 251-301. [21] J. Crank and G.S. Parker (eds.), Diffusion in Polymers (Academic Press, London, 1968) p. 141-162. [22] D. Krevelin and P. Hoftyzer, Properties of Polymers Corelations with Chemical Structure (Elsevier, Amsterdam, 1972). [23] A. Chambaudet, A. Bernas and J. Roncin, Rad. Eff. 34 (1977) p. 57. [24] G. Marletta, Nucl. Instr. and Meth. B 46 (1990) 295. [25] S.A. Al’tshueer and B.M. Kozyrev, Electron Paramagnetic Resonance (Academic Press, New York, 1964) p. 253-273. [26] E. Ferrain and R. Legras, Rad. Eff. Def. Solids 126 (19931 243. [27] E. Ferrain and R. Legras, Nucl. Instr. and Meth. B 82 (1993), 539.
ESR investigations
on ion beam irradiated
M.I. ChiparS a, V.V. Grecu b, P.V. Notingher
Beam Interactions with Materials 8 Atoms
polycarbonate
‘T*, J. Reyes Romero d and M.D. Chiparg e
a Institute for Physics and Technology of Materials, P.O. Box MG-7, Magurele, Bucharest, R-76900, Romania b University of Bucharest, Faculty of Physics, P.O. Box MG-II, Magurele, Bucharest, R-76900, Romania ’ University Politehnica of Bucharest, Electrotechnical Faculty, 313, Splaiul Independentei, Str., 77206 Bucharest, Romania d Universidad Central de Venezuela, Fact&ad de Ingineria, Dpto de fuica Aplicada, Ciudad Universitaria, Chaguaramos, Caracas, Venezuela e Research Institute for Electrotechnics, 45- 47 Tudor Vladimirescu, Bd., Bucharest, R-79423, Romania
Received 27 July 1993 and in revised form 14 February 1994
Electron spin resonance (ESR) investigations with a polycarbonate solid state nuclear detector, irradiated with oxygen ions, are reported. The nature of the paramagnetic defects induced by irradiation is discussed. The temperature dependence of resonance line parameters is studied. From the experimental data, obtained by ESR, spectroscopy, the activation energy for defect recombination, the average isotropic exchange integral between paramagnetic defects as well as the average distance between defects, are estimated. Correlations with latent tracks structure are discussed.
1. Introduction
Electron spin resonance spectroscopy (ESR) is a powerful tool in the investigation of radiation induced paramagnetic defects (free radicals). From the resonance line parameters (g value, resonance line width, line intensity, shape, anisotropy, resonance line structure, etc.) information concerning the nature of free radicals [1,2], their concentrations [3], the interactions among paramagnetic defects [4], may be derived. The temperature dependence of ESR, spectra allows the estimation of the activation energy for free radicals [5] as well as the evaluation of the Curie temperature, whereas from the time dependence of resonance line parameters the overall reaction order [5] for free radicals generation and/or decay and the associated reaction rates [5] may be obtained. Polycarbonate (lexan) is widely used as a solid state nuclear tracks detector [6]. Under the influence of ionizing radiation, this polymer degradates [7], even after a storing time of about 50 h [8]. Ion beam bombardment of lexan leads to microscopical collective defects, along the incident particle trajectory, known as latent tracks [6], which may be developed using various methods (chemically [6,8], electrochemically [9], etc. Defects generation, as noticed from ESR, data, may be described within the thermal spikes model [lo] or supposing that the main contribution is due to secondary electrons [ll]. * Corresponding author.
This contribution concerns the effects of ion beam bombardment on lexan, as they are revealed by electron spin resonance spectroscopy, in connection with the structure and stability of latent tracks.
2. Experimental
methods
Thin foils of polycarbonate (bisphenol-A polycarbonate, lexan from General Electric, with the chemical formula [(OC,H,)C(CH,),(C,H,O)CO],) with a thickness of about 25 pm, have been irradiated in vacuum, at 10-5-10-6 bar, at room temperature, with oxygen ions accelerated up to 25 MeV, at a fluence of about 5 x 10” cm-2s-1 during 5 min. The sample surface has been about lo2 cm2. The ESR, spectra have been measured, using a JES-ME3X spectrometer, operating in the X-band (9GHz). The samples have been stored in air, at room temperature. The temperature dependence of ESR spectra, in the range - 150°C to 250°C has been investigated using a JES-VT3X, variable temperature accessory. Freshly recrystallized DPPH has been used as field and intensity marker. The unirradiated sample gives no resonance signal.
3. Experimental
results and discussions
The resonance spectrum of polycarbonate, irradiated with accelerated oxygen ions, at room tempera-
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0168-583X(94)00149-P
M.I. Chipara’et al. /Nucl. Instr. and Meth. in Phys. Res. B 88 (1994) 418-422
I
&
250
Fig. 3. The temperature
Fig. 1. The resonance spectra of polycarbonate irradiated with oxygen ions, at various temperatures; (a) T = - llO”C, (b) T = - 6o”C, (c) T = - 2O”C,(d) T = 9o”C, (e) T = 230°C.
ture, is a slightly asymmetric singlet, located at g = 2.020 & 0.015 with a peak to peak linewidth of about 9.00 f 0.25 Gs (1 Gs = lop4 T). Some resonance spectra of oxygen irradiated lexan, at various, temperatures, are represented in Fig. 1. The same resonance spectra have been recorded in gamma irradiated polycarbonate, at long irradiation times [1,12-151 neutron [I], electron beam irradiated [13] samples and heavy ion irradiated [26,27] polycarbonate. The small differences noticed in the peak to peak line width of the resonance spectra (for example in gamma irradiated lexan, the reported peak to peak linewidth is about 7 Gs [1,12-H], may be ascribed to the competition between dipolar and exchange interactions [2,4]. Consequently, in agreement with Grosescu et al. [14], we have ascribed the resonance spectrum to a convolution of two resonance lines, assigned to . R, and .R, free radicals (Fig. 2).
“-y-0.
R,
-\=
-<->.
R, D
Fig. 2. The structure of the free radicals induced by oxygen bombardment of lexan.
419
I
350
,
TIKI
LSO
dependence of the peak to peak line width, Hpp.
The time stability of these radicals, at room temperature, is very high, as only a 10% decrease of defects concentration has been noticed after few months, for samples stored in air, at room temperature. The temperature dependence of the peak to peak linewidth HPP, in gauss, is given in Fig. 3. The resonance linewidth of irradiated lexan decreases as the sample temperature is increased due to the motional narrowing of the resonance line [4] and to defects decay. As may be noticed from Fig. 3, the slope of the peak to peak linewidth dependence on the sample temperature, changes at about 50°C and 15O”C, respectively. This behaviour may be related to ESR data on irradiated polycarbonate [12-151 according to which around 90°C the . R, free radicals are converted in *R, free radicals and at about 16O”C, the . R, free radicals disappear. The double integral of the ESR spectrum, S, has been estimated using the relation [l-3]. S =KIH&
(1)
where K is the resonance line shape factor, and I is the resonance line amplitude. The temperature dependence of S, in relative units, given in Fig. 4, is analogous to the temperature dependence of Hpp, confirming that the main free radicals, induced by oxygen irradiation in lexan, are . R, and .R,. The parameter S is proportional to the sample static susceptibility [16] and consequently to the free radical concentration c [l-4]. As the free radicals lifetime, at room temperature, is very high, it is possible to neglect the free radicals recombination, below room
temperature.
Accordingly,
a Curie-Weiss-like
M.I. Chipara’ et al. / Nucl. Instr. and Meth. in Phys. Res. B 88 (1994) 418-422
420
From T,, the isotropic exchange integral J, may be extracted [l], J=
%A zS(S + 1) ’
(3)
where k, is the Boltzmann constant, z is the number of the nearest neighbours, and S is the spin (i in our case). Supposing that z = 2, the maximum value of the isotropic exchange integral is about lO-*l J (10 Gs). Due to the dipolar nature of the exchange interaction, it is possible to estimate the minimum average distance (r > between the free radicals, coupled by antiferromagnetic (T, < 0 K) exchange interactions, using the relation [ 181.
Fig. 4. The temperature dependence of the resonance line double integral S, in relative units.
dependence of S on the sample temperature pected [16],
is ex-
where C is a constant, and T, is the Curie temperature. As may be noticed from Fig. 5, the temperature dependence of S-‘, in the range - 120°C to -20°C is linear. From the experimental data, using a linear regression program written within the least squares approximation, we have obtained T, = -76 f 8 K. The correlation coefficient of this dependence is high (99.79%), supporting a Curie-Weiss dependence of S, on the sample temperature, in the low temperature range.
S=S,
t
I
150
where J is the exchange interaction expressed in gausse and (r) is the average distance between free radicals, expressed in Angstroms. Consequently, our experimental data are consistent with a value of the average distance between free radicals of about 30 A suggesting that almost all paramagnetic defects are located within the latent tracks [19] and strongly coupled by antiferromagnetic exchange interactions. As (r) corresponds to the electron range in plastics [19], we suppose that the main contribution to free radicals generation is due to secondary electrons. This hypothesis is supported by the resonance line features, which allow us to identify the same free radicals *R, and *R, as in electron beam irradiated lexan. In the high temperature range (SO’C to 230”(Z), the main contribution to the decrease of resonance line double integral is related to free radicals recombination. Consequently, we have supposed that in this temperature range, S obeys an Arrhenius-like dependence,
1
I
250
350
_
T[Kl
Fig. 5. The temperature dependence of S-‘, temperature range.
in the low
exp-
$,
where So is a constant, and EA is the activation energy for free radicals recombination. The dependence of In S versus the reciprocal temperature is almost linear, as may be noticed from Fig. 6. The correlation coefficient, estimated using the least squares method, by linear regression, is high (97.99%), supporting an Arrhenius-like dependence of S on the sample temperature, in the high temperature range, and confirming that in this temperature range the Curie-Weiss contribution is negligible. The experimental data are consistent with an activation energy, for free radicals recombination, of 6 f 1 kcal/mole. This value represents about 30% from the activation energy
M.1. Chip& et al. / Nucl. Instr. and Meth. in Phys. Res. B 88 (1994) 418-422
0.6
421
1151 on polycarbonate irradiated with gamma, 252Cf and 241Am supports our data unarming the change in the free radicals ~ncentration at about 160°C. The polymer pyrolysis leads to the enhancement of the electrical conductivity [25]. For a degenerate semiconductor, the spin susceptibility is almost independent of temperature [16] whereas for a conducting system the resonance line has a Dysonian shape f4], due to the skin effect. No evidence for these effects has been noticed in our case.
4. Conclusions
Fig. 6. The dependence of the logarithm of S on the reciprocal temperature.
calculated from the temperature dependence of etching velocity (E, = 17.3 f 0.8 kcai/mole [20]), and may be associated to diffusion processes 1211. The glass transition temperature, in polycarbonate, is located around 145°C [22]. It is therefore possible to suppose that below the glass transition temperature, the free radicals are frozen and consequently their recombination is controlled by the diffusion process, which is the slowest step. Above 145”C, free radicals recombination is suddenly increased, explaining the reported disappearance of free radicals above 160°C [12-151. In the case of polycarbonate irradiated with oxygen ions, the resonance spectrum ascribed to .R, and *R2 free radicals disappears around 230°C. The higher thermal stability of free radicals in the case of our experiment may be due to a cage effect, as the free radicals are coupled by strong exchange interactions, suggesting a cluster of defects along the incident ion trajectory, within the polymer. However, some differences in the sensitivity of ESR spectrometers may also play an jmportant role. Around 220°C a new resonance line develops, as may be noticed from Fig. le. This line, located close to g = 2.0 is a narrow singlet which may be ascribed to polymer thermo-oxidative degradation. This result gives a further support to the hypothesis that the free radicals induced by ion beam bombardment of polymers are not p~olysis-like free radicals, generated by thermal spikes. All experimental data are consistent with the presence of -R, and *R, free radicals, suggesting that the nature of free radicals generated by irradiation does not depend on the incident particle nature, in agreement with the h~thesis of Chipara and Georgescu [11] but at variance with the descriptions suggested by Chambaudet et al. [23] and Marietta [24]. The thermoluminescence data reported by Edmond and Durrani
From the time and temperature dependence of resonance spectra, as well as from the resonance spectra parameters, we may conclude that the nature of free radicals induced by oxygen bombardment of polycarbonate coincides to the nature of free radicals generated by gamma, neutrons, electrons irradiation. We failed to notice the presence of p~ol~is-like active centers or of any other contribution susceptible to indicate their contribution. The free radicals are located within the latent track:, with an average distance between them of about 30 A, and strongly coupled by antiferromagnetic interactions. The close relation between the free radicats and the latent tracks is further supported by the thermal fading of free radicals and latent tracks. From the temperature dependence of the resonance line, Curie temperature, the average isotropic exchange integral, the average distance between free radicais, the activation energy for defects recombination, have been estimated. The activation energy for free radicals recombination is lower than the activation energy estimated from the temperature dependence of etching velocity. This result is expected as chemical etching does not affect, usually, the free radicals. The experimental data suggest that the free radical recombjnation is dominated by the dif~sion step. From the ESR data, obtained by us, it is possible to conclude that the free radicals are produced mainly by &electrons 1111rather than by thermal spikes 1231and that the free radicals are preferentially trapped within the latent tracks.
References [l] B. Ranby and J.F. Rabek, ESR Spectroscopy in Polymer Research (Springer, Berlin, 1977) p. 227. 121P.B. Aysgouch, Electron Spin Resonance in Chemistry, (Methue & Co, London, 1967) p. 105-168, p. 361-398. [3] C.P. Poole, Electron Spin Resonance - A Comprehensive treatise on Experimental Techniques, (Interscience, New York,19671 p. 550-560.
422
M.I. Chipara’ et al. / Nucl. Instr. and Meth. in Phys. Res. B 88 (1994) 418-422
[4] C.P. Poole and H.A. Farach, The Theory of Magnetic
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