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SBS blends irradiated with gamma rays
Nuclear Instruments and Methods in Physics Research B 226 (2004) 320–326 www.elsevier.com/locate/nimb
An electron paramagnetic resonance study of PP and PP/SBS blends irradiated with gamma rays Pedro Silva a,*, Carmen Albano b, Rosestela Perera c, Jeanette Gonz alez c, Miren Ichazo c a
Centro de Fısica, Instituto Venezolano de Investigaciones Cientıficas (IVIC), Carretera Panamericana, Km. 11, Apartado Postal 21827, Caracas 1020-A, Venezuela b Centro de Quımica, Instituto Venezolano de Investigaciones Cientıficas (IVIC), Caracas 1020-A, Venezuela c Departamento de Mecanica, Universidad Simon Bolıvar, Caracas 1080-A, Venezuela Received 8 March 2004; received in revised form 30 April 2004
Abstract Electron paramagnetic resonance (EPR) measurements of Polypropylene (PP) and its blends with a Styrene– Butadiene–Styrene (SBS) copolymer in 10, 20, 30 and 40 wt.% of SBS were carried out. The samples were irradiated in the 0 6 D 6 100 kGy range at a dose rate of 4.8 kGy/h. Typical spectra indicative of the formation of peroxy and alkyl radicals were obtained. The dynamics of formation and recombination of radicals in these samples could be explained using a mixed zero and first order generation–recombination fit of the total spin concentration as a function of the integral dose. The time dependence of the spin concentration was studied assuming a mixed first order fit in the decay process. The parameters obtained from the fitting are interpreted in terms of the rate of formation of free radicals at the irradiation time and in terms of the decay time of the total free radical concentration. 2004 Elsevier B.V. All rights reserved. PACS: 61.82.P; 82.35.L Keywords: Electron paramagnetic resonance; Gamma irradiation; Dose rate
1. Introduction In recent years, elastomeric rubber/thermoplastic blends have become technologically important for various applications [1]. Different structural systems can be produced when different
*
Corresponding author. Tel.: +58-212-5041589; fax: +58212-5041148. E-mail address: [email protected] (P. Silva).
polymers are mixed, depending on the affinity of the components involved [2]. Most blends, either elastomer/elastomer blends or elastomer/plastic blends, are usually vulcanized using conventional chemical methods. However, the use of gamma radiation as an aid for inducing vulcanization of these blends has attracted the attention of some investigators [3,4]. Ionizing radiation induces chemical reactions in polymers that result in changes in both, molecular structure and microscopic properties. The energy transfer from the
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.05.035
P. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 320–326
2. Materials and methods In this work, a Polypropylene (PP) supplied by PROPILVEN C.A. and a Styrene–Butadiene– Styrene radial copolymer (SBS) (Solprene S-416) with a styrene content of 30 wt.% were used. Four blends with 10, 20, 30 and 40 wt.% of SBS were prepared. The PP and SBS were premixed and then blended in a Werner & Pfleiderer ZSK-30 corotating twin-screw extruder at 110 rpm and at a temperature profile of 150–170–190–180–165 C. Strands of the blends were cooled in a water bath and then pelletized. Specimens were then irradiated with c-rays from a 60 Cobalt source in air at a dose rate of 4.8 kGy/h. Integral doses were in the 0 6 D 6 100 kGy range. They were 0, 5, 25, 50, 75 and 100 kGy. The samples for EPR were cut from the extruded strands into small cylinders (2 mm of diameter for approximately 3 mm length). The EPR spectra were obtained in a X-Band EMX BRUKER spectrometer at room temperature. The total number of spin per gram (total free radical concentration) on the samples was obtained by performing, with the ORIGIN software, the second integral of the recorded spectrum and com-
paring it with the one of the 4-(2-iodoacetamide)2,2,6,6-tetramethylpiperidinoxyl radical, which was used as standard of comparison. The experimental conditions (microwave power and modulation field) were adjusted to avoid effects of saturation in the EPR spectrum. No EPR signal could be obtained at zero dose, because of the detection limits of the spectrometer.
3. Results and discussion Fig. 1 shows the EPR spectra of PP/SBS (20%) irradiated at an integral dose of 100 kGy; in the horizontal axis the applied external magnetic field is shown and in the vertical axis the first derivative with respect to the applied magnetic field of the imaginary part of the magnetic susceptibility ðv00 Þ, which is proportional to the first derivative of the absorbed power by the sample respect to the applied magnetic field. The inset in the figure shows the spectrum of pure PP at the same integral dose of 100 kGy. These spectra are representative of the results obtained from almost all the studied blends. According to the literature, the spectra displayed in Fig. 1 are typical of peroxy radicals [6–8]. All the spectra of the blends show the characteristic signal of peroxy radicals in PP with an octahedral symmetry. Those radicals are randomly distributed
g|| = 2.034 g⊥ = 2.008
dχ"/dH [Arb. Units.]
radiation to the polymer does not take place selectively relative to the mixed components. The probability of the generation of free radicals depends on the strength of the interatomic bonds. The lower the bond energy the easier the bond scission will be. Radiochemical studies on crosslinking or degradation of polymer blends are important for designing new materials. When oxygen is present, radiation-induced changes are often quite different from those produced by irradiation under inert atmosphere [5]. It is well known that electron paramagnetic resonance (EPR), a technique based on the microwave absorption for an unpaired electron, is an excellent technique for the characterization and quantification of the free radicals. In the present paper EPR measurements in c-irradiated PP and PP/SBS blends are reported. The spin concentration as a function of the integral dose and the time decay of this spin concentration for all integral doses were studied in all the samples.
321
g = 2.034 1
g = 2.008 2
g = 1.992 3
330
325
340 H [mT]
330
350
335 340 H [mT]
345
350
Fig. 1. EPR spectrum of PP/SBS (20%) irradiated at 100 kGy of integral dose. The inset shows the spectrum for pure PP at the same integral dose. Unnormalized spectra.
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P. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 320–326
in their surroundings. The spectrum of pure PP is typical of those with an octahedral symmetry with a rhombic distortion [9]. Due to the stability displayed by these radicals, there are authors suggesting that such radicals are trapped in a phase with a restricted mobility, corresponding to the crystalline phase of PP [6,10]. From the spectra of the PP/SBS (20%) two g values can be clearly obtained, g? ¼ 2:008 0:002 and gk ¼ 2:034 0:002. In pure PP, three g values can be identified, g1 ¼ 2:034 0:002, g2 ¼ 2:008 0:002 and g3 ¼ 1:992 0:002, due to the anisotropy. The rest of the samples show very similar g values, taking into account the experimental error. These values are in perfect agreement with those reported elsewhere [6]. In the inset, as can be seen, there are other signals than that of the peroxy radical. When the PP sample is irradiated in air, the oxidation processes begin simultaneously as a consequence of the oxygen diffusion during the irradiation. An intermediate spectrum between alkyl and peroxy is observed and the last one becomes more important as the oxygen concentration increases. The wider signal in the spectra can be ascribed to polyenyl radicals, which are left after the alkyl radicals decay [7,11]. The same behavior is also observed in the PP/SBS blends. Fig. 2 shows the total spin concentration in grams as a function of the integral dose of irradi-
5
0% 10% 20% 30% 40%
Spin/gr. x10-17
4 3 2 1 0
0
25
50
75
100
Integral Dose (kGy) Fig. 2. Total free radical concentration as a function of integral dose for all the samples after 4 h of being irradiated. The lines correspond to the best fit obtained with Eq. (2).
ation for all studied samples in the integral dose range of 5 6 DI 6 100 kGy. An increase in the number of free radicals with the integral dose of irradiation is observed in all the samples. For the lowest dose the number of generated radicals is similar, within the experimental error, in all the blends including that of pure polypropylene. At the highest doses, a slight decrease in the number of radicals when SBS is present in the sample was obtained. That is, the sample of pure PP shows a higher free radical concentration for ID P 50 kGy. The experimental results were fitted assuming that the total radical concentration responds to a mixed zero and first order generation–recombination processes [12,13]: dN ¼ K0G þ K1G N K0R K1R N ; dt
ð1Þ
where N is the number of total free radicals generated during the irradiation process and the K’s are the zero and first order generation and recombination rate constants. Defining K0
ðK0G K0R Þ and K1 ðK1R K1G Þ, and assuming that these new constants depend on the energy deposited into the sample by the incident radiation in the time unit, the total free radical concentration as a function of the total incident dose was obtained using the following expression: 0
N ¼ N0 e K1 DI þ
K00 0 ð1 e K1 DI Þ; K10
ð2Þ
where K00 and K10 are proportionality constants, and N0 gives an idea of the radical concentration at zero dose [12,13]. The concavity of the curve depends on the sign of these constants. For positive values of both K00 and K10 , the curve will be concave down and the rate of recombination would be higher than that of the generation of free radicals. For a positive and a negative value of K00 and K10 , respectively, the curve is concave up (convex). This means that the rate of generation of free radicals is higher than that of their recombination. Table 1 displays the best fit parameters obtained from the fitting of all the data showed in Fig. 2 The curve concavity for pure PP and for the PP/SBS (20%) blend suggests that the rate of radical recombination increases with the integral dose, implying a high free radical recombination
P. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 320–326
323
N0
K10
K00 K10
0 10 20 30 40
6.39 · 1015 3.34 · 1016 9.08 · 1015 9.35 · 1016 5.18 · 1016
0.01431 0.00396 0.01948 1.27 · 10 9 )0.00161
5.99 · 1017 1.18 · 1018 4.75 · 1016 2.24 · 1024 )2.01 · 1018
rate constant. In all the other blends a different behavior is observed. For both PP/SBS (10%) and PP/SBS (30%), a slightly linear fitting is obtained which could be interpreted as a single near zero order generation–recombination process. For the PP/SBS (40%) sample, the concavity of the curve and the parameters obtained from the fitting suggest that the generation rate of the free radicals is higher than the recombination rate in the first order process. The mobility and the generation rate of the free radicals is responsible for the change in the concavity of the curve; a decrease in the free radicals mobility limits their recombination. In pure PP, the recombination rate of free radicals is higher than their generation rate, as is observed in the curve. In the blends, except in the PP/SBS (20%), the generation of radicals increases with the percentage of SBS, being for the PP/SBS (40%) the highest. Hence, it can be said that the latter blend is the most sensible to radiation at the highest integral dose, displaying an even comparable free radical concentration to that of pure PP. Fig. 3 shows the EPR spectrum of pure SBS just after being irradiated and after 110 h. The noise/signal ratio is very high as noticed, and even higher after 110 h of the irradiation process, as a consequence of the very high recombination rate of its radicals. SBS has a higher concentration of double bonds in butadiene facilitating a higher formation of free radicals when the radiation dose and the SBS percentage are increased. It is expected that the curves corresponding to the blends cross with the curve corresponding to the pure PP at a specific dose that will depend on the percentage of SBS. Above this dose the total free radical concentration will be higher than that for the pure PP in the specific blend.
325
325
330
330
335 340 H [mT]
335 340 H [mT]
345
345
350
350
Fig. 3. EPR spectrum of pure SBS just after being irradiated at 50 kGy of integral dose. The inset shows the spectrum after 110 h of being irradiated.
Fig. 4 shows the total free radical concentration as a function of the SBS percentage for all the samples just after being irradiated. For simplicity, only three doses are shown. As expected, a displacement in the curves is observed, increasing the spin concentration as the dose is increased. As seen in the same figure, a slight decrease in the total free radical concentration is noticed when the percentage of SBS is increased. It is clear from this graph that pure PP has a higher free radical concentration than its blends with SBS. This fact
6.0
5 kGy 75 kGy 100 kGy
5.0
Spin/gr x 10-17
SBS (%)
dχ"/dH [Arb. Units.]
Table 1 Fitting parameters obtained from graph in Fig. 2
4.0 3.0 0.6 0.5 0.4 0.3
0
10
20 30 % of SBS
40
Fig. 4. Total free radical concentration as a function of the percentage of SBS just after being irradiated. The lines are guides for the eyes.
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P. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 320–326
could be attributed to the higher degree of crystallinity of PP, whereas the SBS is amorphous [14]. The radicals forming in the crystalline zones must diffuse towards the amorphous–crystalline interface in order to react with the oxygen and thus form the peroxides. In general, for all the cases shown in Fig. 4, the free radical concentration in PP is much higher than that in the blends, probably due to the fact that the latter was already decreased at the moment of taking the measurements, as a consequence of the higher recombination rate in pure SBS. However, it is noteworthy that having a higher concentration of double bonds in butadiene as the amount of SBS is increased, it is possible that the amount of initially formed radicals was higher, and even though they would decay, were in higher quantities at the moment of being irradiated. This is in agreement with the relatively higher peroxide concentration in the blends, as spectra in Fig. 5 displays, where the EPR spectra of the samples irradiated at 100 kGy is reported. That figure shows that the radical peroxy signal becomes more defined and the contribution of the alkyl radical signal to the spectra is decreased as the SBS concentration in the blend is increased. On the other hand, it is also possible that the benzene ring, due to stability and steric reasons, hinders the oxygen diffusion and its reaction in the irradiated materials, thus decreas-
ing the amount of peroxy radicals formed when SBS is present at concentrations of 10%. Additionally, the SBS acts as a diluent in the PP crystallization process, increasing its amorphous character, which brings about the fact that if the radicals are indeed formed, they will recombine quickly forming stable structures. That is the reason why at low SBS concentrations the measured radical concentrations decrease [5]. A study of the radical decay of the studied samples irradiated at 100 kGy was done. Its results are displayed in Fig. 6. A similar behavior is observed for the other doses. The decay of the total radical concentration was fitted assuming a mixed first and zero order processes, as those described by Eq. (3): N ¼ p1 e p2 t þ p3 ð1 e p2 t Þ þ p4 e p5 t ;
ð3Þ
where p1 , p2 , p3 , p4 and p5 are fitting parameters, the inverse of p2 and p5 are somehow exponential time decays s1 and s2 of the total free radical concentration. The best fitting parameters of the data in Fig. 6 are shown in Table 2. From that table it is immediately inferred that there are effectively two mechanisms involved in the recombination of the free radicals. The first one, associated with the exponential decay time constant s2 , shorter in time 44 < s2 < 51 h, which can be associated with the reaction of the radicals with
16
dχ"/dH [Arb. Units.]
(a)
Spin/gr x 10-17
(b) (c) (d) (e)
SBS 0 % SBS 10 % SBS 20 % SBS 40 %
12 8 4
325
330
335 340 H [mT]
345
350
Fig. 5. EPR spectra of all the studied samples: (a) PP, (b) PP/ SBS (10%), (c) PP/SBS (20%), (d) PP/SBS (30%) and (e) PP/SBS (40%), at 100 kGy of integral dose. Unnormalized spectra.
0
0
400
800
1200
Time (h) Fig. 6. Time decay of the total free radical concentration for most of the studied samples irradiated at 100 kGy. The lines correspond to the best fit obtained with Eq. (3).
P. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 320–326
325
Table 2 Fitting parameters obtained using Eq. (3) for all the samples irradiated at 100 kGy SBS (%)
P1 17
4.11 · 10 4.83 · 1017 3.85 · 1017 3.41 · 1017
0 10 20 40
P2
P3
P4
0.00281 0.00299 0.00213 0.00146
)1.69 · 10 )2.65 · 1016 )1.32 · 1016 1.35 · 1016 16
18
1.03 · 10 5.63 · 1017 6.15 · 1017 6.64 · 1017
P5
s1
s2
0.02237 0.02945 0.02211 0.01965
355.87 334.45 469.48 684.93
44.70 33.96 45.23 50.89
Table 3 Fitting parameters obtained using Eq. (3) for the PP/SBS (40%) blend for the most integral doses of irradiation Integral dose (kGy)
P1
P2
P3
P4
P5
s1
s2
5 25 75 100
5.91 · 1016 1.12 · 1017 2.98 · 1017 3.41 · 1017
0.00388 0.00157 0.00216 0.00146
5.08 · 1015 2.12 · 1015 )1.25 · 1016 1.35 · 1016
8.47 · 1016 2.33 · 1017 4.72 · 1017 6.64 · 1017
0.05703 0.01551 0.02403 0.01965
257.73 636.94 462.96 684.93
17.53 64.48 41.62 50.89
their nearest neighbors, and the other one, related to the mobility of the radicals, associated with s1 , which is larger in time 350 < s1 < 700 h. In Table 2, the fact that these exponential decay time constants increase when the percentage of SBS is increased can be observed. This is due to the higher free radical concentration in the blends as the percentage of the SBS increases. This is in agreement with the results displayed in Fig. 2. These results can be interpreted assuming that a group of radicals react rapidly with their neighbors, may it be the alkyl and allyl radicals that disappear at
12 5 kGy 25 kGy 75 kGy 100 kGy
Spin/gr x 10-17
10 8 6
very short times and other group of radicals that take much more time, because they are impeded by their mobility (maybe the peroxy radicals). As can be seen, s2 is similar for pure PP and for the PP/ SBS (20%) sample. This is in agreement with the behavior observed in Fig. 2. Fig. 7 shows the time decay of the total free radical concentration for the PP/SBS (40%) blend for some of the studied integral doses. For clarity purposes, only four doses are shown; the other samples show similar behavior. Table 3 displays the best fitting parameter obtained for the data in Fig. 7 using Eq. (3). Except for the 25 kGy dose, an increase in the exponential decay time is observed when the integral dose is increased. Certainly, this behavior cannot be associated with the mobility. Instead, this behavior must be associated with the concentration of free radicals generated at each dose. There is no explanation for the behavior obtained at the dose of 25 kGy.
4
4. Conclusions 2 0 0
400
800 Time (h)
1200
1600
Fig. 7. Time decay of the total free radical concentration for the sample PP/SBS (40%) for most of the integral doses studied. The lines correspond to the best fit obtained with Eq. (3).
Pure PP and its blends with SBS shows the presence of allyl, alkyl and peroxy radicals after irradiation in air in their EPR spectra. Dynamics ruling free radical formation in PP and its blends with SBS is a zero and first order process. The time decay of the total free radical concentration may be fitted using a mixed first and zero order
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P. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 320–326
processes. There are two exponential time decay constants associated with two different processes of recombination of the free radicals. The radical decay dynamics is the same for both pure PP and its blends with SBS. The sole presence of SBS decreases the total free radical concentration in the sample after 4 h of being irradiated, and, in general, an increment in the SBS concentration leads to an increment in the generation rate of free radicals.
Acknowledgements This work was supported in part by FONACIT G-2001000817 project and by funds from the Instituto Venezolano de Investigaciones Cientıficas. References [1] N.R. Choudhury, A.K. Bhoce, J. Mater. Sci. 23 (1988) 2187.
[2] A.Y. Cozan, in: Handbook of Elastomers, New Development and Technology, John Wiley, New York, 1987, p. 177. [3] A. Singh, J. Silverman, Radiation Processing of Polymers, Hanser Verlag, New York, 1992. [4] G. Spadaro, A. Valenza, Polym. Degrad. Stab. 67 (2000) 449. [5] T. Zaharescu, M. Chipara, M. Portolache, Polym. Degrad. Stab. 68 (1999) 5. [6] I.L. Dogue, N. Mermilliod, F. Genoud, J. Polym. Sci. A 32 (1994) 2193. [7] T.S. Dunn, J.L. Williams, H. Sugg, V.T. Stannet, in: D. Allara, W. Hawkins (Eds.), Stabil. and Degrad. Polym., 169 (1978) p. 151. [8] A.M. Hassan Resk, M.M. Senna, E.M. Abdel-Barry, Polym. Int. 28 (1992) 265. [9] P.F. Knowles, D. Marsh, H.W.E. Ratle, Magnetic Resonance of Biomolecules, John Wiley, 1976. [10] M. Dole, The Radiation Chemistry of Macromolecules, Vol. 1, 2, Academic Press, London, 1973. [11] P. Silva, C. Albano, D. Lovera, R. Perera, Rev. Mex. Fis. 49 (S3) (2002) 192. [12] V.V. Grecu, M.D. Chipara, L. Georgescu, M.I. Chipara, Rom. Rep. Phys. 46 (1994) 535. [13] M.D. Chipara, V.V. Grecu, M.I. Chipara, C. Ponta, J. Reyes, Nuc. Instr. and Meth. B 151 (1999) 444. [14] C. Albano, G. Sanchez, A. Ismayel, J. Macromol. Sci.-Pure Appl. Chem. A 35 (7–8) (1998) 1349.
An electron paramagnetic resonance study of PP and PP/SBS blends irradiated with gamma rays Pedro Silva a,*, Carmen Albano b, Rosestela Perera c, Jeanette Gonz alez c, Miren Ichazo c a
Centro de Fısica, Instituto Venezolano de Investigaciones Cientıficas (IVIC), Carretera Panamericana, Km. 11, Apartado Postal 21827, Caracas 1020-A, Venezuela b Centro de Quımica, Instituto Venezolano de Investigaciones Cientıficas (IVIC), Caracas 1020-A, Venezuela c Departamento de Mecanica, Universidad Simon Bolıvar, Caracas 1080-A, Venezuela Received 8 March 2004; received in revised form 30 April 2004
Abstract Electron paramagnetic resonance (EPR) measurements of Polypropylene (PP) and its blends with a Styrene– Butadiene–Styrene (SBS) copolymer in 10, 20, 30 and 40 wt.% of SBS were carried out. The samples were irradiated in the 0 6 D 6 100 kGy range at a dose rate of 4.8 kGy/h. Typical spectra indicative of the formation of peroxy and alkyl radicals were obtained. The dynamics of formation and recombination of radicals in these samples could be explained using a mixed zero and first order generation–recombination fit of the total spin concentration as a function of the integral dose. The time dependence of the spin concentration was studied assuming a mixed first order fit in the decay process. The parameters obtained from the fitting are interpreted in terms of the rate of formation of free radicals at the irradiation time and in terms of the decay time of the total free radical concentration. 2004 Elsevier B.V. All rights reserved. PACS: 61.82.P; 82.35.L Keywords: Electron paramagnetic resonance; Gamma irradiation; Dose rate
1. Introduction In recent years, elastomeric rubber/thermoplastic blends have become technologically important for various applications [1]. Different structural systems can be produced when different
*
Corresponding author. Tel.: +58-212-5041589; fax: +58212-5041148. E-mail address: [email protected] (P. Silva).
polymers are mixed, depending on the affinity of the components involved [2]. Most blends, either elastomer/elastomer blends or elastomer/plastic blends, are usually vulcanized using conventional chemical methods. However, the use of gamma radiation as an aid for inducing vulcanization of these blends has attracted the attention of some investigators [3,4]. Ionizing radiation induces chemical reactions in polymers that result in changes in both, molecular structure and microscopic properties. The energy transfer from the
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.05.035
P. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 320–326
2. Materials and methods In this work, a Polypropylene (PP) supplied by PROPILVEN C.A. and a Styrene–Butadiene– Styrene radial copolymer (SBS) (Solprene S-416) with a styrene content of 30 wt.% were used. Four blends with 10, 20, 30 and 40 wt.% of SBS were prepared. The PP and SBS were premixed and then blended in a Werner & Pfleiderer ZSK-30 corotating twin-screw extruder at 110 rpm and at a temperature profile of 150–170–190–180–165 C. Strands of the blends were cooled in a water bath and then pelletized. Specimens were then irradiated with c-rays from a 60 Cobalt source in air at a dose rate of 4.8 kGy/h. Integral doses were in the 0 6 D 6 100 kGy range. They were 0, 5, 25, 50, 75 and 100 kGy. The samples for EPR were cut from the extruded strands into small cylinders (2 mm of diameter for approximately 3 mm length). The EPR spectra were obtained in a X-Band EMX BRUKER spectrometer at room temperature. The total number of spin per gram (total free radical concentration) on the samples was obtained by performing, with the ORIGIN software, the second integral of the recorded spectrum and com-
paring it with the one of the 4-(2-iodoacetamide)2,2,6,6-tetramethylpiperidinoxyl radical, which was used as standard of comparison. The experimental conditions (microwave power and modulation field) were adjusted to avoid effects of saturation in the EPR spectrum. No EPR signal could be obtained at zero dose, because of the detection limits of the spectrometer.
3. Results and discussion Fig. 1 shows the EPR spectra of PP/SBS (20%) irradiated at an integral dose of 100 kGy; in the horizontal axis the applied external magnetic field is shown and in the vertical axis the first derivative with respect to the applied magnetic field of the imaginary part of the magnetic susceptibility ðv00 Þ, which is proportional to the first derivative of the absorbed power by the sample respect to the applied magnetic field. The inset in the figure shows the spectrum of pure PP at the same integral dose of 100 kGy. These spectra are representative of the results obtained from almost all the studied blends. According to the literature, the spectra displayed in Fig. 1 are typical of peroxy radicals [6–8]. All the spectra of the blends show the characteristic signal of peroxy radicals in PP with an octahedral symmetry. Those radicals are randomly distributed
g|| = 2.034 g⊥ = 2.008
dχ"/dH [Arb. Units.]
radiation to the polymer does not take place selectively relative to the mixed components. The probability of the generation of free radicals depends on the strength of the interatomic bonds. The lower the bond energy the easier the bond scission will be. Radiochemical studies on crosslinking or degradation of polymer blends are important for designing new materials. When oxygen is present, radiation-induced changes are often quite different from those produced by irradiation under inert atmosphere [5]. It is well known that electron paramagnetic resonance (EPR), a technique based on the microwave absorption for an unpaired electron, is an excellent technique for the characterization and quantification of the free radicals. In the present paper EPR measurements in c-irradiated PP and PP/SBS blends are reported. The spin concentration as a function of the integral dose and the time decay of this spin concentration for all integral doses were studied in all the samples.
321
g = 2.034 1
g = 2.008 2
g = 1.992 3
330
325
340 H [mT]
330
350
335 340 H [mT]
345
350
Fig. 1. EPR spectrum of PP/SBS (20%) irradiated at 100 kGy of integral dose. The inset shows the spectrum for pure PP at the same integral dose. Unnormalized spectra.
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P. Silva et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 320–326
in their surroundings. The spectrum of pure PP is typical of those with an octahedral symmetry with a rhombic distortion [9]. Due to the stability displayed by these radicals, there are authors suggesting that such radicals are trapped in a phase with a restricted mobility, corresponding to the crystalline phase of PP [6,10]. From the spectra of the PP/SBS (20%) two g values can be clearly obtained, g? ¼ 2:008 0:002 and gk ¼ 2:034 0:002. In pure PP, three g values can be identified, g1 ¼ 2:034 0:002, g2 ¼ 2:008 0:002 and g3 ¼ 1:992 0:002, due to the anisotropy. The rest of the samples show very similar g values, taking into account the experimental error. These values are in perfect agreement with those reported elsewhere [6]. In the inset, as can be seen, there are other signals than that of the peroxy radical. When the PP sample is irradiated in air, the oxidation processes begin simultaneously as a consequence of the oxygen diffusion during the irradiation. An intermediate spectrum between alkyl and peroxy is observed and the last one becomes more important as the oxygen concentration increases. The wider signal in the spectra can be ascribed to polyenyl radicals, which are left after the alkyl radicals decay [7,11]. The same behavior is also observed in the PP/SBS blends. Fig. 2 shows the total spin concentration in grams as a function of the integral dose of irradi-
5
0% 10% 20% 30% 40%
Spin/gr. x10-17
4 3 2 1 0
0
25
50
75
100
Integral Dose (kGy) Fig. 2. Total free radical concentration as a function of integral dose for all the samples after 4 h of being irradiated. The lines correspond to the best fit obtained with Eq. (2).
ation for all studied samples in the integral dose range of 5 6 DI 6 100 kGy. An increase in the number of free radicals with the integral dose of irradiation is observed in all the samples. For the lowest dose the number of generated radicals is similar, within the experimental error, in all the blends including that of pure polypropylene. At the highest doses, a slight decrease in the number of radicals when SBS is present in the sample was obtained. That is, the sample of pure PP shows a higher free radical concentration for ID P 50 kGy. The experimental results were fitted assuming that the total radical concentration responds to a mixed zero and first order generation–recombination processes [12,13]: dN ¼ K0G þ K1G N K0R K1R N ; dt
ð1Þ
where N is the number of total free radicals generated during the irradiation process and the K’s are the zero and first order generation and recombination rate constants. Defining K0
ðK0G K0R Þ and K1 ðK1R K1G Þ, and assuming that these new constants depend on the energy deposited into the sample by the incident radiation in the time unit, the total free radical concentration as a function of the total incident dose was obtained using the following expression: 0
N ¼ N0 e K1 DI þ
K00 0 ð1 e K1 DI Þ; K10
ð2Þ
where K00 and K10 are proportionality constants, and N0 gives an idea of the radical concentration at zero dose [12,13]. The concavity of the curve depends on the sign of these constants. For positive values of both K00 and K10 , the curve will be concave down and the rate of recombination would be higher than that of the generation of free radicals. For a positive and a negative value of K00 and K10 , respectively, the curve is concave up (convex). This means that the rate of generation of free radicals is higher than that of their recombination. Table 1 displays the best fit parameters obtained from the fitting of all the data showed in Fig. 2 The curve concavity for pure PP and for the PP/SBS (20%) blend suggests that the rate of radical recombination increases with the integral dose, implying a high free radical recombination
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323
N0
K10
K00 K10
0 10 20 30 40
6.39 · 1015 3.34 · 1016 9.08 · 1015 9.35 · 1016 5.18 · 1016
0.01431 0.00396 0.01948 1.27 · 10 9 )0.00161
5.99 · 1017 1.18 · 1018 4.75 · 1016 2.24 · 1024 )2.01 · 1018
rate constant. In all the other blends a different behavior is observed. For both PP/SBS (10%) and PP/SBS (30%), a slightly linear fitting is obtained which could be interpreted as a single near zero order generation–recombination process. For the PP/SBS (40%) sample, the concavity of the curve and the parameters obtained from the fitting suggest that the generation rate of the free radicals is higher than the recombination rate in the first order process. The mobility and the generation rate of the free radicals is responsible for the change in the concavity of the curve; a decrease in the free radicals mobility limits their recombination. In pure PP, the recombination rate of free radicals is higher than their generation rate, as is observed in the curve. In the blends, except in the PP/SBS (20%), the generation of radicals increases with the percentage of SBS, being for the PP/SBS (40%) the highest. Hence, it can be said that the latter blend is the most sensible to radiation at the highest integral dose, displaying an even comparable free radical concentration to that of pure PP. Fig. 3 shows the EPR spectrum of pure SBS just after being irradiated and after 110 h. The noise/signal ratio is very high as noticed, and even higher after 110 h of the irradiation process, as a consequence of the very high recombination rate of its radicals. SBS has a higher concentration of double bonds in butadiene facilitating a higher formation of free radicals when the radiation dose and the SBS percentage are increased. It is expected that the curves corresponding to the blends cross with the curve corresponding to the pure PP at a specific dose that will depend on the percentage of SBS. Above this dose the total free radical concentration will be higher than that for the pure PP in the specific blend.
325
325
330
330
335 340 H [mT]
335 340 H [mT]
345
345
350
350
Fig. 3. EPR spectrum of pure SBS just after being irradiated at 50 kGy of integral dose. The inset shows the spectrum after 110 h of being irradiated.
Fig. 4 shows the total free radical concentration as a function of the SBS percentage for all the samples just after being irradiated. For simplicity, only three doses are shown. As expected, a displacement in the curves is observed, increasing the spin concentration as the dose is increased. As seen in the same figure, a slight decrease in the total free radical concentration is noticed when the percentage of SBS is increased. It is clear from this graph that pure PP has a higher free radical concentration than its blends with SBS. This fact
6.0
5 kGy 75 kGy 100 kGy
5.0
Spin/gr x 10-17
SBS (%)
dχ"/dH [Arb. Units.]
Table 1 Fitting parameters obtained from graph in Fig. 2
4.0 3.0 0.6 0.5 0.4 0.3
0
10
20 30 % of SBS
40
Fig. 4. Total free radical concentration as a function of the percentage of SBS just after being irradiated. The lines are guides for the eyes.
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could be attributed to the higher degree of crystallinity of PP, whereas the SBS is amorphous [14]. The radicals forming in the crystalline zones must diffuse towards the amorphous–crystalline interface in order to react with the oxygen and thus form the peroxides. In general, for all the cases shown in Fig. 4, the free radical concentration in PP is much higher than that in the blends, probably due to the fact that the latter was already decreased at the moment of taking the measurements, as a consequence of the higher recombination rate in pure SBS. However, it is noteworthy that having a higher concentration of double bonds in butadiene as the amount of SBS is increased, it is possible that the amount of initially formed radicals was higher, and even though they would decay, were in higher quantities at the moment of being irradiated. This is in agreement with the relatively higher peroxide concentration in the blends, as spectra in Fig. 5 displays, where the EPR spectra of the samples irradiated at 100 kGy is reported. That figure shows that the radical peroxy signal becomes more defined and the contribution of the alkyl radical signal to the spectra is decreased as the SBS concentration in the blend is increased. On the other hand, it is also possible that the benzene ring, due to stability and steric reasons, hinders the oxygen diffusion and its reaction in the irradiated materials, thus decreas-
ing the amount of peroxy radicals formed when SBS is present at concentrations of 10%. Additionally, the SBS acts as a diluent in the PP crystallization process, increasing its amorphous character, which brings about the fact that if the radicals are indeed formed, they will recombine quickly forming stable structures. That is the reason why at low SBS concentrations the measured radical concentrations decrease [5]. A study of the radical decay of the studied samples irradiated at 100 kGy was done. Its results are displayed in Fig. 6. A similar behavior is observed for the other doses. The decay of the total radical concentration was fitted assuming a mixed first and zero order processes, as those described by Eq. (3): N ¼ p1 e p2 t þ p3 ð1 e p2 t Þ þ p4 e p5 t ;
ð3Þ
where p1 , p2 , p3 , p4 and p5 are fitting parameters, the inverse of p2 and p5 are somehow exponential time decays s1 and s2 of the total free radical concentration. The best fitting parameters of the data in Fig. 6 are shown in Table 2. From that table it is immediately inferred that there are effectively two mechanisms involved in the recombination of the free radicals. The first one, associated with the exponential decay time constant s2 , shorter in time 44 < s2 < 51 h, which can be associated with the reaction of the radicals with
16
dχ"/dH [Arb. Units.]
(a)
Spin/gr x 10-17
(b) (c) (d) (e)
SBS 0 % SBS 10 % SBS 20 % SBS 40 %
12 8 4
325
330
335 340 H [mT]
345
350
Fig. 5. EPR spectra of all the studied samples: (a) PP, (b) PP/ SBS (10%), (c) PP/SBS (20%), (d) PP/SBS (30%) and (e) PP/SBS (40%), at 100 kGy of integral dose. Unnormalized spectra.
0
0
400
800
1200
Time (h) Fig. 6. Time decay of the total free radical concentration for most of the studied samples irradiated at 100 kGy. The lines correspond to the best fit obtained with Eq. (3).
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325
Table 2 Fitting parameters obtained using Eq. (3) for all the samples irradiated at 100 kGy SBS (%)
P1 17
4.11 · 10 4.83 · 1017 3.85 · 1017 3.41 · 1017
0 10 20 40
P2
P3
P4
0.00281 0.00299 0.00213 0.00146
)1.69 · 10 )2.65 · 1016 )1.32 · 1016 1.35 · 1016 16
18
1.03 · 10 5.63 · 1017 6.15 · 1017 6.64 · 1017
P5
s1
s2
0.02237 0.02945 0.02211 0.01965
355.87 334.45 469.48 684.93
44.70 33.96 45.23 50.89
Table 3 Fitting parameters obtained using Eq. (3) for the PP/SBS (40%) blend for the most integral doses of irradiation Integral dose (kGy)
P1
P2
P3
P4
P5
s1
s2
5 25 75 100
5.91 · 1016 1.12 · 1017 2.98 · 1017 3.41 · 1017
0.00388 0.00157 0.00216 0.00146
5.08 · 1015 2.12 · 1015 )1.25 · 1016 1.35 · 1016
8.47 · 1016 2.33 · 1017 4.72 · 1017 6.64 · 1017
0.05703 0.01551 0.02403 0.01965
257.73 636.94 462.96 684.93
17.53 64.48 41.62 50.89
their nearest neighbors, and the other one, related to the mobility of the radicals, associated with s1 , which is larger in time 350 < s1 < 700 h. In Table 2, the fact that these exponential decay time constants increase when the percentage of SBS is increased can be observed. This is due to the higher free radical concentration in the blends as the percentage of the SBS increases. This is in agreement with the results displayed in Fig. 2. These results can be interpreted assuming that a group of radicals react rapidly with their neighbors, may it be the alkyl and allyl radicals that disappear at
12 5 kGy 25 kGy 75 kGy 100 kGy
Spin/gr x 10-17
10 8 6
very short times and other group of radicals that take much more time, because they are impeded by their mobility (maybe the peroxy radicals). As can be seen, s2 is similar for pure PP and for the PP/ SBS (20%) sample. This is in agreement with the behavior observed in Fig. 2. Fig. 7 shows the time decay of the total free radical concentration for the PP/SBS (40%) blend for some of the studied integral doses. For clarity purposes, only four doses are shown; the other samples show similar behavior. Table 3 displays the best fitting parameter obtained for the data in Fig. 7 using Eq. (3). Except for the 25 kGy dose, an increase in the exponential decay time is observed when the integral dose is increased. Certainly, this behavior cannot be associated with the mobility. Instead, this behavior must be associated with the concentration of free radicals generated at each dose. There is no explanation for the behavior obtained at the dose of 25 kGy.
4
4. Conclusions 2 0 0
400
800 Time (h)
1200
1600
Fig. 7. Time decay of the total free radical concentration for the sample PP/SBS (40%) for most of the integral doses studied. The lines correspond to the best fit obtained with Eq. (3).
Pure PP and its blends with SBS shows the presence of allyl, alkyl and peroxy radicals after irradiation in air in their EPR spectra. Dynamics ruling free radical formation in PP and its blends with SBS is a zero and first order process. The time decay of the total free radical concentration may be fitted using a mixed first and zero order
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processes. There are two exponential time decay constants associated with two different processes of recombination of the free radicals. The radical decay dynamics is the same for both pure PP and its blends with SBS. The sole presence of SBS decreases the total free radical concentration in the sample after 4 h of being irradiated, and, in general, an increment in the SBS concentration leads to an increment in the generation rate of free radicals.
Acknowledgements This work was supported in part by FONACIT G-2001000817 project and by funds from the Instituto Venezolano de Investigaciones Cientıficas. References [1] N.R. Choudhury, A.K. Bhoce, J. Mater. Sci. 23 (1988) 2187.
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