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Dose rate effect on internal friction and structural transformations in electron-irradiated carbon-armored composites
ARTICLE IN PRESS
Radiation Physics and Chemistry 76 (2007) 1399–1403 www.elsevier.com/locate/radphyschem
Dose rate effect on internal friction and structural transformations in electron-irradiated carbon-armored composites Yu.A. Zaikina,, D.B. Aimuratova, M. Al-Sheikhlyb a
Al Farabi Kazakh National University, 96a Tole bi, 480012 Almaty, Kazakhstan b University of Maryland, College Park, USA
Abstract Temperature dependence of internal friction and specific electric resistance of multi-layer carbon-armored epoxy-based composites is experimentally studied in the temperature range of 20–300 1C before and after irradiation with 2 MeV electrons. It is shown that carbon penetration into the polymer matrix causes intense polymer cross-linking in the basic layers of the composite even at low irradiation doses. The strong effect of dose rate on radiation-induced structural transformations was observed. r 2007 Elsevier Ltd. All rights reserved. Keywords: Carbon-armored composites; Internal friction; Mechanical relaxation; Electric resistance; Scission; Cross-linking; Electron irradiation; Dose; Dose rate
1. Introduction Recent works (Zaikin and Koztaeva, 2000, 2002; Zaikin et al., 2003) have shown that temperature dependence of internal friction in heterogeneous systems with polymer binder can be a source of important information on the structural changes in different components and intermediate layers of composites (glass–cloth laminates, paperbased laminates, foiled dielectrics, etc.) under the action of radiation and other external factors. In this work, spectra of mechanical relaxation are studied in the carbon-armored epoxy-filled composite for obtaining quantitative characteristics of thermally activated and radiation-induced processes in this class of composite materials. 2. Experimental The two types of specially prepared composite samples were used in the experiments. The first series of the samples Corresponding author. Present address: PetroBeam Inc., R&D Director, 212 Powel Dr., Unit 130, Raleigh, NC 27670, USA. Tel.: +1 919 515 2324; fax: +1 919 515 3465. E-mail address: [email protected] (Yu.A. Zaikin).
0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.02.041
studied was the pressed laminated material consisting of epoxy-impregnated carbon fibers. Samples of the second type included several layers oriented under different angles and sewed by polyester threads. The method of vacuum infusion of liquid epoxy to the block of dry fibers was applied for the material manufacture. The carbon fibers used in the experiment were of the cylindrical form from 5 to 50 mm in diameter. Structure of this type of samples studied by transmission electron microscopy is shown in Figs. 1 and 2. Fig. 1a demonstrates carbon fiber lying in the longitudinal and the transverse sections of the sample. Fig. 1b shows the structure of the carbon-depleted region between the composite layers that considerably affects radiation properties of the material. The composite samples were irradiated by 2 MeV electrons at the linear electron accelerator ELU-4. The pulse width was 5 ms, the time interval between pulses— 5 ms. The samples were irradiated by different doses of electrons up to the time-averaged dose of 60 MGy at the room temperature. Temperature dependence of internal friction and shear modulus was measured using the torsion pendulum facility at the oscillation frequency of about 1 s1 in the temperature range of 20–300 1C.
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Fig. 1. Carbon fibers packing in epoxy-filled carbon-armored composite: (a) longitudinal section and (b) cross-section of the carbon-depleted region.
Fig. 2. Temperature dependences of internal friction (a) and shear modulus (b) in the two-layer samples of epoxy-filled carbon-armored composite irradiated by electrons to doses, D: D ¼ 0 (1), D ¼ 6 MGy (2), D ¼ 12 MGy (3), D ¼ 24 MGy (4) and D ¼ 60 MGy (5). The layer thickness: 0.4 mm.
Temperature dependence of the steady-state internal friction and shear modulus after irradiation of the 0.4-mmthick two-layer composite samples by different electron doses at the same time-averaged dose rate of 2 kGy/s is shown in Fig. 2. Two big relaxation peaks are observed in the temperature dependence of internal friction (Fig. 2a). The experiment has shown (Zaikin et al., 2004) that these twin peaks always appear in the multi-layer samples. They have the same fine structure independently on the method of sample manufacture, dimensions and orientation of carbon fibers. The high-temperature maximum (a2-peak) earlier observed in epoxy-filled composites and neat epoxy (Zaikin and Koztaeva, 2000, 2002; Zaikin et al., 2003) is associated with the transition of the polymer matrix from glassy to high-elastic state. This interpretation is confirmed by the drop in the shear modulus characteristic for the glassy transition (Fig. 2b). A special experiment on separation of the composite layers was conducted for identification of the lowtemperature maximum of internal friction (a1-peak). The pronounced high-temperature a2-peak is observed as before in the temperature dependence of internal friction measured for the separate layer while a1-peak disappears. It shows that a1-peak is associated with the glassy transition in carbon-depleted intermediate region between
the composite layers. The relaxation process in this region is characterized by lower activation energy and higher mobility of macromolecule segments compared with the polymer matrix. On the contrary, increase in the number of composite layers leads to the sample deformation concentrated in the intermediate composite layers. As a result, a1-peak is suppressed while a2-peak is well pronounced. Activation energy of a-relaxation characterizes thermal mobility of the free macromolecule segments. It was determined from the position of the internal friction maximum in the temperature scale and the frequency of the sample oscillations. The values of the activation energy (0.89 eV for a1-peak and 1.23 eV for a2-peak) do not depend on the electron irradiation dose. Fig. 2 shows that two characteristic contrary flexures caused by superposition of two additional maximums of internal friction (a0 -peaks) are observed in the both of the a-peaks. Similar peaks of internal friction were observed in experimental studies of epoxy-filled glass–cloth laminates by Zaikin and Koztaeva (2000, 2002) and Zaikin et al. (2003). Coming from comparison with the internal friction data in the neat polymer, it was suggested that the lowtemperature a0 -peak is associated with the changes in the molecular segment mobility in the crystalline phase of the polymer binder, and the high-temperature a0 -peak appears due to the structural transition in the contact region binder-filler, epoxy-carbon fiber in our case.
ARTICLE IN PRESS Yu.A. Zaikin et al. / Radiation Physics and Chemistry 76 (2007) 1399–1403
Changes in the heights of the internal friction peaks observed in the irradiated composite samples are proportional to the number of radiation-induced scission and cross-linking events. Therefore, measurements of internal friction temperature dependences supply exact quantitative information on radiation-induced structural alterations in the different structural components of the composite. It follows from the graphs in Fig. 2 that at low doses radiation scission predominates both in the ‘‘free’’ polymer and in the intermediate regions of the composite. It is remarkable that the scissoring rate decreases with irradiation dose until scissoring and cross-linking compensate for each other in the dose range of 12–48 MGy (Zaikin et al., 2004). Changes in the heights of the additional a0 -peaks with irradiation dose demonstrate the same direction of radiation-induced structural alterations in free polymer and intermediate regions of the composite material. 3. Temperature dependence of internal friction in electronirradiated composites Important information on radiation-initiated transformation in polymer materials can be obtained from the
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dependence of internal friction parameters on irradiation dose rate. Fig. 3 shows temperature dependence of internal friction in the samples of the epoxy-filled carbon-armored composite irradiated at the different electron dose rates to the doses of 6 and 12 MGy. The dose rate values indicated in Fig. 3 and in the subsequent text are the time-averaged characteristics of the pulse electron irradiation. Fig. 3 demonstrates considerable changes in the internal friction peak characteristics as the dose rate increases. Dependence of the heights of internal friction maximums on the dose rate for the composite sample irradiated to the dose of 6 MGy is shown in Fig. 4. A similar dependence was observed for the higher dose of 12 MGy. Fig. 4 shows that, at low dose rates, increase in the dose rate causes increase in the scissoring rate in all the structural components of the composite. The peak heights reach maximum at a certain critical dose rate and then gradually decrease approaching the peak height before irradiation. Such behavior of internal friction maxima can be associated with the effect of spur overlapping that leads to suppression of radical reactions at the heightened dose
Fig. 3. Temperature dependence of internal friction in two-layer samples of epoxy-filled carbon-armored composite irradiated by electrons to doses 6 MGy (a) and 12 MGy (b) at different dose rates.
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Fig. 4. Dependence of the difference in the heights of low-temperature (a) and high-temperature (b) peaks of internal friction in irradiated and unirradiated composite samples on the dose rate of electron irradiation. Irradiation dose, D ¼ 6 MGy.
Fig. 5. Temperature dependences of internal friction (a) and specific electric resistance (b) in the two-layer sample of carbon-armored composite irradiated by electrons to the dose of 12 MGy.
rates. The limited volume for the reactions releases steric difficulties for recombination of radical appearing as a result of C–C bond breaks. Therefore, polymer scissoring is suppressed stronger than cross-linking processes. It results in decrease in the difference in the numbers of scission and the cross-linking events and, therefore, in decrease in the heights of internal friction peaks, which are proportional to this quantity. Another essential contribution to suppression of the internal friction peaks is associated with the additional cross-linking provoked by carbon diffusion into the polymer matrix. 4. Electric properties of carbon-armored composites The radiation-initiated structural transformations described above define not only structural alterations but also changes in the electric properties of the composite (Zaikin and Shirokaya, 2005). Fig. 5 shows temperature dependence of internal friction and specific electric resistance measured in the direction normal to the plane of fiber packing for the two-layer composite sample irradiated to the dose of 12 MGy. Comparison shows direct correlation of electric and dissipative properties of the material. Any of the peaks in the temperature dependence of specific electric resistance
corresponds to the appropriate internal friction peak. The observed correlation shows that current carriers in the carbon-armored polymer composite scatter predominantly at the oscillations of the free macromolecule segments. Simultaneously, they play the role of the elements that dissipate elastic energy of the sample’s mechanical oscillations. The relative increment of the specific electric resistance of the two-layer composite samples is approximately equal to the relative changes in the heights of the internal friction a-peaks up to the dose of 24 MGy. For the higher doses, correlation of radiation alterations of internal friction and specific electric resistance gets broken due to the considerable increase in the defectiveness of the intermediate composite layers and partial exfoliation of the composite. In the one-layer samples made by the partition of the two-layer composite samples, the high peak of specific electric resistance observed in the vicinity of the glassy transition temperature corresponds to the a2-peak of internal friction. After removal of the remainder of the intermediate layer, electric resistance of the one-layer sample considerably decreases, and the dose dependence of internal friction and specific electric resistance behaves as shown in Fig. 6.
ARTICLE IN PRESS Yu.A. Zaikin et al. / Radiation Physics and Chemistry 76 (2007) 1399–1403
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Fig. 6. Temperature dependence of internal friction (a) and specific electric resistance (b) in the one-layer sample of carbon-armored composite (without intermediate layer) irradiated to doses, D: (1) D ¼ 0, (2) 6 MGy, (3) 12 MGy, (4) 24 MGy, (5) 30 MGy, (6) 40 MGy and (7) 50 MGy.
Fig. 6 shows that pronounced maxima of electric resistance associated with electron scattering at the oscillations of the macromolecule segments are observed in the one-layer samples. However, correlation of radiation-induced changes in internal friction and electric resistance gets broken even at lower doses compared with the two-layer samples (about 12 MGy) due to intense carbon diffusion that causes considerable decrease in the electric resistance. Maximal electric resistance of the composite was reached at the temperature of about 90 1C. It increases up to the dose of 12 MGy that corresponds to epoxy radiation scission. At higher irradiation doses, electric resistance drops down. It is evident from Fig. 6 that specific electric resistance of the one-layer samples decreases as temperature and irradiation dose increase and tends to the limiting value of about 1 102 O m. Such behavior of the electric resistance indicates to intense carbon diffusion in onelayer samples densely packed with carbon fibers. Carbon diffusion intensifies as temperature and electron irradiation dose increase. We shall note that not only electric resistance but also the absolute values of its radiation alterations are much lower in the one-layer samples compared with the twolayer composites. It can be explained by the heterogeneity of the intermediate layer removed in the one-layer samples. Comparison with data of Fig. 5 shows that specific electric resistance is much higher in the two-layer samples. This result comes from the high resistance of the carbondepleted intermediate layers of multi-layer composites where the macromolecule segmental mobility is close to that observed in the neat polymer. It is obvious that these regions make the main contribution to radiation changes of electric resistance. Generally, the observed changes in the specific electric resistance depend on carbon fiber distribution in the basic and intermediate layers of the composite and diffusion-
controlled atomic carbon distribution in the polymer matrix. In the case of low concentrations of the atomic carbon in the polymer matrix, jumps of specific electric resistance in the vicinity of glassy transition can be very high. In our experiments, these alterations in the specific resistance reached 100 O m. An increase in the dose rate of electron irradiation, as well as an increase in dose, provokes enhanced carbon diffusion and additional cross-linking of the polymer matrix. As a result, both peaks in the temperature dependence of internal friction and peaks of electric resistance get suppressed. 5. Conclusion Radiation-initiated structural alterations observed in polymer-based carbon-armored composites lead to nonstandard dose and temperature dependences of the electric resistance that can find interesting technical applications. References Zaikin, Yu.A., Koztaeva, U.P., 2000. Radiation-induced processes and internal friction in polymer-based composite materials. Radiat. Phys. Chem. 58 (4), 387–395. Zaikin, Yu.A., Koztaeva, U.P., 2002. Radiation resistance and structural transitions in polymer-based composites irradiated by electrons. Radiat. Phys. Chem. 63 (2), 547–550. Zaikin, Yu.A., Shirokaya, N.A., 2005. On the problem of nano-structures formation in carbon-armored polymer composites irradiated by electrons. In: Proceedings of the 8th International Conference on ‘‘Solid State Physics’’. Institute of Nuclear Physics, Almaty, pp. 245–253. Zaikin, Yu.A., Potanin, A.S., Koztaeva, U.P., 2003. Kinetics of radiationinduced structural alterations in electron-irradiated polymer-based composites. Radiat. Phys. Chem. 64 (3/4), 431–435. Zaikin, Yu.A., Shirokaya, N.A., Sabitova, M.Sh., Koztaeva, U.P., 2004. Structural transformations and mechanical relaxation in carbonarmored composites irradiated by electrons. In: Proceedings of the fourth International Conference on ‘‘Nuclear and Radiation Physics,’’ vol. 2. Institute of Nuclear Physics, Almaty, pp. 358–366.
Radiation Physics and Chemistry 76 (2007) 1399–1403 www.elsevier.com/locate/radphyschem
Dose rate effect on internal friction and structural transformations in electron-irradiated carbon-armored composites Yu.A. Zaikina,, D.B. Aimuratova, M. Al-Sheikhlyb a
Al Farabi Kazakh National University, 96a Tole bi, 480012 Almaty, Kazakhstan b University of Maryland, College Park, USA
Abstract Temperature dependence of internal friction and specific electric resistance of multi-layer carbon-armored epoxy-based composites is experimentally studied in the temperature range of 20–300 1C before and after irradiation with 2 MeV electrons. It is shown that carbon penetration into the polymer matrix causes intense polymer cross-linking in the basic layers of the composite even at low irradiation doses. The strong effect of dose rate on radiation-induced structural transformations was observed. r 2007 Elsevier Ltd. All rights reserved. Keywords: Carbon-armored composites; Internal friction; Mechanical relaxation; Electric resistance; Scission; Cross-linking; Electron irradiation; Dose; Dose rate
1. Introduction Recent works (Zaikin and Koztaeva, 2000, 2002; Zaikin et al., 2003) have shown that temperature dependence of internal friction in heterogeneous systems with polymer binder can be a source of important information on the structural changes in different components and intermediate layers of composites (glass–cloth laminates, paperbased laminates, foiled dielectrics, etc.) under the action of radiation and other external factors. In this work, spectra of mechanical relaxation are studied in the carbon-armored epoxy-filled composite for obtaining quantitative characteristics of thermally activated and radiation-induced processes in this class of composite materials. 2. Experimental The two types of specially prepared composite samples were used in the experiments. The first series of the samples Corresponding author. Present address: PetroBeam Inc., R&D Director, 212 Powel Dr., Unit 130, Raleigh, NC 27670, USA. Tel.: +1 919 515 2324; fax: +1 919 515 3465. E-mail address: [email protected] (Yu.A. Zaikin).
0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.02.041
studied was the pressed laminated material consisting of epoxy-impregnated carbon fibers. Samples of the second type included several layers oriented under different angles and sewed by polyester threads. The method of vacuum infusion of liquid epoxy to the block of dry fibers was applied for the material manufacture. The carbon fibers used in the experiment were of the cylindrical form from 5 to 50 mm in diameter. Structure of this type of samples studied by transmission electron microscopy is shown in Figs. 1 and 2. Fig. 1a demonstrates carbon fiber lying in the longitudinal and the transverse sections of the sample. Fig. 1b shows the structure of the carbon-depleted region between the composite layers that considerably affects radiation properties of the material. The composite samples were irradiated by 2 MeV electrons at the linear electron accelerator ELU-4. The pulse width was 5 ms, the time interval between pulses— 5 ms. The samples were irradiated by different doses of electrons up to the time-averaged dose of 60 MGy at the room temperature. Temperature dependence of internal friction and shear modulus was measured using the torsion pendulum facility at the oscillation frequency of about 1 s1 in the temperature range of 20–300 1C.
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Fig. 1. Carbon fibers packing in epoxy-filled carbon-armored composite: (a) longitudinal section and (b) cross-section of the carbon-depleted region.
Fig. 2. Temperature dependences of internal friction (a) and shear modulus (b) in the two-layer samples of epoxy-filled carbon-armored composite irradiated by electrons to doses, D: D ¼ 0 (1), D ¼ 6 MGy (2), D ¼ 12 MGy (3), D ¼ 24 MGy (4) and D ¼ 60 MGy (5). The layer thickness: 0.4 mm.
Temperature dependence of the steady-state internal friction and shear modulus after irradiation of the 0.4-mmthick two-layer composite samples by different electron doses at the same time-averaged dose rate of 2 kGy/s is shown in Fig. 2. Two big relaxation peaks are observed in the temperature dependence of internal friction (Fig. 2a). The experiment has shown (Zaikin et al., 2004) that these twin peaks always appear in the multi-layer samples. They have the same fine structure independently on the method of sample manufacture, dimensions and orientation of carbon fibers. The high-temperature maximum (a2-peak) earlier observed in epoxy-filled composites and neat epoxy (Zaikin and Koztaeva, 2000, 2002; Zaikin et al., 2003) is associated with the transition of the polymer matrix from glassy to high-elastic state. This interpretation is confirmed by the drop in the shear modulus characteristic for the glassy transition (Fig. 2b). A special experiment on separation of the composite layers was conducted for identification of the lowtemperature maximum of internal friction (a1-peak). The pronounced high-temperature a2-peak is observed as before in the temperature dependence of internal friction measured for the separate layer while a1-peak disappears. It shows that a1-peak is associated with the glassy transition in carbon-depleted intermediate region between
the composite layers. The relaxation process in this region is characterized by lower activation energy and higher mobility of macromolecule segments compared with the polymer matrix. On the contrary, increase in the number of composite layers leads to the sample deformation concentrated in the intermediate composite layers. As a result, a1-peak is suppressed while a2-peak is well pronounced. Activation energy of a-relaxation characterizes thermal mobility of the free macromolecule segments. It was determined from the position of the internal friction maximum in the temperature scale and the frequency of the sample oscillations. The values of the activation energy (0.89 eV for a1-peak and 1.23 eV for a2-peak) do not depend on the electron irradiation dose. Fig. 2 shows that two characteristic contrary flexures caused by superposition of two additional maximums of internal friction (a0 -peaks) are observed in the both of the a-peaks. Similar peaks of internal friction were observed in experimental studies of epoxy-filled glass–cloth laminates by Zaikin and Koztaeva (2000, 2002) and Zaikin et al. (2003). Coming from comparison with the internal friction data in the neat polymer, it was suggested that the lowtemperature a0 -peak is associated with the changes in the molecular segment mobility in the crystalline phase of the polymer binder, and the high-temperature a0 -peak appears due to the structural transition in the contact region binder-filler, epoxy-carbon fiber in our case.
ARTICLE IN PRESS Yu.A. Zaikin et al. / Radiation Physics and Chemistry 76 (2007) 1399–1403
Changes in the heights of the internal friction peaks observed in the irradiated composite samples are proportional to the number of radiation-induced scission and cross-linking events. Therefore, measurements of internal friction temperature dependences supply exact quantitative information on radiation-induced structural alterations in the different structural components of the composite. It follows from the graphs in Fig. 2 that at low doses radiation scission predominates both in the ‘‘free’’ polymer and in the intermediate regions of the composite. It is remarkable that the scissoring rate decreases with irradiation dose until scissoring and cross-linking compensate for each other in the dose range of 12–48 MGy (Zaikin et al., 2004). Changes in the heights of the additional a0 -peaks with irradiation dose demonstrate the same direction of radiation-induced structural alterations in free polymer and intermediate regions of the composite material. 3. Temperature dependence of internal friction in electronirradiated composites Important information on radiation-initiated transformation in polymer materials can be obtained from the
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dependence of internal friction parameters on irradiation dose rate. Fig. 3 shows temperature dependence of internal friction in the samples of the epoxy-filled carbon-armored composite irradiated at the different electron dose rates to the doses of 6 and 12 MGy. The dose rate values indicated in Fig. 3 and in the subsequent text are the time-averaged characteristics of the pulse electron irradiation. Fig. 3 demonstrates considerable changes in the internal friction peak characteristics as the dose rate increases. Dependence of the heights of internal friction maximums on the dose rate for the composite sample irradiated to the dose of 6 MGy is shown in Fig. 4. A similar dependence was observed for the higher dose of 12 MGy. Fig. 4 shows that, at low dose rates, increase in the dose rate causes increase in the scissoring rate in all the structural components of the composite. The peak heights reach maximum at a certain critical dose rate and then gradually decrease approaching the peak height before irradiation. Such behavior of internal friction maxima can be associated with the effect of spur overlapping that leads to suppression of radical reactions at the heightened dose
Fig. 3. Temperature dependence of internal friction in two-layer samples of epoxy-filled carbon-armored composite irradiated by electrons to doses 6 MGy (a) and 12 MGy (b) at different dose rates.
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Fig. 4. Dependence of the difference in the heights of low-temperature (a) and high-temperature (b) peaks of internal friction in irradiated and unirradiated composite samples on the dose rate of electron irradiation. Irradiation dose, D ¼ 6 MGy.
Fig. 5. Temperature dependences of internal friction (a) and specific electric resistance (b) in the two-layer sample of carbon-armored composite irradiated by electrons to the dose of 12 MGy.
rates. The limited volume for the reactions releases steric difficulties for recombination of radical appearing as a result of C–C bond breaks. Therefore, polymer scissoring is suppressed stronger than cross-linking processes. It results in decrease in the difference in the numbers of scission and the cross-linking events and, therefore, in decrease in the heights of internal friction peaks, which are proportional to this quantity. Another essential contribution to suppression of the internal friction peaks is associated with the additional cross-linking provoked by carbon diffusion into the polymer matrix. 4. Electric properties of carbon-armored composites The radiation-initiated structural transformations described above define not only structural alterations but also changes in the electric properties of the composite (Zaikin and Shirokaya, 2005). Fig. 5 shows temperature dependence of internal friction and specific electric resistance measured in the direction normal to the plane of fiber packing for the two-layer composite sample irradiated to the dose of 12 MGy. Comparison shows direct correlation of electric and dissipative properties of the material. Any of the peaks in the temperature dependence of specific electric resistance
corresponds to the appropriate internal friction peak. The observed correlation shows that current carriers in the carbon-armored polymer composite scatter predominantly at the oscillations of the free macromolecule segments. Simultaneously, they play the role of the elements that dissipate elastic energy of the sample’s mechanical oscillations. The relative increment of the specific electric resistance of the two-layer composite samples is approximately equal to the relative changes in the heights of the internal friction a-peaks up to the dose of 24 MGy. For the higher doses, correlation of radiation alterations of internal friction and specific electric resistance gets broken due to the considerable increase in the defectiveness of the intermediate composite layers and partial exfoliation of the composite. In the one-layer samples made by the partition of the two-layer composite samples, the high peak of specific electric resistance observed in the vicinity of the glassy transition temperature corresponds to the a2-peak of internal friction. After removal of the remainder of the intermediate layer, electric resistance of the one-layer sample considerably decreases, and the dose dependence of internal friction and specific electric resistance behaves as shown in Fig. 6.
ARTICLE IN PRESS Yu.A. Zaikin et al. / Radiation Physics and Chemistry 76 (2007) 1399–1403
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Fig. 6. Temperature dependence of internal friction (a) and specific electric resistance (b) in the one-layer sample of carbon-armored composite (without intermediate layer) irradiated to doses, D: (1) D ¼ 0, (2) 6 MGy, (3) 12 MGy, (4) 24 MGy, (5) 30 MGy, (6) 40 MGy and (7) 50 MGy.
Fig. 6 shows that pronounced maxima of electric resistance associated with electron scattering at the oscillations of the macromolecule segments are observed in the one-layer samples. However, correlation of radiation-induced changes in internal friction and electric resistance gets broken even at lower doses compared with the two-layer samples (about 12 MGy) due to intense carbon diffusion that causes considerable decrease in the electric resistance. Maximal electric resistance of the composite was reached at the temperature of about 90 1C. It increases up to the dose of 12 MGy that corresponds to epoxy radiation scission. At higher irradiation doses, electric resistance drops down. It is evident from Fig. 6 that specific electric resistance of the one-layer samples decreases as temperature and irradiation dose increase and tends to the limiting value of about 1 102 O m. Such behavior of the electric resistance indicates to intense carbon diffusion in onelayer samples densely packed with carbon fibers. Carbon diffusion intensifies as temperature and electron irradiation dose increase. We shall note that not only electric resistance but also the absolute values of its radiation alterations are much lower in the one-layer samples compared with the twolayer composites. It can be explained by the heterogeneity of the intermediate layer removed in the one-layer samples. Comparison with data of Fig. 5 shows that specific electric resistance is much higher in the two-layer samples. This result comes from the high resistance of the carbondepleted intermediate layers of multi-layer composites where the macromolecule segmental mobility is close to that observed in the neat polymer. It is obvious that these regions make the main contribution to radiation changes of electric resistance. Generally, the observed changes in the specific electric resistance depend on carbon fiber distribution in the basic and intermediate layers of the composite and diffusion-
controlled atomic carbon distribution in the polymer matrix. In the case of low concentrations of the atomic carbon in the polymer matrix, jumps of specific electric resistance in the vicinity of glassy transition can be very high. In our experiments, these alterations in the specific resistance reached 100 O m. An increase in the dose rate of electron irradiation, as well as an increase in dose, provokes enhanced carbon diffusion and additional cross-linking of the polymer matrix. As a result, both peaks in the temperature dependence of internal friction and peaks of electric resistance get suppressed. 5. Conclusion Radiation-initiated structural alterations observed in polymer-based carbon-armored composites lead to nonstandard dose and temperature dependences of the electric resistance that can find interesting technical applications. References Zaikin, Yu.A., Koztaeva, U.P., 2000. Radiation-induced processes and internal friction in polymer-based composite materials. Radiat. Phys. Chem. 58 (4), 387–395. Zaikin, Yu.A., Koztaeva, U.P., 2002. Radiation resistance and structural transitions in polymer-based composites irradiated by electrons. Radiat. Phys. Chem. 63 (2), 547–550. Zaikin, Yu.A., Shirokaya, N.A., 2005. On the problem of nano-structures formation in carbon-armored polymer composites irradiated by electrons. In: Proceedings of the 8th International Conference on ‘‘Solid State Physics’’. Institute of Nuclear Physics, Almaty, pp. 245–253. Zaikin, Yu.A., Potanin, A.S., Koztaeva, U.P., 2003. Kinetics of radiationinduced structural alterations in electron-irradiated polymer-based composites. Radiat. Phys. Chem. 64 (3/4), 431–435. Zaikin, Yu.A., Shirokaya, N.A., Sabitova, M.Sh., Koztaeva, U.P., 2004. Structural transformations and mechanical relaxation in carbonarmored composites irradiated by electrons. In: Proceedings of the fourth International Conference on ‘‘Nuclear and Radiation Physics,’’ vol. 2. Institute of Nuclear Physics, Almaty, pp. 358–366.