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The conduction mechanism of carbon black-filled poly(vinylidene fluoride) composite
Materials Letters 57 (2003) 3082 – 3088 www.elsevier.com/locate/matlet
The conduction mechanism of carbon black-filled poly(vinylidene fluoride) composite Zhudi Zhao *, Wenxue Yu, Xiujuan He, Xinfang Chen Department of Materials Science and Alan G. MacDiarmid Lab., Jilin University, Changchun 130023, People’s Republic of China Received 17 September 2002; accepted 15 December 2002
Abstract The positive temperature coefficient (PTC) effect of carbon black (CB)-filled poly(vinylidene fluoride) (PVDF) composite and the changes in resistivity under different conditions were studied. It was found that there were some coherence between the volume expansion and the crystallite melting of PVDF. It is believed that the homogenization diffusion of CB particles results in increasing resistivity during volume expansion and crystallite melting. At high temperature, the resistivity decrease is due to the agglomeration of CB particles or aggregates, which results in a new CB distribution of better conductivity. On the base of the experimental evidence, it is concluded that the PTC effect of PVDF/CB composite is dominated by the volume expansion of composite matrix, the diffusion of CB particles from amorphous region to melted crystalline region and the agglomeration of CB particles. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Conductive polymer composite; Positive temperature coefficient; Conduction mechanism
1. Introduction Polymer materials incorporated conductive fillers exhibiting the effect of positive temperature coefficient (PTC) of resistance were first discovered by Frydman in 1945 [1]. Up to now, polymer PTC materials have been studied extensively because of their numerous high technological electrical and electronic applications [2 –5]. In theory, the mechanism of electric current conduction in the polymer materials have been studied for a long time, but there is still not a satisfactory explanation for PTC effect. Kohler [6] suggested that the PTC effect is caused by thermal
* Corresponding author. E-mail address: [email protected] (Z. Zhao).
expansion. He proposed that the conductive particles were initially spread through the polymer forming a network of conducting chains. As the material is heated, the conductive particles are separated further, thus increasing the resistance. The sudden expansion of polymer at the crystalline melting point is assumed to be the cause of the strong transition to the highresistive state. Ohe and Natio [7] assume that, at lower temperatures, the interparticle gaps are uniformly distributed, but change in a random distribution at higher temperature, thereby increasing the average gap width and thus increasing the resistivity. Meyer [8,9] supposed that thin crystalline forms of polymer were much better conductors than amorphous films, which appear after crystalline melting. In the case of negative temperature coefficient (NTC), it was reported by Narkis and Vaxman [10] and Tang et al. [11] that the
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01440-4
Z. Zhao et al. / Materials Letters 57 (2003) 3082–3088
resistivity decrease is due to the reagglomeration of carbon black (CB) aggregates. However, because these cannot be observed by experimental techniques, there has been difficulty in finding a comprehensible explanation for the effect and, thus, most of mechanism still remain controversial. In the present work, the influence of volume expansion and DSC curves of carbon black-filled poly(vinylidene fluoride) (PVDF) on PTC characteristic curves were studied. The changes of the composites with temperature were measured before and after restricting volume expansion, and the relationship between resistivity and time were also measured at high temperature. A possible mechanism was proposed based on the experimental results.
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thermal properties of the sample. The heating rate was 5 jC/min. The percentage of insoluble gel after irradiation was measured with a Soxhlet apparatus. Approximately 250 mg of sample wrapped in nickel mesh was exposed to refluxing N,N-dimethyl acetamide (DMA) until the sample attained a constant weight, which usually took 56 h. After that, the samples was dried in a vacuum oven at 100 jC for 48 h and the weight (corresponding to the weight of the network structure polymer) was measured. The gel fraction was calculated by g% ¼
w1 wc w wc
where g% is the gel fraction, w1 is the weight of the sample after refluxing, w is the weight of the sample before refluxing and wc is the weight of CB.
2. Experimental PVDF (product of America Pennwalt) with a melting point of 168 jC was used as polymer matrix. CSF carbon black was used as a conductive filler. Its average particle size is 70 nmm, surface area 230 m2/g, dibutyl phthalate (DBP) value 280 ml/100 g and pH value 7– 9. PVDF and CB were melt-mixed in a Brabender for 15 min at 200 jC, then kneaded on two-roll mill for 5 min at 180 jC. The samples were compression molded at 200 jC for 10 min, then quenched in icewater and slowly cooled under pressure to room temperature, respectively. The irradiation of composites was carried out in vacuum at room temperature with a 60Co g-ray source. All of the irradiated composites were held for 1 day after irradiation to eliminate trap radicals. The volume resistivities of the composites were measured by a digital multimeter at a progressively elevated temperature. The two sides of the samples were bonded with metallic foil to reduce the contact resistance. The heating rate was 5 jC/min. The volume expansion coefficient of the samples was obtained using a capillary dilatometer at a heating rate of 5 jC/min. Some samples were sealed in ceramic tube with electrodes to measure their thermal expansion coefficient under the condition of restricting volume expansion. A Perkin-Elmer differential scanning DSC-7 calorimeter was used to measure the
3. Results and discussion The resistivity – temperature curves of quenched and slowly cooled samples with 8 wt.% CB content are shown in Fig. 1. It can be seen that the PTC curves for two samples give different profiles. For the slowly cooled sample, the peak of PTC curves shifts to higher temperature, the width of peak becomes narrow and PTC intensity (the ratio of peak resistivity to room temperature resistivity) increases compared with the quenched composite. Fig. 2 shows the influence of temperature on thermal expansion coefficients of the composites. Before NTC effect appears, the shapes of two thermal expansion coefficients – temperature curves are approximately similar to those of corresponding resistivity– temperature curves (see Fig. 1). The thermal expansion initial and final jumping temperatures for the quenched sample are lower, whereas those of the cooled slowly sample are higher. For same kinds of samples, the transition temperatures in the resistivity –temperature curves are basically consistent with those of the corresponding thermal expansion curves. The consistency reveals that there exists a inherent relationship between PTC characteristic and volume expansion. In order to examine the influence of volume expansion on PTC effect, we sealed a slowly cooled sample in a ceramic tube to measure its PTC charac-
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Fig. 1. Resistivity – temperature behaviors of PVDF/CB composites.
teristic. As shown in Fig. 3, the PTC intensity drops 1.5 orders of magnitude. The thermal expansion coefficient of the ceramic tube is much less that of
the sealed polymer sample, so it can consider that the volume of the sealed sample remains almost unchanged at various temperatures. That restricting
Fig. 2. Volume expansion coefficient – temperature curves of PVDF/CB composites.
Z. Zhao et al. / Materials Letters 57 (2003) 3082–3088
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Fig. 3. Effect of restricting volume expansion on PTC curve of the PVDF/CB composite cooled slowly.
volume expansion results in dropping PTC intensity indicates that PTC effect is really caused by the volume expansion of polymer matrix. When volume expansion is restricted, the PTC intensity is decreased markedly but not eliminated fully, which still remains two orders of magnitude (see Fig. 3). It indicates that some other factors should also influence volume expansion. From Figs. 1 and 4, it can be seen that, as is common, the PTC transition regions are in the vicinity of the crystallite melting regions of the polymer matrixes. The quenched sample has a wider crystalline melting range and a lower melting point, and the corresponding resistivity –temperature curve has also a wider temperature range and lower PTC intensity, but the contrary is true for the slowly cooled sample. Thus, the resistivity increase should also be related to crystallite melting. The non-uniform distribution of CB particles in PVDF is expected since they have been preferentially rejected from crystalline regions during the crystallization of PVDF, thus CB particles were aggregated in amorphous regions and formed a network of conducting chains. As the composite is heated and the viscosity is reduced with rise in temperature, an enhanced redistribution process is taking place toward
a more uniform distribution permitting more and more CB particles or aggregates to diffuse into melted crystalline regions, which will results in an increase in average particle or aggregate distance of carbon black and destroy partially the original conductive network. This may be another cause appearing PTC effect. By comparison of Figs. 1 and 3, it can also be seen that, when temperature is in the vicinity of the melting points of samples, the resistivity drops sharply, i.e. a NTC effect appears, but the volume expansion still is proceeding, which indicates that some crystallites still have not been melted fully (see Fig. 2). It implies that CB particles or aggregates will move continuously toward the melted crystalline regions, which makes the conductive networks disaggregate further. According to the above-mentioned viewpoint, it should lead to increase in resistivity whether polymer matrix volume expansion or disaggregation of CB particles. It is obvious that the NTC effect taking place in the vicinity of melting point indicates that there is another factor affecting resistivity– temperature characteristic except volume expansion and CB particles moving toward the melted crystalline region. To understand the influence of this factor on PTC or NTC effect, the
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Z. Zhao et al. / Materials Letters 57 (2003) 3082–3088
Fig. 4. DSC thermograms for PVDF/CB composites: (a) cooled slowly and (b) quenched.
changes in resistivity with time at different constant temperatures for the slowly cooled samples were measured and shown in Fig. 5. At 145 jC (below
168 jC), the resistivity changes little with temperature, but at 200 jC (above 168 jC), the resistivity decreases gradually and then keeps an approximately
Fig. 5. Resistivity – time curves of PVDF/CB composites during isothermal course.
Z. Zhao et al. / Materials Letters 57 (2003) 3082–3088
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Fig. 6. Effect of radiation dose on the gel fraction of PVDF/CB composites.
constant value with time. Miyasaka et al. [12] discovered that polymer/CB composite has a lower resistance if CB particle has a agglomerative distribution in the polymer matrix, and a higher resistance if it has a random or uniform distribution in the polymer
matrix. Since there are different surface characteristic between CB particles and PVDF [13], the CB particles will have agglomerative trend in the polymer matrix to decrease superfluous surface energy. At high temperature(200 jC), PVDF segments have sufficient
Fig. 7. Resistivity – time curves of PVDF/CB composites during isothermal course.
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mobility, thus the agglomeration of CB particles could take place by the segment motion, which makes the resistivity of the sample decrease. At low temperature(145 jC), a large number of crystallites will restrict the movement of PVDF segments, resulting in the mobility of CB particles decreases, which makes the resistivity remain almost unchanged. If PVDF is quenched from the melt one would expect a lower degree crystallinity. If it is slow cooled from the melt, one would expect a greater crystallinity. If judging from the repeat unit (– CH2 – CF2 – ), PVDF should has the behaviors between polyethylene and polytetrafluoroethylene. It is well known that polyethylene belongs to crosslinking polymer, polytetrafluoroethylene to scission polymer. Therefore PVDF should both produce intermolecular crosslinking and main-chain scission during irradiation. In order to examine if crosslinking takes place in the amorphous region, the samples were irradiated with grays and gel fractions measured, as can be seen in Fig. 6. It is clear that the resulting corosslinking network is nonhomogeneous, crosslinking having taken place predominantly in the amorphous regions. Resistivity –temperature characteristics before and after irradiation (730 kGy) for the slowly cooled samples have been investigated at 200 jC (see Fig. 7). The resistivity of the two samples decreases drastically in the initial time stage and then change little with increase in time, but the resistivity of the irradiated sample is higher than that of the unirradiated sample in the entire measured time. The movement of the CB particles decreases since intermolecular crosslinking could decrease markedly the movement of segments. Therefore, the agglomeration of CB particles can be restricted, then a higher resistivity is expected for the irradiated sample. Klason and Kubat [14] reported that PTC intensity increased with temperature rising rate for polystyrene/CB and polypropylene/CB composites. This is because the agglomeration of CB particles takes some time. When temperature rising rate increases, the PTC
intensity increases since CB particles are of less agglomeration at the same temperature. The phenomenon shows that the agglomeration of CB particles exists not only in NTC process but also over the entire temperature rising range at different degree. Therefore, if restricting the agglomeration of CB particles, the PTC intensity will be improved effectively. In summary, the PTC effect of PVDF/CB composite is dominated by the volume expansion of composite matrix, the diffusion of CB particles from amorphous region to melt crystalline region and the agglomeration of particles.
Acknowledgements The authors wish to thank Dr. L. Rao (Changchun Institute of Applied Chemistry) for her help in gradiation.
References [1] E. Frydman, UK Patent specification, 604 695 I 718 14S (1948). [2] S.K. Bhattacharya, A.C. Chaklder, Polym. Plast. Technol. Eng. 19 (1982) 21. [3] M. Narkis, A. Ram, F. Flashner, Polym. Eng. Sci. 18 (1978) 649. [4] L. Benguigyi, J. Yacubowich, M. Narkis, J. Polym. Sci., Part B, Polym. Phys. 25 (1987) 127. [5] X.Y. Liang, L.C. Ling, C.X. Lu, L. Liu, Mater. Lett. 43 (2000) 144. [6] F. Kohler, US Patent No. 3,243,753 (1966). [7] K. Ohe, Y. Natio, Jpn. J. Appl. Phys. 10 (1971) 99. [8] J. Meyer, Polym. Eng. Sci. 13 (1973) 462. [9] J. Meyer, Polym. Eng. Sci. 14 (1974) 706. [10] M. Narkis, A. Vaxman, J. Appl. Polym. Sci. 29 (1984) 1639. [11] H. Tang, J.H. Piao, X.F. Chen, Y.X. Luo, S.H. Li, J. Appl. Polym. Sc. 48 (1993) 1795. [12] K. Miyasaka, K. Watanabe, E. Jojima, H. Aida, M. Sumita, K. Ishikawa, J. Mater. Sci. 17 (1982) 1610. [13] G.Z. Wu, C. Zhang, T. Miura, S. Asai, M. Sumita, J. Appl. Polym. Sci. 80 (2001) 1063. [14] C. Klason, J. Kubat, J. Appl. Polym. Sci. 19 (1975) 831.
The conduction mechanism of carbon black-filled poly(vinylidene fluoride) composite Zhudi Zhao *, Wenxue Yu, Xiujuan He, Xinfang Chen Department of Materials Science and Alan G. MacDiarmid Lab., Jilin University, Changchun 130023, People’s Republic of China Received 17 September 2002; accepted 15 December 2002
Abstract The positive temperature coefficient (PTC) effect of carbon black (CB)-filled poly(vinylidene fluoride) (PVDF) composite and the changes in resistivity under different conditions were studied. It was found that there were some coherence between the volume expansion and the crystallite melting of PVDF. It is believed that the homogenization diffusion of CB particles results in increasing resistivity during volume expansion and crystallite melting. At high temperature, the resistivity decrease is due to the agglomeration of CB particles or aggregates, which results in a new CB distribution of better conductivity. On the base of the experimental evidence, it is concluded that the PTC effect of PVDF/CB composite is dominated by the volume expansion of composite matrix, the diffusion of CB particles from amorphous region to melted crystalline region and the agglomeration of CB particles. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Conductive polymer composite; Positive temperature coefficient; Conduction mechanism
1. Introduction Polymer materials incorporated conductive fillers exhibiting the effect of positive temperature coefficient (PTC) of resistance were first discovered by Frydman in 1945 [1]. Up to now, polymer PTC materials have been studied extensively because of their numerous high technological electrical and electronic applications [2 –5]. In theory, the mechanism of electric current conduction in the polymer materials have been studied for a long time, but there is still not a satisfactory explanation for PTC effect. Kohler [6] suggested that the PTC effect is caused by thermal
* Corresponding author. E-mail address: [email protected] (Z. Zhao).
expansion. He proposed that the conductive particles were initially spread through the polymer forming a network of conducting chains. As the material is heated, the conductive particles are separated further, thus increasing the resistance. The sudden expansion of polymer at the crystalline melting point is assumed to be the cause of the strong transition to the highresistive state. Ohe and Natio [7] assume that, at lower temperatures, the interparticle gaps are uniformly distributed, but change in a random distribution at higher temperature, thereby increasing the average gap width and thus increasing the resistivity. Meyer [8,9] supposed that thin crystalline forms of polymer were much better conductors than amorphous films, which appear after crystalline melting. In the case of negative temperature coefficient (NTC), it was reported by Narkis and Vaxman [10] and Tang et al. [11] that the
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01440-4
Z. Zhao et al. / Materials Letters 57 (2003) 3082–3088
resistivity decrease is due to the reagglomeration of carbon black (CB) aggregates. However, because these cannot be observed by experimental techniques, there has been difficulty in finding a comprehensible explanation for the effect and, thus, most of mechanism still remain controversial. In the present work, the influence of volume expansion and DSC curves of carbon black-filled poly(vinylidene fluoride) (PVDF) on PTC characteristic curves were studied. The changes of the composites with temperature were measured before and after restricting volume expansion, and the relationship between resistivity and time were also measured at high temperature. A possible mechanism was proposed based on the experimental results.
3083
thermal properties of the sample. The heating rate was 5 jC/min. The percentage of insoluble gel after irradiation was measured with a Soxhlet apparatus. Approximately 250 mg of sample wrapped in nickel mesh was exposed to refluxing N,N-dimethyl acetamide (DMA) until the sample attained a constant weight, which usually took 56 h. After that, the samples was dried in a vacuum oven at 100 jC for 48 h and the weight (corresponding to the weight of the network structure polymer) was measured. The gel fraction was calculated by g% ¼
w1 wc w wc
where g% is the gel fraction, w1 is the weight of the sample after refluxing, w is the weight of the sample before refluxing and wc is the weight of CB.
2. Experimental PVDF (product of America Pennwalt) with a melting point of 168 jC was used as polymer matrix. CSF carbon black was used as a conductive filler. Its average particle size is 70 nmm, surface area 230 m2/g, dibutyl phthalate (DBP) value 280 ml/100 g and pH value 7– 9. PVDF and CB were melt-mixed in a Brabender for 15 min at 200 jC, then kneaded on two-roll mill for 5 min at 180 jC. The samples were compression molded at 200 jC for 10 min, then quenched in icewater and slowly cooled under pressure to room temperature, respectively. The irradiation of composites was carried out in vacuum at room temperature with a 60Co g-ray source. All of the irradiated composites were held for 1 day after irradiation to eliminate trap radicals. The volume resistivities of the composites were measured by a digital multimeter at a progressively elevated temperature. The two sides of the samples were bonded with metallic foil to reduce the contact resistance. The heating rate was 5 jC/min. The volume expansion coefficient of the samples was obtained using a capillary dilatometer at a heating rate of 5 jC/min. Some samples were sealed in ceramic tube with electrodes to measure their thermal expansion coefficient under the condition of restricting volume expansion. A Perkin-Elmer differential scanning DSC-7 calorimeter was used to measure the
3. Results and discussion The resistivity – temperature curves of quenched and slowly cooled samples with 8 wt.% CB content are shown in Fig. 1. It can be seen that the PTC curves for two samples give different profiles. For the slowly cooled sample, the peak of PTC curves shifts to higher temperature, the width of peak becomes narrow and PTC intensity (the ratio of peak resistivity to room temperature resistivity) increases compared with the quenched composite. Fig. 2 shows the influence of temperature on thermal expansion coefficients of the composites. Before NTC effect appears, the shapes of two thermal expansion coefficients – temperature curves are approximately similar to those of corresponding resistivity– temperature curves (see Fig. 1). The thermal expansion initial and final jumping temperatures for the quenched sample are lower, whereas those of the cooled slowly sample are higher. For same kinds of samples, the transition temperatures in the resistivity –temperature curves are basically consistent with those of the corresponding thermal expansion curves. The consistency reveals that there exists a inherent relationship between PTC characteristic and volume expansion. In order to examine the influence of volume expansion on PTC effect, we sealed a slowly cooled sample in a ceramic tube to measure its PTC charac-
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Z. Zhao et al. / Materials Letters 57 (2003) 3082–3088
Fig. 1. Resistivity – temperature behaviors of PVDF/CB composites.
teristic. As shown in Fig. 3, the PTC intensity drops 1.5 orders of magnitude. The thermal expansion coefficient of the ceramic tube is much less that of
the sealed polymer sample, so it can consider that the volume of the sealed sample remains almost unchanged at various temperatures. That restricting
Fig. 2. Volume expansion coefficient – temperature curves of PVDF/CB composites.
Z. Zhao et al. / Materials Letters 57 (2003) 3082–3088
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Fig. 3. Effect of restricting volume expansion on PTC curve of the PVDF/CB composite cooled slowly.
volume expansion results in dropping PTC intensity indicates that PTC effect is really caused by the volume expansion of polymer matrix. When volume expansion is restricted, the PTC intensity is decreased markedly but not eliminated fully, which still remains two orders of magnitude (see Fig. 3). It indicates that some other factors should also influence volume expansion. From Figs. 1 and 4, it can be seen that, as is common, the PTC transition regions are in the vicinity of the crystallite melting regions of the polymer matrixes. The quenched sample has a wider crystalline melting range and a lower melting point, and the corresponding resistivity –temperature curve has also a wider temperature range and lower PTC intensity, but the contrary is true for the slowly cooled sample. Thus, the resistivity increase should also be related to crystallite melting. The non-uniform distribution of CB particles in PVDF is expected since they have been preferentially rejected from crystalline regions during the crystallization of PVDF, thus CB particles were aggregated in amorphous regions and formed a network of conducting chains. As the composite is heated and the viscosity is reduced with rise in temperature, an enhanced redistribution process is taking place toward
a more uniform distribution permitting more and more CB particles or aggregates to diffuse into melted crystalline regions, which will results in an increase in average particle or aggregate distance of carbon black and destroy partially the original conductive network. This may be another cause appearing PTC effect. By comparison of Figs. 1 and 3, it can also be seen that, when temperature is in the vicinity of the melting points of samples, the resistivity drops sharply, i.e. a NTC effect appears, but the volume expansion still is proceeding, which indicates that some crystallites still have not been melted fully (see Fig. 2). It implies that CB particles or aggregates will move continuously toward the melted crystalline regions, which makes the conductive networks disaggregate further. According to the above-mentioned viewpoint, it should lead to increase in resistivity whether polymer matrix volume expansion or disaggregation of CB particles. It is obvious that the NTC effect taking place in the vicinity of melting point indicates that there is another factor affecting resistivity– temperature characteristic except volume expansion and CB particles moving toward the melted crystalline region. To understand the influence of this factor on PTC or NTC effect, the
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Z. Zhao et al. / Materials Letters 57 (2003) 3082–3088
Fig. 4. DSC thermograms for PVDF/CB composites: (a) cooled slowly and (b) quenched.
changes in resistivity with time at different constant temperatures for the slowly cooled samples were measured and shown in Fig. 5. At 145 jC (below
168 jC), the resistivity changes little with temperature, but at 200 jC (above 168 jC), the resistivity decreases gradually and then keeps an approximately
Fig. 5. Resistivity – time curves of PVDF/CB composites during isothermal course.
Z. Zhao et al. / Materials Letters 57 (2003) 3082–3088
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Fig. 6. Effect of radiation dose on the gel fraction of PVDF/CB composites.
constant value with time. Miyasaka et al. [12] discovered that polymer/CB composite has a lower resistance if CB particle has a agglomerative distribution in the polymer matrix, and a higher resistance if it has a random or uniform distribution in the polymer
matrix. Since there are different surface characteristic between CB particles and PVDF [13], the CB particles will have agglomerative trend in the polymer matrix to decrease superfluous surface energy. At high temperature(200 jC), PVDF segments have sufficient
Fig. 7. Resistivity – time curves of PVDF/CB composites during isothermal course.
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Z. Zhao et al. / Materials Letters 57 (2003) 3082–3088
mobility, thus the agglomeration of CB particles could take place by the segment motion, which makes the resistivity of the sample decrease. At low temperature(145 jC), a large number of crystallites will restrict the movement of PVDF segments, resulting in the mobility of CB particles decreases, which makes the resistivity remain almost unchanged. If PVDF is quenched from the melt one would expect a lower degree crystallinity. If it is slow cooled from the melt, one would expect a greater crystallinity. If judging from the repeat unit (– CH2 – CF2 – ), PVDF should has the behaviors between polyethylene and polytetrafluoroethylene. It is well known that polyethylene belongs to crosslinking polymer, polytetrafluoroethylene to scission polymer. Therefore PVDF should both produce intermolecular crosslinking and main-chain scission during irradiation. In order to examine if crosslinking takes place in the amorphous region, the samples were irradiated with grays and gel fractions measured, as can be seen in Fig. 6. It is clear that the resulting corosslinking network is nonhomogeneous, crosslinking having taken place predominantly in the amorphous regions. Resistivity –temperature characteristics before and after irradiation (730 kGy) for the slowly cooled samples have been investigated at 200 jC (see Fig. 7). The resistivity of the two samples decreases drastically in the initial time stage and then change little with increase in time, but the resistivity of the irradiated sample is higher than that of the unirradiated sample in the entire measured time. The movement of the CB particles decreases since intermolecular crosslinking could decrease markedly the movement of segments. Therefore, the agglomeration of CB particles can be restricted, then a higher resistivity is expected for the irradiated sample. Klason and Kubat [14] reported that PTC intensity increased with temperature rising rate for polystyrene/CB and polypropylene/CB composites. This is because the agglomeration of CB particles takes some time. When temperature rising rate increases, the PTC
intensity increases since CB particles are of less agglomeration at the same temperature. The phenomenon shows that the agglomeration of CB particles exists not only in NTC process but also over the entire temperature rising range at different degree. Therefore, if restricting the agglomeration of CB particles, the PTC intensity will be improved effectively. In summary, the PTC effect of PVDF/CB composite is dominated by the volume expansion of composite matrix, the diffusion of CB particles from amorphous region to melt crystalline region and the agglomeration of particles.
Acknowledgements The authors wish to thank Dr. L. Rao (Changchun Institute of Applied Chemistry) for her help in gradiation.
References [1] E. Frydman, UK Patent specification, 604 695 I 718 14S (1948). [2] S.K. Bhattacharya, A.C. Chaklder, Polym. Plast. Technol. Eng. 19 (1982) 21. [3] M. Narkis, A. Ram, F. Flashner, Polym. Eng. Sci. 18 (1978) 649. [4] L. Benguigyi, J. Yacubowich, M. Narkis, J. Polym. Sci., Part B, Polym. Phys. 25 (1987) 127. [5] X.Y. Liang, L.C. Ling, C.X. Lu, L. Liu, Mater. Lett. 43 (2000) 144. [6] F. Kohler, US Patent No. 3,243,753 (1966). [7] K. Ohe, Y. Natio, Jpn. J. Appl. Phys. 10 (1971) 99. [8] J. Meyer, Polym. Eng. Sci. 13 (1973) 462. [9] J. Meyer, Polym. Eng. Sci. 14 (1974) 706. [10] M. Narkis, A. Vaxman, J. Appl. Polym. Sci. 29 (1984) 1639. [11] H. Tang, J.H. Piao, X.F. Chen, Y.X. Luo, S.H. Li, J. Appl. Polym. Sc. 48 (1993) 1795. [12] K. Miyasaka, K. Watanabe, E. Jojima, H. Aida, M. Sumita, K. Ishikawa, J. Mater. Sci. 17 (1982) 1610. [13] G.Z. Wu, C. Zhang, T. Miura, S. Asai, M. Sumita, J. Appl. Polym. Sci. 80 (2001) 1063. [14] C. Klason, J. Kubat, J. Appl. Polym. Sci. 19 (1975) 831.