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Distinct positive temperature coefficient effect of polymer–carbon fiber composites evaluated in terms of polymer absorption on fiber surface† Xi Zhang,a Shaodi Zheng,a Xiaofang Zheng,a Zhengying Liu,*a Wei Yangab and Mingbo Yang*ab In this article, the positive temperature coefficient (PTC) effect was studied for high-density polyethylene (HDPE)/carbon fiber (CF) composites. All of the samples showed a significant PTC effect during the heating processes without a negative temperature coefficient (NTC) effect, even at a temperature much higher than the melting point of the polymer matrix. An ever-increasing PTC intensity with increasing thermal cycles was observed in our study that had never been reported in previous research. The absence of a NTC effect resulted from the increased binding force between the matrix and fillers that contributed to the very special structure of CF surface. We incorporated thermal expansion theory and
Received 19th January 2016, Accepted 23rd February 2016
quantum tunneling effects to explain PTC effect. From the SEM micrographs for the HDPE/CF
DOI: 10.1039/c6cp00398b
with a layer of polymer which resulted in a change in the gap length between CF and HDPE and its
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distribution. We believed that the gap change induced by polymer absorption on the fiber surface had a great effect on the PTC effect.
composites before and after the different thermal cycles, we found that the surface of CF was covered
1. Introduction Electrical conductive filler filled semi-crystalline polymer composites usually exhibit an important insulator–conductor transition related to the temperature dependence of the electrical resistivity. This transition is the so called positive temperature coefficient (PTC) effect, which indicates a rapid increase of resistivity when the temperature is close to the melting point of the polymer matrix. If the resistivity decreases drastically with further increases in temperature, it is called the negative temperature coefficient (NTC) effect.1 The mechanism of the PTC effect has been widely researched for many years and several theories have been proposed, such as the conductive chain and thermal expansion theory,2 quantum tunneling effect theory,3 microcrystalline theory,4,5 and so on.6,7 However, these theories still have some problems explaining the experimental results clearly. Up to now, polymer based PTC materials have been widely used in over-current protectors, self-regulating heaters and shielding materials8–15 due to their advantages of excellent a
College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China. E-mail: [email protected], [email protected]; Fax: +86-28-8540-5324; Tel: +86-28-8540-5324 b State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cp00398b
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formability, flexibility and light weight. Even so, the application of PTC materials is greatly limited because of disadvantages that include (1) their relatively high room temperature resistivity; (2) their bad reproducibility due to irregular changes of the conductive networks during heating and cooling cycles; (3) their comparatively low PTC intensity (IPTC = log(rmax/rRT), where rmax is the maximum resistivity during the heating process and rRT is the resistivity at room temperature) and the companioned NTC effect beyond the melting point due to the re-aggregation of fillers.1,16–18 In recent years, many efforts have been made to overcome these drawbacks. The frequently used method to obtain high IPTC is to reduce the volume fraction of conductive fillers. Gao et al.19 successfully enhanced IPTC by forming brittle conductive pathways through decreasing the conductive filler loading since volume expansion of the polymer matrix can destroy the brittle conductive network more easily and consequently results in higher IPTC. However, this method may lead to poor reproducibility of the electrical properties and the NTC effect cannot be eliminated. Recently, Lu et al.16 demonstrated that the PTC effect could be enhanced and the NTC effect could be eliminated by the selective distribution of CB at the interface of nylon6/polystyrene (PA6/PS) blends. Xu et al.20 designed a ternary-continuous structure or double-percolation structure in modified nanoscale carbon black (MNCB) filled high density polyethylene/polypropylene (HDPE/PP), which showed relatively
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low room temperature resistivity and remarkably increased PTC intensity. Shen et al.9 reported an appreciably increased PTC intensity by preparing a hybrid conductive particle (CF and CB) filled polyethylene or a polyethylene/polypropylene composite with the combined effects of CF and CB. In both composites, electrons were transported over long distances by CFs with little loss of energy, whereas CB particles improved the interfiber contact by forming CB particle bridges. By the way, the two network structures could both increase the density of the conductive paths, which reduced the resistivity of the composites. Even though a lot of work has been done to pursue a desirable PTC material, there is far from enough deep research on the factors affecting the PTC effect. To research the factors that affect PTC characteristics, some efforts have been made by us. In our previous work,21,22 we found that the networks provided by carbon black were unstable and the particles always flocculated in the composite melts, which led to a significant NTC effect when the temperature was beyond the melting point. Compared with CB filled composites, the PTC intensity and reproducibility of nanocomposites were dramatically improved by the addition of a small number of multi-walled nanotubes (MWNTs), due to the high aspect ratio of the nanotubes and their segregated distribution inside the polymer matrix.23–26 Just like carbon nanotubes, CFs possess a certain aspect ratio and the addition of CF to the polymer aids the transport of electrons over long distances. By the way, the dispersion and distribution of CF is relatively favorable. In this study, CFs were used to form distinct conductive networks in the polymer based CPCs (conductive polymer composites) and unique PTC effects with no NTC effect have been achieved synchronously. Thermal expansion theory and electron tunneling effects were combined to further explore the mechanism of the PTC effect and explain the unusual temperature dependence of the resistivity in the CF filled high-density polyethylene conductive composite. Images from scanning electron microscopy (SEM) confirmed that the gap change induced by thermal expansion had a great effect on the PTC effect.
2. Experimental 2.1.
Materials
The principal raw materials were HDPE (2911, Fushun petroleum Chemical Co., Ltd, China, r = 0.960 g cm3 and 5000S, Lanzhou petroleum Chemical Co., Ltd, China. r = 0.953 g cm3) and CF (T300, Toray Company, Japan; diameter = 7.8 mm; initial length, 0.1–1 mm). The results with CF/HDPE (2911) are discussed in the paper and the results with CF/HDPE (5000S) are shown in the ESI† as a comparison. 2.2.
Sample preparation
HDPE was melt mixed with CF in a torque rheometer (XSS-300, Shanghai, China) at 180 1C for 10 min. A copper screen was pre-embedded on both sides of the mixture, which was then compressed into a plate for 5 min under a pressure of 10 MPa at
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180 1C. All of the sheets were cut into testing samples (10 mm 30 mm 2 mm) and rested overnight to release stress. 2.3.
Characterization
The room-temperature volume resistivity of the samples was measured using a two-probe method with an electrometer (Keithley 6517B) when the resistances were lower than 108 O. At both ends of the sample, the surface (10 2 mm2) in contact with the copper electrodes was silver painted to ensure good contact, and a high-resistivity meter (ZC36, Shanghai, China) was used for samples with a higher resistance (beyond 108 O). The resistivity r was calculated using the equation r = RS/d for the given values of resistance R, sample area S, and sample thickness d. A resistivity-temperature characteristic instrument controlled by a computer was employed for the temperature-dependent resistivity measurements. The samples fixed in the thermostatic oil-bath (Minstat 230, Huber, Germany) were heated from 60 1C to 200 1C at a rate of 5 1C min1 and kept at 200 1C for 10 minutes, then cooled to 60 1C at a rate of 2 1C min1. At the same time, the resistivity of the samples was synchronously monitored by an electrometer (Keithley 6517B, America). Four continuous thermal cycles were carried out. The computer recorded statistics every 2 seconds. The samples were fixed in a cuboid container to avoid a dramatic change of the sample state when the temperature was much higher than the melting point of polymer matrix. The melting behavior of the composites was determined using differential scanning calorimetry (DSC; TA Q20, Cranston, Rhode Island, America) under a nitrogen gas atmosphere at a flow rate of 30 ml min1. The samples were cut into small pieces (about 5 mg), weighed in aluminum pans and then they were heated up to 200 1C at a rate of 5 1C min1. The thermal volume expansion was measured by a Gnomix Inc. PVT apparatus using the isothermal mode of operation with 5 1C temperature and 10 MPa steps. The dry sample was surrounded by a thin Ni-foil and mercury (confining fluid), and was sealed in a bellows-type dilatometer. Specific volume changes down to 0.0002 cm3 g1 could be resolved. An FEI Inspect F scanning electron microscope (Inspect F, FEI Company, USA) with an acceleration voltage of 20 kV was used to observe the morphology of the composites. The samples were fractured in liquid nitrogen and then the fractured surface was coated with gold. A stress-controlled rheometer (AR 2000ex, TA Instruments, Ltd) equipped with parallel-plate geometry (diameter = 25 mm) was used. The test temperature was 180 1C, and the gap between the plates was fixed at 1.2 mm. A dynamic o sweep was conducted from 0.005 to 500 rad s1 under a 0.1% strain.
3. Results and discussion 3.1. Percolation behavior of the HDPE/CF composites and morphology of the particle network Fig. 1(a) plotted the relationship between the filler content and resistivity at room temperature for the HDPE/CF composites. In the figure, the resistivity of HDPE/CF at room temperature
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Fig. 1 (a) Relationship between filler content and resistivity at room temperature for HDPE/CF composites. Inset: Relationship between log(j jc) and log resistivity; SEM micrographs of the HDPE/CF composites with different CF content. (b) Frequency (o) dependence of dynamic storage (G 0 ) for the HDPE/CF composites with different CF content. The rheological experiments were performed at 180 1C and at 0.1% strain.
decreased with increasing filler content and declined drastically at a content of 4 vol% CF. The dramatic decrease of resistivity implied the formation of a conductive network. According to percolation theory, the probability of physical contact of CFs became higher by adding more CF into the matrix.27 The scaling law was demonstrated in eqn (1), taken from ref. 27 r = r0(j jc)t(j > jc),
(1)
where r is the resistivity of composites, r0 is a constant, jc is the percolation threshold, j is the volume content of CF (when j > jc), and the exponent characterizes the relationship between r and j. According to eqn (1), jc is 4.41 vol% for HDPE/CF composites, shown as the inset in Fig. 1(a). SEM micrographs were used to probe the CF network in the composites, as shown in the inset of Fig. 1(a). When the volume content was relatively low, there were few CFs that could be seen and they were separated from one and another by the resin layer, so it was almost impossible to form a conductive network. With the increase of CF content, the probability of contact for CF was increased. As shown in the circle marked area of the inset in Fig. 1(a), many CF were in contact with each other or were close enough to each other to form a conductive
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network, resulting in a drastic decrease of resistivity. With a further increase of CF content, a better conductive network penetrating the whole sample has been formed and low resistivity was achieved. The room temperature resistivity decreased to tens of O cm when the CF content was beyond the percolation threshold. A rheology experiment was carried out to further explore the formation of a conductive network with increasing conductive filler. Fig. 1(b) showed the o dependence of dynamic storage (G 0 ) for the HDPE/CF composites as a function of CF content. G 0 increased with the increase of o and the plot of G 0 versus o deviated from linearity at low values for o. When the CF content was higher than 3.4 vol%, G 0 was almost independent of o at low frequencies, indicating a transition from liquid-like to solid-like viscoelastic behavior. This non-terminal low-frequency behavior could be attributed to a fiber network that restrains the long-range motion of polymer chains. At high frequencies, the effect of fibers on the rheological behavior was relatively weak, which suggested that the CFs did not significantly influence the short-range dynamics of the HDPE chains, especially on the length scales comparable to the entanglement length. It should be noticed that the filler loading at which a significant plateau appeared was higher than the conductivity percolation threshold. It was widely accepted that a conductive network existed in the composites, in which the filler formed conductive paths when the filler loading reached the conductive percolation threshold. However, the filler network could effectively restrain polymer motion as long as the filler–filler distance was comparable to the diameter of random coils of polymer chains.28 In a word, the different filler–filler distances were required for rheological and electrical percolation, resulting in a different CF loading at the transition point. On the other hand, rheological percolation happened at a CF content a little higher than the electrical percolation threshold, indicating weak binding forces between the matrix and filler. Additionally, there were fewer functional groups on the surface of CF compared with CB and the interaction between the polymer matrix and CF was much weaker than that between CB and the matrix, so the three-dimensional network was hard to detect and the characteristic module plateau was not so obvious as that for the HDPE/CB composites. 3.2.
PTC effect of HDPE/CF composites
Fig. 2(a) plotted the temperature dependence of resistivity as a function of CF content for HDPE/CF composites during the heating process and Fig. 2(b) showed the relevant DSC scans. The value of resistance of the 6.9 vol% composite read from the electrometer was beyond the maximum of the electrometer (108 O), so the resistivity (r) at high temperature for the sample with 6.9 vol% CF could not be obtained. It was obvious that the resistivity of all of the composites increased slightly with increasing temperature when it was relatively low, then increased rapidly once the temperature was about 138 1C (close to the melting temperature shown in Fig. 2(b)), showing significant PTC effects. The DSC peak remained constant even for different CF content. Each peak temperature was related to the melting point
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Fig. 2 Temperature dependence of the resistivity as a function of CF content for the HDPE/CF composites during the heating process (a), and DSC scans of the HDPE/CF composites with different CF content at a heating rate of 5 1C min1 (b).
of HDPE (135 1C). The temperature threshold of the PTC effect for HDPE/CF composites corresponded with the melting point of the polymer matrix, which agreed with previous work. For the composite filled with 6.9 vol% CF, the significant PTC effect and the absence of resistivity data could indicate that the conductive network was brittle in this composite and easily broken with the increase in temperature. For the samples with higher CF content (8.8 vol%, 10.7 vol%, and 12.7 vol%), the variation trends of the resistivity–temperature curves were similar and they were independent of CF content. The transition temperature of the composites with different CF content remained the same, which corresponded to the constant DSC peak. When the CF content was higher than 6.9 vol%, the conductive network was relatively complete and destruction of the conductive network caused by thermal expansion of the polymer matrix was not so serious. On the other hand, the increase of CF content contributed little to the conductive network when the CF content was high, which conformed to the electrical behavior shown in Fig. 1(a). Another interesting phenomenon in Fig. 2(a) was that no NTC effect was observed, even at a temperature much higher than the melting point of HDPE. This is not the usual case for non-crosslinked conventional CB/HDPE composites, which usually exhibit severe NTC effects above the melting point of HDPE.29,30 According to the widely accepted explanation, the NTC effect was attributed to the formation of flocculated structures of the conductive fillers when the viscosity of the matrix was sufficiently low at elevated temperatures.30 But this was not reasonable for HDPE/CF considering the large size of the CFs used in the experiment. For the HDPE/CF composites,
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the absence of NTC effects might result from the large sizes of CF and the very special structure of CF. As shown in SEM micrographs (Fig. S2, ESI†), the surface of CF was very coarse and there existed many grooves on it, which increased the binding force between the matrix and filler and blocked the re-aggregation at high temperature. The deeper reasons should be further investigated. It was obvious in Fig. 2(a) that the PTC intensity was almost the same for the composite with 8.8 vol% and 12.7 vol%. The resistivity of the 8.8 vol% CF sample should be higher than that of the composite with 12.7 vol%. However, we should notice that with the further increase in CF content, the decrease of resistivity was not so obvious because the increased CF loading contributed little to the formation of a new conductive network after the electrical percolation threshold. In our study, the CF content of 8.8 vol% and 12.7 vol% were much higher than the electrical percolation threshold (4.41 vol%) so the conductive networks in both of them were relatively perfect and their resistivities were very similar, which could be seen in Fig. 1(a). When undergoing the heating process, the similar conductive networks showed semblable responses to temperature. In other words, the degree of damage for the composite with 8.8 vol% and 12.7 vol% CF was very similar during the heating process. As a result, the PTC intensity of them was almost the same. We analyzed the PTC effect of the HDPE/CF composites considering the volume expansion of HDPE, and the PVT curve was plotted as Fig. 3(a). The volume of HDPE in the composites increased with temperature and began to increase rapidly at 138 1C, which resulted from the change from mostly crystalline to amorphous at the melting point of the base polymer. In terms of the volume expansion of HDPE, the actual filler
Fig. 3 (a) PVT curve of HDPE. (b) Relationship between actual CF content and the temperature, calculated using eqn (2).
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content in the composite during temperature increase can be expressed as a function of temperature as eqn (2):
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jðTÞ ¼
Vfiller 100; Vfiller þ VHDPE
(2)
where j(T) is the actual filler content at temperature T, Vfiller is the volume of CF in the composite, and VHDPE is the volume of HDPE at temperature T, which can be calculated from the PVT curve in Fig. 3(a). Considering that the thermal expansion rate of CF is much less than that of HDPE, Vfiller is constant and irrespective of temperature.31 The change of the actual CF content (j(T)) with increasing temperature was plotted Fig. 3(b). As shown in the figure, j(T) decreased with the increase in temperature, which was due to the thermal expansion of the HDPE matrix. The decrease in the CF content became very dramatic at the temperature of the melting point of HDPE. We could conclude that the thermal expansion of the polymer matrix diluted the concentration of CF in the composites, which was equal to the expansion of the conductive network. There was no doubt that the expansion of the conductive network would affect the gap length between adjacent CFs. We tried to determine the mechanism of the PTC effect by combining thermal expansion theory and quantum tunneling effects. According to quantum tunneling effects, there still existed the problem about the formation of a conductive network in conductive composites. The conductivity of the CF conductive network with HDPE was contributed to not only by the perfectly in contact CFs, but also those CFs with a small spacing between adjacent CFs via tunneling effects. The tunneling resistance between two neighboring CFs can be approximately estimated as32 V h2 d 4pd pffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi exp ¼ 2ml ; (3) Rtunnel ¼ SJ Se2 2ml h
to investigate the reproducibility of the HDPE/CF composites and the results were shown in Fig. 6(a). It was obvious that the initial and final resistivity in each heating process was markedly increased compared with those in the previous heating process. This phenomenon indicated that the conductive network became more brittle with increasing thermal cycles. The PTC intensity increased significantly with the increase of thermal cycles, as shown in Fig. 4(b), and the same situation was also observed in the composites with different CF content (shown as Fig. S3, ESI†). This phenomenon has never been reported before. CF filling another HDPE composite showed almost, but not quite, an equivalent phenomenon (shown as Fig. S4, ESI†). As described before, we believed that the increase of resistivity during the heating process was related to the increase of gap length induced by thermal expansion. To investigate the change in the gaps between CFs and the effect on the distinct PTC effect after consecutive thermal cycles, SEM micrographs of HDPE/CF composites before and after different thermal cycles were taken and are shown in Fig. 5. The CF was well distributed in the original composites, as shown in Fig. 5, and the CFs had more chances to form a good conductive network before the thermal cycles. With the increasing thermal cycles, the distribution of CF became worse and worse and the areas containing no CF (marked with dotted circles in the SEM micrographs) were significantly increased. As a result, the distribution of the gaps between CFs became more random, and a significant number of inter-filler gaps were too large to allow appreciable electron tunneling, although the average gap length did not change considerably. Some parts of the conductive network were cut off and the resistivity of the composite was increased for this reason. With the increasing
where J is the tunneling current density, V is the electrical potential difference, e is the quantum of electricity, m is the static mass of electron, h is Planck’s constant, d is the distance between CFs, l is the height of the barrier and A is the cross-sectional area of the tunnel (the cross-sectional area of CF was used as an approximate here). According to eqn (3), the resistance depended exponentially on the gap length between fillers and increased drastically with an increased gap length at the nanometer level (the resistance between two adjacent CFs was plotted as a function of gap length in Fig. S1, ESI†). Therefore, if the gap length between CFs increased with rising temperature, the resistivity would increase accordingly. As mentioned before, the increase in temperature would result in the volume expansion of HDPE and the expanded volume would no doubt cause the gap length between CFs to become bigger. As a result, partially conductive paths were cut off and the conductive network was broken, leading to the occurrence of the PTC effect. 3.3. Reproducibility of the PTC effect for the HDPE/CF composites Good reproducibility is a very important quality for desired PTC materials. Four continuous heating processes were carried out
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Fig. 4 Temperature dependence of resistivity for the 8.8 vol% CF filled HDPE composites during four continuous heating processes (a); relationship between PTC intensity and cycle times during the heating processes (b).
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Fig. 5 SEM micrographs of the 8.8 vol% CF filled HDPE/CF composites with increasing number of thermal cycles.
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Fig. 7 Schematic mechanism for the change in interactions between CF and the polymer matrix with increasing number of thermal processes.
each other well before thermal cycles. The surface of CF will absorb some polymer chains during the heating process and will be covered by a layer of HDPE matrix. The resin layer becomes thicker with subsequent thermal cycles, which results in a larger gap length and higher barrier height between CFs. As a result, the conductive network becomes brittle and can be broken much more easily.
4. Conclusions
Fig. 6 Magnified SEM micrographs of 8.8 vol% CF filled HDPE/CF composites with increasing thermal cycles.
number of thermal cycles, more of the conductive network was broken up, leading to the ever-increasing resistivity. Fig. 6 shows the magnified SEM micrographs of the HDPE/ CF composites before the thermal cycles and after consecutive thermal cycles. With the original HDPE/CF composites, the space between the CFs and the HDPE matrix (marked with a dotted circle) was very obvious, indicating poor interaction between the HDPE and CFs before thermal cycles. The CF was covered with some HDPE during the consecutive thermal cycles and the space between them became less obvious, which indicated a better interaction. After the composites had undergone four continuous thermal cycles, the CF was covered with a layer of the HDPE matrix. As a result, the gap between adjacent CFs and the barrier height of the tunneling gap increased. Considering eqn (3), the gap length between CF and HDPE and the barrier height of the tunneling gap had a great influence on the conductivity of the composites. The increase of the gap length and the barrier height would undoubtedly imply a remarkably broken conductive network, namely, the conductive network became more brittle with the increasing number of thermal cycles (Fig. S5, ESI†). As a result, the PTC was everincreasing with subsequent thermal cycles. As aforementioned, we can explain the distinct PTC effect with the scheme in Fig. 7. The interaction between CF and the polymer matrix is relatively weak and the adjacent CFs can connect with
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In this study, the PTC effect of HDPE/CF was investigated. Conductivity paths grew with increasing filler content at room temperature, so the room temperature resistivity of HDPE/CF and maximum resistivity decreased. The appearance of a plateau at low o indicated the forming of a conductive network in the HDPE/CF composites. All of the samples showed significant a PTC effect during the heating processes and we believed that thermal expansion theory and electron tunneling effects should be incorporated to interpret the PTC effect. The resistivity between two CFs depended on the gap length between the fillers and the increase in gap length resulted from thermal expansion of the matrix during the heating process and brought about a significant decrease in conductivity, leading to the appearance of the PTC effect. No NTC effect could be observed, even at temperatures much higher than the melting temperature of HDPE, which was related to the unique structure of CF. The surface of CF was very coarse and there existed many grooves on it, which increased the binding force between the matrix and filler and blocked re-aggregation at high temperatures. The interaction between the polymer matrix and CF became better with increasing thermal cycles, which resulted in larger gap distribution and a higher barrier height of the tunneling gap, and the significant number of inter-filler gaps was too large to allow appreciable electron tunneling, although the average gap width didn’t change considerably. These factors led to the rising resistivity and ever-increasing PTC intensity.
Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant No. 51103087, 51421061). Authors wish
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to acknowledge Dr Chaoliang Zhang (West China College of Stomatology, Sichuan University) for the SEM observation.
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Distinct positive temperature coefficient effect of polymer–carbon fiber composites evaluated in terms of polymer absorption on fiber surface† Xi Zhang,a Shaodi Zheng,a Xiaofang Zheng,a Zhengying Liu,*a Wei Yangab and Mingbo Yang*ab In this article, the positive temperature coefficient (PTC) effect was studied for high-density polyethylene (HDPE)/carbon fiber (CF) composites. All of the samples showed a significant PTC effect during the heating processes without a negative temperature coefficient (NTC) effect, even at a temperature much higher than the melting point of the polymer matrix. An ever-increasing PTC intensity with increasing thermal cycles was observed in our study that had never been reported in previous research. The absence of a NTC effect resulted from the increased binding force between the matrix and fillers that contributed to the very special structure of CF surface. We incorporated thermal expansion theory and
Received 19th January 2016, Accepted 23rd February 2016
quantum tunneling effects to explain PTC effect. From the SEM micrographs for the HDPE/CF
DOI: 10.1039/c6cp00398b
with a layer of polymer which resulted in a change in the gap length between CF and HDPE and its
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distribution. We believed that the gap change induced by polymer absorption on the fiber surface had a great effect on the PTC effect.
composites before and after the different thermal cycles, we found that the surface of CF was covered
1. Introduction Electrical conductive filler filled semi-crystalline polymer composites usually exhibit an important insulator–conductor transition related to the temperature dependence of the electrical resistivity. This transition is the so called positive temperature coefficient (PTC) effect, which indicates a rapid increase of resistivity when the temperature is close to the melting point of the polymer matrix. If the resistivity decreases drastically with further increases in temperature, it is called the negative temperature coefficient (NTC) effect.1 The mechanism of the PTC effect has been widely researched for many years and several theories have been proposed, such as the conductive chain and thermal expansion theory,2 quantum tunneling effect theory,3 microcrystalline theory,4,5 and so on.6,7 However, these theories still have some problems explaining the experimental results clearly. Up to now, polymer based PTC materials have been widely used in over-current protectors, self-regulating heaters and shielding materials8–15 due to their advantages of excellent a
College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China. E-mail: [email protected], [email protected]; Fax: +86-28-8540-5324; Tel: +86-28-8540-5324 b State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cp00398b
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formability, flexibility and light weight. Even so, the application of PTC materials is greatly limited because of disadvantages that include (1) their relatively high room temperature resistivity; (2) their bad reproducibility due to irregular changes of the conductive networks during heating and cooling cycles; (3) their comparatively low PTC intensity (IPTC = log(rmax/rRT), where rmax is the maximum resistivity during the heating process and rRT is the resistivity at room temperature) and the companioned NTC effect beyond the melting point due to the re-aggregation of fillers.1,16–18 In recent years, many efforts have been made to overcome these drawbacks. The frequently used method to obtain high IPTC is to reduce the volume fraction of conductive fillers. Gao et al.19 successfully enhanced IPTC by forming brittle conductive pathways through decreasing the conductive filler loading since volume expansion of the polymer matrix can destroy the brittle conductive network more easily and consequently results in higher IPTC. However, this method may lead to poor reproducibility of the electrical properties and the NTC effect cannot be eliminated. Recently, Lu et al.16 demonstrated that the PTC effect could be enhanced and the NTC effect could be eliminated by the selective distribution of CB at the interface of nylon6/polystyrene (PA6/PS) blends. Xu et al.20 designed a ternary-continuous structure or double-percolation structure in modified nanoscale carbon black (MNCB) filled high density polyethylene/polypropylene (HDPE/PP), which showed relatively
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low room temperature resistivity and remarkably increased PTC intensity. Shen et al.9 reported an appreciably increased PTC intensity by preparing a hybrid conductive particle (CF and CB) filled polyethylene or a polyethylene/polypropylene composite with the combined effects of CF and CB. In both composites, electrons were transported over long distances by CFs with little loss of energy, whereas CB particles improved the interfiber contact by forming CB particle bridges. By the way, the two network structures could both increase the density of the conductive paths, which reduced the resistivity of the composites. Even though a lot of work has been done to pursue a desirable PTC material, there is far from enough deep research on the factors affecting the PTC effect. To research the factors that affect PTC characteristics, some efforts have been made by us. In our previous work,21,22 we found that the networks provided by carbon black were unstable and the particles always flocculated in the composite melts, which led to a significant NTC effect when the temperature was beyond the melting point. Compared with CB filled composites, the PTC intensity and reproducibility of nanocomposites were dramatically improved by the addition of a small number of multi-walled nanotubes (MWNTs), due to the high aspect ratio of the nanotubes and their segregated distribution inside the polymer matrix.23–26 Just like carbon nanotubes, CFs possess a certain aspect ratio and the addition of CF to the polymer aids the transport of electrons over long distances. By the way, the dispersion and distribution of CF is relatively favorable. In this study, CFs were used to form distinct conductive networks in the polymer based CPCs (conductive polymer composites) and unique PTC effects with no NTC effect have been achieved synchronously. Thermal expansion theory and electron tunneling effects were combined to further explore the mechanism of the PTC effect and explain the unusual temperature dependence of the resistivity in the CF filled high-density polyethylene conductive composite. Images from scanning electron microscopy (SEM) confirmed that the gap change induced by thermal expansion had a great effect on the PTC effect.
2. Experimental 2.1.
Materials
The principal raw materials were HDPE (2911, Fushun petroleum Chemical Co., Ltd, China, r = 0.960 g cm3 and 5000S, Lanzhou petroleum Chemical Co., Ltd, China. r = 0.953 g cm3) and CF (T300, Toray Company, Japan; diameter = 7.8 mm; initial length, 0.1–1 mm). The results with CF/HDPE (2911) are discussed in the paper and the results with CF/HDPE (5000S) are shown in the ESI† as a comparison. 2.2.
Sample preparation
HDPE was melt mixed with CF in a torque rheometer (XSS-300, Shanghai, China) at 180 1C for 10 min. A copper screen was pre-embedded on both sides of the mixture, which was then compressed into a plate for 5 min under a pressure of 10 MPa at
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180 1C. All of the sheets were cut into testing samples (10 mm 30 mm 2 mm) and rested overnight to release stress. 2.3.
Characterization
The room-temperature volume resistivity of the samples was measured using a two-probe method with an electrometer (Keithley 6517B) when the resistances were lower than 108 O. At both ends of the sample, the surface (10 2 mm2) in contact with the copper electrodes was silver painted to ensure good contact, and a high-resistivity meter (ZC36, Shanghai, China) was used for samples with a higher resistance (beyond 108 O). The resistivity r was calculated using the equation r = RS/d for the given values of resistance R, sample area S, and sample thickness d. A resistivity-temperature characteristic instrument controlled by a computer was employed for the temperature-dependent resistivity measurements. The samples fixed in the thermostatic oil-bath (Minstat 230, Huber, Germany) were heated from 60 1C to 200 1C at a rate of 5 1C min1 and kept at 200 1C for 10 minutes, then cooled to 60 1C at a rate of 2 1C min1. At the same time, the resistivity of the samples was synchronously monitored by an electrometer (Keithley 6517B, America). Four continuous thermal cycles were carried out. The computer recorded statistics every 2 seconds. The samples were fixed in a cuboid container to avoid a dramatic change of the sample state when the temperature was much higher than the melting point of polymer matrix. The melting behavior of the composites was determined using differential scanning calorimetry (DSC; TA Q20, Cranston, Rhode Island, America) under a nitrogen gas atmosphere at a flow rate of 30 ml min1. The samples were cut into small pieces (about 5 mg), weighed in aluminum pans and then they were heated up to 200 1C at a rate of 5 1C min1. The thermal volume expansion was measured by a Gnomix Inc. PVT apparatus using the isothermal mode of operation with 5 1C temperature and 10 MPa steps. The dry sample was surrounded by a thin Ni-foil and mercury (confining fluid), and was sealed in a bellows-type dilatometer. Specific volume changes down to 0.0002 cm3 g1 could be resolved. An FEI Inspect F scanning electron microscope (Inspect F, FEI Company, USA) with an acceleration voltage of 20 kV was used to observe the morphology of the composites. The samples were fractured in liquid nitrogen and then the fractured surface was coated with gold. A stress-controlled rheometer (AR 2000ex, TA Instruments, Ltd) equipped with parallel-plate geometry (diameter = 25 mm) was used. The test temperature was 180 1C, and the gap between the plates was fixed at 1.2 mm. A dynamic o sweep was conducted from 0.005 to 500 rad s1 under a 0.1% strain.
3. Results and discussion 3.1. Percolation behavior of the HDPE/CF composites and morphology of the particle network Fig. 1(a) plotted the relationship between the filler content and resistivity at room temperature for the HDPE/CF composites. In the figure, the resistivity of HDPE/CF at room temperature
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Fig. 1 (a) Relationship between filler content and resistivity at room temperature for HDPE/CF composites. Inset: Relationship between log(j jc) and log resistivity; SEM micrographs of the HDPE/CF composites with different CF content. (b) Frequency (o) dependence of dynamic storage (G 0 ) for the HDPE/CF composites with different CF content. The rheological experiments were performed at 180 1C and at 0.1% strain.
decreased with increasing filler content and declined drastically at a content of 4 vol% CF. The dramatic decrease of resistivity implied the formation of a conductive network. According to percolation theory, the probability of physical contact of CFs became higher by adding more CF into the matrix.27 The scaling law was demonstrated in eqn (1), taken from ref. 27 r = r0(j jc)t(j > jc),
(1)
where r is the resistivity of composites, r0 is a constant, jc is the percolation threshold, j is the volume content of CF (when j > jc), and the exponent characterizes the relationship between r and j. According to eqn (1), jc is 4.41 vol% for HDPE/CF composites, shown as the inset in Fig. 1(a). SEM micrographs were used to probe the CF network in the composites, as shown in the inset of Fig. 1(a). When the volume content was relatively low, there were few CFs that could be seen and they were separated from one and another by the resin layer, so it was almost impossible to form a conductive network. With the increase of CF content, the probability of contact for CF was increased. As shown in the circle marked area of the inset in Fig. 1(a), many CF were in contact with each other or were close enough to each other to form a conductive
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network, resulting in a drastic decrease of resistivity. With a further increase of CF content, a better conductive network penetrating the whole sample has been formed and low resistivity was achieved. The room temperature resistivity decreased to tens of O cm when the CF content was beyond the percolation threshold. A rheology experiment was carried out to further explore the formation of a conductive network with increasing conductive filler. Fig. 1(b) showed the o dependence of dynamic storage (G 0 ) for the HDPE/CF composites as a function of CF content. G 0 increased with the increase of o and the plot of G 0 versus o deviated from linearity at low values for o. When the CF content was higher than 3.4 vol%, G 0 was almost independent of o at low frequencies, indicating a transition from liquid-like to solid-like viscoelastic behavior. This non-terminal low-frequency behavior could be attributed to a fiber network that restrains the long-range motion of polymer chains. At high frequencies, the effect of fibers on the rheological behavior was relatively weak, which suggested that the CFs did not significantly influence the short-range dynamics of the HDPE chains, especially on the length scales comparable to the entanglement length. It should be noticed that the filler loading at which a significant plateau appeared was higher than the conductivity percolation threshold. It was widely accepted that a conductive network existed in the composites, in which the filler formed conductive paths when the filler loading reached the conductive percolation threshold. However, the filler network could effectively restrain polymer motion as long as the filler–filler distance was comparable to the diameter of random coils of polymer chains.28 In a word, the different filler–filler distances were required for rheological and electrical percolation, resulting in a different CF loading at the transition point. On the other hand, rheological percolation happened at a CF content a little higher than the electrical percolation threshold, indicating weak binding forces between the matrix and filler. Additionally, there were fewer functional groups on the surface of CF compared with CB and the interaction between the polymer matrix and CF was much weaker than that between CB and the matrix, so the three-dimensional network was hard to detect and the characteristic module plateau was not so obvious as that for the HDPE/CB composites. 3.2.
PTC effect of HDPE/CF composites
Fig. 2(a) plotted the temperature dependence of resistivity as a function of CF content for HDPE/CF composites during the heating process and Fig. 2(b) showed the relevant DSC scans. The value of resistance of the 6.9 vol% composite read from the electrometer was beyond the maximum of the electrometer (108 O), so the resistivity (r) at high temperature for the sample with 6.9 vol% CF could not be obtained. It was obvious that the resistivity of all of the composites increased slightly with increasing temperature when it was relatively low, then increased rapidly once the temperature was about 138 1C (close to the melting temperature shown in Fig. 2(b)), showing significant PTC effects. The DSC peak remained constant even for different CF content. Each peak temperature was related to the melting point
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Fig. 2 Temperature dependence of the resistivity as a function of CF content for the HDPE/CF composites during the heating process (a), and DSC scans of the HDPE/CF composites with different CF content at a heating rate of 5 1C min1 (b).
of HDPE (135 1C). The temperature threshold of the PTC effect for HDPE/CF composites corresponded with the melting point of the polymer matrix, which agreed with previous work. For the composite filled with 6.9 vol% CF, the significant PTC effect and the absence of resistivity data could indicate that the conductive network was brittle in this composite and easily broken with the increase in temperature. For the samples with higher CF content (8.8 vol%, 10.7 vol%, and 12.7 vol%), the variation trends of the resistivity–temperature curves were similar and they were independent of CF content. The transition temperature of the composites with different CF content remained the same, which corresponded to the constant DSC peak. When the CF content was higher than 6.9 vol%, the conductive network was relatively complete and destruction of the conductive network caused by thermal expansion of the polymer matrix was not so serious. On the other hand, the increase of CF content contributed little to the conductive network when the CF content was high, which conformed to the electrical behavior shown in Fig. 1(a). Another interesting phenomenon in Fig. 2(a) was that no NTC effect was observed, even at a temperature much higher than the melting point of HDPE. This is not the usual case for non-crosslinked conventional CB/HDPE composites, which usually exhibit severe NTC effects above the melting point of HDPE.29,30 According to the widely accepted explanation, the NTC effect was attributed to the formation of flocculated structures of the conductive fillers when the viscosity of the matrix was sufficiently low at elevated temperatures.30 But this was not reasonable for HDPE/CF considering the large size of the CFs used in the experiment. For the HDPE/CF composites,
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the absence of NTC effects might result from the large sizes of CF and the very special structure of CF. As shown in SEM micrographs (Fig. S2, ESI†), the surface of CF was very coarse and there existed many grooves on it, which increased the binding force between the matrix and filler and blocked the re-aggregation at high temperature. The deeper reasons should be further investigated. It was obvious in Fig. 2(a) that the PTC intensity was almost the same for the composite with 8.8 vol% and 12.7 vol%. The resistivity of the 8.8 vol% CF sample should be higher than that of the composite with 12.7 vol%. However, we should notice that with the further increase in CF content, the decrease of resistivity was not so obvious because the increased CF loading contributed little to the formation of a new conductive network after the electrical percolation threshold. In our study, the CF content of 8.8 vol% and 12.7 vol% were much higher than the electrical percolation threshold (4.41 vol%) so the conductive networks in both of them were relatively perfect and their resistivities were very similar, which could be seen in Fig. 1(a). When undergoing the heating process, the similar conductive networks showed semblable responses to temperature. In other words, the degree of damage for the composite with 8.8 vol% and 12.7 vol% CF was very similar during the heating process. As a result, the PTC intensity of them was almost the same. We analyzed the PTC effect of the HDPE/CF composites considering the volume expansion of HDPE, and the PVT curve was plotted as Fig. 3(a). The volume of HDPE in the composites increased with temperature and began to increase rapidly at 138 1C, which resulted from the change from mostly crystalline to amorphous at the melting point of the base polymer. In terms of the volume expansion of HDPE, the actual filler
Fig. 3 (a) PVT curve of HDPE. (b) Relationship between actual CF content and the temperature, calculated using eqn (2).
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content in the composite during temperature increase can be expressed as a function of temperature as eqn (2):
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jðTÞ ¼
Vfiller 100; Vfiller þ VHDPE
(2)
where j(T) is the actual filler content at temperature T, Vfiller is the volume of CF in the composite, and VHDPE is the volume of HDPE at temperature T, which can be calculated from the PVT curve in Fig. 3(a). Considering that the thermal expansion rate of CF is much less than that of HDPE, Vfiller is constant and irrespective of temperature.31 The change of the actual CF content (j(T)) with increasing temperature was plotted Fig. 3(b). As shown in the figure, j(T) decreased with the increase in temperature, which was due to the thermal expansion of the HDPE matrix. The decrease in the CF content became very dramatic at the temperature of the melting point of HDPE. We could conclude that the thermal expansion of the polymer matrix diluted the concentration of CF in the composites, which was equal to the expansion of the conductive network. There was no doubt that the expansion of the conductive network would affect the gap length between adjacent CFs. We tried to determine the mechanism of the PTC effect by combining thermal expansion theory and quantum tunneling effects. According to quantum tunneling effects, there still existed the problem about the formation of a conductive network in conductive composites. The conductivity of the CF conductive network with HDPE was contributed to not only by the perfectly in contact CFs, but also those CFs with a small spacing between adjacent CFs via tunneling effects. The tunneling resistance between two neighboring CFs can be approximately estimated as32 V h2 d 4pd pffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi exp ¼ 2ml ; (3) Rtunnel ¼ SJ Se2 2ml h
to investigate the reproducibility of the HDPE/CF composites and the results were shown in Fig. 6(a). It was obvious that the initial and final resistivity in each heating process was markedly increased compared with those in the previous heating process. This phenomenon indicated that the conductive network became more brittle with increasing thermal cycles. The PTC intensity increased significantly with the increase of thermal cycles, as shown in Fig. 4(b), and the same situation was also observed in the composites with different CF content (shown as Fig. S3, ESI†). This phenomenon has never been reported before. CF filling another HDPE composite showed almost, but not quite, an equivalent phenomenon (shown as Fig. S4, ESI†). As described before, we believed that the increase of resistivity during the heating process was related to the increase of gap length induced by thermal expansion. To investigate the change in the gaps between CFs and the effect on the distinct PTC effect after consecutive thermal cycles, SEM micrographs of HDPE/CF composites before and after different thermal cycles were taken and are shown in Fig. 5. The CF was well distributed in the original composites, as shown in Fig. 5, and the CFs had more chances to form a good conductive network before the thermal cycles. With the increasing thermal cycles, the distribution of CF became worse and worse and the areas containing no CF (marked with dotted circles in the SEM micrographs) were significantly increased. As a result, the distribution of the gaps between CFs became more random, and a significant number of inter-filler gaps were too large to allow appreciable electron tunneling, although the average gap length did not change considerably. Some parts of the conductive network were cut off and the resistivity of the composite was increased for this reason. With the increasing
where J is the tunneling current density, V is the electrical potential difference, e is the quantum of electricity, m is the static mass of electron, h is Planck’s constant, d is the distance between CFs, l is the height of the barrier and A is the cross-sectional area of the tunnel (the cross-sectional area of CF was used as an approximate here). According to eqn (3), the resistance depended exponentially on the gap length between fillers and increased drastically with an increased gap length at the nanometer level (the resistance between two adjacent CFs was plotted as a function of gap length in Fig. S1, ESI†). Therefore, if the gap length between CFs increased with rising temperature, the resistivity would increase accordingly. As mentioned before, the increase in temperature would result in the volume expansion of HDPE and the expanded volume would no doubt cause the gap length between CFs to become bigger. As a result, partially conductive paths were cut off and the conductive network was broken, leading to the occurrence of the PTC effect. 3.3. Reproducibility of the PTC effect for the HDPE/CF composites Good reproducibility is a very important quality for desired PTC materials. Four continuous heating processes were carried out
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Fig. 4 Temperature dependence of resistivity for the 8.8 vol% CF filled HDPE composites during four continuous heating processes (a); relationship between PTC intensity and cycle times during the heating processes (b).
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Fig. 5 SEM micrographs of the 8.8 vol% CF filled HDPE/CF composites with increasing number of thermal cycles.
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Fig. 7 Schematic mechanism for the change in interactions between CF and the polymer matrix with increasing number of thermal processes.
each other well before thermal cycles. The surface of CF will absorb some polymer chains during the heating process and will be covered by a layer of HDPE matrix. The resin layer becomes thicker with subsequent thermal cycles, which results in a larger gap length and higher barrier height between CFs. As a result, the conductive network becomes brittle and can be broken much more easily.
4. Conclusions
Fig. 6 Magnified SEM micrographs of 8.8 vol% CF filled HDPE/CF composites with increasing thermal cycles.
number of thermal cycles, more of the conductive network was broken up, leading to the ever-increasing resistivity. Fig. 6 shows the magnified SEM micrographs of the HDPE/ CF composites before the thermal cycles and after consecutive thermal cycles. With the original HDPE/CF composites, the space between the CFs and the HDPE matrix (marked with a dotted circle) was very obvious, indicating poor interaction between the HDPE and CFs before thermal cycles. The CF was covered with some HDPE during the consecutive thermal cycles and the space between them became less obvious, which indicated a better interaction. After the composites had undergone four continuous thermal cycles, the CF was covered with a layer of the HDPE matrix. As a result, the gap between adjacent CFs and the barrier height of the tunneling gap increased. Considering eqn (3), the gap length between CF and HDPE and the barrier height of the tunneling gap had a great influence on the conductivity of the composites. The increase of the gap length and the barrier height would undoubtedly imply a remarkably broken conductive network, namely, the conductive network became more brittle with the increasing number of thermal cycles (Fig. S5, ESI†). As a result, the PTC was everincreasing with subsequent thermal cycles. As aforementioned, we can explain the distinct PTC effect with the scheme in Fig. 7. The interaction between CF and the polymer matrix is relatively weak and the adjacent CFs can connect with
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In this study, the PTC effect of HDPE/CF was investigated. Conductivity paths grew with increasing filler content at room temperature, so the room temperature resistivity of HDPE/CF and maximum resistivity decreased. The appearance of a plateau at low o indicated the forming of a conductive network in the HDPE/CF composites. All of the samples showed significant a PTC effect during the heating processes and we believed that thermal expansion theory and electron tunneling effects should be incorporated to interpret the PTC effect. The resistivity between two CFs depended on the gap length between the fillers and the increase in gap length resulted from thermal expansion of the matrix during the heating process and brought about a significant decrease in conductivity, leading to the appearance of the PTC effect. No NTC effect could be observed, even at temperatures much higher than the melting temperature of HDPE, which was related to the unique structure of CF. The surface of CF was very coarse and there existed many grooves on it, which increased the binding force between the matrix and filler and blocked re-aggregation at high temperatures. The interaction between the polymer matrix and CF became better with increasing thermal cycles, which resulted in larger gap distribution and a higher barrier height of the tunneling gap, and the significant number of inter-filler gaps was too large to allow appreciable electron tunneling, although the average gap width didn’t change considerably. These factors led to the rising resistivity and ever-increasing PTC intensity.
Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant No. 51103087, 51421061). Authors wish
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to acknowledge Dr Chaoliang Zhang (West China College of Stomatology, Sichuan University) for the SEM observation.
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