Easy Fabrication and Resistivity-Temperature Behavior of an

Dec 23, 2009 - An easy fabrication method comprising a slit die extrusion-hot stretch-quench process was used to make carbon nanotubes (CNTs) filled w...
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J. Phys. Chem. B 2010, 114, 689–696

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Easy Fabrication and Resistivity-Temperature Behavior of an Anisotropically Conductive Carbon Nanotube-Polymer Composite Bo Li, Yi-Chuan Zhang, Zhong-Ming Li,* Sha-Ni Li, and Xiao-Na Zhang College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan UniVersity, 127, YiHuanLu, NanYiDuan, Chengdu, 610065, Sichuan, People’s Republic of China ReceiVed: May 6, 2009; ReVised Manuscript ReceiVed: NoVember 25, 2009

An easy fabrication method comprising a slit die extrusion-hot stretch-quench process was used to make carbon nanotubes (CNTs) filled with anisotropically conductive polymer composite (ACPC). CNTs were first premixed with polycarbonate (PC) by coagulation and then melt mixed with polyethylene (PE). During extrusion, the CNT/PC/PE composite was subjected to hot stretching to make the CNT/PC phase form in situ an oriented conductive fibril assembly in the PE matrix. Finally the aligned CNT/PC short fibrils were quenched to preserve their structure. The resultant CNT/PC/PE composite exhibited strong anisotropy in conductivity. This method has the advantages of giving a highly oriented structure with good control of electrical anisotropy as well as the ability to be fabricated in a high rate manner. Temperature-resistivity behavior was investigated by observing the resistivity during isothermal treatment (IT) as well as nonisothermal treatment (NIT). Percolation behavior was seen in the isolated direction during the first IT at 180 °C. This was a result of a disordering-induced conductive network. In addition, the positive temperature coefficient (PTC) effect attenuated with IT duration. This was seen in contrast to the remaining negative temperature coefficient (NTC). The unique evolution of PTC and NTC effects originated from the ACPC’s special conductive network. It can be seen that this is composed of the originally connected “intrinsic pathway” and isolated “potential pathway”. 1. Introduction Anisotropically conductive polymer composites (ACPCs) resulting from preferential alignment of a conductive filler in a polymer matrix have received considerable attention due to the anisotropic conductivity and potential applications in the semiconductor industry.1 Of the available fillers, such as carbon nanotubes (CNTs), carbon fibers, or carbon black (CB), CNTs are an ideal one-dimensional conductive filler with high electrical conductivity, large aspect ratio, and very high flexibility. Therefore it is expected that they will lead to ACPCs with desirable properties.2,3 Several methods have been employed to control the preferential alignment of CNTs in ACPCs, such as cutting,3 mechanical stretching,4 melt-spinning,5,6 injection molding,7 doctor blade technique,8 blow bubble technique,9 electrical field inducement,10,11 and magnetic field inducement.12,13 In addition, growing aligned CNTs in situ and then compounding with polymer can also produce ACPCs.14 However, the long, entangled structure of CNTs causes great difficulties in the engineering process of manufacturing ACPCs. For one, CNTs are difficult to disperse uniformly with good alignment in a polymer composite system. Various methods utilizing strong mechanical force or applying electromagnetic fields with deliberate control of the viscosity of the polymer melt and solution have been used to obtain CNTs-filled ACPCs.5,11 Not only are these manufacturing processes inefficient, they are also highly expensive. Moreover, with an increased concentration the intertwined structure of CNTs greatly diminishes the anisotropy in the conductivity of ACPCs.8,12 The entanglement of the CNTs also presents a challenge to predict and control the properties of the ACPC material. For * To whom correspondence should be addressed. E-mail address: [email protected]. Tel./Fax: +86 28 8540 5324.

example, how does the alignment influence the electrical conductivity of the material? With aligned CNTs, Lanticse et al.8 obtained a million times lower resistivity in the direction of orientation compared with the randomly oriented composite. However, Choi et al.13 reported only a 10 time decrease of electrical resistivity in the orientation direction relative to that of the random sample. It was found that for short single-walled carbon nanotubes with increased degree of anisotropy the electrical resistivity first decreases before increasing.5 The difficulties of directly aligning CNTs are the largest barrier in having them as a good candidate for ACPCs. In this work we present a CNTs-filled binary polymer system that was used to make ACPC with PC/PE as the target system. Po¨tschke et al.15 did fundamental research on the CNTs-filled PC/PE system obtained by the method of melting mixing and then focused on the morphology and electrical properties. It should be noted, however, that melt mixing is an isotropic method and can hardly produce anisotropic materials. Therefore, we present a new fabrication strategy to make ACPC as shown schematically in Figure 1 with the following steps. First, the CNTs are first mixed with polycarbonate (PC) through coagulation. Second, the master batch (CNT/PC) is further melt mixed with polyethylene (PE) and extruded through a narrow slit die. Then the hot stretching is exerted on the extruded sheet along the extrusion direction, during which the dispersed PC droplets filled with CNTs form in situ oriented conductive fibrils. Finally, the oriented structure is frozen by quenching. The CNTs that initially compounded with PC cannot migrate significantly into the PE matrix and impart conductivity to the CNT/PC short fibrils. The interconnection of short fibrils leads to a conductive network throughout the PE matrix. By controlling the alignment of these short fibrils, an ACPC is obtained with a conductive longitudinal direction and insulating transverse direction. We define Cartesian coordinates in which X, Y, and Z correspond

10.1021/jp9042396  2010 American Chemical Society Published on Web 12/23/2009

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Figure 1. Schematic of the ACPC manufacturing process.

to the transverse direction, longitudinal direction, and thickness direction accordingly as shown in Figure 1. This method has several advantages. First, the in situ formed CNT/PC short fibrils can be oriented easily in the insulating PE matrix. In traditional CNT/polymer composites, the rotation and disentanglement of CNTs is laborious. Our in situ forming process is free of those two procedures. Second, the degree of conductive anisotropy is controllable by changing the hot stretch ratio (the ratio of stretch speed to extrusion speed). In this way the electrical anisotropy of ACPC can be tuned. Third, it was seen that the entanglement and curvature of the CNTs do not influence the conductivity anisotropy. It is postulated that this is because the anisotropy is achieved by the alignment of CNT/ PC short fibrils rather than CNTs themselves. Finally, they may be manufactured in a high rate manner because all of the manufacturing processes (coagulation, extrusion, and stretching) are industrialized techniques. In addition to fabrication of the new ACPC, we also investigated its responses to different thermal treatments which give insight into the relation of the structure of this material to its electrical characteristics. This is of crucial importance for its realization for use in electronic devices. The material was repeatedly subjected to the isothermal treatment (IT) and the nonisothermal treatment (NIT) with contact heating and cooling while observing resistivity. This is how the influence of intensive thermal treatment on the conductive network resistivity evolution was studied. Moreover, resistivity-temperature behavior such as positive temperature coefficient (PTC) and negative temperature coefficient (NTC) effects were investigated. It has been seen that PTC and NTC effects are structure-related and are mainly dominated by factors such as the crystalline state of the matrix, the conductive network structure, and the viscosity of the matrix in the melt condition.16-22 The origin of the PTC effect is the breakdown of the conductive network during heating. The effect has been illustrated by a drastic increase of resistivity when the temperature of a semicrystalline polymer matrix such as PE increases to the melting point.16,19 As the temperature increases further, the NTC effect is seen as the resistivity begins to decrease. This corresponds to the reconstruction of the conductive network under melt conditions of the matrix.21,22 Therefore, by investigation of both PTC and NTC effects, the dynamic interaction between the conductive network and the polymer matrix can be explored. During the IT presented in this work, it is interesting to see how the disordering of the oriented conductive network leads to percolation behavior in the isolated direction. During NITs, PTC and NTC effects of adjacent NITs evolve with the duration of IT between them. The oriented network of CNT/PC short fibrils as well as its disordering process is accounted for in the conductive response of NIT.

2. Experimental Section 2.1. Materials. CNTs with diameters of 20-40 nm and lengths ranging from 0.5 to 50 µm were purchased from Nano Harbor Co. Ltd., China. Polycarbonate (K1300) with number average molecular weight (Mn) of approximately 2.8 × 104 to 3.2 × 104 g · mol-1 and molecular weight distribution index 2.1 (by gel permeation chromatography) was purchased from Teijin Co. Ltd., Japan. Polyethylene (5000S) with Mn 5.28 × 105 g · mol-1 was purchased from Daqing Petroleum Chemical Co. Ltd., China. The weight ratio CNT:PC:PE was fixed as 6:24: 70, and the content of CNTs in the CNT/PC master batch was 20 wt %. From our reference experiments, it was known that this content of CNTs is well above the percolation threshold of the isotropic CNT/PC/PE conductive system. 2.2. Sample Preparation. Both the CNTs and PC were initially oven-dried for 10 h at 120 °C. Four grams of CNTs was then sonicated in 250 mL of dichloromethane (CH2Cl2) and 16 g of PC was dissolved separately in 300 mL of CH2Cl2. The two solutions were then mixed in a three-necked bottle in a water bath at 50 °C and sonicated with mechanical stirring for 15 min. Then 400 mL of ethanol was added to the mixture with the water bath cooled rapidly to about 30 °C by adding cold water (20 °C) to the bath. It was seen that the CNT/PC master batch precipitated quickly and was filtered and oven-dried for 24 h at 120 °C. The CNT/PC masterbatch and PE (30:70 by weight) were blended in a single screw extruder (SJ-20AXJ × 25, with screw length to diameter ratio of 25; the screw diameter was 20 mm). The temperature profile used for the extruder was 140, 250, 280, and 270 °C from hopper to die. The die for the extruder was 40 mm wide with a 1.5 mm high slit. The extrudate was stretched by a take-up device with two chill rolls. The tangential velocity of the chill rolls was 2.55 cm/s. The stretched strip with rectangular cross section was then quenched in a water bath at 20 °C to stabilize the aligned morphology of CNT/PC short fibrils. The CNT/PC/PE composite strip could be produced continuously along the Y direction. 2.3. Scanning Electronic Microscope. Scanning electronic microscope (SEM) observation was performed using a JEOL JSM-5900LV. Electron micrographs were taken of the cryofractured cross sections parallel to the Y-Z plane and the X-Z plane as seen in the schematic of Figure 1. The samples were immersed in liquid nitrogen for 30 min to freeze the polymer chains prior to fracture. 2.4. Two-Dimensional Wide Angle X-ray Diffraction. A two-dimensional wide angle X-ray diffraction (2D-WAXD) pattern of ACPC was obtained using a synchrotron light source (wavelength λ ) 1.4809 Å) and 2D Mar345 CCD X-ray detector (MarResearchi Co. Ltd., Germany) at the National Synchrotron

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Radiation Laboratory, Hefei, China. The incident radiation was perpendicular to the sample surface (X-Y plane), so that the orientation of PE crystallites could be measured. The sample was taken from original extruded strips and was prepared by polishing to form a 1 mm thick slide. The data was collect from the center of the slide. The orientation of PE crystals in ACPC was calculated using the Hermans orientation parameter defined as

〈P2(cos φ)〉 ) (3〈cos2 φ〉 - 1)/2

(1)

where an orientation factor, cos2 φ, is defined as

〈cos2 φ〉 )

∫0π/2 I(φ) cos2 φ sin φ dφ/ ∫0π/2 I(φ) sin φ dφ (2)

I(φ) is the scattering intensity at angle φ. The orientation parameter has the value unity when all of the crystals are oriented parallel to the reference direction. For our sample, the orientation parameter was calculated using Picken’s method from the (110) WAXD reflection for PE.23 2.5. Resistivity Measurement. The samples for determination of electrical resistivity in the Y direction (F|) and X direction (F⊥) were cut consecutively from the extruded strip to minimize error. Resistivity was measured in one direction for each sample. The geometry of the sample was 15 (Y) × 10 (X) × 0.6 (Z) mm. A two-probe method was used to measure volume resistivity. Copper electrodes were attached to corresponding cross sections with silver paint. Baking at 100 °C was necessary to cure the silver paint. A TH2683 high resistivity meter was used with applied voltage 10 V. 2.6. Thermal Treatment. All thermal treatments were performed sequentially on the samples to monitor the evolution of electrical properties of the ACPC. For the first IT (IT1), samples with attached electrodes were immersed in a silicone oil bath at a constant temperature of 180 °C for a period of 4 h. It should be noted that the samples were put into the oil bath at room temperature, and both heatings before IT and cooling after IT were conducted at a constant rate of 2 K · min-1. The heating and cooling data are not shown here, so the end resistivities (Fe) from IT1 in both directions captured before cooling were different from the initial resistivities (F0) in the following NIT. Following IT1, the first NIT (NIT1) was performed on each sample. The temperature of the system was ramped from 20 to 180 °C at 2 K · min-1, held at 180 °C for 3 min, and then cooled to 20 °C at 2 K · min-1. The resistance was monitored in real time. After NIT1, a second 10 h IT (IT2) was performed, followed by a second NIT (NIT2). The second NIT had the same parameters as the first. The measurement was performed by using a computer as a controller as well as a data acquisition instrument. The computer interfaced a TH2683 high resistivity meter and a programmable silicone oil bath with tunable temperature range 20-200 °C. It should be noted that the sample was immersed in silicone oil to prevent oxidation. 2.7. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) analysis of ACPC was carried out using a Perkin-Elmer thermoanalyzer. The thermal treatment follows the same procedure of NIT. The sample for DSC analysis was cut from a controlled ACPC sample strip that had been subjected to IT1. 3. Results and Discussion 3.1. Formation Mechanism of ACPC Material. The representative morphology of the ACPC is shown in Figure 2. It

Figure 2. (a) SEM images of ACPC on the cryofractured cross section parallel to the Y-Z plane with low magnification. (b) Surface morphology of CNT/PC short fibrils with high magnification. (c) SEM images of ACPC on cryofractured cross section parallel to the X-Z plane. (d) Cross section of a single broken CNT/PC short fibril.

can be seen in Figure 2a that the aligned CNT/PC short fibrils are embedded in the PE matrix in the Y-Z plane. It is hard to find embedded cylinders with both ends exposed on the cross section. We calculated the length and diameter of seven representative cylinders with both ends exposed from nonembedded samples and found that the length of the cylinders range from 18 to 50 µm with aspect ratios from 10 to 20. In Figure 2b many CNTs appear arching on the surface of CNT/PC short fibrils. Figure 2c illustrates many CNT/PC short fibrils with broken tips in the X-Z cross section; this indicates that the short fibrils are well aligned. The fibril diameters are nonuniform, scattering from several hundreds of nanometers to about 5 µm, and the separation distance between them is zero to several micrometers. Also, there is a degree of misalignment of the short fibrils. The cross section of a single CNT/PC short fibril can be seen in Figure 2d showing that a lot of CNTs are selectively localized in the PC phase. As seen in Figure 2c and 2d, the preferential enrichment of CNTs in the PC phase can be found. This enrichment dictates the conductive mechanism of ACPC, because it ensures that CNT/PC short fibrils are the main conductive component in the PE matrix instead of the CNTs scattered in PE. Selective localization of CNTs in polymer blends has been seen in several systems such as CNT/PC/PE, CNT/polyamide/PE, and CNT/ PC/poly(styrene-acrylonitrile).24,25 Based on Young’s equation,19 the wetting coefficient, ωa, is given as

ωa )

σCNT/PC - σCNT/PE σPE/PC

(3)

in which σCNT/PC is the interfacial energy between CNTs and PC, σCNT/PE is the interfacial energy between CNTs and PE, and σPE/PC is the interfacial energy between PC and PE. If ωa > 1, CNTs tend to stay in the PE phase. However, if ωa< -1, CNTs tend to localize in the PC phase; otherwise, the CNTs are preferentially located at the PE/PC interface, and the interfacial energy can be calculated using the harmonic and geometric mean equations.17 Po¨tschke et al.17 calculated -2.50 for ωa for the CNT/PC/PE system using the harmonic mean equation and -3.83 using the geometric mean equation. Both results showed that the CNTs can preferentially localize in the PC phase. In addition, the premixing procedure for CNTs and PC in our experiments further enhances this selective localization.

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Lc ) f0{[(P| /P⊥) + (P| /P⊥)-1]/2}1/2rs

Figure 3. Schematic of electron transportation between CNT/PC short fibrils. The polymer layer sandwiched between CNTs can be PC or PE.

TABLE 1: Volume Resistivity Resulting from Different Thermal Treatments before IT1 (25 °C) Fea of IT1 F0b of NIT1 F0 of NIT2

F| (Ω · cm)

F⊥ (Ω · cm)

F⊥/F|

6.2 × 10 1.5 × 103 1.5 × 103 9.9 × 102

>10 2.0 × 104 3.9 × 104 1.1 × 104

>105 13 26 11

4

10

a Fe resistivity at the end of a thermal treatment. b F0 resistivity at the beginning of a thermal treatment.

The existence of CNTs on the surface of the PC short fibrils as shown in Figure 2b is another key factor in the conduction mechanism. The fibrillar morphology of the CNT/PC phase demonstrates that the CNT/PC phase is not a continuous phase in PE matrix. Thus, the conductive network can be only built by the interconnection of many short fibrils or more specifically the interaction of CNTs on the surface of connected short fibrils as illustrated in Figure 3.26 The connected region between fibril A and B is illustrated in the top portion of the figure, whereas the lower portion shows how a single electron is transported from A to B through the connection between a CNT on fibril A to a CNT on fibril B. As seen in the figure, if the two CNTs are in contact directly the electron can easily cross the junction. However, if there happens to be a thin polymer layer covering one of the CNTs, the electron may penetrate this layer by tunneling. Finally, if the polymer layer on the CNTs is thick, the two short fibrils are insulated even though they are in intimate contact.27 To ensure that the CNTs are exposed on the surface of the short fibril, the content of the CNTs should exceed the maximum packing factor. In this case the filler can no longer stay in the polymer matrix,; instead, the CNTs are forced to migrate to the interface or to another polymer phase.26 Therefore, a high content of CNTs (20 wt %) was added to the PC to ensure that they exceed the maximum packing factor. Table 1 lists the resistivity in different directions before and during thermal treatment. As was expected, before IT1 F| was 6.2 × 104 Ω · cm while F⊥ exceeded the upper limit (1010 Ω · cm) of the measurement range of the high resistivity meter used. Therefore F⊥ was at least 5 orders of magnitude higher than F|. The measurement of the traverse direction F⊥ began at 5.5 × 109 Ω · cm which occurred at the beginning of IT1 at 180 °C. The mechanism of anisotropic conductance of the ACPC can be elucidated, to some degree, by Balberg’s percolation theory of the rigid stick filled anisotropic conductive system.28 In this theory, preferential alignment of a conductive filler leads to different percolation thresholds in the Y and X directions. The percolation threshold is defined as a critical value at which the conductive network begins to form throughout the insulating matrix leading to a sharp decrease of electrical resistivity of the system.29,30 This is usually related to the content of conductive filler or the average fibril length in the conductive

(4)

where rs ) 1/(πN)1/2 is half the effective intersite distance, f0 ) 4.2 is a constant, and P|/P⊥ is the degree of anisotropy of the conductive network and has a value less than 1 in the anisotropic case.20 According to eq 4, Lc increases with increase of P|/P⊥. In the case of N goes to infinity, the critical stick length in the longitudinal direction, Lc|, equals both Lc⊥ in the transverse direction and Lc. However, in reality N f ∞ is impossible and Lc⊥ diverges from Lc| with the increase of P|/P⊥, whereas the dependences of both Lc⊥ and Lc| on P|/P⊥ are unchanged. As seen in Monte Carlo simulations, the rate of increase of Lc⊥ is much higher.28 This indicates that in a rigid stick system, the alignment of filler will lead to ACPC. In the PC/PE immiscible polymer system, the PC droplets containing CNTs are deformed and oriented during hot stretching. This is how the oriented CNT/PC short fibril assembly is formed.31 The anisotropy of the fibril assembly can be controlled by changing the hot stretch ratio so that the length of the CNT/PC short fibrils formed can be varied in the range between Lc⊥ and Lc|. The interdependence of hot stretch ratio and resistivities in different directions will be reported in a subsequent paper. It is worthwhile to note that better alignment here does not necessarily mean better conductivity. In fact, the alignment of short fibrils leads to weaker interaction between short fibrils and thus higher resisitivity. A similar conclusion was drawn by Villmow et al.7 from a CNT/ PC composite. There exists a conflict between Balberg and Du concerning the dependence of the percolation threshold on anisotropy in the low oriented region.5,28 Based on Balberg’s deduction,28 a monotonic increase of percolation threshold with increasing anisotropy should occur, and even his Monte Carlo simulation showed some disturbance in the low oriented region. Du et al.5 proved with experiment and Monte Carlo simulations that a small extent of alignment in a random CNT system can increase the conductivity, thus lowering the percolation threshold. However, it was shown that further orientation decreases the conductivity. Compared with that of Balberg, Du’s prediction is more precise and more soundly based; however, the divergence only happened in the slightly aligned region. At a higher degree of orientation, Balberg’s theory more closely matches our results. The formation of oriented CNT/PC short fibrils can be attributed to the unique hot stretching-quenching technique. Hot stretching under melt conditions imposes shear and elongation forces on the matrix polymer (PE). This further deforms the droplets of PC-dispersed polymer and therefore forms stick assembly oriented along the stretch direction.32 Since the orientation of CNT/PC phase is generated from the shearing force of the PE matrix, monitoring the orientation of PE crystals might reflect the resulting orientation of the CNT/PC short fibrils. In Figure 4a, the 2D-WAXD pattern of a sample strip shows that the Bragg intensity is preferentially concentrated in two opposite directions. The (110) intensity distribution along the azimuth angle is shown in Figure 4b. Here, two peaks were found centered at about Φ ) 90 and 270°. The degree of orientation of PE crystals is calculated by Picken’s method,23 and it was seen that the (110) reflection of PE was about 0.14. Though the calculated value is not very high, it indicates a significant amount of PE crystal orientation in one direction. The relatively small orientation of PE can be attributed to the

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Figure 4. (a) Two-dimensional WAXD pattern of sample strip; (b) the (110) intensity distribution along the azimuth angle.

Figure 5. (a) Time dependence of F| and F⊥ normalized by the end resistivity (Fe) of each sample during IT1. SEM image of the cross section parallel to the Y-Z plane before IT1 (b) and after IT1 (c).

low stretch ratio and the relaxation of PE chains. However, this does not necessarily mean that the CNT/PC short fibrils cannot attain high preferential alignment. It is well-known that the PE polymer chains relax easily due to their flexible structure. Thus it is possible that some oriented PE chains relaxed in the interval between hot stretching and quenching. In the alignment of CNT/ PC short fibrils, the recovery of an elongated CNT/PC phase is limited by its size and the intertwined networks of CNTs. Consequently, the orientation of CNT/PC stick assemblies should be higher than that of the PE crystals. However, the orientation parameter of the CNT/PC stick assembly is hard to evaluate due to the limitation of testing technology and the large size of the CNT/PC short fibrils. 3.2. Disordering-Induced Percolation. The oriented structure of CNT/PC short fibrils in PE matrix is not thermodynamically stable. This may influence the stability of ACPC during its use in applications, especially in high temperature applications. IT1 was done on both longitudinal and transverse samples to check their thermal stability. Figure 5a shows the resistivity of ACPC samples as a function of holding time at 180 °C. F| and F⊥ were normalized by the Fe of each sample during IT1. The values of Fe are summarized in Table 1. Interestingly, at the starting stage of IT1, F⊥ which was initially undetectable, suddenly became as high as 5.5 × 109 Ω · cm followed by typical percolation behavior. Traditionally, percolation is achieved by inducing agglomeration of conductive components.33,34 However, it has been seen that the large size of the CNT/PC short fibrils prevents them from undergoing large scale aggregation.35 Consequently, there must be a different mechanism for percolation. Here we propose a disordering-induced percolation mechanism based on the unique oriented structure of ACPC. By comparing the SEM images of the cross section parallel to the Y-Z plane before (Figure 5b) and after IT1 (Figure 5c),

it is apparent that the oriented CNT/PC short fibrils are disturbed to some extent. The disordering greatly increases the possibility of nearby short fibrils connecting each other leading to a decrease of resistivity in both directions. On the other hand, the disturbance of CNT/PC short fibrils decreases the degree of electrical anisotropy (P|/P⊥). This leads to the decrease of both Lc⊥ and Lc|. Note that before IT1 the stick length, L, is larger than Lc| and smaller than Lc⊥. However, during IT1 as Lc⊥ decreases to a value smaller than L, percolation behavior occurs and a sharp decrease of F⊥ was observed. The origin of the disordering process can be attributed to the thermodynamic rule that the highly oriented structure is at an unstable transition state and has the tendency to disorder to increase entropy. In our experiment, the oriented short fibrils are frozen and therefore stabilized during the quenching process. The temperature of isothermal treatment is above the melting point of PE (Tm ) 129 °C). Thus, during the thermal treatment, the system is unfrozen and therefore able to reorganize as the matrix is above melt temperature and the CNT/PC phase is above its glass transition temperature (Tg ) 152 °C).36 From a microscopic viewpoint, Brownian movement of the PE chains will occur and facilitate the disordering of CNT/PC short fibrils. It should be noted that since the temperature of the thermal treatment (180 °C) was above the Tg of PC, it is possible that CNT/PC short fibrils deformed to some extent. However, by comparing Figure 5b and 5c, the CNT/PC short fibrils maintain their shape during the treatment, and no apparent bending was seen. This may be attributed to the high content of CNTs in the PC phase (20 wt %) such that the intertwined network of CNTs greatly stiffened the filled PC phase even in the rubbery state.37 Consequently, during thermal treatment the CNT/PC short fibrils are rigid, which matches the assumption in Balberg’s theory. 3.3. The Evolution of Resistivity-Temperature Behavior. For comparison, time dependences of volume resistivity during NITs are given in Figure 6. These were shown instead of temperature dependences for convenience. Since all the NITs follow the same procedure, the temperature-time profile is only provided in Figure 6a shown as the blue curve. F| and F ⊥ are normalized by F0 of each sample during the NITs, and the values of F0 can be seen in Table 1. Figure 6a and 6b show the response of F| in NITs before and after the 10 h IT. Figure 6c and 6d summarize the corresponding F⊥ data. During NIT1, F| seen as squares in Figure 6a exhibits a typical PTC effect around the melting point of PE (Tm ) 129 °C) and is followed by a strong NTC effect. Interestingly enough, after a further 10 h of IT, the PTC effect disappears. However, the NTC effect still remains, as shown in Figure 6b. Figure 6c and 6d show that F⊥ has the same transition as one might expect.

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Figure 7. Schematic illustration of the evolution of anisotropic conductive network during thermal treatment: (a) in the original state; (b) after IT1; (c) after IT2. Black fibrils which are not connected to each other and have the potential to form conductive pathways compose the potential pathways. Red fibrils are connected to each other to form the intrinsic pathways. Figure 6. Time dependence of volume resistivity normalized by F0 at the beginning of each NIT for F| and F⊥. Time dependences of F| in NIT1 and NIT2 are shown in (a) and (b), respectively. Time dependences of F⊥ in NIT1 and NIT2 are shown in (c) and (d), respectively. The temperature-time of all the NITs follow the same curve shown in (a). The characteristic temperatures such as Tg of PC, Tm of PE, and Tc of PE are marked by a dash line in (a).

The PTC effect can be attributed to the thermal expansion of the PE matrix near its melting temperature. It has been seen that this expansion can separate the connecting conductive particles.16 On the other hand, the NTC effect is caused by the reaggregation of the separated conductive particles under the melting condition of PE.21,22 The influence of PC expansion should also be evaluated. However, because electrical resistivity does not show any disturbance at the Tg of PC during the NITs, this indicates that the PC’s expansion is not a critical factor for the PTC and NTC effects as demonstrated by Figure 6. The elimination of the PTC effect after intensive IT is reasonable because the IT can motivate the aggregation conductive component under melting conditions and construct more reliable conductive pathways which do not break down easily in the heating process during the following NIT.38 The unique remaining NTC effect is caused by the reconstruction of conductive pathways. The elimination of PTC indicates that the conductive pathways are robust and cannot easily break. Logically, the contribution of the reconstruction of conductive pathways (NTC effect) should also be greatly attenuated. This is supported by our previous work on CB/PET/PE fibrillar conductive composites. It was seen that in an isotropic conductive network, intensive IT with long duration led to great attenuation of the PTC effect and the NTC effect was eliminated.38 However, the same conclusion cannot apply to this anisotropically conductive network. To explain this new phenomenon, we must first confirm that the conductive pathways along the Y and X directions share the same basic property of the entire conductive network. As is the tendency for PTC and NTC effects to occur during NIT, the evolution of resistivity-temperature behavior is seen in Figure 6. Because these conductive pathways belong to the same conductive network of the composite, they may also share some parts of the conductive network. Thus, electrical differences between Y and X directions may result from the number of existing conductive pathways. Figure 7 illustrates our model to explain the evolvement electrical property in both directions. The original state seen in Figure 7a shows that most of the oriented CNT/PC short fibrils are well isolated by PE matrix and only a number of conductive pathways are constructed through the Y direction. This leads to anisotropic conductivity.

The interconnected pathway composed of red fibrils was named as the intrinsic pathway and the isolated pathway composed of black fibrils as the potential pathway as shown in Figure 7. During IT1, some of the potential pathways disorder and transform to intrinsic pathways so conductive pathways are built in both Y and X directions as shown in Figure 7b. The evidence of the transformation can be seen in Figure 5c where the disordering causes the interconnection of fibrils. In Figure 7c, more and more intrinsic pathways are formed during IT2, and some potential pathways remain. The number of conductive pathways dictates the resistivitytemperature behavior of the system. The potential pathways which were originally isolated cannot further break apart and contribute to the increasing resistivity during heating. Therefore the PTC effect can be only attributed to the behavior of intrinsic pathways when the temperature approaches the melting temperature of PE (Tm). At stage b in Figure 7, the number of intrinsic pathways is small; thus, it is susceptible thermal expansion during heating of NIT, and this leads to a PTC effect in both directions. However, once the intrinsic pathways form a conductive network strong enough to resist thermal effects during heating, the PTC effect is attenuated. The NTC effect is influenced by both the intrinsic and potential network. This is because at temperatures higher than Tm of the PE, both types of pathways are movable. Once the intrinsic pathways are strong enough to resist the separation during PTC, the NTC effect relating to the intrinsic pathway will attenuate. This is because further aggregation does not cause much difference compared to the originally connected network. However, the potential pathway can influence the NTC effect continuously. As previously mentioned, the oriented structure is not stable thermodynamically, so when the temperature is above the melting point of PE the insulating layer between the oriented CNT/PC short fibrils begins to melt. Then the oriented CNT/PC short fibrils tend to disorder and connect to each other to reduce the energy of the system. In ACPC, many potential pathways exist in the system, and the large size of the CNT/PC short fibrils as well as the viscosity of the PE limits the movement of the short fibrils. It takes a long time for the oriented structure to migrate to a completely random system due to the large size of short fibrils. Thus a significant number of potential pathways remain after the intensive ITs, as shown in Figure 7c. This explains why the NTC effect remains after the elimination of the PTC effect. In summary, the evolution of the potential pathway to the intrinsic pathway during IT causes attenuation of the PTC effect, and the remaining potential pathway accounts for the existence of the NTC effect.

Anisotropically Conductive CNT-Polymer Composite

Figure 8. Heating and cooling DSC curves of the control sample after IT1.

Another interesting phenomenon in NIT can verify the existence of the potential pathway and its interaction with the PE matrix. As seen in Figure 6, during cooling each curve shows a steep increase at 121 °C; this is in contrast to the sharp decrease of resistivity in the isotropically conductive composite.35 The DSC analysis of the ACPC in Figure 8 shows that the maximum crystallization temperature (Tc) of PE is 120.9 °C. We only showed the DSC curve ranging from 80 to 140 °C. This is because only a straight line is observed out of this range. These similar temperatures indicate that the crystallization of PE influences the structure of the conductive network. As was explained, the intrinsic pathway is inert to the influence of the thermal treatment after several ITs. It follows that the interaction between the potential pathway and crystallization accounts for this abnormal behavior. Crystallization during cooling has two main effects on the conductive network. On one hand, it results in significant volume shrinkage. This increases the density of the conductive network and reduces the resistivity. On the other hand, the growth of crystallites expels the conductive particles that penetrate the crystalline region.39 This in turn breaks down the conductive pathway. In the isotropic network, the effect of volume shrinkage at Tc surmounts that of particle exclusion. This leads to a sharp decrease in resistivity.35 It should be noted, however, that in this anisotropic network, the latter effect surmounts the former as shown in Figure 6. During NIT when the temperature is above the Tc of PE, the potential pathway disorders. Displacements of the short fibrils are limited by the short duration of NIT when the temperature is above the Tc of PE, as well as the large stick size and the high melt viscosity of PE. Correspondingly, the positions and even number of nucleating sites among the conductive stick networks do not change much when compared with their condition before disordering. Therefore, when the temperature falls back to the Tc of PE during cooling, it is possible that the growth of crystallites breaks the potential pathway. It also has a tendency to reorder the stick networks back to the isolated structure, which leads to the steep increase of both F|and F⊥.28 In Figure 6, the end resistivities of both F| and F⊥ after cooling are similar to their initial resistivities before heating, thus verifying this reordering effect. Conclusion A highly controllable and efficient method was used to manufacture anisotropically conductive polymer composite by in situ formation of aligned composite fiber based on a CNTsfilled PC/PE polymer system. The electrical resistivity in the aligned direction is 5 orders of magnitude higher than that in

J. Phys. Chem. B, Vol. 114, No. 2, 2010 695 the transverse direction. In addition, the present work extends insight into the dynamic interaction between the oriented conductive network and polymer matrix under different thermal treatments. It was found that the oriented network is not stable thermodynamically. This was seen in the disordering of the oriented CNT/PC short fibrils during intensive isothermal treatment at 180 °C which induced percolation of the insulated transverse direction. Moreover, the long isothermal treatment can eliminate the positive temperature coefficient effect in both directions. However, it is unusual that the negative temperature coefficient effect remained. A complex conductive network model was developed in which the positive temperature coefficient effect is controlled mainly by the intrinsic pathways. On the other hand, the negative temperature coefficient effect is greatly influenced by the potential pathways. During intensive isothermal treatment, potential pathways can transfer to intrinsic pathways, and the growth of these intrinsic pathways leads to the elimination of the positive temperature coefficient effect. However, the remaining potential pathways contribute to the existing negative temperature coefficient effect. Finally, the interaction between the conductive network and the PE matrix was proved by the existence of a transition point at the Tc of PE during cooling. Acknowledgment. The authors are indebted to Dr. XiangBin Xu and Master Lei Gao for their helpful discussion of modeling, and to Mr. Gan-Ji Zhong, Ms. Yan Wang, and Mr. Zhe Ma for their help in 2D-WAXD measurement. Thanks are also given to Mr. Peter Ryan from the Department of Mechanical and Industrial Engineering, Northeastern University, and Prof. C. V. Krishnan, from the Department of Chemistry, Stony Brook University, for refining the English language and some useful suggestions. This work is financially supported by the Nature Science Foundation of China (Grant No. 50573049) and the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20060610026). References and Notes (1) Tai, X. Y.; Wu, G. Z.; Tominaga, Y.; Asai, S.; Sumita, M. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 184. (2) Ijima, S. Nature 1991, 354, 56. (3) Ajayan, P. M.; Stephan, O.; Colliex, C. Science 1994, 265, 1212. (4) Jin, L.; Bower, C.; Zhou, O. Appl. Phys. Lett. 1998, 73, 1197. (5) Du, F. M.; Fischer, J. E.; Winey, K. I. Phys. ReV. B 2005, 72, 121304(R). (6) Fornes, T. D.; Baur, J. W.; Sabba, Y.; Thomas, E. L. Polymer 2006, 47, 1704. (7) Villmow, T.; Pegel, S.; Potschke, P.; Wagenknecht, U. Compos. Sci. Technol. 2008, 68, 777. (8) Lanticse, L. J.; Tanabe, Y.; Matsui, K.; Kaburagi, Y.; Suda, K.; Hoteida, M.; Endo, M.; Yasuda, E. Carbon 2006, 44, 3078. (9) Yu, G. H.; Cao, A. Y.; Lieber, C. M. Nat. Nanotechnol. 2007, 2, 372. (10) Park, C.; Wilkinson, J.; Banda, S.; Ounaies, Z.; Wise, K. E.; Sauti, G.; Lillehei, P. T.; Harrison, J. S. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 1751. (11) Sandler, J. K. W.; Kirk, J. E.; Kinloch, I. A.; Shaffer, M. S. P.; Windle, A. H. Polymer 2003, 44, 5893. (12) Kimura, T.; Ago, H.; Tobita, M.; Ohshima, S.; Kyotani, M.; Yumura, M. AdV. Mater. 2002, 14, 1380. (13) Choi, E. S.; Brooks, J. S.; Eaton, D. L.; Al-Haik, M. S.; Hussaini, M. Y.; Garmestani, H.; Li, D.; Dahmen, K. J. Appl. Phys. 2003, 94, 6034. (14) Jung, Y. J.; Kar, S.; Talapatra, S.; Soldano, C.; Viswanathan, G.; Li, X. S.; Yao, Z. L.; Ou, F. S.; Avadhanula, A.; Vajtai, R.; Curran, S.; Nalamasu, O.; Ajayan, P. M. Nano Lett. 2006, 6, 413. (15) Potschke, P.; Bhattacharyya, A. R.; Janke, A. Polymer 2003, 44, 8061. (16) Kohler, F. United States Patent 3243753 I, 1966; p 753. (17) Mayer, J. Polym. Eng. Sci. 1973, 13, 462. (18) Mayer, J. Polym. Eng. Sci. 1974, 14, 706. (19) Yi, X. S.; Wang, B. X.; Pan, Y. J. Mater. Sci. Lett. 1997, 16, 1381. (20) Narkis, M.; Ram, A.; Stein, Z. Polym. Eng. Sci. 1978, 21, 1049.

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