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Electric field induced conductive network formation of MWNTs and MWNTs-COOH in polycarbonate composites
Materials Chemistry and Physics 133 (2012) 1034–1039
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Electric field induced conductive network formation of MWNTs and MWNTs-COOH in polycarbonate composites Chang Su, Lihuan Xu, Rong-jun Yan, Meng-qi Chen, Cheng Zhan ∗ State Key Laboratory Breeding Base for Green Chemistry Synthesis Technology, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 4 October 2011 Received in revised form 15 January 2012 Accepted 26 January 2012 Keywords: Electrical conductivity Electrical properties Composite materials Electronic materials Bend and torsion test
a b s t r a c t Using polycarbonate (PC) as a matrix, and multi-wall carbon nanotubes (MWNTs) and acid-treated MWNTs (MWNTs-COOH) as fillers, PC/MWNTs and PC/MWNTs-COOH conductive composites were prepared, respectively. The conductive network formation induced by electric field in both the PC/MWNTs and PC/MWNTs-COOH composites were investigated by the dynamic percolation measurement. It was found that the electrical resistivity–time curves showed a certain self-similarity under various electric fields, i.e., the electrical resistivity decreased sharply as the percolation time (tp ) was reached. And the dynamic percolation time was shorten with the increase of the electric field intensity for both MWNTs and MWNTs-COOH filled PC composites. However, the percolation time and the formation activation energy of conductive network under electric field for MWNTs filled composites decreased greatly as compared to those of the MWNTs-COOH filled composites. These results indicated that the interaction between MWNTs and PC molecules played an important role in the conductive network formation under the electric field. Furthermore, the tp values under the electric field for PC/MWNTs and PC/MWNTsCOOH composites were successfully predicted by a modified thermodynamic percolation model. And this dynamic percolation measurement method could be recommended to investigate the zero-shear-rate viscosity at the different temperatures. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, anisotropically electrical conductive materials, which were achieved by the directional alignment of conductive fillers in an insulating polymer matrix, have aroused intensive attention, due to their potential application in the electronic industries, such as semiconductor art and electrostatic discharging. Various approaches have been applied to align conductive fillers, such as carbon black (CB) [1–3], carbon fibers (CF) [4,5], and carbon nanotubes (CNTs) [6–10], in polymer matrix. Among these approaches, the electric field induced alignment of fillers in a matrix has been considered as an effective route. CNTs, as a promising conductive filler, have been extensively explored in various fields due to their extremely high aspect ratio, outstanding electrical and thermal conductivity, and excellent mechanical properties. As CNTs were aligned highly in a polymer matrix, a remarkable anisotropy of the electrical property could be expected for the polymer/CNTs composite, which could been applied in the field emission cathodes [11]. Various efforts had been carried out to align CNTs in the matrix [12,13], and recently, special
∗ Corresponding author. Tel.: +86 571 88320253; fax: +86 571 88320253. E-mail address: [email protected] (C. Zhan). 0254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.01.129
attention had been paid to align CNTs induced by an electric field in liquid polymer monomers or organic solvents [14–16]. For example, Martin et al. [14] successfully applied both static and alternate electric field to align nanotube forming the conductive networks in epoxy resin. In our previous work [17–20], a dynamic percolation measurement method was developed to investigate the dynamic process of conductive network formation in high-density polyethylene (HDPE)/isotactic polypropylene (iPP)/vapor grown carbon fiber (VGCF) composite by recording the variation of electrical resistivity with time as the composite was annealed isothermally in the molten state [17]. Furthermore, this method had also been adopted as a tool to trace the self-assembly process of VGCFs in the poly(vinylidene fluoride) melt [18] and the dynamic percolation activity for conductive network formation of MWNTs in PVDF [19]. Recently, we further investigated the conductive network formation of MWCTs in the PC induced by electric field [20], in which the field-induced dipole–dipole interaction between the MWCTs resulted in the MWCTs alignment in the electric field direction [21]. Moreover, the dynamic percolation method might provide an essentially real-time monitor tool to measure the kinetics process of particle alignment under different electric fields. In this paper, in order to study how the interaction between the polymer matrix and conductive fillers-MWNTs influences the
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dynamic percolation behaviors, the MWNTs was oxidation-treated to introduced carboxylic acid groups ( COOH) or hydroxyl groups ( OH) on the surface of the MWNTs and the comparable investigation was adopted to study the kinetic characteristics of dynamic percolation of MWNTs and MWNTs-COOH filled conductive polymer composites under the electric field. 2. Experimental 2.1. Materials Polycarbonate (PC) (IR2200, from Idemitsu Kosan Co. Ltd., Japan) was used as a matrix. MWNTs were supplied by the Chengdu Institute of Organic Chemistry, with diameter of 20–30 nm and length of 0.5–2 m. The oxidative reagents used were nitric acid (HNO3 ) (65%, Zhejiang Zhongxing Chemical Reagent Co. Ltd., China) and sulfuric acid (H2 SO4 ) (96.8%, Quzhou Juhua Reagent Co. Ltd., China). 2.2. Samples preparation The chemical treatment of MWNTs was carried out by refluxing MWNTs in a concentrated mixture of HNO3 and H2 SO4 with volume ratio of 1:3 at the temperature of 100 ◦ C for 0.5 h. After the treatment, acid-treated MWNTs (MWNTs-COOH) were filtrated and then washed with de-ionized water for several times until the pH of the filtrate approached to 7.0. MWNTs, MWNTs-COOH and PC were dried at 80 ◦ C for 24 h under vacuum before used. During the processing, PC was first melted in a Haake Rheomix R600 at 250 ◦ C for 5 min, then MWNTs or MWNTs-COOH were added respectively into the polymer and then mixed for 15 min. The resulting PC/MWNTs and PC/MWNTs-COOH samples were melted at 250 ◦ C for 2 min and were compressed under a pressure of 15 MPa for 3 min at cooling rate of 50 ◦ C min−1 to achieve the sheets with a thickness of 1.5 mm. 2.3. Measurements The functional groups on the surface of the acid treatment MWNTs were measured by FT-IR (Thermo Fisher Nicolet, USA) in the transmission mode. The variation of electrical resistivity with time was measured using a GZ-1 picoammeter equipped with a direct current voltage source and two parallel copper electrodes when the samples were annealed in the molten state. Specimens with a diameter of 30 mm were firstly cut from the center area of the molded sheets, and then put into a temperature-controlled chamber and subjected to various electric fields. The variation of the electrical resistivity with the annealing time was recorded by a computer with a time interval of 5 s. Nitrogen gas was introduced during measurement in order to prevent oxidation of the samples.
Fig. 1. FT-IR spectra of: (a) pristine and (b) acid-treated MWNTs.
dispersed stably in water for a couple of days, compared to agglomeration and sediment of the crude MWNTs in water, which further indicates that the carboxylic acid groups ( COOH) or hydroxyl groups ( OH) has been successfully introduce on the surface of the MWNTs and the surface of the treated MWNTs are well polarized. 3.2. Evaluation of interaction by torque rhemetry Fig. 2 shows typical curve of torque with time in a Haake torque rheometer for Pure PC, 0.5 phr pristine MWNTs filled PC composite, and 0.5 phr MWNTs-COOH filled PC composite. When the polymer is firstly introduced in the mixing chamber, the solid samples offer a certain resistance to the free rotation of the blades and therefore the torque increases. As this resistance is overcome, the torque required to rotate the blades at the fixed speed decreases and reaches a steady-state after a short time. Therefore, the steady-state torque is considered as a measure of melt viscosity for a stabilized morphology. It can be observed that the stabilized torque of the samples show a tendency of PC/MWNTs-COOH > PC/MWNTs > Pure PC. It can be explained as the introduction of MWNTs increases the PC molecular inter twist on the MWNTs, which results in the increase of the melt viscosity. As the treated MWNTs is added, the carboxylic acid groups ( COOH) or hydroxyl groups ( OH) on
3. Results and discussion 3.1. The surface treatment of MWNTs To investigate the effect of the different surface properties of fillers on the formation of conductive network in the composites under the electric field, the MWNTs were treated in HNO3 /H2 SO4 as reported in the previous papers [22,23]. Fig. 1 displays FT-IR spectra of: (a) pristine MWNTs and (b) acid-treated MWNTs. As shown in Fig. 1(b), the presence of characteristic peaks at 1542.6 cm−1 , 1650.9 cm−1 and 3279.4 cm−1 indicates the carboxylic group ( COOH) have been successfully grafted on the surface of MWNTs, as reported in the literatures [24,25]. Dispersion experiment also displays the surface-functionalized MWNTs can
Fig. 2. Typical curve of torque with time in a Haake torque rheometer for Pure PC, 0.5 phr pristine MWNTs filled PC composite, and 0.5 phr MWNTs-COOH filled PC composite.
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Fig. 4. A linear relationship of ln(tp ) versus electric fields for PC/MWNTs-COOH and PC/MWNTs composites.
According to the results shown in Fig. 3, we get the relationship between the percolation time (ln tp ) and the electric field (E). As shown in Fig. 4, the linear relationship is well present for the PC/MWNTs-COOH and PC/MWNTs composites, respectively. The correlative equation is well characterized as following: ln tp = k E + d
(1)
where tp is the percolation time, k and d are the slope and intercept of the line (k < 0). From Eq. (1), we get the percolation time as function of electric field: tp = A exp(k E)
Fig. 3. Dynamic percolation curves of 2 phr (a) MWNTs and (b) MWNTs-COOH filled PC composites annealed at 270 ◦ C under various electric fields.
the surface of the MWNTs cause the stronger interaction with carbonyl of the PC molecules, resulting in the further increase of the stabilized torque. 3.3. Effect of the intensity of electric fields on dynamic percolation Fig. 3 shows the time dependence of resistivity for 2 wt% MWNTs and MWNTs-COOH filled PC composites annealed at 270 ◦ C with various electric fields. It is found that the percolation time all decrease with the increasing the intensity of electrical field in both the composites, indicating that the formation of a conductive path is greatly accelerated under the stronger electrical field. A similar result was reported by Tai et al. for the LDPE/CB system [26]. Moreover, the dynamic percolation curves have similar features in their shape, which indicates that the primary characteristics of the conductive pathways do never change with the electric-field intensity. Comparatively, the percolation time for PC/MWNTs-COOH sample is longer than that of the PC/MWNTs sample under the same electric field intensity, for example that the percolation time of PC/MWNTsCOOH composites under 100 V cm−1 is about ten times larger than that of PC/MWNTs composites. This indicates that the conductive network formation for the MWNTs-COOH filled PC matrix is more difficult than that of the pristine MWNTs due to the hydrogen bond interaction between the carboxyl groups ( COOH) on the surface of the treated MWNTs and carbonyl ( C O) of PC molecules, which limits the movement of MWNTs-COOH to form conductive network in the PC matrix.
(2)
Here A = exp (d). Accordingly, it provides a possible method to predict the percolation time for a given system at the certain electric field intensity by Eq. (2). Compared with the PC/MWNTs composites, the dependence of ln(tp ) on electric field intensity (E) in the PC/MWNTs-COOH composites is less sensitive. This difference is due to that the stronger interaction between the surface-functionalized MWNTs and PC delays the conductive networks formation in the polymer matrix. In general, it has already been confirmed, in the presence of electric field, that the static polarizability in the direction of the tube axis is much larger than in the across the diameter for MWNTs [14]. The polarization leads to a torque acting on MWNTs, which aligns MWNTs against the viscous drag of the surrounding medium along the direction of the electric field. However, for MWNTs-COOH system, in addition to viscous drag of the matrix, it exist an intensely interfacial interaction force by the hydrogen bonding between the carboxyl of MWNTs-COOH and the carbonyl of PC molecules, resulting in an weakening dominant function of electric field on the conductive network formation. Accordingly, the sensitive degree of the percolation time on the electric field intensity is weaken for the MWNTs-COOH system. Therefore, we can monitor the interfacial interaction through measuring the dynamic percolation behaviors of the composites under the electric field: for a given system, the lower the k value of a composites system becomes, the weaker sensitivity of ln(tp ) on electric field intensity becomes, and the more strongly interfacial interaction between fillers and matrix becomes. 3.4. Effect of temperature on dynamic percolation and the activation energy (Ea ) of continuous conductive path formation Fig. 5(a) and (c) shows dynamic percolation curves of resistivity versus time at various temperatures under the certain electric
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Fig. 5. Time dependence of resistivity for 2 phr (a) MWNTs and (c) MWNTs-COOH filled PC composites at different temperatures under 500 V cm−1 ; Arrhenius plots of tp versus the inverse of annealing temperature for (b) PC/MWNTs and (d) PC/MWNTs-COOH composites with different electric fields.
field for PC/MWNTs and PC/MWNTs-COOH composites, respectively. It is found that a shorter percolation time is observed when the annealing temperature increases in both the composites. This is reasonable because the increase in the temperature accelerates both the relaxation of PC macromolecules and the Brownian motion of fillers, resulting in both a decrease in the viscosity resistance of the polymer melt and an increase in the assembly speed of MWNTs. As the activation energy (Ea ) of continuous conductive path formation in a non-electric field had been calculated by Wu et al. [27–29], according to the relationship between ln(tp ) and 1/T, i.e., Arrhenius equation: ln(tp ) = ln A − Ea /RT , we use the same way to calculate the activation energy of continuous conductive path formation for PC/MWNTs and PC/MWNTs-COOH composites under various electric fields. The result is shown in Fig. 5(c) and (d). It is found that Ea decreases gradually with the increase of electric field intensity for both the composites. A similar result was also reported on a published literature [20]. Moreover, we find the Ea value of PC/MWNTs-COOH composites is much higher than that of PC/MWNTs composites under the same electric field. It can be explained based on the kinetic difficulties of forming the conductive network due to the stronger interaction between the carboxyl of MWNTs-COOH and carbonyl of the PC molecules. Otherwise, in PC/MWNTs composite, as the electrical field increases from 100 V cm−1 to 500 V cm−1 , the activation energy decreases obviously from 108 kJ mol−1 to 82 kJ mol−1 , and then remains at about 82 kJ mol−1 nearly invariable with the further increase of electric field up to 1000 V cm−1 . The result strongly demonstrates that, in PC/MWNTs system, 500 V cm−1 is the thermodynamic prerequisite electric field (E*) for 1D orientation, below which it is impossible to build a thoroughly 1D orientation paralleling to the direction of electric field. However, this phenomenon is never
observed for PC/MWNTs-COOH composite, even as the electric field changes from 100 V cm−1 to 1000 V cm−1 . It is due to stronger interaction between the MWNTs-COOH and the PC molecules, which requires electric field intensity even larger than 1000 V cm−1 to orient MWNTs-COOH in the PC matrix. 3.5. Effect of fillers volume fraction on dynamic percolation As reported in our previous work [17] a thermodynamic percolation model has been proposed to predict the percolation time of the HDPE/iPP/VGCF system, and the relationship between tp and ˚ is given as follows: tp = −
1 − [(1 − ˚)/˚][˚∗ /(1 − ˚∗ )] ln c 1 − P(0)/P(∞)
(3)
where is the viscosity of the matrix at an annealing temperature, c is a constant, and ˚ is the volume fraction of fillers in the matrix. ˚* is the volume fraction at the thermodynamic equilibrium state, and P(0) and P(∞) are the fractions of the fillers which join the conduction networks at t = 0 and at equilibrium state (t = ∞), respectively. Considering the similarities of the conductive path formation, the thermodynamic percolation model is also applied to analyze the percolation time and the assembly velocity of MWNTs and MWNTsCOOH in the PC melt as the sample is exposed to a high electrical field of 500 V cm−1 . Fig. 6(a) and (b) illustrates the relationship between the volume fraction of fillers and 1/tp for PC/MWNTs and PC/MWNTs-COOH composites, as annealed at various temperatures under an electrical field of 500 V cm−1 . The values of ˚* can be obtained for these systems as 1/tp is extrapolated to 0, and the results are listed in Tables 1 and 2. It is found that the ˚* value of the PC/MWNTs
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Fig. 6. Plots of filler volume fraction in the matrix versus 1/tp for (a) PC/MWNTs and (b) PC/MWNTs-COOH composites; Plots of ln[1 − P(tp )/P(∞)] versus tp for (c) MWNTs and (d) MWNTs-COOH filled PC composites, annealed at various temperatures under a high electrical field of 500 V cm−1 .
Table 1 Parameters of the percolation model for PC/MWNTs composites. Temperature (◦ C)
˚*
250 260 270
0.002 0.002 0.002
c/ 2 × 10−4 3 × 10−4 4 × 10−4
ln[1 − P(0)/P(∞)] −0.144 −0.135 −0.127
system is lower than that of the PC/MWNTs-COOH system. The result further indicates that the conductive pathway formation of MWNTs in PC matrix is more easier than that of MWNTs-COOH. Moreover, the parameters P(0)/P(∞) and c/ can be estimated from Eq. (4) [17].
P(tp ) ln 1 − P(∞)
c P(0) = − tp + ln 1 − P(∞)
values listed in Tables 1 and 2. For the PC/MWNTs and PC/MWNTsCOOH composites, the relationships between ˚ and 1/tp are shown as solid curves in Fig. 6(a) and (b), respectively. It is found that Eq. (3) fits the experimental results very well. This indicates that the thermodynamic percolation model can be well applied for PC/MWNTs and PC/MWNTs-COOH system under the electric field. It is very interesting to find that the linear relationship is well presented between the ln(c/) and the inverse of different annealing temperatures for the PC/MWNTs and PC/MWNTs-COOH
(4)
where P(tp ) is the fraction of the fillers in the matrix which join in the conduction networks at t = tp . P(tp )/P(∞) is given as follows: P(tp ) ˚∗ 1 − ˚ = P(∞) 1 − ˚∗ ˚
(5)
From the plots of ln[1 − P(tp )/P(∞)] versus tp (Fig. 6(c) and (d)), a linear relationship between ln[1 − P(tp )/P(∞)] and tp can be observed. The parameters P(0)/P(∞) and c/ can be calculated from the intercept and the slope of the line (Tables 1 and 2). The relationship between tp and ˚ can be calculated from Eq. (2) using the Table 2 Parameters of the percolation model for PC/MWNTs-COOH composites. Temperature (◦ C)
˚*
270 280 290
0.0033 0.0033 0.0033
c/ 7 × 10−5 1 × 10−4 2 × 10−4
ln[1 − P(0)/P(∞)] −0.122 −0.120 −0.132
Fig. 7. A linear relationship of ln(c/) versus the inverse of annealing temperature under 500 V cm−1 for PC/MWNTs and PC/MWNTs-COOH composites.
C. Su et al. / Materials Chemistry and Physics 133 (2012) 1034–1039
composites, as shown in Fig. 7. The correlative equation is characterized as following: ln
c 1 =k· T
(6)
We get the viscosity of the system as function of the annealing temperature: ln = −k ·
1 + ln c T
(7)
where is the viscosity of the system, i.e., zero-shear-rate viscosity in this study, the k (k < 0) and ln c are the slope and intercept of the line. As well known, the relationship between the zero-shear-rate viscosity, 0 , and the measurement temperature is characterized as following:
0 = A exp
E0 RT
(8)
where A is a constant, E0 is the activation energy of 0 , therefore, ln 0 can be calculated: ln 0 =
E0 1 · + ln A T R
(9)
Comparing Eq. (7) and Eq. (9), we can get an equation: k=−
E0 R
(10)
Through Eq. (10), the activation energy of 0 is obtained. From Fig. 7, we can obtain that the slop for PC/MWNTs-COOH (−16.02 for PC/MWNTs-COOH) is larger than that of PC/MWNTs (−9.86 for PC/MWNTs), indicating that E0 for PC/MWNTs-COOH is larger than that of PC/MWNTs, which results from the stronger interaction between the treated MWNTs and PC molecules. Moreover, for a given system, as the b E0 is fixed, we can also calculate the values of the zero-shear-rate viscosity at different temperatures by Eq. (7). Therefore, the dynamic percolation measurement method can also been used as a tool to investigate the zero-shear-rate viscosity at the different temperatures.
in the mobility of the fillers. The dependence of ln(tp ) on the electric field intensity in the MWNTs-COOH system was less sensitive than that in the pristine MWNTs system. Therefore, it provided a valid method for the monitor and analysis the interfacial interaction of the composites using the dynamic percolation measurement under electric field. A modified thermodynamic percolation model was used to predict the values of tp under electric field for the PC/MWNTs and PC/MWNTs-COOH filled PC composites, respectively. The predicted data fitted the experimental results very well. Specially, the model could also be applied to predict the activation energy of 0 and the zero-shear-rate viscosity at the different temperatures. Acknowledgements This work is supported by the Program for New Century Excellent Talents in University (NCET-06-0536), the National Natural Science Foundation of China (NSFC, 50773071) and the National Natural Science Foundation of China (NSFC, 51003095). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
4. Conclusions Electric field induced alignment and conductive path formation of MWNTs and MWNTs-COOH in the PC melt were investigated by in situ tracing the time dependence of the electrical resistivity during the isothermal treatment. It was found that electric fieldinduced fillers alignment caused the decrease in the activation energy of conductive network formation and percolation time. A good linear relationship between electric field intensity E and ln tp could be observed. The percolation time and activation energy of the conductive network formation in the MWNTs-COOH filled composite were much larger than those in the MWNTs filled composite under various electric fields. The reason was that the strong interaction between MWNTs-COOH and PC molecules caused a reduction
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Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Electric field induced conductive network formation of MWNTs and MWNTs-COOH in polycarbonate composites Chang Su, Lihuan Xu, Rong-jun Yan, Meng-qi Chen, Cheng Zhan ∗ State Key Laboratory Breeding Base for Green Chemistry Synthesis Technology, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 4 October 2011 Received in revised form 15 January 2012 Accepted 26 January 2012 Keywords: Electrical conductivity Electrical properties Composite materials Electronic materials Bend and torsion test
a b s t r a c t Using polycarbonate (PC) as a matrix, and multi-wall carbon nanotubes (MWNTs) and acid-treated MWNTs (MWNTs-COOH) as fillers, PC/MWNTs and PC/MWNTs-COOH conductive composites were prepared, respectively. The conductive network formation induced by electric field in both the PC/MWNTs and PC/MWNTs-COOH composites were investigated by the dynamic percolation measurement. It was found that the electrical resistivity–time curves showed a certain self-similarity under various electric fields, i.e., the electrical resistivity decreased sharply as the percolation time (tp ) was reached. And the dynamic percolation time was shorten with the increase of the electric field intensity for both MWNTs and MWNTs-COOH filled PC composites. However, the percolation time and the formation activation energy of conductive network under electric field for MWNTs filled composites decreased greatly as compared to those of the MWNTs-COOH filled composites. These results indicated that the interaction between MWNTs and PC molecules played an important role in the conductive network formation under the electric field. Furthermore, the tp values under the electric field for PC/MWNTs and PC/MWNTsCOOH composites were successfully predicted by a modified thermodynamic percolation model. And this dynamic percolation measurement method could be recommended to investigate the zero-shear-rate viscosity at the different temperatures. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, anisotropically electrical conductive materials, which were achieved by the directional alignment of conductive fillers in an insulating polymer matrix, have aroused intensive attention, due to their potential application in the electronic industries, such as semiconductor art and electrostatic discharging. Various approaches have been applied to align conductive fillers, such as carbon black (CB) [1–3], carbon fibers (CF) [4,5], and carbon nanotubes (CNTs) [6–10], in polymer matrix. Among these approaches, the electric field induced alignment of fillers in a matrix has been considered as an effective route. CNTs, as a promising conductive filler, have been extensively explored in various fields due to their extremely high aspect ratio, outstanding electrical and thermal conductivity, and excellent mechanical properties. As CNTs were aligned highly in a polymer matrix, a remarkable anisotropy of the electrical property could be expected for the polymer/CNTs composite, which could been applied in the field emission cathodes [11]. Various efforts had been carried out to align CNTs in the matrix [12,13], and recently, special
∗ Corresponding author. Tel.: +86 571 88320253; fax: +86 571 88320253. E-mail address: [email protected] (C. Zhan). 0254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.01.129
attention had been paid to align CNTs induced by an electric field in liquid polymer monomers or organic solvents [14–16]. For example, Martin et al. [14] successfully applied both static and alternate electric field to align nanotube forming the conductive networks in epoxy resin. In our previous work [17–20], a dynamic percolation measurement method was developed to investigate the dynamic process of conductive network formation in high-density polyethylene (HDPE)/isotactic polypropylene (iPP)/vapor grown carbon fiber (VGCF) composite by recording the variation of electrical resistivity with time as the composite was annealed isothermally in the molten state [17]. Furthermore, this method had also been adopted as a tool to trace the self-assembly process of VGCFs in the poly(vinylidene fluoride) melt [18] and the dynamic percolation activity for conductive network formation of MWNTs in PVDF [19]. Recently, we further investigated the conductive network formation of MWCTs in the PC induced by electric field [20], in which the field-induced dipole–dipole interaction between the MWCTs resulted in the MWCTs alignment in the electric field direction [21]. Moreover, the dynamic percolation method might provide an essentially real-time monitor tool to measure the kinetics process of particle alignment under different electric fields. In this paper, in order to study how the interaction between the polymer matrix and conductive fillers-MWNTs influences the
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dynamic percolation behaviors, the MWNTs was oxidation-treated to introduced carboxylic acid groups ( COOH) or hydroxyl groups ( OH) on the surface of the MWNTs and the comparable investigation was adopted to study the kinetic characteristics of dynamic percolation of MWNTs and MWNTs-COOH filled conductive polymer composites under the electric field. 2. Experimental 2.1. Materials Polycarbonate (PC) (IR2200, from Idemitsu Kosan Co. Ltd., Japan) was used as a matrix. MWNTs were supplied by the Chengdu Institute of Organic Chemistry, with diameter of 20–30 nm and length of 0.5–2 m. The oxidative reagents used were nitric acid (HNO3 ) (65%, Zhejiang Zhongxing Chemical Reagent Co. Ltd., China) and sulfuric acid (H2 SO4 ) (96.8%, Quzhou Juhua Reagent Co. Ltd., China). 2.2. Samples preparation The chemical treatment of MWNTs was carried out by refluxing MWNTs in a concentrated mixture of HNO3 and H2 SO4 with volume ratio of 1:3 at the temperature of 100 ◦ C for 0.5 h. After the treatment, acid-treated MWNTs (MWNTs-COOH) were filtrated and then washed with de-ionized water for several times until the pH of the filtrate approached to 7.0. MWNTs, MWNTs-COOH and PC were dried at 80 ◦ C for 24 h under vacuum before used. During the processing, PC was first melted in a Haake Rheomix R600 at 250 ◦ C for 5 min, then MWNTs or MWNTs-COOH were added respectively into the polymer and then mixed for 15 min. The resulting PC/MWNTs and PC/MWNTs-COOH samples were melted at 250 ◦ C for 2 min and were compressed under a pressure of 15 MPa for 3 min at cooling rate of 50 ◦ C min−1 to achieve the sheets with a thickness of 1.5 mm. 2.3. Measurements The functional groups on the surface of the acid treatment MWNTs were measured by FT-IR (Thermo Fisher Nicolet, USA) in the transmission mode. The variation of electrical resistivity with time was measured using a GZ-1 picoammeter equipped with a direct current voltage source and two parallel copper electrodes when the samples were annealed in the molten state. Specimens with a diameter of 30 mm were firstly cut from the center area of the molded sheets, and then put into a temperature-controlled chamber and subjected to various electric fields. The variation of the electrical resistivity with the annealing time was recorded by a computer with a time interval of 5 s. Nitrogen gas was introduced during measurement in order to prevent oxidation of the samples.
Fig. 1. FT-IR spectra of: (a) pristine and (b) acid-treated MWNTs.
dispersed stably in water for a couple of days, compared to agglomeration and sediment of the crude MWNTs in water, which further indicates that the carboxylic acid groups ( COOH) or hydroxyl groups ( OH) has been successfully introduce on the surface of the MWNTs and the surface of the treated MWNTs are well polarized. 3.2. Evaluation of interaction by torque rhemetry Fig. 2 shows typical curve of torque with time in a Haake torque rheometer for Pure PC, 0.5 phr pristine MWNTs filled PC composite, and 0.5 phr MWNTs-COOH filled PC composite. When the polymer is firstly introduced in the mixing chamber, the solid samples offer a certain resistance to the free rotation of the blades and therefore the torque increases. As this resistance is overcome, the torque required to rotate the blades at the fixed speed decreases and reaches a steady-state after a short time. Therefore, the steady-state torque is considered as a measure of melt viscosity for a stabilized morphology. It can be observed that the stabilized torque of the samples show a tendency of PC/MWNTs-COOH > PC/MWNTs > Pure PC. It can be explained as the introduction of MWNTs increases the PC molecular inter twist on the MWNTs, which results in the increase of the melt viscosity. As the treated MWNTs is added, the carboxylic acid groups ( COOH) or hydroxyl groups ( OH) on
3. Results and discussion 3.1. The surface treatment of MWNTs To investigate the effect of the different surface properties of fillers on the formation of conductive network in the composites under the electric field, the MWNTs were treated in HNO3 /H2 SO4 as reported in the previous papers [22,23]. Fig. 1 displays FT-IR spectra of: (a) pristine MWNTs and (b) acid-treated MWNTs. As shown in Fig. 1(b), the presence of characteristic peaks at 1542.6 cm−1 , 1650.9 cm−1 and 3279.4 cm−1 indicates the carboxylic group ( COOH) have been successfully grafted on the surface of MWNTs, as reported in the literatures [24,25]. Dispersion experiment also displays the surface-functionalized MWNTs can
Fig. 2. Typical curve of torque with time in a Haake torque rheometer for Pure PC, 0.5 phr pristine MWNTs filled PC composite, and 0.5 phr MWNTs-COOH filled PC composite.
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Fig. 4. A linear relationship of ln(tp ) versus electric fields for PC/MWNTs-COOH and PC/MWNTs composites.
According to the results shown in Fig. 3, we get the relationship between the percolation time (ln tp ) and the electric field (E). As shown in Fig. 4, the linear relationship is well present for the PC/MWNTs-COOH and PC/MWNTs composites, respectively. The correlative equation is well characterized as following: ln tp = k E + d
(1)
where tp is the percolation time, k and d are the slope and intercept of the line (k < 0). From Eq. (1), we get the percolation time as function of electric field: tp = A exp(k E)
Fig. 3. Dynamic percolation curves of 2 phr (a) MWNTs and (b) MWNTs-COOH filled PC composites annealed at 270 ◦ C under various electric fields.
the surface of the MWNTs cause the stronger interaction with carbonyl of the PC molecules, resulting in the further increase of the stabilized torque. 3.3. Effect of the intensity of electric fields on dynamic percolation Fig. 3 shows the time dependence of resistivity for 2 wt% MWNTs and MWNTs-COOH filled PC composites annealed at 270 ◦ C with various electric fields. It is found that the percolation time all decrease with the increasing the intensity of electrical field in both the composites, indicating that the formation of a conductive path is greatly accelerated under the stronger electrical field. A similar result was reported by Tai et al. for the LDPE/CB system [26]. Moreover, the dynamic percolation curves have similar features in their shape, which indicates that the primary characteristics of the conductive pathways do never change with the electric-field intensity. Comparatively, the percolation time for PC/MWNTs-COOH sample is longer than that of the PC/MWNTs sample under the same electric field intensity, for example that the percolation time of PC/MWNTsCOOH composites under 100 V cm−1 is about ten times larger than that of PC/MWNTs composites. This indicates that the conductive network formation for the MWNTs-COOH filled PC matrix is more difficult than that of the pristine MWNTs due to the hydrogen bond interaction between the carboxyl groups ( COOH) on the surface of the treated MWNTs and carbonyl ( C O) of PC molecules, which limits the movement of MWNTs-COOH to form conductive network in the PC matrix.
(2)
Here A = exp (d). Accordingly, it provides a possible method to predict the percolation time for a given system at the certain electric field intensity by Eq. (2). Compared with the PC/MWNTs composites, the dependence of ln(tp ) on electric field intensity (E) in the PC/MWNTs-COOH composites is less sensitive. This difference is due to that the stronger interaction between the surface-functionalized MWNTs and PC delays the conductive networks formation in the polymer matrix. In general, it has already been confirmed, in the presence of electric field, that the static polarizability in the direction of the tube axis is much larger than in the across the diameter for MWNTs [14]. The polarization leads to a torque acting on MWNTs, which aligns MWNTs against the viscous drag of the surrounding medium along the direction of the electric field. However, for MWNTs-COOH system, in addition to viscous drag of the matrix, it exist an intensely interfacial interaction force by the hydrogen bonding between the carboxyl of MWNTs-COOH and the carbonyl of PC molecules, resulting in an weakening dominant function of electric field on the conductive network formation. Accordingly, the sensitive degree of the percolation time on the electric field intensity is weaken for the MWNTs-COOH system. Therefore, we can monitor the interfacial interaction through measuring the dynamic percolation behaviors of the composites under the electric field: for a given system, the lower the k value of a composites system becomes, the weaker sensitivity of ln(tp ) on electric field intensity becomes, and the more strongly interfacial interaction between fillers and matrix becomes. 3.4. Effect of temperature on dynamic percolation and the activation energy (Ea ) of continuous conductive path formation Fig. 5(a) and (c) shows dynamic percolation curves of resistivity versus time at various temperatures under the certain electric
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Fig. 5. Time dependence of resistivity for 2 phr (a) MWNTs and (c) MWNTs-COOH filled PC composites at different temperatures under 500 V cm−1 ; Arrhenius plots of tp versus the inverse of annealing temperature for (b) PC/MWNTs and (d) PC/MWNTs-COOH composites with different electric fields.
field for PC/MWNTs and PC/MWNTs-COOH composites, respectively. It is found that a shorter percolation time is observed when the annealing temperature increases in both the composites. This is reasonable because the increase in the temperature accelerates both the relaxation of PC macromolecules and the Brownian motion of fillers, resulting in both a decrease in the viscosity resistance of the polymer melt and an increase in the assembly speed of MWNTs. As the activation energy (Ea ) of continuous conductive path formation in a non-electric field had been calculated by Wu et al. [27–29], according to the relationship between ln(tp ) and 1/T, i.e., Arrhenius equation: ln(tp ) = ln A − Ea /RT , we use the same way to calculate the activation energy of continuous conductive path formation for PC/MWNTs and PC/MWNTs-COOH composites under various electric fields. The result is shown in Fig. 5(c) and (d). It is found that Ea decreases gradually with the increase of electric field intensity for both the composites. A similar result was also reported on a published literature [20]. Moreover, we find the Ea value of PC/MWNTs-COOH composites is much higher than that of PC/MWNTs composites under the same electric field. It can be explained based on the kinetic difficulties of forming the conductive network due to the stronger interaction between the carboxyl of MWNTs-COOH and carbonyl of the PC molecules. Otherwise, in PC/MWNTs composite, as the electrical field increases from 100 V cm−1 to 500 V cm−1 , the activation energy decreases obviously from 108 kJ mol−1 to 82 kJ mol−1 , and then remains at about 82 kJ mol−1 nearly invariable with the further increase of electric field up to 1000 V cm−1 . The result strongly demonstrates that, in PC/MWNTs system, 500 V cm−1 is the thermodynamic prerequisite electric field (E*) for 1D orientation, below which it is impossible to build a thoroughly 1D orientation paralleling to the direction of electric field. However, this phenomenon is never
observed for PC/MWNTs-COOH composite, even as the electric field changes from 100 V cm−1 to 1000 V cm−1 . It is due to stronger interaction between the MWNTs-COOH and the PC molecules, which requires electric field intensity even larger than 1000 V cm−1 to orient MWNTs-COOH in the PC matrix. 3.5. Effect of fillers volume fraction on dynamic percolation As reported in our previous work [17] a thermodynamic percolation model has been proposed to predict the percolation time of the HDPE/iPP/VGCF system, and the relationship between tp and ˚ is given as follows: tp = −
1 − [(1 − ˚)/˚][˚∗ /(1 − ˚∗ )] ln c 1 − P(0)/P(∞)
(3)
where is the viscosity of the matrix at an annealing temperature, c is a constant, and ˚ is the volume fraction of fillers in the matrix. ˚* is the volume fraction at the thermodynamic equilibrium state, and P(0) and P(∞) are the fractions of the fillers which join the conduction networks at t = 0 and at equilibrium state (t = ∞), respectively. Considering the similarities of the conductive path formation, the thermodynamic percolation model is also applied to analyze the percolation time and the assembly velocity of MWNTs and MWNTsCOOH in the PC melt as the sample is exposed to a high electrical field of 500 V cm−1 . Fig. 6(a) and (b) illustrates the relationship between the volume fraction of fillers and 1/tp for PC/MWNTs and PC/MWNTs-COOH composites, as annealed at various temperatures under an electrical field of 500 V cm−1 . The values of ˚* can be obtained for these systems as 1/tp is extrapolated to 0, and the results are listed in Tables 1 and 2. It is found that the ˚* value of the PC/MWNTs
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Fig. 6. Plots of filler volume fraction in the matrix versus 1/tp for (a) PC/MWNTs and (b) PC/MWNTs-COOH composites; Plots of ln[1 − P(tp )/P(∞)] versus tp for (c) MWNTs and (d) MWNTs-COOH filled PC composites, annealed at various temperatures under a high electrical field of 500 V cm−1 .
Table 1 Parameters of the percolation model for PC/MWNTs composites. Temperature (◦ C)
˚*
250 260 270
0.002 0.002 0.002
c/ 2 × 10−4 3 × 10−4 4 × 10−4
ln[1 − P(0)/P(∞)] −0.144 −0.135 −0.127
system is lower than that of the PC/MWNTs-COOH system. The result further indicates that the conductive pathway formation of MWNTs in PC matrix is more easier than that of MWNTs-COOH. Moreover, the parameters P(0)/P(∞) and c/ can be estimated from Eq. (4) [17].
P(tp ) ln 1 − P(∞)
c P(0) = − tp + ln 1 − P(∞)
values listed in Tables 1 and 2. For the PC/MWNTs and PC/MWNTsCOOH composites, the relationships between ˚ and 1/tp are shown as solid curves in Fig. 6(a) and (b), respectively. It is found that Eq. (3) fits the experimental results very well. This indicates that the thermodynamic percolation model can be well applied for PC/MWNTs and PC/MWNTs-COOH system under the electric field. It is very interesting to find that the linear relationship is well presented between the ln(c/) and the inverse of different annealing temperatures for the PC/MWNTs and PC/MWNTs-COOH
(4)
where P(tp ) is the fraction of the fillers in the matrix which join in the conduction networks at t = tp . P(tp )/P(∞) is given as follows: P(tp ) ˚∗ 1 − ˚ = P(∞) 1 − ˚∗ ˚
(5)
From the plots of ln[1 − P(tp )/P(∞)] versus tp (Fig. 6(c) and (d)), a linear relationship between ln[1 − P(tp )/P(∞)] and tp can be observed. The parameters P(0)/P(∞) and c/ can be calculated from the intercept and the slope of the line (Tables 1 and 2). The relationship between tp and ˚ can be calculated from Eq. (2) using the Table 2 Parameters of the percolation model for PC/MWNTs-COOH composites. Temperature (◦ C)
˚*
270 280 290
0.0033 0.0033 0.0033
c/ 7 × 10−5 1 × 10−4 2 × 10−4
ln[1 − P(0)/P(∞)] −0.122 −0.120 −0.132
Fig. 7. A linear relationship of ln(c/) versus the inverse of annealing temperature under 500 V cm−1 for PC/MWNTs and PC/MWNTs-COOH composites.
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composites, as shown in Fig. 7. The correlative equation is characterized as following: ln
c 1 =k· T
(6)
We get the viscosity of the system as function of the annealing temperature: ln = −k ·
1 + ln c T
(7)
where is the viscosity of the system, i.e., zero-shear-rate viscosity in this study, the k (k < 0) and ln c are the slope and intercept of the line. As well known, the relationship between the zero-shear-rate viscosity, 0 , and the measurement temperature is characterized as following:
0 = A exp
E0 RT
(8)
where A is a constant, E0 is the activation energy of 0 , therefore, ln 0 can be calculated: ln 0 =
E0 1 · + ln A T R
(9)
Comparing Eq. (7) and Eq. (9), we can get an equation: k=−
E0 R
(10)
Through Eq. (10), the activation energy of 0 is obtained. From Fig. 7, we can obtain that the slop for PC/MWNTs-COOH (−16.02 for PC/MWNTs-COOH) is larger than that of PC/MWNTs (−9.86 for PC/MWNTs), indicating that E0 for PC/MWNTs-COOH is larger than that of PC/MWNTs, which results from the stronger interaction between the treated MWNTs and PC molecules. Moreover, for a given system, as the b E0 is fixed, we can also calculate the values of the zero-shear-rate viscosity at different temperatures by Eq. (7). Therefore, the dynamic percolation measurement method can also been used as a tool to investigate the zero-shear-rate viscosity at the different temperatures.
in the mobility of the fillers. The dependence of ln(tp ) on the electric field intensity in the MWNTs-COOH system was less sensitive than that in the pristine MWNTs system. Therefore, it provided a valid method for the monitor and analysis the interfacial interaction of the composites using the dynamic percolation measurement under electric field. A modified thermodynamic percolation model was used to predict the values of tp under electric field for the PC/MWNTs and PC/MWNTs-COOH filled PC composites, respectively. The predicted data fitted the experimental results very well. Specially, the model could also be applied to predict the activation energy of 0 and the zero-shear-rate viscosity at the different temperatures. Acknowledgements This work is supported by the Program for New Century Excellent Talents in University (NCET-06-0536), the National Natural Science Foundation of China (NSFC, 50773071) and the National Natural Science Foundation of China (NSFC, 51003095). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
4. Conclusions Electric field induced alignment and conductive path formation of MWNTs and MWNTs-COOH in the PC melt were investigated by in situ tracing the time dependence of the electrical resistivity during the isothermal treatment. It was found that electric fieldinduced fillers alignment caused the decrease in the activation energy of conductive network formation and percolation time. A good linear relationship between electric field intensity E and ln tp could be observed. The percolation time and activation energy of the conductive network formation in the MWNTs-COOH filled composite were much larger than those in the MWNTs filled composite under various electric fields. The reason was that the strong interaction between MWNTs-COOH and PC molecules caused a reduction
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[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
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