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Original article
12 (
3
); 440-446
doi:
10.1016/j.arabjc.2015.10.015

Synthesis and dielectric characterization of polycarbonate/multi-wall carbon nanotubes nanocomposite

, , , , , ,
a School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon 440-746, South Korea
b Division of Electronics Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, South Korea
c Nanotechnology Application Centre, University of Allahabad, Allahabad 211002, India
d Physics Department, University of Allahabad, Allahabad 211002, India
e Department of Chemical Engineering, Konkuk University, Seoul 143-701, South Korea

⁎Corresponding authors. srivastava_anoop@rediffmail.com (Anoop K. Srivastava), jihoonlee@jbnu.ac.kr (Ji-Hoon Lee)

Received: , Accepted: ,
© The Author(s) 2015
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Multi-wall carbon nanotubes (MWCNTs) and Polycarbonate/Multi-wall carbon nanotubes nanocomposite were prepared by chemical vapor deposition (CVD) and twin-screw compounding extruder method, respectively. The morphology and dispersion were characterized by Scanning Electron Microscopy. The Raman analysis confirmed good interaction between multi-wall carbon nanotubes and polycarbonate. The dielectric impedance spectroscopy of polycarbonate/multi-wall carbon nanotubes nanocomposite varying from 0.25 to 1.5 weight percentages has been carried out in the frequency range of 1.0 Hz–1.0 MHz. One dielectric relaxation mode is observed in pristine polycarbonate at low frequency region (∼10 Hz), whereas polycarbonate loaded with multi-wall carbon nanotubes induced one more dielectric relaxation in kHz region (∼10 kHz). The percolation threshold of the polycarbonate/multi-wall carbon nanotubes composites was observed at 0.75 wt.% of MWCNTs.

Keywords

Nanocomposite
Polymers
Multi-wall carbon nanotubes
Dielectric spectroscopy
1

1 Introduction

The polymer nanocomposite consists of a polymer matrix with nano-sized fillers. Different fillers have been tested to improve or modify the properties of polymers and also utilized for commercial applications (Li, 2014, 2012, 2010a,b, 2009, 2008a,b). In the last few years, conductive nanofiller/polymer composites (PC) have been widely studied in academia and industry because of their extraordinary properties compared to conventional conductive polymer composites (Breuer and Sundararaj, 2004; Winey and Vaia, 2007). In this context, carbon nanotubes (CNTs) are one of the most promising fillers for future applications due to their unique and useful characteristics such as low mass density, large interfacial contact area, high stiffness, chemical stability, and superb electrical characteristics. Because of their large aspect ratio, a small amount of CNTs can have an unusually large influence on the properties of the composites (Wang et al., 2015; Xie et al., 2005). These opportunities include the use of CNTs as conductive filler in insulating polymer matrices and as reinforcement in structural materials (Allaoui et al., 2002; Putz et al., 2004). Although single wall carbon nanotubes (SWCNTs) are dispersed well in the polymer matrix by solution mixing, relatively weak Van der Waals forces affect the bonding between nanotubes and polymer matrix. Therefore, multiwall carbon nanotubes (MWCNTs) are preferred fillers rather than SWCNTs for polymer matrices (Zhang et al., 2006; Man et al., 2009).

Polymer nanocomposites indeed attracted much scientific and technological interest (Shaffer and Windle, 1999; Andrews et al., 2002; Cooper et al., 2002; Haggenmueller et al., 2000; Jin et al., 2001; Sandler et al., 1999; Osińska and Czekaj, 2013; Patsidis and Psarras, 2013; Ulrich, 2004; Bai et al., 2000; Wan et al., 2015) mainly because incorporation of nanomaterials enhances the capacitance or dielectric constant of polymer and epoxy matrices and produces a network of nanocapacitors (Sandler et al., 1999; Osińska and Czekaj, 2013; Patsidis and Psarras, 2013; Ulrich, 2004; Bai et al., 2000; Wan et al., 2015). The embedded nanocapacitors could act as energy storing devices. The large capacitance or dielectric constant allows the system to function as power supply decoupling device and thus large external capacitors can be avoided from circuit boards surface.

Complex impedance spectroscopy is a most suitable and versatile technique to study collective and molecular dynamics of crystalline materials and characterize the electrical and dielectric properties of materials (Wan et al., 2015; Srivastava et al., 2015, 2008, 2005; Chawla et al., 2014; Rizos et al., 1999; Campbell et al., 2001; Pradhan et al., 2008; Man et al., 2009; Kum et al., 2006; Potschke et al., 2004, 2003; Shrivastava et al., 2014). The charge transport process causes a number of polarization mechanisms, which give rise to frequency dispersion or dielectric relaxation in the materials under an AC field. It is useful for evaluating high ionic conductivity, molecular relaxation dynamics, and orientation fluctuations. Dielectric impedance spectroscopy also provides detailed insight of the molecular and cooperative dynamics on various time and length scales. Literature survey reveals that the dielectric studies on polycarbonate (PC) filled with carbon nanotubes have not been studied extensively (Man et al., 2009; Kum et al., 2006; Potschke et al., 2004, 2003; Shrivastava et al., 2014). Man et al. (2009) have reported that the dielectric constant and the permittivity of the PC/MWCNTs increase dramatically above 1.0 wt.% of MWCNTs. The percolation threshold of the PC/MWNTs was observed between 1.5 and 2.5 wt.% MWNTs content by Kum et al. (2006). Potschke et al. (2004, 2003) and Allaoui et al. (2002) have also reported similar results for the PC/MWNT composites by electrical conductivity and rheological measurements, respectively.

In the present work, our aim was to synthesize MWCNTs by chemical vapor deposition (CVD) technique and PC/MWCNTs nanocomposite by twin-screw compounding extruder method and to investigate the effect on dielectric properties of PC/MWCNTs nanocomposite for small wt.% varying from 0.25 to 1.5 wt.% of PC/MWCNTs nanocomposite. The dielectric investigation of pristine PC and PC/MWCNTs nanocomposite has been carried out in frequency range of 1.0 Hz to 1.0 MHz.

2

2 Experimental

PC was obtained in the form of granules, and MWCNT was prepared by the CVD technique. The purity of MWCNT was characterized by the Raman spectroscopy. Melt compounding was carried out using a twin-screw compounding extruder (DACA Instruments, USA) with a screw speed of 150 rpm at 260–270 °C. About 20 g PC was introduced into the preheated rotating compounder, followed by the addition of 0.25, 0.55, 0.75, 1.00 and 1.50 wt.% of MWCNTs. A rotational speed of 150 rpm was applied. Half of the strand was used as one time extruded (OTE), and the rest was again extruded by twin screw compounding extruder maintaining the same temperature, the resulting nanocomposite being two time extruded (TTE). Then, the strand was stacked into a desired size (60 × 20 × 2.5 mm3) followed by compression molding at 150 °C. The morphology of the as produced MWCNTs and nanocomposite for 0.55 and 0.75 wt.% of MWCNTs was observed by the scanning electron microscope (SEM) (Model LEO 440, USA). The Raman analysis of the nanocomposite was carried out for 0.55 and 0.75 wt.% of MWCNTs by using a Renishaw India Reflex Micro Raman Spectrometer equipped with the charge-coupled device (CCD) detector at room temperature in air. The Red laser (excitation line 785 nm) was used to excite the nanocomposites with laser power of 25 mW for 10 s. Dielectric studies with the pellet of specimen, sandwiched between brass plates as capacitors have been carried out in the frequency range of 1.0 Hz to 1.0 MHz by using impedance gain phase analyzer of Solartron model SI-1260 coupled with Solartron dielectric interface model-1296. The dielectric constants of the samples were calculated by measuring the capacitance of the pellet and its geometry.

3

3 Results and discussion

The morphology and the extent of nanotube dispersion were characterized by SEM. The SEM images of the synthesized MWCNTs and the fractured surface of the PC/MWCNTs nanocomposite are shown in Fig. 1(a)–(c). The fracture surface of the PC/MWCNTs nanocomposite reveals the interaction between MWCNTs and PC matrices. Fig. 1(b) and (c) shows that MWCNTs achieved good dispersion in PC (Yadav and Lee, 2014).

SEM images of (a) MWCNT and fracture surface of (b) 0.55% and (c) 0.75% PC/MWCNTs nanocomposite.
Figure 1
SEM images of (a) MWCNT and fracture surface of (b) 0.55% and (c) 0.75% PC/MWCNTs nanocomposite.

In order to further investigate the purity of as produced MWCNTs and interaction between MWCNTs and PC nanocomposite, the Raman spectroscopic analysis was carried out on MWCNTs and PC/MWCNTs nanocomposite. The results are interpreted on the basis of changes in Raman shift (G band, 1578 cm−1 in 0.55% and 1581 cm−1 in 0.75%), as shown in Fig. 2(b). Raman shift can provide insight into dispersion and interaction of MWCNTs in PC matrices (Lordi and Yao, 2000; Lucas and Young, 2004; Heller et al., 2004; Xia and Song, 2005). The shifting of G-band peak of MWCNTs in PC/MWCNTs nanocomposite showed the interaction between MWCNTs and PC. A significant increase in the Raman shift indicated good dispersion and interaction between MWCNTs and PC.

Raman spectra of (a) pure MWCNT, and (b) 0.55% and 0.75% PC/MWCNTs nanocomposite.
Figure 2
Raman spectra of (a) pure MWCNT, and (b) 0.55% and 0.75% PC/MWCNTs nanocomposite.

To analyze measured data, the dielectric spectra have been fitted with the generalized Cole–Cole equation (Srivastava et al., 2015, 2008, 2005).

(1)
ε ∗ = ε ′ - j ε ″ = ε ′ ( ∞ ) + ∑ i = 1 k Δ ε i 1 + j f f ri ( 1 - h i ) where ε ( ∞ ) is the relative permittivity in high frequency limit, Δεi, fri and hi are dielectric strength, relaxation frequency and distribution parameter (0 ⩽ hi ⩽ 1) of ith mode, respectively. The real (ɛ′) and imaginary (ɛ″) parts of complex dielectric permittivity in Eq. (1) can be written as
(2)
ε ′ = ε ′ ( ∞ ) + ∑ i Δ ε i 1 + ω f f ri ( 1 - h i ) sin ( h i π / 2 ) 1 + ω f f ri 2 ( 1 - h i ) + 2 ω f f ri ( 1 - h i ) sin ( h i π / 2 )
(3)
ε ″ = ∑ i Δ ε i ω f f ri ( 1 - h i ) cos ( h i π / 2 ) 1 + ω f f ri 2 ( 1 - h i ) + 2 ω f f ri ( 1 - h i ) sin ( h i π / 2 ) The measured relative dielectric permittivity and dielectric loss are separately fitted with Eqs. (2) and (3) by using program developed in Origin software. The aim of nonlinear fitting is to estimate the parameter values that best describe the data and minimum deviations. Here the best fitting is characterized by the correlation coefficient (R2). For best fitted curve, the value of correlation factor (R2) should tend to 1. By fitting process, the best fitted values of relaxation frequency, dielectric strength and distribution parameter are obtained from Eqs. (2) and (3). The value of R2 was 0.999. Fig. 3 shows the variation of dielectric loss with increasing frequency for different wt.% of MWCNTs. One relaxation peak at low frequency region is clearly visible, whereas for high frequency, relaxation peaks are not clearly visible except for 1.50 wt.% of MWCNTs. To visualize the relaxation peak for high frequency region, the Cole–Cole graph has been plotted as shown in Fig. 4. Fig. 4 clearly depicts that only semicircle is present in case of pristine PC, whereas induction of carbon nanotubes induces one more semicircle. In other words, only one relaxation phenomenon is present in case of pristine PC, whereas two relaxation phenomena have been manifested in dielectric spectra in case of PC/MWCNTs nanocomposite. Therefore for fitting, the value of k = 2 has been taken in Eqs. (1)–(3) for PC/MWCNTs nanocomposite. The variation of dielectric permittivity with increasing wt.% of MWCNTs is shown in Fig. 5. The observed dielectric constant increases until 0.75 wt.% of PC/MWCNTs nanocomposite, and thereafter it decreases. Particularly at 0.75 wt.% of PC/MWCNTs nanocomposite, the dielectric constant increases up to five times than that of pure PC. At high frequency limit, the dielectric constant of pure PC was ∼2, whereas in case of 0.75 wt.% of PC/MWCNTs nanocomposite the value of dielectric constant was ∼9. The increase in the dielectric constant in the polymeric matrices by insertion of graphene and different nanoparticles such as Ag, Au, and ZnO has also been reported by Alzari et al. (2014). Potschke et al. (2003, 2004) have reported two distinguished behavior between 1.0 and 1.5 wt.% of MWCNTs for PC/MWCNTs nanocomposite. In our case the materials show significantly different behavior from 0.75 wt.% of MWCNTs. From fitting, two separate relaxation peaks can be resolved as shown in the Fig. 6. Low frequency relaxation peaks for pristine PC and PC/MWCNTs nanocomposites are shown in Fig. 6(a), whereas high frequency relaxation peaks for different wt.% of MWCNTs are shown in Fig. 6(b). As shown in Fig. 6, the relaxation peak observed in low frequency region grows very fast up to 0.75 wt.% MWCNTs, and thereafter it ceases. In dielectric loss dispersion spectra, the frequency at which the dielectric loss exhibits its maximum value is known as relaxation frequency. The estimated values of relaxation frequency with increasing wt.% of MWCNTs are shown in Fig. 7. For low frequency relaxation process, the relaxation frequency particularly at 0.75 wt.% of MWCNTs drastically increases, and thereafter it decreases with increase in wt.% of MWCNTs. The relaxation frequency for high frequency relaxation mode first decreases with increase in wt.% of MWCNTs and after 0.75 wt.% of MWCNTs, it increases with increasing concentration of MWCNTs. As the relaxation frequency is directly proportional to its activation energy (Srivastava et al., 2008, 2005), it seems that activation energy of high frequency relaxation mode competes with the activation energy of low frequency relaxation mode and above 0.75 wt.% of MWCNTs, the relaxation peak due to MWCNTs dominates. The values of dielectric strength with increasing wt.% of MWCNTs are shown in Fig. 8. The dielectric strength of both the modes increases up to 0.75 wt.% of MWCNTs and thereafter it becomes constant. The origin of low frequency mode is due to the rotation of polarized PC molecules. However, the origin of high frequency relaxation mode can be attributed to reorientation of PC molecules which arises due to interfacial polarization because of the accumulation of charges at the systems’ interface under an AC electric field. The distribution parameters of PC/MWCNTs nanocomposite for low and high frequency mode vary from 0.15 to 0.25 and 0.55 to 0.60, respectively. Therefore both modes do not follow the Debye-type relaxation mode and the center of semicircles in Fig. 4 would lie below the x-axis. In Debye type dielectric loss dispersion spectra, the dielectric strength should exactly be the double of dielectric loss value occurs at relaxation frequency. For non-Debye type dielectric loss dispersion spectra, the dielectric strength should be more than the double of dielectric loss value occurs at relaxation frequency. Therefore the estimated dielectric strengths shown in Fig. 8, agree well with the dielectric loss spectra of Fig. 6.
Variation of dielectric loss of pristine PC and PC/MWCNTs nanocomposite of different wt.%.
Figure 3
Variation of dielectric loss of pristine PC and PC/MWCNTs nanocomposite of different wt.%.
Cole–Cole plot of pristine PC and PC/MWCNTs nanocomposite of different wt.%.
Figure 4
Cole–Cole plot of pristine PC and PC/MWCNTs nanocomposite of different wt.%.
Variation of dielectric permittivity of PC/MWCNTs nanocomposite of different wt.% in high frequency limit.
Figure 5
Variation of dielectric permittivity of PC/MWCNTs nanocomposite of different wt.% in high frequency limit.
Resolved peaks: (a) low frequency dielectric loss peaks, after subtracting the high frequency cole–cole term in the whole dielectric spectra. (b) High frequency dielectric loss peaks, after subtracting the low frequency cole–cole term in the whole dielectric spectra.
Figure 6
Resolved peaks: (a) low frequency dielectric loss peaks, after subtracting the high frequency cole–cole term in the whole dielectric spectra. (b) High frequency dielectric loss peaks, after subtracting the low frequency cole–cole term in the whole dielectric spectra.
Variation of relaxation frequency of pristine PC and PC/MWCNTs nanocomposite of different wt.%.
Figure 7
Variation of relaxation frequency of pristine PC and PC/MWCNTs nanocomposite of different wt.%.
Variation of dielectric strength of pristine PC and PC/MWCNTs nanocomposite with different wt.%.
Figure 8
Variation of dielectric strength of pristine PC and PC/MWCNTs nanocomposite with different wt.%.

Fig. 9 shows the frequency dependent electrical conductivity behavior of nanocomposites and PC at room temperature. The net AC conductivity (σac) of synthesized nanocomposites at room temperature is the linear superposition of DC conductivity and AC conduction component (Aωs; A is constant and s is power law exponent), which is called universal dynamic behavior (UDR) (Wan et al., 2015). The second component (Aωs) is the representative of the electrons tunneling and hopping in PC nanocomposite structures which contribute to electrical conductivity. In the present case, it is clear that the DC component of electrical conductivity (ω → 0) is less dominant than the AC part. Nevertheless, the effect of DC component is relatively more pronounced at 0.75 wt.% of MWCNTs at low frequencies. The increase in electrical conductivity at higher frequencies is attributed to space charge polarization removal. Electrical conductivity rises from 10−11 S/m in PC to 10−10 S/m in 1.5 wt.% MWCNTs nanocomposite at 1.0 Hz. Though the conductivity is very low due to very small wt.% of MWCNTs, high conducting polymer nanocomposite of the order of ∼1.0 S/m can also be obtained by increasing the loading amount of greater than 2.0 wt.% of MWCNTs (Potschke et al., 2003). Recently, the conductivity of the order of ∼50 S/m was reported for graphene PC nanocomposite (Yoonessi and Gaier, 2010).

Variation of conductivity of pristine PC and PC/MWCNTs nanocomposite with different wt.%.
Figure 9
Variation of conductivity of pristine PC and PC/MWCNTs nanocomposite with different wt.%.

4

4 Conclusions

In summary, we have synthesized the MWCNTs and PC/MWCNTs nanocomposite by CVD and twin-screw compounding extruder, respectively. The Raman analysis confirmed good interaction between carbon nanotubes and polycarbonate. Single dielectric relaxation mode has been observed in pristine polycarbonate, whereas dispersion of MWCNTs induced one more dielectric relaxation mode in PC/MWCNTs nanocomposite. The percolation threshold of the polycarbonate/multi-wall carbon nanotubes composite was observed at 0.75 wt.% of MWCNTs. The low frequency relaxation mode is due to the rotation of PC molecules, whereas the origin of high frequency relaxation mode is attributed to reorientation of PC molecules which arises due to interfacial polarization because of accumulation of charges at the systems’ interface under an AC electric field.

Acknowledgements

This work was supported by the Human Resource Development Program (No. 20134030200360) of the Korea Institute of Energy Technology Evaluation and Planning – South Korea. One of authors (AKS) would like to thank the Department of Science and Technology, New Delhi, India, for financial support under Fast Track Young Scientist Project SR/FTP/PS-037/2011.

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