PEO electrospun fibers for shielding effectiveness of electromagnetic interference

Colloids and Surfaces A: Physicochem. Eng. Aspects 364 (2010) 151–157

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Enhanced adhesion and dispersion of carbon nanotube in PANI/PEO electrospun fibers for shielding effectiveness of electromagnetic interference Ji Sun Im a , Jong Gu Kim a , Sei-Hyun Lee b , Young-Seak Lee a,∗ a b

Department of Fine Chemical Engineering and Applied Chemistry, BK21-E2 M, Chungnam National University, Gung-dong 220, Yuseong-gu, Daejeon 305-764, Republic of Korea Department of Electrical and Electronic Engineering, Korea Polytechnic IV College, Daejeon 300-702, Republic of Korea

a r t i c l e

i n f o

Article history: Received 10 February 2010 Received in revised form 16 April 2010 Accepted 11 May 2010 Available online 16 May 2010 Keywords: Fibers Polymer-matrix composites Adhesion Surface properties

a b s t r a c t Polyaniline-based fibers were fabricated with multi-walled carbon nanotubes and polyethylene oxide by the electrospinning method to be employed as an electromagnetic interference shielding material. To improve the electromagnetic interference shielding efficiency, the dispersion and adhesion of the carbon nanotubes in the polyaniline electrospun fibers were enhanced through surface modification by the direct fluorination method. This fluorination treatment enhanced the electron donor–acceptor reaction between the fibers and nanotubes, causing adhesive bonding for high electromagnetic interference shielding efficiency. The electrical conductivity improved with the addition of the carbon nanotubes and the fluorination treatment of the carbon nanotubes, reaching up to 4.8 × 103 S/m. The scanning electron microscope images showed well-oriented carbon nanotubes inside of the polyaniline fibers based on the effects of the fluorination. Investigating the electromagnetic interference shielding efficiency mechanism confirmed that absorption was the main reaction that shielded the electromagnetic interference. Eventually, an improved electromagnetic interference shielding efficiency of 42 dB was obtained. © 2010 Elsevier B.V. All rights reserved.

1. Introduction In modern society, the proliferation of electronic devices and instruments has become a serious problem because they generate electromagnetic interference (EMI). EMI can cause the operational malfunction and miscarriage of electric devices, leading to the loss of valuable time, energy, resources, money or even life. The interference can also bring about diseases such as leukemia and breast cancer. Thus, attenuation of EMI has been investigated by many researchers [1–3]. Metals are the most common materials used for EMI shielding. However, they suffer from disadvantages such as high density, susceptibility to corrosion, complexity and expensive processing. Furthermore, metals mainly reflect radiation and cannot be used in applications where absorption is desired such as in stealth technology [4,5]. This has led to a great deal of interest in the development of materials that can absorb EMI radiation. In particular, conducting polymers have obtained a special status because of their high EMI shielding efficiency (SE), efficient surface modification and easy formation of composites [6,7]. Carbon nanotubes (CNTs) have also been regarded as a promising candidate for shielding EMI due to their excellent electrical conductivity, high aspect ratio

and excellent mechanical properties. Some important reports have considered the effect of the shape of an EMI shielding material, indicating that a high aspect ratio of CNTs positively affects the enhancement in EMI SE [8,9]. Under the above considerations, the composites of conducting polymers and CNTs have become the promising material for EMI SE [5,7]. However, the expected EMI SE has not been achieved so far, because the heterogeneous interface between the two components of polymers and CNTs negatively affects the EMI SE [4]. In this paper, polyaniline (PANI) fibers were prepared by the electrospinning method in order to obtain a continuously-shaped conducting polymer. Polyethylene oxide (PEO) was used to enhance the shape of the uniform PANI electrospun fibers. Multi-walled carbon nanotubes (MWCNTs) were selected as a conducting carbon filler. The surface modification of the MWCNTs was carried out by means of an easy and efficient fluorination method before adding them to the PANI fibers. This method resulted in a better match between the MWCNTs and the PANI fiber surfaces, leading to better adhesion of them.

2. Materials and methods 2.1. Preparation of the polymer solution

∗ Corresponding author. Tel.: +82 42 821 7007; fax: +82 42 822 6637. E-mail address: [email protected] (Y.-S. Lee). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.05.015

In order to prepare the polymer solution for electrospinning, 100 mg of high-molecular weight emeraldine-base PANI (Mw

152

J.S. Im et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 364 (2010) 151–157

65,000) was dissolved in 10 ml of chloroform. In this step, 128 mg of 10-camphorsulfonic acid (HCSA) was added for protonation of the PANI, which increased the conductivity and solubility. The resulting solution was filtered and 20 mg of PEO (Mw 600,000) was dissolved in the filtered solution, because it is helpful to enhance the shape of the uniform PANI electrospun fibers without negative effects for high electrical properties [10,11]. The dissolution was carried out at ambient temperature with stirring. The MWCNTs (NK Co., 75 Korea) were selected as the additive material in order to increase the conductivity. To enhance the dispersion of the MWCNTs in the hydrophobic polymer solution, fluorination was used to introduce the hydrophobic functional groups onto the MWCNT surface. The conditions for fluorination (such as the reaction temperature, time and pressure) were determined by trial and error with a dispersion test of fluorinated MWCNT in chloroform. The dispersion test was carried out using a UV-spectrometer at 635 nm with the general knowledge that a lower transmittance value indicates a higher dispersion [12,13]. Eventually, the fluorination of the MWCNTs was carried out with a mixed gas (fluorine:argon = 1:9 vol%) at 298 K for 10 min under 1 bar pressure [14–17]. In this study, both the non-treated MWCNTs and the fluorinated MWCNTs were used as an additive to investigate the effects of the fluorination treatment. In the case of the unmodified MWCNTs, the MWCNTs could only be added up to 100 wt% of MWCNTs/PANI. However, the fluorination treatment made it possible to add up to 120 wt% of MWCNTs/PANI.

Fig. 1. Structure of (a) the EMI SE holder and (b) the shape and dimensions of the EMI SE test specimen.

F1529-97 method. The electrical conductivity () was calculated using the following two equations [24]:  = Rs × t =

1 

(m)

(S/m)

(1) (2)

Electrospinning was accomplished by loading each polymer solution into a 30 cm3 syringe with a capillary tip (18 G, 1.27 mm inner diameter) placed in a KD scientific syringe pump (Model 100) for metered dispensing at 1 cm3 /h. The positive output lead of a high voltage power supply (NT-PS-25K, NTSEE Co., Korea) set to 15–21 kV was attached to a blunt needle on the syringe. A grounded target (17 cm wide × 30 cm long × 0.3 cm thick; 303 polished stainless steel) was placed 10 cm away from the tip of the needle, rotated at 100 rpm and translated at 3 cm/s over a 17 cm travel distance. This process evenly created a mat of uniform thickness without imparting a high degree of alignment to the deposited fibers [18–23].

where  is the bulk resistivity and t is the sample thickness. Permittivity (relative complex permittivity), magnetic permeability (relative complex magnetic pemeability) and EMI SE were obtained according to the ASTM D-4935-99 method using a network analyzer (Agilent, E5071A) equipped with an amplifier and a scattering parameter (S parameter) test set over a frequency range from 800 MHz to 8.5 GHz. Annular disks were prepared with a punching machine and were installed into the test tool as shown in Fig. 1. The EMI SE was calculated using the S parameters [25]. In order to investigate the surface morphology of the resultant samples based on the effects of fluorination treatment of MWCNTs, SEM images were obtained using a field emission scanning electron microscope (FE-SEM, Hitachi, S-5500). Each sample was analyzed without any treatment (such as coating).

2.3. Characterization of the resultant samples

3. Results and discussion

The oriented and defected carbon structures were examined by Raman analysis in order to determine the effects of fluorination. Raman spectral analysis was conducted with an excitation power of 10 mW at 514 nm (RM 1000-InVia, Renishaw). The X-ray photoelectron spectroscopy (XPS) of the MWCNTs used in this study was obtained with a MultiLab 2000 spectroscopy (Thermo Electron Co., England) to evaluate changes in the chemical species on the surface of the MWCNTs before and after fluorination. Al-K␣ (1485.6 eV) X-rays were used with a 14.9 keV anode voltage, a 4.6 A filament current and a 20 mA emission current. All samples were treated at 10−9 mbar in order to remove impurities. The survey spectra were obtained with a 50 eV pass energy and a 0.5 eV step size. Core level spectra were obtained at a 20 eV pass energy with a 0.05 eV step size. Ultraviolet (UV) spectrometry (Optizen 2120 UV, Mecasys, Korea) was used to investigate the dispersion of the MWCNTs in chloroform. This measurement was carried out by following the general method presented by other groups [12,13]. Measurements were acquired at 635 nm after sonication for 1 h. Sheet resistance (Rs , ) was measured five times with the fourprobe method at room temperature under ambient conditions using the probe head station (DASOL ENG, Korea) and the ASTM

3.1. Structural changes by fluorination determined using Raman spectra

2.2. Electrospinning of the prepared solution

Raman spectra are presented in Fig. 2 to investigate the effects of fluorination on the structure of the MWCNTs. D (defect) and G (graphite) peaks were observed at 1356 and 1586 cm−1 , respectively. The fluorination treatment resulted in a slight intensity increase in the D peak and a decrease in the G peak, indicating that the fluorination treatment did not seriously destroy the graphite structure of the MWCNTs, as the peaks show less than a 10% change. This is an important finding in order to determine which graphite structure contributes to the high electrical conductivity. 3.2. Chemical analysis by fluorination using XPS The elemental survey data of non-treated and fluorinated MWCNTs is shown in Fig. 3. In both cases, C 1s and O 1s peaks were observed, indicating that the carbon atoms from the MWCNTs and the oxygen on the MWCNTs are from the ambient atmosphere. The F 1s and F KL1 peaks confirm that the fluorine atoms were introduced into the MWCNTs. In order to investigate the chemical structures in more detail, Fig. 4 shows the deconvolu-

J.S. Im et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 364 (2010) 151–157

153

Fig. 2. Structural analysis by Raman spectra of the pristine and fluorinated MWCNTs.

Fig. 4. Chemical bonding analysis by C 1s deconvolution of the (a) pristine and (b) fluorinated MWCNTs.

Fig. 3. Elemental survey of the pristine and fluorinated MWCNTs.

tion of the C 1s peaks to several Pseudo-Vogit functions (sum of Gaussian–Lorentzian function) using a peak analysis program obtained from Unipress Co., USA. The pseudo-Vogit function is given by [26]:



F(E) = H

(1 − S) exp



 − ln(2)

E − E0 FWHM

2 

+



S 1 + ((E − E0 )/FWHM)

2

where F(E) is the intensity at energy E, H is the peak height, E0 is the peak center, FWHM is the full width at half maximum, and S is the shape function related to the symmetry and the Gaussian–Lorentzian mixing ratio. The assignments for the different components of the C 1s spectra are listed in Table 1. The

C(1) peak corresponds to non-functionalized sp2 carbon atoms that come from the aromatic structure of the MWCNTs. This component’s concentration was 84.12% and 72.25% in the pristine MWCNTs and the fluorinated MWCNTs, respectively. The C(2) component assigned to the aliphatic sp3 carbon atoms was present at 11.81% and 18.18%. Some oxygenated sp3 carbons were detected in both cases, which suggests that the chemical binding may be due to the ambient atmosphere. The C(5) and C(6) components correspond to covalent bonding of the carbon and fluorine, which indicates that the sp2 hybridized carbon covalently linked to the F atoms (C(5)) and the CF groups (C(6)) in the CF2 groups [27,28]. The C(5) and C(6) components accounted for 2.27% and 2.84%. These results indicate that some sp2 carbons of the aromatic structure were changed to sp3 carbon structure with C–F chemical bonds. Based on the percentage (less than 10%) of chemical bond changes, these results suggest that the fluorination treatment did not change the nature of the MWCNTs.

Table 1 C 1s spectra and peak parameters of the pristine and fluorinated MWCNTs. Component

Peak position (eV)

Assignment

Concentration of MWCNTs (%)

Concentration of fluorinated MWCNTs (%)

C(1) C(2) C(3) C(4) C(5) C(6)

284.5 285.1 286.2 287.5 288.85 290.7

sp2 carbon sp3 carbon O–C (carboxyl) Semi-ionic C–F Nearly covalent C–F Covalent C–F (CF2 , CF3 )

84.12 11.81 4.07

72.25 18.18 5.43 1.87 2.27 2.84

154

J.S. Im et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 364 (2010) 151–157

Fig. 5. Fluorination effects on the dispersion of MWCNTs.

3.3. Effect of fluorination on the dispersion of CNTs The effect of fluorination on improving the dispersion of the MWCNTs was investigated in Fig. 5, which presents the transmittance of the pristine and fluorinated MWCNT/chloroform solution at 635 nm over time. Comparing the two cases demonstrates that the transmitted wave intensity decreased through the fluorination treatment indicated that the dispersion degree was enhanced by the fluorination. 3.4. Electrical conductivity of the composite Fig. 6 reports on the electrical conductivity of the samples. In both cases, the conducting additives enhanced the electrical conductivity by 3.5 × 103 and 4.8 × 103 S/m for the non-treated and fluorinated MWCNTs. In the case of adding non-treated MWCNTs, the electrical conductivity increased linearly up to 3.4 × 103 S/m at 80 wt% of the MWCNTs/PANI. However, there was no significant change upon adding more MWCNTs, which resulted in a small increase around 1.0 × 102 S/m. It seems that the surplus MWCNTs does not have a significant effect after an optimized MWCNT conducting network has been formed with the fibers. This phenomenon was also observed in the composite of fluorinated MWCNTs and PANI. One more interesting

Fig. 6. Electrical conductivity of the pristine and fluorinated MWCNT/PANI/PEO electrospun fiber sheets.

Fig. 7. Permittivity of the samples.

result is that the electrical conductivity was higher for the same MWCNT/PANI weight ratio of fluorinated MWCNT samples than for the pristine MWCNT samples. This result is caused by enhanced interface between the MWCNTs and the PANI upon fluorination. 3.5. Permittivity, magnetic permeability samples In order to investigate the permittivity, magnetic permeability and EMI SE, four samples were selected based on the electrical conductivity results. PANI/PEO fibers were used as a reference and were designated sample 1. Two samples with 100 wt% of pristine and fluorinated MWCNTs embedded PANI/PEO fibers were selected in order to investigate the effects of the fluorination treatment and were designated as samples 2 and 3. Finally, a sample with 120 wt% of fluorinated MWCNTs embedded PANI/PEO fibers that showed the highest electrical conductivity was used and designated sample 4. The permittivity of the samples is presented in Fig. 7. Samples 1 and 2 showed a real permittivity of about 2 and 7.2, which indicates that the addition of MWCNTs significantly improved the permittivity. This effect can be attributed to the excellent electrical conductivity of the MWCNTs. The fluorination effects were observed by comparing samples 2 and 3, demonstrating that fluorination increased the real permittivity from 7.2 to 9.7. These results demonstrate that the enhancement in adhesion between the MWCNTs and the PANI fibers by the fluorination surface treatment leads to better permittivity. The highest real permittivity of about 12 was observed in sample 4. The imaginary permittivity showed a similar trend as the real permittivity because changes in the imaginary permittivity are strongly related to changes in the real permittivity. Fig. 8 presents the magnetic permeability of the samples. The improved magnetic permeability was observed based on the effects of MWCNT additives and fluorination treatment by samples 2 and 3, respectively. Despite the same amount of MWCNT additives, the real magnetic permeability of sample 3 showed the almost 1.7 times value more than sample 2 indicating the fluorination effects. Based on the excellent magnetic properties of the MWCNTs, these results suggest that the magnetic permeability can be significantly improved by improving the adhesion between the conducting PANI polymer and the MWCNT additives [29,30]. The highest real magnetic permeability was observed in sample 4 with higher amount of MWCNT additives and fluorination treatment.

J.S. Im et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 364 (2010) 151–157

155

Fig. 10. Average EMI SE with the fraction of reflection and absorption effects. Fig. 8. Magnetic permeability of the samples.

and absorbance (A) coefficients were obtained using the S parameters [33]:

3.6. EMI SE of the samples EMI SE is defined as the attenuation of the propagating electromagnetic waves, which is expressed as [31]:





PI  EI  EMI SE = 10 log = 20 log   PT ET

(dB)

(3)

where PI (EI ) and PT (ET ) are the power (electric field) of the incident and transmitted electromagnetic waves. Fig. 9 shows the EMI SE of samples in the frequency range of 800–4000 MHz. The EMI SE of sample 1 was 8.5 dB, and did not significantly change with variations in the frequency. Sample 2 showed an improved EMI SE based on the effects of adding the conductive MWCNTs. The effects of the fluorination treatment were also observed by comparing samples 2 and 3. The EMI SE of sample 2 abruptly decreased over 3000 MHz, while the EMI SE of sample 3 was higher across the whole frequency range and did not decrease as dramatically at high frequencies. This may be attributed to the enhanced adhesion between the PANI polymer and the MWCNTs provided by fluorination. The highest EMI SE was observed in sample 4, which is based on the effects of the largest amount of added MWCNTs and the fluorination treatment. In order to investigate the contribution of the absorption and reflection to the total EMI SE of the samples, S parameters were used for the measurement [32]. The transmittance (T), reflectance (R)

1=A+R+T

(4)

 2  ET  T =   = |S12 |2 E I

 2  ER   = |S11 |2 E

R=

I

(= |S21 |2 )

(5)

(= |S22 |2 )

(6)

In this study, the multiple reflection effects were not considered separately because the measured reflected power includes not only the power that has been reflected from the external surface but also a positive contribution from internal surface reflection and a negative contribution from multiple reflections [4]. Eventually, shielding efficiency by reflection (SER ) and absorption (SEA ) were calculated by the following equations [5,34]: SER = 10 log

I I−R

(7)

SEA = 10 log

I−R T

(8)

Total SE = SER + SEA = 10 log

I T

(9)

The EMI SE for reflection and absorption is presented in Fig. 10. The SEA s against the total SE were 57%, 72%, 79% and 82%. Thus, the EMI shielding percentage by absorption increased with the addition of the MWCNTs and the fluorination treatment. This is due to the excellent electrical conductivity of the MWCNTs and the enhanced adhesion by surface modification. 3.7. Suggested mechanism of enhanced adhesion between the MWCNTs and the PANI polymer by fluorination treatment

Fig. 9. EMI SE of the samples from 800 to 4000 MHz.

Recently, in the manufacturing process of composites that use aniline and MWCNTs, there have been some important reports that the MWCNTs act as an electron acceptor while the aniline acts as an electron donor, which is explained by a weak charge transfer. These reports have explained that the interface between them can be chemically well matched by the electron acceptor–donor reaction, causing the improved dispersion of the MWCNTs in the aniline solution [5]. Based on the above knowledge, the fluorine on the MWCNTs plays an essential role in enhancing the ability of the MWCNTs to accept electrons. This is because the fluorine has the highest electronegativity in comparison to the rest of the atoms. Improving the ability of the MWCNTs to accept electrons

156

J.S. Im et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 364 (2010) 151–157

Fig. 11. Suggested mechanism about the enhanced electron acceptor–donor reaction between MWCNTs and PANI by fluorination treatment.

may lead to strong adhesion between the MWCNTs and the aniline. The suggested mechanism is depicted in Fig. 11. The formation of the complex is facilitated by the unshared electron pair on the amine nitrogen and the electronegativity of the fluorine. This mechanism is supported by FT-IR and SEM images. The FT-IR results of sample 2 and sample 3 were presented in Fig. 12. In case of sample 2, pristine MWCNT embedded PANI/PEO fibers were confirmed by peaks [PEO peaks: 1730 cm−1 (C O STR), 1350 cm−1 (C–H bend), 1238 cm−1 (O–H bend), 1093 and 850 cm−1 (C–O rocking) and 1560 cm−1 (N–H bend)] [35] [PANI/MWCNT composite peaks: 1585–1380 cm−1 (C–C bond of the quinoid versus benzenoid groups) and 1105–950 cm−1 (ring-deformations of quinoid and benzenoid groups)] [36] as shown in Fig. 12a. Three new peaks were observed in sample 3 indicating the N–F and C–F bonds [37] as presented in Fig. 12b; [1020 and 900 cm−1 (N–F groups) and 1220 cm−1 (C–F groups)]. This electron donor–acceptor reaction may lead to welloriented MWCNTs in the PANI fibers, as shown in Fig. 13. The used

Fig. 12. FT-IR of (a) sample 2 and (b) sample 3.

fluorinated MWCNTs showed the well-oriented carbon structure without any significant defect in Fig. 13a as explained by Raman results in Fig. 2. The MWCNTs are observed on the inside of the PANI fibers in Fig. 13b and c. Even though the same amount of additives was used, sample 3 shows the better uniformly oriented MWCNTs inside of the fibers (see Fig. 13d). This image shows the MWCNTs arranged inside of the PANI/PEO fibers like well-aligned electric wires in a cable. This suggests that the fluorination treatment enhanced the adhesion between the PANI and the MWCNTs by a weak charge transfer as explained in Figs. 11 and 12. The adhesive bonding enhanced conducting complex resulted in alignment

Fig. 13. Images of (a) fluorinated MWCNT, (b and c) sample 2 and (d) sample 3.

J.S. Im et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 364 (2010) 151–157

during the electrospinning process by a high electric power. While it is generally known that the orientation of a conducting filler is one of the main factors that affects the EMI shielding of a polymer composite [4,5], the fluorination treatment definitely played an important role in improving the EMI SE. 4. Conclusions MWCNT additives embedded PANI/PEO-based fibers were prepared by the electrospinning method as the EMI shielding material. The dispersion and adhesion of the MWCNTs in the PANI fibers were improved using a fluorination treatment on the MWCNTs by enhancing the electron donor–acceptor reaction between the two components. The electrical conductivity increased with the addition of MWCNTs, more so when the MWCNTs were fluorinated, reaching up to 4.8 × 103 S/m. The permittivity and magnetic permeability which works for EMI SE increased also significantly based on the effects of electrical and magnetic property of MWCNT additives and fluorination treatment. In conclusion, a high EMI SE of 42 dB was obtained by preparation of a conducting composite with enhanced absorption. Acknowledgements This research was supported by the Consortium Program from the Small and Medium Business Administration, Republic of Korea. The authors thank Mr. Tae-Sung Bae at KBSI (Korea Basic Science Institute) in Jeonju center for providing FE-SEM images. References [1] L.L. Wang, B.K. Tay, K.Y. See, Z. Sun, L.K. Tan, D. Lua, Electromagnetic interference shielding effectiveness of carbon-based materials prepared by screen printing, Carbon 47 (2009) 1905–1910. [2] C.J.V. Klemperer, D. Maharaj, Composite electromagnetic interference shielding materials for aerospace applications, Compos. Struct. 91 (2009) 467– 472. [3] Y. Li, C. Chen, S. Zhang, Y. Ni, J. Huang, Electrical conductivity and electromagnetic interference shielding characteristics of multiwalled carbon nanotube filled polyacrylate composite films, Appl. Surf. Sci. 254 (2008) 5766–5771. [4] M.H. Al-Saleh, U. Sundararaj, Electromagnetic interference shielding mechanisms of CNT/polymer composites, Carbon 47 (2009) 1738–1746. [5] P. Saini, V. Choudhary, B.P. Singh, R.B. Mathur, S.K. Dhawan, Polyaniline–MWCNT nanocomposites for microwave absorption and EMI shielding, Mater. Chem. Phys. 113 (2009) 919–926. [6] E. Håkansson, A. Amiet, S. Nahavandi, A. Kaynak, Electromagnetic interference shielding and radiation absorption in thin polypyrrole films, Eur. Polym. J. 43 (2007) 205–213. [7] A. Kaynak, E. Håkansson, A. Amiet, The influence of polymerization time and dopant concentration on the absorption of microwave radiation in conducting polypyrrole coated textiles, Synthetic Met. 159 (2009) 1373–1380. [8] Y. Huang, N. Li, Y. Ma, F. Du, F. Li, X. He, L. Xiao, G. Hongjun, C. Yongsheng, The influence of single-walled carbon nanotube structure on the electromagnetic interference shielding efficiency of its epoxy composites, Carbon 45 (2007) 1614–1621. [9] Z. Liu, G. Bai, Y. Huang, Y. Ma, F. Du, F. Li, G. Tianying, C. Yongsheng, Reflection and absorption contributions to the electromagnetic interference shielding of single-walled carbon nanotube/polyurethane composites, Carbon 45 (2007) 821–827. [10] I.D. Norris, M.M. Shaker, F.K. Ko, A.G. Macdiarmid, Electrostatic fabrication of ultrafine conducting fibers: polyaniline/polyethylene oxide blends, Synthetic Met. 114 (2000) 109–114. [11] P.K. Kahol, N.J. Pinto, An EPR investigation of electrospun polyaniline–polyethylene oxide blends, Synthetic Met. 140 (2004) 269–272.

157

[12] O.K. Park, T. Jeevananda, N.H. Kim, S.I. Kim, J.H. Lee, Effects of surface modification on the dispersion and electrical conductivity of carbon nanotube/polyaniline composites, Scripta Mater. 60 (2009) 551–554. [13] S. Arepalli, P. Nikolaev, O. Gorelik, V.G. Hadjiev, W. Holmes, B. Files, Y. Leonard, Protocol for the characterization of single-wall carbon nanotube material quality, Carbon 42 (2004) 1783–1791. [14] J.S. Im, S.J. Kim, P.H. Kang, Y.S. Lee, The improved electrical conductivity of carbon nanofibers by fluorinated MWCNTs, J. Ind. Eng. Chem. 15 (2009) 699–702. [15] J.W. Lim, J.M. Lee, S.M. Yun, B.J. Park, Y.S. Lee, Hydrophilic modification of polyacrylonitrile membranes by oxyfluorination, J. Ind. Eng. Chem. 15 (2009) 876–882. [16] J.M. Lee, S.J. Kim, J.W. Kim, P.H. Kang, Y.C. Nho, Y.S. Lee, A high resolution XPS study of sidewall functionalized MWCNTs by fluorination, J. Ind. Eng. Chem. 15 (2009) 66–71. [17] M.J. Jung, J.W. Kim, J.S. Im, S.J. Park, Y.S. Lee, Nitrogen, hydrogen adsorption of activated carbon fibers modified by fluorination, J. Ind. Eng. Chem. 15 (2009) 410–414. [18] S.J. Kim, S.M. Yun, Y.S. Lee, Characterization of nanocrystalline porous SiC powder by electrospinning and cabothermal reduction, J. Ind. Eng. Chem. 16 (2010) 273–277. [19] J.S. Im, J.S. Jang, Y.S. Lee, Synthesis and characterization of mesoporous electrospun carbon fibers derived from silica template, J. Ind. Eng. Chem. 15 (2009) 914–918. [20] J.S. Im, S.W. Woo, M.J. Jung, Y.S. Lee, Improved capacitance characteristics of electrospun ACFs by pore size control and vanadium catalyst, J. Colloid Interf. Sci. 327 (2008) 115–119. [21] J.S. Im, S.J. Park, Y.S. Lee, Preparation and characteristics of electrospun activated carbon materials having meso- and macropo, J. Colloid Interf. Sci. 314 (2007) 32–37. [22] J.S. Im, S.J. Park, T.J. Kim, Y.H. Kim, Y.S. Lee, The study of controlling pore size on electrospun carbon nanofibers for hydrogen absorption, J. Colloid Interf. Sci. 318 (2008) 42–49. [23] J.S. Im, S.J. Park, Y.S. Lee, The metal–carbon–fluorine system for improving hydrogen storage by using metal and fluorine with different levels of electronegativity, Int. J. Hydrogen Energy 34 (2009) 1423–1428. [24] J.S. Im, S.J. Park, T. Kim, Y.S. Lee, Hydrogen storage evaluation based on investigations of the catalytic properties of metal/metal oxides in electrospun carbon fibers, Int. J. Hydrogen Energy 34 (2009) 3382–3388. [25] Z. Wu, J. Li, D. Timmer, K. Lozano, S. Bose, Study of processing variables on the electrical resistivity of conductive adhesives, Int. J. Adhes. Adhes. 29 (2009) 488–494. [26] A.M. Nicolson, G.F. Ross, Measurement of the intrinsic properties of materials by time-domain techniques, IEEE Trans. Instrum. Meas. 19 (1970) 377–382. [27] J.S. Im, J.G. Kim, Y.S. Lee, Fluorination effects of carbon black additives for electrical properties and EMI shielding efficiency by improved dispersion and adhesion, Carbon 47 (2009) 2640–2647. [28] Y. Hattori, H. Tanaka, F. Okino, H. Touhara, Y. Nakahigashi, S. Utsumi, H. Kanoh, K. Kaneko, Quantum sieving effect of modified activated carbon fibers on H2 and D2 absorption at 20 K, J. Phys. Chem. B 110 (2006) 9764–9767. [29] S.J. Park, B.J. Kim, Ammonia removal of activated carbon fibers produced by oxyfluorination, J. Colloid Interf. Sci. 291 (2005) 597–599. [30] P.C. Ma, B.Z. Tang, J.K. Kim, Effect of CNT decoration with silver nanoparticles on electrical conductivity of CNT-polymer composites, Carbon 46 (2008) 1497–1505. [31] X. Zhang, J. Zhang, R. Wang, Z. Liu, Cationic surfactant directed polyaniline/CNT nanocables: synthesis, characterization, and enhanced electrical properties, Carbon 42 (2004) 1455–1461. [32] American Society for Testing and Materials, Standard Test Method for Measuring the Electromagnetic Shielding Effectiveness of Planar Materials, ASTM, Philadelphia, PA, 1999, p. D-4935. [33] D.K. Ghodgaonkar, V.V. Varadan, V.K. Varadan, Free-space measurement of complex permittivity and complex permeability of magnetic materials at microwave frequencies, Trans. Instrum. Meas. 39 (1990) 387–394. [34] Y.K. Hong, C.Y. Lee, C.K. Jeong, S.H. Sim, K. Kim, J. Joo, M.S. Kim, J.Y. Lee, S.H. Jeong, S.W. Byun, Electromagnetic interference shielding characteristics of fabric complexes coated with conductive polypyrrole and thermally evaporated Ag, Curr. Appl. Phys. 1 (2001) 439–442. [35] H. Zhang, J. Wang, X. Xuan, K. Zhuo, FT-IR studies on interactions of AlCl3 with PEO–NaSCN polymer electrolytes, Electrochim. Acta 51 (2006) 3244–3248. [36] W.K. Maser, R. Sainz, M.T. Martinez, A.M. Benito, Electroactive polymer-carbon nanotube composites: smart organic materials for optoelectronic applications, Contrib. Sci. 4 (2008) 187–192. [37] M. Koh, H. Yumoto, H. Higashi, T. Nakajima, Fluorine intercalation in carbon alloy CX N, J. Fluorine Chem. 97 (1999) 239–246.