Multiwalled Carbon Nanotubes

Article pubs.acs.org/IECR

Preparation of Polyaniline/Multiwalled Carbon Nanotubes Nanocomposites by High Gravity Chemical Oxidative Polymerization Yi-bo Zhao,† Wei Wu,*,† Jian-feng Chen,†,‡ Hai-kui Zou,†,‡ Lu-lu Hu,† and Guang-wen Chu*,‡ †

State Key laboratory of Organic Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, China

‡

ABSTRACT: This study first reports the synthesis of polyaniline (PANI)/multiwalled carbon nanotubes (MWNTs) nanocomposites carried out by the high gravity chemical oxidative polymerization (HGCOP) method in a rotating packed bed (RPB). The results show that the uniform core−shell tubular structure with diameters of 30−50 nm was successfully formed and PANI was covalently bonded to MWNTs. The conductivity of PANI/MWNTs prepared by RPB is higher than that by the stirred tank reactor (STR) when the content of MWNTs is lower, and the conductivities of the two nanocomposites get close with the increase of MWNTs content. This method provides a significant improvement to the rapid mixing method to prepare PANI/MWNTs nanocomposites in large scale. Moreover, it has potential applications in the synthesis of other polymer nanocomposites.

1. INTRODUCTION Owing to the unique structural, mechanical, electronic, and thermal properties, carbon nanotubes (CNTs) have attracted a great deal of interest since Iijima initially identified their structures in 1991.1 Recently, much attention is paid to the synthesis of CNTs/conducting polymer nanocomposites which are considered as a promising approach to explore synergetic effects arising from the components. The nanocomposites show potential in many fields, such as antistatic materials,2,3 gas sensor applications,4,5 and supercapacitors.6−8 The combination of the CNTs with conducting polymers offers an attractive route to reinforce polymer as well as enhancing electronic properties based on morphological modification or electronic interaction between two components. Among the conductive polymers, polyaniline (PANI) is a promising candidate for practical applications due to its good processability, environmental stability, and reversible control of electrical properties by both charge-transfer doping and protonation.9 Because of the synergies between PANI and CNTs, dramatic improvement in the mechanical, thermal, electrical, optical, and redox properties of these nanocomposites are expected. Many approaches have been made to fabricate PANI/CNTs nanocomposites. Generally, the methods used to incorporate nanotubes into PANI matrix include emulsion polymerization,10 microwave-assisted polymerization,11 ultrasonic irradiation method,12 electrochemical polymerization,13 in situ chemical oxidation polymerization,14 etc. Among these approaches, in situ chemical oxidation polymerization is widely used in the synthesis of PANI/CNTs nanocomposites for its convenience (no need of organic solvent and auxiliary equipments). However, there still exist some problems for the in situ chemical oxidation polymerization. For example, the oxidants were usually added by dripping slowly in the polymerization progress to avoid unequably nanocomposites or need low © 2012 American Chemical Society

temperature operation to get better morphology which required longer reaction time and strict reaction condition and limited its scale-up synthesization. We have prepared PANI nanofiber using high gravity chemical oxidative polymerization (HGCOP) method in large scale, and the products have good morphology and conductivity within wide temperature range and concentration range.15 The HGCOP method is based on the rapid mixing polymerization, but HGCOP has some advantages over the rapid mixing polymerization, including the potential to industrial production, intermittent or constant operation, good product performance, ease for operation, etc. This paper will report the synthesis of PANI/multiwalled carbon nanotubes (MWNTs) nanocomposites by HGCOP. To investigate the influence of rotating packed bed (RPB) on the nanocomposites, a stirred tank reactor (STR) was also used for comparison. The characterization results suggest that RPB is suitable for the preparation of nanocomposites in large scale. In addition, we expect that this method could be appropriate for preparing other polymer nanocomposites.

2. EXPERIMENTAL DETAILS 2.1. Reagents. All the reagents used in this work were of analytical grade except for MWNTs. The MWNTs (FloTube 9000, purity ≥95 wt %, 20 nm in diameter, 10 μm in length) used in this work were provided by CNano Technology Ltd. Anline (Tianjin Fuchen Chemical Reagent Company, China) was distilled under vacuum and stored in a refrigerator prior to use. Ammonium peroxydisulfate (APS, (NH4)2S2O8), hydrochloric acid (HCl), sulphuric acid (H2SO4), tetrahydrofuran (THF), methanol, and andanhydrous alcohol were purchased from Beijing Chemical Works (China). The para-phenylenediReceived: Revised: Accepted: Published: 3811

October 18, 2011 January 10, 2012 February 8, 2012 February 8, 2012 dx.doi.org/10.1021/ie202394c | Ind. Eng. Chem. Res. 2012, 51, 3811−3818

Industrial & Engineering Chemistry Research

Article

Figure 1. Chemical routes for the modification of MWNTs.

procedure so as to keep the reaction at 20 °C. Then, the asprepared solutions were pumped into RPB by two peristaltic pumps (at 100 mL/min each one), premixed by a tee joint, and injected into the RPB (at a high-gravity level of 343 m/s2). The two solutions were mixed sufficiently through the RPB, and the mixture was collected with a conical flask and left undisturbed in the water bath for 25 min. At last, the green product was filtered and rinsed several times with deionized water and anhydrous alcohol and dried at 60 °C in an oven overnight. For comparison, another product was prepared by rapid mixing polymerization in a stirred tank reactor (STR, consisting of a three-neck flask and a stirrer) under the same operating conditions. In this procedure, the two solutions were poured into the STR and stirred vigorously using a magnetic stir bar. The stir was stopped when the color of the black mixture was somewhat blue which suggests the end of the induction period of polymerization.19 In order to investigate the impact of the modification of the MWNTs, the o-MWNTs were also used to prepare PANI/ MWNTs nanocomposites under the same conditions as those above. 2.4. Characterization. The morphologies and structures of the treated MWNTs and the nanocomposites were characterized by a JSM-6701F (JEOL, Japan) scanning electron microscope (SEM), H-800 (Hitachi) transmission electron microscopy (TEM), and JEM-3010 (JEOL) high-resolution transmission electron microscopy (HRTEM). The Fourier transform infrared (FTIR) analysis was carried out with a Nicolet model 8700 spectrometer (Nicolet Instrument Corporation, USA). UV−vis spectra were recorded between 200 and 800 nm on a Lambda 950 UV/vis spectrophotometer (PerkinElmer, USA) by dispersing the samples of PANI in water via ultrasonication for 30 min. The surface elements analysis of p-MWNTs and PANI/MWNTs were conducted using an ESCALAB 250 X-ray photoelectron spectroscopy instrument (XPS). The X-ray diffraction (XRD) analysis was performed with a powder X-ray diffractometer (Bruker D8 Advance, Germany) over a range from 5° to 50°. The samples of PANI/MWNTs nanocomposites powder were compressed into tablets under a pressure of 8.0 MPa on a DY-30 (Tianjin Keqi High and New Technology Company, China) desktop tablet pressing machine driven by motor. The electrical conductivity of each PANI tablet was determined using a four-probe technique on a SX1934 digital multimeter (Suzhou Telecommunication Factory, China).

amine and the thionyl chloride were purchased from Tianjin Guangfu Fine Chemical Research Institute and Xilong chemical Co., Ltd., respectively. 2.2. Chemical Modification of MWNTs. In order to increase the interfacing binding between the MWNTs and PANI, the MWNTs was surface modified by a series of processes. As shown in Figure 1, the detailed chemical process for the modification of MWNTs is like other literature:16−18 500 mg of MWNTs was previously treated with 100 mL of mixed acid (mixture of concentrated H2SO4/HNO3 = 3:1) at 60 °C and stirred for 12 h; the resultant suspensions were filtrated and washed thoroughly with deionized water until the pH value was close to 7 (Step a). Then the o-MWNTs were refluxed with 150 mL of thionyl chloride and 6 mL of N,N-dimethylformamide (DMF) for 24 h; the powders were separated by centrifugation and washed with tetrahydrofuran (THF) to get the a-MWNTs (Step b). After this, the a-MWNTs were stirred with 0.75 g of p-PDA at 120 °C in 150 mL of DMF for 72 h in a nitrogen atmosphere. The resulting products p-MWNTs were filtered and washed with DMF, methanol, 1.0 M HCl solution, and deionized water successively and dried in vacuum (Step c). 2.3. Synthesis of HCl doped PANI Grafted MWNTs Nanocomposites. The nanocomposites were synthesized by in situ oxidation polymerization of aniline in the presence of pMWNTs using RPB as a reactor. The whole experiment process is similar with other literature.15 The reaction apparatus is illustrated in Figure 2. Typically, 1.86 g (0.02M) of aniline

3. RESULTS AND DISCUSSION Figure 2. Schematic diagram of reaction apparatus (1, 5: reactants tank; 2, 6: peristaltic pump; 3, 7: valve; 4, 8: flowmeter; 9: RPB; 10: product tank; 11: outlet; 12: wire mesh packing).

3.1. The Effects of Reactors and the Surface Properties of MWNTs. The SEM images for the p-MWNTs and PANI/MWNTs nanocomposites were shown in Figure 3. From the Figure 3a, the tubes of p-MWNTs coil around each other with smooth surface and closed ends, forming a network. The diameter of p-MWNTs is about 10−20 nm, keeping no obvious change after modification. From Figure 3b, the nanocomposites exhibit smooth surface and the diameter increase to 30−50 nm

and 1.14 g (0.005M) of APS were dissolved in 100 mL of HCl (1.0 M), respectively. Then, various weight ratios of p-MWNTs were dissolved in the aniline solutions and ultrasonicated for 1 h. Circulator bath systems were provided for the whole 3812

dx.doi.org/10.1021/ie202394c | Ind. Eng. Chem. Res. 2012, 51, 3811−3818


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Figure 3. SEM images of (a) p-MWNTs and (b) PANI/MWNTs nanocomposites (MWNTs/aniline mass ratio: 1:10).

Figure 4. TEM images of p-MWNTs (in the top left inset) and PANI/MWNTs nanocomposites obtained in different reactors (A, C: RPB; B: STR), different MWNTs (A, B: p-MWNTs; C: o-MWNTs), and different MWNTs/aniline mass ratio (Left: 1:20; Right: 1:10).

after polymerization assuring that the PANI was coated on the surfaces of MWNT. The TEM images for as-synthesized nanocomposites by RPB are shown in Figure 4. For comparison, the TEM image of pMWNTs was inserted in Figure 4 as a contrast. As shown in

Figure 4A, the nanocomposites exhibit core−shell tubular structure with diameter 30−50 nm and all the shell of the nanocomposites is relatively smooth and uniform. When the pMWNTs content is lower, the excessive aniline polymerizes and forms some pure PANI nanofibers. The generation of the 3813



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Figure 5. HRTEM image of PANI/MWNTs nanocomposites obtained in different reactors (A, C: RPB; B: STR) and different MWNTs/aniline mass ratio (Left: 1:20; Right: 1:10).

To investigate the influence of the different MWNTs on the product, we prepared the nanocomposites using o-MWNTs in a RPB. In Figure 4C, the shell of the nanocomposites is much thinner (about 30 nm in diameter) and the surface is badly rough. Philip et al.18 reported that, in a well dispersed solution of carbon nanotube, the adsorption of aniline hydrochloride will be uniform, which can lead to the formation of a thicker uniform coating of PANI. Since p-MWNTs can be well dispersed in the HCl solution compared to o-MWNTs, a thick and uniform shell was formed. Besides, the phenyl amino groups on the surface of p-MWNTs can initiate polymerization, and the concentration of the monomeric species is higher on the surface of p-MWNTs. These two points can explain the formation of a thicker and more uniform coating on the surface of p-MWNTs in the polymerization process. From the HRTEM images shown in Figure 5, it is obvious that the nanocomposite exhibit core−shell structure, indicating

pure PANI nanofibers in the prepared process is unavoidable. The surplus PANI nanofibers can be dissolve in Nmethylpyrrolidone (NMP) or other solvents under certain conditions. At the same time, because of the covalent bonds between PANI and MWNTs, the nanocomposites can be remained. For comparison, the nanocomposites which were prepared by STR (Figure 4B) present poor dispersity and coating rate. Jiaxing Huang et al.20 point out that, in the induction period, if the aniline and APS molecules are evenly distributed, all the reactants are consumed at the mean time; secondary growth is suppressed, and this contributes to form uniform core−shell tubular structure. The mass transfer and micromixing were greatly intensified in RPB,21,22 and the even distribution can be easily realized. As a result, the products keep smooth and uniform nanofibers. On the other hand, using STR can only get poor morphology because of its weak micromixing. 3814



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that the PANI was attached strongly to the MWNTs. From Figure 5A and 5B, there is a distinct difference between RPB and STR. Apparently, the surface of the nanocomposites obtained by RPB look smoother and does not have much protuberants. As for RPB, when the MWNTs/aniline mass ratio increased from 1:20 to 1:10, the coating became smoother. However, there have no obvious differences for STR when the MWNTs/aniline mass ratio varied. It follows that higher concentration of p-MWNTs is easier for the formation of smooth surface when nanocomposites are prepared by RPB. In order to better observe the structure of the nanocomposites, a high magnification HRTEM image of PANI/ MWNTs obtained by RPB was shown in Figure 5C. The structure of MWNTs in the nanocomposites can be seen clearly. The protuberants on the surface (arrows in Figure 5C) may be due to the higher temperature and aniline concentration in our experiment, leading to fast nucleation and growth. 3.2. The Composition of PANI/MWNTs Nanocomposites. Figure 6 shows the FTIR spectrum of the MWNTs, o-

Figure 7. (a) FTIR spectra of PANI/MWNTs nanocomposites and pure PANI prepared by HGCOP. (b) Form of the I1580/I1480 values about PANI and PANI/MWNTs.

benzenoid rings around 1570 and 1490 cm−1, the C−N stretching mode at 1300 cm−1, respectively, are all observed in the two samples, which are similar to other literature.15,24,26 The bands at 798 and 1130 cm−1 can be attributed to the bending of C−H (out of plane) in benzene ring π-disubstituted and the stretching of CN (NquinoidN). Yijun Yu et al.24 studied the I1580/I1480 values of PANI and PANI/ MWNTs which revealed that the PANI in the MWNT/PANI nanocomposites is richer in quinoid units than the pure PANI. We also calculated this value, as shown in Figure 7b; the value of the nanocomposite is a little larger than the value of PANI. This reveals that the chain of polyaniline deposited on the surface of MWNTs has longer conjugation lengths, which is corresponding with the molecular structure of aniline groups on the surface of MWNTs shown in Figure 1. The UV−vis spectra of p-MWNTs and PANI/MWNTs were showed in Figure 8. The p-MWNTs have no absorbance in this Figure 6. FTIR spectra of (a) MWNTs, (b) o-MWNTs, (c) aMWNTs, and (d) p-MWNTs.

MWNTs, a-MWNTs, and p-MWNTs. Except the pure MWNTs, the rest of the MWNTs exhibit the peak at around 1700 cm−1 which corresponds to the CO stretching vibration of the carboxylic acid groups (Curve b in Figure 6).23 This indicates the existence of the carboxylic acid groups at the surface of the MWNTs. According to the curve c in Figure 6, the small peak at around 618 cm−1 indicates the C−Cl stretching vibration of the acyl chloride. The amide bond was confirmed by the peak at around 1575 cm−1which corresponds to the N−H bending vibration (amide II bond). The peaks at 1512 cm−1 (benzene ring CC) and 820 cm−1 (benzene ring 1, 4 asymmetric replace) show that the structure of the pMWNTs is exactly what we want. The PANI/MWNTs nanocomposites and pure PANI prepared by HGCOP are also characterized by FTIR. As shown in Figure 7a, all the samples show nearly identical numbers and positions of the main IR bonds. PANI has a strong absorption peak for N−H stretching vibration at 3430 cm−1. Three characteristic bands of the emeraldine salt of PANI, the CC stretching vibration of the quinonoid and

Figure 8. UV−vis absorbance spectra of PANI/MWNTs prepared by RPB (solid line) and p-MWNTs (dash line).

interval. For PANI/MWNTs, three main peaks were observed located at 336.1, 429.6, and 781.4 nm, which were attributed to π−π* transition of the benzenoid rings, polaron-π*, and π3815



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polaron transitions, respectively.25,27 Furthermore, the peaks around 400−420 nm and 788−860 nm are related to doping level and formation of polaron.25 These results suggest that the PANI coated on MWNTs is in the doped state. To study the covalent bond between the p-MWNTs and polyaniline matrix in the nanocomposites, the p-MWNTs and nanocomposites were subjected to the XPS examination and the results were shown in Figure 9 and Table 1.

other nitrogen in the PANI were shown in Figure 9b, and the relative content of the two peaks are almost equal. The detailed analysis of nitrogen was shown in Table 1. According to the calculation of XPS results, the content of aniline groups on the surface of p-MWNTs is 2.68%. The shift of NH almost remains the same position between the two samples, which from the other side of that MWNTs and PANI connect with amido bond. The peak at 401.8 eV derived from N and NH in the PANI matrix has a positive shift of 1.17 after the polymerization; this is because NH2 groups were changed to NC (quinonoid ring carbon) and the ability of attracting the electron of carbon is stronger than that of hydrogen. 3.3. The Structure of Nanocomposites Prepared by RPB. The crystallinity of the products was compared using Xray diffraction (XRD). Figure 10 shows the X-ray diffraction

Figure 10. X-ray diffraction patterns for PANI/MWNTs prepared by HGCOP in RPB with different MWNTs/aniline mass ratio ((a): pure PANI; (b): 1:100; (c): 1:20; (d): 1:10).

patterns of pure PANI and the resulting nanocomposites. All the patterns exhibit nearly the same position. For pure PANI, the Bragg diffraction peaks appear at 2θ = 14.8°, 20.5°, and 25.6° which indicate the resultant polymers are in the highly doped emeraldine salt form.27 The peaks at around 2θ = 25.6° and 43.1° are derived from the graphite-like structure of pMWNTs.17,28 The Bragg diffraction peaks of PANI and pMWNTs are overlapping at 2θ = 25.6°. The intensity of the strong peak at 2θ = 25.6° and the low peak at 2θ = 43.1° increases with the increase of the MWNTs/aniline mass ratio which suggests the increasing p-MWNTs content.29 At the same time, other major characteristic peaks of PANI have little change with the increase of MWNTs/aniline mass ratio. This phenomenon shows that the percentage crystallinity of the shell of the PANI/MWNTs nanocomposites is almost the same as for pure PANI molecules.14 3.4. The Conductivity of Nanocomposites. Carbon nanotube can enhance the electrical conductivity of polymeric nanocomposites, and the conductivity of the nanocomposites can be controlled by changing the content of the carbon nanotubes.30−32 The conductivity of PANI/MWNTs prepared using RPB and STR with different content of p-MWNTs is shown in Figure 11. With the increase of the content of pMWNTs, the conductivity of PANI/MWNTs keeps going up,

Figure 9. High-resolution XPS spectra of N1s for (a) p-MWNTs and (b) PANI/MWNTs.

Table 1. XPS Binding Energies and Relevant Bond Assignment for Nitrogen samples p-MWNTs PANI/ MWNTs

binding energy (eV)

bond assignment

shift (eV)

399.6 400.6 399.6 401.8

NH NH2 NH N and NH in the PANI matrix

0 0 0 +1.17

From Figure 9, N1s spectra of the two samples were both fit to two components. In Figure 9a, the peak at 399.6 eV from  NH is more intensive than the peak at 400.6 eV derived from NH2 and the relative surface of the former is obviously larger than the latter which indicated that some byproduct exists at the surface of MWNTs. For PANI/MWNTs, there is a peak at 399.6 eV from NH and a peak at 401.8 eV from all the 3816



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into cation radicals by losing electrons in the presence of APS and then react with cation radicals in solution. The monomer molecules keep polymerization like that. At last, a thick and uniform shell formed. When o-MWNTs were used for preparing PANI/MWNTs nanocomposites, the reaction process can be explained as shown in Figure 13. The aniline first adsorb on the surface of o-

Figure 13. The formation of PANI/MWNTs prepared by HGCOP method using o-MWNTs.

Figure 11. Variation of conductivity of PANI/MWNTs prepared using RPB and STR with different content of p-MWNTs.

MWNTs and then become into cation radicals and keep polymerization in the presence of APS. Because the adsorption is a physical process, the distribution of aniline on the surface of o-MWNTs is not very well which eventually leads to a thin and rough shell formed. By comparison, the phenylamino groups on the surface of MWNTs can act as some active sites which are vital to forming the uniform in the polymerization process.

and the conductivity of the nanocomposites prepared using STR exhibits lower conductivity on the whole. For the pure PANI, the conductivity of the two samples is almost the same. According to the TEM analysis before, when the content of the MWNTs is lower, the ability of STR’s dispersity is worse; hence, there is more pure PANI that have not coated on the surface of the MWNTs in the nanocomposites. Many researchers have proved that the MWNTs facilitate chargetransfer processes between the two components.33−35 Therefore, the dispersity of the MWNTs in the nanocomposites is vital to the conductivity. This is the reason why conductivity of nanocomposites prepared by RPB is higher. However, when the content of the MWNTs is higher, the conductivity of the two nanocomposites is very close. In this case, the MWNTs crosslink each other and form a conductive network. The contribution to the conductivity is dominated by MWNTs and the differences between the RPB and the STR are secondary. Therefore, according to the TEM images before, we suggest that, when the content of p-MWNTs is higher, the conductivity of the products prepared by the two methods will become close to each other, but the morphology of the nanocomposites made by PRB is much better, which means good coating rate and dispersity. 3.5. Mechanism Analysis. The growth mode of PANI on the surface of MWNTs can be divided into two ways: lamellar growth18 and granular growth.24 In this work, the growth mode of PANI belongs to lamellar growth, which can be seen from the HRTEM images in Figure 5. As shown in Figure 12, in the

4. CONCLUSION In summary, we successfully fabricated PANI/MWNTs nanocomposites with core−shell tubular structure using RPB as the reactor for the first time. The advantages of this method lie in the lower reaction time than most other research and the convenience for industrialization. From the comparison between RPB and STR, the results obtained by these two methods indicated that RPB is a more suitable reactor for the preparation of PANI/MWNTs nanocomposites than that of STR. The nanocomposites prepared using p-MWNTs have a more uniform shell compared to o-MWNTs. The conductivity of PANI/MWNTs prepared by RPB is higher than STR when the content of p-MWNTs is lower, but when the content of the p-MWNTs is higher, the conductivity of the two nanocomposites is very close. Our research provides a simple and fast method to prepare PANI/MWNTs nanocomposites and has potential applications in the synthesis of other polymer nanocomposites.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-64443134 (W.W.); +86-10-64449453 (G.W.C.). Fax: +86-10-64434784 (W.W.); +86-10-64449453 (G.W.C.). E-mail: [email protected] (W.W.); [email protected]. edu.cn (G.W.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly acknowledge financial support from the Natural Science Foundation of China (No. 21076021 and 20934001) and thank Wei-zhong Qian (Department of Chemical Engineering, Tsinghua University) for the MWNTs used in this work.

Figure 12. The formation of PANI/MWNTs prepared by HGCOP method using p-MWNTs.

whole course of reaction, the phenylamino groups on the surface of p-MWNTs and the aniline molecules first become 3817



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Multiwalled Carbon Nanotubes

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