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Facile and green method for polystyrene grafted multi-walled carbon nanotubes and their electroresponse
Colloids and Surfaces A: Physicochem. Eng. Aspects 396 (2012) 177–181
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Facile and green method for polystyrene grafted multi-walled carbon nanotubes and their electroresponse Huating Hu a , K.N. Hui b,∗∗ , K.S. Hui a,∗ , S.K. Lee c , W. Zhou d a
Department of Systems Engineering and Engineering Management, City University of Hong Kong, Hong Kong Department of Materials Science and Engineering, Pusan National University, Republic of Korea Energy Policy Research Center, Korea Institute of Energy Research, Republic of Korea d School of Engineering, Sun Yat-sen University, China b c
a r t i c l e
i n f o
Article history: Received 15 November 2011 Received in revised form 21 December 2011 Accepted 22 December 2011 Available online 30 December 2011 Keywords: Electrorheological fluid Functionalization Grafting Microsphere Multi-walled carbon nanotube
a b s t r a c t Polystyrene (PS) microspheres with a mean diameter of 69 nm were prepared by emulsion polymerization. The PS microspheres were successfully decorated to multi-walled carbon nanotubes (MWNTs) induced by benzoyl peroxide in water during a high temperature refluxing process under nitrogen atmosphere. The as-prepared multi-walled carbon nanotube/polystyrene (MWNT/PS) nanocomposites were characterized by FTIR spectroscopy, Raman spectroscopy, SEM, TEM, differential scanning calorimetry (DSC), and electrical resistance measurements. The results reveal that PS microspheres are mainly covalently grafted onto the walls of MWNTs. The PS-decorated MWNTs show high solubility in toluene and xylene without sedimentation after 24 h. With MWNTs in the hybrids, PS achieved a considerable increase in electrical conductivity and glass-transition temperature (Tg ). In particular, the obtained MWNT/PS composites were examined for electrorheological (ER) fluids, which showed thin and dense chains of particles after the application of an electric field. The facile and environmentally friendly technique presented in the present study could be an effective and promising method for the functionalization of MWNTs by other polymers. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) have attracted considerable interest from both the academia and industry since their discovery by Iijima [1]. CNTs have been reported to exhibit effective and promising applications in various areas such as sensors [2], transistors [3,4], devices [5], catalysts [6,7], bioluminescent probes [8,9], and high-performance composites [10] because of their remarkable mechanical, electrical, and thermal characteristics [11–13]. With their extraordinary structural and physical properties, CNTs have been chosen as ideal reinforcing materials in polymer nanocomposites [14,15]. However, because of their strong Van der Waals interactions, CNTs tend to form aggregated bundles in a matrix polymer, hindering the realization of the full potential of CNTs as reinforcing agents. Thus, functionalizing the surface of CNTs with functional groups or polymers can be an effective way to enhance the compatibility between CNTs and a polymer matrix. This process could achieve homogeneous dispersion of CNTs in
∗ Corresponding author. Tel.: +852 3442 4759; fax: +852 3442 0172. ∗∗ Corresponding author. Tel.: +82 051 510 2467; fax: +82 051 514 4457. E-mail addresses: [email protected] (K.N. Hui), [email protected] (K.S. Hui). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.12.066
the matrix and efficient load transfer across the polymer/CNTs interface. Various methods have been developed for the functionalization of CNTs with polymer, including layer-by-layer self assembly [16], physical adsorption [17], Friedel–Craft acylation pre-reaction [18], radical coupling [19], latex technology [20], and in situ polymerization [21]. Recently, benzoyl peroxide (BPO) was used to functionalize single-walled CNTs (SWNTs) with polypropylene in situ. Free radicals allow for the linkage of the SWNTs to the surrounding polypropylene matrix via a covalent bond, upon the decomposition of BPO during the high shear and hightemperature processing phase [22]. Furthermore, polymer/CNT composites prepared by various methods in the presence of CNTs have exhibited electrorheological (ER) behavior under an applied electric field when dispersed in insulating silicone oil [17,21,23–25]. However, most of the reported techniques to functionalize CNTs are usually conducted in toxic organic solvents. Moreover, these techniques involved tedious processes and multi-step reactions. As a result, exploring a green and facile technique for functionalization of CNTs with polymer remains a challenge. In the current study, a facile and environmentally friendly approach for the covalent graft of polystyrene (PS) microspheres onto the sidewall of multi-walled CNTs (MWNTs) induced by BPO in water during a high temperature refluxing process under nitrogen
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H. Hu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 396 (2012) 177–181
atmosphere is proposed. In addition, the ER effect of multi-walled carbon nanotube/polystyrene (MWNT/PS) nanocomposites was also examined. 2. Experimental 2.1. Materials MWNTs (OD: 30–50 nm) were supplied by Chengdu Organic Chemical Co. Ltd. (China). The MWNTs were pretreated by heating in a water-rich atmosphere at 225 ◦ C, followed by washing with HCl solution to remove impurities. Styrene monomer (St), sodium dodecyl sulfate (SDS), potassium persulfate (K2 S2 O8 , KPS), and BPO were supplied by Shanghai Chemical Company (China). St was purified with diluent NaOH solution to remove polymerization inhibitors before use. Other reagents were obtained from commercial sources and used as received.
2.4. Characterization Fourier transform infrared (FTIR) spectra were recorded on a PerkinElmer spectrometer using KBr pellets. The morphologies and structures of the samples were examined by transmission electron microscopy (TEM) using FEI Tecnai G20 instrument at 200 kV and field emission scanning electron microscopy (Nova 400 Nano SEM). Raman spectra were recorded on a laser confocal Raman microspectroscopy (LabRAM HR 800 UV, HPRIBA JOBIN YVON) with 632.8 nm laser excitation. The glass transition temperature of PS and MWNT/PS nanocomposites was determined using simultaneous DTA-TG apparatus (SHIMADZU, DTG-60). The samples were heated to 220 ◦ C at a heating rate of 10 ◦ C/min and cooled to ambient temperature. The samples were then subjected to a second heating cycle with a heating rate of 10 ◦ C/min in a temperature range of 25–180 ◦ C. Solubility measurements were conducted by dispersing the samples into toluene and xylene by an ultrasonic generator. Electrical conductivity of the nanocomposites was measured by a four-probe method using pressed disc-type specimens.
2.2. Preparation of PS microspheres Typically, PS microspheres were prepared by an emulsion polymerization method. About 0.1 g SDS was dispersed in 60 ml deionized water (DI) water in a 250 ml round-bottom flask. A total of 10 g St was added into the flask, followed by 10 min of ultrasonic irradiation. Then, 0.03 g KPS was added into the mixture. The reaction mixture was refluxed at 80 ◦ C for 6 h under a nitrogen atmosphere. At the end of the reaction, the obtained emulsions were de-emulsified with anhydrous CaCl2 . The precipitate was filtered, washed several times with methanol and DI water, and dried in a vacuum oven at 60 ◦ C overnight as white powder. 2.3. Preparation of MWNT/PS nanocomposites Approximately 20 mg purified MWNTs were dispersed in 150 ml DI water containing 0.5 g BPO and 0.1 g PS microspheres in a 250 ml three-neck round-bottom flask under continuous ultrasonication for 15 min. Afterwards, the mixture was refluxed at 80 ◦ C for 5 h under a nitrogen atmosphere. Finally, the mixture was cooled to room temperature, filtered through a 0.22 m PTFE membrane, and then washed with DI water and ether to remove the impurities. The purified product was dried in a vacuum oven at 60 ◦ C overnight as black powder.
2.5. ER measurements To prepare the ER fluid, the MWNT/PS nanocomposites were first dispersed in silicone oil (0.5 wt.%) by sonication. No stabilizers were added to the silicone oil. A DC high voltage source (600 V) was applied to the samples to observe the ER effect. The gap between the two parallel electrodes was fixed at 5 mm. The images of the ER fluids were obtained using a digital camera. 3. Results and discussion As a free radical initiator, BPO can be introduced into the processing stages of composites to facilitate generation of radical sites along polymer chains. This process enables the polymers to interact with the aromatic -electron system of CNTs, resulting in the formation of a covalent bond between the polymer chains and the surface of CNTs [22]. In the proposed system, PS microspheres were grafted onto the sidewall of MWNTs via two processing steps. First, PS microspheres with a mean diameter of 69 mm were prepared by emulsion polymerization. The PS microspheres were covalently decorated to the sidewalls of MWNTs induced by BPO in an aqueous medium during a high-temperature (100 ◦ C) refluxing process under nitrogen atmosphere. In this process,
Fig. 1. TEM images of (a) pure PS microspheres (inset shows the particle size distribution of as-synthesized PS microspheres) and (b) MWNT/PS nanocomposites (inset indicates a high-magnification TEM image of MWNT/PS repeatedly washed with chloroform).
H. Hu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 396 (2012) 177–181
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Fig. 2. SEM images of (a) purified MWNTs and (b) MWNT/PS.
characteristic vibration peaks appear in the curve. The FTIR spectrum of PS microspheres exhibits the characteristic adsorption peaks at 3025 cm−1 caused by the C H stretching vibrations of an aromatic compound. The adsorption peaks at 2921 and 2854 cm−1 are attributed to the CH2 symmetric and asymmetric stretching vibrations, respectively, whereas the ring C C stretching vibrations were observed in the region of 1430–1620 cm−1 . After the free polymer was removed by washing, some of the characteristic bands of PS could still be found in the IR spectra of MWNT/PS nanocomposites (Fig. 3), which further proves that PS microspheres attach to sidewall of MWNTs via chemical bonds. This result agrees well with that depicted in the inset of Fig. 1b. Raman spectroscopy is an effective tool to probe the structural characteristics of carbon-based materials, providing essential information for evaluating the covalent modification of MWNTs [26]. The Raman spectra of MWNTs and MWNT/PS nanocomposites are shown in Fig. 4. With respect to the Raman spectra of MWNTs, two intense features are assigned to the D band at 1328 cm−1 and the G band at 1583 cm−1 . The G band should be ascribed to the first order scattering of the E2g phonon of sp2 C atoms, whereas the D band is related to the sp3 states of carbon. The intensity ratios of the D–G band (ID /IG ) can be used as proof of disruption in the aromatic -electrons system of CNTs [22]. The ID /IG for the MWNT/PS composites is 1.20, which is larger than that of the purified MWNTs
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phenyl radicals generated from the thermal decomposition of BPO capture protons from the C H units on the PS main chains to create polymer radical sites. These polymer radical sites then react with the aromatic -electron system of CNTs, resulting in the direct covalent grafting of PS microspheres to the sidewalls of MWNTs via C C chemical bonds. A TEM image of PS microspheres synthesized by emulsion polymerization is illustrated in Fig. 1a. The inset shows the particle size distribution curve of the as-synthesized PS microspheres counted from the TEM image. The TEM image reveals the mean diameter of PS microspheres to be 69 nm, with a standard deviation of 6 nm. A TEM image of PS microsphere-grafted MWNTs nanocomposites is shown in Fig. 1b, which illustrates that many PS microspheres are densely decorated to the sidewall of MWNTs. Notably, the morphology of PS microspheres attached onto MWNTs is apparently slightly different from that of the initial ones in Fig. 1a. This condition is thought to be related to the following reasons: (1) MWNTs with super high curved surfaces can change the shape of PS microspheres when they are linked to the sidewall of MWNTs via chemical bonds; and (2) the high reflux temperature (100 ◦ C) can affect the morphology of PS microspheres during the grafting reaction process. After the MWNT/PS nanocomposites were repeatedly washed with chloroform to remove any physically absorbed PS microspheres (inset of Fig. 1b), the PS layers were still firmly wrapped around the MWNTs sidewall. This phenomenon suggests that the PS microspheres were mainly covalently grafted to the sidewall of MWNTs. The SEM images of purified MWNT and MWNT/PS nanocomposites are displayed in Fig. 2, from which it can be clearly seen that PS microspheres are successfully grafted onto the sidewalls of MWNTs. The MWNTs tend to aggregate together because of their hydrophobic nature and the Van der Waals interactions (Fig. 2a). However, the PS microsphere-decorated MWNTs in Fig. 2b seem to have a better dispersion performance, because the PS microspheres can reduce the Van der Waals interactions and prevent MWNTs from approaching each other. PS microsphere-decorated MWNTs with improved dispersion and wettability are expected to act as fine nanofillers within the polymer matrix to enhance their mechanical properties. FTIR spectra were analyzed to prove the successful graft of the PS microspheres to the MWNT sidewall and the interaction between the PS and MWNTs. Before testing, the MWNTs/PS nanocomposite samples were washed repeatedly with chloroform to remove the potential free polymer absorbed on the sidewall of MWNTs. The FTIR spectra of purified MWNTs, PS microspheres, and MWNT/PS nanocomposites are shown in Fig. 3. For the purified MWNTs, no
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Fig. 4. Raman spectra of MWNTs and MWNT/PS nanocomposites.
Fig. 5. DSC curves for PS and MWNT/PS nanocomposites.
(1.09). This result indicates that a number of sp2 hybridized carbons have been converted to sp3 hybridization carbons caused by the covalent attachment of PS chains onto the sidewall of MWNTs. To elucidate the role of MWNTs in the PS matrix, a study of the glass-transition region of the nanocomposites was conducted by differential scanning calorimetry (DSC). The DSC plots for PS and MWNTs/PS are shown in Fig. 5. The neat PS microspheres have a low glass-transition temperature (Tg ) of 101 ◦ C and a broad glasstransition region. The import of MWNTs increases the Tg of PS to 112 ◦ C. The movements of the polymer chains are hindered because the PS microspheres graft to the sidewall of MWNTs and the density of the tethered polymer chain is higher than that of pure polymer, resulting in higher glass transition temperature. The electrical conductivity of the MWNT/PS nanocomposites was measured by a four-probe method using pressed disc-type specimens at room temperature. The four-probe electric measurements of the specimens gave conductivities of 5.9 × 10−4 S/cm for the MWNT/PS nanocomposites, which indicate the as-formed composites are semiconductor hybrids. When only pure PS is considered, the conductivity is much lower than that of the MWNT/PS nanocomposites. An earlier research has reported that the conductivity of PS is approximately 1.0 × 10−12 S/cm [20]. This phenomenon reveals that the conductivity of PS was improved greatly by MWNTs in the hybrids.
MWNTs dispersed in toluene (a) and xylene (b) began to precipitate immediately after sonication (Fig. 6A). However, the PSdecorated MWNTs remained stable after a prolonged period (24 h). This behavior results from the formation of a network where the soluble PS chains extend into the solution and create high steric hindrance, preventing the MWNTs from approaching each other. Generally, ER fluids are a kind of suspension which consists of semiconductive solid particles (polymers or inorganic materials or hybrids) and low conducting oil (silicone oil, transformer oil, etc.) [27]. In this paper, PS microspheres with a mean diameter of 69 nm were successfully covalently grafted onto the sidewalls of MWNTs to form a novel “shish kebab” semiconductor heterostructure. The presence of MWNT enhances the conductivity of PS microspheres-based hybrids and thus influences their ability to be polarized. The ER behaviors of MWNT/PS nanocomposites are shown in Fig. 6B. Without an applied electric field, the nanocomposites were randomly dispersed in silicone oil in a random, liquid-like state [24,25,28]. However, when an electric field (1.2 kV/cm) was applied, the particles began to move rapidly toward the electrodes and formed many paralleled paths across the electrodes within seconds. The paths display the typical structures of ER materials with thin and dense chains of particles along the orientation of the applied electric fields [17,21,23–25,28]. The structure remained stable, as long as the electric fields were applied, indicating that the primitive conductivity and polarization property of MWNT/PS
Fig. 6. (A) Different samples with similar MWNT concentrations of 1 mg/ml: MWNTs in toluene (a) and xylene (b), MWNTs/PS in toluene (c) and xylene (d); (B) ER behaviors of MWNT/PS nanocomposites dispersed in silicon oil (0.5 wt.%) between two electrodes under an applied electric field of (a) 0 kV/cm and (b) 1.2 kV/cm.
H. Hu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 396 (2012) 177–181
hybrids mainly affect its ER behavior under an applied electric field. On the one hand, it was found that the MWNT/PS hybrids could congregate and move regularly, and then immediately form thin and dense chains of particles within seconds as a path for the mobile carrier transport along the direction of the applied electric fields. On the other hand, when the electric field was switched off, polarization of particles disappeared. After several minutes, because of the electrostatic interaction and gravity effect, the particles returned to random positions and especially some of them began to sedimentate. However, the particles can be dispersed well again and returned to the original form (as shown in Fig. 6B-a) by sonication. These properties imply that the as-prepared MWNT/PS hybrids may be a potential candidate for controlled targeted drug delivery and release [28]. 4. Conclusions PS microsphere-grafted MWNT nanocomposites were successfully prepared via a two-step process. PS microspheres disrupted the van der Waals interactions of MWNTs and caused a perfect dispersion of MWNTs in the matrix. MWNTs in the hybrids also markedly improved the thermal and electrical properties of PS. The resulting nanocomposites were examined as particle materials for ER fluids, showing thin and dense chain paths along the orientation of the electric field. The MWNT/PS nanocomposites serve as eminent ER materials and fine polymer nanofillers, indicating great promise for various prospective applications. Acknowledgments This project is funded by the ARG of City University of Hong Kong (project no. of 9667051) and SRG of City University of Hong Kong (project no. of 7002470). We also gratefully acknowledge that portions of this work were carried out at Faculty of Materials Science and Engineering, Hubei University, China. References [1] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [2] H.-Y. Chiu, P. Hung, H.W.C. Postma, M. Bockrath, Atomic-scale mass sensing using carbon nanotube resonators, Nano Letters 8 (2008) 4342–4346. [3] S. Kim, S. Ju, J.H. Back, Y. Xuan, P.D. Ye, M. Shim, D.B. Janes, S. Mohammadi, Fully Transparent Thin© Film Transistors based on aligned carbon nanotube arrays and indium tin oxide electrodes, Advanced Materials 21 (2009) 564–568. [4] K. Xiao, Y. Liu, P.a. Hu, G. Yu, Y. Sun, D. Zhu, n-Type field-effect transistors made of an individual nitrogen-doped multiwalled carbon nanotube, Journal of the American Chemical Society 127 (2005) 8614–8617. [5] A. Vijayaraghavan, S. Blatt, D. Weissenberger, M. Oron-Carl, F. Hennrich, D. Gerthsen, H. Hahn, R. Krupke, Ultra-large-scale directed assembly of singlewalled carbon nanotube devices, Nano Letters 7 (2007) 1556–1560. [6] P. Gajendran, R. Saraswathi, Enhanced electrochemical growth and redox characteristics of poly(o-phenylenediamine) on a carbon nanotube modified glassy carbon electrode and its application in the electrocatalytic reduction of oxygen, The Journal of Physical Chemistry C 111 (2007) 11320–11328.
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[7] Y. Tang, B.L. Allen, D.R. Kauffman, A. Star, Electrocatalytic activity of nitrogendoped carbon nanotube cups, Journal of the American Chemical Society 131 (2009) 13200–13201. [8] R. Yang, J. Jin, Y. Chen, N. Shao, H. Kang, Z. Xiao, Z. Tang, Y. Wu, Z. Zhu, W. Tan, Carbon nanotube-quenched fluorescent oligonucleotides: probes that fluoresce upon hybridization, Journal of the American Chemical Society 130 (2008) 8351–8358. [9] S.H. Yoshimura, S. Khan, H. Maruyama, Y. Nakayama, K. Takeyasu, Fluorescence labeling of carbon nanotubes and visualization of a nanotube-protein hybrid under fluorescence microscope, Biomacromolecules 12 (2011) 1200–1204. [10] L. Ci, J. Suhr, V. Pushparaj, X. Zhang, P.M. Ajayan, Continuous carbon nanotube reinforced composites, Nano Letters 8 (2008) 2762–2766. [11] M.F. Yu, O. Lourie, M.J. Dyer, K. Moloni, T.F. Kelly, R.S. Ruoff, Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load, Science 287 (2000) 637. [12] N.M. Piper, Y. Fu, J. Tao, X. Yang, A.C. To, Vibration promotes heat welding of single-walled carbon nanotubes, Chemical Physics Letters 502 (2011) 231–234. [13] R. Ramasubramaniam, J. Chen, H. Liu, Homogeneous carbon nanotube/polymer composites for electrical applications, Applied Physics Letters 83 (2003) 2928–2930. [14] M. Moniruzzaman, K.I. Winey, Polymer nanocomposites containing carbon nanotubes, Macromolecules 39 (2006) 5194–5205. [15] K.H. Kim, W.H. Jo, Synthesis of polythiophene-graft-PMMA and its role as compatibilizer for poly(styrene-co-acrylonitrile)/MWCNT nanocomposites, Macromolecules 40 (2007) 3708–3713. [16] H. Paloniemi, M. Lukkarinen, T. ritalo, S. Areva, J. Leiro, M. Heinonen, K. Haapakka, J. Lukkari, Layer-by-layer electrostatic self-assembly of single-wall carbon nanotube polyelectrolytes, Langmuir 22 (2006) 74–83. [17] H.J. Jin, H.J. Choi, S.H. Yoon, S.J. Myung, S.E. Shim, Carbon nanotube-adsorbed polystyrene and poly(methyl methacrylate) microspheres, Chemistry of Materials 17 (2005) 4034–4037. [18] R.R. Nayak, A.M. Shanmugharaj, S.H. Ryu, A novel route for polystyrene grafted Single© Walled Carbon Nanotubes and their characterization, Macromolecular Chemistry and Physics 209 (2008) 1137–1144. [19] Y. Liu, Z. Yao, A. Adronov, Functionalization of single-walled carbon nanotubes with well-defined polymers by radical coupling, Macromolecules 38 (2005) 1172–1179. [20] J. Yu, K. Lu, E. Sourty, N. Grossiord, C.E. Koning, J. Loos, Characterization of conductive multiwall carbon nanotube/polystyrene composites prepared by latex technology, Carbon 45 (2007) 2897–2903. [21] S.M. Kwon, H.S. Kim, S.J. Myung, H.J. Jin, Poly(methyl methacrylate)/multiwalled carbon nanotube microspheres fabricated via in situ dispersion polymerization, Journal of Polymer Science Part B: Polymer Physics 46 (2008) 182–189. [22] D. McIntosh, V.N. Khabashesku, E.V. Barrera, Benzoyl peroxide initiated in situ functionalization, processing, and mechanical properties of single-walled carbon nanotube–polypropylene composite fibers, The Journal of Physical Chemistry C 111 (2007) 1592–1600. [23] R. Jung, S.H. Yoon, M. Kang, H.-S. Kim, H.-J. Jin, Preparation of carbon nanotubesincorporated polymeric microspheres for electrorheological fluids, Current Applied Physics 8 (2008) 807–809. [24] K. Zhang, H.J. Choi, Carboxylic acid functionalized MWNT coated poly(methyl methacrylate) microspheres and their electroresponse, Diamond and Related Materials 20 (2011) 275–278. [25] K. Zhang, J.Y. Lim, H.J. Choi, Y. Seo, Core–shell structured carbon nanotube/poly(methyl methacrylate) composite and its electrorheological characteristics, Diamond and Related Materials 17 (2008) 1604–1607. [26] M. Farbod, S.K. Tadavani, A. Kiasat, Surface oxidation and effect of electric field on dispersion and colloids stability of multiwalled carbon nanotubes, Colloids and Surfaces A: Physicochemical and Engineering Aspects 384 (2011) 384. [27] J.W. Goodwin, G.M. Markham, B. Vincent, Studies on model electrorheological fluids, The Journal of Physical Chemistry B 101 (1997) 1961–1967. [28] S.-H. Baek, B. Kim, K.-D. Suh, Chitosan particle/multiwall carbon nanotube composites by electrostatic interactions, Colloids and Surfaces A: Physicochemical and Engineering Aspects 316 (2008) 292–296.
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Facile and green method for polystyrene grafted multi-walled carbon nanotubes and their electroresponse Huating Hu a , K.N. Hui b,∗∗ , K.S. Hui a,∗ , S.K. Lee c , W. Zhou d a
Department of Systems Engineering and Engineering Management, City University of Hong Kong, Hong Kong Department of Materials Science and Engineering, Pusan National University, Republic of Korea Energy Policy Research Center, Korea Institute of Energy Research, Republic of Korea d School of Engineering, Sun Yat-sen University, China b c
a r t i c l e
i n f o
Article history: Received 15 November 2011 Received in revised form 21 December 2011 Accepted 22 December 2011 Available online 30 December 2011 Keywords: Electrorheological fluid Functionalization Grafting Microsphere Multi-walled carbon nanotube
a b s t r a c t Polystyrene (PS) microspheres with a mean diameter of 69 nm were prepared by emulsion polymerization. The PS microspheres were successfully decorated to multi-walled carbon nanotubes (MWNTs) induced by benzoyl peroxide in water during a high temperature refluxing process under nitrogen atmosphere. The as-prepared multi-walled carbon nanotube/polystyrene (MWNT/PS) nanocomposites were characterized by FTIR spectroscopy, Raman spectroscopy, SEM, TEM, differential scanning calorimetry (DSC), and electrical resistance measurements. The results reveal that PS microspheres are mainly covalently grafted onto the walls of MWNTs. The PS-decorated MWNTs show high solubility in toluene and xylene without sedimentation after 24 h. With MWNTs in the hybrids, PS achieved a considerable increase in electrical conductivity and glass-transition temperature (Tg ). In particular, the obtained MWNT/PS composites were examined for electrorheological (ER) fluids, which showed thin and dense chains of particles after the application of an electric field. The facile and environmentally friendly technique presented in the present study could be an effective and promising method for the functionalization of MWNTs by other polymers. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) have attracted considerable interest from both the academia and industry since their discovery by Iijima [1]. CNTs have been reported to exhibit effective and promising applications in various areas such as sensors [2], transistors [3,4], devices [5], catalysts [6,7], bioluminescent probes [8,9], and high-performance composites [10] because of their remarkable mechanical, electrical, and thermal characteristics [11–13]. With their extraordinary structural and physical properties, CNTs have been chosen as ideal reinforcing materials in polymer nanocomposites [14,15]. However, because of their strong Van der Waals interactions, CNTs tend to form aggregated bundles in a matrix polymer, hindering the realization of the full potential of CNTs as reinforcing agents. Thus, functionalizing the surface of CNTs with functional groups or polymers can be an effective way to enhance the compatibility between CNTs and a polymer matrix. This process could achieve homogeneous dispersion of CNTs in
∗ Corresponding author. Tel.: +852 3442 4759; fax: +852 3442 0172. ∗∗ Corresponding author. Tel.: +82 051 510 2467; fax: +82 051 514 4457. E-mail addresses: [email protected] (K.N. Hui), [email protected] (K.S. Hui). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.12.066
the matrix and efficient load transfer across the polymer/CNTs interface. Various methods have been developed for the functionalization of CNTs with polymer, including layer-by-layer self assembly [16], physical adsorption [17], Friedel–Craft acylation pre-reaction [18], radical coupling [19], latex technology [20], and in situ polymerization [21]. Recently, benzoyl peroxide (BPO) was used to functionalize single-walled CNTs (SWNTs) with polypropylene in situ. Free radicals allow for the linkage of the SWNTs to the surrounding polypropylene matrix via a covalent bond, upon the decomposition of BPO during the high shear and hightemperature processing phase [22]. Furthermore, polymer/CNT composites prepared by various methods in the presence of CNTs have exhibited electrorheological (ER) behavior under an applied electric field when dispersed in insulating silicone oil [17,21,23–25]. However, most of the reported techniques to functionalize CNTs are usually conducted in toxic organic solvents. Moreover, these techniques involved tedious processes and multi-step reactions. As a result, exploring a green and facile technique for functionalization of CNTs with polymer remains a challenge. In the current study, a facile and environmentally friendly approach for the covalent graft of polystyrene (PS) microspheres onto the sidewall of multi-walled CNTs (MWNTs) induced by BPO in water during a high temperature refluxing process under nitrogen
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atmosphere is proposed. In addition, the ER effect of multi-walled carbon nanotube/polystyrene (MWNT/PS) nanocomposites was also examined. 2. Experimental 2.1. Materials MWNTs (OD: 30–50 nm) were supplied by Chengdu Organic Chemical Co. Ltd. (China). The MWNTs were pretreated by heating in a water-rich atmosphere at 225 ◦ C, followed by washing with HCl solution to remove impurities. Styrene monomer (St), sodium dodecyl sulfate (SDS), potassium persulfate (K2 S2 O8 , KPS), and BPO were supplied by Shanghai Chemical Company (China). St was purified with diluent NaOH solution to remove polymerization inhibitors before use. Other reagents were obtained from commercial sources and used as received.
2.4. Characterization Fourier transform infrared (FTIR) spectra were recorded on a PerkinElmer spectrometer using KBr pellets. The morphologies and structures of the samples were examined by transmission electron microscopy (TEM) using FEI Tecnai G20 instrument at 200 kV and field emission scanning electron microscopy (Nova 400 Nano SEM). Raman spectra were recorded on a laser confocal Raman microspectroscopy (LabRAM HR 800 UV, HPRIBA JOBIN YVON) with 632.8 nm laser excitation. The glass transition temperature of PS and MWNT/PS nanocomposites was determined using simultaneous DTA-TG apparatus (SHIMADZU, DTG-60). The samples were heated to 220 ◦ C at a heating rate of 10 ◦ C/min and cooled to ambient temperature. The samples were then subjected to a second heating cycle with a heating rate of 10 ◦ C/min in a temperature range of 25–180 ◦ C. Solubility measurements were conducted by dispersing the samples into toluene and xylene by an ultrasonic generator. Electrical conductivity of the nanocomposites was measured by a four-probe method using pressed disc-type specimens.
2.2. Preparation of PS microspheres Typically, PS microspheres were prepared by an emulsion polymerization method. About 0.1 g SDS was dispersed in 60 ml deionized water (DI) water in a 250 ml round-bottom flask. A total of 10 g St was added into the flask, followed by 10 min of ultrasonic irradiation. Then, 0.03 g KPS was added into the mixture. The reaction mixture was refluxed at 80 ◦ C for 6 h under a nitrogen atmosphere. At the end of the reaction, the obtained emulsions were de-emulsified with anhydrous CaCl2 . The precipitate was filtered, washed several times with methanol and DI water, and dried in a vacuum oven at 60 ◦ C overnight as white powder. 2.3. Preparation of MWNT/PS nanocomposites Approximately 20 mg purified MWNTs were dispersed in 150 ml DI water containing 0.5 g BPO and 0.1 g PS microspheres in a 250 ml three-neck round-bottom flask under continuous ultrasonication for 15 min. Afterwards, the mixture was refluxed at 80 ◦ C for 5 h under a nitrogen atmosphere. Finally, the mixture was cooled to room temperature, filtered through a 0.22 m PTFE membrane, and then washed with DI water and ether to remove the impurities. The purified product was dried in a vacuum oven at 60 ◦ C overnight as black powder.
2.5. ER measurements To prepare the ER fluid, the MWNT/PS nanocomposites were first dispersed in silicone oil (0.5 wt.%) by sonication. No stabilizers were added to the silicone oil. A DC high voltage source (600 V) was applied to the samples to observe the ER effect. The gap between the two parallel electrodes was fixed at 5 mm. The images of the ER fluids were obtained using a digital camera. 3. Results and discussion As a free radical initiator, BPO can be introduced into the processing stages of composites to facilitate generation of radical sites along polymer chains. This process enables the polymers to interact with the aromatic -electron system of CNTs, resulting in the formation of a covalent bond between the polymer chains and the surface of CNTs [22]. In the proposed system, PS microspheres were grafted onto the sidewall of MWNTs via two processing steps. First, PS microspheres with a mean diameter of 69 mm were prepared by emulsion polymerization. The PS microspheres were covalently decorated to the sidewalls of MWNTs induced by BPO in an aqueous medium during a high-temperature (100 ◦ C) refluxing process under nitrogen atmosphere. In this process,
Fig. 1. TEM images of (a) pure PS microspheres (inset shows the particle size distribution of as-synthesized PS microspheres) and (b) MWNT/PS nanocomposites (inset indicates a high-magnification TEM image of MWNT/PS repeatedly washed with chloroform).
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Fig. 2. SEM images of (a) purified MWNTs and (b) MWNT/PS.
characteristic vibration peaks appear in the curve. The FTIR spectrum of PS microspheres exhibits the characteristic adsorption peaks at 3025 cm−1 caused by the C H stretching vibrations of an aromatic compound. The adsorption peaks at 2921 and 2854 cm−1 are attributed to the CH2 symmetric and asymmetric stretching vibrations, respectively, whereas the ring C C stretching vibrations were observed in the region of 1430–1620 cm−1 . After the free polymer was removed by washing, some of the characteristic bands of PS could still be found in the IR spectra of MWNT/PS nanocomposites (Fig. 3), which further proves that PS microspheres attach to sidewall of MWNTs via chemical bonds. This result agrees well with that depicted in the inset of Fig. 1b. Raman spectroscopy is an effective tool to probe the structural characteristics of carbon-based materials, providing essential information for evaluating the covalent modification of MWNTs [26]. The Raman spectra of MWNTs and MWNT/PS nanocomposites are shown in Fig. 4. With respect to the Raman spectra of MWNTs, two intense features are assigned to the D band at 1328 cm−1 and the G band at 1583 cm−1 . The G band should be ascribed to the first order scattering of the E2g phonon of sp2 C atoms, whereas the D band is related to the sp3 states of carbon. The intensity ratios of the D–G band (ID /IG ) can be used as proof of disruption in the aromatic -electrons system of CNTs [22]. The ID /IG for the MWNT/PS composites is 1.20, which is larger than that of the purified MWNTs
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phenyl radicals generated from the thermal decomposition of BPO capture protons from the C H units on the PS main chains to create polymer radical sites. These polymer radical sites then react with the aromatic -electron system of CNTs, resulting in the direct covalent grafting of PS microspheres to the sidewalls of MWNTs via C C chemical bonds. A TEM image of PS microspheres synthesized by emulsion polymerization is illustrated in Fig. 1a. The inset shows the particle size distribution curve of the as-synthesized PS microspheres counted from the TEM image. The TEM image reveals the mean diameter of PS microspheres to be 69 nm, with a standard deviation of 6 nm. A TEM image of PS microsphere-grafted MWNTs nanocomposites is shown in Fig. 1b, which illustrates that many PS microspheres are densely decorated to the sidewall of MWNTs. Notably, the morphology of PS microspheres attached onto MWNTs is apparently slightly different from that of the initial ones in Fig. 1a. This condition is thought to be related to the following reasons: (1) MWNTs with super high curved surfaces can change the shape of PS microspheres when they are linked to the sidewall of MWNTs via chemical bonds; and (2) the high reflux temperature (100 ◦ C) can affect the morphology of PS microspheres during the grafting reaction process. After the MWNT/PS nanocomposites were repeatedly washed with chloroform to remove any physically absorbed PS microspheres (inset of Fig. 1b), the PS layers were still firmly wrapped around the MWNTs sidewall. This phenomenon suggests that the PS microspheres were mainly covalently grafted to the sidewall of MWNTs. The SEM images of purified MWNT and MWNT/PS nanocomposites are displayed in Fig. 2, from which it can be clearly seen that PS microspheres are successfully grafted onto the sidewalls of MWNTs. The MWNTs tend to aggregate together because of their hydrophobic nature and the Van der Waals interactions (Fig. 2a). However, the PS microsphere-decorated MWNTs in Fig. 2b seem to have a better dispersion performance, because the PS microspheres can reduce the Van der Waals interactions and prevent MWNTs from approaching each other. PS microsphere-decorated MWNTs with improved dispersion and wettability are expected to act as fine nanofillers within the polymer matrix to enhance their mechanical properties. FTIR spectra were analyzed to prove the successful graft of the PS microspheres to the MWNT sidewall and the interaction between the PS and MWNTs. Before testing, the MWNTs/PS nanocomposite samples were washed repeatedly with chloroform to remove the potential free polymer absorbed on the sidewall of MWNTs. The FTIR spectra of purified MWNTs, PS microspheres, and MWNT/PS nanocomposites are shown in Fig. 3. For the purified MWNTs, no
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Fig. 4. Raman spectra of MWNTs and MWNT/PS nanocomposites.
Fig. 5. DSC curves for PS and MWNT/PS nanocomposites.
(1.09). This result indicates that a number of sp2 hybridized carbons have been converted to sp3 hybridization carbons caused by the covalent attachment of PS chains onto the sidewall of MWNTs. To elucidate the role of MWNTs in the PS matrix, a study of the glass-transition region of the nanocomposites was conducted by differential scanning calorimetry (DSC). The DSC plots for PS and MWNTs/PS are shown in Fig. 5. The neat PS microspheres have a low glass-transition temperature (Tg ) of 101 ◦ C and a broad glasstransition region. The import of MWNTs increases the Tg of PS to 112 ◦ C. The movements of the polymer chains are hindered because the PS microspheres graft to the sidewall of MWNTs and the density of the tethered polymer chain is higher than that of pure polymer, resulting in higher glass transition temperature. The electrical conductivity of the MWNT/PS nanocomposites was measured by a four-probe method using pressed disc-type specimens at room temperature. The four-probe electric measurements of the specimens gave conductivities of 5.9 × 10−4 S/cm for the MWNT/PS nanocomposites, which indicate the as-formed composites are semiconductor hybrids. When only pure PS is considered, the conductivity is much lower than that of the MWNT/PS nanocomposites. An earlier research has reported that the conductivity of PS is approximately 1.0 × 10−12 S/cm [20]. This phenomenon reveals that the conductivity of PS was improved greatly by MWNTs in the hybrids.
MWNTs dispersed in toluene (a) and xylene (b) began to precipitate immediately after sonication (Fig. 6A). However, the PSdecorated MWNTs remained stable after a prolonged period (24 h). This behavior results from the formation of a network where the soluble PS chains extend into the solution and create high steric hindrance, preventing the MWNTs from approaching each other. Generally, ER fluids are a kind of suspension which consists of semiconductive solid particles (polymers or inorganic materials or hybrids) and low conducting oil (silicone oil, transformer oil, etc.) [27]. In this paper, PS microspheres with a mean diameter of 69 nm were successfully covalently grafted onto the sidewalls of MWNTs to form a novel “shish kebab” semiconductor heterostructure. The presence of MWNT enhances the conductivity of PS microspheres-based hybrids and thus influences their ability to be polarized. The ER behaviors of MWNT/PS nanocomposites are shown in Fig. 6B. Without an applied electric field, the nanocomposites were randomly dispersed in silicone oil in a random, liquid-like state [24,25,28]. However, when an electric field (1.2 kV/cm) was applied, the particles began to move rapidly toward the electrodes and formed many paralleled paths across the electrodes within seconds. The paths display the typical structures of ER materials with thin and dense chains of particles along the orientation of the applied electric fields [17,21,23–25,28]. The structure remained stable, as long as the electric fields were applied, indicating that the primitive conductivity and polarization property of MWNT/PS
Fig. 6. (A) Different samples with similar MWNT concentrations of 1 mg/ml: MWNTs in toluene (a) and xylene (b), MWNTs/PS in toluene (c) and xylene (d); (B) ER behaviors of MWNT/PS nanocomposites dispersed in silicon oil (0.5 wt.%) between two electrodes under an applied electric field of (a) 0 kV/cm and (b) 1.2 kV/cm.
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hybrids mainly affect its ER behavior under an applied electric field. On the one hand, it was found that the MWNT/PS hybrids could congregate and move regularly, and then immediately form thin and dense chains of particles within seconds as a path for the mobile carrier transport along the direction of the applied electric fields. On the other hand, when the electric field was switched off, polarization of particles disappeared. After several minutes, because of the electrostatic interaction and gravity effect, the particles returned to random positions and especially some of them began to sedimentate. However, the particles can be dispersed well again and returned to the original form (as shown in Fig. 6B-a) by sonication. These properties imply that the as-prepared MWNT/PS hybrids may be a potential candidate for controlled targeted drug delivery and release [28]. 4. Conclusions PS microsphere-grafted MWNT nanocomposites were successfully prepared via a two-step process. PS microspheres disrupted the van der Waals interactions of MWNTs and caused a perfect dispersion of MWNTs in the matrix. MWNTs in the hybrids also markedly improved the thermal and electrical properties of PS. The resulting nanocomposites were examined as particle materials for ER fluids, showing thin and dense chain paths along the orientation of the electric field. The MWNT/PS nanocomposites serve as eminent ER materials and fine polymer nanofillers, indicating great promise for various prospective applications. Acknowledgments This project is funded by the ARG of City University of Hong Kong (project no. of 9667051) and SRG of City University of Hong Kong (project no. of 7002470). We also gratefully acknowledge that portions of this work were carried out at Faculty of Materials Science and Engineering, Hubei University, China. References [1] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [2] H.-Y. Chiu, P. Hung, H.W.C. Postma, M. Bockrath, Atomic-scale mass sensing using carbon nanotube resonators, Nano Letters 8 (2008) 4342–4346. [3] S. Kim, S. Ju, J.H. Back, Y. Xuan, P.D. Ye, M. Shim, D.B. Janes, S. Mohammadi, Fully Transparent Thin© Film Transistors based on aligned carbon nanotube arrays and indium tin oxide electrodes, Advanced Materials 21 (2009) 564–568. [4] K. Xiao, Y. Liu, P.a. Hu, G. Yu, Y. Sun, D. Zhu, n-Type field-effect transistors made of an individual nitrogen-doped multiwalled carbon nanotube, Journal of the American Chemical Society 127 (2005) 8614–8617. [5] A. Vijayaraghavan, S. Blatt, D. Weissenberger, M. Oron-Carl, F. Hennrich, D. Gerthsen, H. Hahn, R. Krupke, Ultra-large-scale directed assembly of singlewalled carbon nanotube devices, Nano Letters 7 (2007) 1556–1560. [6] P. Gajendran, R. Saraswathi, Enhanced electrochemical growth and redox characteristics of poly(o-phenylenediamine) on a carbon nanotube modified glassy carbon electrode and its application in the electrocatalytic reduction of oxygen, The Journal of Physical Chemistry C 111 (2007) 11320–11328.
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