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Amphiphilic Multiwalled Carbon Nanotube Polymer Hybrid with Improved Conductivity and Dispersibility Produced by Functionalization with Poly(vinylbenzyl)triethylammonium Chloride Eagambaram Murugan* and Vimala Gopi Department of Physical Chemistry, School of Chemical Sciences, University of Madras, Maraimalai Campus, Guindy, Chennai 600025, Tamil Nadu, India ABSTRACT: Amphiphilic multiwalled carbon nanotube polymer hybrids were prepared by functionalization of multiwalled carbon nanotubes (MWCNTs) with different loads of poly(vinylbenzyl)triethylammonium chloride (PVBTEAC) by surface initiated polymerization followed by quaternization. The resulting hybrids were characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, ζ potential measurements, four-probe electrical conductivity, scanning and high-resolution transmission electron microscopy, energydispersive spectroscopy, atomic force microscopy, and gel permeation chromatography. Among the 10 hybrids, 4 wt % amphiphilic MWCNT PVBTEAC hybrid showed an electrical conductivity 78 times higher than pristine MWCNTs. Its dispersibility in organic and aqueous phases was very high, and the dispersion was stable for 8 months.
’ INTRODUCTION The study of carbon nanotubes (CNTs) and its related materials are continuously growing since the landmark paper by Iijima in 1991. This is due to CNTs having excellent mechanical, electrical, and thermal properties due to their remarkable structure. They have potential applications in a variety of fields such as chemicalbiological sensors, nanoelectronic devices,1 hydrogen storage, field emitters, catalyst supports, nanotube-reinforced materials, and supercapacitors.2 Although the CNT composites have applications in numerous fields, the problem of effective dispersion of CNTs in polymers as well as its insolubility in solvents is still a continuous challenge. The insolubility of CNTs in aqueous/organic solvents is due to its strong intertube van der Waals attraction. Therefore, the improvement of dispersibility and solubility of CNTs in aqueous and organic phases has become a challenge, and hence any techniques that address this issue can enhance the applicability of CNTs. Generally, noncovalent or covalent functionalization of CNTs with organic molecules can improve the solubility. The noncovalent functionalization of CNTs was performed through dispersion with low molar mass or block copolymer surfactants,3 polymer wrapping, and polymer absorption4 methods, and in all cases the dispersion of polymers is achieved by in situ ring-opening polymerization or emulsion polymerization. The advantage of noncovalent attachment of organic molecules is that the nanotube structure and its electronic properties are not altered, but at the same time the surfactants and polymers that can be used for this method are limited. In contrast, covalent functionalization can be performed by either modification of carboxylic acid groups on the CNTs or direct addition of reagents to the sidewalls of the CNTs. r 2011 American Chemical Society
Diffferent methods have been adopted to anchor the polymers to the surface of CNTs, viz., esterification and amination, radical coupling, addition of nucleophilic carbenes, and cycloaddition of nitrenes.5 In all of the methods, the degree of functionalization density is low. However, high functionalization density is necessary to promote solubility since the attached organic molecules are very small compared with the tube length. A limitation in high functionalization is that the electronic and spectroscopic properties of CNTs are altered by conversion of sidewall carbon atoms from trigonal to tetrahedral bonding. To preclude this problem, polymer grafted CNT brushes came into the picture so as to facilitate the solubility of CNTs in aqueous/organic phases. A polymer brush is an assembly of polymer chains that are tethered by one end to a substrate, such as silicon wafer,6 gold, latex, and carbon black particles, and the other end with long polymer backbones.7 Specifically, polymer brushes with CNTs have received considerable attention because of their novel structure and properties.8 Modifying the CNTs by covalently tethering polymers has proven to be an excellent method to improve their dispersibility. In other words, polymer-grafted CNTs have fairly good dispersibility in various solvents and most importantly preserve the tube structure as well as the outstanding electronic properties of CNTs in the polymer/CNT composites. Therefore, grafting polymer molecules using surface-initiated polymerization is an attractive method for the preparation of CNT polymer brush. Received: May 23, 2011 Revised: August 25, 2011 Published: August 30, 2011 19897
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The Journal of Physical Chemistry C This technique was performed by growing the polymer chains through covalently bonding the surface of CNTs and has provided an excellent approach to enhance the solubility of CNTs with negligible loss of the electronic properties, dispersion stability, and wettability of CNTs in polymer composites. Shim et al.9 reported that polystyrene and poly(4-vinylpyridine) brushes can be functionalized with MWCNTs through solution polymerization. Further, they also observed that the said polymer fabrication derived from MWCNTs was dispersed only in hydrophobic solvent such as toluene. It may be mentioned here that the importance of solubilization of CNTs in aqueous/ organic phase has gained much attention due to its numerous applications, specifically in biomedical applications which include biosensors, antimicrobial, anticancer activity, nanoprobes, and nanotweezers, etc. Similarly, several conducting CNTpolymer composites, viz., CNTpoly(3-octylthiophene), CNT polyaniline (CNTPANI), and MWCNTsulfonated polyaniline (MWCNTSPAN),10 were reported. Herein, we report the covalent functionalization of MWCNTs with amphiphilic hybrid, viz., poly(vinylbenzyl)triethylammonium chloride (PVBTEAC). Although few studies on amphiphilic hybrid nanospheres and macromolecular nanowires were reported,11 amphiphilic nanotubes, particularly those based on covalent linkage of polymer building blocks, have been rarely studied. Hence, in this study, we are reporting the preparation of 10 amphiphilic MWCNTPVBTEAC hybrids by varying the feed ratios of functional monomer, i.e., vinylbenzyl chloride (VBC), and keeping the MWCNTs load constant through solution polymerization technique. The resulting amphiphilic MWCNTPVBTEAC hybrids with different functional loads of PVBTEAC were thoroughly characterized with spectral, thermal, microscopic, electrical conductivity, and dispersibility studies to identify the superior amphiphilic MWCNTPVBTEAC hybrid material with high functionalized yield, good dispersibility, and improved electrical conductivity. Particularly, our intention in this study is to focus the right composition of MWCNTs and amphiphilic PVBTEAC load to yield the smart amphiphilic MWCNTPVBTEAC hybrid which may be used for fabrication of electronic materials, sensors, and materials for biomedical applications.
’ EXPERIMENTAL SECTION Materials. MWCNTs with purity higher than 95% were obtained from Sigma-Aldrich. The diameter of MWCNT was 140 nm and the length 7080 μm. Hydrochloric acid (HCl, Merck), potassium permanganate (KMnO4, Merck), methylene chloride (CH2Cl2, Merck), tetrabutylammonium bromide (TBAB, Alfa aesar), acetic acid (CH3COOH, 99.8%, Merck), hydroquinone (Merck), 3-methacryloxypropyltrimethoxysilane (3-MPTMS, Alfa aesar), 2,20 -azobis(isobutyronitrile) (AIBN, Alfa aesar), and vinylbenzyl chloride (VBC, Sigma-Aldrich) were used as such on the reactions. Characterization. Fourier transform infrared spectra (FTIR) were recorded on a Bruker Tensor-27 FTIR spectrophotometer with OPUS software in the range of 4004000 cm1. The pellet for analysis was made by taking equal amounts of each amphiphilic MWCNTPVBTEAC hybrid and KBr (1:1 ratio). Similarly, MWCNTPVBTEAC hybrids were used for the thermogravimetric analysis (TGA) and Raman studies. The TGA was carried out on SDT Q600 V20.5 Build 15 instrument at a heating rate of 10 °C/min from 50 to 800 °C under nitrogen atmosphere.
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Raman spectra were recorded on a Witec Confocal Raman instrument (CRM 200) with argon ion laser (514.5 nm). The molecular weights of MWCNTPVBC and MWCNT PVBTEAC were measured with Breeze GPC (gel permeation chromatography) equipped with 510 differential refractometer and Viscotek T50 differential viscometer. The MWCNTPVBC and MWCNTPVBTEAC dissolved in tetrahydrofuran (THF) was injected at a flow rate of 1.0 mL/min. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) measurements were carried out on a HITACHI S-3000H scanning electron microscope instrument interfaced with EDS DX-4 energy diffraction spectrometer, and both analyses were performed using the amphiphilic MWCNTPVBTEAC hybrid through accelerating voltage of 2 kV. That is, the samples for analyses were prepared by taking equal amounts and were spread on the surface of double-sided adhesive tape, one side being already adhered to the surface of a circular copper disk pivoted by a rod, and the spread samples were sputtered with gold prior to SEM observation. After the SEM observation, using the amphiphilic MWCNTPVBTEAC hybrid, the elemental analysis was also performed with EDS and the percentage of elements was observed (semiquantitative). High-resolution transmission electron microscopy (HRTEM) analysis was performed on JEOL 3010 transmission electron microscope operating at 200 kV. The sample to be analyzed was initially sonicated with acetone for a few minutes and then one drop of each sample (suspension) was placed on a glow discharged carbon-coated grid, and the sample used for HRTEM observation after evaporating the solvent. The electrical conductivity of MWCNT was measured using a fourprobe resistivity/Hall measurement system (HL5500PC, BioRad). Sheet samples of 4060 μm thickness were prepared by pumping 5 mg of MWCNTs between two iron plates at a pressure of 150 KN/cm2. The bulk electrical conductivity of each amphiphilic MWCNTPVBTEAC hybrid was measured at room temperature using a programmable curve tracer (Sony Tektronix 370A). Specimens were polished on both sides into a thickness of 1 mm, and a very small amount of silver paste (of thickness about 0.05 mm) was applied on the sample surface to reduce the contact resistance between the samples and electrodes. To minimize any potential problems associated with silver paste, the samples were heated at 40 °C to remove solvent quickly. Then, the edges of the samples were ground again to remove silver paste attached on them. The tapping mode of atomic force microscopy (AFM) images was taken with Agilent technologies (Digital instrument) using silicon tips. The dispersibility of all the amphiphilic hybrids were studied commonly with water, and especially 4 wt % MWCNTPVBTEAC was specifically examined with different organic solvents. Synthesis. Functionalization of MWCNTs. Pristine MWCNTs (50 mg) and 15 mL of CH2Cl2 were taken in a 100 mL roundbottomed flask, and the mixture was dispersed by Cole Parmer ultrasonication for 10 min. Then, 0.25 g of TBAB, dissolved in 5 mL of H2O, 5 mL of acetic acid, and 0.065 g of KMnO4 dissolved in 5 mL of H2O, was added to it.9bd Then the mixture was stirred vigorously at 25 °C for 48 h. The mixture was then diluted with 1000 mL of deionized water, and the resulting product was filtered under vacuum by 0.2 μm Teflon membrane. The dispersion, washing, and centrifugation of the resulting materials were repeated continuously until the filtrate showed a value of pH 7 (at least 10 cycles were required). After vacuum drying the filtrate, 0.048 g of hydroxyl group functionalized MWCNT (MWCNTOH) was obtained. 19898
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Scheme 1. Synthesis of MWCNTPVBTEAC
Synthesis of MWCNTPVBTEAC. MWCNTPVBTEAC was synthesized by three different steps which included (1) coupling of 3-MPTMS onto the MWCNTOH, (2) functionalization of VBC onto MWCNT3-MPTMS, and (3) quaternizaion of MWCNTPVBC. In the first step, 0.05 g of MWCNTOH
was taken in a 100 mL round-bottomed flask and dispersed in 20 mL of toluene by ultrasonication for 10 min. An excess of 3-MPTMS together with 0.5 g of hydroquinone was added, and the suspension was refluxed under N2 at 100 °C for 12 h. The resulting suspension was washed repeatedly with methanol to 19899
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Figure 1. FTIR spectra of (a) pristine MWCNTs, (b) MWCNTOH, (c) MWCNT3-MPTMS, (d) MWCNTPVBC, and (e) MWCNT PVBTEAC.
remove the unreacted MPTMS and hydroquinone and then filtered, to obtain MWCNT3-MPTMS. From this, 0.025 g (MWCNT3-MPTMS) was taken in a 100 mL round-bottomed flask; to that 10 mL of toluene was added, and the mixture ultrasonicated for 10 min. After the dispersion, 0.6 103 mol (0.1 g) of VBC was dissolved in the reaction mixture and 0.01 g of AIBN was added to the reaction vessel. The mixture was then allowed for solution polymerization through stirring under N2 atmosphere at 70 °C for 24 h. After polymerization, the resulting solution containing MWCNTs functionalized with PVBC was washed with acetone followed by centrifugation of the suspension for at least 510 times to remove the unreacted VBC and homopolymer of PVBC. The resulting blackish powder was dried in a vacuum oven, and thus MWCNTPVBC was obtained. Similarly, the other nine different types of MWCNTPVBC hybrids were also prepared individually by varying the VBC load as 0.98 103 mol (0.15 g), 1.31 103 mol (0.2 g), and 1.63 103 mol (0.25 g) to 3.9 103 mol (0.6 g) with 0.025 g of MWCNTMPTMS; the other reagents and conditions were maintained as constant. All 10 MWCNTPVBC hybrids were converted into amphiphilic hybrids through quaternization reaction. For quaternization, 0.025 g of MWCNTPVBC was taken in a 100 mL round-bottomed flask and dissolved in 30 mL of dry toluene, followed by deaeration under N2 atmosphere and addition of 20 mL of triethylamine to the solution. Again, the reaction mixture was gently refluxed for 48 h under nitrogen atmosphere at 70 °C, and then the solvent was removed by rotary evaporator. The resulting quaternized product was centrifuged and dried under vacuum to obtain the MWCNTPVBTEAC (Scheme 1).
’ RESULTS AND DISCUSSION Ten types of MWCNTPVBTEAC were prepared by varying the feed ratio of VBC from 0.6 103 to 3.6 103 mol, keeping the MWCNT weight constant. The functionalization of OH, MPTMS, the quantuam of PVBC and PVBTEAC in each type of amphiphilic MWCNT hybrids were investigated using spectroscopic and microscopic techniques. The common functionalized samples such as MWCNTOH, MWCNTMPTMS, and
Figure 2. Raman spectra of (a) pristine MWCNTs, (b) MWCNT PVBC, and (c) MWCNTPVBTEAC.
their corresponding 10 types of amphiphilic MWCNTPVBTEAC hybrids were thoroughly characterized by spectroscopy, microscopy, thermal, electrical, and dispersibility techniques to determine the optimized MWCNTPVBTEAC hybrid with higher quantum of 19900
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The Journal of Physical Chemistry C PVBTEAC functionalized yield having increased electrical conductivity and dispersibility, and the observed results are discussed below: FTIR Study. The FTIR spectra of pristine MWCNTs, MWCNT OH, MWCNT3-MPTMS, MWCNTPVBC, and MWCNT PVBTEAC are shown in Figure 1ae, respectively. Although the FTIR spectra were recorded for all 10 types of MWCNTPVBC and MWCNTPVBTEAC hybrids, only the FTIR spectra of the representative hybrid, viz., 4 wt % MWCNTPVBC, and its amphiphilic MWCNTPVBTEAC hybrid are shown. Parts d and e of Figure 1 are the representative spectra of MWCNTPVBC and MWCNT PVBTEAC. The FTIR spectrum of pristine MWCNTs (Figure 1a) shows the stretching vibrations of the CH group at 2922 and 2851 cm1, and the other peaks at 1632 and 1380 cm1 are due to the stretching vibration of CdC and CC groups, respectively. The spectrum for MWCNTOH is shown in Figure 1b, and it shows new broad and intense peaks at 3369, 1716, and 1262 cm1 and these peaks due to OH, CdO, and CO groups indicate the surface functionalization of the -OH groups onto the MWCNTs. In the spectrum of silane-functionalized MWCNT (MWCNT3-MPTMS, Figure 1c), the addition of 3-MPTMS to MWCNTOH is established with different new peaks appearing at 1698, 1635, 1170, and 1084 cm1, these peaks are attributed to the stretching vibration of CdO, CdC, and SiOSi groups, respectively, and this in turn confirms the reaction of trimethoxy groups of 3-MPTMS with SiOH groups of silica and the formation of a greater number of 3-methacroxypropyls on the surface of the MWCNTs. In Figure 1d the intense peaks of CCl can be observed at 700 cm1, and the decreasing of the CH and CdC peak intensity at 2956 and 1635 cm1 confirmed the covalent functionalization of PVBC onto the surface of MWCNT 3-MPTMS. With a view to generate the amphiphilic character, the MWCNTPVBC derived from different loads of VBC (110 wt %) were quaternized with triethylamine yielding the corresponding amphiphilic MWCNTPVBTEAC hybrid. The generation of amphiphilic character or functionalization of PVBTEAC on MWCNTs has been established from FTIR. As a model, Figure 1e is the FTIR spectrum of representative MWCNTPVBTEAC (4 wt %), the new intense peak observed at 1154 cm1 attributed to the presence of CN+(str), the shift of CH stretching from 2956 to 2958 cm1, and the appearance of the more intense CdC (str) peak at 1631 cm1 confirming the functionalization of PVBTEAC onto the MWCNTs. Similar characteristic features were noticed in all of the other amphiphilic MWCNT hybrids; the only variation noticed in the comparative study is the peak intensity of CN+ is gradually increased from 1 wt % MWCNTPVBTEAC to 10 wt % MWCNT PVBTEAC and thus indicates the increased covalent functionalization of PVBTEAC. Normally, the peak intensity of CN+ is directly related to the quantum of functionalized yield.12 Raman Spectral Study. A Raman spectrum can provide qualitative and quantitative information about the structural change and change of electronic properties of MWCNTs due to functionalization of PVBC and PVBTEAC. Hence, all 10 types of amphiphilic MWCNTPVBTEAC hybrids were analyzed by Raman spectroscopy along with pristine MWCNTs and MWCNTPVBC. The observed ID/IG values for six samples along with the control are presented in Table 1. The spectra of pristine MWCNTs, the representative hybrid and its amphiphilic hybrid, viz., 4 wt % MWCNTPVBC and 4 wt % MWCNT PVBTEAC, are shown in Figure 2ac, respectively. Generally, in Raman spectra the peaks observed at 1331 and 1559 cm1 correspond to a defective carbon band due to disordered sp3hybridized carbons in the nanotube walls D-band and a graphite carbon band from the sp2-hybridized G-band of MWCNTs,
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Table 1. Results of MWCNTPVBTEAC electrical
MWCNT
PVBTEAC, wt % yield,a % ID/IG b ζ potential, mV conductivity, S/cm 1
59.2
1.20
5.70
8.20
2
60.5
1.28
15.40
19.40
3
66.2
1.60
23.60
29.90
4
68.8
1.80
40.00
40.00
5
69.0
3.30
35.70
35.20
6
71.7
2.50
26.20
20.00
14.90 7.20
3.50 3.10
7 8
a
9
3.90
2.80
10
7.80
1.80
Determined by TGA analysis. b Determined by Raman studies.
respectively. The area ratio of D-band to G-band of MWCNTs is a direct indication for the degree of modification of MWCNT. The calculated ID/IG ratio for pristine MWCNTs (Figure 2a), MWCNTPVBC (Figure 2b), and MWCNTPVBTEAC (Figure 2c) was 0.3, 1.2, and 1.8, respectively. This observation confirms the covalent functionalization of PVBC and PVBTEAC onto the MWCNTs. The quantum of covalent functionalization of organic molecules in CNTs is directly related to the damage of CNTs structure and thereby drastic disturbance in electronic properties, and this can be ascertained through increased values of ID/IG in the Raman spectrum.13 In fact, through this value only the nature of covalent or noncovalent functionalization on MWCNTs is identified. In our case, the enhancement of the ID/IG ratio from 0.3 (pristine MWCNTs) to 1.2 for MWCNT PVBC hybrid sufficiently confirmed the higher degree of covalent functionalization of PVBC, and similarly further enhancement to 1.8 supported the covalent functionalization of PVBTEAC. The ID/IG values (Table 1) revealed that on increasing the VBC load the covalent functionalization yield of PVBC and PVBTEAC also increased and thus reflected increased ID/IG values. In the Raman analysis, ID/IG increased with respect to the PVBTEAC load, at the same time after the 4 wt %, although the ID/IG values and quantum of functionalized yield determined through TGA increased continuously, their ζ potential and electrical conductivity sharply decreased from 5 to 10 wt % of hybrids. This observation indicated that, from 5 to 10 wt %, the salient features of MWCNTs,.viz., the electronic properties, were affected largely due to structural change and hence further loading of PVBTEAC might not be good, although they produced more functionalized yield. Themogravimetric Analysis. TGA was performed for all of the samples to quantitative determination of functionalized molecules onto the MWCNTs, and the results are presented in Table 1. The weight-loss curves for pristine MWCNTs; MWCNTOH; MWCNT3-MPTMS; the representative hybrid, viz., MWCNTPVBC; and its 4 wt % amphiphilic MWCNTPVBTEAC hybrid are shown in Figure 3. For pristine MWCNTs (Figure 3a), no weight loss was noticed up to 800 °C. For MWCNTOH (Figure 3b), 3.0% weight loss was observed at 200300 °C due to decomposition of hydroxyl groups. For MWCNT3-MPTMS hybrid (Figure 3c), 7.98% weight loss was observed due to the decomposition of grafted silane units at 200400 °C.12a The representative hybrid 4 wt % MWCNTPVBC (Figure 3d) showed the 57.9% weight loss 19901
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Figure 3. TGA curves of (a) pristine MWCNTs, (b) MWCNTOH, (c) MWCNT3- MPTMS, (d) MWCNTPVBC, and (e) MWCNT PVBTEAC.
Figure 4. ζ potential vs MWCNTPVBTEAC content (wt %) at pH 7.
from 300 to 420 °C, and this in turn confirmed the higher degree of PVBC functionalization onto the MWCNTs. The 4 wt % amphiphilic MWCNTPVBTEAC hybrid (Figure 3e) showed about 68.8% of weight loss from 200 to 420 °C, which proved decomposition of quaternary ammonium groups.14 In other words, it proved that this particular 4 wt % amphiphilic MWCNTPVBTEAC hybrid contained a high load of functionalized yield with improved conductivity (Table 1). The other amphiphilic hybrids with more than 4 wt % loading showed reduced electrical conductivity, and hence they were not suitable for fabrication of conducting materials. ζ Potential Measurements. To study the aqueous dispersion stability of MWCNTs, the ζ potentials were measured. The ζ potentials for all 10 amphiphilic MWCNTPVBTEAC hybrids at pH 7 were measured, and the values have been presented in Table 1. Here too, the ζ potentials for pristine MWCNTs and MWCNTOH were +20 and 9.5 mV, respectively. For MWCNTPVBTEAC, the observed potential values in Table 1 and Figure 4 showed that the ζ potential increased from 1 to 4 wt %
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Figure 5. Electrical conductivity of MWCNTPVBTEAC.
MWCNTPVBTEAC due to the appropriate/balanced load ratio of MWCNTs to poly(quaternary)ammonium ions and also the electrostatic attraction between the coated PVBTEAC and MWCNT. In contrast, the amphiphilic hybrids derived from 5 to 10 wt % of MWCNTPVBTEAC gave a decreased ζ potentials, although their quanta of poly(quaternary)ammonium ions (functionalized load) was high. That is, when the PVBTEAC load was increased from 5 to 10 wt %, the severity of the damage on MWCNTs gradually increased due to ratio of covalent functionalization of PVBTEAC and hence lowered the ζ potential. The decreased trend of ζ potential for the amphiphilic MWCNTPVBTEAC hybrids derived from 5 to 10 wt % mainly depends on the (i) the amount of damaged MWCNTs due to the PVBTEAC load, (ii) the change of CNTs structure due to higher quantum of covalent functionalization, and (iii) electrostatic repulsion between MWCNTs and coated polymer, thus leading to stabilization of the MWCNTs against van der Waals attractions. All of these factors greatly contributed to the reduction of electrical conductivity. In other words, from the results of Table 1 and Figure 4, it is understood that, from 1 to 10 wt %, the constant increase of ID/IG noticed in Raman spectrum and continuous weight loss of PVBTEAC observed in TGA strongly supported the increased covalent functionalization of PVBTEAC onto MWCNTs. Therefore, the amphiphilic MWCNT PVBTEAC hybrid derived from 4 wt % proved to be an appropriate/effective load without affecting much damage on MWCNTs and even the conductivity loss due to damage was compensated for by the addition of an optimized functional load of poly(quaternary)ammonium ion (PVBTEAC). Electrical Conductivity Measurements. To confirm the observations established in the ζ potential study, all 10 amphiphilic MWCNTPVBTEAC hybrids were again studied for the determination of electrical conductivity measurements using the four-probe method at room temperature. The observed values of electrical conductivity for each type of amphiphilic hybrids along with pristine MWCNTs and MWCNTPVBC are presented in Table 1. Electrical conductivity measurements could provide information about the geometric configuration of the CNTs that cannot be extracted by other measurements such as thermal conductivity or optical spectrum. The recent studies showed that the CNTs were dispersed into polymers to increase the electrical 19902
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Figure 6. SEM images of (a) pristine MWCNTs, (b) MWCNTPVBC, and (C) MWCNTPVBTEAC.
conductivity of the composites. Liu et al. and Linsunova et al. studied the electrical conductivity of MWCNTs in aqueous solution fluids. From Table 1 and Figure 5, it is seen that the conductivities of the pristine MWCNTs and MWCNTPVBC were determined as 5.2 and 6.5 S/cm, respectively. The conductivity of the 14 wt % of amphiphilic MWCNTPVBTEAC hybrid fell between 8.2 and 40 S/cm, which was 8 times higher than that of the pristine MWCNTs and 67 times higher than that of MWCNTPVBC. As discussed in the ζ potential measurements, the conductivity gradually increased from 1 to 4 wt % loading of MWCNT PVBTEAC. Above 4 wt % loading, the conductivity gradually decreased from 5 to 10 wt % of the PVBTEAC load; this trend again confirms the ζ potential observation. The maximum conductivity noticed in the 4 wt % of the amphiphilic MWCNT PVBTEAC hybrid was due to (i) an appropriate load ratio of MWCNT and PVBTEAC and (ii) orderly percolation of MWCNT in the PVBTEAC matrix thus forming good conduction network structure throughout the polymer matrix (Figure 6c). It is also known that MWCNTs are an excellent electron acceptor15 and PVBTEAC can be a good electron donor as well as electron acceptor. Therefore, it is inferred that the enhanced doping effect is associated with MWCNTs (or) effective charge transfer from PVBTEAC to the MWCNT through induced chemical bonding. This kind of perfect association/interactions, homogeneity in mixing, and compatibility enabled electron delocalization and enhanced the conductivity of the amphiphilic hybrid. On the contrary, in the case of 510 wt % of MWCNTPVBTEAC, although the functionalized load on
MWCNTs was high, due to formation of poor network structure, a lesser load of more conducting MWCNTs as compared to increased PVBTEAC load (due to increased functionalization), improper homogeneity, and lower compatibility has led to lower charge transfer/delocalization, thus reflecting the lower conductivity. SEM, EDS, HRTEM, AFM, and GPC. The surface morphologies of pristine MWCNTs, representative hybrid, viz., 4 wt % MWCNTPVBC, and its corresponding amphiphilic MWCNT PVBTEAC hybrid were studied with SEM, EDS, HRTEM, AFM, and GPC techniques. The SEM image (Figure 6a) suggested that the pristine MWCNTs entangled together with a distribution such as fine threads/ropes, whereas the image of MWCNT PVBC (Figure 6b) and MWCNTPVBTEAC (Figure 6c) showed clear, intense white patches distributed heterogeneously, and the existence of MWCNTs was not visibe. It confirmed that in this particular composition the MWCNTs was homogeneously mixed with a complete coverage of polymers onto the surface. It also strongly supported the functionalization of PVBC and PVBTEAC onto the MWCNTs. The EDS analysis is one of the most effective surface characterization techniques for identifying and quantifying (semiquantitatively) the surface elements. The percentage of elements in pristine MWCNTs, representative hybrid MWCNT PVBC, and its amphiphilic MWCNTPVBTEAC hybrid was determined with EDS, and the observed spectra along with the percentage of elements are shown in Figure 7ac, respectively. The results suggested that the percentage of carbon gradually decreased from pristine MWCNT to MWCNTPVBTEAC with the induction of a sizable percentage of newer elements in each 19903
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Figure 7. EDS spectra of (a) pristine MWCNTs, (b) MWCNTPVBC, and (c) MWCNTPVBTEAC. 19904
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Figure 8. HRTEM images of (a) pristine MWCNTs, (b) MWCNTPVBC, and (c) MWCNTPVBTEAC.
step of the functionalization. Comparison of Figure 7a,b, showed that the amount of carbon atom in MWCNTPVBC was lower (61.36%) than the pristine MWCNTs (99.97%) and it was still lowered in MWCNTPVBTEAC (57.91%, Figure 7c). Since the percentage of elements was determined by taking equal amounts of each sample, it is logical to compare the quanta of elements (semiquantitatively) in the analysis. The decrease in carbon content from pristine MWCNT to MWCNTPVBTEAC and appearance of newer peaks, viz., oxygen (25.57%), Si (61.3%), and chloride (6.94%) noticed in MWCNTPVBC (Figure 7b) confirmed the functionalization of MPTMS-based PVBC onto the MWCNTs. The generation of amphiphilic character was achieved through quaternization in MWCNTPVBC, and it is confirmed from the appearance of nitrogen peaks (6.21%) in MWCNT PVBTEAC. The functionalization of PVBC and PVBTEAC onto the MWCNTs was also visualized through the HRTEM images. The image of the pristine MWCNTs (Figure 8a) showed a smooth surface. In contrast, the image of MWCNTPVBC (Figure 8b) revealed that it was a cluster of isolated CNTs with heterogeneous thick white layers onto the surface, and this observation confirmed the functionalization of PVBC onto the MWCNTs. Similarly, the image recorded from MWCNT PVBTEAC (Figure 8c) suggested well-distributed/-dispersed MWCNTs with heterogeneous coverage of layers on the surface, thus proving the functionalization of PVBTEAC onto the
MWCNTs. In other words, the degree of debundled CNTs was more due to increased functionalization of PVBTEAC. AFM is an effective tool to explore the contour lengths and diameters of functionalized CNTs16 in tapping mode. The pristine MWCNTs (Figure 9a) aggregated into large bundles several micrometers in length and several tens to a hundred nanometers in diameter, whereas Figure 9b shows the AFM image of the representative 4 wt % amphiphilic MWCNT PVBTEAC hybrid. It indicated enhanced dispersion of MWCNT PVBTEAC and the contour length ranges from 200 to 2000 nm with the average height of about 23 nm, and this in turn supports the HRTEM observation. The degree of functionalization of PVBC and PVBTEAC onto MWCNTs was also measured through the increase of molecular weight determined by GPC analysis. The molecular weight for representative 4 wt % MWCNTPVBC hybrid and its amphiphilic MWCNTPVBTEAC hybrid were measured separately by GPC, and the results are shown in Figure 10a,b. The obtained results revealed that the molecular weight of MWCNTPVBC was 7878 Da (Mw) with the polydispersity index of 2.08. The molecular weight of 4 wt % MWCNTPVBTEAC was 8550 Da (Mw) with the polydispersity index at 2.59. Hence, the GPC data clearly showed enhanced molecular weight and thus confirmed the functionalization of PVBC and PVBTEAC onto the MWCNTs. Dispersibility of MWCNTPVBTEAC. CNTs readily aggregate in aqueous/organic solutions due to high surface energy, which makes it difficult to suspend them in liquids especially in 19905
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Figure 10. Molecular weights of (a) MWCNTPVBC and (b) MWCNTPVBTEAC.
Figure 9. AFM image of (a) pristine MWCNTs and (b) MWCNT PVBTEAC.
aqueous phase.17 Dispersed CNTs in aqueous/organic phase have remarkable applications in the field of medicinal chemistry, particularly as a sensor,18 in drug delivery, for example. Hence, several researchers attempted to debundle the MWCNTs through covalent/noncovalent functionalization to facilitate the dispersibility in aqueous and organic solvents. Of course, even if the dispersion of CNTs is achieved, sustaining the stability of dispersed MWCNTs without aggregation in aqueous and various organic solvents is a challenging task. To address this problem, 10 amphiphilic MWCNT hybrids, viz., MWCNTPVBTEACs with different amounts of PVBTEAC functionalized loads, were prepared by increasing the feed ratios of VBC, and subsequently they were dispersed in water to study the degree of dispersibility
without sonication under identical experimental conditions at ambient temperature. To ascertain the degree of dispersibility and stability, the respective MWCNTPVBTEAC solutions were periodically monitored under undisturbed condition up to 8 months. The dispersibilities of pristine MWCNT and representative 4 wt % amphiphilic MWCNTPVBTEAC hybrid were studied not only in aqueous phase but also in various organic solvents, viz., n-butyl chloride, toluene, THF, and dimethyl sulfoxide (DMSO), and the corresponding photographs are shown in Figure 11ae. The photograph of pristine MWCNTs dispersed in water (Figure 11a) showed that the MWCNTs settled down in water due to its higher surface energy, van der Waals force, and high aspect ratio.19 In contrast, the degree of dispersibility of MWCNTPVBTEACs derived from 1 to 10 wt % in aqueous phase gradually improved. In other words, the dispersibility increased with the quantum of PVBTEAC functionalized on MWCNTs. This is because, the increased PVBTEAC load increases the hydrophilic character and hence promotes the dispersibility. To check the dispersibility and stability, 4 wt % amphiphilic MWCNTPVBTEAC hybrid was dispersed in various organic solvents and compared with the pristine MWCNT and MWCNTPVBC. The results in n-butyl chloride, toluene, THF, and DMSO (Figure 11be) indicated that the functionalized MWCNTs partially dispersed in n-butyl chloride (Figure 11b), but homogeneous dispersibility was noticed in toluene, THF, and DMSO (Figure 11ce). In water, it gave a clear homogeneous solution confirming the effective dispersibility of MWCNTs. The dispersibility of MWCNT PVBTEAC depends on the polarity of the medium and the hydrophobic and hydrophilic character. The 4 wt % MWCNT PVBTEAC gained amphiphilic character, but in spite of that partial dispersibility was noticed in n-butyl chloride due to its lower polarity (1.0). In contrast, the other organic solvents such as toluene, THF, and DMSO facilitate the dispersibility of MWCNTs due to alkyl groups present in the amphiphilic hybrids. More specifically, the effective dispersibility observed in water (Figure 11f) is due to the increased polarity (10.2) as well as hydrophilic attraction due to poly(quaternary onium ions) in the amphiphilic hybrid. Thus the synergetic action of 19906
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Figure 11. Photograph of dispersibility measurements of MWCNTPVBTEAC: (a) pristine MWCNTs (b) n-butyl chloride, (c) toluene, (d) THF, (e) DMSO, and (f) water. The concentrations of all samples were fixed to be 5 mg/mL.
(i) attraction of high polar solvents with amphiphilic hybrid and hydrophilic attraction between MWCNTPVBTEAC and water due to enriched functionalization of poly(quaternary onium ions) led to improved dispersibility. Hence, the 4 wt % amphiphilic MWCNTPVBTEAC hybrid was stable for 8 months in aqueous and other organic solvents. Shim et al.9bd found that poly(styrene), poly(4-vinylpyridine), poly(acrylic acid), and poly(methacrylic acid) brushes were functionalized with MWCNT and noticed the dispersibility only in toluene. Xu et al. showed that poly(N-isopropylacrylamide) functionalized MWCNTs and dispersed in aqueous solution alone. Wang et al. reported that the MWCNTs functionalized with double-hydrophilic block copolymer and poly(ethylene oxide)-bpoly[2-(N,N-dimethylamino)ethylmethacrylate] showed better dispersibility in DMSO and ethanolwater mixtures. The carboxylic acid-terminated hyperbranched poly(ether-ketone) functionalized MWCNT composite enabled the dispersibility in polar solvents. Murugan et al.20 reported that the MWCNTs functionalized with amphiphilic poly(propyleneimine) dendrimer showed better dispersibility in aqueous and organic solvents. In contrast, in the present study, the 4 wt % amphiphilic MWCNTPVBTEAC hybrid contains both hydrophobic and hydrophilic properties which enabled dispersion of the MWCNTs effectively in aqueous/ organic media with improved conductivity.
’ CONCLUSIONS Ten amphiphilic MWCNTPVBTEACs hybrids were prepared by different loads of VBC through surface initiated polymerization followed by quaternization. Among them 4 wt % amphiphilic MWCNTPVBTEAC hybrid was superior in terms of (i) high functionalized yield, (ii) higher electrical conductivity, and (iii) better dispersibility in aqueous and organic solvents. Functionalization of MWCNTs with PVBC and PVBTEAC was confirmed by FTIR analysis. The quantum and nature of the functionalization of MWCNTs with PVBTEAC was established through TGA and Raman studies, in which the percentage of decomposition of PVBTEAC in TGA decreased from 1 to 10 wt %, thus reflecting the increased trend of functionalization. Similarly, the increased ratio of ID/IG in the Raman spectrum from 1 to 10 wt % strongly supported the covalent functionalization of PVBTEAC. However, the ζ potential and electrical conductivity values steadily increased from 1 to 4 wt % of PVBTEAC load, but further loading decreased the conductivity. On the basis of TGA results the 4 wt % of MWCNTPVBTEAC was functionalized with 68.8% of PVBTEAC and confirmed the covalent functionalization of MWCNTs. It was supported by EDS results for nitrogen and chlorine, change of surface morphology from smooth to heterogeneous with thick white patches (or complete coverage of the PVBTEAC layer observed in SEM, HRTEM, and AFM), and enhancement of molecular weight 19907
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The Journal of Physical Chemistry C from MWCNTPVBC to MWCNTPVBTEAC recorded in GPC. Further, the 4 wt % amphiphilic MWCNTPVBTEAC hybrid has proved to be 78 times superior in electrical conductivity than pristine MWCNTs and 67 times higher than MWCNT PVBC as shown by four-probe conductivity measurements. It also showed better dispersibility in aqueous and various polar and nonpolar organic solvents, and the dispersion of MWCNT PVBTEAC was homogeneous and stable for 8 months. In a nutshell, these new 4 wt % amphiphilic MWCNTPVBTEAC hybrids have a potential application for fabrication of conducting materials in the fields of electrical, biomedical, and biosensors applications.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +91 4422202818/22202819. Fax: +91 44 22300488. E-mail:
[email protected];
[email protected].
’ ACKNOWLEDGMENT The financial assistance provided by DST under Nanomission Scheme (DST-NSTI), New Delhi, Government of India and Prof. C. N. R. Rao, Honorary President of the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India, are gratefully acknowledged. ’ REFERENCES (1) (a) Iijima, S. Nature 1991, 354, 56–58. (b) Kong, J.; Franklin, N. R. Science 2000, 287, 622–625. (c) Belavoine, F.; Schul, P.; Richard, C. Angew. Chem., Int. Ed. 1999, 38, 1912–1915. (d) Frank, P. S.; Poncharal, Z. L.; Wang, W. A. Science 1998, 280, 1744–1746. (e) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49–52. (2) (a) Mu, S.; Tang, H.; Qian, S.; Pan, M.; Yuan, R. Carbon 2006, 44, 762–767. (b) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512–514. (c) Li, L.; Wu, G.; Xu, B. Carbon 2006, 44, 2973–2983. (d) Wong, E. W.; Sheehan, P. E. Science 1997, 277, 1971–1975. (e) Zhao, C.; Hu, G.; Justice, R.; Schaefer, D. W.; Zhang, S.; Yang, M. Polymer 2005, 46, 5125–5132. (f) Boccaccini, A. R.; Cho, J.; Roether, J. A.; Thomas, B. J. C.; Minay, E. J.; Shaffer, M. S. P. Carbon 2006, 44, 3149–3160. (3) (a) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787–792. (b) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. Nano Lett. 2003, 3, 269–273. (c) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H. Science 2002, 297, 593–596. (d) Kang, Y.; Taton, T. A. J. Am. Chem. Soc. 2003, 125, 5650–5651. (4) (a) O’Connell, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y.; Haroz, E. Chem. Phys. Lett. 2001, 342, 265–271. (b) Star, A.; Stoddart, J. F.; Steuerman, D.; Diehl, M.; Boukai, A.; Wong, E. W. Angew. Chem., Int. Ed. 2001, 40, 1721–1725. (c) Star, A.; Steuerman, D. W.; Heath, J. R.; Stoddart, J. F. Angew. Chem., Int. Ed. 2002, 41, 2508–2512. (d) Gomez, F. J.; Chen, R. J.; Wang, D.; Waymouth, R. M.; Dai, H. Chem. Commun. (Cambridge, U. K.) 2003, 190–191. (e) Barraza, H. J.; Pompeo, F.; O’Rear, E. A.; Resasco, D. E. Nano Lett. 2002, 2, 797–802. (5) (a) Hill, D. E.; Lin, Y.; Rao, A. W.; Allard, L. F.; Sun, Y. P. Macromolecules 2002, 35, 9466–9471. (b) Shaffer, M. S. P.; Koziol, K. Chem. Commun. (Cambridge, U. K.) 2002, 2074–2075. (c) Wu, W.; Zhang, S.; Li, Y.; Li, J.; Liu, L.; Qin, Y. Macromolecules 2003, 36, 6286–6288. (d) Yao, Z. L.; Braidy, N.; Botton, G. A.; Adronov, A. J. Am. Chem. Soc. 2003, 125, 16015–16024. (6) (a) Milner, S. T. Science 1991, 251, 905–914. (b) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677–710. (c) Huang, W.; Skanth, G.; Baker, G. L.; Bruening, M. L. Langmuir 2001, 17, 1731–1736. (d) Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B. Macromolecules 1999, 32, 8716–8724.
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