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Synthetic Metals 158 (2008) 900–907

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

ncapsulation of multi-walled carbon nanotubes by poly(4-vinylpyridine)nd its dispersion stability in various solvent media

anghyun Honga, Myunghun Kima, Chang Kook Hongb, Dongsoo Jungc, Sang Eun Shima,∗

Department of Chemical Engineering, Inha University, 253 Yonghyundong, Namgu, Incheon 402-751, South KoreaSchool of Applied Chemical Engineering, Chonnam National University, 300 Yongbong-dong, Puk-gu, Gwangju 500-757, South KoreaDepartment of Mechanical Engineering, Inha University, 253 Yonghyundong, Namgu, Incheon 402-751, South Korea

r t i c l e i n f o

rticle history:eceived 28 January 2008eceived in revised form 15 June 2008

a b s t r a c t

MWNTs were effectively functionalized with KMnO4 in the presence of a phase transfer catalyst atroom temperature. The hydroxyl functionalized MWNTs were reacted with a vinyl group-carrying silane-coupling agent and the terminal vinyl groups were used to fabricate poly(4-vinylpyridine) brushes by

ccepted 18 June 2008

eywords:arbon nanotubeshase transfer catalysisicroencapsulationodification

solution polymerization. Finally, P4VP-encapsulated MWNTs were obtained. The resulting materials wereanalyzed using TEM, TGA and their dispersion stability in various solvents was characterized by Turbiscan.It was found that the dispersion stability of P4VP-encapsulated MWNTs is dramatically improved in alco-holic medium due to the chemical affinity of P4VP with alcohol. This functionalization technique wouldprovide a facile route to prepare various polymer brushes on the surface of MWNTs in order to improvethe dispersion of MWNTs for potential applications.

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ispersion stability

. Introduction

Carbon nanotubes (CNTs) have brought considerable attentiono a wide range of science and technology since their discovery in991 [1–4]. Due to their superior mechanical, thermal, electrical,nd optical properties, it is expected that CNTs are expected to sub-titute a variety of classical materials in near future [5]. However, its crucial to find an economic and effective method to increase dis-ersibility in liquids or solid matrices against self-agglomeration.

n order to overcome the problem, chemical modification on theNTs’ surface is thought to be an effective way to improve theirettability and adhesion with host matrix materials [6]. Primary

hemical covalent functionalization of CNTs such as hydroxyl groupr carboxyl group is a promising practice to solve the problem. Onhe other hand, wrapping CNTs with polymer molecules using thenterfacial activity of CNTs is the method to treat the surface of CNTs

ith minimizing damage of CNTs. But their adhesive energy is notigh enough to transfer the stress between materials [7]. Therefore,

rafting polymer molecules using surface-initiated polymerizationn which growing polymer chains are covalently coupled onto theurface of CNTs has become the most adaptable pathway to improveispersion stability and wettability of CNTs in polymer nanocom-

∗ Corresponding author. Tel.: +82 32 860 7475; fax: +82 32 872 0959.E-mail address: seshim@inha.ac.kr (S.E. Shim).

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379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2008.06.023

© 2008 Elsevier B.V. All rights reserved.

osites [8]. The CNTs uniformly dispersed in solvents can be utilizedn direct conductive coating, further sol–gel process, and nanocom-osite manufacturing.

Recently, a novel chemical oxidation process using KMnO4 in theresence of a phase transfer catalyst (PTC) has been explored. Therexist a few publications in which PTCs are employed for the surfaceodification of fullerene or CNTs consisting of a planar wall and a

emispherical cap [9–11] It has been found that PTC methodologyossesses several advantages over conventional oxidation methodssing strong acids: (1) efficiency of oxidation drastically increases,2) product selectivity between carboxylic acid and hydroxyl groups improved, (3) reaction condition is very mild, and (4) dam-ge of CNTs is minimized [9]. Their tendency of locating at thenterface of two phases (liquid–liquid or solid–liquid) introducesontinuity between the two different phases [12]. In this experi-ent PTC assists to extract permanganate from the water phase to

rganic liquid phase. Permanganate is a powerful oxidizing agentn organic reaction [13,14]. Hydroxyl group with high selectivity isenerated by contact of CNTs in methylene chloride, KMnO4, aceticcid employing tetrapropyl ammonium bromide (TPABr) as a PTCissolved in water.

In our previous report, we devised a new method to synthe-

ize the surface-initiated polymer brush in which polymerizableerminal vinyl groups were covalently attached on the surfacef CNTs, named CNT-mer. The CNT-mer was used as a reactiveonomer in the subsequent in situ solution polymerization [15]. In

Metals 158 (2008) 900–907 901

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his work, hydroxyl groups were introduced on the CNTs by usingetrapropyl ammonium bromide (TPABr) as a PTC in conjunctionith potassium permanganate. By reacting with a vinyl group-

arrying silane-coupling agent, methacryloxypropyl trimethoxyilane (MPTMS), CNTs carrying vinyl groups were synthesizednd used to prepare poly(4-vinylpyridine)-encapsulated CNTs byhe followed solution polymerization. And dispersion stability ofoly(4-vinylpyridine)-encapsulated CNTs in various solvents was

nvestigated by a multiple light scattering method.

. Experimental

.1. Materials

The multi-walled CNTs (MWNTs) synthesized by a thermalhemical vapor deposition (CVD) method were used (approx-mately 95% pure, Iljin Nanotech, Korea). The CNTs have aiameter of 10–20 nm and length of 10–50 �m. Hydrochloriccid (36%), potassium permanganate (99.3%), methanol, ethanol99.9%), and methylene chloride (99%) were purchased fromamchun Chemical Co., Korea. Tetrapropyl ammonium bro-ide (TPABr) (99%) was purchased from Aldrich. Acetic acid

99.8%) and hydroquinone (99%) were purchased from Duk-an Chemical Co., Korea. 3-MethacryloxypropyltrimethoxysilaneMPTMS) was purchased from Shin-Etsu Chemical Co., Japan.,2′-Azobis(isobutyronitrile) (AIBN) (Junsei Chemical, Japan) wassed as received. 4-Vinylpyridine monomer (Acros Organics) wasurified using inhibitor-removal column (Aldrich) then storedt −4 ◦C.

.2. Synthesis

.2.1. Grafting of hydroxyl groups on the surface of MWNTsThe impurities such as amorphous carbon and metallic cata-

yst in the MWNTs were removed by 37% HCl at 60 ◦C for 4 h prioro experiment. 0.2 g raw MWNTs and 100 ml methylene chlorideere added into 250 ml flask and the mixture was dispersed byltrasonication (Bandelin Sonoplus) for 10 min in the flask. TPABr1 g) dissolved in 10 ml H2O, 10 ml acetic acid, and KMnO4 solution0.25 g KMnO4 dissolved in 5 ml H2O) were added in the flask. The

ixture was stirred vigorously at 25 ◦C for 24 h. The mixture wasashed with HCl and filtered with methanol through 0.2 �m Teflonlter (Adventec MFS). The methanol-washing step was repeated at

east four cycles. After vacuum drying the filtrate, 0.18 g hydroxylroup functionalized MWNTs (MWNTs-OH) were achieved.

.2.2. Modification of MWNTs-OH with MPTMSThe as-prepared hydroxyl functionalized MWNTs (0.05 g) were

ispersed in 40 ml toluene by ultrasonication for 10 min. Afterdding an excess of MPTMS together with 1.0 g hydroquinone, theuspension was refluxed under N2 at 100 ◦C for 6 h. After reaction,he resultant was washed repeatedly with methanol to removeesidual MPTMS and hydroquinone.

.2.3. Synthesis of P4VP-grafted MWNTsMPTMS-functionalized MWNTs (0.015 g) and 20 ml of toluene

ere charged into the 100 mL flask then exposed the mix-ure in ultrasonication for 10 min. After dispersion, 0.25–2.0 g of-vinylpyridine monomer was dissolved in the mixture. AIBN

0.0025–0.02 g, 1 wt% to 4-vinylpyridine) was injected into theask and the reaction vessel was heated to 70 ◦C to initiate solu-ion polymerization for 24 h with stirring under N2 atmosphere.fter polymerization, the solution was washed with acetone oncend free poly(4-vinylpyridine) (P4VP) was extracted out in Soxh-

2ftas

ig. 1. (a) TGA weight loss curves of P4VP-grafted MWNTs and (b) weight losses ofrafted polymer on the MWNTs for the different amount of 4-vinylpyridine.

et apparatus using boiling methanol for 36 h. Finally, P4VP-graftedWNTs were prepared and characterized.

.3. Characterization

The Raman spectra were recorded with Brucker RFS 100/S FT-aman instrument. The excitation wavelength was 1064 nm from ad/YAG laser power of 20 mW. Transmission electron microscope

TEM) micrographs were obtained from pristine and encapsulatedWNTs on a carbon film coated copper grid employing Philips CM

00, and energy dispersive spectroscopy (EDS) data was achievedrom EDS DX-4. Thermal gravimetric analysis (TGA) characteriza-ion was performed with a heating rate of 20 ◦C/min under nitrogentmosphere on a TA Q50 instrument. Measurement of the FT-IRpectra was performed using a Bruker 48 series FT-IR spectroscopy.

902 S. Hong et al. / Synthetic Metals 158 (2008) 900–907

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cheme 1. Synthetic route used to prepare (a) hydroxyl groups functionalized MW

ll infrared spectra recorded at 4 cm−1; 21 resolution and 16 scansere accumulated in 20 s for each specimen. Turbiscan (Fomula-

ion Turbiscan lab) was employed to investigate dispersion stabilityf raw and modified MWNTs in various solvents. The migration

bi(c

Fig. 2. TEM microphotographs of (a and b) raw MWN

) silane-coupling agent-grafted MWNTs, and (c) P4VP-encapsulated MWNTs.

ehavior of the CNTs in various liquids was monitored by measur-ng the backscattering and transmission of monochromatic light� = 880 nm) from the suspension. Suspensions in flat-bottomedylindrical glass tubes (70 mm height, 27.5 mm external diame-

Ts and (c and d) P4VP-encapsulated MWNTs.

Metals 158 (2008) 900–907 903

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er) were placed in the instrument and the transmission of lightrom suspensions was then periodically measured along the heightt room temperature. The transmission detector receives the lightoing out of the sample at 0◦ from the incident beam, while theackscattering detector receives the light scattered by the sam-le at 135◦ from the incident beam. The results from transmissionre presented as the sedimentation profile i.e., �transmission fluxersus time. The backscattering (BS) and transmittance (T) of inci-ent light are measured by calculating transport mean free pathf photons (l*) throughout the medium. Based on Mie theory,he BS and T can be obtained for a concentrated suspension asollows [16]:

S ≈(

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≈ exp(−r

l

)(2)

here r is an internal radius of a measurement cell. The photonransport mean free path (l*) and photon mean free path (l) areefined:

∗ = 2d

3˚(1 − g)QS(3)

= 2d

2˚QS(4)

here d, ˚, g, and QS denote a particle mean diameter, the volumeraction of a dispersed phase, asymmetry factor, and scattering effi-iency factor, respectively. The obtained results are presented as theedimentation profile i.e., �transmission versus time.

. Results and discussions

.1. Synthesis of poly4-vinylpyridine-encapsulated MWNTs

The entire procedure from the oxidation of MWNTs with per-anganate in the presence of a PTC to the synthesis of P4VP-graftedWNTs is illustrated in Scheme 1. The surface modification of car-

on nanotubes or fullerene using phase transfer catalysts can bearried out in very mild conditions so that strong acids are noteeded and the reaction takes place at room temperature. Fur-hermore, it offers a high concentration of surface acid groups andigh yield of hydroxyl group [10]. It is noted that the modificationeaction is carried out in mild conditions so that the damage ofWNTs can be minimized. During the surface modification reac-

ion in the presence of a PTC, the color of KMnO4 changed fromark purple to dark brown. This fact implies the transformation ofn + 7 to Mn + 4. The surface hydroxyl groups developed by a phase

ransfer catalyst, TPABr, are subsequently reacted with MPTMSo give vinyl group terminated carbon nanotubes, so-called CNT-

er. The vinyl groups in the CNT-mer are then used to fabricateP4VP brush by a simple in situ polymerization. Since the CNT-er can be considered as a polymerizable gigantic monomer due

o the reactive vinyl groups, the utilization of the CNT-mer woulde potentially advantageous in several aspects: (1) most vinylonomers can be polymerized to give wide varieties of polymer

rushes covalently attached to CNTs so that the choice of matrixolymer can be broadened for making CNT/polymer nanocompos-

tes; (2) the chain length of the polymer brushes can be controlled

y performing controlled/living free radical polymerizations with-ut attaching initiator fragment on CNTs as frequently practiced;3) block copolymers can be also attached by performing livingfree radical) polymerizations together with the CNT-mer; (4) bothn situ homogeneous and heterogeneous (suspension, emulsion,

lmat6

Fig. 3. EDS of (a) raw MWNTs and (b) P4VP-encapsulated MWNTs.

nd dispersion) polymerizations are possible to fabricate polymerrushes.

.2. Characterization of poly4vinylpyridine-encapsulated MWNTs

Thermogravimetric analysis (TGA) was performed for the quan-itative analysis of functionalization of MWNTs. Prior to TGA

easurements, freely adsorbed P4VP molecules were removed byoxhlet extraction using boiling methanol for 36 h. The weightoss curves of modified MWNTs where increasing amounts of 4VP

onomer was incorporated in the in situ polymerization processre shown in Fig. 1a. In our previous work, we confirmed thathe amount of silane-coupling agent grafted on MWNTs is about.9 wt% in this oxidation process [17]. For P4VP-grafted MWNTs,

904 S. Hong et al. / Synthetic Metals 158 (2008) 900–907

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tPootta

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tlM

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Fig. 4. FT-IR spectra of (a) raw MWNTs, (b) P4V

he main weight loss is obtained for the decomposition of grafted4VP in the temperature range of 300–420 ◦C. The weight lossesf grafted polymer on the MWNTs are shown in Fig. 1b. 14–20 wt%f P4VP relative to the mass of CNTs is successfully grafted usinghis method as illustrated in Scheme 1. Fig. 1b shows a tendencyhat the amount of grafted P4VP on the MWNTs linearly increasesccording to increasing the monomer concentration to the MWNTs.

Fig. 2 shows TEM morphologies of raw MWNTs (Fig. 2a and) and P4VP-encapsulated MWNTs (Fig. 2c and d). P4VP-grafted

ayer was observed in the latter images from c and d. The diameterf raw MWNTs is 10–20 nm. Observing the TEM image in Fig. 2c,he MWNTs’ average diameter is 9.0 nm but the average diame-

(ttoe

Fig. 5. Dispersion stability of (a–c) raw MWNTs and (d–f) P4VP-encapsulate

opolymer, and (c) P4VP-encapsulated MWNTs.

er of P4VP-encapsulated MWNTs is 11.2 nm. As a result, the P4VPayer with 1.1 nm thickness on average is formed on the surface of

WNTs.EDS was used to investigate the surface chemical state of

4VP-encapsulated MWNTs and determine the evidence of surfacencapsulation by the simple solution polymerization of MPTMS-rafted MWNTs and 4-vinylpyridine. EDS spectra of raw MWNTs

Fig. 3a) and P4VP-encapsulated MWNTs (Fig. 3b) are shown andhe analysis results are summarized in each table. The two spec-ra are similar with the naked eye, however, the analyzed amountsf carbon, nitrogen, oxygen, and silicon in raw MWNTs and P4VP-ncapsulated MWNTs reveals the different chemical states of each

d MWNTs dispersed in toluene, ethanol, distilled water, respectively.

S. Hong et al. / Synthetic Metals 158 (2008) 900–907 905

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sifg[eTMcatpsPuPgacc1wpiStae

3

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apeumWsotctdrceMtAMteeuPdasbdtMstp

Fig. 6. Turbiscan spectra of (a–c) raw MWNTs and (d–f) P4VP-enca

ample. It is noted that the inclusion of oxygen and silicon atomsn raw CNTs originates from the oxygen and silicon contaminantsrom silicon oxide substrates of the CVD process or inclusion of oxy-en atoms during the chemical purification process of crude CNTs18,19]. The amount of carbon atom is lower (79.3 wt%) in P4VP-ncapsulated MWNTs than the value (83.7 wt%) of raw MWNTs.he ratios of carbon to other components in P4VP-encapsulatedWNTs are calculated as follows: nitrogen 5.3:1, oxygen 15:1, sili-

on 198:1. On the contrary, the ratios of raw MWNTs are calculateds follows: nitrogen 7.4:1, oxygen 18.2:1, silicon 209:1. It indicateshe decrease of the amount of carbon related to the other com-onents in P4VP-encapsulated MWNTs. Considering the chemicaltructure of P4VP-encapsulated MWNTs as seen in Scheme 1c, the4VP molecules are chemically bound with a silicon–oxygen spacernit derived from MPTMS on the surface of MWNTs and grafted4VP has the nitrogen component in their side chain. The chemicalrafting of P4VP chains on MWNTs is verified by means of FT-IRnalyses as shown in Fig. 4 where P4VP-encapsulated MWNTs areompared with raw MWNTs and P4VP homopolymer. The P4VPhains have characteristics peaks of pyridine ring at 1415 and597 cm−1 as detected in Fig. 4(b) which shows a good agreementith a previous report (1413 and 1599 cm−1) [20]. The characteristic

yridine peaks are clearly observed in P4VP-encapsulated MWNTsn Fig. 4(c). It is noted that free P4VP molecules are removed byoxhlet extraction using boiling ethanol. Therefore, it is evidenthat the higher atomic contents of nitrogen, oxygen, and siliconre observed, which indicates the successful synthesis of P4VP-ncapsulated MWNTs using MPTMS as proposed in Scheme 1.

.3. Dispersion stability of pristine and modified MWNTs

The dispersion stability of raw and P4VP-funtionalized MWNTsn model polar (water) and non-polar (toluene) solvents is tested inig. 5. The photographic images were taken 2 months after making

he dispersion. Raw MWNTs are severely aggregated and com-letely sedimented after 2 weeks in water, ethanol, and toluene.4VP-grafted MWNTs have the similar behavior as raw MWNTs inater, toluene however, it has much improved dispersion stability

n ethanol even after 2 months. In order for a quantitative analysis,

lOoe

ted MWNTs dispersed in toluene, ethanol, distilled water in order.

novel optical analyzer, Turbiscan®, was employed to measure dis-ersion stability of raw and P4VP-encapsulated MWNTs in water,thanol, and toluene. The interpretation of the transmission profilesses the change in the light transmission caused by the sedi-entation of the MWNTs occurring over an entire sample cell.hen sedimentation takes place in the suspension, the transmis-

ion profiles vary with the height of the sample with time. On thether hand, if the dispersion is stable, no discernible change in theransmission profile with time is observed over an entire sampleell. It is natural that sedimentation behavior of a colloidal sys-em is affected by the surface nature of a solute. X-Axis in Fig. 6enotes the height of sample cell from bottom and Y-axis rep-esents the variation of transmitted light throughout the sampleell. The aggregation and sedimentation behavior was monitoredvery 1 h plotted as transmission intensity versus time in Fig. 7. ForWNTs in toluene (Fig. 7a), MWNTs instantly aggregate and set-

le down even after 1 h and sedimentation continues with time.fter 10 h, most of raw MWNTs precipitate and P4VP-encapsulatedWNTs slowly precipitate for 24 h. For MWNTs in ethanol (Fig. 7b),

he transmission intensity of raw MWNTs is steadily increased inthanol. However, stable dispersion state is maintained for P4VP-ncapsulated MWNTs so that its transmission intensity remainsnchanged with time due to the enhanced compatibility of grafted4VP fragments on MWNTs with ethanol. Since P4VP completelyissolves in ethanol, P4VP molecules serve as steric barrier againstggregation among the MWNTs, which finally imparts ideal disper-ion state. This stable dispersion implies that crosslinking reactionetween MWNT bundles is negligible. In addition, Fig. 7c shows theispersion stability of raw and P4VP-encapsulated MWNTs in dis-illed water. The both samples, raw MWNTs and P4VP-encapsulated

WNTs, slowly aggregate and precipitate so that the transmis-ion intensities monotonously increase with time. The variations ofransmission intensities are small for 24 h but the samples finallyrecipitate after 2 months.

These results suggest that the dispersion stability of MWNTs iniquids can be finely tuned by the proper choice of grafted polymers.

n this report, we confirmed the improved dispersion stabilityf MWNTs grafted with alcohol-soluble polymers dispersed inthanol.

906 S. Hong et al. / Synthetic Metals 158 (2008) 900–907

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Fig. 7. Transmission intensity variations with time of raw MWNTs and P4VP-e

. Conclusions

In this study, MWNTs were effectively functionalized withotassium permanganate, in the presence of a phase transferatalyst, tetrapropyl ammonium bromide, by liquid–liquid extrac-ion at room temperature. The hydroxyl functionalized MWNTsere reacted with a vinyl group-carrying silane-coupling agent,ethacryloxypropyl trimethoxy silane. The terminal vinyl groupsere used to fabricate poly(4-vinylpyridine) (P4VP) brushes fromWNTs by simple in situ solution polymerization. Finally, P4VP-

ncapsulated MWNTs were obtained. The resulting materials wereerified from TGA, EDS, and TEM. The amount of grafted P4VP was4–20 wt% relative to MWNTs and P4VP outer shell was formedn 1.1 nm thickness on MWNTs. The dispersion stability of P4VP-

A

F

ulated MWNTs in (a) toluene, (b) ethanol, and (c) distilled water, respectively.

ncapsulated MWNTs is dramatically improved in alcohol mediumecause grafted P4VP interacts with the medium and has improvedompatibility. Since P4VP completely dissolves in ethanol, P4VPolecules serve as steric barrier against aggregation among theWNTs, which finally imparts ideal dispersion state. It is thought

hat this technique would provide a facile route to prepare variousolymer brushes on the surface of MWNTs in order to improve theispersion of MWNTs for potential applications.

cknowledgement

This work was supported by the Korea Science and Engineeringoundation (grant no. R01-2007-000-20055-0).

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