MODIFICATION AND FUNCTIONALIZATION OF MULTIWALLED CARBON NANOTUBE (MWCNT) VIA FISCHER ESTERIFICATION

Faraj A. Abuilaiwi, Tahar Laoui, Mamdouh Al-Harthi, and Muataz Ali Atieh

June 2010 The Arabian Journal for Science and Engineering, Volume 35, Number 1C 37

MODIFICATION AND FUNCTIONALIZATION OF MULTIWALLED CARBON NANOTUBE (MWCNT) VIA

FISCHER ESTERIFICATION

Faraj A. Abuilaiwi1, 2, Tahar Laoui1, 4, Mamdouh Al-Harthi1, 3, and Muataz Ali Atieh*1, 3

1Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, 31261 Dhahran, Saudi Arabia

2Hafr Al-Batin Community College, King Fahd University of Petroleum and Minerals 31991 Hafr Al-Batin, Saudi Arabia

3Department of Chemical Engineering, King Fahd University of Petroleum and Minerals 31261 Dhahran, Saudi Arabia

4Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, 31261 Dhahran, Saudi Arabia

:الخالصـة

م استخدا تمو .سترةلألفيشر أسلوباستخدام مجموعات وظيفية مختلفة من خالل بإضافة ) MWCNTs(أنابيب الكربون النانوية لقد تم توظيف وقد . انابيب الكربون النانويةسطحأ على ربوآسيليالكاحمض ال ات وإدخال مجموع الكربون النانويةأنابيب محفز منال إلزالة أوًال حمض النيتريك

، آحول و، أمينوفينول، : هيأربع مجموعات وظيفيةفي توظيف أسطح انابيب الكربون النانوية مع عالئط الكربوآسيلية آاتاستخدمت هذه المجموعالمجموعات يز يتمول. األسترةباستخدام طريقة ةتساهميابط النانوية برو على أسطح انابيب الكربونبنجاح إضافتها ، وتمت وبولي ايثيلين جاليكول

مقياس الثقل النوعي ؛ آتحليلوتقنيات التحليل الحرارية) FTIR(األشعة تحت الحمراء نابيب الكربون النانوية تم استخدام مطياف أ أسطحالوظيفية على )TGA( ،الكم الحراري قياس التفاضلي لتحليلالو)DSC(.ستخدامها في ال لديها مرونة أآبر بون النانوية المرتبطة بأجزاء عضوية الكرنابيب إن أ

. النفط والبتروآيماوياتتقنيات و ومعالجة المياه، ،تراآيب النانويةالوتطبيقات مختلفة مثل علم األحياء،

*Corresponding Author: E-mail: [email protected] ____________________________________________________________________________________________________

Paper Received March 29, 2010; Paper Revised June 11, 2010; Paper Accepted July 3, 2010


The Arabian Journal for Science and Engineering, Volume 35, Number 1C June 2010 38

ABSTRACT

Multiwalled carbon nanotubes (MWCNTs) were functionalized by different functional groups via the Fischer Esterification method. Nitric acid treatment was first used to remove the catalyst from MWCNTs and introduce carboxylic acid groups onto the surface of MWCNTs. These carboxylic groups were used as reaction precursors in the functionalization. Four functional groups―phenol, dodecylamine, 1-octadecanol, and polyethylene glycol―were successfully covalently attached to MWCNTs via amidation or esterification. The functionalized MWCNTs were then characterized by Fourier Transform Infrared (FTIR) spectroscopy, thermal analysis techniques, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The MWCNTs attached to the organofunctional moieties have greater versatility for further utilization in different application fields such as biology, nanocomposites, water treatment, petroleum, and petrochemical technologies.

Key words: functionalized CNTs, Fischer Esterification, carboxylic acid groups, phenol, dodecylamine, 1-octadecanol, polyethylene glycol



MODIFICATION AND FUNCTIONALIZATION OF MULTIWALLED CARBON NANOTUBE (MWCNT) VIA FISCHER ESTERIFICATION

1. INTRODUCTION

Over the last twenty years, carbon nanotubes (CNTs) have received considerable attention from many researchers [1–5] due to their interesting properties and wide applications. In addition to their outstanding mechanical characteristics, CNTs exhibit excellent electrical and thermal properties. These superior properties provide exciting opportunities to produce advanced materials for new applications [6–9]. A common use of CNT is to disperse it in other media as a reinforcement to enhance and modify their original characteristics. However, the degree of efficiency of the CNT in improving the properties of the host medium is strongly linked to CNT dispersion. In fact, the homogeneous dispersion of CNTs into the host media, which can be in the form of liquid or solid materials, is one of the major challenges encountered in the area of CNT applications. CNT is known to be an agglomerated material that will bundle together and entangle, causing many site defects in the composite [10]. Functionalization of the CNT is one of several ways utilized to improve the compatibility of CNT and the host material. The solubility of CNT in common solvents can be improved by the presence of functional groups on the surface of the CNT. The presence of fluorine atoms on the surface of multi-wall CNT (MWCNT) increased its solubility in alcoholic solvents [11]. Similarly, dispersion of CNT in THF and other common solvents was achieved by replacing the fluorine atoms with alkyl groups [12–14]. The addition of amino groups to CNT sidewalls increased its solubility in water, which has potential use in biological applications [15]. Highly customized and individually suspended water-soluble SWCNTs can be produced by the oleum method in which sulfonated 4-chlorophenyl moieties are functionalized to the sidewalls of the SWCNTs by diazonium salts to provide a bimodal functionality to the sidewalls of the SWCNTs [16,17]. The functional groups which can be attached to the CNT surface range from small molecules, such as the above examples, to macromolecules that can be used in different applications, especially in the nanocomposites field. The compatibility and dispersion of CNTs in polypropylene and isotactic polypropylene (iPP) matrixes can be significantly improved using CNTs chemically modified with alkyl chains [18–20]. CNTs act as fillers to achieve improved mechanical properties by changing the iPP crystal form [21]. Functionalized CNT with different initiator moieties can be used in in situ polymerizations to produce composites that have covalent bonds between the filler and the polymer chains. The presence of the chemical bonding leads to improvement in the mechanical properties of the final mixture. For instance, CNTs functionalized by butyl lithium act as an initiator for polymerization to produce grafted CNT polymer nanocomposites where the anions disperse the nanotubes due to electrostatic repulsive force between the tubes. When CNTs are functionalized by oxy radicals, they also act as initiators for polymerization of polyethylene chains grafted on the nanotubes [22]. Highly dispersible, green, low-cost, and efficient functionalized SWCNTs were produced by incorporating various reactive groups such as −OH, −NH2, −COOH, and −Br onto the convex surfaces of CNTs by adjusting the feed ratio of azide compounds to CNTs and were used in surface-initiated polymerizations, amidation, and reduction of metal ions affording various CNT-polymer and CNT-Pt nanohybrids [23,24].

It has been shown in the literature that functionalized CNT has several applications in different fields, as mentioned in the above few examples, which makes this area of research very attractive indeed for many researchers. The present study presents a detailed methodology for functionalizing MWCNTs with different functional groups of value for a variety of different purposes.

2. METHODOLOGY FOR FUNCTIONALIZING MWCNT

2.1. Carboxylation Treatment of Multiwalled Carbon Nanotubes (MWCNT)

Multi-walled carbon nanotubes were purchased from Nanostructured & Amorphous Materials, Inc. USA. The purity of the as-received MWCNT material was > 95%, and its outside and inside diameters were 10–20 nm and 5–10 nm, respectively. The length of these MWCNTs was 10–30 µm. These specification details were given by the manufacturer. Three hundred ml of concentrated nitric acid (69%, AnalaR grade) were added to 2g of as-received MWCNT. The mixture was refluxed for 48 hours at 120°C. After cooling to room temperature, the reaction mixture was diluted with 500 ml of deionized water and then vacuum-filtered through a 3 µm porosity filter paper. This washing operation was repeated until the pH became the same as that of deionized water and was followed by drying in a vacuum oven at 100°C. Such conditions lead to the removal of the catalyst from the carbon nanotubes and opened the tube caps. In addition, holes were formed in the sidewalls followed by an oxidative etching along the walls with the concomitant release of carbon dioxide. These less vigorous conditions minimized the shortening of the tubes, and the chemical modification was then limited mostly to the opening of the tube caps and the formation of functional groups at defect sites along the sidewalls. The final products were nanotube fragments whose ends and sidewalls were decorated with various oxygen-containing groups (mainly carboxyl groups) (Figure 1). Moreover, the



percentage of carboxylic functional groups on the oxidized MWCNT surface does not exceed 4% in the best cases, which corresponds to the percentage of MWCNT structural defects [25].

Figure 1: Chemical modification of carbon nanotubes (CNTs) through thermal oxidation

2.2. Esterification and Amidation of Carbon Nanotubes

The Fischer Esterification is an equilibrium reaction, whereas other esterification routes do not involve equilibrium. To shift the equilibrium to favor the production of esters, it is customary to use an excess of one of the reactants, either the alcohol or the acid. In the present reactions, an excess of the phenol (Aldrich, 98% purity), dodecylamine (Merck, 97% purity), 1-octadecanol (Merck, 97% purity), and polyethylene glycol (Fluka with 98% purity and an average molecular mass of 14000) have been used, because it is cheaper and easier to remove than the carbon nanotubes. Another way to drive a reaction toward its products is to remove one of the products as it forms. Water formed in this reaction was removed by evaporation during the reaction.

The oxidatively introduced carboxyl groups represent useful sites for further modifications [26], as they enable the covalent coupling of molecules through the creation of ester (Figure 2) or amine bonds (Figure 3). In a 250 ml beaker, 10 g of the functionalizing reactant were melted on a hotplate at 90°C, 1 g of MMWCNT (modified MWCNT) was added, and the mixture was stirred for 10 minutes before a few drops of sulfuric acid as a catalyst were added. After addition of the catalyst, the mixture was kept on the hotplate and stirred for 2 hours. Upon completion of the reaction, the mixture was poured into 250 ml of benzene and vacuum-filtered through a filter paper (3 µm porosity). This washing operation was repeated five times and followed by washing with petroleum ether three times and THF three times. The product was then washed with deionized water and acetone, and the functionalized MMWCNT material produced was dried in a vacuum oven at 90°C.

Figure 2: Chemical esterification of modified carbon nanotubes (MMWCNTs). For clarity only the outer wall of the multi-walled carbon nanotube is shown.



H+O

HO

OH

O

MMWCNT

H2N(CH2)11CH3

dodecylamine

O

NH

NH

O(CH2)11H3C

(CH2)11CH3

H2O

MMWCNT-dodecylamide

Figure 3: Chemical amidation of modified carbon nanotubes (MMWCNT). For clarity only the outer wall of the multi-walled carbon nanotube is shown.

The mechanism for this reaction involves the nucleophilic addition of the alcohol or amine to the carbonyl group of the protonated acid of the carbon nanotubes, followed by elimination of a proton. The tetrahedral intermediate is unstable under the acidic conditions of the reaction and undergoes dehydration to form the ester or amide.

The key steps of this mechanism involve activation of the carbonyl group by protonation of the carbonyl oxygen, nucleophilic addition to the protonated carbonyl to form a tetrahedral intermediate, and elimination of water from the tetrahedral intermediate to restore the carbonyl group.

3. EXPERIMENTAL ANALYSIS

The functionalized MWCNTs obtained using different functional groups and prepared using the Fischer Esterification method detailed above were analyzed using Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy and thermal analysis, specifically thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).

3.1. FTIR Measurements

Fourier Transform Infrared Spectroscopy (FTIR) has shown some limited ability to probe the structure of MWCNTs. A factor that has hindered the advancement of FTIR as a tool for MWCNT analysis is the poor infrared transmittance of MWCNTs [27]. A solution to this was the use of KBr preparations of nanotube samples.

Because of their black character, the MWCNTs exhibit a strong absorbance and often are unable to be distinguished from the background noise, making it necessary to use a very weak concentration of the nanotubes in a KBr powder. However, the greater vibrational freedom of attached polymeric species presents much more pronounced peaks that are typically the focus of attention in FTIR results. Despite this, with very careful sample preparation, some researchers have managed to elucidate peaks corresponding to surface bound moieties, such as carboxylic acid groups at wavenumbers of 1791, 1203, and 1080cm−1 [28].

In this study, the spectra of the analyzed samples were recorded using a PERKIN ELMER 16F PC FT-IR instrument. Samples for FTIR measurements were prepared by grinding dry material into potassium bromide at a concentration of ~0.03 %wt. As indicated above, this very low concentration of MWCNTs is necessary due to the strong absorption characteristic of the nanotubes.

3.2. Thermal Analysis

Thermogravimetric analysis (TGA) measures changes in weight of a sample with increasing temperature. Moisture content and the presence of volatile species can be determined with this technique. Computer-based methods can be used to calculate weight percent losses.

The dynamic thermogravimetric experiments were carried out using a Netzsch model STA 449 F3 Jupiter® simultaneous thermal analyzer, allowing measurement of mass change and associated phase transformation energetics. The system employed for this work was equipped with a PtRh furnace capable of operation from 25 to 1500°C, the temperature being measured using type S thermocouple. The system is vacuum tight, allowing measurements to be conducted under controlled atmosphere. In these experiments, differential scanning calorimetry (DSC) measurements were also recorded to study phase transitions and exothermic/endothermic decompositions taking place in the samples investigated.

TGA-DSC analysis was performed on small samples (about 6 mg), mounted on platinum crucibles with Al2O3 liners and pierced lids, in an inert Ar atmosphere (flow rate of argon gas, 70 ml/min). The temperature range was varied from room temperature to 1400°C and the typical heating rate was 20°C/min, while the digital resolution of the balance was 1 µg/digit.



4. RESULT AND DISCUSSION

4.1. Fourier Transform Infrared Spectroscopy

FTIR spectra from the as-received MWCNTs show a broad peak at ~3450 cm−1, which refers to the O-H stretch of the hydroxyl group (Figure 4a), which can be ascribed to the oscillation of carboxyl groups. Carboxyl groups on the surfaces of as-received MWCNTs could be due to the partial oxidation of the surfaces of MWCNTs during purification by the manufacturer. This feature moves to 1730 cm−1 and is associated with the stretch mode of carboxylic groups as observed in the IR spectrum of the acid-treated MMWNTs (Figure 4b) [29], indicating that carboxylic groups are formed due to the oxidation of some carbon atoms on the surfaces of the MWNTs by nitric acid [30,31]. The IR spectra of oxidized MWCNTs shows four major peaks, located at 3750, 3450, 2370, and 1562 cm−1. The peak at 3750 cm−1 is attributed to free hydroxyl groups [32]. The peak at 3445 cm−1 can be assigned to the O-H stretch from carboxyl groups (O=C−OH and C−OH), while the peak at 2364 cm−1 can be associated with the O−H stretch from strongly hydrogen-bonded −COOH [33]. The peak at 1565 cm−1 is related to the carboxylate anion stretch mode [34]. It should be noticed that the as-received MWCNTs were purified by the manufacturer and part of the catalytic metallic nanoparticles were possibly eliminated during the purification process cutting the nanotube cap. Thus, the presence of carboxylic groups in these commercial MWCNTs can be expected.

The peak at 1635 cm−1 can be associated with the stretching of the carbon nanotube backbone [35]. Increased strength of the signal at 1165 cm−1 may be associated with C–O stretching in the same functionalities [28]. The peaks at around 2877 and 2933 cm-1 correspond to the H−C stretch modes of H−C=O in the carboxyl group.

In Figure 4c, the MMWCNT−COO(CH2)17CH3 shows IR absorptions at 2924 cm−1 and 2872 cm−1 associated with C−H stretch modes, which is indicative of the presence of a long-chain alkyl molecule - (octadecanol) - while the peak at 1701 cm−1 can be attributed to the C=O stretch of the ester. The peak observed at 1560 cm−1 is the C=C stretch of the MMWCNTs, while the peak at 1461 cm−1 originates from the C−H bend of the alkyl chain, and the peak at 1108 cm−1 arises from the C−O stretch of the ester group. Many of these vibrational modes have been reported previously for functionalized SWCNTs [36]. Also, spectra of MMWCNT–octadecanoate exhibit typical bands originating from −CH2 rocking at 760 cm-1 [37].

The peaks at 3000–2800 cm-1 in Figure 4d are due to C−H stretching vibrations: antisymmetric and symmetric modes of methylene groups of polyethylene glycol (PEG). The peak at 2281 cm-1 is from the N−C=O asymmetric vibration (Figure 4e), while a new peak is observed at 1544 cm-1 which is attributed to the overlapping of a signal from the N−H, N−C bands and N−C=O group [38]. Moreover, all figures show peaks between 1300 and 1100 cm-1, which are ascribed to the C–C bond stretch [39].

As depicted in Figure 4f, the spectrum corresponding to the phenyl structure can be described as follows: the peaks between 3000 and 3140 cm-1 correspond to stretching of the C−H group of the aromatic ring; peaks between 1600 and 2000 cm-1 correspond to the overtones of the phenyl ring group; the peaks at 685 and 890 cm-1 are bands originating from the phenyl ring; at 1565, 1505, and 1460 cm-1 , the C=C bands of the aromatic ring can be observed [40].



Figure 4: FTIR spectra of (a) as-received MWCNT, (b) MMWCNT, (c) MMWCNT−octadecanoate, (d) MMWCNT−PEG ester, (e) MMWCNT−dodecylamide, and (f) MMWCNT−Phenyl ester

4.2. Thermal Degradation Analysis of Functionalized Carbon Nanotubes

The study of the thermal degradation of materials is of major importance since it can, in many cases, determine the upper temperature limit of use for a material. In addition, considerable attention has been directed towards the exploitation of thermogravimetric data for the determination of functional groups. For this purpose, thermogravimetric analysis (TGA) is a technique widely used because of its simplicity and the information afforded by a simple thermogram.

Figures 5 through 8 depict the TGA-DSC results for the carbon nanotubes functionalized with four different organic groups, namely CNT−COOH, CNT−Amine, CNT−Octadecanoate, and CNT−Phenol. In these figures, TG% refers to the temperature-dependent mass change in percent, DTG (%/min) to the rate of mass change (the derivative of the TG curve), and DSC (mW/mg) to the heat flow rate into or out of the sample.

Several mass loss steps were observed below ~150°C which are due to the release of moisture and the decomposition of the associated organic groups. The mass loss steps were accompanied by endothermic peaks visible in the DSC signal, except for the sample CNT−COOH which exhibits one exothermic peak (Figure 8). As revealed by the DTG curve shown in Figure 5, the initial degradation of −COOH starts at approximately 170°C and reaches a maximum weight loss of this acidic group at about 321ºC and completes at about 480°C. The second peak appearing at about 783°C corresponds to the oxidation of CNT due to the decomposition of the carboxylic group releasing oxygen into the chamber of the TGA–DSC system.

As shown in Figure 6, for the CNT−Amine, the two initial peaks, located at 287°C and 387°C, in the DTG curve correspond to the maximum degradation of the amine group (dodecylamine) and carboxylic acid group, respectively. Figure 7 displays the degradation of CNT−Octadecanoate. Two peaks appear at 265°C and approximately 400°C, corresponding to the maximum degradation of the octadecanol and carboxylic acid group, respectively. It seems from Figure 8 that a small amount of phenol has been attached to the nanotubes through the carboxyl group, therefore yielding a small DTG peak appearing at about 250°C, corresponding to maximum degradation of phenol, followed by a peak at 376°C showing the maximum degradation of the carboxylic acid group.

a

b

c

d

e

f



0

1

2

3

4

5

6

DSC /(mW/mg)

85

90

95

100

105

110

TG /%

200 400 600 800 1000 1200 1400Temperature /°C

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

DTG /(%/min)

Sample: CNT COOH

-13.58 J/g

12.35 J/g

143.3 °C354.1 °C

-0.75 %

-3.20 %

-2.19 %-0.89 %

321.5 °C

782.7 °C

[5]

↓ exo

Figure 5: Thermogravimetric (TG) and Derivative Thermogravimetric (DTG) analysis

of modified carbon nanotubes with carboxylic group

0

1

2

3

4

5

6

DSC /(mW/mg)

40

50

60

70

80

90

100

110

120

130

TG /%

200 400 600 800 1000 1200 1400Temperature /°C

-14

-12

-10

-8

-6

-4

-2

0

DTG /(%/min)

Sample: CNT Amine

274.1 °C

222.8 J/g

-0.68 %

-12.02 %

-23.06 %

-0.70 % -2.22 %

287.4 °C386.6 °C

771.9 °C

[5]

↓ exo


of modified carbon nanotubes with Amine group



-10

-8

-6

-4

-2

0

DTG /(%/min)

200 400 600 800 1000 1200 1400Temperature /°C

1

2

3

4

5

6

DSC /(mW/mg)

60

70

80

90

100

110

120

TG /%

Sample: CNT C18

53.84 J/g

71.94 J/g

285.7 °C

733.1 °C

-0.29 %

-10.08 %

-2.70 %

-10.04 %

-1.47 %

265.3 °C

731.2 °C

[5]↓ exo


of modified carbon nanotubes with Octadecanoate group

0

1

2

3

4

5

6

DSC /(mW/mg)

75

80

85

90

95

100

105

TG /%

200 400 600 800 1000 1200 1400Temperature /°C

-5

-4

-3

-2

-1

0

DTG /(%/min)

Sample: CNT Phenol

49.44 J/g

391.5 °C

-0.72 %

-9.27 %

-1.52 % -1.51 %

376.0 °C

762.1 °C113.5 °C

[5]↓ exo


of modified carbon nanotubes with Phenol group



5. CONCLUSION

A detailed methodology for the modification and functionalization of multiwalled carbon nanotube (MWCNT) via Fischer Esterification has been presented. Different compounds containing different functional groups were introduced onto the surface of MWCNT using Fischer Esterification after oxidation of the MWCNT surface. Oxidation of MWCNT surface introduces carboxyl and hydroxyl groups enabling further chemical elaboration of this material. Moreover, amidation of oxidized surface MWCNT was also achieved using the Fischer Esterification approach. The functionalized MWCNTs obtained using different functional groups were analyzed using both Fourier Transform Infrared (FTIR) spectroscopy and thermal analysis (TGA–DSC). The thermogravimetric analysis (TGA) data has shown the presence of the functional groups attached to the surface of MWCNT and their corresponding degradation with increasing temperature in inert atmosphere.

ACKNOWLEDGMENTS

The author(s) would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No: AR-28-118. In addition, the authors would like to acknowledge Naizak/Netzsch for the TGA analysis performed using their instrument.

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MODIFICATION AND FUNCTIONALIZATION OF MULTIWALLED CARBON NANOTUBE (MWCNT) VIA FISCHER ESTERIFICATION

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