- Email: [email protected]
Epoxide-terminated carbon nanotubes
Letters to the Editor / Carbon 45 (2007) 3042–3059
3047
Epoxide-terminated carbon nanotubes Shiren Wang
a,*
, Richard Liang b, Ben Wang b, Chuck Zhang
b
a
b
Department of Industrial Engineering, Texas Tech University, Lubbock, TX 79409, USA High-Performance Materials Institute (HPMI), Department of Industrial and Manufacturing Engineering, Florida State University, Tallahassee, FL 32310, USA Received 20 August 2007; accepted 10 October 2007 Available online 24 October 2007
Carbon nanotubes (CNTs) demonstrate many exceptional properties and show great promise for numerous applications, including electronics [1], optics [2], thermal management [3] and high-performance composite applications [4,5]. However, application attempts have been hindered by technical roadblocks such as difficulties in the dispersion and inert surface characteristics. Nanotubes’ tendency to form large bundles or big ropes significantly limits potential applications. Some researchers used surfactant to aid in the dispersion of CNTs and acquired small carbon nanotube ropes [6]. Many attempts in the chemical functionalization of CNTs were reported [7–11]. These functionalization approaches facilitated the dispersion of nanotubes, but they may have difficulty in acquiring individual tubes by large scale. Even though some progress has been made in the past decade, many efforts are still needed for the dispersion of CNTs. Mass-production of soluble and isolated CNTs is very desirable for industrial applications. In addition, none of researchers tried epoxide-grafting. The epoxide group is of great interest due to its rich chemistry. The epoxy group could be converted into different areas of functionalities through a ring-opening reaction [12]. This ring-opening reaction provides various opportunities for applications involving composite interfacial improvements, polymer surface modifications and high-performance membranes [12]. The cross-linking reaction of the epoxy groups under electron-beam exposures creates the potential for a high-sensitivity negative-tone electron-beam resistance [12]. Therefore, grafting epoxy groups onto CNTs would open the door for wide applications using CNTs in functional devices. This paper presents an effective method to functionalize CNTs using epoxide group and shows that functionalized nanotubes can be easily dissolved in the organic solvent with excellent dispersion. Not only does this methodology graft powerful epoxide group onto CNT surfaces, but it also provides the added advantages of being scalable and cost-effective to separate CNT bundles into individual tubes.
*
Corresponding author. E-mail address: [email protected] (S. Wang).
0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.10.014
Benzoyl peroxide (BPO) is widely used as a free radical initiator in conventional polymerizations. The peroxide bond is easy to dissociate under low temperatures (<150 C). The half-life time for BPO in benzene is about 7–10 h at 75 C [13]. The formation of benzoyl radicals initializes the polymerization of glycidyl methacrylate (GMA), similar to the steps in conventional polymerization [12]. Fig. 1 shows the functionalization method. Termination happens through the reaction of the propagating long-chain radical with SWCNTs. Hence, the epoxidegrafting was realized with free radical additions. Single-walled carbon nanotubes (SWCNTs), produced by a HiPco process, were first ground with a small quantity of benzene, and then transferred to a small beaker that contained GMA. SWCNTs were dispersed in the GMA using ultrasonic processing for 15 min at a power of 30– 40 w/m3. The mixture was transferred to a rotated flask and BPO was added by stirring. The mixture was slowly heated to 75 C and strongly stirred for 24 h. Subsequently the mixture was diluted with benzene and filtered through 0.2 lm PTFE membrane, resulting in a thin film. The resultant film was substantially washed with acetone until it is thoroughly cleaned. The functionalized SWCNTs were characterized with FT-IR (Nexus 470, Thermal Nicole Inc.). Although no visible peaks were seen in the FT-IR spectra of pristine SWCNTs, there existed several vibration peaks in the spectra of functionalized SWCNTs, as shown in Fig. 2. The peaks appeared in the frequency of 1719 cm1 indicating ester group existed in the functionalized SWCNTs and this group originated from the monomer GMA. In addition, three small peaks appearing at 987, 901 and 874 cm1 were attributed to the vibration of epoxy group [12,14]. RAMAN spectroscope is a powerful tool capable of characterizing functionalized SWCNTs. Functionalization reactions might introduce many SP3 hybrids so that intensity of the disorder band (1290 cm1) might be enlarged in comparison with the pristine carbon nanotubes. Non-covalent functionalization, such as polymer wrapping, usually does not affect the disorder band since it does not change the molecular structure of tubes. Fig. 3 shows the Raman spectroscope of epoxy-grafted SWCNTs. An increase in the disorder band intensity of functionalized SWCNTs
3048
Letters to the Editor / Carbon 45 (2007) 3042–3059
Fig. 1. Functionalization scheme for epoxy grafting onto SWCNTs.
b
a
1.0
1719.47
0.035
0.9
0.030 0.025
Absorbance
Absorbance
0.8 0.7 0.6
0.020
1111.95
0.015 0.010
841.47 987.82 901.18
0.005
0.5
709.45
1262.46
0.000
0.4
-0.005
4000
3500
3000
2500
2000
1500
1000
4000
500
3500
2500
3000
2000
1500
1000
500
Wave Numbers (cm-1)
Wave Numbers (cm-1)
Fig. 2. FT-IR spectrum of (a) pristine SWCNTs and (b) x-SWCNTs.
Intensity (a.u.)
3000
(c) Ratio of Raman band intensity
2500 2000
(b) SWCNTs
I RBM/I G+ (RBM at 267cm-1)
I D/I G+
I G-/I G+
pristine
0.36
0.04
0.16
Epoxidegrafted
0.08
0.35
0.09
1500 1000 5000
(a)
0 500
1000
1500
2000
2500
3000
Wave Numbers (cm-1)
Fig. 3. Raman spectroscopy of (a) pristine SWCNTs and (b) epoxide-grafted SWCNTs; (c) table of the ratio of Raman band intensity.
120
Weight (%)
was observed, indicating covalent bond formations between the SWCNTs and poly-(glycidyl methacrylate) (PGMA) molecules. It also showed that the intensity of the RBM model was significantly reduced and almost disappeared because the radius vibration of CNTs was restricted by the grafted molecules. The evident undermined G-mode in x-SWCNT further validated the covalent functionalization through in situ polymerization. The relative amounts of grafted polymer on SWCNTs were determined by TGA in the nitrogen environment. Fig. 4 shows the TGA results of pristine SWCNT and xSWCNT. Obviously, the grafted poly-(glycidyl methacrylate) (PGMA) was completely decomposed at 600 C while the SWCNTs basically remained the same. Through adjusting the chemical ratios, the grafted-PGMA percentage was tailored from 28% to 54% in the functionalized x-SWCNTs. The x-SWCNTs recovered their original state after annealing at 600 C. This result suggests that the
100
a
80
b c
60 40
——————— ——— — ————— · ——— — —
Pristine SW NT X_SWNT/BPO= 1:8 X_SWNT/BPO= 1:2 X- SWNT/BPO= 1:1
d
20 0
200
600 400 Temperature (°C)
800
1000
Fig. 4. TGA results of pristine SWCNTs and x-SWCNTs. (a) Pristine SWCNT; (b) x-SWCNT when SWCNT: BPO = 1:8; (c) x-SWCNT when SWCNT: BPO = 1:2 and (d) x-SWCNT when SWCNT: BPO = 1:1. (Note: The ratios are based on the mole of carbon atoms in SWCNTs and BPO molecules.)
grafted polymers were broke down and the functionalized CNTs returned to their pristine state.
Letters to the Editor / Carbon 45 (2007) 3042–3059
a
5.00
b
c
3049
40 35
2.50
Percent (%)
30 25 20 15 10 5
0
2.50
0 5.00 μm
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Diameter of dispersed nanotube ropes (nm)
Fig. 5. Dispersion and solubility of pristine SWCNTs and x-SWCNTs. (a) Pristine SWCNT (left) and x-SWCNT (right) in DMF; (b) AFM image of dispersed x-SWCNT; (c) diameter distribution of dispersed x-SWCNT.
The experimental results indicated that the functionalized SWCNTs can be dissolved with 0.53 mg/ml in the organic solvents, including dimethyl formamide (DMF), chloroform, and methylene chloride. The solution was stable and uniform, as shown in the Fig. 5a. The solution was sampled and characterized with AFM, as shown in Fig. 5b. The AFM image was further investigated by section analysis, and the height of the nanotube ropes (diameter of the dispersed SWCNT rope) was extracted to the histogram, as shown in Fig. 5c. The statistical results indicated that more than 60% of the ropes were well dispersed with diameters less than 2 nm. Generally, the nanotube ropes with diameters less than 2 nm are regarded as individual tubes. Therefore, most of the SWCNTs were dispersed into an individual state. In summary, CNTs were functionalized with epoxidegrafting, resulting in desired solubility and special functionality. The grafted epoxide group enriches the chemistry of the CNTs and offers various potential applications. These functionalized CNTs offer the potential for fabricating multifunctional high-performance epoxy polymer composites. They may also be used as chemical sensors or atomic force microscope tips with high-sensitivity. Of course, they are also very promising in the application of multifunctional membranes. Further applications of epoxide-functionalized nanotubes are being explored. Most importantly, this investigation provides a cost-effective way to achieve isolated CNTs by large scale. Acknowledgements The authors gratefully acknowledge the support of AFOSR, AFRL ML, and AFRL MN. References [1] Javey A, Qi P, Wang Q, Dai H. Ten- to 50-nm-long quasi-ballistic carbon nanotube devices obtained without complex lithography. Proc Natl Acad Sci 2004;101(37):13408–10.
[2] Misewich JA, Martel R, Avouris P, Tsang J, Heinze S, Tersoff J. Electrically induced optical emission from a carbon nanotube FET. Science 2003;300:783–6. [3] Wang S, Liang Z, Gonnet P, Liao Y, Wang B, Zhang C. Effect of nanotube functionalization on the coefficient of thermal expansion of nanocomposites. Adv Funct Mater 2007;17:87–92. [4] Ajayan PM, Schadler LS, Giannaris C, Rubio A. Single-walled carbon nanotube-polymer composites: strength and weakness. Adv Mater 2000;12(10):750–3. [5] Wang S, Liang Z, Pham G, Park Y, Wang B, Zhang C, et al. Controlled nanostructure and high-loading of single-walled carbon nanotubes reinforced polycarbonate nanocomposite. Nanotechnology 2007;18:095708–15. [6] Wang H, Zhou W, Ho DL, Winey KI, Fisher JE, Glinka CJ, et al. Dispersing single-walled carbon nanotubes with surfactants: a small angle neutron scattering study. Nano Lett 2004;4(9):1789–93. [7] Chen J, Hamon M, Hu H, Chen Y, Rao A, Eklumd P, et al. Solution properties of single-walled carbon nanotubes. Science 1998; 282(5386):95–8. [8] Hu H, Zhao B, Hamon M, Kamaras K, Itkis M, Haddon R. Sidewall functionalization of single-walled carbon nanotubes by addition of Dichlorocarbene. J Am Chem Soc 2003;125(48):14893–900. [9] Hamon M, Hu H, Bhowmik P, Niyogi S, Zhao B, Itkis M, et al. Endgroup and defect analysis of soluble single-walled carbon nanotubes. Chem Phys Lett 2001;347:8–12. [10] Wong S, Joselevich E, Woolley AT, Cheung C, Lieber CM. Covalently functionalized nanotubes as nanometre-sized probes in chemistry and biology. Nature 1998;394:52–5. [11] Zhu J, Peng H, Rodriguez-Macias F, Margrave JL, Khabashesku VN, Imam AM, et al. Reinforcing epoxy polymer composites through covalent integration of functionalized nanotubes. Adv Funct Mater 2004;14:643–8. [12] Mao Y, Gleason KK. Hot filament chemical vapor deposition of poly(glycidyl methacrylate) thin films using tert-butyl peroxide as an initiator. Langmuir 2004;20:2484–8. [13] Hawker CJ. Molecular weight control by living free-radical polymerization process. J Am Chem Soc 1994;116:11185–6. [14] Vandijkwolthuis WN, Franssen O, Talsma H, Vansteenbergen MJ, Vandenbosch JJ, Hennink WE. Synthesis, characterization, and polymerization of glycidyl methacrylate derivatized dextran. Macromolecules 1995;28:6317–22.
3047
Epoxide-terminated carbon nanotubes Shiren Wang
a,*
, Richard Liang b, Ben Wang b, Chuck Zhang
b
a
b
Department of Industrial Engineering, Texas Tech University, Lubbock, TX 79409, USA High-Performance Materials Institute (HPMI), Department of Industrial and Manufacturing Engineering, Florida State University, Tallahassee, FL 32310, USA Received 20 August 2007; accepted 10 October 2007 Available online 24 October 2007
Carbon nanotubes (CNTs) demonstrate many exceptional properties and show great promise for numerous applications, including electronics [1], optics [2], thermal management [3] and high-performance composite applications [4,5]. However, application attempts have been hindered by technical roadblocks such as difficulties in the dispersion and inert surface characteristics. Nanotubes’ tendency to form large bundles or big ropes significantly limits potential applications. Some researchers used surfactant to aid in the dispersion of CNTs and acquired small carbon nanotube ropes [6]. Many attempts in the chemical functionalization of CNTs were reported [7–11]. These functionalization approaches facilitated the dispersion of nanotubes, but they may have difficulty in acquiring individual tubes by large scale. Even though some progress has been made in the past decade, many efforts are still needed for the dispersion of CNTs. Mass-production of soluble and isolated CNTs is very desirable for industrial applications. In addition, none of researchers tried epoxide-grafting. The epoxide group is of great interest due to its rich chemistry. The epoxy group could be converted into different areas of functionalities through a ring-opening reaction [12]. This ring-opening reaction provides various opportunities for applications involving composite interfacial improvements, polymer surface modifications and high-performance membranes [12]. The cross-linking reaction of the epoxy groups under electron-beam exposures creates the potential for a high-sensitivity negative-tone electron-beam resistance [12]. Therefore, grafting epoxy groups onto CNTs would open the door for wide applications using CNTs in functional devices. This paper presents an effective method to functionalize CNTs using epoxide group and shows that functionalized nanotubes can be easily dissolved in the organic solvent with excellent dispersion. Not only does this methodology graft powerful epoxide group onto CNT surfaces, but it also provides the added advantages of being scalable and cost-effective to separate CNT bundles into individual tubes.
*
Corresponding author. E-mail address: [email protected] (S. Wang).
0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.10.014
Benzoyl peroxide (BPO) is widely used as a free radical initiator in conventional polymerizations. The peroxide bond is easy to dissociate under low temperatures (<150 C). The half-life time for BPO in benzene is about 7–10 h at 75 C [13]. The formation of benzoyl radicals initializes the polymerization of glycidyl methacrylate (GMA), similar to the steps in conventional polymerization [12]. Fig. 1 shows the functionalization method. Termination happens through the reaction of the propagating long-chain radical with SWCNTs. Hence, the epoxidegrafting was realized with free radical additions. Single-walled carbon nanotubes (SWCNTs), produced by a HiPco process, were first ground with a small quantity of benzene, and then transferred to a small beaker that contained GMA. SWCNTs were dispersed in the GMA using ultrasonic processing for 15 min at a power of 30– 40 w/m3. The mixture was transferred to a rotated flask and BPO was added by stirring. The mixture was slowly heated to 75 C and strongly stirred for 24 h. Subsequently the mixture was diluted with benzene and filtered through 0.2 lm PTFE membrane, resulting in a thin film. The resultant film was substantially washed with acetone until it is thoroughly cleaned. The functionalized SWCNTs were characterized with FT-IR (Nexus 470, Thermal Nicole Inc.). Although no visible peaks were seen in the FT-IR spectra of pristine SWCNTs, there existed several vibration peaks in the spectra of functionalized SWCNTs, as shown in Fig. 2. The peaks appeared in the frequency of 1719 cm1 indicating ester group existed in the functionalized SWCNTs and this group originated from the monomer GMA. In addition, three small peaks appearing at 987, 901 and 874 cm1 were attributed to the vibration of epoxy group [12,14]. RAMAN spectroscope is a powerful tool capable of characterizing functionalized SWCNTs. Functionalization reactions might introduce many SP3 hybrids so that intensity of the disorder band (1290 cm1) might be enlarged in comparison with the pristine carbon nanotubes. Non-covalent functionalization, such as polymer wrapping, usually does not affect the disorder band since it does not change the molecular structure of tubes. Fig. 3 shows the Raman spectroscope of epoxy-grafted SWCNTs. An increase in the disorder band intensity of functionalized SWCNTs
3048
Letters to the Editor / Carbon 45 (2007) 3042–3059
Fig. 1. Functionalization scheme for epoxy grafting onto SWCNTs.
b
a
1.0
1719.47
0.035
0.9
0.030 0.025
Absorbance
Absorbance
0.8 0.7 0.6
0.020
1111.95
0.015 0.010
841.47 987.82 901.18
0.005
0.5
709.45
1262.46
0.000
0.4
-0.005
4000
3500
3000
2500
2000
1500
1000
4000
500
3500
2500
3000
2000
1500
1000
500
Wave Numbers (cm-1)
Wave Numbers (cm-1)
Fig. 2. FT-IR spectrum of (a) pristine SWCNTs and (b) x-SWCNTs.
Intensity (a.u.)
3000
(c) Ratio of Raman band intensity
2500 2000
(b) SWCNTs
I RBM/I G+ (RBM at 267cm-1)
I D/I G+
I G-/I G+
pristine
0.36
0.04
0.16
Epoxidegrafted
0.08
0.35
0.09
1500 1000 5000
(a)
0 500
1000
1500
2000
2500
3000
Wave Numbers (cm-1)
Fig. 3. Raman spectroscopy of (a) pristine SWCNTs and (b) epoxide-grafted SWCNTs; (c) table of the ratio of Raman band intensity.
120
Weight (%)
was observed, indicating covalent bond formations between the SWCNTs and poly-(glycidyl methacrylate) (PGMA) molecules. It also showed that the intensity of the RBM model was significantly reduced and almost disappeared because the radius vibration of CNTs was restricted by the grafted molecules. The evident undermined G-mode in x-SWCNT further validated the covalent functionalization through in situ polymerization. The relative amounts of grafted polymer on SWCNTs were determined by TGA in the nitrogen environment. Fig. 4 shows the TGA results of pristine SWCNT and xSWCNT. Obviously, the grafted poly-(glycidyl methacrylate) (PGMA) was completely decomposed at 600 C while the SWCNTs basically remained the same. Through adjusting the chemical ratios, the grafted-PGMA percentage was tailored from 28% to 54% in the functionalized x-SWCNTs. The x-SWCNTs recovered their original state after annealing at 600 C. This result suggests that the
100
a
80
b c
60 40
——————— ——— — ————— · ——— — —
Pristine SW NT X_SWNT/BPO= 1:8 X_SWNT/BPO= 1:2 X- SWNT/BPO= 1:1
d
20 0
200
600 400 Temperature (°C)
800
1000
Fig. 4. TGA results of pristine SWCNTs and x-SWCNTs. (a) Pristine SWCNT; (b) x-SWCNT when SWCNT: BPO = 1:8; (c) x-SWCNT when SWCNT: BPO = 1:2 and (d) x-SWCNT when SWCNT: BPO = 1:1. (Note: The ratios are based on the mole of carbon atoms in SWCNTs and BPO molecules.)
grafted polymers were broke down and the functionalized CNTs returned to their pristine state.
Letters to the Editor / Carbon 45 (2007) 3042–3059
a
5.00
b
c
3049
40 35
2.50
Percent (%)
30 25 20 15 10 5
0
2.50
0 5.00 μm
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Diameter of dispersed nanotube ropes (nm)
Fig. 5. Dispersion and solubility of pristine SWCNTs and x-SWCNTs. (a) Pristine SWCNT (left) and x-SWCNT (right) in DMF; (b) AFM image of dispersed x-SWCNT; (c) diameter distribution of dispersed x-SWCNT.
The experimental results indicated that the functionalized SWCNTs can be dissolved with 0.53 mg/ml in the organic solvents, including dimethyl formamide (DMF), chloroform, and methylene chloride. The solution was stable and uniform, as shown in the Fig. 5a. The solution was sampled and characterized with AFM, as shown in Fig. 5b. The AFM image was further investigated by section analysis, and the height of the nanotube ropes (diameter of the dispersed SWCNT rope) was extracted to the histogram, as shown in Fig. 5c. The statistical results indicated that more than 60% of the ropes were well dispersed with diameters less than 2 nm. Generally, the nanotube ropes with diameters less than 2 nm are regarded as individual tubes. Therefore, most of the SWCNTs were dispersed into an individual state. In summary, CNTs were functionalized with epoxidegrafting, resulting in desired solubility and special functionality. The grafted epoxide group enriches the chemistry of the CNTs and offers various potential applications. These functionalized CNTs offer the potential for fabricating multifunctional high-performance epoxy polymer composites. They may also be used as chemical sensors or atomic force microscope tips with high-sensitivity. Of course, they are also very promising in the application of multifunctional membranes. Further applications of epoxide-functionalized nanotubes are being explored. Most importantly, this investigation provides a cost-effective way to achieve isolated CNTs by large scale. Acknowledgements The authors gratefully acknowledge the support of AFOSR, AFRL ML, and AFRL MN. References [1] Javey A, Qi P, Wang Q, Dai H. Ten- to 50-nm-long quasi-ballistic carbon nanotube devices obtained without complex lithography. Proc Natl Acad Sci 2004;101(37):13408–10.
[2] Misewich JA, Martel R, Avouris P, Tsang J, Heinze S, Tersoff J. Electrically induced optical emission from a carbon nanotube FET. Science 2003;300:783–6. [3] Wang S, Liang Z, Gonnet P, Liao Y, Wang B, Zhang C. Effect of nanotube functionalization on the coefficient of thermal expansion of nanocomposites. Adv Funct Mater 2007;17:87–92. [4] Ajayan PM, Schadler LS, Giannaris C, Rubio A. Single-walled carbon nanotube-polymer composites: strength and weakness. Adv Mater 2000;12(10):750–3. [5] Wang S, Liang Z, Pham G, Park Y, Wang B, Zhang C, et al. Controlled nanostructure and high-loading of single-walled carbon nanotubes reinforced polycarbonate nanocomposite. Nanotechnology 2007;18:095708–15. [6] Wang H, Zhou W, Ho DL, Winey KI, Fisher JE, Glinka CJ, et al. Dispersing single-walled carbon nanotubes with surfactants: a small angle neutron scattering study. Nano Lett 2004;4(9):1789–93. [7] Chen J, Hamon M, Hu H, Chen Y, Rao A, Eklumd P, et al. Solution properties of single-walled carbon nanotubes. Science 1998; 282(5386):95–8. [8] Hu H, Zhao B, Hamon M, Kamaras K, Itkis M, Haddon R. Sidewall functionalization of single-walled carbon nanotubes by addition of Dichlorocarbene. J Am Chem Soc 2003;125(48):14893–900. [9] Hamon M, Hu H, Bhowmik P, Niyogi S, Zhao B, Itkis M, et al. Endgroup and defect analysis of soluble single-walled carbon nanotubes. Chem Phys Lett 2001;347:8–12. [10] Wong S, Joselevich E, Woolley AT, Cheung C, Lieber CM. Covalently functionalized nanotubes as nanometre-sized probes in chemistry and biology. Nature 1998;394:52–5. [11] Zhu J, Peng H, Rodriguez-Macias F, Margrave JL, Khabashesku VN, Imam AM, et al. Reinforcing epoxy polymer composites through covalent integration of functionalized nanotubes. Adv Funct Mater 2004;14:643–8. [12] Mao Y, Gleason KK. Hot filament chemical vapor deposition of poly(glycidyl methacrylate) thin films using tert-butyl peroxide as an initiator. Langmuir 2004;20:2484–8. [13] Hawker CJ. Molecular weight control by living free-radical polymerization process. J Am Chem Soc 1994;116:11185–6. [14] Vandijkwolthuis WN, Franssen O, Talsma H, Vansteenbergen MJ, Vandenbosch JJ, Hennink WE. Synthesis, characterization, and polymerization of glycidyl methacrylate derivatized dextran. Macromolecules 1995;28:6317–22.