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Polymer-grafted multi-walled carbon nanotubes through surface-initiated ring-opening polymerization and click reaction
Polymer 52 (2011) 2180e2188
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Polymer journal homepage: www.elsevier.com/locate/polymer
Polymer-grafted multi-walled carbon nanotubes through surface-initiated ring-opening polymerization and click reaction Ren-Shen Lee a, *, Wen-Hsin Chen a, Jarrn-Horng Lin b a b
The Center of General Education, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan, Tao-Yuan 333, Taiwan, ROC Department of Material Science, National University of Tainan, 33, Sec. 2, Shu-Lin St., Tainan 700, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 January 2011 Received in revised form 9 March 2011 Accepted 13 March 2011 Available online 21 March 2011
Multi-walled carbon nanotubes (MWNTs) are modified to possess the hydroxyl groups and are used as coinitiators to polymerize poly(e-caprolactone) (PCL) or poly(a-chloro-e-caprolactone) (PaClCL) by surface-initiated ring-opening polymerization. Pendent chlorides were converted into azides by the reaction with sodium azides. Finally, various types of terminal alkynes were reacted with pendent azides by copper-catalyzed Huisgen’s 1,3-dipolar cycloaddition (click reaction). Chemical structure of resulting product and the quantities of grafted polymer were determined by Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS). High-resolution transmission electron microscopy (HR-TEM) images clearly indicate that the nanotubes were coated with a polymer layer. The MWNT-g-PCLs and MWNT-g-(PaN3CL-g-alkyne)s are well dispersed in the organic solvent. The dispersability of MWNTs with grafted organic moieties is easier in CHCl3 than in THF. The average thickness of the enwrapped polymer layer is approximately 8e10 nm for MWNT-g-PCL and 3 nm for MWNT-g-(PaN3CL-g-PBA). Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Carbon nanotubes Functionalization of polymers Click
1. Introduction Carbon nanotubes (CNTs) are potential modifiers for polymer matrices to improve mechanical properties and electrical conductivity, and are actively explored as multipurpose innovative carriers for drug delivery and diagnostic applications [1e5]. However, preparation of valuable polymer/CNT nanocomposites strongly depends on the extent of the CNT dispersion and the strength of the interfacial adhesion. Although polymer/CNT nanocomposites can be prepared by merely mixing CNTs with the polymers in the melt [6], an appropriate chemical treatment of the nanotube surfaces or the use of a compatibilizer is required for a fine dispersion to be observed. For this reason, surface properties of CNTs have been modified by the grafting of organic molecules, including polymer chains [7e9]. Methods involving “Grafting to”, and “grafting from” have been reported for the covalent bonding of polymers onto CNTs. The covalent grafting of organic or polymeric molecules on to carbon nanotubes has been accomplished by a grafting technique through esterification or amidation reactions [10,11]. However, if a long-chain polymer were used as the grafting substance, it would be difficult to control the functionalization density by the “graft to” approach because of stereo-hindrance, leading to inefficient grafting [12]. * Corresponding author. E-mail address: [email protected] (R.-S. Lee). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.03.020
The “grafting from” technique relies on the immobilization of initiators onto tubes followed by the surface-initiated polymerization and formation of possibly dense polymer brushes. For instance, carbanions were generated at the CNT surface by reaction with an anionic compound (sec-butyl lithium), followed by the surfaceinitiated polymerization of styrene [13]. In addition, atom-transfer radical polymerization (ATRP) initiators have been attached to CNTs and poly(butyl methacrylate), and poly(methyl methacrylate) chains have been grown from the surface resulting in nanotube structures for which the solubility was controlled by the nature of the attached polymers [14e18]. A strategy for covalent grafting of biocompatible polyacrylamides onto sidewalls of MWNT via UV-initiated freeradical polymerization was presented [19]. Yoon et al. expanded this general polymerization strategy to undertake the investigation of non-radical approaches to polymer growth. One such approach involves obtaining CNT-graft-poly(L-lactide) (PLLA) using surfaceinitiated ring-opening polymerization [20]. Amine-functionalized MWNTs are used to initiate the ring-opening polymerization of g-benzyl-L-glutamate N-carboxyanhydride (BLG-NCA), resulting in polypeptide-grafted MWNTs [21]. The catalyst-functionalized carbon nanotubes were used for the surface-initiated titaniummediated coordination polymerizations L-lactide, e-caprolactone, and n-hexyl isocyanate employing the grafting from technique [22]. The copper-catalyzed 1,3-dipolar cycloaddition of azides and terminal alkynes is potentially a highly interesting reaction for
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modifying block-graft copolymers with hydrophilicehydrophobic properties [23e25]. This work presents a novel in situ ring-opening polymerization (ROP) and click reaction “grafting from” approach to functionalize multi-walled carbon nanotubes (MWNTs). This approach grafts poly(e-caprolactone) (PCL) or poly(a-azo-e-caprolactone-graft-alkyne) (PaN3CL-g-alkyne) to the nanotube surface, allowing for the modification of the nanotube properties. This compounding of a CNT-g-(PaN3CL-g-alkyne) hybrid with PaN3CL-g-alkyne may simultaneously optimize the CNT dispersion and enhance interfacial bonding through covalent grafting between the polymer chain and the functionality of the CNT; thereby improving dispersion properties. This study examined the influence of graft polymer chain length, type of graft alkyne on the dispersion, and thickness of the polymer shell around the CNTs.
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and the purity was >95%. 2-Chlorocyclohexanone, 2-propynyl benzoate (PBA), 1-hexyne, b-D-glucose pentaacetate, e-caprolactone (e-CL), and sodium azide were purchased from Aldrich Chemical Co. (Milwaukee, WI, USA). m-Chloroperoxybenzoic acid (m-CPBA) was purchased from Fluka Chemical Co. (Buchs SG1, Switzerland). Stannous octoate (SnOct2) was purchased from Strem Chemical Co. (Newburyport, MA). aClCL was prepared according to the reported method [25]. Organic solvents such as tetrahydrofuran (THF), N,Ndimethyl formamide (DMF), chloroform, toluene, and n-hexane were a high-pressure liquid chromatography (HPLC) grade and were purchased from Merck Chemical (Darmstade, Germany). Ultrapure water was used in this study was purified with a Milli-Q Plus purification system (Waters, Milford, MA). 2.2. Acid treatment of MWNTs
2. Experimental 2.1. Materials The MWNTs were provided by Iljin Nanotech Co. (South Korea). The MWNTs were obtained using a chemical vapor deposition method
A 500 mL flask charged with 1 g of the crude MWNTs and 200 mL of 60% HNO3 aqueous solution was sonicated in a bath (28 KHz) for 30 min. The mixture was then stirred for 12 h under reflux. After cooling to room temperature, the solution was diluted with 400 mL of deionized water and vacuum-filtered through
Scheme 1. Route for the preparation of the (A) MWNT-g-PCLs, and (B) MWNT-g-(PaN3CL-g-alkyne).
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a 0.22 mm polycarbonate membrane. The solid was washed with deionized water until the pH of the filtrate reached approximately 7.0. The solid was then dried under vacuum for 24 h at 60 C to yield 0.6 g of the carboxylic acid-functionalized MWNTs (MWNT-COOH). 2.3. Synthesis of hydroxyl-functionalized MWNTs The MWNT-COOH (0.6 g) was reacted with excess SOCl2 (20 mL) for 24 h under reflux, and then the residual SOCl2 was removed by reduced pressure distillation equipped with a liquid nitrogen trap, to yield an acyl chloride-functionalized MWNT (MWNT-COCl). The MWNT-COCl was added to chloroform and the mixture was sonicated for 20 min to create a homogeneous dispersion. Ethylene glycol (20 mL) was then added to the mixture under nitrogen and the reaction was allowed to proceed at 120 C for 48 h and 1 atm. The resulting reaction medium was dissolved in excess chloroform and vacuum-filtered three times through a 0.22 mm polycarbonate membrane to yield hydroxyl-functionalized MWNTs (MWNT-OH) (1.26 g). Then, the MWNT-OH was dried in vacuo at 60 C for 24 h and analyzed. The reaction is schematicized in Scheme 1. 2.4. Functionalization of MWNTs with PCL or PaClCL The resulting MWNT-OH (20 mg) was allowed to swell in 20 mL of dry DMF for 1 h at 140 C. The swollen MWNT-OHs were sonicated in a bath (28 KHz) for 30 min and then SnOct2 (3 wt%) was added under a nitrogen atmosphere followed by sonication for another 10 min. A predetermined amount (50 eq.) of e-CL (or aClCL) was then added under a nitrogen atmosphere, and the reaction was allowed to proceed at 140 C for 24 h. The reaction is schematicized in Scheme 1. After cooling to room temperature, the reaction medium was washed with THF several times to remove the unbound PCL or PaClCL and vacuum-filtered through a 0.22 mm polycarbonate membrane to yield 26 mg PCL (or PaClCL)-grafted MWNT (MWNT-g-PCL, or MWNT-g-PaClCL). The PCL (or PaClCL)grafted MWNT was dried in vacuo at 60 C for 24 h and then analyzed. 2.5. Synthesis of MWNT-g-PaN3CL The MWNT-g-PaClCL (101 mg) was allowed to swell in 10 mL of dry DMF in a glass reactor followed by the addition of NaN3 (5 eq.). The mixture was allowed to proceed under a nitrogen atmosphere at 60 C for 24 h. After cooling to room temperature, the product was separated by centrifugation and washed with THF and DI-water several times to remove the unreacted NaN3. The MWNTg-PaN3CL (130 mg) was dried in vacuo at 60 C for 24 h and then analyzed.
Fig. 1. FT-IR spectra: (a) MWNT-g-CO2H, (b) MWNT-g-CO2CH2CH2OH(MWNT-OH), (c) MWNT-g-PCL4, (d) MWNT-g-PCL8, and (e) MWNT-g-PCL11.
2.6. Typical click reaction The MWNT-g-PaN3CL (50 mg) was transferred into a glass reactor containing 10 mL of dry THF. Propynyl benzoate (PBA) (2.5 eq.), CuI (0.1 mol), and triethyl amine (0.1 mol) were then added to the reactor. The solution was stirred under a nitrogen atmosphere at 60 C for 24 h. The cycloaddition of the MWNT-g(PaN3CL-g-PBA) was separated by centrifugation and washed with THF and DI-water several times. The purified MWNT-g-(PaN3CLg-PBA) (57.7 mg) was dried in vacuo at 60 C for 24 h and then analyzed. 2.7. Measurements High-resolution TEM (HR-TEM) analysis was carried out on a FEI Tecnai G2 F20 operating at 300 KeV. The samples after pretreatment were dispersed in methanol, and the solution was mixed ultrasonically at room temperature. A part of solution this solution was dropped on the grid for the measurement of TEM images. The grafting behavior of the PCL or PaN3CL-g-PBA onto the MWNT was monitored by Fourier transform infrared (FT-IR) spectroscopy (Bruker TENSOR 27, German). The FT-IR transmission spectra of the samples were measured with a disk of KBr. Raman spectra were obtained by neodymium (Nd) laser excitation at 1064 nm (Bruker RFS 100/s) using an InGaAs detector. Thermogravimetric analysis (TGA) was performed using a TGA-1 (Mettler-Toledo, Switzerland)
Table 1 Result of the polymer-grafted MWNTs and TGA data. MWNT-g-polymer
Monomer/MWNT-OHa molar ratio in feed
Weight lossb (%)
Grafted unit
MWNT-g-PCL4 MWNT-g-PCL8 MWNT-g-PCL11 MWNT-g-PaClCL11 MWNT-g-PaN3CL11 MWNT-g-(PaN3CL11-g-Hex)c MWNT-g-(PaN3CL11-g-PBA)c
10/1 30/1 50/1 10/1 11/1 11/1 11/1
34.1 51.9 67.0 85.6 86.1 88.9 88.5
4 8 11 11 11 7.5d 3.3d
a The mol% of initiator present in the MWNTs was calculated as follows: [I]MWNT ¼ [(weight loss % of initiator from TGA/mol. wt. of initiator fragment)/(100/ mol. wt. of carbon)] 100. b Determined by TGA. c Abbreviations : Hex ¼ 1-hexyne; PBA ¼ 2-propynyl benzoate. d The grafting efficiency (%) of MWNT-g-PaN3CL with alkyne by click reaction.
Fig. 2. FT-IR spectra: (a) MWNT-g-PaClCL11, (b) MWNT-g-PaN3CL11, and (c) MWNT-g(PaN3CL11-g-PBA).
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by scanning from 25 to 800 C at a heating rate of 10 C min1 under a nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) analyses were conducted on an ESCA Lab 250, and all the samples were used directly without further treatment. The solubility of the samples was tested as follows: the samples were ground, and 0.5 mg of the sample was added with 1.0 mL of the solvent into a tube. The tubes were treated by supersonic radiation for 50 min and kept still for 3 h. 3. Results and discussion 3.1. Synthesis of MWNT-g-PCL The general strategy for grafting polymers from multi-walled carbon nanotubes (MWNT) via ring-opening polymerization is described in Scheme 1. Three steps are involved: (1) carbonyl
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chloride groups of functionalized MWNT (MWNT-COCl) were prepared via reaction of thionyl chloride with carboxyl-contained MWNT (MWNT-COOH) previously made by oxidation of the crude MWNT with 60% HNO3; (2) hydroxyl groups were introduced onto the surface of the MWNT by reaction of MWNT-COCl with glycol, generating MWNT-OH; and (3) grafting polymerization of e-CL or aClCL from MWNT-OH was conducted using ROP, resulting in an MWNT-g-PCL or an MWNT-g-PaClCL. Results of the polymer-grafted MWNT are shown in Table 1. The quantity of the initiator attached to the surface was determined by thermo-gravimetric analysis (TGA) of the MWNT-CO2H, showing a 15.8% weight loss. The mole percent of the initiator ([I]MWNT ¼ 4.2 mol% with respect to carbon) on the MWNT surface was calculated using the weight loss % and the initiator fragment molecular weight (45 g mol1) [26]. Grafting of PCL chains from the MWNT-OH surface by ringopening polymerization was attempted in a controlled manner.
Fig. 3. 1H NMR spectra MWNT graft polymers in CDCl3; (A) MWNT-g-PCL8 and, (B) MWNT-g-(PaN3CL11-g-PBA). The star indicates solvent signal.
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SnOct2 was employed as the catalyst for the e-CL polymerizations because of the high efficiency, and relatively low toxicity. It is permissible as a food additive in numerous countries. Scheme 1(A) illustrates the synthesis of MWNT-g-PCL using ROP of e-CL with initiator MWNT-OH (at molar ratios 10/1, 30/1, and 50/1) in the presence of SnOct2 in C6H5Cl at reflux for 24 h. After the grafting reaction, the product was washed thoroughly with THF, and filtered to remove THF-soluble substances such as the PCL not bound to the MWNT. The PCL-functionalized MWNT (MWNT-g-PCL) emitted a black color. Considerable weight gain was observed after the grafting reaction, confirming that a substantial amount of PCL had been grafted to the MWNT. Washing and filtration were continued
until no trace of the THF-soluble substances was detected in the filtrate, and the PCL not bound to the MWNT was considered to be completely removed from the product. The ungrafted PCL formed that it is possible that there is trace water adsorbed on the nanotube surface, and each adsorbed water molecule can act as an initiator of the ring-opening polymerization, generating as a result of ungrafted PCL. Based on this, the FT-IR, and 1H NMR spectra of the MWNT-g-PCL were analyzed to prove the polymerization of e-CL from the MWNT-OH surface. Fig. 1 shows the FT-IR spectra of the MWNT-CO2H, MWNT-OH and MWNT-g-PCL. The FT-IR spectrum of the MWNT-CO2H (Fig. 1 (a)) exhibits the expected C]O stretching at 1736 cm1,
Fig. 4. TGA traces of (A) MWNT-CO2H (trace 1), MWNT-g-PCL3 (trace 2), MWNT-g-PCL4 (trace 3), MWNT-g-PCL8 (trace 4), and MWNT-g-PCL11 (trace 5), (B) MWNT-g-PaClCL11 (trace 1), MWNT-g-PaN3CL11 (trace 2), MWNT-g-(PaN3CL11-g-PBA) (trace 3), and MWNT-g-(PaN3CL11-g-hexyne) (trace 4).
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corresponding to the incorporated carboxylic acid groups as a result of the acid treatment process. In Fig. 1(b), the FT-IR band observed at 3400, 1735, and 1050 cm1 are attributed, respectively, to the stretching of OH, C]O (ester), and CeO vibrations that arose from the esterification. The strong carbonyl stretching band at 1735 cm1 clearly reveals the ester bands that came from the PCL grafted to the MWNT. The CeH stretching frequencies at 2853 and 2925 cm1 that correspond to the alkyl groups were observed. The intensity of the carbonyl and CeH stretching band were more intense in the MWNT-g-PCL than the MWNT-OH and increased with the incorporated into the PCL length, and again confirming the occurrence of the grafting reaction. The presence of surfaces initiated polymer (PCL) was clearly seen in the 1H NMR spectrum of the MWNT-g-PCL (Fig. 3(A)). The resonance peaks were assigned to the corresponding hydrogen atoms of the PCL. The amount of the PCL grafted to the MWNT, defined as the ratio of the mass of the immobilized PCL to that of the MWNT-g-PCL was estimated by TGA as shown in Fig. 4(A); and the weight-loss data at 800 C are listed in Table 1. All of the MWNT-g-PCLs exhibited multiple decomposition stages. The first decomposition stage, in the range of 50e180 C, is attributed to the loss of absorbed water. The second decomposition stage, in an interval of 200e300 C, corresponds to the dehydration (anhydrification of vicinal acid functions) plus the decarboxylation of the polymer, which results in inter- and intramolecular bridging. The third decomposition stage, in the range 300e470 C, is a result of the degradation of the polymer. The residual fraction at the temperature above 700 C corresponds mainly to the MWNTs. The amount of the PCL bound to the MWNT ranged from 34 to 67 wt% when the molar ratios increased from 10/1 to 50/1. The amount of grafted PCL increased with the molar ratio of e-CL to MWNT-OH. In terms of the PCL contents and the hydroxyl group content of MWNT-OH, one can estimate the average molecular weights of the grafted PCL chains respective to the MWNT-g-PCL samples, which are MWNT-g-PCL3: 307, MWNT-g-PCL4: 436, MWNT-g-PCL8: 890, and MWNT-g-PCL11: 1219 g mol1. The average molecular weights of grafted PCLs ¼ [(weight loss % of MWNTg-PCL) (weight loss % of the MWNT-CO2H)]/[I]MWNT. The number of hydroxyl groups in MWNT-OH is limited while the amount of e-CL is abundant in the reaction solution; therefore, the reacted e-CL is not proportional to the feed ratio. The traces in Fig. 4(A) show that the weight loss of MWNT-CO2H is more rapid than that of MWNT-gPCL under 250 C. This phenomenon suggests that the chemical bonds of the hydroxyl groups with the MWNTs are more easily broken, and the small segments broken away from the MWNT-OH can then be easily removed by the N2 flow.
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appealing because of a mild condition. Various types of alkynes, for example, 1-hexyne, 2-propynyl benzoate (PBA), and 4-pentynyl2,3,4,6-tetra-O-acetyl-b-D-gluco-pyranoside (PAG) can be attached onto azides containing MWNT-g-PaN3CL by click reaction. The IR absorption at 2106 cm1, characteristic of the azide, completely disappears. The 1H NMR of MWNT-g-(PaN3CL-g-PBA) is shown in Fig. 3(B). The observed proton signals are entirely due to surfaceinitiated polymers grown from MWNTs. However, considerable line broadening was observed as a result of the existence of paramagnetic substances in the MWNTs. The amount of the PaClCL grafted to the MWNT, defined as the ratio of the mass of the immobilized PaClCL to that of the MWNTg-PaClCL, was estimated by TGA, as shown in Fig. 4(B); and the weight-loss data at 800 C are listed in Table 1. The TGA analysis of MWNT-g-PaClCL in the presence of nitrogen showed two major decomposition stages. The first decomposition stage, in the range of 250e400 C, corresponds to the dehydrochlorination (eliminate the aCl and bH of PaClCL) plus the decarboxylation of the MWNT. The second decomposition stage, in the range of 500e625 C, corresponding to the decompositions of surface grown polymers (Fig. 4 (B) trace 1). The weight loss is 85.6%. The mass loss stops at approximately 14% and remains stable to 800 C, indicating that the sample was composed of 14% nanotubes by weight. The graft units of aClCL bound to the MWNT of which approximately 11 correspond to the molar ratio. Fig. 4(B) trace 2 shows the TGA decomposition of MWNT-g-PaN3CL. The weight loss is 86.1%. The graft units of aN3CL
3.2. Synthesis of MWNT-g-(PaN3CL-g-alkyne) The synthesis of the MWNT-g-(PaN3CL-g-alkyne) consists of three consecutive steps from the MWNT-OH surface (Scheme 1(B)). First, the ring-opening polymerization of aClCL was initiated by hydroxyl-terminated MWNT-OH with SnOct2 as the catalyst. In a second step, pendent chlorides were converted into azides by the reaction with sodium azide. Finally, various types of terminal alkynes were reacted with pendent azides by copper-catalyzed Huisgen’s 1,3dipolar cycloaddition. Fig. 2 shows the FT-IR spectra of the MWNT-gPaClCL, MWNT-g-PaN3CL, and MWNT-g-(PaN3CL-g-PBA). The strong carbonyl stretching band at 1737 cm1 clearly reveals the ester bonds that come from the PaClCL grafted to the MWNT. Then, the pendent chlorides of MWNT-g-PaClCL were converted into azides by reaction with sodium azide. The MWNT-g-PaClCL was thus reacted with 1 eq. of sodium azide in DMF at 60 C overnight. The IR spectrum expectedly shows a new absorption at 2106 cm1, characteristic of the azide (Fig. 2(b)). To functionalize the MWNT, chemical transformation by click reaction between alkynes and azides is highly
Fig. 5. Raman spectra of polymer-grafted MWNTs (514.5 nm excitation): (A) MWNTOH (curve 1), MWNT-g-PCL11 (curve 2), MWNT-g-PCL8 (curve 3), MWNT-g-PCL4 (curve 4), pristine MWNT (curve 5), (B) MWNT-g-PaClCL11 (curve 1), MWNT-g-(PaN3CL11g-PBA) (curve 2), MWNT-g-PaN3CL11 (curve 3).
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bound to the MWNT of which approximately 11 correspond to the molar ratio of MWNT-g-PaN3CL. For the MWNT-g-(PaN3CL-galkyne), the TGA analysis showed two major decompositions in the temperature range of 250e325 C and 300e450 C. However, the grafting efficiency of alkyne bound to azide are low; only 7.5 and 3.3% for 1-hexyne and PBA, respectively; this may be due to the stereohindrance of MWNT-g-PaN3CL (Fig. 4(B), traces 3 and 4).
3.3. Raman analysis Raman spectroscopy is a powerful tool used for the characterization of functionalized CNTs. As shown in Fig. 5(A), the D and G bands of the MWNTs at 1287 and 1598 cm1, attributed to the defects and disorder-induced peaks and tangential-mode peaks that were clearly observed for the MWNT-OH, and the MWNT-g-
Fig. 6. Digital photos: (A) A: MWNT-OH, A1: MWNT-g-PCL8, A2: MWNT-g-PCL4, A3: MWNT-g-PCL11 in CHCl3, (B) B: MWNT-OH, B1: MWNT-g-(PaN3CL11-g-hexyne), B2: MWNTg-PaClCL11, B3: MWNT-g-PaN3CL11, B4: MWNT-g-(PaN3CL11-g-PBA) in CHCl3, (C) C: MWNT-OH, C1: MWNT-g-(PaN3CL11-g-hexyne), C2: MWNT-g-PaClCL11, C3: MWNT-g-PaN3CL11, C4: MWNT-g-(PaN3CL11-g-PBA) in THF, (D) D: MWNT-g-(PaN3CL11-g-PBA), D1: MWNT-g-(PaN3CL30-g-PBA), D2: MWNT-g-(PaN3CL15/PCL15-g-PBA), D3: MWNT-g-(PaN3CL11-g-PAG) in CHCl3. The content of the sample is around 1 mg of sample per 1 mL of solvent.
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Fig. 7. HR-TEM images: (A) pristine MWNT, (B) MWNT-g-PCL11, (C) MWNT-g-PCL8, and (D) MWNT-g-(PaN3CL11-g-PBA). (Scare bar 10 nm).
PCL. The intensity ratio of the D to G band (ID/IG) for MWNT-g-PCLs was 1.59, 1.39, and 1.35, with PCL units 4, 8, and 11, respectively; all of which are larger than that of the pristine MWNT (1.07) as a result of PCL grafting. However, the peak intensity of the MWNT-g-PCL was weaker than that of the MWNT-OH (ID/IG ¼ 1.64). In other words, the characteristic absorption peaks were strongly attenuated by the grafted PCL. The absorption attenuation as a function of the amount of the grafted polymers may be due to the grafted polymers lowering the crystal symmetry of the nanotube lattice. For MWNT-g-PaClCL, MWNT-g-PaN3CL, and MWNT-g-(PaN3CL-gPBA) grafted MWNT, the ID/IG ranged from 1.00 to 1.25, lower than that of MWNT-OH (Fig. 5(B)). For the covalently functionalized CNTs, the polymer anchored to the CNT surface forms a nanometer-scale layer of coverage. The decrease of the Raman signals of the grafted polymer MWNT came neither from the low CNT content nor from the translucence of the polymer layer. It can possibly be attributed to the energy transfer between the CNT and the polymer layer, or to the influence of the grafted polymers on the electronic properties of the CNT. Further studies are required to understand completely the effects of polymer grafting to CNTs on the intensity of Raman signals. 3.4. The dispersability of polymer-grafted MWNTs Dispersion of pristine MWNTs into aqueous solution or organic solvents was highly difficult even after it was sonicated. However, dispersion of MWNT-polymer into organic solvents was extremely easy. Therefore, the solubility or dispersability of the functionalized MWNTs strongly depends on the structure and amounts of the grafted organic moieties. Fig. 6(A) provides the digital photos of MWNT-OH and MWNT-g-PCL (with the units of CL: 4, 8, and 11) (1 mg) in CHCl3 (1 mL). It is clear that the dispersability of MWNTg-PCL is larger than MWNT-OH, and increases with longer PCL moieties. The dispersability of MWNT-OH, MWNT-g-PaClCL, MWNT-g-PaN3CL, MWNT-g-(PaN3CL-g-hexyne), and MWNT-g(PaN3CL-g-PBA) were investigated in CHCl3 and THF (Fig. 6(B) and
Fig. 8. XPS analysis of MWNT-OH, and MWNT-g-PCL: (A) C1s, and (B) O1s.
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(C)). The dispersability of the MWNT with grafted organic moieties are easier in CHCl3 than in THF, due to the interaction of the PCL chains on the MWNT surface with CHCl3 is stronger [27]. For comparison, we attempted to disperse the MWNT-g-(PaN3CL10g-PBA), MWNT-g-(PaN3CL30-g-PBA), MWNT-g-(PaN3CL15/PCL15g-PBA), and MWNT-g-(PaN3CL10-g-PAG) in CHCl3, showing that MWNT-g-(PaN3CL30-g-PBA) is a favorable dispersant in CHCl3 (Fig. 6(D)). All of the evidence indicates that PaN3CL-g-PBA chains were successfully grafted from the surface of MWNTs. 3.5. Morphology The morphological structures of pristine, PCL-grafted and PaN3CL-g-PBA-grafted MWNTs were examined by HR-TEM images. Fig. 7 shows HR-TEM images of pristine MWNTs, MWNT-g-PCL11, MWNT-g-PCL8, and MWNT-g-(PaN3CL10-g-PBA). From the image of a pristine MWNT, as shown in Fig. 7(A), it is clear that the pristine MWNT shell is quite shallow. Fig. 7(B)e(D) displays the images of modified MWNTs with various thicknesses of the polymer layers having somewhat shallower and more transparent color. The average thickness of the enwrapped polymer layer is approximately 8e10 nm for MWNT-g-PCL and 3 nm for MWNT-g-(PaN3CL-g-PBA). By comparison, the MWNT shell with a polymer layer is thicker than the pristine MWNT. These results indicate that the MWNT can be grafted by surface-initiated ring-opening polymerization and click reaction. The outcome obtained from TEM effectively accords with that resulting from TGA. 3.6. XPS analysis The results of XPS analysis are shown in Fig. 8. The C1s, and O1s spectra provide some interesting information [11]. The C1s spectrum of MWNT-OH mainly shows that the CeC peak of the CNTs at 284.2 eV, and O]CeO peak (288.7 eV) is relatively weak. The C1s spectrum of MWNT-g-PCL has a similar appearance: the peak at 284.5 eV belongs to CeC and C]C bonds. The peaks of the ester C]O (288.8 eV) are mild peaks. The O1s spectrum of MWNT-OH shows a mild and wide peak that results from the carbonyl (532.5 eV) and CeO (535.7 eV) bond introduced by the oxidation treatment. The MWNT-g-PCL show mainly two peaks: C]O (532.7 eV), and CeO (536 eV). The peak shift may also be attributed to hydrogen bonding. 4. Conclusion Poly(e-caprolactone) (PCL) and PaN3CL-g-alkyne modified MWNTs have been prepared by the “graft-from” approach. FT-IR, 1H NMR, XPS,
and TGA data verifies that the surface modification of MWNTs is successful. The HR-TEM images of the products show that the thickness of the polymer shell around the MWNT is approximately 8e10 nm for MWNT-g-PCL and 3 nm for MWNT-g-(PaN3CL-g-PBA). Therefore, a facile route for the covalent surface functionalization of CNTs by PCL and PaN3CL-g-alkyne was developed in this work.
Acknowledgments The research was supported by grants from Chang Gung University (UMRPD590091). The authors are grateful to Advanced Instrumentation Center, National Taiwan University for XPS measurement.
References [1] Qin S, Qin D, Ford WT, Herrera JE, Resasco DE, Bachilo SM, et al. Macromolecules 2004;37:3965. [2] Zengin H, Zhou W, Jin J, Czerw R, Smith DW, Echegoyen L, et al. Adv Mater 2002;14:1480. [3] Prato M, Kostarelos K, Bianco A. Acc Chem Res 2008;41:60. [4] Saito N, Usui Y, Aoki K, Narita N, Shimizu M, Hara K, et al. Chem Soc Rev 2009; 38:1897. [5] Germarie SP, Lenibel SR, Carlos RC. J Nanosci Nanotechnol 2009;9:2175. [6] Hashiwagi T, Grulke E, Hilding J, Harries R, Awad W, Douglas J. Macromol Rapid Commun 2002;23:761. [7] Liu Y, Yao Z, Adronov A. Macromolecules 2005;38:1172. [8] Li H, Cheng F, Duft AM, Adronov A. J Am Chem Soc 2005;127:14518. [9] Qin S, Qin D, Ford WT, Herrera JE, Resasco DE. Macromolecules 2004;37:9963. [10] Zhang Y, Li J, Shen Y, Wang M, Li J. J Phys Chem B 2004;108:15343. [11] Ramanathan T, Fisher FT, Ruoff RS, Brinson LC. Chem Mater 2005;17:1290. [12] Baskaran D, Mays JW, Bratcher MS. Angew Chem Int Ed 2004;43:2138. [13] Viswanathan G, Chakrapani PM, Yang H, Wei B, Chung H, Cho K, et al. J Am Chem Soc 2003;125:9258. [14] Qiu S, Qin D, Ford WT, Resasco DE, Herrera JE. J Am Chem Soc 2004;126:170. [15] Yao Z, Braidy N, Batton GA, Adronov A. J Am Chem Soc 2003;125:16015. [16] Kong H, Gao C, Yan D. J Am Chem Soc 2004;126:412. [17] Vestal CR, Zhang ZJ. J Am Chem Soc 2002;124:14312. [18] Wang G, Liu Y. Macromol Chem Phys 2009;210:2070. [19] Petro P, Georgiev G, Momekova D, Momekov G, Tsvetanov CB. Polymer 2010; 51:2465. [20] Chen GX, Kim HK, Park BH, Yoon JS. Macromol Chem Phys 2007;208:389. [21] Yao Y, Li W, Wang S, Yan D, Chen X. Macromol Rapid Commun 2006;27:2019. [22] Priftis D, Petzetakis N, Sakellariou G, Pitsikalis M, Baskaran D, Mays JW, et al. Macromolecules 2009;42:3340. [23] Lutz JF. Angew Chem Int Ed 2007;46:1018. [24] Nandivada BH, Jiaqng X, Lahann J. Adv Mater 2007;19:2197. [25] Lee RS, Huang YT. J Polym Sci Part A Polym Chem 2008;46:4320. [26] [I]MWNT ¼ [(weight loss % of initiator from TGA/mol. wt. of initiator fragment)/ (100/mol. wt. of carbon)] 100. [27] Yang LP, Pan CY. Macromol Chem Phys 2008;209:783.
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Polymer-grafted multi-walled carbon nanotubes through surface-initiated ring-opening polymerization and click reaction Ren-Shen Lee a, *, Wen-Hsin Chen a, Jarrn-Horng Lin b a b
The Center of General Education, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan, Tao-Yuan 333, Taiwan, ROC Department of Material Science, National University of Tainan, 33, Sec. 2, Shu-Lin St., Tainan 700, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 January 2011 Received in revised form 9 March 2011 Accepted 13 March 2011 Available online 21 March 2011
Multi-walled carbon nanotubes (MWNTs) are modified to possess the hydroxyl groups and are used as coinitiators to polymerize poly(e-caprolactone) (PCL) or poly(a-chloro-e-caprolactone) (PaClCL) by surface-initiated ring-opening polymerization. Pendent chlorides were converted into azides by the reaction with sodium azides. Finally, various types of terminal alkynes were reacted with pendent azides by copper-catalyzed Huisgen’s 1,3-dipolar cycloaddition (click reaction). Chemical structure of resulting product and the quantities of grafted polymer were determined by Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS). High-resolution transmission electron microscopy (HR-TEM) images clearly indicate that the nanotubes were coated with a polymer layer. The MWNT-g-PCLs and MWNT-g-(PaN3CL-g-alkyne)s are well dispersed in the organic solvent. The dispersability of MWNTs with grafted organic moieties is easier in CHCl3 than in THF. The average thickness of the enwrapped polymer layer is approximately 8e10 nm for MWNT-g-PCL and 3 nm for MWNT-g-(PaN3CL-g-PBA). Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Carbon nanotubes Functionalization of polymers Click
1. Introduction Carbon nanotubes (CNTs) are potential modifiers for polymer matrices to improve mechanical properties and electrical conductivity, and are actively explored as multipurpose innovative carriers for drug delivery and diagnostic applications [1e5]. However, preparation of valuable polymer/CNT nanocomposites strongly depends on the extent of the CNT dispersion and the strength of the interfacial adhesion. Although polymer/CNT nanocomposites can be prepared by merely mixing CNTs with the polymers in the melt [6], an appropriate chemical treatment of the nanotube surfaces or the use of a compatibilizer is required for a fine dispersion to be observed. For this reason, surface properties of CNTs have been modified by the grafting of organic molecules, including polymer chains [7e9]. Methods involving “Grafting to”, and “grafting from” have been reported for the covalent bonding of polymers onto CNTs. The covalent grafting of organic or polymeric molecules on to carbon nanotubes has been accomplished by a grafting technique through esterification or amidation reactions [10,11]. However, if a long-chain polymer were used as the grafting substance, it would be difficult to control the functionalization density by the “graft to” approach because of stereo-hindrance, leading to inefficient grafting [12]. * Corresponding author. E-mail address: [email protected] (R.-S. Lee). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.03.020
The “grafting from” technique relies on the immobilization of initiators onto tubes followed by the surface-initiated polymerization and formation of possibly dense polymer brushes. For instance, carbanions were generated at the CNT surface by reaction with an anionic compound (sec-butyl lithium), followed by the surfaceinitiated polymerization of styrene [13]. In addition, atom-transfer radical polymerization (ATRP) initiators have been attached to CNTs and poly(butyl methacrylate), and poly(methyl methacrylate) chains have been grown from the surface resulting in nanotube structures for which the solubility was controlled by the nature of the attached polymers [14e18]. A strategy for covalent grafting of biocompatible polyacrylamides onto sidewalls of MWNT via UV-initiated freeradical polymerization was presented [19]. Yoon et al. expanded this general polymerization strategy to undertake the investigation of non-radical approaches to polymer growth. One such approach involves obtaining CNT-graft-poly(L-lactide) (PLLA) using surfaceinitiated ring-opening polymerization [20]. Amine-functionalized MWNTs are used to initiate the ring-opening polymerization of g-benzyl-L-glutamate N-carboxyanhydride (BLG-NCA), resulting in polypeptide-grafted MWNTs [21]. The catalyst-functionalized carbon nanotubes were used for the surface-initiated titaniummediated coordination polymerizations L-lactide, e-caprolactone, and n-hexyl isocyanate employing the grafting from technique [22]. The copper-catalyzed 1,3-dipolar cycloaddition of azides and terminal alkynes is potentially a highly interesting reaction for
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modifying block-graft copolymers with hydrophilicehydrophobic properties [23e25]. This work presents a novel in situ ring-opening polymerization (ROP) and click reaction “grafting from” approach to functionalize multi-walled carbon nanotubes (MWNTs). This approach grafts poly(e-caprolactone) (PCL) or poly(a-azo-e-caprolactone-graft-alkyne) (PaN3CL-g-alkyne) to the nanotube surface, allowing for the modification of the nanotube properties. This compounding of a CNT-g-(PaN3CL-g-alkyne) hybrid with PaN3CL-g-alkyne may simultaneously optimize the CNT dispersion and enhance interfacial bonding through covalent grafting between the polymer chain and the functionality of the CNT; thereby improving dispersion properties. This study examined the influence of graft polymer chain length, type of graft alkyne on the dispersion, and thickness of the polymer shell around the CNTs.
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and the purity was >95%. 2-Chlorocyclohexanone, 2-propynyl benzoate (PBA), 1-hexyne, b-D-glucose pentaacetate, e-caprolactone (e-CL), and sodium azide were purchased from Aldrich Chemical Co. (Milwaukee, WI, USA). m-Chloroperoxybenzoic acid (m-CPBA) was purchased from Fluka Chemical Co. (Buchs SG1, Switzerland). Stannous octoate (SnOct2) was purchased from Strem Chemical Co. (Newburyport, MA). aClCL was prepared according to the reported method [25]. Organic solvents such as tetrahydrofuran (THF), N,Ndimethyl formamide (DMF), chloroform, toluene, and n-hexane were a high-pressure liquid chromatography (HPLC) grade and were purchased from Merck Chemical (Darmstade, Germany). Ultrapure water was used in this study was purified with a Milli-Q Plus purification system (Waters, Milford, MA). 2.2. Acid treatment of MWNTs
2. Experimental 2.1. Materials The MWNTs were provided by Iljin Nanotech Co. (South Korea). The MWNTs were obtained using a chemical vapor deposition method
A 500 mL flask charged with 1 g of the crude MWNTs and 200 mL of 60% HNO3 aqueous solution was sonicated in a bath (28 KHz) for 30 min. The mixture was then stirred for 12 h under reflux. After cooling to room temperature, the solution was diluted with 400 mL of deionized water and vacuum-filtered through
Scheme 1. Route for the preparation of the (A) MWNT-g-PCLs, and (B) MWNT-g-(PaN3CL-g-alkyne).
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a 0.22 mm polycarbonate membrane. The solid was washed with deionized water until the pH of the filtrate reached approximately 7.0. The solid was then dried under vacuum for 24 h at 60 C to yield 0.6 g of the carboxylic acid-functionalized MWNTs (MWNT-COOH). 2.3. Synthesis of hydroxyl-functionalized MWNTs The MWNT-COOH (0.6 g) was reacted with excess SOCl2 (20 mL) for 24 h under reflux, and then the residual SOCl2 was removed by reduced pressure distillation equipped with a liquid nitrogen trap, to yield an acyl chloride-functionalized MWNT (MWNT-COCl). The MWNT-COCl was added to chloroform and the mixture was sonicated for 20 min to create a homogeneous dispersion. Ethylene glycol (20 mL) was then added to the mixture under nitrogen and the reaction was allowed to proceed at 120 C for 48 h and 1 atm. The resulting reaction medium was dissolved in excess chloroform and vacuum-filtered three times through a 0.22 mm polycarbonate membrane to yield hydroxyl-functionalized MWNTs (MWNT-OH) (1.26 g). Then, the MWNT-OH was dried in vacuo at 60 C for 24 h and analyzed. The reaction is schematicized in Scheme 1. 2.4. Functionalization of MWNTs with PCL or PaClCL The resulting MWNT-OH (20 mg) was allowed to swell in 20 mL of dry DMF for 1 h at 140 C. The swollen MWNT-OHs were sonicated in a bath (28 KHz) for 30 min and then SnOct2 (3 wt%) was added under a nitrogen atmosphere followed by sonication for another 10 min. A predetermined amount (50 eq.) of e-CL (or aClCL) was then added under a nitrogen atmosphere, and the reaction was allowed to proceed at 140 C for 24 h. The reaction is schematicized in Scheme 1. After cooling to room temperature, the reaction medium was washed with THF several times to remove the unbound PCL or PaClCL and vacuum-filtered through a 0.22 mm polycarbonate membrane to yield 26 mg PCL (or PaClCL)-grafted MWNT (MWNT-g-PCL, or MWNT-g-PaClCL). The PCL (or PaClCL)grafted MWNT was dried in vacuo at 60 C for 24 h and then analyzed. 2.5. Synthesis of MWNT-g-PaN3CL The MWNT-g-PaClCL (101 mg) was allowed to swell in 10 mL of dry DMF in a glass reactor followed by the addition of NaN3 (5 eq.). The mixture was allowed to proceed under a nitrogen atmosphere at 60 C for 24 h. After cooling to room temperature, the product was separated by centrifugation and washed with THF and DI-water several times to remove the unreacted NaN3. The MWNTg-PaN3CL (130 mg) was dried in vacuo at 60 C for 24 h and then analyzed.
Fig. 1. FT-IR spectra: (a) MWNT-g-CO2H, (b) MWNT-g-CO2CH2CH2OH(MWNT-OH), (c) MWNT-g-PCL4, (d) MWNT-g-PCL8, and (e) MWNT-g-PCL11.
2.6. Typical click reaction The MWNT-g-PaN3CL (50 mg) was transferred into a glass reactor containing 10 mL of dry THF. Propynyl benzoate (PBA) (2.5 eq.), CuI (0.1 mol), and triethyl amine (0.1 mol) were then added to the reactor. The solution was stirred under a nitrogen atmosphere at 60 C for 24 h. The cycloaddition of the MWNT-g(PaN3CL-g-PBA) was separated by centrifugation and washed with THF and DI-water several times. The purified MWNT-g-(PaN3CLg-PBA) (57.7 mg) was dried in vacuo at 60 C for 24 h and then analyzed. 2.7. Measurements High-resolution TEM (HR-TEM) analysis was carried out on a FEI Tecnai G2 F20 operating at 300 KeV. The samples after pretreatment were dispersed in methanol, and the solution was mixed ultrasonically at room temperature. A part of solution this solution was dropped on the grid for the measurement of TEM images. The grafting behavior of the PCL or PaN3CL-g-PBA onto the MWNT was monitored by Fourier transform infrared (FT-IR) spectroscopy (Bruker TENSOR 27, German). The FT-IR transmission spectra of the samples were measured with a disk of KBr. Raman spectra were obtained by neodymium (Nd) laser excitation at 1064 nm (Bruker RFS 100/s) using an InGaAs detector. Thermogravimetric analysis (TGA) was performed using a TGA-1 (Mettler-Toledo, Switzerland)
Table 1 Result of the polymer-grafted MWNTs and TGA data. MWNT-g-polymer
Monomer/MWNT-OHa molar ratio in feed
Weight lossb (%)
Grafted unit
MWNT-g-PCL4 MWNT-g-PCL8 MWNT-g-PCL11 MWNT-g-PaClCL11 MWNT-g-PaN3CL11 MWNT-g-(PaN3CL11-g-Hex)c MWNT-g-(PaN3CL11-g-PBA)c
10/1 30/1 50/1 10/1 11/1 11/1 11/1
34.1 51.9 67.0 85.6 86.1 88.9 88.5
4 8 11 11 11 7.5d 3.3d
a The mol% of initiator present in the MWNTs was calculated as follows: [I]MWNT ¼ [(weight loss % of initiator from TGA/mol. wt. of initiator fragment)/(100/ mol. wt. of carbon)] 100. b Determined by TGA. c Abbreviations : Hex ¼ 1-hexyne; PBA ¼ 2-propynyl benzoate. d The grafting efficiency (%) of MWNT-g-PaN3CL with alkyne by click reaction.
Fig. 2. FT-IR spectra: (a) MWNT-g-PaClCL11, (b) MWNT-g-PaN3CL11, and (c) MWNT-g(PaN3CL11-g-PBA).
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by scanning from 25 to 800 C at a heating rate of 10 C min1 under a nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) analyses were conducted on an ESCA Lab 250, and all the samples were used directly without further treatment. The solubility of the samples was tested as follows: the samples were ground, and 0.5 mg of the sample was added with 1.0 mL of the solvent into a tube. The tubes were treated by supersonic radiation for 50 min and kept still for 3 h. 3. Results and discussion 3.1. Synthesis of MWNT-g-PCL The general strategy for grafting polymers from multi-walled carbon nanotubes (MWNT) via ring-opening polymerization is described in Scheme 1. Three steps are involved: (1) carbonyl
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chloride groups of functionalized MWNT (MWNT-COCl) were prepared via reaction of thionyl chloride with carboxyl-contained MWNT (MWNT-COOH) previously made by oxidation of the crude MWNT with 60% HNO3; (2) hydroxyl groups were introduced onto the surface of the MWNT by reaction of MWNT-COCl with glycol, generating MWNT-OH; and (3) grafting polymerization of e-CL or aClCL from MWNT-OH was conducted using ROP, resulting in an MWNT-g-PCL or an MWNT-g-PaClCL. Results of the polymer-grafted MWNT are shown in Table 1. The quantity of the initiator attached to the surface was determined by thermo-gravimetric analysis (TGA) of the MWNT-CO2H, showing a 15.8% weight loss. The mole percent of the initiator ([I]MWNT ¼ 4.2 mol% with respect to carbon) on the MWNT surface was calculated using the weight loss % and the initiator fragment molecular weight (45 g mol1) [26]. Grafting of PCL chains from the MWNT-OH surface by ringopening polymerization was attempted in a controlled manner.
Fig. 3. 1H NMR spectra MWNT graft polymers in CDCl3; (A) MWNT-g-PCL8 and, (B) MWNT-g-(PaN3CL11-g-PBA). The star indicates solvent signal.
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SnOct2 was employed as the catalyst for the e-CL polymerizations because of the high efficiency, and relatively low toxicity. It is permissible as a food additive in numerous countries. Scheme 1(A) illustrates the synthesis of MWNT-g-PCL using ROP of e-CL with initiator MWNT-OH (at molar ratios 10/1, 30/1, and 50/1) in the presence of SnOct2 in C6H5Cl at reflux for 24 h. After the grafting reaction, the product was washed thoroughly with THF, and filtered to remove THF-soluble substances such as the PCL not bound to the MWNT. The PCL-functionalized MWNT (MWNT-g-PCL) emitted a black color. Considerable weight gain was observed after the grafting reaction, confirming that a substantial amount of PCL had been grafted to the MWNT. Washing and filtration were continued
until no trace of the THF-soluble substances was detected in the filtrate, and the PCL not bound to the MWNT was considered to be completely removed from the product. The ungrafted PCL formed that it is possible that there is trace water adsorbed on the nanotube surface, and each adsorbed water molecule can act as an initiator of the ring-opening polymerization, generating as a result of ungrafted PCL. Based on this, the FT-IR, and 1H NMR spectra of the MWNT-g-PCL were analyzed to prove the polymerization of e-CL from the MWNT-OH surface. Fig. 1 shows the FT-IR spectra of the MWNT-CO2H, MWNT-OH and MWNT-g-PCL. The FT-IR spectrum of the MWNT-CO2H (Fig. 1 (a)) exhibits the expected C]O stretching at 1736 cm1,
Fig. 4. TGA traces of (A) MWNT-CO2H (trace 1), MWNT-g-PCL3 (trace 2), MWNT-g-PCL4 (trace 3), MWNT-g-PCL8 (trace 4), and MWNT-g-PCL11 (trace 5), (B) MWNT-g-PaClCL11 (trace 1), MWNT-g-PaN3CL11 (trace 2), MWNT-g-(PaN3CL11-g-PBA) (trace 3), and MWNT-g-(PaN3CL11-g-hexyne) (trace 4).
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corresponding to the incorporated carboxylic acid groups as a result of the acid treatment process. In Fig. 1(b), the FT-IR band observed at 3400, 1735, and 1050 cm1 are attributed, respectively, to the stretching of OH, C]O (ester), and CeO vibrations that arose from the esterification. The strong carbonyl stretching band at 1735 cm1 clearly reveals the ester bands that came from the PCL grafted to the MWNT. The CeH stretching frequencies at 2853 and 2925 cm1 that correspond to the alkyl groups were observed. The intensity of the carbonyl and CeH stretching band were more intense in the MWNT-g-PCL than the MWNT-OH and increased with the incorporated into the PCL length, and again confirming the occurrence of the grafting reaction. The presence of surfaces initiated polymer (PCL) was clearly seen in the 1H NMR spectrum of the MWNT-g-PCL (Fig. 3(A)). The resonance peaks were assigned to the corresponding hydrogen atoms of the PCL. The amount of the PCL grafted to the MWNT, defined as the ratio of the mass of the immobilized PCL to that of the MWNT-g-PCL was estimated by TGA as shown in Fig. 4(A); and the weight-loss data at 800 C are listed in Table 1. All of the MWNT-g-PCLs exhibited multiple decomposition stages. The first decomposition stage, in the range of 50e180 C, is attributed to the loss of absorbed water. The second decomposition stage, in an interval of 200e300 C, corresponds to the dehydration (anhydrification of vicinal acid functions) plus the decarboxylation of the polymer, which results in inter- and intramolecular bridging. The third decomposition stage, in the range 300e470 C, is a result of the degradation of the polymer. The residual fraction at the temperature above 700 C corresponds mainly to the MWNTs. The amount of the PCL bound to the MWNT ranged from 34 to 67 wt% when the molar ratios increased from 10/1 to 50/1. The amount of grafted PCL increased with the molar ratio of e-CL to MWNT-OH. In terms of the PCL contents and the hydroxyl group content of MWNT-OH, one can estimate the average molecular weights of the grafted PCL chains respective to the MWNT-g-PCL samples, which are MWNT-g-PCL3: 307, MWNT-g-PCL4: 436, MWNT-g-PCL8: 890, and MWNT-g-PCL11: 1219 g mol1. The average molecular weights of grafted PCLs ¼ [(weight loss % of MWNTg-PCL) (weight loss % of the MWNT-CO2H)]/[I]MWNT. The number of hydroxyl groups in MWNT-OH is limited while the amount of e-CL is abundant in the reaction solution; therefore, the reacted e-CL is not proportional to the feed ratio. The traces in Fig. 4(A) show that the weight loss of MWNT-CO2H is more rapid than that of MWNT-gPCL under 250 C. This phenomenon suggests that the chemical bonds of the hydroxyl groups with the MWNTs are more easily broken, and the small segments broken away from the MWNT-OH can then be easily removed by the N2 flow.
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appealing because of a mild condition. Various types of alkynes, for example, 1-hexyne, 2-propynyl benzoate (PBA), and 4-pentynyl2,3,4,6-tetra-O-acetyl-b-D-gluco-pyranoside (PAG) can be attached onto azides containing MWNT-g-PaN3CL by click reaction. The IR absorption at 2106 cm1, characteristic of the azide, completely disappears. The 1H NMR of MWNT-g-(PaN3CL-g-PBA) is shown in Fig. 3(B). The observed proton signals are entirely due to surfaceinitiated polymers grown from MWNTs. However, considerable line broadening was observed as a result of the existence of paramagnetic substances in the MWNTs. The amount of the PaClCL grafted to the MWNT, defined as the ratio of the mass of the immobilized PaClCL to that of the MWNTg-PaClCL, was estimated by TGA, as shown in Fig. 4(B); and the weight-loss data at 800 C are listed in Table 1. The TGA analysis of MWNT-g-PaClCL in the presence of nitrogen showed two major decomposition stages. The first decomposition stage, in the range of 250e400 C, corresponds to the dehydrochlorination (eliminate the aCl and bH of PaClCL) plus the decarboxylation of the MWNT. The second decomposition stage, in the range of 500e625 C, corresponding to the decompositions of surface grown polymers (Fig. 4 (B) trace 1). The weight loss is 85.6%. The mass loss stops at approximately 14% and remains stable to 800 C, indicating that the sample was composed of 14% nanotubes by weight. The graft units of aClCL bound to the MWNT of which approximately 11 correspond to the molar ratio. Fig. 4(B) trace 2 shows the TGA decomposition of MWNT-g-PaN3CL. The weight loss is 86.1%. The graft units of aN3CL
3.2. Synthesis of MWNT-g-(PaN3CL-g-alkyne) The synthesis of the MWNT-g-(PaN3CL-g-alkyne) consists of three consecutive steps from the MWNT-OH surface (Scheme 1(B)). First, the ring-opening polymerization of aClCL was initiated by hydroxyl-terminated MWNT-OH with SnOct2 as the catalyst. In a second step, pendent chlorides were converted into azides by the reaction with sodium azide. Finally, various types of terminal alkynes were reacted with pendent azides by copper-catalyzed Huisgen’s 1,3dipolar cycloaddition. Fig. 2 shows the FT-IR spectra of the MWNT-gPaClCL, MWNT-g-PaN3CL, and MWNT-g-(PaN3CL-g-PBA). The strong carbonyl stretching band at 1737 cm1 clearly reveals the ester bonds that come from the PaClCL grafted to the MWNT. Then, the pendent chlorides of MWNT-g-PaClCL were converted into azides by reaction with sodium azide. The MWNT-g-PaClCL was thus reacted with 1 eq. of sodium azide in DMF at 60 C overnight. The IR spectrum expectedly shows a new absorption at 2106 cm1, characteristic of the azide (Fig. 2(b)). To functionalize the MWNT, chemical transformation by click reaction between alkynes and azides is highly
Fig. 5. Raman spectra of polymer-grafted MWNTs (514.5 nm excitation): (A) MWNTOH (curve 1), MWNT-g-PCL11 (curve 2), MWNT-g-PCL8 (curve 3), MWNT-g-PCL4 (curve 4), pristine MWNT (curve 5), (B) MWNT-g-PaClCL11 (curve 1), MWNT-g-(PaN3CL11g-PBA) (curve 2), MWNT-g-PaN3CL11 (curve 3).
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bound to the MWNT of which approximately 11 correspond to the molar ratio of MWNT-g-PaN3CL. For the MWNT-g-(PaN3CL-galkyne), the TGA analysis showed two major decompositions in the temperature range of 250e325 C and 300e450 C. However, the grafting efficiency of alkyne bound to azide are low; only 7.5 and 3.3% for 1-hexyne and PBA, respectively; this may be due to the stereohindrance of MWNT-g-PaN3CL (Fig. 4(B), traces 3 and 4).
3.3. Raman analysis Raman spectroscopy is a powerful tool used for the characterization of functionalized CNTs. As shown in Fig. 5(A), the D and G bands of the MWNTs at 1287 and 1598 cm1, attributed to the defects and disorder-induced peaks and tangential-mode peaks that were clearly observed for the MWNT-OH, and the MWNT-g-
Fig. 6. Digital photos: (A) A: MWNT-OH, A1: MWNT-g-PCL8, A2: MWNT-g-PCL4, A3: MWNT-g-PCL11 in CHCl3, (B) B: MWNT-OH, B1: MWNT-g-(PaN3CL11-g-hexyne), B2: MWNTg-PaClCL11, B3: MWNT-g-PaN3CL11, B4: MWNT-g-(PaN3CL11-g-PBA) in CHCl3, (C) C: MWNT-OH, C1: MWNT-g-(PaN3CL11-g-hexyne), C2: MWNT-g-PaClCL11, C3: MWNT-g-PaN3CL11, C4: MWNT-g-(PaN3CL11-g-PBA) in THF, (D) D: MWNT-g-(PaN3CL11-g-PBA), D1: MWNT-g-(PaN3CL30-g-PBA), D2: MWNT-g-(PaN3CL15/PCL15-g-PBA), D3: MWNT-g-(PaN3CL11-g-PAG) in CHCl3. The content of the sample is around 1 mg of sample per 1 mL of solvent.
R.-S. Lee et al. / Polymer 52 (2011) 2180e2188
Fig. 7. HR-TEM images: (A) pristine MWNT, (B) MWNT-g-PCL11, (C) MWNT-g-PCL8, and (D) MWNT-g-(PaN3CL11-g-PBA). (Scare bar 10 nm).
PCL. The intensity ratio of the D to G band (ID/IG) for MWNT-g-PCLs was 1.59, 1.39, and 1.35, with PCL units 4, 8, and 11, respectively; all of which are larger than that of the pristine MWNT (1.07) as a result of PCL grafting. However, the peak intensity of the MWNT-g-PCL was weaker than that of the MWNT-OH (ID/IG ¼ 1.64). In other words, the characteristic absorption peaks were strongly attenuated by the grafted PCL. The absorption attenuation as a function of the amount of the grafted polymers may be due to the grafted polymers lowering the crystal symmetry of the nanotube lattice. For MWNT-g-PaClCL, MWNT-g-PaN3CL, and MWNT-g-(PaN3CL-gPBA) grafted MWNT, the ID/IG ranged from 1.00 to 1.25, lower than that of MWNT-OH (Fig. 5(B)). For the covalently functionalized CNTs, the polymer anchored to the CNT surface forms a nanometer-scale layer of coverage. The decrease of the Raman signals of the grafted polymer MWNT came neither from the low CNT content nor from the translucence of the polymer layer. It can possibly be attributed to the energy transfer between the CNT and the polymer layer, or to the influence of the grafted polymers on the electronic properties of the CNT. Further studies are required to understand completely the effects of polymer grafting to CNTs on the intensity of Raman signals. 3.4. The dispersability of polymer-grafted MWNTs Dispersion of pristine MWNTs into aqueous solution or organic solvents was highly difficult even after it was sonicated. However, dispersion of MWNT-polymer into organic solvents was extremely easy. Therefore, the solubility or dispersability of the functionalized MWNTs strongly depends on the structure and amounts of the grafted organic moieties. Fig. 6(A) provides the digital photos of MWNT-OH and MWNT-g-PCL (with the units of CL: 4, 8, and 11) (1 mg) in CHCl3 (1 mL). It is clear that the dispersability of MWNTg-PCL is larger than MWNT-OH, and increases with longer PCL moieties. The dispersability of MWNT-OH, MWNT-g-PaClCL, MWNT-g-PaN3CL, MWNT-g-(PaN3CL-g-hexyne), and MWNT-g(PaN3CL-g-PBA) were investigated in CHCl3 and THF (Fig. 6(B) and
Fig. 8. XPS analysis of MWNT-OH, and MWNT-g-PCL: (A) C1s, and (B) O1s.
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(C)). The dispersability of the MWNT with grafted organic moieties are easier in CHCl3 than in THF, due to the interaction of the PCL chains on the MWNT surface with CHCl3 is stronger [27]. For comparison, we attempted to disperse the MWNT-g-(PaN3CL10g-PBA), MWNT-g-(PaN3CL30-g-PBA), MWNT-g-(PaN3CL15/PCL15g-PBA), and MWNT-g-(PaN3CL10-g-PAG) in CHCl3, showing that MWNT-g-(PaN3CL30-g-PBA) is a favorable dispersant in CHCl3 (Fig. 6(D)). All of the evidence indicates that PaN3CL-g-PBA chains were successfully grafted from the surface of MWNTs. 3.5. Morphology The morphological structures of pristine, PCL-grafted and PaN3CL-g-PBA-grafted MWNTs were examined by HR-TEM images. Fig. 7 shows HR-TEM images of pristine MWNTs, MWNT-g-PCL11, MWNT-g-PCL8, and MWNT-g-(PaN3CL10-g-PBA). From the image of a pristine MWNT, as shown in Fig. 7(A), it is clear that the pristine MWNT shell is quite shallow. Fig. 7(B)e(D) displays the images of modified MWNTs with various thicknesses of the polymer layers having somewhat shallower and more transparent color. The average thickness of the enwrapped polymer layer is approximately 8e10 nm for MWNT-g-PCL and 3 nm for MWNT-g-(PaN3CL-g-PBA). By comparison, the MWNT shell with a polymer layer is thicker than the pristine MWNT. These results indicate that the MWNT can be grafted by surface-initiated ring-opening polymerization and click reaction. The outcome obtained from TEM effectively accords with that resulting from TGA. 3.6. XPS analysis The results of XPS analysis are shown in Fig. 8. The C1s, and O1s spectra provide some interesting information [11]. The C1s spectrum of MWNT-OH mainly shows that the CeC peak of the CNTs at 284.2 eV, and O]CeO peak (288.7 eV) is relatively weak. The C1s spectrum of MWNT-g-PCL has a similar appearance: the peak at 284.5 eV belongs to CeC and C]C bonds. The peaks of the ester C]O (288.8 eV) are mild peaks. The O1s spectrum of MWNT-OH shows a mild and wide peak that results from the carbonyl (532.5 eV) and CeO (535.7 eV) bond introduced by the oxidation treatment. The MWNT-g-PCL show mainly two peaks: C]O (532.7 eV), and CeO (536 eV). The peak shift may also be attributed to hydrogen bonding. 4. Conclusion Poly(e-caprolactone) (PCL) and PaN3CL-g-alkyne modified MWNTs have been prepared by the “graft-from” approach. FT-IR, 1H NMR, XPS,
and TGA data verifies that the surface modification of MWNTs is successful. The HR-TEM images of the products show that the thickness of the polymer shell around the MWNT is approximately 8e10 nm for MWNT-g-PCL and 3 nm for MWNT-g-(PaN3CL-g-PBA). Therefore, a facile route for the covalent surface functionalization of CNTs by PCL and PaN3CL-g-alkyne was developed in this work.
Acknowledgments The research was supported by grants from Chang Gung University (UMRPD590091). The authors are grateful to Advanced Instrumentation Center, National Taiwan University for XPS measurement.
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