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Ultrasound-promoted direct functionalization of multi-walled carbon nanotubes in water via Diels-Alder “click chemistry”
Accepted Manuscript Ultrasound-promoted direct functionalization of multi-walled carbon nanotubes in water via Diels-Alder “click chemistry” Cuong M.Q. Le, Xuan Thang Cao, Kwon Taek Lim PII: DOI: Reference:
S1350-4177(17)30207-9 http://dx.doi.org/10.1016/j.ultsonch.2017.04.042 ULTSON 3678
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
14 March 2017 27 April 2017 27 April 2017
Please cite this article as: C.M.Q. Le, X.T. Cao, K.T. Lim, Ultrasound-promoted direct functionalization of multiwalled carbon nanotubes in water via Diels-Alder “click chemistry”, Ultrasonics Sonochemistry (2017), doi: http:// dx.doi.org/10.1016/j.ultsonch.2017.04.042
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Ultrasound-promoted direct functionalization of multi-walled carbon nanotubes in water via Diels-Alder “click chemistry” Cuong M.Q. Le, Xuan Thang Cao, Kwon Taek Lim* Department of Display Engineering, Pukyong National University, Busan, South Korea *
Address correspondence to Kwon Taek Lim, Department of Display Engineering, Pukyong National
University, 599-1 Daeyeon 3-Dong, Nam-Gu, Busan 608-737, South Korea. Tel.: (+82)-51-629-6409; Fax: (+82)-51-629-6408; E-mail address: [email protected] Abstract A facile and environmentally friendly strategy for grafting polymers onto the surface of multiwalled carbon nanotubes (CNTs) was demonstrated by Diels-Alder “click chemistry”. Firstly, the copolymers of poly(styrene-alt-maleic anhydride) (PSM) were prepared by the reversible additionfragmentation chain transfer (RAFT) polymerization and subsequently functionalized with furfuryl amine to introduce anchoring groups. The copolymers were then grafted on CNTs via the Diels-Alder reaction in water through a conventional heating-stirring route and ultrasound-assisted method. The obtained nanocomposite materials were characterized by thermogravimetric analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and transmission electron microscopy. The results indicated that the reaction rate under ultrasound irradiation was accelerated about 12 times than the one under the conventional heating-stirring condition without losing the grafting efficiency. The direct functionalization of CNTs formed a stably dispersed solution in water, promising a green and effective method for industrial process. Keywords: carbon nanotubes; reversible addition-fragmentation chain transfer polymerization; Diels-Alder; grafting; ultrasound-assisted.
1. INTRODUCTION Since the discovery of carbon nanotubes (CNTs) in 1991[1], they have attracted great attention in material science due to their unique properties such as electrical, mechanical, thermal, and structural properties. CNTs offer a wide variety of applications in many areas including electricity (e.g. transistors, sensors, and conductive thin films) [2, 3], catalysis [4], drug delivery [5-7], and absorption [8, 9]. However, their low dispersibility in organic solvents and aqueous media as well as a high tendency for aggregation of bundles limits their scope of applications [10]. To overcome these obstacles, scientists have developed surface modification methods of CNTs [11, 12]. Till date, two broad categories of methods have been proposed for the surface modification including non-covalent and covalent functionalization [13-19]. The stable conjugated structure of CNTs is mostly associated with covalent functionalization strategy, which is more advantageous than non-covalent routes [20], especially for bio1
medical applications, drug delivery and imaging [21, 22]. Various chemical treatments have been used to prepare covalent surface functionalization of CNTs such as oxidation [23], fluorination [24], free radical addition [25, 26], 1,3-cycloaddition [27, 28], nucleophilic addition [29], alkylation [30], and plasma modification [31]. Some of these approaches, especially acid treatment, involve harsh reaction conditions and take multiple steps [19]. So far, it is extremely desirable to modify the CNTs by a simple, low-cost, and effective route. Among different methods for covalent functionalization of CNTs, the Diels-Alder (DA) clicktype reaction has great potential due to some interesting features such as a mild reaction condition, no metal catalyst, and a benign solvent used. Additionally, the DA reaction allows to directly occur without surface pretreatment of CNTs [15, 32]. The DA reaction has also been employed for the ligation of functionalized polymer on the surface of graphene [33], CNTs [34], and fullerene [35]. In a general process for covalent functionalization, CNTs were suspended in organic solvents (e.g. N-methyl-2pyrrolidone and dimethylformamide) under sonication to improve dispersibility of CNTs. The mixture was then stirred over a long period of time to proceed the reaction [36-40]. In fact, ultrasonication was used for not only effective dispersion of CNTs but also functionalization and polymer grafting of CNTs [41-44]. The chemical effects of ultrasound come from acoustic cavitation [45, 46] which can activate carbon-carbon bonds and initiate chemical reactions on the surface of CNTs [47]. Thus, the use of ultrasound provided some attractive features such as low reaction temperature and short reaction time compared to other methods. Besides, water as a green solvent is known to accelerate the DA reaction significantly by comparison with other organic solvents [48-50]. Therefore, there is a high motivation to apply utrasound irradiation for modification the surface of CNTs by the DA reaction in fast, efficient, mild, and environmentally benign way. In this study, we proposed a procedure for direct grafting of polymers on CNTs via the DA reaction in water using ultrasonication. Poly(styrene-alt-maleic anhydride) (PSM) copolymers were synthesized by the reversible addition-fragmentation chain transfer (RAFT) polymerization. The PSM was subsequently derivatized with furfuryl amine to form poly(styrene-alt-maleic anhydride) furfurylamide (PSMF) derivative as a highly water-soluble polymer. Then, the PSMF was directly grafted on the surface of pristine CNTs in water under ultrasound assistance to afford PSMF/CNTs composites. The effect of reaction conditions on the chain grafting density and the dispersibility of PSMF/CNTs was investigated as well. The PSMF/CNTs hybrid materials were characterized by various analytical methods. To the best our knowledge, this is the first report on the preparation of polymer grafted CNTs in water by DA reaction under ultrasonication.
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Fig. 1. A schematic representation for the preparation of PSMF/CNTs composite.
2. EXPERIMENTAL DETAILS 2.1. Materials The multi-walled carbon nanotubes (CM-95 diameter 10-15 nm length 10-20 µ m, specific area 2
200 m .g-1) were purchased from Hanwha Nanotech (Korea). Styrene (99%, Alfa) was passed through a neutral alumina column and 2,2’-azobis(isobutyronitrile) (AIBN) (98%, Sigma-Aldrich) was recrystallized in methanol prior to use. 2-(Dodecylthiocarbonothioylthio)-2-methylpropanoic acid (DDMAT) was synthesized according to the previous literature [51]. Furfuryl amine (98%, TCI), maleic anhydride (≥ 99%, Sigma-Aldrich) and other chemicals of analytical grade were used as received. 2.2. Synthesis of poly(styrene-alt-maleic anhydride) (PSM) The overall synthesis strategy is illustrated in Fig. 1. In the first step, PSM copolymers were prepared by the RAFT polymerization using DDMAT as a chain transfer agent. Briefly, DDMAT (0.36 g, 1 mmol), styrene (5.2 g, 50 mmol), maleic anhydride (4.9 g, 50 mmol), AIBN (0.08 g, 0.5 mmol), and 1,4-dioxane ( 25 mL) were charged into a 100 mL round bottom flask with a magnetic stirrer. The flask was carefully purged with nitrogen for 1 h and placed in a preheated oil bath at 60 oC for 3 h. After the reaction completion, the solution was quenched by cooling it in ice water. The product was purified by dissolving it in THF and precipitating into an excess amount of ethyl ether. The copolymer was dried in a vacuum oven at 40 oC overnight, yielding a white solid. 2.3. Preparation of poly(styrene-alt-maleic anhydride) furfurylamide derivative (PSMF) The high reactive maleic anhydride units on PSM were functionalized with furfuryl amine as follows: PSM (4.14 g, 20 mmol of maleic anhydride groups) was dissolved in anhydrous DMF (20 mL) homogeneously. Then, furfuryl amine (2.0 mL, 22.2 mmol) was added dropwise to the solution under stirring. The reaction was conducted in an oil bath at 60 oC for 6 h under nitrogen. The mixture was iso3
lated by precipitation in ethyl ether and the final product was dried under vacuum (6.07 g, yield 98.5%). For the control experiment, the PSM copolymer without furfuryl pendant groups was prepared by hydrolysis of PSM with KOH solution. The PSM (2.0 g) was dissolved in 20 mL of 1.0 M KOH solution. The mixture was stirred in an oil bath at 60 oC for 6 h. The solution was then poured into 1.0 M HCl solution to precipitate. The product was filtered and washed with distilled water. The final product was dried at 40 oC under vacuum for 24 h. 2.4. General procedure for the functionalization of CNTs by Diels-Alder reaction In a typical procedure, 1.0 g of PSMF and 0.2 g of KOH were dissolved in 200 mL of solvent (DMF, NMP or deionized water) in a 250 mL round-bottom flask. Then, 100 mg of CNTs was added to the polymer solution and the mixture was sonicated using an ultrasonic bath (Bandelin ultrasonic bath DK1028P 250W at frequency 35 kHz). After 4 h, the mixture was cooled to room temperature. The solid was filtered and rinsed with 200 mL of deionized water and 2 x 200 mL of methanol to remove ungrafted polymers. The products were dried at 40 oC under vacuum for 24 h. 2.5. Characterization The number average molecular weight (Mn) and molecular weight distribution (Mw/Mn) were determined using gel permeation chromatography (GPC) which was equipped with an HP 1100 pump, a RID detector, and PL gel columns (5µ m; 104, 103, and 102 Ao). Dimethylacetamide with 50 mM LiCl was used as the eluent at 50 o C with a flow rate of 1 mL/min. Polystyrene standards were used to construct the calibration curve. 1H NMR spectra were recorded using a 400 MHz JEOL NMR spectrometer. Fourier transform infrared (FTIR) spectra were measured on an Agilent Cary640 spectrometer in the range 4000-500 cm-1 with KBr pellets. The Raman spectra were obtained by measuring the solution after reaction directly using Raman system (PeakSeeker PRO-785), with a 785 nm diode laser. X-ray diffraction measurements were made by a Philips X’pert-MPD system diffractometer (Netherlands) with Cu Kα radiation. Scanning electron microscopy (SEM) images were captured by HITACHI (Japan) S-2700. The amount of polymers coated on MWNTs were determined by thermogravimetric analysis (TGA) Perkin-Elmer Pyris 1 analyzer at a heating rate of 10 oC/min under continuous nitrogen flow. The X-ray photoelectron spectroscopy (XPS) experiments were performed at KRATOS-AXIS SUPRA system with monochromic Al Kα (1486.6 eV) as the X-ray source. The binding energies were calibrated based on the carbon 1s peak at 284.3 eV. The dispersed material on carbon coated copper grid was characterized using a transmission electron microscopy (TEM) JEOL JEM-2010.
3. RESULT AND DISCUSSION Copolymers with three different degrees of polymerization were prepared by RAFT. The RAFT method resulted in controlled radical polymerization with high conversion (> 80%) in a good agreement with theoretical Mn (8,600 – 29,000 g/mol) and low dispersity (PDI < 1.3) as indicated in Table 1. 4
Furfuryl pendant groups were introduced to the polymer chains by the amidation reaction between anhydride groups and furfuryl amine. The 1H NMR spectrum confirmed the structure of copolymers as shown in Fig. 2. PSMF copolymers clearly showed the typical peaks of furfuryl at 4.1, 6.4 and 7.5 ppm (c, d, e), indicating the successful functionalization of copolymers. Additionally, the GPC chromatogram of PSMF copolymers exhibited a clean shift toward high molecular weight as compared with PSM, implying the anchoring of furfuryl amine moieties (Fig. 3). Due to the high reactivity of the maleic anhydride ring, the conversion of the ring-opening reaction was very high (≥ 98%) in water, which was similar to the organic solvents such as DMF and THF references [52, 53]. Table 1. Characteristics of the PSM and PSMF prepared by RAFT polymerization. Polymer
Feed ratioa [M]SM/[M] RAFT
Conv.b (%)
Mn (1H NMR) (g/mol)
Polymer
Feed ratio [M]FA/[M]PSM
Conv.b (%)
Mn (1H NMR) (g/mol)
PDI (GPC)
PSM1
50
82.6
8,600
PSM1F
45 :1
98.5
12,600
1.21
PSM2
100
93.4
18,900
PSM2F
100 :1
98.0
27,800
1.22
PSM3
150
94.5
29,000
PSM3F
150 :1
99.0
42,600
1.26
a
Molar ratio [SMA]:[AIBN] = [100]: [1]
b
Conversion was determined by gravimetric analysis.
Fig. 2. 1 H-NMR spectra of PSM (a) and PSMF copolymers (b) in DMSO-d6.
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Fig. 3. GPC chromatograms of PSM and PSMF copolymers. CNTs were directly functionalized with PSMF in aqueous alkaline solution through ultrasonication, followed by washing steps with deionized water and methanol to remove unreacted PSMF. The washing procedure was repeated until no free PSMF was detected from the product. The efficiency of washing was monitored by TGA of PSMF/CNTs after each washing step (Fig. 4). The PSMF/CNTs which was directly isolated after reaction without washing exhibited two thermal degradation steps at 200 oC and 370 oC, and the weight loss of 24% at 650 oC since the product contained a large amount of unreacted PSMF. After the first washing step with water, the amount of the product decreased considerably due to the removal of free polymers from the PSMF/CNTs, showing reduced amount of PSMF degraded around 350 oC with 14.7% weight loss. After the second and third washing steps with methanol, a less fraction of PSMF degraded around 330 o C with 11.5% weight loss. However, the percentage of PSMF did not further change for the sample after the fourth washing step. The results suggested that free PSMF was completely removed during the repeated washing procedure.
Fig. 4. TGA thermograms of CNTs modified with PSMF after repeated washing steps. 6
PSMF and PSMF/CNTs composites were characterized by FTIR spectroscopy as shown in Fig. 5. The FTIR spectrum of PSM displayed characteristic bands of the cyclic anhydride at 1780 and 1853 cm-1 (Fig. 5a). After ring-opening reaction, signals of the cyclic anhydride ring at 1780 and 1853 cm-1 disappeared, whereas new characteristic bands of amides at 1603 and 1710 cm-1, and furfuryl at 600 and 750 cm-1 were generated (Fig. 5b). In the spectrum of CNTs (Fig. 5c), the weak absorption band at 1637 cm-1 was assigned to C=C vibrations. After functionalization with PSMF (Fig. 5d), the spectrum of PSMF/CNTs composites exhibited new bands at 600, 700, 750, 1595, 1625, and 1706 cm-1 which are similar to the characteristic bands of pure PSMF. The results indicate the successful modification of CNTs with PSMF.
Fig. 5. FTIR spectra of (a) PSM, (b) PSMF, (c) CNTs, and (d) PSMF/CNTs. TGA was used to identify the amount of grafted polymers on CNTs. The derivative thermogravimetric (DTG) curve of the pure PSMF indicated that its loss of mass happened in two different steps. The first one started around 140 oC, while the second one started around 320 oC, which reached a maximum at 395 oC and mainly ended around 500 oC (Fig. 6). However, the TGA/DTG of PSMF/CNTs composites showed only one peak between 260 oC and 450 oC because there was a slight amount of grafted PSMF as calculated around 11.2 % from TGA.
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Fig. 6. TGA and DTG curves of PSMF and PSMF/CNTs Table 2. Reaction conditions for functionalization of CNTs a
Feed ratio PSMF/CNTs [mg/mg]
Solvent
Grafted Temp. Method-Reaction time Grafting density c polymer b o ( C) (h) (chains. nm-2) (wt%)
Sample
Polymer
1
PSM1F
10
Water
80
Stirring- 48h
9.80
0.023
2
PSM1F
10
Water
50
Stirring- 48h
10.40
0.025
3
PSM1F
10
Water
r.t.
Ultrasonication- 4h
9.23
0.022
4
PSM1F
10
Water
80
Ultrasonication- 4h
12.52
0.030
5
PSM1F
10
Water
50
Ultrasonication- 4h
10.87
0.026
6
PSM1F
10
DMF
50
Ultrasonication- 4h
3.80
0.009
7
PSM1F
10
NMP
50
Ultrasonication- 4h
5.47
0.013
8
PSM2F
10
Water
50
Ultrasonication- 4h
10.98
0.012
9
PSM3F
10
Water
50
Ultrasonication- 4h
11.70
0.008
10
PSM1F
1
Water
50
Ultrasonication- 4h
7.11
0.017
11
PSM1F
2
Water
50
Ultrasonication- 4h
8.23
0.020
12
PSM1F
5
Water
50
Ultrasonication- 4h
8.29
0.020
13
PSM1
10
Water
50
Ultrasonication- 4h
3.66
0.009
a
Molecular weight of polymers was indicated in Table 1.
b
Grafted polymer was determined from TGA.
c
Grafting density was calculated by using an equation reported by Zydziak et al. [36]. In order to gain a better understanding of the influence of the initial condition on the grafting of
polymers onto CNTs via DA reaction, a series of functionalized reactions were performed under differ8
ent solvents, temperatures, chain lengths of copolymers. The results are summarized in Table 2. The percentage of grafted polymers and the grafting density of various PSMF/CNTs composites can be calculated by evaluating the quantity of sample degrade from 140 to 500 oC. The effects of solvents on DA reactions were reported by Otto et al. [50]. Briefly, an aqueous medium could accelerate the rate of DA reaction significantly as compared to organic solvents. Under the given reaction condition, the grafting density value in water was about two and three times as higher as the one in NMP and DMF, respectively (Fig. 7a and Table 2). On the other hand, the reaction rate by ultrasound irradiation was accelerated about 12 times than the one by the stirring condition with the same grafting efficiency. The grafting density increased with the reaction temperature in the range of room temperature to 80 o C as shown in Table 2 for sample 3, 4, and 5. At the temperature higher than 80 oC, the retro-DA was favorable, so as to the grafting efficiency could be lost [54]. Thus, in this study, the retro-DA reaction of the PSMF/CNTs composites was conducted at 150 oC in DMF in order to de-attach the grafted PSMF from the surface of CNTs. The TGA result of sample 5 after retro-DA reaction showed a significant reduction of grafting density, i.e. 0.006 (chains. nm-2) (Fig. 7b). The effect of the feeding ratio of the polymer to CNTs and the polymer chain length on the grafting density were also investigated. The high adsorption capacity of PSMF as a dispersant leads to higher grafting density when the PSMF/CNT ratio is increased (Fig. 7c). Moreover, it was found that the polymer chain length had no significant effect on the grafting density in term of wt.% polymer grafted on CNTs (Fig. 7d). As a result, the number of polymer chains per nm2 CNTs linearly dropped with increasing polymer chain length (Fig. 8a). It could be explained by the fact that the longer polymer chain was used with respect to the fewer number of polymer chains for covering the specific surface area of the CNTs (Fig. 8b). These results coincide well with previous reports [55-57]
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Fig. 7. TGA weight loss curves of PSMF-MWCNT under various reaction conditions: solvent (a), temperature (b), feed ratio PSMF/CNTs (c) and chain length (d). Sample 1-12 defined in Table 3.
Fig. 8. The effect of polymer chain length on grafting density (a), and schematic explanation (b) The DA reaction between PSMF copolymers and CNTs was further confirmed by Raman analysis [58]. The D band at 1310 cm-1 and the G bands at 1610 cm-1 were clearly detected on CNTs and PSMF/CNTs (Fig. 9). The relative intensity ratio of the D band to the G band (ID/IG) describes the degree of defect on CNTs due to the transformation from C-C (sp2) to C-C (sp3) bonds on nanotubes. After DA reaction, the values of ID/IG ratio of PSMF/CNTs increased from 1.09 to 1.23 corresponding to feed ratios of PSMF/CNTs from 1 to 10 (Fig. 9c-g, and Table 2), respectively, which were higher than the value of CNTs (i.e. ID/IG = 1.03) (Fig 9a). The result indicates the increase of defects induced by the DA reaction. Furthermore, the ID/IG ratio (Fig. 9b) of the control sample (i.e. 1.02) was quite similar to the one of CNTs, implying that PSM did not react with CNTs and the structure of CNTs was well pre10
served after 4 h ultrasound irradiation[59]. To verify whether the DA reaction could be reversible, we investigated the thermal treatment of PSMF/CNTs at 150 oC. As mentioned above, the retro-DA reaction of PSMF/CNTs occurred at 150 oC for the removal of grafted PSMF. The Raman spectrum of obtained CNTs showed that the ID/IG ratio of 1.03 which was the same as pure CNTs (Fig. 9h).
Fig. 9. Raman spectra of dispersed CNTs in water (a) CNTs, (b) control sample (sample 13), (c,d,e,f) PSMF/CNTs (sample 10, 11, 12, and 5, respectively), (g) PSMF/CNTs (sample 2), and retroDA product of sample 5 (h). The XRD patterns of CNTs and PSMF/CNTs are shown in Fig. 10. The strong diffraction peak at 2θ = 25.8o can be indexed as the (002) reflection of the hexagonal graphite structure and a peak at approximately 2θ = 42.8° assigned to the diffraction of the (100)-graphite base plane. The relatively sharp diffraction peaks indicate that the graphitic structures are still maintained in the carbon nanotubes. A comparison of SEM images before and after 4 h sonication also revealed no distinguishable changes in morphology.
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Fig. 10. XRD patterns of CNTs and PSMF/CNTs after 4 h ultrasonication. XPS was employed to evaluate the chemical bonds formed on the surface of CNTs before and after the DA reaction. Fig. 11 shows the XPS spectra of pristine CNTs and PSMF/CNTs. The binding energy peaks of C 1s, N 1s, and O 1s are observed at 284.3, 400, and 532 eV, respectively. Pristine CNTs exhibit a strong peak at 284.3 eV of C 1s and a very weak signal at 532 eV of O 1s which can be attributed to the defect and moisture, atmosphere O2, or CO2 adsorbed on the CNTs. After functionalization, a new peak of N 1s is observed at 400 eV and the O 1s peak at 532 eV increases significantly. The increased intensity of O and N is definitely attributed to the presence of PSMF on CNTs. The highresolution spectra of C, O, N are shown in Fig. 12. For the pristine CNTs, C 1s spectrum shows five peaks representing graphitic carbon atoms, i.e. C=C bond at 284.3 eV, C-C bond at 284.4 eV, carbonoxygen single bond (C-O) at 285 eV, carbon-oxygen double bond (C=O) at 287.8 eV, and π-π* transition level at 290.7 eV. The C 1s spectrum of PSMF/CNTs shows similar peaks with the one of CNTs. However, the intensity peak of O-C=O bond of PSMF/CNTs is much higher than that of CNTs, indicating the presence of carboxylate groups from PSMF. Moreover, deconvolution of O 1s spectrum displays two binding energies at 532.4 eV of O-C and 530.8 eV of O=C, confirming the existence of carboxylate. Finally, the appearance of nitrogen peaks at 400.1 eV is attributed to the N-C from furfuryl amide groups, suggesting the covalent functionalization of CNTs by PSMF.
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Fig. 11. XPS survey spectra of pristine CNTs and PSMF/CNTs.
Fig. 12. High-resolution spectra of pristine CNTs and PSMF/CNTs. 13
We examined the dispersibility of CNTs in solvents to evaluate the effect of surface modification via DA reaction. The samples were dispersed in deionized water and methanol at the concentration of 0.5 mg/mL. The mixture was sonicated for 3 min and then centrifuged for 20 min at 5000 rpm. The upper phase was carefully isolated and stored in a vial for observation. The pristine CNTs could not disperse in both solvents, whereas the PSMF/CNTs exhibited a better dispersibility in water and methanol (Fig. 13a, b, and c). The DA reaction between CNTs and hydrolyzed PMS instead of PSMF was carried out under the same experimental conditions as a control sample. The hydrolyzed PSM is well known as a good dispersant for CNTs due to the high adsorption capacity based on the π-π stacking of aromatic rings on CNTs [60, 61]. Nevertheless, these PSM polymer chains could be easily removed from the composites by washing with solvents. Consequently, the control sample could not be redispersed in water (Fig. 13d). The results demonstrate the CNTs are covalently functionalized with the PSMF copolymers by the DA reaction.
Fig. 13 Photograph of the supernatant of PSMF/CNTs solution after centrifugation: pure CNTs (a); PSMF/CNTs prepared by stirring for 48 h (b) and ultrasonication (c); and control sample (d). TEM analysis was performed on both samples prepared by the stirring and ultrasonication method. After the DA reaction, the pristine CNTs with an initial diameter of 10-15 nm were covered by PSMF copolymers as observed in Fig. 14. The polymer layer thickness could be calculated around 15 nm. The results from both methods yield similar morphology of polymer on CNTs but ultrasonication method has an advantage of shorter reaction time.
Fig. 14. TEM images of PSMF/CNTs prepared by stirring (a) and ultrasonication (b) method
14
CONCLUSION An efficient and green approach for the modification of CNTs was achieved by DA reaction. The PSMF copolymers were covalently grafted on the surface of CNTs in water by the ultrasound-assisted method at 50 oC without any catalyst. The reaction with ultrasonication could reduce the reaction time up to 12 times in comparison with the stirring method. In addition, the grafting reaction in water gave a grafting efficiency of two to three times higher than that in the organic solvents such as DMF and NMP, as evidenced by TGA. The grating reaction between CNTs and PSMF was confirmed by the increase of the ID/IG ratio in the Raman spectra. The chemical bonds between furfural rings and C-sp2 were clearly demonstrated by the XPS analysis. The PSMF layer around 15 nm in thickness was observed on the surface of CNTs in TEM images. The PSMF/CNTs hybrid materials could be well redispersed in water and methanol.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Education (NRF-2015R1D1A3A01019109) and the Technology Innovation Program (No. 10052923, The Development of Light Extraction Complex Substrate for Curved-OLED Lighting with High-CRI) funded by the Ministry of Trade, Industry and Energy.
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19
Highlights •
The reaction rate was significantly accelerated by ultrasound irradiation.
•
Direct functionalization of carbon nanotubes in water without catalyst.
•
The process is simple and is not involving any hazardous chemicals.
20
S1350-4177(17)30207-9 http://dx.doi.org/10.1016/j.ultsonch.2017.04.042 ULTSON 3678
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
14 March 2017 27 April 2017 27 April 2017
Please cite this article as: C.M.Q. Le, X.T. Cao, K.T. Lim, Ultrasound-promoted direct functionalization of multiwalled carbon nanotubes in water via Diels-Alder “click chemistry”, Ultrasonics Sonochemistry (2017), doi: http:// dx.doi.org/10.1016/j.ultsonch.2017.04.042
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Ultrasound-promoted direct functionalization of multi-walled carbon nanotubes in water via Diels-Alder “click chemistry” Cuong M.Q. Le, Xuan Thang Cao, Kwon Taek Lim* Department of Display Engineering, Pukyong National University, Busan, South Korea *
Address correspondence to Kwon Taek Lim, Department of Display Engineering, Pukyong National
University, 599-1 Daeyeon 3-Dong, Nam-Gu, Busan 608-737, South Korea. Tel.: (+82)-51-629-6409; Fax: (+82)-51-629-6408; E-mail address: [email protected] Abstract A facile and environmentally friendly strategy for grafting polymers onto the surface of multiwalled carbon nanotubes (CNTs) was demonstrated by Diels-Alder “click chemistry”. Firstly, the copolymers of poly(styrene-alt-maleic anhydride) (PSM) were prepared by the reversible additionfragmentation chain transfer (RAFT) polymerization and subsequently functionalized with furfuryl amine to introduce anchoring groups. The copolymers were then grafted on CNTs via the Diels-Alder reaction in water through a conventional heating-stirring route and ultrasound-assisted method. The obtained nanocomposite materials were characterized by thermogravimetric analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and transmission electron microscopy. The results indicated that the reaction rate under ultrasound irradiation was accelerated about 12 times than the one under the conventional heating-stirring condition without losing the grafting efficiency. The direct functionalization of CNTs formed a stably dispersed solution in water, promising a green and effective method for industrial process. Keywords: carbon nanotubes; reversible addition-fragmentation chain transfer polymerization; Diels-Alder; grafting; ultrasound-assisted.
1. INTRODUCTION Since the discovery of carbon nanotubes (CNTs) in 1991[1], they have attracted great attention in material science due to their unique properties such as electrical, mechanical, thermal, and structural properties. CNTs offer a wide variety of applications in many areas including electricity (e.g. transistors, sensors, and conductive thin films) [2, 3], catalysis [4], drug delivery [5-7], and absorption [8, 9]. However, their low dispersibility in organic solvents and aqueous media as well as a high tendency for aggregation of bundles limits their scope of applications [10]. To overcome these obstacles, scientists have developed surface modification methods of CNTs [11, 12]. Till date, two broad categories of methods have been proposed for the surface modification including non-covalent and covalent functionalization [13-19]. The stable conjugated structure of CNTs is mostly associated with covalent functionalization strategy, which is more advantageous than non-covalent routes [20], especially for bio1
medical applications, drug delivery and imaging [21, 22]. Various chemical treatments have been used to prepare covalent surface functionalization of CNTs such as oxidation [23], fluorination [24], free radical addition [25, 26], 1,3-cycloaddition [27, 28], nucleophilic addition [29], alkylation [30], and plasma modification [31]. Some of these approaches, especially acid treatment, involve harsh reaction conditions and take multiple steps [19]. So far, it is extremely desirable to modify the CNTs by a simple, low-cost, and effective route. Among different methods for covalent functionalization of CNTs, the Diels-Alder (DA) clicktype reaction has great potential due to some interesting features such as a mild reaction condition, no metal catalyst, and a benign solvent used. Additionally, the DA reaction allows to directly occur without surface pretreatment of CNTs [15, 32]. The DA reaction has also been employed for the ligation of functionalized polymer on the surface of graphene [33], CNTs [34], and fullerene [35]. In a general process for covalent functionalization, CNTs were suspended in organic solvents (e.g. N-methyl-2pyrrolidone and dimethylformamide) under sonication to improve dispersibility of CNTs. The mixture was then stirred over a long period of time to proceed the reaction [36-40]. In fact, ultrasonication was used for not only effective dispersion of CNTs but also functionalization and polymer grafting of CNTs [41-44]. The chemical effects of ultrasound come from acoustic cavitation [45, 46] which can activate carbon-carbon bonds and initiate chemical reactions on the surface of CNTs [47]. Thus, the use of ultrasound provided some attractive features such as low reaction temperature and short reaction time compared to other methods. Besides, water as a green solvent is known to accelerate the DA reaction significantly by comparison with other organic solvents [48-50]. Therefore, there is a high motivation to apply utrasound irradiation for modification the surface of CNTs by the DA reaction in fast, efficient, mild, and environmentally benign way. In this study, we proposed a procedure for direct grafting of polymers on CNTs via the DA reaction in water using ultrasonication. Poly(styrene-alt-maleic anhydride) (PSM) copolymers were synthesized by the reversible addition-fragmentation chain transfer (RAFT) polymerization. The PSM was subsequently derivatized with furfuryl amine to form poly(styrene-alt-maleic anhydride) furfurylamide (PSMF) derivative as a highly water-soluble polymer. Then, the PSMF was directly grafted on the surface of pristine CNTs in water under ultrasound assistance to afford PSMF/CNTs composites. The effect of reaction conditions on the chain grafting density and the dispersibility of PSMF/CNTs was investigated as well. The PSMF/CNTs hybrid materials were characterized by various analytical methods. To the best our knowledge, this is the first report on the preparation of polymer grafted CNTs in water by DA reaction under ultrasonication.
2
Fig. 1. A schematic representation for the preparation of PSMF/CNTs composite.
2. EXPERIMENTAL DETAILS 2.1. Materials The multi-walled carbon nanotubes (CM-95 diameter 10-15 nm length 10-20 µ m, specific area 2
200 m .g-1) were purchased from Hanwha Nanotech (Korea). Styrene (99%, Alfa) was passed through a neutral alumina column and 2,2’-azobis(isobutyronitrile) (AIBN) (98%, Sigma-Aldrich) was recrystallized in methanol prior to use. 2-(Dodecylthiocarbonothioylthio)-2-methylpropanoic acid (DDMAT) was synthesized according to the previous literature [51]. Furfuryl amine (98%, TCI), maleic anhydride (≥ 99%, Sigma-Aldrich) and other chemicals of analytical grade were used as received. 2.2. Synthesis of poly(styrene-alt-maleic anhydride) (PSM) The overall synthesis strategy is illustrated in Fig. 1. In the first step, PSM copolymers were prepared by the RAFT polymerization using DDMAT as a chain transfer agent. Briefly, DDMAT (0.36 g, 1 mmol), styrene (5.2 g, 50 mmol), maleic anhydride (4.9 g, 50 mmol), AIBN (0.08 g, 0.5 mmol), and 1,4-dioxane ( 25 mL) were charged into a 100 mL round bottom flask with a magnetic stirrer. The flask was carefully purged with nitrogen for 1 h and placed in a preheated oil bath at 60 oC for 3 h. After the reaction completion, the solution was quenched by cooling it in ice water. The product was purified by dissolving it in THF and precipitating into an excess amount of ethyl ether. The copolymer was dried in a vacuum oven at 40 oC overnight, yielding a white solid. 2.3. Preparation of poly(styrene-alt-maleic anhydride) furfurylamide derivative (PSMF) The high reactive maleic anhydride units on PSM were functionalized with furfuryl amine as follows: PSM (4.14 g, 20 mmol of maleic anhydride groups) was dissolved in anhydrous DMF (20 mL) homogeneously. Then, furfuryl amine (2.0 mL, 22.2 mmol) was added dropwise to the solution under stirring. The reaction was conducted in an oil bath at 60 oC for 6 h under nitrogen. The mixture was iso3
lated by precipitation in ethyl ether and the final product was dried under vacuum (6.07 g, yield 98.5%). For the control experiment, the PSM copolymer without furfuryl pendant groups was prepared by hydrolysis of PSM with KOH solution. The PSM (2.0 g) was dissolved in 20 mL of 1.0 M KOH solution. The mixture was stirred in an oil bath at 60 oC for 6 h. The solution was then poured into 1.0 M HCl solution to precipitate. The product was filtered and washed with distilled water. The final product was dried at 40 oC under vacuum for 24 h. 2.4. General procedure for the functionalization of CNTs by Diels-Alder reaction In a typical procedure, 1.0 g of PSMF and 0.2 g of KOH were dissolved in 200 mL of solvent (DMF, NMP or deionized water) in a 250 mL round-bottom flask. Then, 100 mg of CNTs was added to the polymer solution and the mixture was sonicated using an ultrasonic bath (Bandelin ultrasonic bath DK1028P 250W at frequency 35 kHz). After 4 h, the mixture was cooled to room temperature. The solid was filtered and rinsed with 200 mL of deionized water and 2 x 200 mL of methanol to remove ungrafted polymers. The products were dried at 40 oC under vacuum for 24 h. 2.5. Characterization The number average molecular weight (Mn) and molecular weight distribution (Mw/Mn) were determined using gel permeation chromatography (GPC) which was equipped with an HP 1100 pump, a RID detector, and PL gel columns (5µ m; 104, 103, and 102 Ao). Dimethylacetamide with 50 mM LiCl was used as the eluent at 50 o C with a flow rate of 1 mL/min. Polystyrene standards were used to construct the calibration curve. 1H NMR spectra were recorded using a 400 MHz JEOL NMR spectrometer. Fourier transform infrared (FTIR) spectra were measured on an Agilent Cary640 spectrometer in the range 4000-500 cm-1 with KBr pellets. The Raman spectra were obtained by measuring the solution after reaction directly using Raman system (PeakSeeker PRO-785), with a 785 nm diode laser. X-ray diffraction measurements were made by a Philips X’pert-MPD system diffractometer (Netherlands) with Cu Kα radiation. Scanning electron microscopy (SEM) images were captured by HITACHI (Japan) S-2700. The amount of polymers coated on MWNTs were determined by thermogravimetric analysis (TGA) Perkin-Elmer Pyris 1 analyzer at a heating rate of 10 oC/min under continuous nitrogen flow. The X-ray photoelectron spectroscopy (XPS) experiments were performed at KRATOS-AXIS SUPRA system with monochromic Al Kα (1486.6 eV) as the X-ray source. The binding energies were calibrated based on the carbon 1s peak at 284.3 eV. The dispersed material on carbon coated copper grid was characterized using a transmission electron microscopy (TEM) JEOL JEM-2010.
3. RESULT AND DISCUSSION Copolymers with three different degrees of polymerization were prepared by RAFT. The RAFT method resulted in controlled radical polymerization with high conversion (> 80%) in a good agreement with theoretical Mn (8,600 – 29,000 g/mol) and low dispersity (PDI < 1.3) as indicated in Table 1. 4
Furfuryl pendant groups were introduced to the polymer chains by the amidation reaction between anhydride groups and furfuryl amine. The 1H NMR spectrum confirmed the structure of copolymers as shown in Fig. 2. PSMF copolymers clearly showed the typical peaks of furfuryl at 4.1, 6.4 and 7.5 ppm (c, d, e), indicating the successful functionalization of copolymers. Additionally, the GPC chromatogram of PSMF copolymers exhibited a clean shift toward high molecular weight as compared with PSM, implying the anchoring of furfuryl amine moieties (Fig. 3). Due to the high reactivity of the maleic anhydride ring, the conversion of the ring-opening reaction was very high (≥ 98%) in water, which was similar to the organic solvents such as DMF and THF references [52, 53]. Table 1. Characteristics of the PSM and PSMF prepared by RAFT polymerization. Polymer
Feed ratioa [M]SM/[M] RAFT
Conv.b (%)
Mn (1H NMR) (g/mol)
Polymer
Feed ratio [M]FA/[M]PSM
Conv.b (%)
Mn (1H NMR) (g/mol)
PDI (GPC)
PSM1
50
82.6
8,600
PSM1F
45 :1
98.5
12,600
1.21
PSM2
100
93.4
18,900
PSM2F
100 :1
98.0
27,800
1.22
PSM3
150
94.5
29,000
PSM3F
150 :1
99.0
42,600
1.26
a
Molar ratio [SMA]:[AIBN] = [100]: [1]
b
Conversion was determined by gravimetric analysis.
Fig. 2. 1 H-NMR spectra of PSM (a) and PSMF copolymers (b) in DMSO-d6.
5
Fig. 3. GPC chromatograms of PSM and PSMF copolymers. CNTs were directly functionalized with PSMF in aqueous alkaline solution through ultrasonication, followed by washing steps with deionized water and methanol to remove unreacted PSMF. The washing procedure was repeated until no free PSMF was detected from the product. The efficiency of washing was monitored by TGA of PSMF/CNTs after each washing step (Fig. 4). The PSMF/CNTs which was directly isolated after reaction without washing exhibited two thermal degradation steps at 200 oC and 370 oC, and the weight loss of 24% at 650 oC since the product contained a large amount of unreacted PSMF. After the first washing step with water, the amount of the product decreased considerably due to the removal of free polymers from the PSMF/CNTs, showing reduced amount of PSMF degraded around 350 oC with 14.7% weight loss. After the second and third washing steps with methanol, a less fraction of PSMF degraded around 330 o C with 11.5% weight loss. However, the percentage of PSMF did not further change for the sample after the fourth washing step. The results suggested that free PSMF was completely removed during the repeated washing procedure.
Fig. 4. TGA thermograms of CNTs modified with PSMF after repeated washing steps. 6
PSMF and PSMF/CNTs composites were characterized by FTIR spectroscopy as shown in Fig. 5. The FTIR spectrum of PSM displayed characteristic bands of the cyclic anhydride at 1780 and 1853 cm-1 (Fig. 5a). After ring-opening reaction, signals of the cyclic anhydride ring at 1780 and 1853 cm-1 disappeared, whereas new characteristic bands of amides at 1603 and 1710 cm-1, and furfuryl at 600 and 750 cm-1 were generated (Fig. 5b). In the spectrum of CNTs (Fig. 5c), the weak absorption band at 1637 cm-1 was assigned to C=C vibrations. After functionalization with PSMF (Fig. 5d), the spectrum of PSMF/CNTs composites exhibited new bands at 600, 700, 750, 1595, 1625, and 1706 cm-1 which are similar to the characteristic bands of pure PSMF. The results indicate the successful modification of CNTs with PSMF.
Fig. 5. FTIR spectra of (a) PSM, (b) PSMF, (c) CNTs, and (d) PSMF/CNTs. TGA was used to identify the amount of grafted polymers on CNTs. The derivative thermogravimetric (DTG) curve of the pure PSMF indicated that its loss of mass happened in two different steps. The first one started around 140 oC, while the second one started around 320 oC, which reached a maximum at 395 oC and mainly ended around 500 oC (Fig. 6). However, the TGA/DTG of PSMF/CNTs composites showed only one peak between 260 oC and 450 oC because there was a slight amount of grafted PSMF as calculated around 11.2 % from TGA.
7
Fig. 6. TGA and DTG curves of PSMF and PSMF/CNTs Table 2. Reaction conditions for functionalization of CNTs a
Feed ratio PSMF/CNTs [mg/mg]
Solvent
Grafted Temp. Method-Reaction time Grafting density c polymer b o ( C) (h) (chains. nm-2) (wt%)
Sample
Polymer
1
PSM1F
10
Water
80
Stirring- 48h
9.80
0.023
2
PSM1F
10
Water
50
Stirring- 48h
10.40
0.025
3
PSM1F
10
Water
r.t.
Ultrasonication- 4h
9.23
0.022
4
PSM1F
10
Water
80
Ultrasonication- 4h
12.52
0.030
5
PSM1F
10
Water
50
Ultrasonication- 4h
10.87
0.026
6
PSM1F
10
DMF
50
Ultrasonication- 4h
3.80
0.009
7
PSM1F
10
NMP
50
Ultrasonication- 4h
5.47
0.013
8
PSM2F
10
Water
50
Ultrasonication- 4h
10.98
0.012
9
PSM3F
10
Water
50
Ultrasonication- 4h
11.70
0.008
10
PSM1F
1
Water
50
Ultrasonication- 4h
7.11
0.017
11
PSM1F
2
Water
50
Ultrasonication- 4h
8.23
0.020
12
PSM1F
5
Water
50
Ultrasonication- 4h
8.29
0.020
13
PSM1
10
Water
50
Ultrasonication- 4h
3.66
0.009
a
Molecular weight of polymers was indicated in Table 1.
b
Grafted polymer was determined from TGA.
c
Grafting density was calculated by using an equation reported by Zydziak et al. [36]. In order to gain a better understanding of the influence of the initial condition on the grafting of
polymers onto CNTs via DA reaction, a series of functionalized reactions were performed under differ8
ent solvents, temperatures, chain lengths of copolymers. The results are summarized in Table 2. The percentage of grafted polymers and the grafting density of various PSMF/CNTs composites can be calculated by evaluating the quantity of sample degrade from 140 to 500 oC. The effects of solvents on DA reactions were reported by Otto et al. [50]. Briefly, an aqueous medium could accelerate the rate of DA reaction significantly as compared to organic solvents. Under the given reaction condition, the grafting density value in water was about two and three times as higher as the one in NMP and DMF, respectively (Fig. 7a and Table 2). On the other hand, the reaction rate by ultrasound irradiation was accelerated about 12 times than the one by the stirring condition with the same grafting efficiency. The grafting density increased with the reaction temperature in the range of room temperature to 80 o C as shown in Table 2 for sample 3, 4, and 5. At the temperature higher than 80 oC, the retro-DA was favorable, so as to the grafting efficiency could be lost [54]. Thus, in this study, the retro-DA reaction of the PSMF/CNTs composites was conducted at 150 oC in DMF in order to de-attach the grafted PSMF from the surface of CNTs. The TGA result of sample 5 after retro-DA reaction showed a significant reduction of grafting density, i.e. 0.006 (chains. nm-2) (Fig. 7b). The effect of the feeding ratio of the polymer to CNTs and the polymer chain length on the grafting density were also investigated. The high adsorption capacity of PSMF as a dispersant leads to higher grafting density when the PSMF/CNT ratio is increased (Fig. 7c). Moreover, it was found that the polymer chain length had no significant effect on the grafting density in term of wt.% polymer grafted on CNTs (Fig. 7d). As a result, the number of polymer chains per nm2 CNTs linearly dropped with increasing polymer chain length (Fig. 8a). It could be explained by the fact that the longer polymer chain was used with respect to the fewer number of polymer chains for covering the specific surface area of the CNTs (Fig. 8b). These results coincide well with previous reports [55-57]
9
Fig. 7. TGA weight loss curves of PSMF-MWCNT under various reaction conditions: solvent (a), temperature (b), feed ratio PSMF/CNTs (c) and chain length (d). Sample 1-12 defined in Table 3.
Fig. 8. The effect of polymer chain length on grafting density (a), and schematic explanation (b) The DA reaction between PSMF copolymers and CNTs was further confirmed by Raman analysis [58]. The D band at 1310 cm-1 and the G bands at 1610 cm-1 were clearly detected on CNTs and PSMF/CNTs (Fig. 9). The relative intensity ratio of the D band to the G band (ID/IG) describes the degree of defect on CNTs due to the transformation from C-C (sp2) to C-C (sp3) bonds on nanotubes. After DA reaction, the values of ID/IG ratio of PSMF/CNTs increased from 1.09 to 1.23 corresponding to feed ratios of PSMF/CNTs from 1 to 10 (Fig. 9c-g, and Table 2), respectively, which were higher than the value of CNTs (i.e. ID/IG = 1.03) (Fig 9a). The result indicates the increase of defects induced by the DA reaction. Furthermore, the ID/IG ratio (Fig. 9b) of the control sample (i.e. 1.02) was quite similar to the one of CNTs, implying that PSM did not react with CNTs and the structure of CNTs was well pre10
served after 4 h ultrasound irradiation[59]. To verify whether the DA reaction could be reversible, we investigated the thermal treatment of PSMF/CNTs at 150 oC. As mentioned above, the retro-DA reaction of PSMF/CNTs occurred at 150 oC for the removal of grafted PSMF. The Raman spectrum of obtained CNTs showed that the ID/IG ratio of 1.03 which was the same as pure CNTs (Fig. 9h).
Fig. 9. Raman spectra of dispersed CNTs in water (a) CNTs, (b) control sample (sample 13), (c,d,e,f) PSMF/CNTs (sample 10, 11, 12, and 5, respectively), (g) PSMF/CNTs (sample 2), and retroDA product of sample 5 (h). The XRD patterns of CNTs and PSMF/CNTs are shown in Fig. 10. The strong diffraction peak at 2θ = 25.8o can be indexed as the (002) reflection of the hexagonal graphite structure and a peak at approximately 2θ = 42.8° assigned to the diffraction of the (100)-graphite base plane. The relatively sharp diffraction peaks indicate that the graphitic structures are still maintained in the carbon nanotubes. A comparison of SEM images before and after 4 h sonication also revealed no distinguishable changes in morphology.
11
Fig. 10. XRD patterns of CNTs and PSMF/CNTs after 4 h ultrasonication. XPS was employed to evaluate the chemical bonds formed on the surface of CNTs before and after the DA reaction. Fig. 11 shows the XPS spectra of pristine CNTs and PSMF/CNTs. The binding energy peaks of C 1s, N 1s, and O 1s are observed at 284.3, 400, and 532 eV, respectively. Pristine CNTs exhibit a strong peak at 284.3 eV of C 1s and a very weak signal at 532 eV of O 1s which can be attributed to the defect and moisture, atmosphere O2, or CO2 adsorbed on the CNTs. After functionalization, a new peak of N 1s is observed at 400 eV and the O 1s peak at 532 eV increases significantly. The increased intensity of O and N is definitely attributed to the presence of PSMF on CNTs. The highresolution spectra of C, O, N are shown in Fig. 12. For the pristine CNTs, C 1s spectrum shows five peaks representing graphitic carbon atoms, i.e. C=C bond at 284.3 eV, C-C bond at 284.4 eV, carbonoxygen single bond (C-O) at 285 eV, carbon-oxygen double bond (C=O) at 287.8 eV, and π-π* transition level at 290.7 eV. The C 1s spectrum of PSMF/CNTs shows similar peaks with the one of CNTs. However, the intensity peak of O-C=O bond of PSMF/CNTs is much higher than that of CNTs, indicating the presence of carboxylate groups from PSMF. Moreover, deconvolution of O 1s spectrum displays two binding energies at 532.4 eV of O-C and 530.8 eV of O=C, confirming the existence of carboxylate. Finally, the appearance of nitrogen peaks at 400.1 eV is attributed to the N-C from furfuryl amide groups, suggesting the covalent functionalization of CNTs by PSMF.
12
Fig. 11. XPS survey spectra of pristine CNTs and PSMF/CNTs.
Fig. 12. High-resolution spectra of pristine CNTs and PSMF/CNTs. 13
We examined the dispersibility of CNTs in solvents to evaluate the effect of surface modification via DA reaction. The samples were dispersed in deionized water and methanol at the concentration of 0.5 mg/mL. The mixture was sonicated for 3 min and then centrifuged for 20 min at 5000 rpm. The upper phase was carefully isolated and stored in a vial for observation. The pristine CNTs could not disperse in both solvents, whereas the PSMF/CNTs exhibited a better dispersibility in water and methanol (Fig. 13a, b, and c). The DA reaction between CNTs and hydrolyzed PMS instead of PSMF was carried out under the same experimental conditions as a control sample. The hydrolyzed PSM is well known as a good dispersant for CNTs due to the high adsorption capacity based on the π-π stacking of aromatic rings on CNTs [60, 61]. Nevertheless, these PSM polymer chains could be easily removed from the composites by washing with solvents. Consequently, the control sample could not be redispersed in water (Fig. 13d). The results demonstrate the CNTs are covalently functionalized with the PSMF copolymers by the DA reaction.
Fig. 13 Photograph of the supernatant of PSMF/CNTs solution after centrifugation: pure CNTs (a); PSMF/CNTs prepared by stirring for 48 h (b) and ultrasonication (c); and control sample (d). TEM analysis was performed on both samples prepared by the stirring and ultrasonication method. After the DA reaction, the pristine CNTs with an initial diameter of 10-15 nm were covered by PSMF copolymers as observed in Fig. 14. The polymer layer thickness could be calculated around 15 nm. The results from both methods yield similar morphology of polymer on CNTs but ultrasonication method has an advantage of shorter reaction time.
Fig. 14. TEM images of PSMF/CNTs prepared by stirring (a) and ultrasonication (b) method
14
CONCLUSION An efficient and green approach for the modification of CNTs was achieved by DA reaction. The PSMF copolymers were covalently grafted on the surface of CNTs in water by the ultrasound-assisted method at 50 oC without any catalyst. The reaction with ultrasonication could reduce the reaction time up to 12 times in comparison with the stirring method. In addition, the grafting reaction in water gave a grafting efficiency of two to three times higher than that in the organic solvents such as DMF and NMP, as evidenced by TGA. The grating reaction between CNTs and PSMF was confirmed by the increase of the ID/IG ratio in the Raman spectra. The chemical bonds between furfural rings and C-sp2 were clearly demonstrated by the XPS analysis. The PSMF layer around 15 nm in thickness was observed on the surface of CNTs in TEM images. The PSMF/CNTs hybrid materials could be well redispersed in water and methanol.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Education (NRF-2015R1D1A3A01019109) and the Technology Innovation Program (No. 10052923, The Development of Light Extraction Complex Substrate for Curved-OLED Lighting with High-CRI) funded by the Ministry of Trade, Industry and Energy.
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Highlights •
The reaction rate was significantly accelerated by ultrasound irradiation.
•
Direct functionalization of carbon nanotubes in water without catalyst.
•
The process is simple and is not involving any hazardous chemicals.
20