Dispersion of carbon nanotubes by the branched block ... - Springer Link

Colloid Polym Sci (2013) 291:691–698 DOI 10.1007/s00396-012-2776-x

ORIGINAL CONTRIBUTION

Dispersion of carbon nanotubes by the branched block copolymer Tetronic 1107 in an alcohol–water solution Teng Liu & Guiying Xu & Juan Zhang & Haihong Zhang & Jinyu Pang

Received: 19 June 2012 / Accepted: 15 August 2012 / Published online: 29 August 2012 # Springer-Verlag 2012

Abstract The dispersion of carbon nanotubes (CNTs) by the branched block copolymer Tetronic 1107 was investigated in mixed solvents consisting of water and one of the following alcohols: ethanol, n-propanol, ethylene glycol (EG), or glycerol (GLY). The maximum concentration of dispersed CNTs (Climit) and the optimum T1107 concentration (Copt) to disperse the maximum amount of CNTs in different solvents were obtained from UV–vis–NIR absorbance spectra. The addition of ethanol or n-propanol to water dramatically increases the Climit. The value of Copt follows the order: npropanol–water > ethanol–water > EG–water ≈ GLY–water mixtures. ID/IG was used to characterize the defect density of CNTs dispersed in the mixed solvents, which was investigated by Raman spectroscopy. The ID/IG values in n-propanol– water and ethanol–water mixtures are higher than those in EG–water and GLY–water mixtures. High-resolution transmission electron microscopy is used to confirm a favorable dispersion in the presence of different alcohols. Keywords Carbon nanotubes . Dispersion . Copolymer . Tetronic 1107 (T1107) . Alcohol

Introduction Carbon nanotubes (CNTs), which own unique structural, electrical, thermal, and mechanical properties, have wide applications including nanoscale electronic devices, energy storage, biosensors, and reinforcement for materials [1–5]. However, a major obstacle for the utilization of CNTs is T. Liu : G. Xu (*) : J. Zhang : H. Zhang : J. Pang Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Shandong University, Jinan 250100, People’s Republic of China e-mail: [email protected]

their poor solubility and dispersibility in different solvent media. The as-synthesized CNTs are often bundled together due to strong van der Waals interactions between the nanotubes. The formation of bundles leads to a significant decrease of the promising mechanical and electronic properties proposed to CNTs. To remove this obstacle, continuous contributions from scientists in colloid and interface science and organic chemistry have paved effective ways to detach CNTs from the bundle and disperse them in solutions. These methods can be divided into two categories: noncovalent methods and covalent ones [6, 7]. Compared to the covalent method, the noncovalent method has the advantage of causing no disruption to the structural and electronic properties of native tubes. The noncovalent treatments by surfactants [8–11] or polymers [12–15] have been widely used. When mixed with CNTs, the hydrophobic parts of surfactants or polymers work by adsorption at the interface of CNTs, while the hydrophilic parts stay outside and impede the aggregation of CNTs, which help CNT dispersion retain a stable colloidal state. In particular, block copolymer dispersants have drawn great attention not only because of their excellent dispersion capability but also their intriguing ability to self-assemble into ordered nanostructures that can be utilized further when incorporated with nanotubes [15–18]. The dispersing block copolymers can be further used for shear alignment of the dispersed CNTs [19], preparation of CNT–polymer composites [20], coupling agents [21], and as a targeting agent for CNT assembly at interfaces. In the majority of applications, various solvents or mixed solvents are widely used. While water is the main solvent used in most biochemistry, polar organic solvents (such as glycerol and ethanol) are often added to achieve desired performance requirements [22, 23]. The solvent quality is a controlling factor in the dispersion of CNTs. However, it has not been fully understood currently how solvent conditions affect the quality of nanotube dispersions.

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In our previous work, we have studied the effect of alcohols on the aggregation behaviors of branched block copolymer Tetronic 1107 (T1107) [23]. In this paper, we further investigate the effects of alcohols on the dispersion states of CNTs in the solutions of T1107. The alcohols we have chosen to study (ethanol, n-propanol, ethylene glycol, and glycerol) resemble the monomers of the copolymer blocks (PEO or PPO), and they are commonly used in biomedical applications due to their low toxicity and nonirritating behavior. The maximum concentration of dispersed CNTs in the solution (Climit) and the optimum T1107 concentration (Copt) to disperse the maximum amount of CNTs in different solvents were obtained from ultraviolet–visible– near infrared (UV–vis–NIR) absorbance spectra. ID/IG was used to characterize the defect density of CNTs dispersed in the mixed solvents, which was investigated by Raman spectroscopy. Additionally, the high-resolution transmission electron microscopy (HRTEM) was used to confirm a favorable dispersion in the presence of various alcohols. We believe that our results will be greatly useful for the selection of solvents in the practical applications of CNTs.

accurately into a vitreous bottle before a 5-mL solution was added. The suspensions were sonicated by an ultrasonicator (KQ-250DB, Analytical Instrument Inc., Shanghai, China) with a frequency of 40 kHz and a maximum power of 250 W. After sonication at 100 W and 40 kHz for 1 h, the samples were stored at room temperature for 2 weeks and characterized. Methods UV–vis–NIR measurements were carried out on a Hitachi 4100 spectrometer (Japan). The corresponding copolymer solution was used to get the baseline in the measurements. Raman spectra were obtained from an NXR FT-Raman module (Nexus 670, Nicolet Co.) equipped with a Ge detector. The samples were excited by a laser source with a wavelength of 1,064 nm and a power of 0.103 W. HRTEM observations were carried out on a JEOL JEM-2100 microscope (Japan) at an accelerating voltage of 200 kV. Samples were prepared directly by dipping an ultrathin carbon-coated copper grid into the suspension and then dried by a NIR lamp.

Experimental section Results and discussion Materials The branched block copolymer T1107 was purchased from Sigma–Aldrich Co. and used as received, and the T1107 structure is shown in Fig. 1. CNTs, with a diameter 95 %, were purchased from Shenzhen Nanotech Port Co., Ltd. and used without further purification. Analytical-grade ethanol, n-propanol, ethylene glycol (EG), and glycerol (GLY) were purchased from Sinopharm Chemical Reagent Co. and used as received. Water used in the experiments was triply distilled by a quartz water purification system. Sample preparation All solutions were prepared by weighing. T1107 aqueous solutions with the concentration of 10 wt% were prepared by weighing appropriate amounts of copolymers in water and were stored at 25 °C. Then, the copolymer solutions were further diluted with triply distilled water, and the alcohol was added to achieve the proposed concentration. In a typical experiment, 2.0 mg CNTs was weighed HO(EO) 60(PO)20

(PO)20(EO)60OH N-CH 2-CH2-N

HO(EO) 60(PO)20

(PO)20(EO)60OH

Fig. 1 The chemical structure of T1107. Mw, 15,000 gmol−1, the PEO content is about 70 wt%

Dispersion of CNTs in aqueous solutions with various concentrations of T1107 The dispersion of CNTs in aqueous solutions with T1107 is shown in Fig. 2. The amount of CNTs was fixed to be 2.0 mg, while the concentrations of T1107 were varied from 0.01 to 10 wt%. T1107 has good dispersion abilities when its concentration is higher than 0.03 wt%. After stored at room temperature for more than 1 month, CNT dispersions by T1107 are still stable and show a black color. UV–vis–NIR absorbance spectroscopy is a common technique for characterizing CNT dispersions and has the capacity to probe all species of nanotubes simultaneously [24]. In the experiments, baseline correction was carried out every time by using the corresponding solution without CNTs; therefore, their absorbance values got subtracted from those of CNT dispersions. It has been reported that there is almost no absorption band in the UV–vis–NIR region for bundled CNTs; however, individual CNTs are active in this region, and a strong absorption can be observed [25, 26]. Therefore, the dispersion of CNTs can be characterized by using UV–vis–NIR absorption spectroscopy. The UV–vis–NIR absorptions in the 400–1,300-nm regions for the dispersion of CNTs in T1107 aqueous solutions are shown in Fig. 3. Evidently, the spectra exhibit well-resolved peaks due to the van Hove transitions of metallic and semiconducting CNTs, which reflect a high


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Fig. 2 Images of the dispersion of CNTs in T1107 aqueous solutions. T1107 concentrations from (a) to (h) are 0.01, 0.03, 0.1, 0.3, 1, 2, 5, and 10 wt%, respectively

degree of individual CNTs. The peaks at 900–1,300 nm are assigned to the first van Hove transitions of semiconducting CNTs (S11). The peaks at 550–900 nm are attributed to the second van Hove transitions of semiconducting CNTs (S22); and the peaks at 400–600 nm are ascribed to the first van Hove transitions of metallic CNTs (M11). Bands corresponding to the semiconducting metallic CNTs are much clearer than those of the metallic CNTs. The absorption features of the metallic tubes and the S11, S22 transition semiconducting tubes are obvious, which suggest that T1107 is an effective dispersing agent for CNTs. The T1107 concentration has an influence on dispersing CNTs. In order to reveal this influence, the absorption intensity (λ0600 nm) of aqueous CNTs dispersions in different T1107 concentrations is also shown in Fig. 3. The absorbance curves can be divided into two sections. At the beginning, with the T1107 concentration increasing, the absorbance sharply increases and reaches a maximum, which may be caused by the increase of T1107 molecules for CNT dispersion. Then, when the T1107 concentration further increases, the absorbance decreases slowly. This phenomenon mainly results from the formation of T1107 micelles. At high concentrations, the T1107 molecules form micelles in the solution. There is a competition between the molecules adsorbed on the surface of CNTs and those in micelles, and these molecules reach a balance finally.

Absorbance

2.0

0.03 wt% 0.3 wt% 2 wt% 10 wt%

Absorbance

0.01 wt% 0.1 wt% 1 wt% 5 wt%

2.4

1.6

1.4 1.2 1.0 0.8 0.6 0.4 0.2

1.2

0.0

0.01

0.1

1

10

cT1107 /wt%

0.8 0.4 0.0

400

600

800

1000

1200

1400

Wavelength/nm Fig. 3 UV–vis–NIR absorptions in the 400–1,300-nm regions for the dispersion of CNTs in T1107 aqueous solutions with different concentrations. Inset is variation of absorbance for the CNT dispersion (λ0 600 nm) as a function of T1107 concentration

Therefore, although the concentration of T1107 increases, the amount of T1107 molecules adsorbed on CNTs does not increase. At the same time, the reaggregation of the separated CNTs may occur due to the interactions between T1107 micelles and CNTs. So, the amount of CNTs dispersed in aqueous solutions decreases with further increasing concentration of T1107. A similar behavior has been observed when using other surfactants or copolymers as dispersants [27, 28].

Dispersion of CNTs in mixed solvents Maximum concentration of dispersed CNTs In order to study the influence of alcohols on the dispersion of CNTs in T1107 aqueous solutions, dispersion of CNTs in T1107 aqueous solutions in the presence of different concentrations of alcohols are studied. For example, the images of the dispersion of CNTs in different concentrations of T1107 aqueous solutions with 20 wt% ethanol are shown in Fig. 4. The colors of these dispersions are much darker than those obtained from aqueous solutions without ethanol at the same T1107 concentrations (Fig. 2), which could be easily observed by naked eyes. Furthermore, the transmittance of 2.0 mg CNTs dispersed in a 5-mL solution with 0.03 wt% T1107 at a wavelength of 600 nm is only 0.479, while that in the presence of 20 wt% ethanol treated at the same condition is much higher, which is 1.266. This indicates clearly that the addition of ethanol results in more CNTs dispersed into the solutions. It could be observed in the UV–vis–NIR absorptions of the dispersed CNTs in the 400–1,300-nm regions (Fig. 5) that there are several absorptions which may be attributed to the absorption features of the metallic tubes and the S22 transition semiconducting tubes. And the UV–vis–NIR absorption spectra of CNT dispersions in T1107 aqueous solutions with 20 wt% ethanol show much sharper van Hove transition peaks than those without ethanol, which indicate that CNTs are better dispersed in the presence of ethanol. Generally, our aim is to increase the dispersibility of CNTs in the alcohol–water mixed solvents. However, it is difficult to directly measure the solubility of CNTs due to their extremely low solubility in the solvents. Thus,

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Fig. 4 Images of the dispersion of CNTs in T1107 aqueous solutions in the presence of 20 wt% ethanol. T1107 concentrations from (a) to (h) are 0.01, 0.03, 0.1, 0.3, 1, 2, 5, and 10 wt%, respectively

absorbance measurements are often employed in connection with the Lambert–Beer law, A0ε·c·l, where A is the absorbance at a particular wavelength, ε is the molar extinction coefficient, l is the light path length in centimeters, and c is the concentration. To characterize the dispersion of CNTs in T1107 solutions through UV–vis–NIR spectroscopy, absorbance values at 600 nm as reported in previous studies were used [29]. The concentration of CNTs dispersed into the solution can be determined by using ε of CNTs at 600 nm (ε600 033.4 cm2 mg−1). The concentration of the dispersed CNTs as a function of the T1107 concentration in different solvents is shown in Fig. 6. The concentration of the dispersed CNTs first increases then decreases with the rise of the T1107 concentration. This observation can also be explained in the light of micelle formation in solutions as discussed above. In addition, micelle formation induces an uncompensated force that results in a depletion attraction between adjacent CNTs, leading to the flocculation of dispersed CNTs. Thus, the flocculation of CNTs is another reason why the concentration of dispersed CNTs decreases at high T1107 concentration. It is reported that the dispersion limit (Climit) can be used to characterize the dispersibility of CNTs [30–32], which is defined as the maximum concentration of dispersed CNTs in the solution. The Climit values of CNTs in different solutions are shown in Table 1. Obviously, the Climit values of CNTs 2.8

cT1107/wt%

Absorbance

2.4

0.01 0.1 1 5

2.0 1.6

0.03 0.3 2 10

1.2 0.8 0.4 0.0 400

600

800

1000

1200

Wavelength/nm Fig. 5 UV–vis–NIR absorptions in the 400–1,300-nm regions for the dispersions of CNTs in T1107 aqueous solutions in the presence of 20 wt% ethanol

in ethanol–water or n-propanol–water mixtures with T1107 are much larger than that in aqueous solution (38.8 μg mL−1), while the Climit values of CNTs in EG–water or GLY–water mixtures are similar to that in aqueous solution. It is reported that in good solvent conditions for the copolymers, entropic repulsion among the tethered chains generates a free energy barrier that prevents CNTs from approaching the attractive part of the intertube potential [33]. Due to the steepness and short-ranged nature of the potential, a relatively weak repulsion, such as the osmotic repulsion among tails of tethered copolymers in a good solvent for the tail chains, can stabilize the dispersed CNTs and prevent CNTs from approaching the attractive minimum. This means that the copolymer in good solvent conditions has better ability of dispersing of CNTs. In the case of our solvents, ethanol is known to be a good solvent for both PEO and PPO blocks of copolymers [34]. The ethanol–water mixture becomes a better solvent for T1107 compared to pure water. N-propanol and ethanol have similar structures, but the dielectric constant of npropanol is smaller than that of ethanol, which indicates that the bulk phase of n-propanol–water mixtures will be a better solvent for T1107 molecules than ethanol–water mixtures. GLY and EG have similar structures, and it is found that the miscibility of GLY with water decreases in the presence of block copolymers [35]. At the same time, the solvency conditions for PEO–PPO–PEO become poorer in GLY– water mixed solvent. This means that GLY–water mixed solvent becomes a poor solvent for T1107 compared to pure water. In summary, ethanol–water and n-propanol–water mixtures are better solvents for T1107 molecules than water, while GLY–water and EG–water mixtures become poorer solvents than water. Thus, the Climit values in different mixed solvents represent the order of n-propanol–water> ethanol–water>water>EG–water≈GLY–water mixtures. In order to compare the dispersion efficiency of T1107 in different solvents, we define the optimum copolymer concentration (Copt) at the maximum dispersed amount of CNTs as the characteristic parameter. The Copt value of T1107 in aqueous solution without alcohol is 1 wt%, and the corresponding concentrations are 0.3 wt% in both EG–water and GLY–water mixtures, while those are 2 and 1 wt% in npropanol–water and ethanol–water mixtures, respectively. The different results are possibly due to the variation of the critical micellization concentration (cmc) of T1107 in


695 60

A

Conc. of CNTs / μg⋅mL-1


80 70 60 50 40

T1107 5 wt% n-propanol 10 wt% n-propanol 20 wt% n-propanol 30 wt% n-propanol

30 20 10 0

0.01

0.1

1

B 50 40 30 20

5 wt% ethanol 10 wt% ethanol 20 wt% ethanol 30 wt% ethanol

10 0

10

0.01

0.1

50

10

50

C

D Conc. of CNTs / μg⋅mL-1


1

cT1107 /wt%

cT1107 /wt%

40 30 20

5 wt% EG 10 wt% EG 20 wt% EG 30 wt% EG

10 0 0.01

0.1

1

40 30 20

5 wt% GLY 10 wt% GLY 20 wt% GLY 30 wt% GLY

10 0 0.01

10

0.1

cT1107 /wt%

1

10

cT1107 /wt%

Fig. 6 The concentration of dispersed CNTs as a function of T1107 concentration in different solvents. a In n-propanol–water mixed solvents, b in ethanol–water mixed solvents, c in EG–water mixed solvents, and d in GLY–water mixed solvents

the presence of different alcohols. In our previous work, we have found that the presence of alcohols in the aqueous solution obviously alters the cmc of T1107 [23]. The addition of ethanol and n-propanol to water disfavors the micellization and progressively increases the cmc of T1107, while the presence of EG and GLY in the aqueous solution generally decreases the cmc. The order of cmc is in the order of npropanol–water>ethanol–water>water>EG–water>GLY– water mixtures which is almost consistent with the order of Copt. Since the cmc of T1107 in GLY–water mixture is lower than that in water, the micelles will form at lower T1107 concentrations, so the Copt is expected to be located at a lower concentration. This can be seen in Table 1 that in ethanol–water mixture the cmc is close to that in water, and the Copt is found similar to that in water.

Defect density of CNTs To investigate the dispersion state of the samples more elaborately, Raman spectroscopy was used. The Raman spectrum of CNTs in the absence of alcohol is shown in Fig. 7a and displays all of the common features reported in the literature [36, 37]. The four important Raman absorption peaks of CNTs are observed. The lowest energy feature is the radial breathing mode (RBM), which is known to be sensitive to the diameter but not to the helicity of the tubes. Main three peaks of the RBM are observed for pure CNTs at about 167, 263, and 328 cm−1. These corresponded to the mean diameters of the tubes of 1.34, 0.85, and 0.68 nm, according to the following equation: d ¼ 223:75=RBM . Raman spectroscopy was used to study the electronic

Table 1 The dispersion limit (Climit) of CNTs and optimum copolymer concentration (Copt) in the absence and presence of different alcohols Solvent

Water

n-propanol–water

Calcohol/wt% Climit/μgmL−1 Copt/wt%

0 38.8 1

5 57.6 2

10 69.8 2

20 81.2 2

Ethanol–water 30 60.1 2

5 43.9 1

10 53.7 1

EG–water 20 55.6 1

30 49.3 1

5 38.6 0.3

10 41.1 0.3

GLY–water 20 46.8 0.3

30 41.9 0.3

5 37.4 0.3

10 42.1 0.3

20 47.8 0.3

30 46.3 0.3

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Colloid Polym Sci (2013) 291:691–698 25

A

40

pure CNTs CNTs /0.3 wt% T1107 CNTs /1wt% T1107

G

20

Intensity

Intensity

Intensity

0.6

10

0.4

0.2

D 0.0 100

200

300

400

20 D

15

G

5

0 1000

RBM

25

10

Raman shift /cm

G

0

20 wt% n-propanol 20 wt% ethanol 20 wt% EG 20 wt% GLY

500

-1

5

G

30

RBM

15

B

35

2000

3000

4000

-1

Raman shift /cm

0 0

1000

2000

Raman shift /cm

3000

4000

-1

Fig. 7 a Raman spectra of pure CNTs and CNTs dispersed in 0.3 and 1 wt% aqueous solution of T1107. Inset is the magnified image of the curves in the circle. b Raman spectra of CNTs dispersed in 1 wt% T1107 aqueous solution in the presence of 20 wt% alcohols

structure of CNTs before and after their interaction with T1107. After dispersing in 0.3 and 1 wt%T1107 solutions, these three peaks still exist. Inspection of the position of each RBM peak before and after treatment with T1107 revealed a 1–4-cm−1 shift to higher frequency (blueshift), which are 168, 266, and 332 cm−1, respectively. These shifts suggest that T1107 molecules interact with CNTs, which are wrapped onto the surface of CNTs and counteract the aggregation of CNTs. Thus, the sizes of CNTs change and cause the RBM to move. In addition, the peak located around 266 cm−1 is more obvious, and it seems that T1107 tend to selectively disperse CNTs with certain diameter values. The D peak located at around 1,280 cm−1 is usually caused by defects in CNTs, and its intensity can show the defect level of CNTs. The defects of pure CNTs are rare, so the D peak intensity is nearly negligible. It is also seen from Fig. 7a that the intensity of D peak of the dispersed CNTs is stronger than that of pure CNTs, indicating that the sonication conditions cause the destructions of nanotubes. The G peak and G* peak of the pure CNTs are located around 1,590 and 2,540 cm−1, which are generated by the tangential modes of the CNTs and longrange ordering. After CNTs are dispersed into the bulk aqueous solution with T1107, the positions of the G and G* peak

Table 2 The G and D band intensities in the Raman spectra and defect density of CNTs (ID/IG) in different alcohol–water mixed solvents; the concentration of alcohol is 20 wt%, and the concentration of T1107 is 1 wt% Solvent

ID

IG

ID/IG

Water n-propanol–water Ethanol–water EG–water GLY–water

7.01 12.98 11.13 8.14 8.10

20.94 34.68 30.00 26.37 28.14

0.33 0.38 0.37 0.31 0.29

remain unchanged. The broad peak at 3,300 cm−1 arises from the O–H stretching transitions of water. However, the curves tend to protrude out of the baseline around 500 cm−1 probably due to the resonance effect between dispersed CNTs and adsorbed T1107 molecules. Raman spectroscopy of dispersed CNTs in T1107 aqueous solutions in the presence of 20 wt% different alcohols is shown in Fig. 7b. The structural integrity of the Raman spectroscopy in different mixed solvents is the same. As discussed above, Raman spectroscopy can be used to analyze the change of defect density in CNTs. It is important for the utilization of CNTs because defects can influence the properties of CNTs. The intensities of G and D band can reflect the amounts of CNTs and defective CNTs dispersed in the solution. So, the ratio of ID to IG can be used to characterize the defect density of CNTs [38, 39]. The larger the ID/IG ratio is, the higher the defect density is. The ID/IG values of CNTs in T1107 aqueous solutions in the presence of 20 wt% alcohol are shown in Table 2. Varying the type of alcohols has a significant effect on the defect density of CNTs. The ID/IG values of CNTs in T1107 solutions with ethanol or n-propanol are higher than that in T1107 aqueous solutions, while the ID/ IG ratio values of CNTs in T1107 solution with EG or GLY are much lower than that in T1107 aqueous solutions. It means that defect density of CNTs dispersed in T1107 solution is lower in EG–water and GLY–water mixtures, but it is higher in ethanol–water and n-propanol–water mixtures. Therefore, EG and GLY molecules have better abilities than ethanol and n-propanol to protect CNTs during the ultrasonication process. Since each EG or GLY molecule has more than one donor and acceptor sites, there is a high probability that all molecules are interlinked by a number of hydrogen bonds. EG and GLY interacts favorably with water and strengthens the H-bond network of the mixed solvent. This may be the result of an increase in water structuring which makes better protection of CNTs; consequently, the ID/IG decreases.


The values of ID/IG increase with the addition of ethanol or propanol, which indicates that the amounts of defective CNTs dispersed in ethanol–water and n-propanol–water mixtures are increased. Compared with the results of UV– vis–NIR absorbance spectroscopy, the Climit values in ethanol–water and n-propanol–water mixtures are higher than those in EG–water and GLY–water mixtures. This means that more CNTs and more defective CNTs can be dispersed in ethanol–water and n-propanol–water mixtures. In the case of EG–water and GLY–water mixtures, less CNTs and less defective CNTs can be dispersed. Therefore, the type of alcohols should be paid enough attention in order to reach a reasonable level.

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dispersed in T1107 aqueous solutions with different alcohols is contrastively studied. In the presence of ethanol or npropanol, CNTs are cut eventually, and the length gets shorter. In the presence of EG or GLY, long and isolated CNTs are observed, and CNTs are not entirely seen in the micrographs, so CNTs are longer than 500 nm. Namely, the amount of defective CNTs dispersed by T1107 is less in EG–water and GLY–water mixtures, but more in ethanol– water and n-propanol–water mixtures. HRTEM results are in agreement with the results obtained by Raman spectroscopy measurements.

Conclusions Morphology of the dispersed CNTs In order to visualize the states of CNTs dispersed in the solutions of T1107 with different alcohols, HRTEM measurement has been performed, and the results are shown in Fig. 8. Most of the individual CNTs are observed in the HRTEM images, suggesting that CNTs can be dispersed effectively in mixed solutions with the aid of T1107. Different diameters of CNTs were dispersed by T1107, which is also proved by Raman spectroscopy. As shown in Fig. 8a, there are some bundled CNTs in the aqueous dispersion, while individual CNTs are easy to be observed after adding alcohols (Fig. 8b–e). Then, the morphology of CNTs

The effects of different alcohols on the dispersion of CNTs in the aqueous solutions of T1107 have been investigated as a function of the alcohol content. Climit, Copt, and ID/IG are used to demonstrate the dispersion ability of CNTs in mixed solvents, and they can be changed greatly by the variation of alcohols. The addition of ethanol or n-propanol to water dramatically increases the maximum concentration of dispersed CNTs. The copolymer in good solvent conditions has better dispersibility of CNTs. The addition of ethanol or npropanol to water results in better solvent conditions for T1107, while GLY–water and EG–water mixtures become poor solvents for T1107. The value of Copt follows the

Fig. 8 HRTEM photographs of CNTs dispersed in aqueous solutions of 1 wt% T1107 solution (a), in the presence of 20 wt% ethanol (b), propanol (c), EG (d), and GLY (e)

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order: n-propanol–water>ethanol–water>EG–water≈GLY– water mixtures. The results are ascribed to the variation of cmc of T1107 in the presence of different alcohols. Furthermore, EG and GLY molecules have better abilities than ethanol and n-propanol to protect CNTs during the ultrasonication process because EG and GLY interact favorably with water and strengthen the H-bond network of the mixed solvent. In a word, the dispersibility of CNTs can be modulated by the variation of alcohols. This is a critical issue because controlling CNT dispersions is an important step in the incorporation CNTs in matrix materials to form nanocomposites and fibers with well-defined and optimized properties. We believe that this study will provide useful information on the CNTs dispersed by copolymer in the practical application. Acknowledgments The authors gratefully acknowledge the financial support from the Natural Science Foundation of China (20873077).

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