Introduction

In recent years, there has been attractive interest in single-walled carbon nanotubes (SW-CNTs) due to exhibit unique physical and chemical properties such as a high flexibility, chemical stability, electrical conductivity, which result from the high ratio of length to diameter and π–π connections among adjacent carbons [1]. SW-CNTs have a variety of potential applications such as nanodevices, biochemistry, and reinforcing materials for polymer composites; however, they have a great deal of difficulties to obtain uniform dispersed distribution of SW-CNTs in aqueous and organic media, because SW-CNTs exhibit an extremely poor solubility in these media [2]. Hitherto, we have reported that ABA triblock-type fluoroalkyl end-capped oligomers could exhibit a wide variety of unique properties such as high solubility, surface active properties, biological activities, and nanometer size-controlled self-assembled molecular aggregates which cannot be achieved by the corresponding non-fluorinated and randomly fluoroalkylated ones [3]. Especially, these fluorinated oligomeric aggregates could provide suitable host moieties to interact with a variety of guest molecules such as organic dyes, low-molecular biocides, calcium carbonates, and fullerenes to afford stable fluorinated oligomeric aggregates/guest molecules nanocomposites [4]. In these fluorinated oligomeric aggregates/guest molecules nanocomposites, we have also succeeded in preparing fluorinated oligomeric aggregates/carbon nanotube composites possessing a dispersibility in water [5]. Therefore, the preparation of fluorinated oligomeric aggregates/carbon nanotubes composites possessing a higher dispersibility and stability in water is of particular interest from the developmental viewpoint of new fluorinated functional materials. Very recently, we have found that new fluoroalkyl end-capped dendrimer-type block copolymers can be prepared by the use of fluoroalkanoyl peroxide as a key intermediate [6]. It was also demonstrated that these fluorinated dendrimer-type block copolymers could form the new fluorinated molecular aggregates to have a higher dispersion ability for not only SW-CNTs and fullerenes but also magnetic nanoparticles in water, compared to that of the corresponding ABA triblock-type fluoroalkyl end-capped oligomers [6]. We now give a full account of the dispersion of SW-CNTs in water by the use of new fluorinated dendrimer-type block copolymer.

Experimental

NMR spectra and Fourier-transform infrared (FTIR) spectra were measured using JEOL JNM-400 (400 MHz) FT NMR SYSTEM (Tokyo, Japan) and Shimadzu FTIR-8400 FT-IR spectrophotometer (Kyoto, Japan), respectively. Molecular weights were measured using a Shodex DS-4 (pomp) and Shodex RI-71 (detector) gel permeation chromatography (GPC) calibrated with polystyrene standard using tetrahydrofuran (THF) as the eluent. Dynamic light-scattering (DLS) and static light-scattering (SLS) measurements were performed using Otsuka Electronics DLS-7000 HL (Tokyo, Japan). Ultraviolet-visible (UV-vis) spectra were measured using Shimadzu UV-1600 UV-vis spectrophotometer (Kyoto, Japan). Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-1210—electron microscopy (Tokyo, Japan).

Materials

SW-CNT [purity >50%: diameter: 1–2 nm, length: 0.5–100 μm; impurities: other nanotube, amorphous carbon, carbon-coated metal nanoparticles] and SW-CNT containing carboxy groups [(SW-CNT)-COOH: purity: 80–90%; the coverage of carboxy groups on the side wall of SW-CNT is 3–6 atomic %; diameter: 4–5 nm; length: 500–1,500 μm; impurities: other nanotube, amorphous carbon, carbon-coated metal nanoparticles (5–10%)] were purchased from Sigma-Aldrich Japan Corp. (Tokyo, Japan).

Dendrimer-type copolymer containing fluoroalkyl segments [RF-(TRIV-Si)n-RF-block-RF-(DMAA)q-RF] was prepared according to our previuosly reported method [6]. ABA triblock-type fluoroalkyl end-capped oligomers were prepared by the reactions of fluoroalkanoyl peroxides with the corresponding monomers according to our previously reported method [7].

Dispersion of SW-CNT in water by the use of RF-(TRIV-Si) n -RF-block-RF-(DMAA) q -RF

To an aqueous solution of RF-(TRIV-Si) n -RF-block-RF-(DMAA) q -RF; RF = CF(CF3)OC4F9; 4.0 g/dm3: 2 mL] were added SW-CNTs (2.0 mg). The mixture was stirred with a magnetic stirring bar at 30 °C for 3 days. The aqueous solution thus obtained was centrifuged for 30 min (1000 rpm), and then the residual transparent black SW-CNTs solution was filtered through a 1.2-μm filter membrane to obtain a similar transparent solution. The relative amounts of dispersed SW-CNTs in water were estimated by the optical density at 500 nm (UV-vis. spectra) with the use of the molar absorption coefficient (ε) of SW-CNTs-o-dichlorobenzene solution reported by Smalley et al. [8] and the relative amounts of dispersed SW-CNTs were summarized in Fig. 1.

Fig. 1
figure 1

UV-vis spectra of aqueous solutions of fluorinated polysoaps [RF = CF(CF3)OC4F9] in the presence of SW-CNTs. Concentration of fluorinated polysoaps: 4.0 g/dm3 (2 mL); Stirring conditions: 30 °C/3 days; Used SW-CNT: 2 mg. (a) RF-(TTRV-Si) n -RF-block-RF-(DMAA) q -RF in the presence of SW-CNTs. Dispersed SW-CNTs: 129 μg/mL, (b) RF-(DMAA) n -RF in the presence of SW-CNTs. Dispersed SW-CNTs: 22 μg/mL, (c) RF-(TTRV-Si) n -RF-block-RF-(DMAA) q -RF

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The amounts of dispersed (SW-CNT)-COOH in water with ABA triblock-type fluorinated oligomer were also determined under similar conditions.

Results and discussion

Single-walled carbon nanotubes (2 mg) were added to an aqueous solution of dendrimer-type fluoroalkyl end-capped block copolymer [RF-(TTRV-Si) n -RF-block-RF-(DMAA) q -RF; RF = CF(CF3)OC4F9; Mn = 7840, 4 g/dm3: 2 mL] illustrated in Scheme 1. The mixture was stirred with a magnetic stirring bar at 30 °C for 3 days. The aqueous solution thus obtained was centrifuged, and then the residual SW-CNTs solution was filtered through a 1.2-μm filter membrane to obtain a transparent black solution. We have also studied on the dispersion of SW-CNTs in water by the use of the corresponding ABA triblock-type fluoroalkyl end-capped dimethylacrylamide oligomer [RF-(CH2CHCONMe2)n-RF [RF-(DMAA)n-RF]: RF = CF(CF3)OC4F9; Mn = 6480, 4 g/dm3: 2 mL] under similar conditions, for comparison. The relative amounts of dispersed SW-CNTs in water were estimated by the optical density at 500 nm with the use of the molar absorption coefficient (ε) of SW-CNTs-o-dichlorobenzene solutions reported by Smalley et al. [8]. The original RF-(TTRV-Si) n -RF-block-RF-(DMAA) q -RF and RF-(DMAA) n -RF have no absorbance around 500 nm, and the relative amounts of dispersed SW-CNTs were summarized in Fig. 1.

Scheme 1
scheme 1

Fluorinated block copolymer

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As shown in Fig. 1, RF-(TTRV-Si) n -RF-block-RF-(DMAA) q -RF and RF-(DMAA) n -RF oligomer were able to disperse SW-CNTs in water. This finding suggests that these fluorinated block copolymer and oligomer should provide the suitable host moieties to interact with SW-CNTs in water. The DLS measurements at 30 °C showed that the size (number−average diameter) of molecular assemblies formed in aqueous solutions of RF-(TTRV-Si) n -RF-block-RF-(DMAA) q -RF could increase from 20 to 254 nm by dispersion of SW-CNTs. A similar tendency for the increase of the size (from 11 to 146 nm) was also observed in the dispersion of SW-CNTs in water with RF-(DMAA)n-RF oligomer. More interestingly, RF-(TTRV-Si) n -RF-block-RF-(DMAA) q -RF was able to disperse SW-CNTs quite effectively about six times compared to that of RF-(DMAA)n-RF oligomer under similar conditions.

An extremely higher dispersibility of SW-CNTs thus obtained would be due to the architectures of self-assembly of fluorinated dendrimer-type block copolymers in aqueous solutions. It is well-known that fluoroalkyl end-capped dimethylacrylamide homooligomers [RF-(DMAA) n -RF] can form the nanometer size-controlled self-assembled molecular aggregates imparted by the aggregation between end-capped fluoroalkyl segments in oligomers in aqueous and organic media [7(a), 9]. Therefore, it is suggested that our present fluorinated dendrimer-type block copolymers could also form the molecular aggregates resembling micelles derived from the aggregations of end-capped fluoroalkyl segments in block copolymers in aqueous solutions. We have measured the molecular weights of RF-(TTRV-Si) n -RF-block-RF-(DMAA) q -RF copolymer in water by SLS measurements at 30 °C. These results were shown in Table 1.

Table 1 Molecular weights of RF-(TTRV-Si) n -RF-block-RF-(DMAA)q-RF and RF-(DMAA)n-RF homooligomer determined by GPC and static light-scattering (SLS) measurements in water [RF = CF(CF3)OC4F9]
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As shown in Table 1, the molecular weights of RF-(DMAA) n -RF homooligomeric aggregates determined by SLS and GPC measurements were 569500 and 6480, respectively. This finding indicates that fluorinated molecular aggregate formed by RF-(DMAA) n -RF oligomer in aqueous solutions are considered to consist of around 88 fluorinated oligomeric molecules. Similarly, the molecular weight (199800) of RF-(TTRV-Si) n -RF-block-RF-(DMAA) q -RF copolymer determined by SLS was higher than that (7840) by GPC. Thus, fluorinated dendrimer-type copolymer should form the self-assembled molecular aggregates, which consists of around 25 fluorinated block copolymeric molecules in aqueous solutions, and in particular, the aggregations of end-capped fluoroalkyl segments in block copolymers are strongly involved in establishing the molecular aggregates as shown in Fig 2.

Fig. 2
figure 2

Schematic illustration for the self-assembly of RF-(TTRV-Si) n -RF-block-RF-(DMAA)q-RF

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Thus, such new fluorinated dendrimer-type block copolymers should open new development in the higher dispersion of SW-CNTs, which in general exhibit an extremely poor dispersibility in water, through the encapsulation of SW-CNTs into the fluorinated dendrimer-type copolymeric aggregates in aqueous solutions. Especially, fluorinated dendrimer-type block copolymers should form the more suitable aggregates cores for the encapsulation of SW-CNTs, compared to that of traditional ABA block-type fluorinated RF-(DMAA)n-RF oligomer.

We studied in detail on the dispersion of SW-CNTs in water by the use of RF-(TTRV-Si) n -RF-block-RF-(DMAA) q -RF copolymer and RF-(DMAA)n-RF oligomer, and these results were shown in Fig. 3.

Fig. 3
figure 3

Relationship between the amounts of dispersed SW-CNTs and concentration of fluorinated polysoaps [RF = CF(CF3)OC4F9] in water at 30 °C

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As shown in Fig. 3, RF-(TTRV-Si) n -RF-block-RF-(DMAA) q -RF copolymer was more effective in the dispersion of SW-CNTs in water, compared to that of RF-(DMAA)n-RF oligomer. The amounts of dispersed SW-CNTs in water were increased with the increase of the concentration of block copolymer from 0.5 to 2.5 g/dm3. However, unexpectedly, the amounts of dispersed SW-CNTs were found to decrease by the increase of the concentration of block copolymer from 2.5 to 3.0 g/dm3. Similarly, unexpected results were obtained in the cases of the concentrations of the block copolymer from 3.5 to 4.0 g/dm3 and from 4.0 to 5.0 g/dm3. TEM images show that SW-CNTs could be smoothly encapsulated into dendrimer-type block copolymeric aggregate cores to afford the corresponding fluorinated aggregates/SW-CNTs composites in each concentration of block copolymer, especially above the concentration of 3.0 g/dm3 (block copolymer) (see Fig. 4).

Fig. 4
figure 4

TEM images of RF-(TTRV-Si) n -RF-block-RF-(DMAA) q -RF/SW-CNTs composites in water. (a) Concentration of RF-(TTRV-Si)n-RF-block-RF-(DMAA) q-RF

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In addition, we studied on the relationship between the amounts of dispersed SW-CNTs in water and the dispersion time, and these results were shown in Fig. 5.

Fig. 5
figure 5

Relationship between the amounts of dispersed SW-CNTs in water and dispersing time at 30 °C. Concentrationof fluorinated polysoaps [RF = CF(CF3)OC4F9]: 2.5 g/dm3 (2 mL); Used SW-CNTs: 2 mg

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As shown in Fig. 5, the amounts of dispersed SW-CNTs in water by using block copolymer were found to become extremely higher than that of RF-(DMAA)n-RF oligomer in each dispersion time. Especially, the amounts of dispersed SW-CNTs by using block copolymer increased with increasing the stirring times from 1 to 4 days; however, the amounts of dispersed SW-CNTs decreased with the increase of stirring times from 4 to 6 days. Such similar results were observed in the cases of the increase of stirring times from 7 to 8 days, from 8 to 9 days, and from 9 to 10 days. These interesting findings in Figs. 3 and 5 are not clarified in detail at the present time; however, one though is as follows (see Fig. 6): that is, fluorinated dendrimer-type copolymeric aggregates could provide suitable host moieties to interact with SW-CNTs as guest molecules (Fig. 6a). SW-CNTs should be in part released (to Fig. 6c) from the fluorinated aggregates/SW-CNTs composites (Fig. 6b) or encapsulated (from Fig. 6c) into these composites with increasing the dispersion times. These release and encapsulation behaviors of SW-CNTs would be due to the flexible architectures formed by the aggregation of terminal fluoroalkyl segments in our present fluorinated dendrimer-type block copolymer.

Fig. 6
figure 6

Schematic illustration for the interaction of self-assembled dendrimer-type fluorinated copolymeric aggregates with SW-CNTs in aqueous solutions

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(SW-CNT)-COOH possesses carboxy groups on the sidewall of SW-CNTs. Thus, it is strongly expected that self-assembled fluorinated oligomeric aggregates formed by ABA triblock-type fluoroalkyl end-capped acrylic acid oligomers [RF-(CH2CHCOOH)n-RF; RF = fluoroalkyl groups [RF-(ACA)n-RF)]] should interact more effectively with (SW-CNT)-COOH as a guest molecule through the intermolecular hydrogen bonding between the carboxy groups in both (SW-CNT)-COOH and RF-(ACA)n-RF oligomers, compared to that of block copolymer or RF-(DMAA)n-RF oligomer. In fact, we have studied on the dispersion of (SW-CNT)-COOH in water by the use of RF-(ACA)n-RF oligomer, fluorinated block copolymer and RF-(DMAA)n-RF oligomer, and these results were shown in Fig. 7.

Fig. 7
figure 7

UV-vis. spectra of aqueous solutions of fluorinated polysoaps in the presence of (SW-CNT)-COOH. Concentration of fluorinated polysoaps [RF = CF(CF3)OC4F9]: 2.5 g/dm3 (2 mL); Stirring conditions: 30 °C / 3 days; Used (SW-CNT)-COOH: 2 mg

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As shown in Fig. 7, UV-vis. spectra of dispersed (SW-CNT)-COOH solutions in water show that the relative amounts of dispersed (SW-CNT)-COOH estimated by the optical density at 500 nm were found to increase effectively in the case of RF-(ACA)n-RF oligomer [RF = CF(CF3)OC4F9; Mn = 2550].

Additionally, we summarized the amounts of dispersed (SW-CNT)-COOH and SW-CNTs in water by the use of RF-(ACA)n-RF oligomer, fluorinated block copolymer, and RF-(DMAA)n-RF oligomer including their composites size under similar conditions, and these results were shown in Table 2.

Table 2 The amounts of dispersed (SW-CNT)-COOH and SW-CNTs in water using fluorinated polysoaps and size of fluorinated polysoaps/(SW-CNT)-COOH and /SW-CNTs composites in water determined by dynamic light-scattering measurements at 30 °C
Full size table

As shown in Table 2, the amounts of dispersed (SW-CNT)-COOH in water were estimated to be 136, 76, and 69 μg/ml, respectively, and the highest dispersibility was obtained by the use of RF-(ACA)n-RF oligomer. On the other hand, fluorinated block copolymer was the most effective for the dispersion of SW-CNTs in water. DLS measurements show that the size of self-assembled molecular aggregates formed by RF-(ACA)n-RF oligomer could increase effectively from 11 to 112 nm and 116 nm by the dispersion of (SW-CNT)-COOH and SW-CNTs in water, respectively, as well as fluorinated block copolymer and RF-(DMAA)n-RF oligomer. TEM images also show that (SW-CNT)-COOH should be smoothly encapsulated as a guest molecule into fluorinated acrylic acid oligomeric aggregate cores (Fig. 8a). In contrast, (SW-CNT)-COOH could be tightly encapsulated into fluorinated block copolymeric aggregate cores (Fig. 8b), because dendrimer-type block copolymeric aggregate cores consist of fine networks in water.

Fig. 8
figure 8

TEM Images of dispersed fluorinated polysoap/(SW-CNT)-COOH composites. Concentration of fluorinated polysoaps [RF = CF(CF3)OC4F9]: 2.5 g/dm3; Stirring conditions: 30 °C/3 days; Used (SW-CNT)-COOH: 2 mg

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In this way, it was verified that RF-(ACA)n-RF oligomer was able to disperse (SW-CNT)-COOH quite effectively in water, although this oligomer failed to disperse SW-CNTs effectively in water. This finding indicates that intermolecular hydrogen bonding between the carboxy groups in both oligomer and SW-CNTs could afford the good dispersibility of (SW-CNT)-COOH in water. In order to clarify this dispersion characteristic with RF-(ACA)n-RF oligomer, we have studied in detail on the dispersion of (SW-CNT)-COOH in water by the use of RF-(ACA)n-RF. We also studied on the dispersion of (SW-CNT)-COOH in water by the use of fluorinated block copolymer under similar conditions, for comparison. These results were shown in Fig. 9.

Fig. 9
figure 9

Relationship between the amounts of dispersed (SW-CNT)-COOH in water and the dispersing times at 30 °C. Concentration of fluorinated polysoaps [RF = CF(CF3)OC4F9]: 2.5 g/dm3 (2 mL); Used SW-CNT-COOH: 2 mg

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As shown in Fig. 9, RF-(ACA)n-RF oligomer was able to increase the amounts of dispersed (SW-CNT)-COOH with increasing the stirring times from 1 h to 1 day, and almost constant values (ca. 135 μg/ml) were obtained above 1 day. This dispersion characteristic behavior in RF-(ACA)n-RF oligomer is quite different from that of fluorinated block copolymer, indicating that the dispersion of (SW-CNT)-COOH with RF-(ACA)n-RF oligomer would be depend on the intermolecular hydrogen bonding interaction. Thus, the release of (SW-CNT)-COOH from the fluorinated nanocomposites illustrated in Fig. 6 [from (b) to (c)] could not be occurred due to the hydrogen bonding interactions related to carboxy groups.

In conclusion, dendrimer-type fluoroalkyl end-capped block copolymer was able to disperse SW-CNTs quite effectively in water. On the other hand, the corresponding ABA triblock-type RF-(DMAA)n-RF oligomer was not able to disperse SW-CNTs in water, effectively. DLS measurements and TEM images of fluorinated block copolymer showed that SW-CNTs could be smoothly encapsulated into fluorinated block copolymeric aggregate cores as guest molecules to increase their aggregate core size. On the other hand, dendrimer-type fluorinated block copolymer was not effective for the dispersion of (SW-CNT)-COOH in water; however, (SW-CNT)-COOH could be more effectively dispersed into water by the use of RF-(ACA)n-RF oligomer.