Dispersion of Multi-Walled Carbon Nanotubes by Polymers with

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Dispersion of Multi-Walled Carbon Nanotubes by Polymers with Carbazole Pendants Caizhen Liang, Bin Wang, Jianjun Chen, Qiwen Yong, Yuewen Huang, and Bing Liao J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b05481 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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The Journal of Physical Chemistry

Dispersion of Multi-Walled Carbon Nanotubes by Polymers with Carbazole Pendants Caizhen Liang,1,2 Bin Wang,1 Jianjun Chen,1,2 Qiwen Yong,1,2 Yuewen Huang,1 Bing Liao3, * 1

Key Laboratory of Cellulose and Lignocellulosics Chemistry, Guangzhou Institute

of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, P.R. China 2

University of Chinese Academy of Sciences, Beijing 100039, P.R. China

3

Guangdong Academy of Sciences, Guangzhou 510650, P.R. China

*Corresponding author: [email protected] (B. Liao)

1

ACS Paragon Plus Environment


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ABSTRACT: For various applications, it is essential to enhance the colloidal stabilization of carbon nanotube (CNT) dispersions. Here, the polymers with carbazole pendants of poly(4-(N-carbazolyl)methyl styrene-bl-poly (ethylene glycol) methyl ether methacrylate) (PCMS5-b-PAPEG73 and PCMS16-b-PAPEG43) and PCMS30, synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization, were used for noncovalent functionalization of multi-walled carbon nanotubes (MWCNTs) in Tetrahydrofuran (THF), offering efficient colloidal stabilization. Meanwhile, the adsorption of polymers onto MWCNTs was investigated. The results showed that the MWCNTs decorated with these three polymers in THF exhibited different colloidal stabilization and adsorption capacity. Moreover, the MWCNT dispersions could be stabilized for days and their colloidal stabilization elevated with the increase of polymer concentrations. The block copolymer PCMS16-b-PAPEG43 exhibited the optimal adsorption and dispersion capability for MWCNTs. These findings imply that PCMSm-b-PAPEGn will be a desirable dispersant for optimizing the stabilization of CNT dispersion, making carbon nanotubes (CNTs) achievable in different applications.

2


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1. INTRODUCTION

Carbon nanotubes (CNTs) are of great interest to researchers due to their extraordinary electronic, structure, and mechanical properties, which enable promising applications in various fields such as electronics1, biologies2, composites3, and material sciences4. Nevertheless, it is intractable that the pristine CNTs are prone to entangle and agglomerate owing to the strong intertube van der Waals interaction5 in both aqueous and organic solvents, which greatly limits their application. To widely exploit the CNTs, individualizing CNTs is required to obtain a homogeneous and stable dispersion6-7. Compared with covalent functionalization of nanotube surfaces, dispersant-assisted dispersion via non-covalent interaction with nanotubes is more appealing. Dispersant-assisted dispersion not only improves the dispersibility and stabilization of CNTs but also preserves the intrinsic properties of CNTs8-9. For this strategy,

the

small

molecule-based

surfactants10-12,

polymers13-18,

and

biomacromolecules19 are frequently chosen as dispersants to debundle or disperse CNTs. Generally, the dispersants stabilize the CNT dispersions mainly through electrostatic repulsion, steric repulsion or depletion forces17-18. Copolymers are composed of the solvophobic segments interacting with nanotubes and the solvophilic segments dangling into solvent, which provide steric or electrostatic repulsion preventing the dispersed nanotubes from reaggregating, hence forming dynamically stable dispersion14,

17, 20-23

. In this context, the ones with

aromatic side chains have attracted a lot of interest for CNT dispersion due to the strong π-π interaction between copolymers and nanotubes. Bahun24 et al reported 3



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that the pyrene-containing copolymers were able to stabilize single walled carbon nanotube (SWCNT) dispersions by non-covalent functionalization of SWCNTs. Kim25

et

al

reported

non-covalent

functionalization

of

SWCNTs

with

poly(DMAEMA-co-St) in THF and found that the poly(DMAEMA-co-St) with 70:30 molar ratio of DMAEMA/St was the most efficient dispersant. Wan26 et al found that the polysoap of poly(St-alt-Ma) functionalized with aminopyrene was more effective in dispersing SWCNTs than poly(St-alt-Ma). These reports proved that the aromatic side chains as solvophobic chains have significant effect on SWCNT dispersion. Carbazole is a heterocyclic compound with a dibenzopyrrole system, which endows it with excellent physicochemical properties27. Thus, the polymers with carbazole pendants as solvophobic groups will be good candidates for CNT dispersion. For copolymer-dispersant, the solvophilic chains also play an important role for stabilizing CNTs. Poly(ethylene glycol) (PEG) as a hydrophilic chain in linear or pendant copolymers has shown remarkable solvophilic property in polar solvent, imparting steric barrier for suspending CNTs20, 28. For example, the amphiphilic block copolymer PEG-PPS performed high dispersion and deagglomeration ability for CNTs attributed to the steric stabilization offered by PEG chains 23, 29. In order to obtain excellent colloidal stabilization of CNT dispersion, a valid dispersant should be not only capable of deaggregating the CNT clusters, but also capable of stabilizing the individual or small bundled CNT against reaggregating. Although a myriad of copolymers have been reported for dispersing CNTs in appropriate media, there is still much space to increase the colloidal stabilization of 4


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CNT dispersion for further application. Moreover, for MWCNT dispersion, there are few reports concerning synthesized polymers containing carbazole pendants and PEG side chains as dispersant. Therefore, it is meaningful to implement the study regarding the carbazole-containing polymers dispersing MWCNTs through noncovalent functionalization in specific medium, achieving stable and homogeneous MWCNT dispersions. To reach this goal, the monomer styrene derivative with carbazole pendant 4-(N-carbazolyl)methyl styrene (CMS) was chosen to homopolymerize and then copolymerize with poly (ethylene glycol) methyl ether methacrylate (APEG) via RAFT polymerization. The polymeric products were used to disperse MWCNTs. How do PCMS block and PAPEG block influence the MWCNT dispersion? How much of the polymers absorb onto the nanotube surfaces? How do the polymer concentrations affect the MWCNT dispersion? To

explore

these

questions,

the

polymers

of

PCMS5-b-PAPEG73,

PCMS16-b-PAPEG43 and PCMS30 were selected for noncovalent decoration of MWCNTs in THF. Fourier transform infrared, X-ray photoelectron spectroscopic, and thermogravimetric analysis were carried out to evaluate the adsorption behavior of these three polymers onto MWCNTs. The dispersion morphology of MWCNTs were preliminarily estimated by transmission electron microscopy. Further, the dispersion capabilities of these polymers varying in concentrations for MWCNTs were measured by ultraviolet-visible spectroscopic analysis. This study allows us to provide an efficient dispersant for dispersion MWCNTs with long-term stabilization, which make the MWCNTs possible for more applications. 5



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2. EXPERIMENTAL SECTION

Materials. Multi-walled carbon nanotubes (MWCNTs) with diameters of 8-15 nm and length of 50 µm were purchased from Alddin (Shanghai, China). Carbazole (Aldrich, AR) was recrystallized from methanol and dried under vacuum at 40 °C. 2, 2’- azobis(isobutyronitrile) (AIBN, Aldrich, 98%) was recrystallized from ethanol twice prior to use. 2-(Dodecylthiocarbonothioylthio)-2-methylpropanic Acid (TTCA) and 4-(N-carbazolyl)methyl styrene (CMS) were synthesized according to the reported procedures30-31. Poly (ethylene glycol) methyl ether methacrylate (APEG, Mn = 475, Aladdin) was purified of inhibitors by passing through a basic alumina column. 1-(chloromethyl)-styrene (CS, Aldrich, 90%), chloroform (99%, Aldrich), carbon disulfide (CS2, 98%, Aladdin), 1-dodecanethiol (98%, Aladdin) were used as received. All other chemicals were of analytical grade and used as received. Synthesis of PCMSm Macro-CTA Agents. A typical example (Table 1, entry 3) for the synthesis procedure is as follows: CMS (2.84 g, 10 mmol), TTCA (0.36 g, 0.2 mmol), AIBN (6.57 mg, 0.04 mmol; RAFT/initiator molar ratio = 5/1) and DMF (10 mL) were added into a 50 mL round-bottom septum-sealed flask equipped with a magnetic bar. The reaction mixture was degassed for 30 min with argon at ice bath before being placed in a preheated thermostated oil bath at 70 °C. After 5 h, the polymerization was quenched by cooling the reaction mixture in an ice bath and subsequently exposing it to air. The synthesized PCMS16 was purified by precipitation into excess methanol and further repeated two times. The conversion of CMS monomer was determined by gravimetric analysis. THF GPC analysis indicated an Mn 6


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of 4200 g/mol and an Mw/Mn of 1.13. Synthesis of PCMSm-b-PAPEGn. The present description corresponds to the typical example of PCMS16-b-PAPEG43 as list in Table 1. A 50 mL reaction flask with a magnetic stir bar was added with APEG (4.75 g, 10 mmol), PCMS16 macro-CTA (0.84 g, 0.2 mmol), AIBN (6.57 mg, 0.04 mmol), and DMF (10 mL). The flask was degassed with argon in ice water bath, and then RAFT polymerization was conducted at 70 °C in oil bath for 12 h and finally quenched through cooling the flask in an ice water bath. The monomer conversion of APEG was determined by gravimetric analysis. The synthesized PCMS16-b-PAPEG43 was purified by precipitation into excess diethyl ether and repeated two times, then dried in vacuum overnight. Polymers Characterization. The number-average molecular weight (Mn) and dispersity (Mw/Mn) of PCMSm and PCMSm-b-PAPEGn were determined by gel permeation chromatography (GPC) conducted on a Waters E2695 GPC with the columns in THF operating at the flow rate of 1 mL/min using a Waters 2410 refractive index detector, and the system was calibrated with the narrow polystyrene standards. The polymer compositions were determined by Fourier transform infrared (FT-IR) spectra (KBr, Nicolet 5100) and 1H NMR spectra (400 MHz, CDCl3; supporting information). The optical properties of polymers were recorded with UV-Vis spectra (Shimadzu UV-2550, U.S.A.). Preparation and Characterization CNT Samples. The MWCNT dispersions were prepared by dispersing MWCNTs in THF via sonication for 30 min at a MWCNT concentration of 1 mg/mL. The colloidal stabilization was evaluated in the 7



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presence of PCMS5-b-PAPEG73, PCMS16-b-PAPEG43 or PCMS30. The amounts of polymers added to the dispersions ranged from 0.25 mg/mL to 2 mg/mL. The colliodal assays were carried out for a period of 192 h detected by ultraviolet-visible (UV-Vis) spectroscopy in a diluted MWCNT concentration of 0.03 mg/mL, and all samples remained static at approximately 25 °C. The dispersions were also detected with a transmission electron microscopy (TEM, JEM-100CXII). The adsorptions of polymers on the surface of MWCNTs were analyzed by FT-IR, thermogravimetric analysis (TGA, Pyris, Shimadu) and X-ray photoelectron spectroscopy (XPS). The MWCNT dispersions without or with an equivalent dosage of polymers were centrifuged at 8000 rpm for 30 min and decanted. The deposits were washed with THF two times, and then dried in vacuum at 60 °C for 24 h. The thermogravimetric analysis (TGA, Pyris, Shimadu) measurement was implemented under nitrogen from 30 to 700 °C with a scan rate of 10 °C/min. XPS was characterized using a Thermo Scientific Escalab 250Xi instrument with a scanning microprobe and an Al Kα radiation (1486.8 ev). The colloidal dispersion stabilization was also evaluated by centrifugation study17, 32

. The MWCNTs combined with four different concentrations of polymers were

mixed in THF at final MWCNT concentration of 0.2 mg/mL. After sonication for 30 min, all the samples were centrifuged at the rpm varying from 1000 to 8000 for 5 min, and the supernatants were measured by UV-Vis spectroscopy at a wavelength of 700 nm, which was outside of the absorption window of polymers.

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3. RESULTS AND DISCUSSION Table 1. Experimental Conditions and Results of the RAFT Polymerization of PCMSm and PCMSm-b-PAPEGn in DMF Entry Polymer [M]0:[R]0:[I]0 Time/h Conva/% Mn,thb Mn,GPCc Mw/Mnd 1 PCMS5 15:1:1/5 5 33 1800 1700 1.09 2 PCMS16 50:1:1/5 5 32 4900 4200 1.13 3 PCMS30 100:1:1/5 5 30 8900 7900 1.12 85:1:1/5 12 87 31700 40500 1.48 7 PCMS5-b-PAPEG73 9 PCMS16-b-PAPEG43 50:1:1/5 12 90 18400 30600 1.39 a The monomer conversion determined by gravimetric analysis, bThe theoretical molecular weight determined by monomer conversion. cThe molecular weight measured by GPC. dThe Mw/Mn value measured by GPC. Synthesis and Polymer Characterization. Three trithiocarbonate macro-CTAs of PCMS5, PCMS16, and PCMS30 as listed in Table 1 were synthesized via RAFT solution polymerization of CMS using TTCA as the CTA and AIBN as the initiator in DMF at 70 °C at suitable monomer conversion. The THF GPC characterization (Mn, Mw/Mn), and monomer conversions (gravimetric analysis) of PCMSm macro-CTAs were given in Table 1. A second RAFT polymerization of APEG was subsequently started in the presence of PCMSm acting as a macro-CTA to obtain diblock copolymer PCMSm-b-PAPEGn, as outlined in Figure 1c. Details of GPC analysis and monomer conversions of the block copolymers are also recorded in Table 1.

9



(c) S 10

TTCA

S

S S

COOH AIBN, DMF

+

S

10

S

COOH

m

70 οC N

N

PCMSm

CMS

O

AIBN, DMF, 70 oC

O

O 7-8

O

APEG S 10

S

S

n

m

COOH

O O

O 7-8

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N

O

PCMSm-b-PAPEGn

Figure 1. Synthesis of PCMSm-b-PAPEGn block polymers via RAFT polymerization of APEG using PCMSm as macro-CTA agent: (a) FT-IR spectra, (b) UV-Vis spectra of PCMSm-b-PAPEGn and PCMS30, and (c) RAFT polymerization process of PCMSm-b-PAPEGn.

Figure 1a shows the FT-IR spectra of the synthesized polymers PCMS30, PCMS5-b-PAPEG73, PCMS16-b-PAPEG43, and the monomer CMS. The strong characteristic adsorption peaks at 2900 cm-1, 1720 cm-1 and 1116 cm-1 were assigned to –CH2, C=O, and C–O–C stretching vibration in the PEO side chains of PCMS5-b-PAPEG73 and PCMS16-b-PAPEG43 copolymers. Compared with the FT-IR spectrum of CMS, the characteristic peaks of –CH2 vibration at 2917 cm-1 and 2850 cm-1 in the spectrum of PCMS30 were highly stronger than that in the spectrum of CMS because of the increase of –CH2 from the polymerization of CH2=CH–. These results confirmed that the PCMS30 was successfully polymerized. For PCMS30, 10


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PCMS5-b-PAPEG73, and PCMS16-b-PAPEG43 polymers, the peaks at 3052 cm-1, 752 cm-1 and 723 cm-1 originating from in-plane =C–H stretching vibration and ring out-of-plane bending vibration of aromatic ring were stronger with the PCMS blocks extended in the polymers, which were accorded with the structure of the three polymers. The evidence was also proved in the UV-Vis spectra (Figure 1b). Therefore, the results of FT-IR and UV-Vis spectra are consistent with the molecular structures of PCMS5-b-PAPEG73, PCMS16-b-PAPEG43, and PCMS30.

Figure 2 (a) FT-IR spectra of MWCNT samples; (b) Thermogravimetric analysis of MWCNT samples.

Adsorption of Polymers onto the Surface of MWCNTs. The presence of polymers on the surfaces of MWCNTs was identified by Fourier transform infrared spectroscopic (FT-IR) analysis. As shown in Figure 1a, the FT-IR analysis exhibited significant difference between the pristine MWCNT and the MWCNT samples treated with polymers. The MWCNTs decorated with polymers display distinctive absorption peaks at around 1640-1400 cm-1 belonging to the C=C stretching vibration of the 11



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aromatic group of CMS, which are not observed in the spectrum of pristine MWCNTs. It confirms that the polymers were successfully adsorbed onto the surfaces of MWCNTs. Additionally, the MWCNT samples attached by PCMS5-b-PAPEG73 or PCMS16-b-PAPEG43 displayed a peak at 1070 cm-1 due to the C–O–C stretching vibration of PEO side chain. The adsorptions of polymers on the surfaces of MWCNTs were also conducted by thermogravimetric analysis (TGA), as seen in Figure 2b. The MWCNT/PCMS5-b-PAPEG73 showed a weight loss up to 20 wt %, starting at about 239 °C, while the MWCNT/PCMS30 exhibited a two-step decomposition process at the beginning of 150 °C and 340 °C with weight loss of 4.1 wt % and 15.5 wt %, respectively. The first decomposition is, perhaps, due to the contaminant decomposition of the impurity of original MWCNT or the evaporation and decomposition of the small polymeric fragments of PCMSm. It can be found that the

MWCNT/PCMS16-b-PAPEG43

sample

had

higher

weight

loss

than

MWCNT/PCMS5-b-PAPEG73 and MWCNT/PCMS30 samples, obtaining a weight loss of 26.2 wt % in the decomposition temperature ranging from 233 °C to 700 °C. The pristine MWCNT decomposed steadily with the increase of temperature having a weight loss of about 5.1 wt % probably because of the degradation of disordered or amorphous carbon, and metal impurities introduced during the pristine MWCNT preparation33. The different decomposition process between the MWCNT/polymers and pristine MWCNT proved that the polymers successfully absorbed onto the surface of MWCNTs. It implies that the polymers were able to break the aligned MWCNTs through the non-covalent π-π interaction9, enabling the MWCNTs to 12


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disperse in solvents, and the surface coverage of polymers on the surface of MWCNTs was relied on the APEG/CMS molar ratio of the polymers.

Figure 3 The XPS Wide-scan spectra, N 1s and C 1s high-resolution XPS spectra of pristine

MWCNT

(a),

(b);


(c),

(d);

MWCNT/PCMS16-b-PAPEG43 (e), (f) and MWCNT/PCMS30 (g), (h). In order to further investigate the elemental compositions on the surfaces of MWCNT samples, the X-ray photoelectron spectroscopic (XPS) analysis was carried out in this study. Figure 3 displays the XPS wide-scan spectra, C 1s and N 1s high-resolution XPS spectra of different MWCNT samples. For the MWCNTs decorated with polymers, the peaks with binding energies of 284.8 (C 1s), 400 (N 1s) 13



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and 533 (O 1s) were present in the Survey scans (Figure 3c, 3e, 3g) where the N 1s was absent in the pristine MWCNTs (Figure 3a), debunking the presence of nitrogen is belonged to the PCMS segments of the polymers. The elementary compositions of these MWCNT samples are summarized in Table 2. The content of nitrogen varied from 0.7% to 2.59%, and the oxygen content was in the range of 2.26% - 11.26%. Compared with the pristine MWCNTs, the N/C ratio of MWCNT/PCMS30 was 0.24% and

1.93%

higher

than

that

of


and

MWCNT/PCMS5-b-PAPEG73, respectively. Also, the O/C ratio of MWCNT/PCMS30 was 2.37%, which approached the pristine MWCNTs (2.48%), disclosing the presence of the PCMS30 on the surface of MWCNTs. For MWCNT/PCMS5-b-PAPEG73 and MWCNT/PCMS16-b-PAPEG43, the O/C ratios were 12.30% and 13.10%, which is about 5 times than that of pristine MWCNTs. Based on these results and the scale of PAPEG of the polymers, it can be found that the absorption loading of PCMS16-b-PAPEG43 was the maximum, in accordance with the TGA result. Figure 3b, d, f, and h show the C 1s high resolution spectra of the MWCNT samples. The C 1s peak of pristine MWCNTs shown in Figure 3b was deconvoluted into four binding energies at 284.78 (–C=C–), 285.68 (–C–C–), 286.68 (–C–O) and 287.48 eV (–C=O), while the deconvoluted peak at 287.48 (–C=O) was not observed in the spectrum of MWCNT/PCMS30-TTCA. Figure 3d and f display the curve-fit of C 1s for MWCNT/PCMS5-b-PAPEG73 and MWCNT/PCMS16-b-PAPEG43, with the binding energies of 284.78, 285.68, 286.68, and 287.48 eV originated from the – C=C–, –C–C–, –C–O–C–, and –C=O, respectively. Additionally, the –C–O contents 14


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of


and


were

prominently higher than that of pristine MWCNTs. These results indicate that the polymers we synthesized are effective on interacting with MWCNTs. Table 2. Element Analysis of C, O, N on the Surface of MWCNTs Samples Based on XPS Calculation Element (atomic %) C O N O/C N/C -C-Oa (%) (%) (%) p-MWCNT 97.58 2.42 2.48 5.67 MWCNT/PCMS5-b-PAPEG73 88.42 10.88 0.70 12.30 0.79 18.93 MWCNT/PCMS16-b-PAPEG43 85.93 11.26 2.13 13.10 2.48 18.41 MWCNT/PCMS30 95.16 2.26 2.59 2.37 2.72 3.37 a was calculated from the C 1s deconvoluted peak of high resolution XPS. Colloidal Dispersion. Addition of polymers is a versatile method to hinder the dispersed CNTs from reaggregating, via non-covalent functionalization of CNTs34-35. For increasing the colloidal stabilization and shelf life of CNT dispersions, the polymers with different CMS scale containing carbazole pendants (Table 1, Figure 1) were chosen to disperse MWCNTs in organic solvent. The effects of PCMS5-b-PAPEG73, PCMS16-b-PAPEG43 and PCMS30 on the dispersibility and dispersion stabilization of MWCNTs in THF were detected by Transmission electron microscope (TEM), and ultraviolet-visible (UV-Vis) spectroscopy.

15



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C

Figure 4. (A) Schematic showing the probable dispersion mechanism of MWCNTs in THF with polymers. (B), (C) Photographs and TEM images of MWCNT dispersions: (a) Pristine MWCNT dispersion; (b) MWCNT/PCMS5-b-PAPEG73 dispersion; (c) MWCNT/PCMS16-b-PAPEG43 Dispersion; (d) MWCNT/PCMS30 dispersion. Figure 4 shows the photographs and TEM images of pristine MWCNT and polymers decorated MWCNT dispersions, which were sonicated for 30 min without further manipulating. As seen in Figure 4B and C, the pristine MWCNTs mutually entangled to form precipitates in THF due to their strong van der Waals interaction. The dark ink dispersions were achieved after adding the polymers and sonication (Figure 4B: b, c and d), showing that these polymers are able to disperse MWCNTs effectively. The probable dispersion mechanisms are displayed in Figure 4A. The comparison of the images also reveals that the improved dispersion of MWCNTs was available

through

the

noncovalent

interaction

with

PCMS5-b-PAPEG73,

PCMS16-b-PAPEG43 or PCMS30 compared to pristine MWCNT sample. Interestingly, the MWCNT/PCMS16-b-PAPEG43 dispersion had a better colloidal dispersion compared to the MWCNT/PCMS5-b-PAPEG73 and MWCNT/PCMS30 dispersions. This phenomenon suggests that the block copolymer PCMS16-b-PAPEG43 have higher 16


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competence for dispersing MWCNTs in THF, consistent with its adsorption behavior. More importantly, the MWCNT dispersions with these polymers could keep stable for several months, providing potential for extra application (Figure 4B). To probe the effect of the polymers on the colloidal stability of MWCNT dispersions, the dispersions with a MWCNT concentration of 30 µg/mL were investigated in the presence of polymers by measuring the absorbance using UV-Vis spectroscopy at approximately 25 °C. The sedimentation process was monitored over a period of days under incubation. The absorbance of various dispersions as a function of time is displayed in Figure 5. It can be seen that the polymers exhibited different extent of stabilization for MWCNTs in THF, while the pristine MWCNTs were not initially stable in the absence of polymer. For the polymer-decorated dispersions, the absorbance curves exhibited similar decaying behavior with the time processing. The value decreased significantly at first 24 h, which is likely that the amount of polymers were not enough to separate all the bundled nanotubes so that the steric repulsion was inferior to van der Waals attractions among the suspended MWCNTs initially, resulting in partial MWCNTs entangling and sedimenting out17, 36. Fortunately, when the time exceeded 24 h, the absorbance started dropping slightly, and then tended to be a plateau. This subtle change in MWCNT concentration interprets the steric repulsion induced by polymers was dominant in dynamically stabilizing the MWCNTs.

17



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Figure 5 Colloidal stability assays for MWCNTs with different polymer concentrations in THF for a period of hours. The polymer concentrations of 7.5, 15, 30, and 60 µg/mL were analyzed: (a) Pristine MWCNT dispersion; (b) MWCNT/PCMS5-b-PAPEG73

dispersions;

(c)


dispersions; (d) MWCNT/PCMS30 dispersions. Figure 5 conveys that the polymer concentration played a great impact on dispersion stabilization. When the polymer concentration was 7.5 µg/mL, nearly all MWCNTs sedimented out, similar to the pristine MWCNTs dispersed in THF. It suggests that under low polymer concentration condition, the polymers were invalid in dispersing the nanotubes because of the limit steric barrier, which was inadequate to beat the regnant van der Waals attractions. Continuing adding the polymers, the absorbance positively changed, which bore out that more polymers available interacted with nanotubes, contributing to the increasing steric repulsion between the 18


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nanotubes. Hence, higher fractions of nanotubes were stabilized in dispersions compared to the dispersions with the concentration of 7.5 µg/mL polymers, demonstrating that the polymer concentration exerts a significant effect on the stabilization of dispersion. Figure 5 also points out the intimate affinity of molar ratio of CMS to APEG on stabilizing MWCNT dispersion. Distinctly, PCMS16-b-PAPEG43 exhibited better dispersion ability for MWCNTs compared to PCMS5-b-PAPEG73 and PCMS30. It proves that PCMS16-b-PAPEG43 has a preferable composition of PCMS and PAPEG to afford the steric barrier in this dispersion system. As a result, the MWCNT/PCMS16-b-PAPEG43 dispersion maintained a larger portion of MWCNTs with

long-term

stabilization.

Moreover,

the

effect

of

PCMS16-b-PAPEG43

concentration on dispersion stabilization was more evident. These findings suggest that a more stable MWCNT dispersion can be achieved with less amount of PCMS16-b-PAPEG43, which will be preferable in nanoparticles dispersion. It is believed that the extension of solvophobic block of PCMS could fortify the π-π interaction with nanotubes, and consequently enhance the dispersion stabilization of nanotubes9, 20. However, with regard to MWCNT/PCMS30 dispersions, the colloidal stabilization did not increase markedly as preconception with the increase of PCMS30 concentration, which was slightly lower than that of MWCNT/PCMS5-b-PAPEG73 dispersions. It suggests that the homopolymer PCMS30 was devoid of polar chains to construct the steric hindrance, so it was less effective in precluding the reagglomeration of nanotubes compared to the block copolymers. It proves that the 19



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Page 20 of 31

polymer with appropriate molar ratio of solvophobic moiety of PCMS to solvophilic moiety of PAPEG is a key factor for dispersing and stabilizing MWCNTs. This study also indicates that PCMS16-b-PAPEG43 showed prominent dispersion capability for MWCNTs dispersion.

Figure 6 Colloidal stability of MWCNT dispersions with different polymer concentrations: (a) Pristine MWCNT dispersion, (b) MWCNT/PCMS5-b-PAPEG73 dispersions, (c) MWCNT/PCMS16-b-APEG43 dispersions, (d) MWCNT/PCMS30 dispersions. The curves represent the MWCNT solid content in dispersion after centrifuging at different centrifugal speed recorded by UV-Vis spectroscopy. The dispersion stabilization was further probed through centrifugation assays with the MWCNT concentration of 200 µg/mL and measured the fractions of MWCNTs in dispersions as a function of centrifugal speed, as seen in Figure 6. The curves present the dispersibility and colloidal stabilization of MWCNTs in a new equilibrium state 20


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obtained after centrifugation17. Good agreement was achieved for the colloidal stabilization of MWCNT dispersions obtained from UV-vis analysis results. Similarly, at the same polymer concentration, the dispersion stabilization afforded by PCMS16-b-PAPEG43 was substantially higher than that by PCMS5-b-PAPEG73 and PCMS30. What’s more, PCMS16-b-PAPEG43 with the concentration of 50 µg/mL could stabilize the same amount of nanotubes as PCMS5-b-PAPEG73 or PCMS30 did with the concentration of 400 µg/mL. This indicates that PCMS16-b-PAPEG43 realized the goal that lower concentration of PCMS16-b-PAPEG43 could induce higher dispersion ability for MWCNTs, leading to low cost37. On the other hand, PCMS16-b-PAPEG43 exhibited prominent dispersibility under its concentration range (50-400 µg/mL; see Figure 3c) at initial state or slow centrifugal speed (≤ 2000 rpm). It discloses the preferable composition of PCMS solvophobic block and PAPEG solvophilic block of PCMS16-b-PAPEG43, which created more efficient steric barrier between nanotubes to enhance the colloidal stabilization. It is surprised that MWCNT/PCMS30 dispersions maintained higher fractions of MWCNT relative to MWCNT/PCMS5-b-PAPEG73 at slow speed (≤ 2000 rpm), which hints that more PCMS30 interacted with nanotube38 through π-π interaction under concentrated conditions, thereby stabilizing more MWCNTs. Nevertheless, the contents of suspended MWCNTs reduced rapidly to less than 50 wt % for MWCNT/PCMS16-b-PAPEG43 dispersions, and lower than 30 wt % for MWCNT/PCMS5-b-PAPEG73 and MWCNT/PCMS30 dispersions when the centrifugal speed was up to 8000 rpm. It implies the steric stabilization of polymers for MWCNT dispersions failed to surmount the centrifugal force inducing 21



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Page 22 of 31

agglomeration. Obviously, for MWCNT/PCMS16-b-PAPEG43 and MWCNT/PCMS30 dispersions, the 4000 rpm was a crucial centrifugal speed to disrupt the original dispersion equilibrium, bringing about large amount of sediments since the dominant centrifugal force accelerates the reaggregation of nanotubes. These results suggest that these polymers could be promising in sorting CNTs. Although PCMS30 was able to partially stabilize nanotubes in concentrated dispersion conditions in comparison to the sedimentation survey displayed in Figure 5d, the effect of PCMS30 concentration on the dispersion stabilization was not significant. Besides, the colloidal stabilization offered by PCMS5-b-PAPEG73 is less efficient than that by PCMS16-b-PAPEG43. It confirms that the steric stabilization induced by the polymers is the main mechanism for stabilizing MWCNTs in this study system. It will be difficult to raise the dispersion stabilization through elevating the amount of PCMS30, but valid for PCMS5-b-PAPEG73 and PCMS16-b-PAPEG43. This difference is associated with the chemical structure and composition of these polymers. In MWCNT dispersions, the block copolymers PCMS5-b-PAPEG73 and PCMS16-b-PAPEG43 mainly stabilize the MWCNTs through the steric stabilization imparted by the synergistic effect that PCMS chains interact with MWCNs by π-π interaction and the PAPEG chains stretch into solvent. With the increase in PCMS solvophobic

moiety,

higher

MWCNT/PCMS16-b-PAPEG43 MWCNT/PCMS5-b-PAPEG73

dispersion

stabilization

dispersions dispersions,

but

was

obtained

in

comparison

feeble

improvement

for to for

MWCNT/PCMS30, because of the absence of solvophilic groups of PAPEG. The 22


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evidence demonstrates that the polar moiety of PAPEG as solvent chains is also crucial to produce steric hindrance for MWCNTs. It worth mentioning that the block polymers of PCMSm-b-PAPEGn were also efficient in dispersing CNTs in water (Figure S3), which expands the dispersion abilities of the block polymers for CNTs in aqueous medium. This study provides a kind of valid dispersants in dispersing CNTs in aqueous and organic media, which make the polymers applicable in other nanoparticle dispersion. Furthermore, the CNT dispersions with long-term stabilization afford a prospect in various application.

4. CONCLUSION

The polymers with carbazole pendants of PCMS-b-PAPEG and PCMS30 have been successfully synthesized by RAFT polymerization, following the characterizations by GPC,

1

H NMR, FT-IR and UV-Vis analysis. The adsorption capabilities of

PCMS5-b-PAPEG73, PCMS16-b-PAPEG43 and PCMS30 for MWCNTs were explored and compared through non-covalent decoration of MWCNTs in THF with the evaluation by TGA, FT-IR, XPS, TEM and UV-Vis analysis. It proved that the adsorption of the block copolymer PCMS16-b-PAPEG43 on the surfaces of MWCNTs was maximal, while the homopolymer PCMS30 was minimal. The dispersion stabilization of MWCNT dispersion was not only relied on the polymer structure and composition

but

also

on

the

polymer

concentration,

especially

for

PCMS16-b-PAPEG43. Obviously, the block copolymer PCMS16-b-PAPEG43 has the optimum dynamic dispersion stabilization, which indicates that an efficient 23



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Page 24 of 31

copolymer-dispersant should have the balance between solvophilic moiety and solvophobic moiety, thus enhancing dispersion stabilization. Besides, the steric stabilization is a dominant mechanism for dispersing MWCNTs. With the increase of polymer concentration, the dispersion stabilization changed positively. During the incubation, the diluted MWCNT dispersions were stabilized for more than 192 h despite a fraction of MWCNTs sedimenting out at first 24 h. Additionally, the dispersion stabilization lowered with the increase of centrifugal speed. Typically, the block copolymer PCMS16-b-PAPEG43 exhibit the optimal dispersion ability due to its outstanding molar ratio of APEG to CMS. The combination of solvophilic and solvophobic composition of PCMS-PAPEG generated stable dispersion of MWCNTs, which made it prospective for the next dispersion strategy probing in water of high ionic strength or DMF. Also, these block copolymers have potential applications in sorting CNTs and other nanoparticles dispersion, and the excellent dispersion ability of PCMSm-b-PAPEGn provides possibility for expanding the application of MWCNTs.

ACKNOWLEDGMENTS This work was supported by Key Laboratory of Cellulose and Lignocellulosics Chemistry, Chinese Academy of Sciences and Foshan Centre for Functional Polymer Materials and Fine Chemicals. The support is also acknowledged from Guangzhou Municipal Science and Technology Program (No. 201510010183)

24


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Dispersion of Multi-Walled Carbon Nanotubes by Polymers with

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