Journal of Nanoparticle Research

, Volume 11, Issue 4, pp 1011–1016

Facile graft polystyrene onto multi-walled carbon nanotubes via in situ thermo-induced radical polymerization

Authors

    • State Key Laboratory of Applied Organic Chemistry, Institute of Polymer Science and Engineering, College of Chemistry and Chemical EngineeringLanzhou University
Brief Communication

DOI: 10.1007/s11051-008-9563-0

Cite this article as:
Liu, P. J Nanopart Res (2009) 11: 1011. doi:10.1007/s11051-008-9563-0

Abstract

A facile procedure was developed for the grafting of polystyrene onto the surfaces of multi-walled carbon nanotubes (MWNTs) via the in situ thermo-induced bulk radical polymerization of styrene at the different polymerizing temperatures, in the presence of MWNTs without any initiator added. The grafting products were validated by the dispersibility, TEM, TGA, FT-IR, and Raman analysis. The TGA results also showed the lower polymerizing temperature was propitious to the free radical addition reactions.

Keywords

PolystyreneMulti-walled carbon nanotubesRadical additionThermo-inducedBulk polymerizationMWCNTNanomaterial

Introduction

More and more intense interest is focused on carbon nanotubes (CNTs) since their discovery by Iijima in 1991 because of their unique structure, and mechanical and electrical properties (Falvo et al. 1997; Odom et al. 1998). Many useful applications, such as nanocomposites (Moniruzzaman and Winey 2006), field emission displays (Kim et al. 2004), microcircuit (Jun et al. 2007), diodes and transistors (Arvray et al. 2005), sensors (Sinha et al. 2006), hydrogen storage (Ajayan 1999) have already been demonstrated. However, their lack of solubility and difficulty in processing as thin films in solvents has imposed many barriers for their widespread use in many of these applications.

The functionalization of CNTs via polymer is an efficient way to promote solubility of CNTs in common solvents and organic polymeric matrices (Sun et al. 2002; Tasis et al. 2006). The polymer chains could be attached toward CNTs via noncovalent or covalent methods (Liu 2005). The noncovalent attachment is implemented by polymer wrapping, thus maintaining electronic properties of nanotubes. However, it is workable only for limited species. The covalent grafting of a variety of polymers to the surface of carbon nanotubes was achieved via coupling reactions (Baskaran et al. 2005), controlled/“living” radical polymerization (Hemenick et al. 2007), free radical addition (Park et al. 2003; Petrov et al. 2004; Yan et al. 2005; Xu et al. 2006), and polycondensation (Yang et al. 2006). The resulting hybrid products showed enhanced solubility in common organic solvents.

Recently, the free radical addition method has attracted more and more interests because of its simple process. In this method, CNTs were dispersed into the monomer solutions and then the free radical polymerizations were induced with initiators (Park et al. 2003), 60Co γ-ray irradiation (Xu et al. 2006), ultraviolet irradiation (Yan et al. 2005), or electrochemical initiation (Petrov et al. 2004). In the present communication, the polystyrene grafted multi-walled carbon nanotubes (MWNT-PS) were prepared via the facile in situ thermo-induced bulk polymerization for the first time.

Experimental

Materials

MWNTs, synthesized by a thermal chemical vapor deposition method, were obtained from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China) with an average diameter of 20–40 nm and a purity of 95%. The MWNTs were used without any purification process. Styrene was washed with dilute alkali solution, dried over barium oxide, and distilled twice under reduced pressure. Other solvents used were analytical grade.

Polymerization

MWNTs (0.20 g) and styrene (20 mL) were charged into a polymerizing tube, which was sealed after bubbling N2 for 10 min in an ice water bath. The polymerizing tube was irradiated with ultrasonic vibrations for 30 min and heated to the certain temperature (80 or 100 °C) with oil bath for 2 weeks. The polymerizing tube was irradiated with ultrasonic vibrations for 30 min every day in the first week during the polymerizing period.

The polystyrene grafted multi-walled carbon nanotubes (MWNT-PS80 via polymerization at 80 °C and MWNT-PS100 via polymerization at 100 °C, respectively) were separated by being dispersed in toluene with ultrasonic vibration and centrifuged (15,000 rpm for 30 min) several times until the PS was not found in the solvent (by being precipitated in methanol).

Analysis and characterizations

Bruker IFS 66 v/s infrared spectrometer was used for the Fourier transform infrared (FTIR) spectroscopy analysis. Raman measurements were carried out on the powder samples using FT-Raman spectrometer (BRUKER RFS 100/S) with the excitation laser of Nd:YAG (wavelength: 1064 nm). Thermogravimetric analysis (TGA) was performed with a Perkin–Elmer TGA-7 system at a scan rate of 10 °C min−1–700 °C in N2. The morphologies of the MWNTs and MWNTs-PS were characterized with a JEM-1200 EX/S transmission electron microscope (TEM). The powers were dispersed in toluene in an ultrasonic bath for 5 min, and then deposited on a copper grid covered with a perforated carbon film.

Results and discussion

In many reported works, the MWNTs were treated with acids to eliminate impurities in the MWNT (Park et al. 2003; Choi et al. 2007; Kim et al. 2007). However, these acid-treatments are known to shorten the length of MWNT and introduce hydroxyl and carboxylic functional groups to MWNT (Liu et al. 1998; Chen et al. 2002; Park et al. 2003; Sung et al. 2004). In the present work, the MWNTs were used without any purification process in order to avoid the effect of the hydroxylic functional groups on the graft polymerization. Furthermore, the length of the MWNTs remained the same.

In the present polymerizing conditions, the thermo-induced polymerization of the monomer, styrene, was initiated and the polystyrene chain free radicals formed. The free polystyrene chain radicals could be added onto the surfaces of the MWNTs by opening the π-bonds on MWNT surface to introduce radicals onto the MWNT surface. Sometimes the π-bonds on MWNT surface can be available by an initiator and initiate radical polymerization (Park et al. 2003; Choi et al. 2007). It requires more activation energy to break the π-bonds on MWNT surface. So it is asseverated that most of the polystyrene chains grafted were resulted from the breaking of the π-bonds on MWNT surface with the polystyrene chain free radicals and the graft polymerization was conducted from the active centers on MWNT surface therefore.

Figure 1 exhibits the optical images of MWNTs suspensions after standing for different times. As shown in Fig. 1a, phase separation and formation of sediments were observed after only 1 h. The MWNTs seemed only to be swollen in toluene. As a common solvent, toluene does not surmount the gravity-driven sedimentation of MWNTs bundles. Consequently, toluene fails to sustain stable nanotubes suspension. However, no nanotubes precipitation was observed in the PS-MWNTs solution for at least 1 month (Fig. 1b). Practically, most of the nanotubes remained suspended for over 3 months since the PS-MWNTs suspensions were made. From the above results, it is concluded that polystyrene had been successfully grafted onto the surfaces of the MWNTs.
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Fig. 1

Suspensions of MWNTs in toluene (weight ratio 1.0%). a pristine MWNTs, and b MWNTs-PS80

TEM studies were performed for characterizing the structure of the modified MWNTs. Figure 2 shows the TEM images of MWNTs before and after modification. It is found that the MWNTs before modification (Fig. 2a) were mainly a long pipe and its average diameter is about 15 nm. The hollow core is clearly evident in the TEM image. After polystyrene was covalently grafted from the surface of MWNTs by the proposed thermal induced bulk polymerization approach at 80 °C (MWNTs-PS80, Fig. 2b) or 100 °C (MWNTs-PS100, Fig. 2c), a layer of unevenly coated films can be observed in the TEM images. Some sections of the convex surfaces of MWNTs congregate more polymers. Moreover, the hollow core has been unclear in the TEM image.
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Fig. 2

TEM images of the pristine MWNTs and the MWNTs-PS obtained at different polymerizing temperature. a MWNTs, b MWNTs-PS80, and c MWNTs-PS100

The amount of polystyrene on the surface of modified MWNTs was determined by TGA (Fig. 3). The weight losses of the three samples (MWNTs, MWNTs-PS80, and MWNTs-PS 100) at the temperature lower than 100 °C were attributed to the release of the solvent adsorbed. It could be found that the weight loss of the MWNTs was lower than the MWNTs-PS samples. This is due to the surface grafted polymers. The weight losses at the temperature range of 350 to 500 °C were attributed to the decomposition of the polystyrene grafted. From the data, the percentage of grafting (PG%), the weight ratio of the polymer grafted and the MWNTs, could be calculated to be about 16% for MWNTs-PS80 and 8% for MWNTs-PS100, respectively. It might be due to the higher terminating rate of the free radicals at the higher temperature. In the condition, the terminating reactions between the chain free radicals and the addition reactions between the chain free radicals and the MWNTs were a couple of competing reactions. The weight loss of the MWNTs was due to the decomposition of the impurities and the side wall defects of the pristine MWNTs because the MWNTs were used without any purification process and their purity was only 95%.
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Fig. 3

TGA curves of the pristine MWNTs and the MWNTs-PS obtained at different polymerizing temperature

Fourier transform infrared spectra (FTIR) of the pristine MWNTs and the MWNTs-PS80 were given in Fig. 4. The spectrum of MWNTs had no obvious vibration peaks. The spectra of MWNTs-PS80 modified by the in situ thermo-induced bulk polymerization showed the characteristic vibration peaks of PS. The peaks of 2925 and 2850 cm−1 are asymmetrical and symmetrical stretching vibrations of –CH2, respectively. The peak of 1450 cm−1 is assigned to the flexural vibrations of –CH2. Furthermore, additional bands at 700 cm−1 characteristic of the out of plane C–H vibration, and 1630 cm−1 characteristic of C–C ring stretching vibrations were also present. The presence of these vibration peaks confirms that PS had attached onto the face of MWNTs.
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Fig. 4

FT-IR spectra of the pristine MWNTs and the MWNTs-PS obtained by bulk polymerization at 80 °C

Raman spectrum can provide qualitative information on the status of sidewall modification, which corresponds to the change of properties for MWNTs. The Raman spectra for the MWNTs and modified MWNTs are shown in Fig. 5. Both spectra present similar spectral patterns. The D-band can be found in the 1300–1400 cm−1 region. This peak is attributed to scattering from sp2 carbons containing defects. The band seen in the 1500–1600 cm−1 is the so-called G-band, resulting from the tangential C–C stretching vibrations both longitudinally and transversally in the MWNTs axis (Kukovecz et al. 2002). The change of spectral position of G-band after functionalization with PS could be found. In the spectra of pristine MWNTs, the G-band and D-band were peaking at 1601 and 1288 cm−1, respectively, but, after functionalization with PS, these bands shifted to 1589 and 1291 cm−1, respectively. The observed changes of G-band and D-band could be explained by considering grafting of PS onto MWNTs presented in TEM images. The D/G-band intensity ratio (ID/IG) typically changes when covalent functionalization of the graphite sheet occurs (Bahr and Tour 2001; Dyke and Tour 2003). The ID/IG of pristine MWNTs was 0.74. The ID/IG of modified MWNTs was 2.24. The data presented above indicate that PS had been covalently attached onto the MWNTs surface.
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Fig. 5

Raman spectra of the pristine MWNTs and the MWNTs-PS obtained by bulk polymerization at 80 °C

In summary, the polystyrene grafted multi-walled carbon nanotubes (MWNTs-PS) were successfully prepared via the free radical addition reaction by the in situ thermo-induced bulk polymerization in the presence of MWNTs. The MWNTs had wonderful dispersibility in organic solvents such as toluene. The proposed method could be used for the facile preparation of the polystyrene/multi-walled carbon nanotubes (PS/MWNTs) nanocomposites.

Copyright information

© Springer Science+Business Media B.V. 2008