Facile graft polystyrene onto multi-walled carbon nanotubes via in situ thermo-induced radical polymerization
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- Liu, P. J Nanopart Res (2009) 11: 1011. doi:10.1007/s11051-008-9563-0
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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.
KeywordsPolystyreneMulti-walled carbon nanotubesRadical additionThermo-inducedBulk polymerizationMWCNTNanomaterial
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.
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.
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.
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.