Synthesis and characterization of multiwalled CNT–PAN based composite carbon nanofibers via electrospinning
© Kaur et al. 2016
Received: 22 December 2015
Accepted: 24 March 2016
Published: 19 April 2016
Electrospun fibrous membranes find place in diverse applications like sensors, filters, fuel cell membranes, scaffolds for tissue engineering, organic electronics etc. The objectives of present work are to electrospun polyacrylonitrile (PAN) nanofibers and PAN–CNT nanocomposite nanofibers and convert into carbon nanofiber and carbon-CNT composite nanofiber. The work was divided into two parts, development of nanofibers and composite nanofiber. The PAN nanofibers were produced from 9 wt% PAN solution by electrospinning technique. In another case PAN–CNT composite nanofibers were developed from different concentrations of MWCNTs (1–3 wt%) in 9 wt% PAN solution by electrospinning. Both types of nanofibers were undergone through oxidation, stabilization, carbonization and graphitization. At each stage of processing of carbon and carbon-CNT composite nanofibers were characterized by SEM, AFM, TGA and XRD. It was observed that diameter of nanofiber varies with processing parameters such as applied voltage tip to collector distance, flow rate of solution and polymer concentrations etc. while in case of PAN–CNT composite nanofiber diameter decreases with increasing concentration of CNT in PAN solution. Also with stabilization, carbonization and graphitization diameter of nanofiber decreases. SEM images shows that the minimum fiber diameter in case of 3 wt% of CNT solution because as viscosity increases it reduces the phase separation of PAN and solvent and as a consequence increases in the fiber diameter. AFM images shows that surface of film is irregular which give idea about mat type orientation of fibers. XRD results show that degree of graphitization increases on increasing CNT concentration because of additional stresses exerting on the nanofiber surface in the immediate vicinity of CNTs. TGA results shows wt loss decreases as CNT concentration increases in fibers.
Electrospinning is a most efficient technique to generate fibers with submicron diameters. Electrospun nanofibers mats are more promising because of high surface area and porosity, which have applications in air filtration, tissue engineering, drug delivery, and energy storage materials (Doshi and Reneker 1995; Formhals 1944). Reliable production of porous nanofibers in a simple and inexpensive way has been attempted by number of groups (Xia and Li 2006) showed that, by using a coaxial spinneret with miscible solvents and immiscible polymers, highly porous fibers could be obtained (Xia and Li 2006). Since the beginning of this century, researchers all over the world have been re-looking at a century old process (Cooley and Morton 1902) currently it is known as electrospinning (Hagewood 2004). Probably unknown to most researchers for most of the last century, electrospinning is able to produce continuous fibers from the submicron diameter down to the nanometer diameter. It was not until the mid-1990s with interest in the field of nanoscience and nanotechnology that researchers started to realize the huge potential of the process in nanofiber production (Doshi and Reneker 1995). Nanofibers and nanowires with their huge surface area to volume ratio, about a thousand times higher than that of a human hair, have aspect area.
Carbon fibers are manufactured through heating and stretching treatments (Shenoy et al. 2005). Polyacrylonitrile (PAN) and pitch are the two most common raw products used to produce carbon fibers. PAN is a synthetic fiber that is pre manufactured and wound onto spools, and pitch is a coal-tar petroleum product that is melted, spun, and stretched into fibers. The production of carbon nano fibrils will achieve a unique property (Mechanical, Electrical and Physical) by keeping the same amount of crystallite size from core to skin and increasing the surface area per unit volume which mimic the same structure of MWCNT (Saito et al. 1998).
This paper reports our findings on testing this concept by electrospinning mixtures of Chopped PAN co-polymer micro fibers of diameter 12.5 µm having polyacrylonitrile with a 6 % monomer methyl methacrylate and N,N-dimethylformamide (DMF) of 99 % purity (B. Pt. = 157 °C) and also Functionalized Carbon Nanotubes (CNT) are used as fillers (Haddon and Itkis 2008). Fibers uniformity and diameter (75–1500 nm) have been shown to increase with increasing concentrations of CNT by controlling solution and processing parameters in whole study (Haddon and Itkis 2008).
Chopped PAN co-polymer micro fibers of diameter 12.5 µm having polyacrylonitrile with a 6 % monomer methyl methacrylate was used as source of PAN. Molecular weight of PAN is 53.0626 ± 0.0028 g/mol, C 67.91 %, H 5.7 %, N 26.4 %.
N,N-dimethylformamide (DMF) of 99 % purity (B. Pt. = 157 °C) from Fisher Scientific.
Functionalized Carbon Nanotubes (CNT) from Nanocyl.
Preparation of PAN nanofibers
The 9 wt% PAN copolymer solution was prepared by taking 0.9 g chopped PAN microfibers was socked in 9.1 g of DMF and mix with glass rod. The mix was sonicated in ultrasonicator till it gives a clear solution and stirred continuously on magnetic stirrer for 5 h to mix well the contents.
Preparation of PAN–CNT composite nanofibers
PAN–CNT (1–3 wt%) 9 % solutions were prepared in following steps: at first 0.01 g functionalized CNTs were dispersed in N,N-dimethylformamide. The mixture was kept in sonicator for 5–8 h to break bundles of CNTs and magnetic stirring to disperse well CNTs in DMF. A good dispersion was achieved in 7 h. Calculated quantity of PAN copolymer chopped microfibers were added in dispersion solution and kept on magnetic stirrer for overnight to mix well the polymer in CNT dispersion. Solution was sonicated for 1 h and then electrospun with ESPIN instrument at voltage 15 kV, flow rate 0.2 ml/h, drum speed 2000 rpm, distance 15 cm to produce PAN–CNT composite nanofibers. It was difficult to prepare solution using higher concentration of CNT. The problem faced was that, in high quantity of CNTs polymer not get dispersed well and form a solid mass. So both CNTs and polymer were dispersed in DMF separately. CNTs and DMF mixture was sonicated for 1 h till all bundles breakup and then kept on magnetic stirrer, good dispersion observed in sun light. Polymer was soaked in DMF well and sonicated for 1 h. Then it was kept on magnetic stirrer till a clear solution was obtained. Same procedure was carried out for PAN–CNT concentrations (2–3 wt%).
Both types of nanofibers were undergone through oxidation, stabilization, carbonization and graphitization. At each stage of processing of carbon and carbon-CNT composite nanofibers were characterized by SEM, AFM, XRD and TGA.
Results and discussions
In the present study we have successfully used the electrospinning for the synthesis of PAN–CNT carbon nanofibers. SEM images shows that the fiber diameter varies from 75 to 1500 nm on increasing concentration of CNT. The presence of black color in fiber, it indicates the presence of CNTs in nanofibers. AFM images show that the surface morphology of the composite nanofibers is smooth at lower concentration of MWCNT but rough at high concentration of MWNTs. Since the MWNTs possess a high electron density compared with the PAN polymer matrix, the nanotubes appear as darker tubular structures embedded in the PAN nanofibers. It is also seen that on Stabilization fibers diameter decreases due to shrinkage in fibers. Degree of graphitization for PAN nanofibers is −20 % and for PAN–CNT nanofibers is −41.86. Degree of graphitization increases due to presence of CNT in fibers. TGA results shows that wt loss is small in case of PAN nanofibers because of high surface area of nanofibers while in case of carbonized PAN–MWCNT carbon nanofibers wt loss is small as comparison to PAN–CNT stabilized because of increase in stability in structure, and removal of nitrogen, water and hydrogen. HCN and another gases and also carbon content increases on carbonization.
All authors have equal contribution in carrying out research work as well as in writing work of manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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