MWNT nanofibers by electrospinning

MWNT nanofibers by electrospinning

Sensors and Actuators B 134 (2008) 122–126 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevie...

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Sensors and Actuators B 134 (2008) 122–126

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Enhanced conductivity of aligned PANi/PEO/MWNT nanofibers by electrospinning Min Kyoon Shin a , Yu Jin Kim a , Sun I. Kim a , Sung-Kyoung Kim b , Haiwon Lee b , Geoffrey M. Spinks c , Seon Jeong Kim a,∗ a b c

Center for Bio-Artificial Muscle and Department of Biomedical Engineering, Hanyang University, Seoul 133-791, Republic of Korea Department of Chemistry, Hanyang University, Seoul 133-791, Republic of Korea ARC Center of Excellence in Electromaterials Science and Intelligent Polymer Research Institute, University of Wollongong, Wollongong, NSW 2522, Australia

a r t i c l e

i n f o

Article history: Received 28 November 2007 Received in revised form 8 March 2008 Accepted 16 April 2008 Available online 23 April 2008 Keywords: Multiwalled carbon nanotubes Polyaniline Electrical conductivity Composite nanofibers

a b s t r a c t Conducting composite nanofibers were fabricated from a mixture of multiwalled carbon nanotubes (MWNTs) and a polyaniline (PANi)/poly(ethylene oxide) (PEO) blend using an electrospinning process. We observed a surprising transition in the electrical conductivity of the conducting composite nanofibers while measuring the I–V characteristics of the nanofibers aligned on an electrode when they were exposed to an applied high voltage. We believe that this unexpected transition is closely related to the self-heating of the MWNTs incorporated into the conducting polymer. This type of self-heating method will be very helpful in enhancing the electrical properties of nanoscale conducting composite fibers. © 2008 Elsevier B.V. All rights reserved.

1. Introduction One-dimensional conducting nanofibers fabricated by electrospinning have attracted much attention because of their promise in electronic devices, such as chemical sensors, Schottky nanodiodes, and in field-effect transistors [1–3]. However, because electrospun conducting fibers are stably produced from blend solutions formed by mixing a conducting polymer and a nonconducting polymer [1–4], these fibers can have a handicap in terms of their electrical properties compared to pure conducting nanofibers and nanowires fabricated using methods such as electropolymerization and electrodeposition [5,6]. A good strategy for improving the electrical properties of electrospun conducting fibers is to incorporate carbon nanotubes (CNTs), which have superior electrical conductivity [7], into the conducting polymer. Many investigations into the fabrication of CNT composite fibers using electrospinning have been carried out to improve the electrical properties of polymer fibers. However, all the previous research studied have focused on fabricating CNT/nonconducting polymer composite fibers [8,9], despite many advantages of conducting polymers. In this work, we have fabricated one-dimensional composite nanofibers from

∗ Corresponding author at: Seongdong, P.O. Box 55, Seoul 133-605, Republic of Korea. Tel.: +82 2 2220 2321; fax: +82 2 2291 2320. E-mail address: [email protected] (S.J. Kim). 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.04.021

a mixed solution of multiwalled carbon nanotubes (MWNTs) and a polyaniline (PANi)/poly(ethylene oxide) (PEO) blend using an electrospinning process. We also report on an unexpected and surprising transition in the electrical conductivity of the conducting composite nanofibers. The PANi used in this work is the most studied of the conducting polymers due to its ease of synthesis and its relative environmental stability [10]. The electrical conductivity of PANi is dependent on the temperature [11], solvent, dopant, humidity [12,13], structural defects [14], and the polymer morphology [15]. However, the most significant factor influencing the conductivity of PANi is the proton doping level [16]. The maximum conductivity occurs when PANi is 50% doped by protons. At doping levels above 50%, some amine sites are protonated and, at levels lower than 50% doping, some imine sites remain unprotonated. In both cases, delocalization of the charge carriers over the polymer backbone is disrupted, thereby reducing the overall polymer conductivity. The intrinsic limitation in the improvement of the conductivity of PANi requires the use of CNTs. In particular, electrospun conducting fibers fabricated from a blend solution of a mixture of a conducting polymer and a nonconducting polymer necessitate the introduction of CNTs for use as more effective electronic materials. In addition to their excellent high electrical conductivity, CNTs can withstand remarkable current densities, exceeding 109 A/cm2 , which in part is due to their strong carbon–carbon bonding [17]. However, metallic CNTs can experience electrical breakdown because of self-heating at high

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biases [18]. Although, for a single suspended CNT, the self-heating is a perplexing problem, the thermal dissipation by the self-heating of CNTs incorporated into a conducting polymer provides the possibility of enhancing the electrical properties of a composite material due to potential annealing effect on the conducting polymer. Therefore, the incorporation of CNTs may generate unexpected results in the electrical conductivity of a conducting composite fiber, regardless of the level of CNT loading. 2. Experimental 2.1. Materials PANi emeraldine base (Mw = 65,000), PEO (Mw = 100,000) and 10-camphorsulfonic acid (HCSA) were purchased from Aldrich Chemicals (USA). The MWNTs, produced via a catalytic carbon vapor deposition (CCVD) process, were supplied by Nanocyl (Belgium). The MWNTs had an average diameter of 10 nm, lengths between 0.1 and 1 ␮m, and a purity of 95%. The chloroform used was obtained from J. T. Baker (Germany).

Fig. 1. A schematic diagram showing the electrospinning apparatus used to align the nanofibers.

2.2. Preparation of the solution To prepare the PANi/PEO blend solution, a 0.6:1 mole ratio of HCSA and PANi was dissolved in chloroform to a final concentration of 1 wt%. The solution was then stirred at room temperature at 1000 rpm for a period of 8 h using a mechanical stirrer, and then filtered through a filter paper to remove any particulate matter. Finally, by adding PEO to the filtered solution, the 11 wt% PANi/PEO solution was prepared, which was then stirred for a period of 2 h. The PANi/PEO/MWNT composite solution was made using the following procedure. The MWNTs were dispersed in chloroform in concentrations of 0.25, 0.5, and 1 wt% for a period of 1 h at room temperature using an ultrasonicator. The resulting dispersions were homogeneous and stable, and had a dark ink-like appearance. Then, the MWNT solutions were added to the filtered PANi solution and stirred for a period of 12 h. Finally, by adding PEO to the PANi/MWNT solution, the ∼11 wt% PANi/PEO/MWNT solution was prepared, and this was then stirred for a period of 12 h using a mechanical stirrer. 2.3. Electrospinning The PANi/PEO and PANi/PEO/MWNT nanofibers were aligned on a gold-insulator-gold electrode (1 cm × 1 cm) attached to a rotating drum using a typical literature method [8] (Fig. 1). All the goldinsulator-gold electrodes were attached at the same position. The rotation speed of the drum was 300 rpm. An electric potential difference of 10 kV was applied between the collector and a syringe tip, and the distance between the collector and the tip was 13 cm. The feed rate of the polymer solution was kept steady (3 ␮l/min) using a syringe pump fabricated at KD Scientific in the USA. Although we tried to fabricate the same number of blend nanofibers as composite nanofibers on the electrode, a slightly different number of both fibers were generated and collected. 2.4. Characterization The morphology of the fibers was characterized using scanning electron microscopy (SEM, Hitachi (Japan), Model S4700, accelerating voltage = 15 kV) and transmission electron microscopy (TEM; Philips (Netherlands), Model CN30, accelerating voltage = 300 kV). The I–V characteristics of the fibers were measured at room temperature using a Keithley (USA) Model 2400 electrometer.

3. Results and discussion Fig. 2(a) shows a representative conducting composite nanofiber fabricated from a PANi/PEO composite solution containing 1 wt% MWNTs employing electrospinning. From Fig. 2(a), we confirmed that the MWNTs were dispersed inside the fiber without any severe aggregation, and that the diameter of the MWNTs was in the range 10–20 nm. Although some of the incorporated MWNTs were not completely aligned along the axis of the fiber, MWNTs with lengths greater than several hundred nanometers were completely surrounded by a conducting matrix. This means that MWNTs incorporated into the conducting matrix can form a direct interaction with the conducting polymer. Fig. 2(b) shows the I–V curves of PANi/PEO fibers including 1 wt% MWNTs. The conducting composite fibers showed a stable linear ohmic behavior at applied low voltages of ±1 V. The electrical conductance of the aligned composite fibers was about 0.12 ␮S. We carried out I–V measurements on the conducting composite fibers up to ±5 V, and we confirmed that the I–V curves of the conducting composite fibers were similar. However, we found that the conducting composite fibers showed a sudden increase in current at applied high voltages of >8 V. This would be attributed to a transformation of the internal structure of the composite fiber at high electric powers of 12–16 ␮W. After the conducting composite fibers were exposed to an applied high voltage of ∼9 V, an unexpected and surprising transition in the electrical conductivity was observed, and the measured conductance was 2.77 ␮S [Fig. 3(a)]. This conductance value is 23 times larger than that of the composite fibers that were not exposed to a high electric voltage. This surprising phenomenon was also generated in composite fibers including 0.25 and 0.5 wt% MWNTs. However, the distinct transition in electrical properties was not observed in PANi/PEO blend fibers, even when they were exposed to applied high voltages. In Fig. 3(b), because the repeated I–V curves of the composite fibers exposed to a high electric voltage were almost identical, we confirmed that the electrical properties of the composite fibers were completely transformed after exposing the composite fibers to an applied high voltage. From the viewpoints of both self-heating of the CNTs and electron transfer, we propose that the unexpected transition in the electrical conductivity can be attributed to a change in the inter-

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Fig. 2. (a) TEM image of MWNTs incorporated into a PANi/PEO fiber. (b) I–V characteristics of PANi/PEO/MWNT nanofibers aligned on a gold electrode with an insulating gap at potential of ±1 (blue), ±5 (black), and ±9 V (red). The black dotted circle highlights the instability in the electrical conductivity of PANi/PEO/MWNT nanofibers in applied high voltages. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

actions between the MWNTs and the conducting polymer inside the fiber due to an annealing effect of the PANi/PEO matrix from the thermal dissipation of the CNTs. The conductivity of PANi films or pellets usually follows a quasi-one-dimensional variablerange hopping model (1D-VRH) [19,20], and thus, the electrical conductivity of electrospun films that consist of densely aligned one-dimensional PANi/PEO/MWNT fibers on an electrode can be explained using the 1D-VRH model: (T) =  0 exp[−(T0 /T)1/2 ], where  0 is a constant; T0 = 24/[kB Lc3 N(EF )], and is the characteristic Mott temperature, which generally depends on the hopping barrier; kB is the Boltzmann constant; Lc is the localization length; and N(EF ) is the density of the states at the Fermi level [21]. Because we measured the change in electrical properties of the composite samples at room temperature, we focused on the relationship between the conductivity and the localization length, rather than on the other parameters. The localization length of a CNT is usually >10 nm due to the larger ␲-conjugated structure in carbon nanotubes [22]. However, for poorly conducting, or “amorphous”, PANi, the localization length is no more than 2 nm [23]. Cochet et al. pointed out that synthesis using an in situ process leads to effective site-selective interactions between the quinoid

Fig. 3. (a) Graph showing the transition in the electrical conductivity of PANi/PEO/MWNT nanofibers. (b) I–V characteristics of PANi/PEO/MWNT nanofibers at potentials of ±0.1 (blue), ±0.3 (red), and ±0.5 V (black) after the electrical transition. (c) I–V characteristics of PANi/PEO nanofibers (red line) and PANi/PEO/MWNT nanofibers (black line = before transition, blue line = after transition). The inset shows the I–V characteristics of PANi/PEO/MWNT nanofibers measured before the transition and of PANi/PEO nanofibers. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

ring of the PANi and the CNT, which facilitates charge transfer processes between the two components [24]. This means that a strong interaction between CNTs and the polymer chains can increase the average localization length of the composite. However, it is worth noting that the coupling between a CNT and

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4. Summary

Fig. 4. A scheme showing the change in interactions between a CNT and a polymer chain by the self-heating of a CNT inside a conducting composite nanofiber.

polymer chains is more influenced by the methods used to incorporate a CNT into a polymer. For example, the electrical properties of in situ polymerized composites are better than those of ex situ polymerized samples [24]. The fabrication of our composite fibers using electrospinning is similar to ex situ polymerization, because the composite fibers were produced from a mixture of a doped PANi/PEO blend and CNTs using sonication. Therefore, in the case of the electrospun composite fibers, it is reasonable to assume that no chemical reactions occur between the CNTs and the conducting polymer chains during the electrospinning process. This means that enhancement of the electrical conductivity due to the increase in localization length by the CNTs cannot be expected. The results shown in Fig. 3(c) support the assertion that the composite fibers fabricated by electrospinning have weak interactions between the CNTs and the polymer chains. Although the composite fibers that were not exposed to high voltages showed improved electrical properties compared to those of the PANi/PEO blend fibers, their conductivity was very low compared to that of the composite fibers exposed to high voltages. However, if the thermal dissipation from the self-heating of the CNTs was high enough to anneal the polymer matrix and induce strong interactions between both these materials, the average localization length of the PANi/PEO/MWNT fibers would be larger due to the strong interactions between the CNTs and the conducting matrix. Through Fig. 4, we suggest that after the polymer chains surrounding the CNTs undergo a plastic welding process [25] by the self-heating of the CNTs, the chains are well adhered to the CNTs. This process would act as a main factor of strong interactions between the quinoid ring of the PANi and the CNT. Therefore, electron transfer between the CNTs and the conducting polymer matrix would be facilitated, as illustrated in Fig. 4. Finally, this would result in a marked enhancement in the electrical conductivity of the composite fibers. It is likely that structural changes in the conducting polymer matrix, such as an increased chain ordering due to the thermal annealing, would also contribute to an enhancement of the electrical conductivity, regardless of any interaction of the matrix with the CNTs. The unexpected increase in conductivity of the PANicamphorsulfonic acid (CSA)/polyamic acid (PAA) blend films due to the annealing effect has been reported by other groups [26,27]. Moreover, Kobayashi et al. suggested that based on the observation of wide-angle X-ray diffraction, the enhancement of regularity in the PANi structure is responsible for the increase of the conductivity observed in the initial stage of annealing [27]. However, in addition to the chain ordering arising from the annealing, we propose that the improved interaction between the polymer and the CNTs has an important influence on the unexpected and surprising transition in the electrical conductivity.

In summary, we have fabricated PANi/PEO/MWNT nanofibers to increase the electrical conductivity of PANi/PEO nanofibers. While measuring the conductivity of the conducting composite nanofibers, we observed a new, unexpected transition in the electrical conductivity of the conducting composite nanofibers when the aligned composite fibers were exposed to a high electric field. The surprise transition is closely related to the self-heating of the CNTs inside the conducting polymer, and the transition in the electrical conductivity was attributed to a change in the localization length of the composite fibers. Our experimental data represent one example of using thermal dissipation by the self-heating of CNTs in a nanoscale structure. In the future, the self-heating method will be very helpful in enhancing the electrical properties of conducting composite fibers combined with a method of increasing the amount of CNTs incorporated into a conducting polymer.

Acknowledgments This work was supported by the Creative Research Initiative Center for Bio-Artificial Muscle of the Ministry of Science & Technology (MOST) and the Korea Science and Engineering Foundation (KOSEF) in Korea.

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Biographies Min Kyoon Shin received his MS degree in Biomedical Engineering from Hanyang University, Seoul. Presently he is a Ph.D. student in the department of biomedical

engineering at Hanyang University. His research area of interests includes electrospinning, electrical and mechanical properties of nanofiber based actuators for artificial muscles. Yu Jin Kim received her MS degree in Biomedical Engineering from Hanyang University in 2007. Her research project was involved in the fabrication of conducting polymer/carbon nanotube fibers. Sun I. Kim is a professor of Biomedical Engineering at Hanyang University. His research interests include artificial muscles and neuroscience. Sung-Kyoung Kim received his Ph.D. degree in Chemistry from Hanyang University in 2007. His research interests include carbon nanotube films using electrodeposition. Haiwon Lee is director of Institute of Nano Science and Technology and a professor of Chemistry at Hanyang University. His research interests include organized molecular assembly and nanofabrication of ultrathin organic films. Geoffrey M. Spinks is a professor of Mechanical Engineering at University of Wollongong, and associated with Intelligent Polymer Research Institute of University of Wollongong. His research interests include the development of electromechanical actuators using conducting polymers and carbon nanotubes. Seon Jeong Kim is a professor of Biomedical Engineering at Hanyang University, and director of the Creative Research Initiative Center for Bio-Artificial Muscle. His research interests include stimuli responsive hydrogels, fabrication of multifunctional fibers using electrospinnig and wet-spinning, biofuel cells, and biomaterial based actuators ranging from microscale to nanoscale for artificial muscles.