- Nano Express
- Open Access
Mechanical and electrical properties of electrospun PVDF/MWCNT ultrafine fibers using rotating collector
© Wang et al.; licensee Springer. 2014
- Received: 30 August 2014
- Accepted: 16 September 2014
- Published: 23 September 2014
Poly(vinylidene fluoride) (PVDF) ultrafine fibers with different proportions of multi-walled carbon nanotube (MWCNT) embedded have been fabricated using a modified electrospinning device with a rotating collector. With the increasing of MWCNT content, the β phase was noticeable enhanced, and the fibers became more elastic, which was manifested by Young's modulus decreased drastically. Furthermore, with adding the amounts of MWCNTs, the density of carbon nanotube (CNT)-CNT junctions among the fibers increased accordingly. When the MWCNT content was of 1.2 wt.%, a stable three-dimensional conducting network was formed. After this percolation threshold, the density of CNT-CNT junctions among the fibers tended to be a constant quantity, leading to a stabilized conductivity consequently. It is hoped that our results can be helpful for the fabrication of flexible devices, piezoelectric devices, force transducer, and so on.
- Multi-walled carbon nanotubes
Electrospinning is a simple and versatile technique for fabricating ultrafine fibers with diameters ranging from several micrometers down to a few nanometers. With outstanding properties such as large surface area, high length/diameter ratio, flexible surface functionality, and tunable surface morphologies, the electrospun fibers have an underlying application in optoelectronics, sensors, catalysis, textiles, filters, fiber reinforcement, tissue engineering, drug delivery, wound healing, etc. [1–7]. During electrospinning process, when the electric field force reaches a certain threshold value, polymer droplet overcomes the surface tension and forms a jet trickle from the capillary Taylor cone vertex. After a series of vigorous whipping and/or splitting motion due to fluid instability and electrically driven bending instability, the products are deposited commonly as a nonwoven fibrous web on a collector. In order to improve the further application of the as-spun fibers, numerous researchers and groups have engaged in fabricating morphology-controlled electrospun micro/nanofibers, and it is delighted to notice that apart from fiber membranes without orientation, other fibrous structures and organization (e.g., aligned fiber arrays, helical or wavy fibers, twisted fibrous yarns, patterned fibrous mats) based on not only polymers of synthetic or biological nature but also metals, metal oxides, ceramics, organic/organic, organic/inorganic, as well as inorganic/inorganic composite systems have been electrospun successfully via modified electrospinning process or collectors [8–10], which will extend further application of as-spun fibers in many fields.
As a semicrystalline polymer, poly(vinylidene fluoride) (PVDF) has aroused much attention due to its distinguished electroactive properties, nonlinear optical, strong corrosive, susceptibility, and high dielectric constant [11, 12], which make it useful in a variety of fields such as sensors, actuators, and energy transducers . PVDF consists of four different crystalline phases depending on the chain conformation of trans and gauche linkages: α, β, γ, and δ. Among these phases, the α phase is known as the most abundant form commercially available powders and films, and the β phase has the largest spontaneous polarization per unit cell and thus, exhibits the highest electroactive properties, responsible for most of PVDF's piezoelectric characters . It is reported that electrospinning and blending PVDF with carbon nanotubes (CNTs) can increase the β-phase content in PVDF .
So far, the study of PVDF/CNT composites mainly focuses on the following three aspects: (1) the dielectric property of the composites and its CNT dispersion and loading dependence ; (2) enhancement of the β-phase crystal formation of PVDF in the doping of CNTs and the related property alterations [17, 18]; (3) the electrical conductivity, its percolation behavior and other properties of the composites in the doping of CNTs [18–23]. Although numerous studies on PVDF/CNT nanofibrous composites have been published, new work in this field emerges consistently and continually. In the present work, well-aligned PVDF ultrafine fibers with different multi-walled carbon nanotube (MWCNT) contents (0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, and 2%) have been fabricated using a modified electrospinning device with a rotating collector. It is found that with the increasing content of MWCNTs from 0.6 to 2 wt.%, the β phase has been noticeable enhanced, and the composited fibers become much more elastic lying in the fact that Young's modulus decreased from 4.4 × 10-2 to 9.1 × 10-3 MPa. Moreover, with adding the amounts of MWCNTs, the density of CNT-CNT junctions among the fibers increased accordingly, forming a stable three-dimensional conducting network. After the three-dimensional network has been constructed (the percolation threshold) where MWCNT content was of 1.2 wt.%, the density of CNT-CNT junctions tend to be a constant quantity, resulting in a stabilized conductivity of the fibers.
Preparation of PVDF/MWCNT solution and electrospinning
To study the surface morphology and the size of electrospun fibers, a scanning electron microscope (SEM; JEOL JSM-6390, JEOL Ltd., Akishima-shi, Japan) and a transmission electron microscope (TEM; HITACHI H-9000, Hitachi, Ltd., Chiyoda-ku, Japan) were used. The crystalline phase or phases present in the composited fibers were identified by Fourier transform infrared (FTIR) spectroscopy using a Thermo Scientific Nicolet iN10 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), and absorbance data were processed for the wave number ranging from 600 to 1,600 cm-1. The X-ray diffraction patterns were recorded by a Bruker D8 Advance X-ray diffractometer (XRD; Bruker AXS, Inc., Madison, WI, USA). An electronic tensile testing machine (Jinan Hengrui machine Co. Ltd., Jinan, China) was used for the mechanical characterization of aligned electrospun fibrous membranes, and the electrical properties of the fibers were tested using a Keithley 6485 high-resistance meter system (Keithley Instruments, Inc., Cleveland, OH, USA).
Figure 4b presents the relationship between Young's modulus and MWCNT contents for the electrospun PVDF/MWCNT fibers. With the content of carbon nanotubes increased from 0.6 to 2 wt.%, Young's modulus is decreased from 4.4 × 10-2 to 9.1 × 10-3 MPa, e.g., the products' elasticity has been drastically improved with the increasing of MWCNTs.
Here σ is the electrical conductivity of the PVDF/MWCNT composited fibers, p is MWCNT concentration, p c is the percolation threshold, and t is the critical exponent, which reflects dimensionality of the system and universality class of the problem. Usually, experimental results are fitted by plotting log σ vs. log (p - p c ) and incrementally varying p c until the best linear fit is obtained . In our experiments, the percolation threshold p c and critical exponent t (the slope of the linear relation of log σ to log (p - p c )) were of 1.2 and 4.08 wt.%, respectively, as shown in Figure 5b. Besides, it is reported that for the polymer/CNT composites, the exponent t is frequently associated with the conducting system dimensionality, namely, with values of t ≈ 1.3 (or slightly higher) representing a two-dimensional network while t ≈ 2 (or higher) a three-dimensional one . Here the values of t = 4.08 indicated a three-dimensional conducting network formed among PVDF/MWCNT composited fibers. As increasing the amounts of MWCNTs in the composites, the density of CNT-CNT junctions increased accordingly, with an enhancive conductivity till the MWCNT content of 1.2 wt.%. After the three-dimensional network had been constructed, the density of CNT-CNT junctions tended to be a constant, therefore, the conductivity of the PVDF/MWCNTs remained stabilized, and the value of log σ in Figure 5a was likely to form a platform after the percolation threshold.
In this paper, the ultrafine fibers of PVDF/MWCNTs were fabricated via a modified electrospinning technique. The mechanical and electrical properties of the as-spun fibers were enhanced evidently by incorporating MWCNTs into the PVDF fibers. With the increase of the MWCNT content, an enhancement of the β phase was observed. With the MWCNT mass proportion increased from 0.6 to 2 wt.%, Young's modulus of the composited fibers decreased from 4.4 × 10-2 to 9.1 × 10-3 MPa. At room temperature, the conductivity of the PVDF/CNT fiber membranes with MWCNT content of 0.6 wt.% was 1 × 10-14 S cm-1, however, for the 1.2 wt.% loaded, it changed into 1 × 10-6 S cm-1, and the critical exponent t was of 4.08, which proved that a three-dimensional conducting network constructed among PVDF/MWCNT fibers. After the network formed, the density of CNT-CNT junctions tended to a steady value, which led to the conductivity of the PVDF/MWCNT fibers forming a platform after the percolation threshold (MWCNT content of 1.2 wt.%). It is hoped that our results can be helpful for the fabrication of flexible devices, piezoelectric devices, force transducer, etc.
This work was supported by the Project of Shandong Province Higher Educational Science and Technology Program (J13LJ07), the National Natural Science Foundation of China (11144007), and the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province, China.
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