Electrospun Aligned Fibrous Arrays and Twisted Ropes: Fabrication, Mechanical and Electrical Properties, and Application in Strain Sensors
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Electrospinning (e-spinning) is a versatile technique to fabricate ultrathin fibers from a rich variety of functional materials. In this paper, a modified e-spinning setup with two-frame collector is proposed for the fabrication of highly aligned arrays of polystyrene (PS) and polyvinylidene fluoride (PVDF) nanofibers, as well as PVDF/carbon nanotube (PVDF/CNT) composite fibers. Especially, it is capable of producing fibrous arrays with excellent orientation over a large area (more than 14 cm × 12 cm). The as-spun fibers are suspended and can be easily transferred to other rigid or flexible substrates. Based on the aligned fibrous arrays, twisted long ropes are also prepared. Compared with the aligned arrays, twisted PVDF/CNT fiber ropes show enhanced mechanical and electrical properties and have potential application in microscale strain sensors.
KeywordsElectrospinning Aligned arrays Twisted ropes Electrical properties Strain sensors
Electrospinning (e-spinning) is a simple and versatile technique to fabricate fibers with diameters typically ranging from a few micrometers down to 10 nm or less. In a traditional e-spinning process, a charged jet is ejected from a Taylor cone and rushes onto the grounded collector under the driving force of an electric field. After solvent evaporation, solid fibers with uniform diameter are randomly deposited on the collector [1, 2, 3, 4, 5]. In the past decade, numerous ultrathin fibers originated from polymer, metal, ceramic, and glass have been prepared by e-spinning, and their potential applications in optoelectronics, sensors, catalysis, textiles, filters, fiber reinforcement, tissue engineering, drug delivery, and wound healing have also been extensively explored [1, 6, 7, 8, 9].
Normally, the products fabricated by traditional e-spinning are randomly oriented fibers known as a nonwoven mat. In order to extend the potential applications of e-spinning, a lot of modified e-spinning techniques have been proposed to obtain fibers with desired morphologies such as aligned fibrous arrays and twisted ropes. For example, aligned fibers can be prepared by introducing a gap into the conventional collector , adding an auxiliary electric or magnetic field , double spinning,  near-field e-spinning [13, 14, 15, 16, 17, 18], and rotating collector . Twisted fiber bundles have also been fabricated by a few means such as dual collection rings , AC e-spinning , a modified setup with two collectors , or with an alternating electric field . The twisted nanofiber rope is promising in the applications of artificial muscle, electron devices, and suture materials . Nevertheless, there are few methods by which both well-aligned fibers and twisted ropes can be fabricated.
So far, the applications of electrospun fibers have been paid much attention due to their unique physical, chemical, and even biological properties, especially in the field of electrical sensors. For instance, piezoelectric materials such as ZnO and polyvinylidene fluoride (PVDF) have been electrospun into aligned fibers and then integrated into a nanogenerator. If this device is pressed or bended by a strain, the current will be induced, resulting from the piezoelectric properties of the functional fibers [24, 25]. Lotus et al. modified the traditional e-spinning setup by introducing a rotating and a stationary collector , and obtained semiconducting twisted ZnO/NiO composite yarns, exhibiting a rectifying behavior of a p-n junction . In addition, twisted microropes of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)-polyvinyl pyrrolidone (PEDOT:PSS-PVP) fibers doped with ionic liquid showed a linear correlation of electrical conductivity with a strain up to 35 % and a repeatable cycle loop of tensile-resilience .
Moreover, efforts have been done to study a nanocomposite made of PVDF and carbon nanotube (CNT) and the interfacial interactions between the two components [29, 30, 31]. For example, the difference of the Raman spectra of CNT and CNT dispersed in the PVDF was ascribed to the interfacial interaction between the fluorine of PVDF and the CNTs . The interfacial interaction between the single-walled carbon nanotube (SWCNT) and PVDF and the extensional force experienced by the nanofibers in the e-spinning and collection processes could work synergistically to induce highly oriented β-form crystallites of PVDF extensively . Although the morphological and structural properties and polymorphic behaviors as well as interfacial interactions between PVDF and embedded CNT have been studied, the electrical and mechanical properties of electrospun PVDF/CNT fibers have not been reported intensively.
In this letter, we report on a novel e-spinning device with a set of a modified frame collector, by which highly ordered arrays of polystyrene (PS) and PVDF nanofibers, as well as PVDF/multiwall CNT (MWCNT) composite nanofibers, can be fabricated. It is worthy to note that the spun fibrous arrays show excellent orientation over a large area (more than 14 × 12 cm2). Moreover, twisted ropes can be also prepared successfully with ~10 cm in length by the same technique. The twisted PVDF/CNT composite ropes exhibit improved mechanical performance and conductivity compared with aligned fibrous arrays. In addition, an electrical device has been integrated with high-strain sensitivity based on the twisted rope, indicating good potential in the high-strain sensor.
Preparation of Spun Solution and Electrospinning
Polystyrene (PS) solution was prepared by dissolving 2.0 g PS (average molecular weight of 250,000, ACROS) in 8.0 g tetrahydrofuran (THF). After being stirred for 4 h at room temperature, the PS solution was kept at room temperature for 0.5 h before e-spinning. The PVDF solution was prepared by a similar way by dissolving 1.25 g PVDF (average molecular weight of 275,000, Aldrich) in the mixture of 2.5 g acetone and 2.5 g dimethylformamide (DMF) with 2 h stirring at 60 °C. CNT precursor solution was prepared by dissolving 1.2 g CNTs (Chengdu Organic Chemical Co., LTD) in the mixture of 16.8 g acetone and 2.0 g CNT dispersion (TNWIDS, Chengdu Organic Chemical Co., LTD). PVDF/CNT solution was prepared by dissolving CNT precursor solution and PVDF in the mixture of acetone and DMF (the weight ratio is 1:1). Here, the contents of the CNTs in the final fibers were 8.0, 12.4, and 16.7 wt%, respectively. The PVDF/CNT solution was stirred for 4 h in a water bath at 60 °C before e-spinning.
A high-voltage DC power supply (Tianjin Dongwen, China) was employed to generate voltages. The spun solution was loaded into a 5.0-ml syringe with a stainless spinneret (inner diameter 0.72 mm) which is connected with the anode of the power supply. The applied spinning voltage was 20 kV, and the vertical distance (work distance) between the spinneret and the top of the grounded frame was 8 cm, while the inner frame was placed horizontally at the beginning. The feed rate (e.g., 0.5 ml min−1) could be controlled by a syringe pump. During the electrospinning (ES) process, the rotating speed of the outer frame was set to 600 rpm. All experiments were carried out at room temperature.
A digital camera, an optical microscope (SMZ-168), and a scanning electron microscope (SEM; JEOL JSM-6390) were used to observe morphologies of the ES fibers. All samples are coated with an evaporated gold thin film before SEM imaging to ensure higher conductivity. The PVDF/CNT fibers were characterized by a transmission electron microscope (TEM; HITACHI H-9000), and Fourier transform infrared spectroscopy (FTIR) using a Thermo Scientific Nicolet iN10 spectrometer and absorbance data were processed for the wave number range 700–1000 cm−1. A mechanical test system (Agilent T150 UTM) and a set of an electrical measurement system (Keithley 6220 and Vitech triple output DC power supply) were used to measure the mechanical and electrical properties of the fiber bundles and ropes, separately.
Result and Discussion
Fabrication of Aligned Fibrous Arrays and Twisted Ropes
Influence of the ES Parameters on the Morphology of Aligned Fibrous Arrays and Twisted Ropes
Fabrication of PVDF/CNT Twisted Ropes
Mechanical and Electrical Properties of Aligned PVDF/CNT Bundles and Twisted Ropes
Strain Sensors Based on Twisted Ropes
In summary, a modified e-spinning setup with two-frame collector is introduced for fabrication of highly ordered arrays within an area up to 14 × 10 cm2. Based on the aligned arrays, twisted continuous ropes can be prepared. The results show that both the position of the inner frame and the rotation speed have significant influence on the morphologies of resultant twisted ropes. On the other hand, the mechanical properties of PVDF/CNT composite twisted ropes are improved by one order of magnitude, indicating their potential in many applications, such as artificial muscle and electronic devices. In addition, the strain-sensitive property of these composite twisted ropes indicates their potential use in strain sensors.
This work was supported by the National Natural Science Foundation of China (51373082); the Taishan Scholars Program of Shandong Province, China (ts20120528); the National Key Basic Research Development Program of China (973 special preliminary study plan, 2012CB722705); the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province; and the Postdoctoral Scientific Research Foundation of Qingdao.
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