Figure 1. Setup for characterizing PVDF force sensors.

2. Experimental details

2.1. Materials

Poly(vinylidene fluoride) (PVDF) with a molecular weight of 534 000 (g mol−1), N, N-dimethylformamide (DMF), and acetone were purchased from Sigma Aldrich (USA). They were used without deviating from their original conditions.

2.2. Solution preparation

Measured amounts of PVDF were ultrasonically stirred in a co-solvent of DMF/acetone (2:3 by weight) at 40 ◦C until a transparent and homogeneous polymer solution was formed. The solution was then cooled to ambient temperature for electrospinning. In this study, 12 wt% of PVDF solution was used for electrospinning.

2.3. Electrospinning process

An electrospinning unit (Keskatotech Co., Ltd) was used in our experiments. Nanofibers were collected on an electrically grounded metal plate, which was set 15 cm away from the tip of the anodic needle of the syringe. Other parameters such as the applied voltage and injection flow rate could be precisely controlled as desired. The electrospinning process was carried out in a closed box; the temperature in the box roughly remained at 22 ◦C and the humidity between 50 and 60%.

2.4. Characterization

A scanning electron microscope (FEI Sirion) was used to observe the morphology of the PVDF fabrics. The samples were sputter coated with gold before being scanned. An unpolarized Fourier transform infraredtransmission spectroscope (FTIR-TS) equipped with a BrukerIFS66V spectrometer and an x-ray diffraction spectrometer (XDR) equipped with a diffractometer (Brueker D8 Focus) were used to characterize the crystalline contents.

2.5. Force sensor measurement

The force sensors were experimentally characterized with the setup as shown in figure 1. The setup consists of a

Table 1. Samples prepared at varied conditions.

controller, load cell, amplifier and oscilloscope. The controller controls the up-and-down movement of the load cell with a set frequency. The contact area of the load cell is 19.6 mm2. The load cell is able to detect the impact force when contact is made with the tested sample surface, with a sensitivity of 22.5 mV N−1. On the other hand, the signals from the designated sensor were enhanced by an amplifier (CCA1000) and then recorded by an oscilloscope (Tektronix TDS 540A). In this study, the loading forces used for the tests ranged between 3 and 5 N, and all measurements were carried out on ambient conditions.

3. Results and discussion

Preliminary experiments were carried out to optimize the conditions for electrospinning the PVDF nanofibers. In this study, the PVDF solution used was consistently at the concentration of 12 wt%; the tip-to-plate distance was set at 15 cm. The applied voltage was between 9 and 18 kV. No fibers could be formed below or above the previously mentioned voltages. This implies that forces including electrical force and surface tension compete in the electrospinning process. Additionally, the injection flow rate varied in the range of 0.01– 0.04 ml min−1 for experimental processing.

Two sets of samples were prepared during this study, with variation of either their applied voltage or injection flow rate. The electrospinning conditions are detailed in table 1.

3.1. Morphology and diameter distribution of PVDF nanofiber

The experiment shows that the diameter of the PVDF fibers did not change significantly due to the varying voltages and flow rates; the diameters of the PVDF fibers ranged from 20 to 800 nm. However, the distribution of diameters was somewhat affected by these processing conditions.

Figure 2 shows the SEM images and detailed statistical analysis of the diameter distributions of samples A, B, C and D, electrospun at various applied voltages. Despite a similar morphology of the samples being observed, their fiber diameter distributions varied slightly from each other, with the narrowest one at the applied voltage of 12 kV. The broad diameter distribution may be attributed to an unsteady electric force.

Measurements were also carried out with respect to the flow rate. The results are shown in figure 3. No significant

differences between the SEM images were observed; however, a similar phenomenon to that above regarding diameter distribution is shown in the right side diagram in figure 3, which reveals that the flow rate affected the charge interactions as well. It can be concluded that at a flow rate of 0.02 ml min−1 and an applied voltage of 12 kV, a high population of molecules with the same charge density is deposited on the grounded collector, meaning that there is a narrower diameter distribution. This suggests that there is an optimum condition for each voltage and flow rate related to the optimum charge interaction.

3.2. Crystalline structure of the PVDF nanofibers

Among the five different crystalline structures of PVDF, the β-phase crystalline is the one with the most effective piezoelectricity. To characterize the contents, experiments were carried out by means of FTIR and x-ray diffraction.

FTIR spectra of the samples are shown in figures 4(a) and (b), where the characteristic bands at 474, 510, 1276 cm−1

the applied voltage and the controlled flow rate. X-ray diffraction experiments were carried out to determine the

β-phase contents of the samples. The results are shown in figures 5(a) and (b); diffraction angles (2θ ) of 20◦, 36◦ and 40.4◦ were assigned to the β -phase crystalline. The FTIR spectra and the x-ray diffraction both yielded three β-phase crystalline, however the results of the x-ray diffraction were extremely sharp when compared to the FTIR spectra. Both the x-ray diffraction data and FTIR spectra confirm that the electrospun PVDF nanofibers manufactured by electrospinning have a predominantly β -phase structure. This means that the α-phase of the raw PVDF is converted into β-phase during electrospinning with various process parameters, including different applied voltages and different flow rates.

With respect to the samples A–D shown in figure 5(a), sample B is distinguished from the others with the highest peaks of 2θ = 20◦, indicating more β-phase crystalline contents in the nanofibers, the applied voltage being 12 kV. For samples E–H, their diffraction patterns are compared in figure 5(b). Apparently, sample E, corresponding to the lowest flow rate of 0.01 ml min−1, displays the lowest diffraction intensity at 2θ = 20◦. Among the rest of the samples, the diffraction intensities do not vary significantly, but the shape of sample F (flow rate 0.02 ml min−1) is slightly sharper. These results supported the results from figures 2 and 3, where samples B and F (identical samples) had the narrowest diameter distribution of the nanofibers. Namely, for the same collector distance, the most β-phase formation in the electrospinning fibers was observed when the voltage was at 12 kV. This reveals that an optional electric force applied on the polymer solution elongated the polymer chains from the tip of the needle until the polymer touched the collector.

Figure 7. Sensing response of sensors under loading in terms of voltage.

The possible mechanism for the formation of β-phase PVDF produced by electrospinning might be attributed to several factors. First, the β-phase transition is caused by an

intense stretching of PVDF jets during electrospinning. The effect of the electrically induced stretch may therefore be analogous to that of mechanical stretch used on nanofibers. Second, the strong electric field in the electrospinning process plays a role in the β-phase formation, in a way similar to the poling process. Third, the rapid evaporation of the solvent and condensation of PVDF nanofibers could have led to the transition from α-phase to β-phase.

3.3. Fabrication of a piezoelectric force sensor based on PVDF fibrous sheet

Fabricated force sensors based on PVDF fabric are shown in figure 6(a). These sensors consist of a flexible electrode on the top, a rigid one at the bottom and a PVDF fabric in between,

as shown in the schematic drawing in figure 6(b). The flexible electrode is a piece of plastic film coated with indium tin oxide (ITO), whereas the rigid one is a plate of ITO-coated glass. No pre-loads were added onto the sensor except for the weight of the flexible electrode itself during our measurements.

To determine the sensitivities of the sensors, measurements were carried out by exerting a force onto the sensor surface (see figure 1). A load between 3 and 5 N was applied, and the response of the sensors is shown in figure 7. In this figure, the upper signal is equivalent to the loading force when the load cell comes to contact with, holds on and releases from the sensor’s surface, and the lower one with two opposite peaks is the electric signal of the sensor in response. These two opposite peaks corresponded to the exerting force and the releasing force respectively. The output electrical signal of the