Direct writing of PVDF piezoelectric film based on near electric field added by [Emim]BF4

PVDF/IL piezoelectric film based on [Emim]BF4-added was prepared by near electric field direct writing. The β-phase content and its electric property of the film were analyzed. The results showed that the content of β crystal phase of PVDF film was improved by the synergistic effect of IL ([Emim]BF4) and near electric field. When the amount of [Emim]BF4 added was 1.5 wt%, the relative β-phase content of PVDF film reached 85.09%, and the crystallinity is 47.84%. Increase in [Emim]BF4 content resulted in a significant increase in the relative dielectric constant of the film, from 6 to 285, but also accompanied by a sharp increase in dielectric loss. The d33 value of the film can be as high as −10 pC/N, and the output voltage from the cantilever beam vibration feedback indicates that the voltage strength of the [Emim]BF4-added film is greatly improved compared with the pure PVDF film.


Introduction
Polyvinylidene fluoride (PVDF) has been widely studied since its inception in the late 1960s. Compared to traditional piezoelectric materials, PVDF piezoelectric films have high sensitivity, thin thickness, good impact resistance, wide frequency response range, close to the acoustic impedance of water, good stability and easy processing. As a sensor and actuator, it is widely used in mechanics, acoustics, electronics, healthcare, military, transportation, hydroacoustic measurement, geological exploration, etc [1,2]. PVDF usually has four crystal phases of α, β, γ and δ [3]. Generally, the α-phase is achieved under crystallization conditions because the molecular potential energy is the lowest and it is a thermodynamically stable phase [4]. The α-phase having a helical conformation TGTG is arranged in anti-parallel and has no polarity. The phase of the β-phase of all-trans TTT conformation is oriented and the electroactive is the strongest. Therefore, increasing the content of β-phase is crucial for improving the electrical properties of PVDF piezoelectric film [5,6].
PVDF piezoelectric film with high β-phase content can be prepared by stretching method [7], spin coating method [8], solution casting [9] and traditional electrospinning [10]. However, these methods are not suitable for the preparation of PVDF piezoelectric film with complex shapes. Customized PVDF film with special shapes still requires subsequent cutting and other processes. In order to solve the above problem, and accurate customization of any shape of the film some scholars use a new near-field direct writing method to prepare customizable-PVDF piezoelectric film [11]. Kameoka et al [12] proposed a near-field direct writing technique with a scanning tip. The distance from the tip to the collector is 0.5∼1 cm, which can control the deposition orientation of polymer nanofibers, but the precision is low. Sun [13] et al developed a near electric field direct writing technique by further reducing the tip-to-collector distance to sub-millimeters, with a tip-to-collector distance h ranging from 50 mm to 3 mm. Gupta [14] et al used a tube connected to the reservoir to resolve the solution limit and installed a collector under the tip guide electrode to improve positioning. This near electric field direct writing method can promote the formation of β-phase to a certain extent, and greatly reduces the working voltage [15], avoiding the unstable bending motion stage of the electrospinning Taylor cone ejecting jet.
This achieves accurate writing of the jet on the collecting plate, which saves resources and can produce films of various shapes.
In order to further improve the electrical properties of PVDF piezoelectric film, it is common practice to add some additives such as clay, salts, nanoparticles, ionic liquids, etc to the solution [16]. 1-ethyl-3methylimidazolium tetrafluoroborate ([Emim]BF 4 ) is a hydrophilic ionic liquid (IL) at room temperature, with low vapor pressure, non-flammable, and a strong solvency, high conductivity and stability. Ionic liquids play an active role in improving the uniformity of spinning and promoting the formation of β-phase in PVDF. Seo [17] prepared a certain concentration of PLLA chloroform solution, and then a certain amount of [HMIM]Cl was added for electrospinning. It was noted that the addition of ionic liquid resulted in a sharp decrease in fiber diameter, the pore diameter on the PLLA fiber decreased with the addition of the ionic liquid, and the fiber diameter was reduced and more uniform. Pei et al [18] prepared different proportions of PVDF/IL composites by solution casting method with IL([PhCH 2 MIm]PF 6 ) as nucleating agent. The results showed that IL could be uniformly dispersed in PVDF. The α crystal form of PVDF is significantly decreased or even disappeared compared with pure PVDF, and the content of polar phase (β-phase and γ-phase) is increased, and PVDF/IL composite material mainly based on β crystal form is obtained. Lopes et al [19] prepared PVDF films with 0 wt%, 5 wt% and 10 wt% of [Emim]BF 4 by solvent casting method. And the content of β-phase was significantly improved. When the addition amount was 10 wt%, the β content exceeded 60%.
However, the current reports mainly focus on the preparation of high-content IL-added polymer fibers by electrospinning, and the preparation of high-content IL-added PVDF films by casting. With the near electric direct writing method, under the premise of ensuring the smooth progress of the preparation process, there are not many researches on the effect of the addition of trace IL on the β crystal phase of PVDF film.
In this paper, we have prepared a series of IL-added PVDF piezoelectric films by near electric direct writing method, and reported the preparation process and performance characterization of the films.

Experiment
The raw materials are PVDF (Arkema 721, 99.5%), 1-methyl-2-pyrrolidone (NMP, 99.5%, Shanghai Aladdin Biochemical Technology Co., Ltd), ionic liquid [Emim] BF 4 (98%, Shanghai Maclean Biochemical Technology Co., Ltd), Acetone (Acetone, 99.5%, Sinopharm Chemical Reagent Co., Ltd), tetrabutylammonium chloride (TBAC, Shanghai Maclean Biochemical Technology Co., Ltd). 10 g of printing solution was prepared, wherein the NMP content was 77.0-79.5 wt%, the PVDF content was 15 wt%, the Acetone content was 5 wt%, the TBAC content was 0.5 wt%, and the added [Emim] BF 4 content was 0 wt%, 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt%, and 2.5 wt%. A certain amount of ionic liquid [Emim]BF 4 (0 g, 0.05 g, 0.10 g, 0.15 g 0.20 g, 0.25 g) and tetrabutylammonium chloride (0.05 g TBAC) were added to NMP (7.70-7.95 g) solvent and sonicated at room temperature for 30 min PVDF powder (1.5 g) was added, dissolved in the mixed solution, ultrasonically for 1 h to fully dissolve them, add acetone (0.5 g), and continue ultrasonication for 30 min to obtain a uniform printing solution. Figure 1 is a schematic view of the diagram. A syringe containing 1 ml of print solution is attached to the 3D printer nozzle. The distance between the printing noodle and the writing bed is 2 mm, and the voltage of the DC power supply is set to 6 kV. The heating bed is attached to the copper plate, and the upper side of the copper plate is writing bed. The temperature was set to 70°C and the running speed was 15 mm s −1 . The printing nozzle first moves in the horizontal x direction, then moves in the vertical y direction, and the deposited semi-solid PVDF fibers are dried in the air, and the 20×20 mm film is woven after 70 layers of printing. Finally, a gold electrode is plated on both sides of the film by sputtering.
The surface morphology of the sample was analyzed by a thermal field emission scanning electron microscope (JSM-7001F, Japan), and the crystal phase structure was measured by an x-ray diffractometer (Rigaku D/MAX-rA, Japan). Crystallization properties of films were studied using differential scanning calorimetry (STA 449 F3 Jupiter, Germany). A heating rate of 10°C min −1 over the temperature range of 50°C-200°C, all the experiments were performed under a nitrogen environment. A Fourier transform infrared spectrometer (Nicolet 6700, Nicolet, USA) was utilized to scan and analyze the relative content of the β-phase of the film. The piezoelectric constant(d 33 ) were measured by quasi-static piezoelectric strain constant (d 33 ) measuring instrument (ZJ-6A, China) and the electric signal feedback of PVDF films was detected by manganese steel cantilever beams. As showed in figure 2, the distance from the PZT driver to the cantilever beam is 15 cm, and the distance from the PVDF piezoelectric film to the PZT driver is 7 cm. The voltage and frequency generated by the PZT drive is controlled by the signal generator. The PVDF piezoelectric film captures the vibration signal on the beam and amplifies and analyzes the captured signal to show the amplitude and frequency of the input vibration signal.

Results and discussion
Surface topography of the PVDF thin films Figure 3 is a scanning electron micrograph of a pure PVDF film (a), a PVDF film with 0.5 wt% [Emim]BF 4 added (b), a PVDF film with 1.5 wt% [Emim]BF 4 added (c), and a PVDF film with 2.5 wt% [Emim]BF 4 added (d). As can be seen from the figure, the thickness of the spherulites of the [Emim]BF 4 -added PVDF film is much smaller and denser than each other compared to the pure PVDF film. This may be because the ionic liquid [Emim]BF 4 is less volatile, forming fewer pores, and some of the [Emim]BF 4 is also encapsulated in the pores. Due to the high conductivity caused by the addition of [Emim]BF 4 , the greater the conductivity of the solution when the jet breaks the surface of the Taylor cone, the stronger the electric field force acting on the jet of the solution, resulting in a lower jet radius and faster drying of the solution. PVDF crystallization time becomes shorter, and the growth of the spherulites is suppressed. These effects only need to add a small amount of [Emim]BF 4 , for example, the addition of 0.5 wt% of [Emim]BF 4 can greatly change its morphology. The surface topography between the samples added in an amount of 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, and 2.5 wt% tends to be flattened, and the spherulites become smaller.

Thermal analysis of PVDF film with [Emim]BF 4 added
The DSC curves(heating) of the PVDF film is shown in    Compared with pure PVDF mentioned in related literature [21], the film prepared by direct writing has lower crystallinity. In the process of the direct writing process, the writing solution are rapidly solidified when exposed to air, which will hinder the crystallinity, that is, PVDF molecular chains do not have enough time to fully form crystals before solidification. Although the electrostatic interaction between PVDF and [Emim]BF 4 can induce the α-phase to β-phase transition, it also hinders the crystallization process, resulting in less stable crystalline phase and increased amorphous phase.
From table 1, we can see that the crystallinity also decreases slightly with the increase in the amount of [Emim]BF 4 added, but there is an abnormally increased value at 1.5 wt%. Related studies have demonstrated that the direct addition of ionic liquids in casting methods is often accompanied by a slight decrease in crystallinity [22]. However, in the work, the comprehensive influence of factors such as the electric field, the thickness and stability of the jet are not static. Since [Emim]BF 4 itself is conductive, its addition will affect the writing process. From this point of view, the addition of ionic liquids in this experiment has a positive effect on the degree of crystallinity. However, when the amount of ionic liquid is more than 1.5 wt%, the jet flow is unstable because of the high conductivity under the action of electric field. This affects the uniformity of the film, further reduces the crystallization time, and decreases the crystallinity.

β-phase characterization of PVDF film with [Emim]BF 4 added
The XRD diffraction pattern of PVDF film with different content of ionic liquid [Emim]BF 4 is shown in figure 5. In the figure, the main crystallization peak of the α-phase of PVDF is 2θ=18.4°. (corresponding to (020) crystal plane), and the main crystallization peak of the β-phase of PVDF is 2θ=20.6°(corresponding to (110) crystal plane) [23]. It can be observed that the addition of the ionic liquid [Emim]BF 4 causes the peak intensity corresponding to α-phase to decrease, even disappear. And enhances the peak intensity corresponding to the βphase. This indicates that the addition of the ionic liquid [Emim]BF 4 induces the transformation of the α-phase to the β-phase, and this leads to a decrease in the α-phase content of PVDF and an increase in the β-phase content.  As shown in figure 6(a), PVDF is a homopolymer whose molecular chain is tightly entangled. Due to the applied electric field, Emim + has a certain degree of distribution with directionality. The functional group CF 2 has a large electronegativity, and the taste ring of [Emim]BF 4 is a positively charged planar cation. CF 2 and Emim + , these two substances with a large difference in electronegativity interacted have a strong interaction, which promotes the rotation and rearrangement of PVDF molecular chains. As a plasticizer, TBAC's unique sterically hindered structure can reduce the interaction between PVDF's own molecules and promote the movement of PVDF chains, as shown in figures 6(b) and (c). In addition, [Emim]BF 4 itself can also act as a heterogeneous nucleation point. Under the guidance of the electric field and the stretching of the jet, the rotation and rearrangement of the PVDF molecular chain are synergistically induced to change from a non-polar α-phase to a polar β-phase. It causes a decrease in the α-phase and an increase in the β-phase.
The infrared spectrum of the PVDF film with different contents of [Emim]BF 4 is illustrated in figure 7. The dotted line corresponds to the characteristic peaks of the αphase and the β-phase. In the FTIR spectrum, the corresponding absorption peak positions of α of PVDF is 763 cm −1 (CF 2 bending and skeletal bending); the corresponding absorption peak position of β-phase are: 840 cm −1 (CH 2 rocking) and 1279 cm −1 (CF out-ofplane deformation) [24][25][26][27]. 875 cm −1 represents the stretching vibration of functional groups CF 2 in the amorphous component of pure PVDF. For the thin film sample with [Emim]BF 4 added, the stretching vibration peaks of CF 2 shifted, which indicates that the CF 2 and Emim + have electrostatic effects.
The relative content of the β-phase (F(β)) present in the sample is calculated by the following formula [28]: Where F(β) represents the relative content of the β-phase; A α and A β are at 763 cm −1 (α-phase characteristic peak corresponding to CF 2 bending and skeleton bending) and 840 cm −1 (β-phase characteristic peak corresponding to CF 2 sway) Integral area; K α and K β represent absorption coefficients at 763 cm −1 and 840 cm −1 , respectively. K α = 6.1×10 4 cm 2 mol −1 , K β = 7.7×10 4 cm 2 mol −1 [29] Table 2 shows the effect of different amounts of [Emim]BF 4 on the relative content of β-phase(F(β)). The addition of [Emim]BF 4 does obvious increase the F(β). As the amount of addition increased, the F(β) first increased and then decreased, and reached a maximum at the addition amount of 1.5 wt%, which was 85.09%. And from 0 wt% to 1.5 wt%, the β-phase relative content is increased, from 1.5 wt% to 2.5 wt%, and the β-phase relative content is decreased. This may be when the amount added is 1.5 wt%, its role as a heterogeneous nucleation point synergistic electric field and other factors inducing the formation of the β-phase in PVDF have tended to be saturated. It reaches its maximum at 1.5 wt% of the addition, and more added amount did not further improve the β-phase relative content, and even acts as an impurity, the F(β) is reduced instead.
Dielectric and piezoelectric properties of PVDF film added with[Emim]BF 4 added Figure 8 and figure 9 show the frequency dependence of the relative dielectric constant (ε') and dielectric loss (tanδ) of the PVDF/[Emim]BF 4 film. Compared with pure PVDF film, ε' and tanδ of the sample containing [Emim]BF 4 were strongly improved. And as the [Emim]BF 4 content increases, the ε' increases from 6 to 285 because of the [Emim]BF 4 slight plasticizing effect leading to chain motion and high ion conductivity, but at the same time the tanδ also increases significantly. The large value of films at low frequencies are due to the electrode polarization effect, which is due to the conduction loss caused by the radial and tangential diffusion of free charges or ions near the electrode surface. The dielectric strength of PVDF /[Emim]BF 4 film at higher frequency is controlled by the Maxwell-Wager-Sillars (MWS) interface polarization. The good solubility of [Emim]BF 4 in solution leads to a large heterogeneous interface, when the sample is applied with an electric field, the space charge to generate large phase-to-phase accumulation because of the large difference in conductivity between [Emim]BF 4 phase and PVDF phase. This movement and accumulation of charge carriers in the interface causes the material to produce a large dielectric constant [30], while accumulating induced charges, losses are also produced [31].
As the frequency increases, ε′ and tanδ decrease dramatically. This is caused by the decrease in interface polarization as the frequency increases. Since the movement and accumulation process of charge carriers in [Emim]BF 4 takes a certain time, and the alternating electric field changes slowly at low frequencies, the interface between [Emim]BF 4 and PVDF has sufficient time to accumulate a large amount of the induced charge. However, when the frequency is increased, the rate at which the interface accumulates the induced charge can not keep up with the electric field change, and the induced charge is reduced, so that the interfacial polarization effect is weakened at high frequencies, and the ε' and tanδ are reduced.  The piezoelectric film electrical signal detecting device shown in figure 2 is used to detect the output voltage signal of the film. The signal generator is connected with the PZT driver, and PZT drives the vibration of the cantilever beam, so that the PVDF piezoelectric film generates an electrical signal, which is filtered and displayed on the oscilloscope. Figure 10 shows the results of the test. It can be seen that after inputting 10 V/20 Hz sine   wave, the PVDF film produces an output signal. Compared with the output voltage of 96 mV of pure PVDF film, the output voltage of PVDF film added by 1.5 wt% [Emim]BF 4 reaches 332 mV, which is a huge upgrade. It can also be seen from the figure that when the [Emim]BF 4 content is between 0 wt% (pure) and 1.5 wt%, the output voltage increases, reaching the highest at 1.5 wt%, and decreasing at 2 wt% to 2.5 wt%, which is consistent with the previous phase content analysis. This shows that under the addition of [Emim]BF 4 , the β-phase of the PVDF film does increase and the electrical properties are enhanced, and the film performance is best when the amount of [Emim]BF 4 added is 1.5 wt%.

Conclusion
The near electric field direct writing method is simple and suitable for applicability, and can accurately customize any shape of film. The electrical properties of the PVDF film are enhanced by adding a microscale amount of ionic liquid [Emim]BF 4 and inducing the multiple influencing factors of the β-phase formation. The introduction of [Emim]BF 4 greatly changed the surface morphology of the PVDF film, the spherulite radius was greatly reduced, and the density of the film was increased. When the addition amount of [Emim]BF 4 is 1.5 wt%, the relative β-phase content of PVDF film reached 85.09%, and the crystallinity is 47.84%. The addition of [Emim]BF 4 increases the dielectric and piezoelectric constant of the film. As the [Emim]BF 4 content increases, the ε' increases from 6 to 285, the d 33 value of the film is as high as −10 pC/N, and the output voltage is greatly improved compared with the pure PVDF film.