Recent advances in the preparation of PVDF-based piezoelectric materials

Liangke Wu; Zhaonan Jin; Yaolu Liu; Huiming Ning; Xuyang Liu; Alamusi; Ning Hu

doi:10.1515/ntrev-2022-0082

Open Access Published by De Gruyter March 25, 2022

Recent advances in the preparation of PVDF-based piezoelectric materials

Liangke Wu , Zhaonan Jin , Yaolu Liu , Huiming Ning , Xuyang Liu , Alamusi and Ning Hu

From the journal Nanotechnology Reviews

https://doi.org/10.1515/ntrev-2022-0082

Abstract

In recent years, PVDF(polyvinylidene fluoride) and its copolymers have attracted great attention in the development of energy-harvesting devices because of their unique properties such as good flexibility, environmental friendliness, high halogen and acid resistance, lightweight and good biocompatibility. Compared to the most commonly used PZT (lead zirconate titanate), the piezoelectricity of PVDF and its copolymer-based materials is relatively low. To further expand the applications of PVDF, there is an urgent need for efficient methods to prepare high piezoelectric polymers or composites. In this work, the crystal phases are introduced first. Then, the preparation methods of PVDF and its copolymer-based materials are summarized, which are mainly focused on four determining factors of piezoelectricity. The mechanisms of piezoelectric β-phase formation and α- to β-phase transformation are introduced. The influence parameters of each process and their interactions are discussed in detail. In the last section, the progress of the preparation methods is summarized. This work will provide useful information to researchers working on piezoelectric composites.

Keywords: PVDF; piezoelectric films; stretching; poling; nanocomposites

1 Introduction

Piezoelectric devices are widely used for energy harvesting. Nowadays, PZT (lead zirconate titanate) is the most commonly used piezoelectric material because of its high piezoelectric constant and electromechanical coupling factor. However, the development is limited because it is fragile, expensive, high density, and environmentally hazardous [1,2]. In contrast, PVDF and its copolymers have many advantages such as good flexibility, environmental friendliness, high halogen and acid resistance, lightweight, and good biocompatibility [3,4,5,6]. In particular, these materials can be shaped into curvilinear structures [7,8]. Thus, they are promising candidates to replace PZT to some extent. Compared to PZT, PVDF has relatively low piezoelectricity, which greatly hinders its applications.

The piezoelectric energy-harvesting devices are based on the piezoelectric effect. In contrast, the piezoelectric actuators are based on the reverse piezoelectric effect. PVDF is a semicrystalline polymer with electroactive responses including piezoelectricity, pyroelectricity, and ferroelectricity [9]. The piezoelectric coefficients of PVDF and its copolymers are basically of the same order of magnitude, i.e., about 20–40 pC/N, which is an order lower than that of PZT [10,11,12]. Therefore, improving piezoelectricity is essential for the development of PVDF-based piezoelectric devices. Among the five known phases, the β-phase is the most important because of its excellent piezoelectricity [3,13]. Piezoelectricity is caused by the highly aligned dipole moments in the crystal polymers [14], which depends on the following four factors. (1) The β-phase fraction. The β-phase exhibits the best piezoelectricity, while the α-phase is the most common one. Thus, one of the most important objectives is to improve the formation of the β-phase and the transformation from α- to β-phase. (2) The crystallinity degree. Only the crystalline part of the polymer exhibits piezoelectricity, while the amorphous area does not. Therefore, some processes may enhance the β-phase fraction but reduce the crystallinity degree. The optimal parameters should be carefully considered to balance both effects. (3) The alignment of molecular chains. In the initial crystallization samples, the direction of molecular chains in the matrix is randomized. Therefore, its electrical output capability is extremely weak. (4) The alignment of the dipole moments. In the films without stretching or poling, the dipole moments in the polymer or composites are usually disordered. As a result, the samples show extremely weak or even no piezoelectricity due to the mutual offset of the dipole moments in the opposite direction [15].

In recent decades, researchers have been trying to enhance the piezoelectricity of PVDF and its copolymers or composites. A number of methods have been developed to prepare high piezoelectric polymers or composites. Nevertheless, the phase transformation mechanism is still unclear. Sometimes, even completely opposite results are observed in similar studies, and thus, the conclusions are conflictive. As a result, more work is required to further investigate the application of PVDF.

2 Properties of PVDF and its copolymers

2.1 Microstructure of PVDF

The crystal structure of PVDF can be verified by different spatial arrangements of the CH₂ and CF₂ groups along the polymer chains [8]. PVDF possesses at least five polymorphs, i.e., α, β, γ, δ, and ε. The most common phase is the α-phase, while the most important one is the β-phase because of its excellent piezoelectricity. The α-polymorph possesses the monoclinic unit cell with the TGTG conformation, which is piezoelectrically inactive. In contrast, the β-polymorph possesses the orthorhombic unit cell with the TTTT conformation, which is piezoelectrically active. The γ-polymorph has an orthorhombic unit cell with the T3GT3G conformation. The δ-polymorph is the polar analogue of the α-form, and the ε-polymorph is the anti-polar analogue of the δ-form [3,13]. In the PVDF or its copolymer samples, the formation of the β-phase and the transformation from α- to β-phase play an extremely important role in realizing perfect piezoelectricity. The parameters of α- and β-polymorphs unit cells are listed in Table 1. Figure 1 shows the molecular chains of both α- and β-forms.

Table 1

Parameters of α- and β-polymorphs unit cells [16,17]

	Crystallographic system	Molecular conformation	a (nm)	b (nm)	c (nm) (fiber axis)
α	Monoclinic	TGTG′	0.496	0.964	0.462
β	Orthorhombic	TT	0.858	0.491	0.256

Figure 1

Molecular chains of (a) α- and (b) β-phase [17].

2.2 Characterization of the PVDF crystal structure

2.2.1 XRD

XRD (X-ray diffraction) is a widely used technique to investigate the crystallinity of PVDF and its copolymers. The characteristic peaks are shown in Table 2. It is worth noting that the characteristic peaks may shift little in different reports.

Table 2

Characteristic peaks of the PVDF structure in XRD spectra [3,18]

Peaks (2θ)	17.7°	18.5°	19.9°	26.5°	20.6°	36.3°
Crystal	α	α	α	α	β	β
Plane	100	020	110	021	110	200

XRD can also be applied to calculate the crystal degree of PVDF as shown in Figure 2. The method can be described as below: (1) remove the baseline; (2) resolve the curve into the crystal peak and the amorphous peak; (3) use the following equation to calculate the crystallinity degree [4,19,20]:

(1) X c = I c I a + I c ,

where I _c is the crystal area (C) and I _a is the amorphous area (N).

Figure 2

Deconvolution of the XRD spectra into the crystal peak and the amorphous peak [4].

2.2.2 FTIR

The characteristic peaks of the PVDF structure in FTIR (Fourier transform infrared spectroscopy) spectra are shown in Table 3. Similar to the XRD spectra, the characteristic peaks may shift slightly in different experiments.

Table 3

Characteristic peaks of the PVDF structure in FTIR spectra [3,17,18,21,22]

Peaks (cm⁻¹)	510	615	765	840	1,170	1,220	1,280
Crystal	β	α	α	β	α	β or γ	β

The relative fraction of the β-phase can be calculated by the following equation:

(2) F ( β ) = A β 1.26 A α + A β ,

where A _α is the absorbency at 765/cm and A _β is the absorbency at 840/cm [23]. Figure 3 shows an example of the relative β-phase fraction calculation.

Figure 3

Calculation of the relative β-phase content from FTIR spectra [23].

2.2.3 DSC

The crystallization degree can be calculated by the following equation from DSC (differential scanning calorimetry) curves [24]:

(3) X c = Δ H m Δ H m ∅ × 100 % ,

where Δ H m and Δ H m ∅ are the melting enthalpy of the film (which can be calculated using software Pyris) and the melting enthalpy of the 100% crystal material (104.5 J/g), respectively.

2.3 Phase transformation

The crystallinity of PVDF can be changed under certain conditions. The phase transformation in pure PVDF is concluded as shown in Figure 4 [25]. From Figure 4, it is obvious that stretching and poling are two important methods to transform other phases to β-phase.

Figure 4

Transformation among various phases of PVDF [25].

2.4 Simulation of PVDF molecular chains

The phase evolution in PVDF is a very complicated topic, and it still remains unclear up to now. Besides experiments, simulation methods are also widely applied to investigate this theme. Limited by the computational ability, the number of monomer units in the molecular chains is usually less than 20, which can balance computational costs and computation precision [17,26,27,28,29]. The energy per monomer unit is calculated by Wang et al., which also supports this conclusion, as shown in Figure 5 [26]. Because the α-phase is a thermally stable phase, external energy is required to overcome the energy barrier from α-phase to β-phase. As a result, the energy barrier is a key parameter for phase transformation. DFT (density functional theory) has been applied to calculate the energy barrier [17,26]. For pure PVDF, the energy barrier of α- to β-phase transformation is 16.16 kJ/mol, while the reverse transition requires only 6.25 kJ/mol [26]. The addition of nanofillers can affect the energy barrier dramatically, leading to easy formation of the β-phase, which is one of the reasons that the β-phase content of PVDF/nanofiller composites is higher than that of pure PVDF samples. In PVDF/DMF(N,N-dimethylformamide)/CNT(carbon nanotube) mixed solution in the solution casting process, both α- and β-phase exist. The released energy of the β-phase chain near the CNT surface is larger than that of the α-phase chain, indicating that the β-phase chain is preferred to be adsorbed on the CNT surfaces. From the energy barrier calculation results, it is found that for the α-phase chains adsorbed on the CNT surfaces, the energy barrier is much higher than that far from the CNT surfaces. Therefore, the most likely route for α- to β-phase transformation is that the α-phase chains far from CNT transit into β-phase chains and then are adsorbed to the CNT surface [17]. The electric field poling can also result in phase transformation. The mechanism can be explained as follows: when a constant voltage is applied on the PVDF chains, the chains would be stretched (the dipole orientation is in contrast with that of the voltage) or compressed (the dipole orientation is parallel to that of the voltage). Based on the simulation results, piezoelectricity is mostly attributed to the dimension effect, which is quite different from inorganic piezoelectric materials (dipole fluctuations) [27]. The defects in PVDF are unavoidable. Therefore, the influences of defects on the properties have also been investigated. Based on the analysis of the features of four crystal phases (α, β, γ, and δ) with different types of defects, it has been found that the interstitial defects are not stable in all phases and tend to transform into vacancies. Among the four phases, only in the γ-phase, more energy is required in the second vacancy formation compared to the first one. Similarly, vibration contributions facilitate the second vacancy formation in the α-, β-, and δ-phases, which is different from the γ-phase [29]. Referred to PVDF fibers, the diameter can affect the stability of the β-phase. In ultrathin fibers, the β-phase is easily transformed into a paraelectric phase [30]. Ramos et al. have investigated the effect of the electric field on both α- and β-chains of PVDF by a self-consistent quantum molecular dynamics method. The orientation of the dipolar moments caused by the electric field and the accompanying structural evolution caused by these reorientations are discussed in detail. The simulation results show that when the chain length is increased by 37% (which can be caused by stretching), the α- to β-phase transformation occurs. Meanwhile, the total dipole moment of the stretched chain is much lower than the theoretical value of the β-phase chain due to the dipole moment disorder. When an electric field (perpendicular to the chain axis) over 50 MV/cm is applied to the stretched chain, it may cause the rotation of –CF₂ and –CH₂ around the chain axis in the opposite directions. Thus, by combining stretching and poling, the high piezoelectric β-phase chains are formed. Furthermore, when the electric field is over 100 MV/cm (or i.e., 10 GV/m), the direct α- to β-phase transformation is caused. Although the phase evolution agrees with the experiments, the electric field calculation is completely different. In the experiments, an electric field of no more than 100 MV/m can induce significant phase transformation, and it is extremely difficult or even impossible to apply an electric field of 10 GV/m to the films because breakdown will occur [31].

Figure 5

Energy per monomer unit of the chain in PVDF vs chain lengths [26].

2.5 PVDF-HFP

PVDF-HFP (poly(vinylidene fluoride-hexafluoropropylene)) is one of the most common PVDF copolymers, which can be synthesized by polymerization of VDF (vinylidene fluoride) and HFP (hexafluoropropylene), as shown in Figure 6. Copolymerization with HFP greatly reduces the crystallinity degree because of the bulky CF₃ groups, which has a negative effect on obtaining high piezoelectricity. On the other hand, flexibility increases greatly compared to pure PVDF [32,33,34]. Hydrophobicity is also enhanced because of the increase of the fluorine content caused by the addition of HFP [35]. It is reported that the piezoelectric coefficient (d ₃₁) is comparable to that of PVDF, while its pyroelectric coefficient is higher [36,37].

Figure 6

Polymerization of PVDF-HFP.

2.6 PVDF-TrFE

PVDF-TrFE (poly(vinylidene fluoride-trifluoroethylene)) is another important PVDF copolymer, which can be synthesized by polymerization of VDF (vinylidene fluoride) and TrFE (trifluoroethylene), as shown in Figure 7. The introduction of TrFE modifies the PVDF crystal structure due to the increase of the unit cell size and interplanar distance of the ferroelectric phase [14,38]. Compared to PVDF, PVDF-TrFE has a higher crystallinity and a preferred orientation of good growth crystallinity, which improves the electromechanical coupling factor [39]. The mass ratio of VDF/TrFE and the synthesis conditions can greatly influence the piezoelectricity of copolymers [40]. The addition of TrFE leads to easy formation of β-phase. As a result, it can be polarized without stretching [19,41,42]. Generally speaking, the phase configuration and phase transformation of PVDF-TrFE are very similar to those of PVDF. Thus, the characterization techniques are the same. Compared to PVDF, PVDF-TrFE exhibits lower Curie temperature, which is due to the reduction of the interactions between each unit and dipole moments [40]. The notable difference between PVDF and PVDF-TrFE is that, for PVDF-TrFE, the ferroelectric to paraelectric phase transition occurs at the Curie temperature (T _c) below the melting temperature, while for PVDF, it does not happen [43]. The other important point is that the characteristic peak of β-phase shifts to 19.9° (20.4° for PVDF) and the interplanar spacing b is 0.45 nm (0.491 nm for PVDF). Similarly, in the FTIR spectrum, the peak of β-phase shifts to 850/cm (840/cm for PVDF) [14,44]. In the PVDF-TrFE films, the β-phase crystallinity exhibits the rod-like shape of the grains [14].

Figure 7

Polymerization of PVDF-TrFE.

3 Preparation of PVDF and its copolymer-based piezoelectric composites

3.1 Initial crystallization

3.1.1 Solution casting method

The solution casting method is a widely used method for the preparation of PVDF and its copolymer-based films. It can be divided into the following steps:

(a) Dissolve PVDF pellets in an organic solvent. PVDF and its copolymers can be dissolved in some organic solvents such as DMF [18,45,46,47,48,49], N,N-dimethylacetamide (DMAc) [17,50], dimethyl sulfoxide (DMSO) [51], methyl ethyl ketone (MEK) [52], acetone [53], and 1,3-dioxolane (DXL) [43]. In most cases, the dissolution undergoes at room temperature. Sometimes, a relatively higher temperature is required to improve this process (<70°C) [46,52]. However, spontaneous dissolution is difficult or even impossible (depending on the solvent and the PVDF powder). As a result, reinforcement techniques are usually applied. Ultrasonication is a widely used method to improve dissolution [17,18,45,48,49,50]. The ultrasonication operation time varies case by case, ranging from several minutes to 3 h [17,45,48,50,54], while in most cases it is less than 30 min. As the PVDF powder can be dissolved in an organic solvent easily and ultrasonication can heat the mixed solution, ultrasonication usually works at room temperature without additional heating. Stirring is another method to improve dissolution [52,53,54]. To prepare the PVDF-TrFE/MEK solution, stirring is applied for 45 min at 70°C and the solution is heated to 140°C and boiled for 20 min to increase its viscosity [52]. Different from ultrasonication, stirring usually is done for more than 30 min. Jiang et al. used the solution casting method to prepare PVDF/CoFe₂O₄ composites. In order to obtain 8 g of PVDF completely dissolved in 40 mL of DMF, magnetic stirring should be applied overnight [54]. Besides, the combination of sonication and stirring has also been reported [17,49].

(b) The completely mixed PVDF/organic solution is poured onto the substrate made from different materials (Al [50] and glass [17,41,45,47,48,51,54]), and the initial films can be obtained after evaporation. The evaporation may occur in air [17,47,50,52,54] or in a vacuum chamber [46,49,51]. In order to enhance evaporation, a high temperature is usually required in both cases. The temperature usually ranges from 50°C [17,46] to 120°C [47] and the duration lasts more than 2 h [20]. In some reports, it may last over 3 days [52]. It is worth noting that higher temperatures may increase the evaporation rate and result in fast crystallization, which leads to the preferential formation of nonpolar α-phase [55,56].

Figure 8 is a schematic of the PVDF-HFP/graphene composite preparation process by the solution casting method [20].

Figure 8

A typical solution-casting preparation process of PVDF-HFP films [20].

3.1.2 Hot press

Preparation of PVDF or its copolymers films can also be carried out by hot press [41,57,58,59]. Although solution casting has advantages such as low cost and a simple preparation process, sometimes the evaporation is not complete and can be affected by the environment dramatically. Therefore, a hot press is developed for producing PVDF films by complete evaporation or without dissolution in the organic solvent. A typical hot press for the PVDF film preparation includes a high-temperature press process and a high-pressure cooling process [57,58,59,60,61,62,63,64]. Figure 9 shows a schematic of the hot press process to fabricate PVDF-HFP composite films. The key parameters of the hot press are temperature, pressure, and operation time. For most hot press processes, the temperature is over 150°C, and sometimes it is even higher than 200°C [41,57,62]. The high temperature can ensure the complete melting of the PVDF matrix. The pressures in the hot press are different from each other, which depends on the experimental conditions. From the published reports, the lowest pressure is over 5 MPa and the highest pressure reaches 30 MPa [41]. The hot press time is usually less than 1 h [57,58,59], and sometimes, 5 min is enough [41]. Hot pressure not only works on PVDF or the powder of its copolymers but can also be used to form samples with desired sizes from the prepared samples [60,62,63,64]. Na et al. have used an electrospinning method to prepare aligned ultrafine fibrous membranes from the PVDF/DMF solution. Consequently, a continuous hot press post-treatment is performed on the membranes to improve the mechanical properties [61]. A hot press can eliminate the voids in the matrix due to the rolling of the matrix-induced by high temperature and high pressure [65,66].

Figure 9

The hot press process for fabrication of PVDF-HFP/BT-NW composites [62].

3.1.3 Electrospinning

Electrospinning is a widely used method to prepare PVDF films, especially ultrathin films. The electrospinning process includes the precursor solution preparation and injection. The precursor solution preparation is similar to that of the solution casting method introduced above. The injection setups are shown in Figure 10. Compared to other methods, electrospinning has an obvious advantage: the strain in the injection applied on the mixed solution induces the uniaxial elongation of the PVDF molecular chains along the fiber axis, which leads to β-phase formation [67,68]. The other advantage is that PVDF films prepared by electrospinning can show piezoelectricity without the post-poling process because poling can be carried out in the electrospinning process due to polymer jet elongation and whipping. The poling and stretching process can be finished in the electrospinning by the electric forces [15,69]. To fabricate PVDF membranes with desired crystallinity, heat treatment can be applied during or after the electrospinning process. The α-, β- or γ-phase-dominant PVDF films are obtained by controlling the collector temperature, the flow rate, or heating electrospun nanofibrous samples. Furthermore, by fine-tuning the heating process during or after the electrospinning, cross-linked membranes with perfect mechanical properties can be prepared [70]. As mentioned above, the combination of hot press and electrospinning has also been investigated. Several warp–warp arranged dried electrospun PVDF/SMG (surface-modified graphene) membranes can be compressed to obtain final products [60]. Generally speaking, the influence factors of electrospinning include solution parameters (solution concentration, solvent, and molecular weight of PVDF and its copolymers), voltage, tip-to-collect distance, flow rate, the rotation speed of the collector, temperature, needle diameter, humidity, and added nanofillers [15,60,70,71,72,73]. It is reported that at a higher rotation speed, the β-phase fraction is higher, and the annealing temperature contributes less to the increment of the β-phase. Under optimal conditions (9,000 rpm, 100°C), the β-phase fraction reaches a maximum value of 93% [74]. The molecular weight also affects the β-phase fraction. PVDF fibers with higher molecular weight have longer molecular chains, which results in higher viscosity in the mixed solution and lower evaporation. Therefore, jet elongation (stretching) is more than sufficient and β-phase chains are formed [75,76].

Figure 10

(a) Electrospinning setup [80]; (b) electrospinning system for random nanofibers; and (c) electrospinning system for aligned and stretched nanofibers [5].

Electrospinning can also be used to fabricate PVDF piezoelectric fibers. Near-field electrospinning can be applied to direct-write piezoelectric PVDF nanofibers with in situ mechanical stretching and electrical poling. It can increase the conversion efficiency by an order of magnitude compared to PVDF thin films [77]. Wet-spun PVDF fibers are compressed to eliminate voids and consequently poled to align the molecular dipoles. The influences of the processing parameters on the crystal structure, tensile strength, and the piezoelectric constant have been studied. The optimal condition for piezoelectricity is confirmed as a drawing ratio of 6 and heating temperature of 150°C, at which the β-phase content is about 93% [78]. Similar to the PVDF film preparation, the applied voltage, needle-collector distance, and flow rate are the main factors in the electrospinning process. The crystallinity and fraction of the β-phase increase with the applied voltage until 20 kV and then decrease; meanwhile, they decrease with the needle-tip diameter consistently. With regard to the flow rate, the crystallinity (negative correlation) and the β-phase fraction (positive correlation) are different. The orthogonal experiments demonstrate that the mixed effect of the three parameters is different from a mono-factor. In other words, the flow rate has the greatest effects. It may suggest a new approach to prepare high β-phase fibers by adjusting the electrospinning parameters [79,80]. By modifying the collector, the aligned PVDF fibers can also be prepared [5].

Other methods including tape casting, template, and phase separation have also been reported, which are not discussed here due to the disadvantages such as irregularity of shapes, cost, and complexity of the synthesis processes [15,81,82].

3.2 Improvement of the β-phase formation

In most initially prepared PVDF films, the α-phase dominates due to its stable thermal property. In contrast, the β-phase is difficult to form due to the high energy barriers of the all-trans conformation [3]. In contrast, the piezoelectricity of PVDF or copolymers depends on the electroactive phase, especially the β-phase. As a result, a number of techniques have been developed to improve the β-phase formation or α- to β-phase transformation.

3.2.1 Addition of nanofillers

The addition of nanofillers is a widely applied method to improve β-phase formation. Various nanofillers such as CNT, graphene, GO (graphene oxide), CNF, and CB (carbon black) are utilized as reinforcing agents. Uniform distribution is extremely important in composites, as the aggregation of nanofillers results in not only crystal defects but also lower crystallinity degree. In contrast, uniform dispersion can further improve the property or increase the overall crystallinity [83]. CNT is commonly utilized because of its unique properties [83]. The added CNTs play an important role as nucleating agents, which is in favor of developing β-phase crystal and higher elastic modulus [18]. The addition of CNT may increase the β-phase content due to the rapid crystallization rate caused by the nucleating action of CNT but it will soon reach a plateau [3]. The extremely low content of CNT can significantly change the crystal structure and piezoelectricity of PVDF. In most cases, the threshold of CNT loading is less than 0.5 wt% and the optimal loading is less than 1 wt% [3,18,83,84]. The addition of raw CNTs may lead to problems such as aggregation and instability. As a result, modification of CNTs is necessary as shown in some reports [48]. Another reason why CNT can improve the β-phase content is the induced charge accumulation at the boundary, which forces the PVDF chains to arrange in the TT conformation. In contrast, the addition of CNTs may hinder crystallization during solidification (i.e., crystallinity degree decreases), and thus, β-phase decrement becomes possible [68]. Graphene is another agent filler that attracts much attention in recent years due to its large surface specific area [23,85,86]. Besides the direct addition of graphene, in situ reduction in composites is also utilized to incorporate graphene into PVDF. The in situ reduction can avoid rGO (reduced graphene oxide) agglomeration during the reduction in PVDF/GO solutions, thus improving rGO dispersion [85,86]. The reduction can be completed by heating the prepared PVDF/GO composites at 140°C for 1 h [85], or with hydrobromic acids [86]. The addition of graphene oxide (GO) has also been reported [81,85,87]. Similar to CNTs, modification on the surface of graphene or graphene oxide is an effective method to improve nanofiller dispersion and piezoelectricity [88,89,90]. In PVDF/graphene composites, the β-phase content can reach up to 97% and the power capability voltage can be increased by more than 100% [85,86,89]. Compared to CNTs, graphene can affect piezoelectricity at a lower content, and the optimal graphene content is even less than 0.1 wt% [81]. The insulated modification of the filler can prevent direct connection of GO and limit the leakage current in composites [90]. The addition of graphene may also induce another active phase (γ-phase) rather than the β-phase. Fakhri et al. found that the addition of GO/Cu and GO/Au can enhance the γ-phase due to electrostatic interactions among the CH₂–CF₂ dipoles of PVDF and the nanofillers. The γ-phase of the PVDF/GO/Au (1 wt%) composite reaches 95%, which is about 2.5 times higher than that of pure PVDF. The PVDF/GO/Au composite shows a higher γ-phase content than that of the PVDF/GO/Cu composite containing the same concentration fillers [91]. Carbon black (CB) is also a perfect nanofiller that can double the output voltage by the addition of only 0.5 wt% CB into the PVDF-HFP matrix [92]. Besides functioning as nucleate agents in the crystallization, the nanofillers can also improve the interaction between the nanofiller surfaces and the molecular chains [3,68,81,87,88,90,91,92,93]. The interaction leads to uniform dispersion and nucleation of mobile PVDF crystals due to the conformation defects and kinks [87]. PZT is a highly expected enhancement agent because of its excellent piezoelectricity, which is 10 times higher than that of PVDF and its copolymers. It was found that PVDF/PZT (30 wt%) possesses a piezoelectric coefficient up to 84 pC/N, about 3 times that of pure PVDF [94].

Metal nanowires and particles have also been used as nucleating agents recently. Mandal et al. prepared PVDF films modified by gold nanoparticles (AuNPs) by the solution casting method. It was found that proper AuNPs may improve the formation of the β-phase of PVDF. While, in the composites, spherulitic features have also been observed, indicating the existence of spherulitic structures in the β-phase (β-spherulite) [95]. Other types of nanofillers, such as BaTi₂O₅, were also reported, which can improve the β-phase formation, the piezoelectricity, or/and the power capability [96].

In recent years, hybrid nanofillers are used as nucleate agents to further improve the phase transformation based on the synergistic effects. Compared to monofillers, the hybrid fillers perform better at the same content. Yang et al. developed a three-dimensional composite structure consisting of MnO₂/graphene/MWCNT to improve the piezoelectricity of PVDF. The loading of MnO₂ has an obvious influence on the tunable breakdown strength and piezoelectricity. Besides, the prepared PVDF/MnO₂/graphene/MWCNT composite requires a relatively lower poling electric field (50–80 MV/m) than that of neat PVDF films (>100 MV/m) to induce high piezoelectricity. This result is due to the better compatibility between hybrids and the PVDF matrix caused by the larger specific surface area and rougher surface attached with high coverage of –OH groups [97]. Other hybrid nanofillers include rGO/TiO₂ [98], AgNPs/AgNWs (silver nanoparticles and silver nanowires) [99], CaCO₃/Mt (montmorillonite) [90], BT/GNS (barium titanate/graphene nanosheets) [100], TiO₂@MWCNT [101], and IL (ionic liquid) [102]. The synergistic effects are from the uniform dispersion, which favors the formation of the β-phase rather than the α-phase [98,100].

It is worth noting that the addition of nanofillers may decrease crystallinity, which may limit the enhancement of piezoelectricity [103]. Usually, the effects of nanofillers occur only when the nanofiller content is higher than a certain value (i.e., the percolation threshold). The percolation threshold of nanofillers is different depending on the filler type and process parameters.

Researchers have reached the following common conclusions regarding the functions of nanofillers: (1) the nanofillers may function as nucleate agents in the initial crystallization process, thus improving the β-phase formation; (2) the stress accumulation induced by the added fillers may supply energy for the α- to β-phase transformation during the stretching process; and (3) moderate electrical conductive nanofillers may form a conductive network, which can induce the dipole moments aligned during the subsequent poling process.

However, the improvement mechanism is still unclear, and the effects largely depend on the preparation process. The relationship between the nanofiller properties (size, conductivity, surface modification, dielectricity, type, etc.) is also unclear. The optimal contents of nanofillers of large specific surface areas (CNT, graphene, carbon, VGCF, etc.) for piezoelectricity enhancement are usually less than 1 wt%, while their effects on the improvement of conductivity and mechanical strength are limited due to their extremely low content. For other fillers, the optimal content may be over 20 wt%, while the enhancement effects may be weaker than those of large specific surface areas. Therefore, it must be considered by the combination of the practical processes. Sometimes, the reports from different researchers may have different results, even if the matrix and the nanofillers are the same. Thus, it needs more investigation.

3.2.2 Stretching

Stretching is usually applied to improve the α- to β-phase transformation and rearrange the β-phase molecular chains, which can enhance the voltage output capability dramatically. The parameters influence the crystallinity state including the stretching speed, elongation ratio, and temperature. The temperature is usually set above 50°C but lower than 140°C [23,104,105]. In the nonstretched samples, the crystal area is dominated by the α-phase (spherical structure). During the stretching process, the applied force induces the spherical structure change into the fibrillar-like structure, thus leading to the all trans-planar zigzag conformation (TT, the conformation of β-phase molecular chains), as shown in Figure 11 [104]. Meanwhile, the dipole moments also align along the normal direction of stretching [57]. The stretching ratio is an extremely important factor, and high stretching ratios can lead to sufficient elongation and complete crystal structure change. It has been reported that the elongation ratio should be higher than 4 in order to obtain nearly pure β-phase samples [22,23,104]. However, when the stretching ratio is too high, microcracks or defects may occur, which may lead to failures in poling due to the induced breakdown [104,106]. In the published reports, the optimal conditions are set at less than 80°C, and the elongation λ is about 5 [104]. A higher temperature or elongation ratio leads to more oriented polymer chains but lowers phase transition efficiency. Besides, the higher elongation ratio may break the stretching samples, while stretching may decrease the crystallinity degree, which is different from other reports [104].

Figure 11

SEM (scanning electron microscopy) micrographs obtained from samples stretched with different elongation ratios at 80°C: (a) 1; (b) 2; (c) 3; and (d) 4 [104].

The addition of nanofillers can significantly affect the stretching effects. The addition of 1 wt% CNF can increase the β-phase content to over 96% in stretched films. Meanwhile, the AC conductivity also increases significantly, which is attributed to the re-orientation of CNF [107]. It is widely reported that in the PVDF/nanofiller films, the α-phase is the minority part or does not even exist at a higher content [48,85,86,107,108]. Thus, stretching may not impact much on the phase transformation, while it still plays an important role in the realignment of molecular chains [109]. The added nanofillers may improve further transformation if there is still α-phase in the composites [107]. Similarly, the added nanofillers also function in the subsequent poling process to improve piezoelectricity. When the poling electric field is set at 90 MV/m, the piezoelectric coefficient d ₃₃ for nonstretched samples is 1.2 pC/N. For stretched pure PVDF films, it increases to 12.1 pC/N, and for stretched PVDF/OS(organosilicate) (4 wt%), it increases to 22.2 pC/N [110]. The effects of added nanofillers are affected by the nanofiller’s content obviously. When the content is lower than the threshold value, its effect is little. With the increase of the content, the nanofillers work as nucleate agents to induce β-phase formation. In the stretching process, the stress concentration induced by the added fillers can force the α- to β-phase transformation. However, excessive nanofillers may form agglomeration, hindering the molecular chain mobility and resulting in the crystal structure defects [18,23,48,68,85,86,87,88,89,90,91,92,93].

The influence of stretching on crystallinity depends on the practical process. It may increase crystallinity [111] or decrease it [104]. It is worth noting that stretching may also induce γ-phase formation, depending on the elongation ratio [111]. Although stretching is usually carried out at high temperatures, it is also possible at room temperature. It was found that during stretching, lamellae separation, the crystalline transformation from α- to β-phase, and disappearance of growth crystals during annealing coexist. The strain rate should be less than 0.034/s, and the stretching ratio is less than 100%. A higher strain rate and ratio may lead to deformation and breakage of lamellae structure [112].

According to the optical tensile stress microscopy tester (OTSMT) observation, the α-phase to β-phase transformation in stretching is described as follows: (i) first, a small black field is generated from the middle of the spherulite (crystal structure of the α-phase); (ii) the black field extends to the outside of the spherulite and connects with the neighboring black fields; and (iii) the larger deformation area is formed and brightened again, and the phase formation completes. The whole process is shown in Figure 12 [23]. Some researchers have declared that the phase transformation is carried out via an intermediate approach rather than a direct approach. The stress-induced lamellar fragments are recrystallized into defective α-crystals. Then, the newly formed α-crystal is disturbed, and the metastable β phase is formed under excess stress [105,113].

Figure 12

Schematic drawing of the transformation process from α-crystal to β-crystal of PVDF by mechanical stretching [23].

In conclusion, the functions of stretching include (1) improving the α- to β-phase transformation; (2) realigning the molecular chains as well as the dipole moments; and (3) increasing the crystallinity degree to some extent. The influence factors affecting stretching include (1) elongation rate; (2) temperature; (3) drawing speed; and (4) added nanofillers.

3.2.3 Heat treatment

Temperature plays an extremely important role in the crystal structure formation of PVDF films. The function of heat treatment includes solvent evaporation and an increase in the β-phase crystallinity degree. Therefore, proper heat treatment is an effective method to produce high β-phase fraction samples. There are three important parameters that influence the effects of heat treatment: temperature, operation time, and rate of ramping up and cooling [14]. Among these factors, the temperature is the most important. The effective annealing temperature ranges from 60 to 150°C [21,114,115,116,117]. Arshad et al. annealed PVDF thin films from 70 to 170°C, which shows 70°C is the favorable annealing temperature for PVDF thin films with high dielectric constant, low tangent loss, and high resistivity [21]. Similar reports also indicate that an annealed temperature below 100°C is suitable for the β-phase formation [19,74,114,117,118,119,120,121]. In annealed PVDF-TrFE (70/30) samples, if the annealed temperature is 100°C, the high fraction crystalline β-phase with a rod-like structure can be induced, and the remnant polarization P _r reaches 94 mC/m². A higher annealing temperature (especially near the melting point) induces the elongated needle-like crystal majority, which leads to the crystallinity decrease and the functional electric properties, as shown in Figure 13 [19]. The phase evolution depends on the temperature. When the annealing temperature is too high (>140°C), the amorphous region (no contribution to piezoelectricity) [19] may increase accompanied by the defects formation [21,117,122] and decrease of the β-phase fraction [115,119]. In some reports, heat treatment at high temperatures even leads to the formation of γ-phase due to the interaction between molecular chains and the added nanofillers [118,123]. Besides, the dielectric constant of samples annealed at 80°C and quenched at 20°C is higher than 60, which is much higher than that in other reports [120]. Annealing may also affect the molecular chain alignment. The thermal energy can promote the molecular chains to realign their orientation and position after cooling, which can increase the crystalline degree. For PVDF-TrFE, when the annealing temperature is too high (above the melting temperature T _m), the β-phase crystallinity will be reduced, while the α- and γ-phase crystallinity increases. Hence, the annealing temperature should be set between T _c and T _m [14,124].

Figure 13

FESEM (field electron scanning electron microscope) images of annealed PVDF-TrFE samples at (a) 100°C and (b) 140°C; (c) an enlarged section of (b); (d) an enlarged section of (b); (e) dependence of the degree of the crystalline structure as a function of annealing temperature [19].

The addition of nanofillers also affects heat treatment. In nanofiller-containing composites, both the annealing temperature and nanofiller content affect the crystal structure [114,117]. High-temperature annealing can increase more IL insert into the amorphous region of the polymer matrix to induce phase transformation while promoting IL removal. As a result, the piezoelectric coefficient d ₃₃ can be increased by 50%. Interestingly, crystallinity can be increased by annealing, which is different from the results of other reports. It may be explained that the annealing treatment can improve the PVDF chains in the amorphous area to entirely flop and form more polar crystals [114]. Another function of heat treatment is the reduction of void defects [125].

Although annealing may improve transformation from nonpolar α-phase to polar β- and γ-phase at the initial crystallization and facilitate the PVDF chains in the amorphous region to form more polar phases, it is limited by the side effect of the decrease of crystallinity degree [126].

Annealing should be applied between the Curie temperature (T _c) and the melting temperature (T _m), and thus, the thermal energy promotes the polymer chain rearrangement of their orientation and position, which leads to a higher crystallinity degree in the subsequent cooling process [19]. The operation time is different from each other, which ranges from 10 min to 48 h [19,114,115,117,118,119,120,122,126].

Besides conventional thermal annealing methods, UV-annealing (ultraviolet annealing) has also been developed. The mechanism of UV-annealing is illustrated in Figure 14. This technique exposes the spun PVDF film to a broad spectrum light for microseconds (250–400 µs) to induce the transformation and avoid chemical degradation during long exposures (>400 µs). The optimal exposure time is confirmed as 350 µs, at which the annealed films reach the maximum P _r of 5.4 µC/cm² and a coercive field of around 120 MV/m. The phase transition from α-phase to β-phase is confirmed by FTIR and polarized optical microscopy (POM) imaging [127]. Similar results have been reported by Anand et al. In their experiments, by exposure to ultraviolet-visible light, the output voltage is increased by 13.8% [128].

Figure 14

Schematic representation of the UV-annealing process of PVDF thin films [127].

3.2.4 Poling

As mentioned above, the addition of nanofillers and stretching can induce a high β-phase fraction from evaporation and crystallization without poling [129,130]. While the dipole moments are randomly distributed in the matrix, which leads to the mutual offset in the whole material, and thus, the whole piezoelectricity does not exist. In order to lead the dipole moments in a certain direction to achieve the best piezoelectricity, poling is applied. The applied electric field during poling can force the dipole moments to align along the electric field direction (normal to the film surface), leading to the maximum whole piezoelectricity. The saturation poling electric field is about 200 MV/m, which is hard to achieve since breakdown occurs when electricity exceeds a critical value depending on the process [131]. In the published reports, the maximum electric field is usually set at lower than 160 MV/m [93,97,130,132,133]. The electrodes attached to the films for poling also affect the maximum electric field. Ye et al. applied a poling electric field of 160 MV/m on the Ag electrode (t = 1 µm) sandwiched PVDF films (t = 50 µm) at 90°C for 1 h and then cooled to ambient temperature with the remaining electric field. During the high electric field poling, the negative electric charge implanted into the PVDF film and the diffusion of Ag atoms lead to the formation of the organo-complex F–C–Ag structure. The structure can improve the adhesion capability between the Ag electrodes and PVDF films as well as the breakdown strength. This work proposes a method for applying ultrahigh electric fields in the poling process [132]. Besides, continuous poling is not a moderate method for high electric field poling. Accordingly, some poling strategies have been developed. Stepwise poling is a widely used technology in the preparation of PVDF or its copolymer-based piezoelectric films [134].

For extremely thin films, the poling electric field may be much higher. Hartono et al. applied a 2 GV/m electric field for poling of annealed PVDF films with a thickness of 1 µm prepared by a deep coating method. By XRD characterization, the β-phase fraction reaches 83%. Due to extremely small thickness, the applied voltage is only 2 kV [135].

The poling time also plays an important role in the poling effects. If the poling time is not enough, the dipole moment realignment cannot be finished. However, excessive poling time may lead to structural defects [136] or even failures.

During corona poling, the PVDF films are exposed to an electrically charged needle and a grid electrode generating a high electric field under a heated environment, as shown in Figure 15 [130]. Kim et al. fabricated piezoelectric PVDF films by integrating fused deposition modeling 3D printing and the corona poling technique. Compared to traditional fabrication methods, in this novel work, poling is carried out in the 3D printing process rather than post-poling. The β-phase fraction of the prepared PVDF films and the generated output current are dramatically increased. In contrast, the film without poling generates almost no current. Similar results have been observed in PVDF/Ba₃TiO₃ or PVDF-TrFE/rGO composites [93,129,138].

Figure 15

Setup schematic of stretching and corona poling experiments [130].

Two-step poling (stretching and poling are separated) is widely used as well as the simultaneous poling method. Lee et al. developed the electric poling-assisted manufacturing (EPAM) to directly print PVDF films in a single step, which is illustrated in Figure 16. The molten PVDF solution is extruded from the nozzle tips (stretching) and the dipole moment alignment is realized by the applied strong electric field between the nozzle tip and the printing bed (poling). The poling effects can be controlled by adjusting the printing speed, melting the filament by heaters, and adjusting the applied voltage [137]. However, the application of simultaneous poling and stretching is more complicated. Although it may reduce the preparation time, it can increase the failure risk when a high electric field is applied to the solution.

Figure 16

(a) Contact poling; (b) corona poling; and (c) a schematic of the EPAM process. Stretching and poling are finished in a single step [137].

For electrospun fibers, poling is usually accompanied by an electrospinning process rather than an independent poling process, which is called in situ poling. Poling is finished by the electrospinning voltage, as discussed in the Electrospinning section [133].

Another effect of poling is the improvement of α- to β-phase transformation. As discussed above, the addition of nanofillers and stretching can greatly improve the β-phase fraction. Therefore, poling has little effect on the stretched samples containing high nanofiller content, while it still enhances the β-phase transformation for those samples containing a low content of nanofillers. The improvement is attributed to the dipolar intermolecular interactions between the surface functional groups on the nanofiller and fluorocarbon. In contrast, excessive nanofillers may decrease the β-phase content due to charge accumulation at the interfacial boundaries between the α-phase (or amorphous phase) and nanofillers and the ferroelectric β-phase. The charge accumulation is proportional to the composite conductivity caused by electrical conductive nanofillers [3,93].

3.2.5 Nanofiller alignment induced by the magnetic field or electric field

In most circumstances, nanofillers are randomly dispersed in the PVDF matrix. However, in certain working modes (i.e., 31 mode), the generated voltage is dramatically affected by the alignment of molecular chains. As a result, some technologies have been developed to improve the nanofiller alignment along the stretching direction. Magnetic field-induced nanofiller alignment has been reported by Jiang et al. CoFe₂O₄ nanoparticles are introduced into PVDF under a DC magnetic field to generate piezoelectric materials with a high β-phase content by the solution casting method as illustrated in Figure 17. The β-phase content can be further increased to 95% by applying DC magnetic fields during the solution casting process, while the samples without DC magnetic fields only exhibit an F(β) of 84%. This result indicates that the application of the magnetic field may improve the β-phase formation and α- to β-phase transformation. The mechanism is attributed to the tensile stress at the CoFe₂O₄/PVDF interfaces created by the magnetostriction effect [54]. Besides, the magnetic field can also affect the phase formation on pure PVDF. The formation of the β-phase is attributed to the magnetic-field-induced tensile stress on the diamagnetic PVDF. It is worth noting that the magnetic-field-induced stress applied on the molecular chains is much weaker than that of stretching, indicating that the α- to β-phase transformation is limited. The magnetic field also results in the increase of the crystallinity degree, which can be explained as follows: (a) the tensile strain increases with the magnetic field, which leads to the linear expansion of molecular chains and thus the increase of crystallinity; and (b) on the other hand, the overload magnetic field may suppress the crystallization process. Therefore if (a) is stronger than (b), the magnetic field may lead to a higher crystallinity degree [139]. Up to now, the nanofiller alignment in PVDF or its copolymers by an electric field has not been reported yet. Theoretically, the nanofiller alignment in the matrix may improve piezoelectricity as discussed above. Similar results on nanofiller alignment in epoxy have been reported. Ladani et al. used an alternating current (AC) electric field to align carbon nanofibers (CNFs) in the epoxy matrix. Rotation and alignment of CNFs excited by the applied AC electric field were observed during the curing process of the epoxy, leading to the formation of a chain-like structure. These results demonstrate the promising application of the AC electric field to realign nanofillers in the PVDF matrix [140]. Similar results have been reported by Prasse et al., by which the interaction forces between particles and the external electric field are demonstrated [141].

Figure 17

Schematic illustration of the formation of the β-phase in the PVDF/CoFe₂O₄ composites prepared (a) without and (b) with a DC magnetic field [54].

3.2.6 Summary

To improve the β-phase formation and α- to β-phase transformation, a number of different methods are generally needed to achieve the best results. An individual process is not independent of other processes. For example, it is clear that the high electrical field poling may force the α–β phase transformation, while if the samples are stretched, the α-phase content is extremely low, so the phase transformation may not occur in the poling process. In addition, the poling electric field is limited as the ultrahigh poling electric field will increase the failure risk in the poling. Stretching is an effective method to produce β-phase conformation, while it may also cause problems such as crystal defects and decrease of crystallinity degree. Table 4 summarizes the combined processes in the fabrication of PVDF piezoelectric materials. It is worth noting that although most investigations focus on the formation or transformation of the β-phase, the transition from β-phase to α-phase or γ-phase also exists. Pramanick et al. performed a direct investigation on the phase transformation from β-phase to α-phase. Through in situ XRD and quasielastic neutron scattering measurements at high temperatures, the results demonstrate the structural and dynamical mechanisms for β-phase to α-phase transformation. The transition can be divided into two steps: (i) the nucleation and growth of an intermediate γ-phase; and (ii) the γ-phase to α-phase transformation [142]. Sharma et al. investigated the process conditions (annealing, poling, and mechanical rolling) on the dielectric and ferroelectric properties of PVDF films, which shows that all adopted treatments induce α- to β-phase transformation and β-phase formation. In pure PVDF films, only α-phase exists. In the mechanically rolled samples, the maximum β-phase fraction reaches ca. 85%. The combination involving mechanical rolling followed by poling exhibits a much higher energy density [143]. Tang et al. investigated the effects of nanofillers (CNFs), stretching, and recrystallization on the microstructure, and phase transformation and dielectric properties in PVDF nanocomposites. The experimental results showed that overdosed CNFs (≥1 wt%) may result in lower β-phase fraction, reduced grain size, and a secondary aggregation. Both phase transformation and nanofiller dispersion may affect the dielectric properties dramatically. The isothermal recrystallization reduces the β-phase fraction, which may greatly affect the properties of PVDF [144].

Table 4

Summary of the preparation process of PVDF and its copolymer-based piezoelectric materials

Matrix/filler	Preparation method	Stretching	Poling	Heat treatment	β-Phase content or piezoelectric coefficient	Ref.
P/CNT	SC + HP	R: 4, T: 90°C, ν: 1 mm/s	T: 100°C, t: 30 min, E: 150 MV/m		F(β): P/CNT(0.2 wt%) – 84% (drawn), 96% (drawn and poled), <10% (undrawn and poled); d ₃₃: P/CNT(0.2 wt%) – 23 pC/N (drawn and poled), 5 pC/N (undrawn and poled)	[3]
P/CNT	SC	ν: 1 mm/s,	E: 150 MV/m, T: 100		d ₃₁: P/CNT(0.2 wt%) – 8.8 pC/N, P – 8 pC/N	[18]
P	SC	R: 6.4, T: 145°C			F(β): 98%	[22]
P	HP	R: 3, T: 100°C			F(β): >95%	[23]
P/CoFe₂O₄	SC				F(β): P – 73%, P/CoFe₂O₄(5 wt%) – 84%, P/CoFe₂O₄(5 wt%) – 95% (magnetic field applied)	[54]
P/CNT	ES				d ₃₃: P/CNT(0.5 wt%) – 18.8 pC/N, P – 15.2 pC/N	[68]
P/rGO	ES			(1) T: 80°C, t: 2 h; (2) T: 140°C, t: 1 h	F(β): P/rGO(2 wt%) – 87%, P – 82%; P _d: P/rGO(2 wt%) – 28 mW/m², P – 0.19 mW/m²	[85]
P/rGO	SC			T: 70°C, t: 48 h	F(β): P/rGO(0.1 wt%) – >95%, P – 37%; d ₃₃: P/rGO(0.1 wt%) – 39.3 pC/N, P – 22 pC/N	[86]
P/PFOES-rGO	SC			T: 70°C, t: 48 h	d ₃₃: P/PEOES-rGO(0.3 wt%) – 39.8 pC/N, P – 22 pC/N	[89]
PH/CB	SC	R: 4–5, T: 60°C	E: 90 MV/m		F(β): PH/CB(0.5 wt%) – >80%, P – 59% (nonstretched and nonpoled); V _o: PH/CB(0.5 wt%) – >3.68 V, PH – 1.80 V	[92]
PT/rGO	Coating		V: 3 kV(grid voltage), t: 5 min (corona poling)	T: 140°C, t: 1 h	d ₃₃: PT/rGO – 34 pC/N, PT – 23 pC/N	[93]
P/PZT	SC			T: 80°C, t: 5 h, rapidly quenched: 20°C	F(β): P/PZT(30 vol%) – >75%, P – 69%; d ₃₃: P/PZT(30 vol%) – 84 pC/N, P – 32 pC/N	[94]
P/rGO/TiO₂	SC + HP				F(β): P/rGO(2.5 wt%)/TiO₂(2.5 wt%) – >75.7%, P – 70.4%	[98]
P	SC	R: 5, T: 80°C, ν: 1 mm/min	200 MV/m		F(β): 80% (stretched samples), 84% (stretched and poled samples)	[104]
P/IL	SC			(1) T: 35°C, t: 30 min; (2) T: RT/70°C, t: 2 h	F(β): 92.4%, 60% (annealed at room temperature in the second step); V _o: P/IL(10 wt%, 70°C) – >2.5 V, P/IL(10 wt%, RT) – >2.1 V	[114]
P/CoFe₂O₄	SC	R: 5, T: 80°C, ν: 3 mm/min	T: 80°C, t: 30 min, E: 50 MV/m	T: 210°C, t: 10 min	F(β): P/CoFe₂O₄(5 wt%) – 47.1% (poled), 78.5% (stretched), 29.5% (annealed)	[115]
P/SiC	SC			T: 140°C/80°C, t: 2 h		[117]
P	ES			T: 100°C, t: 4 h	F(β): 87.4%, 58.3% (without annealing)	[121]
P/rGO	SC			UV-annealing	V _o: P – 0.91 V, P/rGO(11 wt%) – >2.18 V, P/rGO(11 wt%, without annealing) – >1.92 V	[128]
P	Commercially	Simultaneous stretching and corona poling: T: 80°C, t: 45 min, R: 4, V: 6 kV			F(β): 83%; d ₃₃: 34.3 pC/N, 1.7 pC/N (without poling), 25.3 pC/N (t: 15 min)	[130]
P	HP	R: 4 (rolling)	T: 100°C, E: 50 MV/m	T: 100°C, t: 3 h	F(β): 46% (poled), 60% (annealed), 85% (rolled), 78% (rolled + poled)	[143]

d ₃₁: piezoelectric strain constant; E: electric field; ES: electrospinning; F(β): relative β-phase fraction; HP: hot press; IL: ionic liquid; P: PVDF; PH: PVDF-HFP; PT: PVDF-TrFE; P _d: power density; R: elongation ratio; RT: room temperature; SC: solution casting; t: operation time; T: temperature; ν: stretching speed; V: voltage; V _o: open-circuit voltage.

In general, the improvement of α- to β-phase transformation and β-phase formation can be realized by several methods. For the preparation of high piezoelectric PVDF or its copolymers-based materials, the preparation process should be determined by concrete conditions and the optimal parameters can be confirmed by the experiments. Based on the above-mentioned and discussed points, a brief summary of the effects of the applied methods is summarized in Table 5.

Table 5

Effects of different methods for improving the phase evolution in PVDF or its copolymer-based piezoelectric films

Methods	Positive effects	Negative effects	Typical parameters
Filler addition	(1) β-phase formation (nucleate agent and charge accumulation in poling) [3,18,68]; (2) lower electric field for poling (synergistic effects of hybrid fillers) [97,98,100]; (3) increase of crystallinity degree(uniform distribution) [83]	(1) Crystal defects (filler aggregation) [83]; (2) failure in poling process (higher conductivity or crystal defects).	Content: <1 wt% (large specific surface area)
Stretching	(1) α- to β-phase transformation (induced stress) [104]; (2) dipole moments alignment (induced stress) [57]; (3) crystallinty degree increase (realignment of molecular chains in the amorphous area) [20]	(1) Crystallinity decrease (chain orientation accompanied with the phase transformation) [104]; (2) crystal defects (higher elongation ratio) [104,106]	T _s < 100°C, R > 4.
Heat treatment	(1) β-phase formation (thermal energy for the CF₂ group rotation and molecular chain mobility) [19,74,114,117,118,119,120,121]; (2) crystallinity degree increase (realignment of molecular chains’ orientation and position by thermal energy) [14,24,114]	(1) Formation of γ-phase (Easier motion of conformers) [118,123]; (2) crystallinity degree increase (amorphous region expansion accompanied with the defects formation) [19]	T < 140°C
Poling	(1) Dipole moments alignment (induced stress by the electric field)[3,131]; (2) α- to β-phase transformation (dipolar interactions between the surface functional groups on the nanofillers and fluorocarbon)[3,93]	Electric breakdown (high electric field/high temperature)	E _max < 160 MV/m
Filler alignment	(1) Molecular chains alignment (induced stress by the magnetic/electric field) [139,140,141]; (2) β-phase formation and α- to β-phase transformation (tensile stress at the interfaces caused by the magnetostriction effect) [54]; (3) crystallinity degree increase (linear expansion of molecular chains by the tensile strain) [139]	Crystallinity degree (the suppression of the crystallization process at high magnetic fields) [139]	Lack of enough experimental data

4 Conclusion

PVDF and its copolymers are important piezoelectric materials of great promise. However, their applications are limited due to the relatively lower piezoelectricity. As a result, many efforts have been made to improve the piezoelectricity, thereby expanding their application areas. The preparation methods include solution casting, electrospinning, hot press, etc. In order to improve the formation of piezoelectric β-phase and the α- to β-phase transformation, processes such as stretching, added fillers, heat treatment, poling, and nanofiller alignment by electric field/magnetic field have been developed, and the influence of the parameters has been investigated. However, as the crystallinity evolution mechanism is still not clear, therefore further work is required.

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Funding information: This work was financially supported by the National Natural Science Foundation of China (No. 51703015) and Fundamental Research Funds for the Central Universities (No. 2020CDJQY-A008).
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.

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Received: 2021-07-24

Revised: 2021-09-11

Accepted: 2022-02-17

Published Online: 2022-03-25

This work is licensed under the Creative Commons Attribution 4.0 International License.

Recent advances in the preparation of PVDF-based piezoelectric materials

Abstract

1 Introduction

2 Properties of PVDF and its copolymers

2.1 Microstructure of PVDF

2.2 Characterization of the PVDF crystal structure

2.2.1 XRD

2.2.2 FTIR

2.2.3 DSC

2.3 Phase transformation

2.4 Simulation of PVDF molecular chains

2.5 PVDF-HFP

2.6 PVDF-TrFE

3 Preparation of PVDF and its copolymer-based piezoelectric composites

3.1 Initial crystallization

3.1.1 Solution casting method

3.1.2 Hot press

3.1.3 Electrospinning

3.2 Improvement of the β-phase formation

3.2.1 Addition of nanofillers

3.2.2 Stretching

3.2.3 Heat treatment

3.2.4 Poling

3.2.5 Nanofiller alignment induced by the magnetic field or electric field

3.2.6 Summary

4 Conclusion

References

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