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Article

Fabrication of PVTF Films with High Piezoelectric Properties Through Directional Heat Treatment

by
Xin Xin
,
Aotian Yee
,
Zhiyuan Zhou
,
Xuzhao He
,
Wenjian Weng
,
Chengwei Wu
* and
Kui Cheng
*
State Key Laboratory of Silicon and Advanced Semiconductor Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
*
Authors to whom correspondence should be addressed.
J. Compos. Sci. 2024, 8(12), 512; https://doi.org/10.3390/jcs8120512
Submission received: 23 October 2024 / Revised: 27 November 2024 / Accepted: 4 December 2024 / Published: 6 December 2024
(This article belongs to the Special Issue Feature Papers in Journal of Composites Science in 2024)
Browse Figures
Figure 1
<p>(<b>a</b>) Schematic diagram of P(VDF-TrFE) obtained by the heating table heat treatment and the SEM images of P(VDF-TrFE) of different thicknesses including (<b>b</b>) 50 μm, (<b>c</b>) 100 μm and (<b>d</b>) 150 μm.</p> "> Figure 2
<p>Composition and crystallization analysis of films of different thicknesses. (<b>a</b>) XRD image, (<b>b</b>) FTIR image, (<b>c</b>) DSC image, (<b>d</b>) schematic diagram of crystallization of films of different thicknesses.</p> "> Figure 3
<p>(<b>a</b>,<b>b</b>) d<sub>33</sub> analysis of films of different thicknesses and (<b>c</b>) contact angle images.</p> "> Figure 4
<p>SEM images of different temperature: (<b>a</b>) 160 °C, (<b>b</b>) 180 °C.</p> "> Figure 5
<p>Composition and crystallization analysis of films of different temperature. (<b>a</b>) XRD image, (<b>b</b>) FTIR image, (<b>c</b>) DSC image (the sample data for the heat treatment temperature of 200 °C here is the same as above), (<b>d</b>) schematic diagram of crystallization of films of different temperature.</p> "> Figure 6
<p>(<b>a</b>,<b>b</b>) d<sub>33</sub> analysis of films of different annealing temperatures and (<b>c</b>) contact angle images (the sample data for the heat treatment temperature of 200 °C here are the same as above).</p> "> Figure 7
<p>(<b>a</b>) FTIR image and (<b>b</b>) DSC image of a 100 μm film obtained by the muffle furnace, (<b>c</b>) melting point between the upper and lower surfaces of a 100 μm film obtained by heating table treatment, KPFM images of (<b>d</b>) 100 μm film obtained by the muffle furnace, (<b>e</b>) the worst-performing sample made from a heating table and (<b>f</b>) the best-performing sample made from a heating table; (<b>g</b>) schematic diagram of crystallization of films obtained from muffle furnace and heating table.</p> ">
Versions Notes

Abstract

:
Piezoelectric materials can realize the mutual conversion of mechanical energy and electric energy, so they have excellent application prospects in the fields of sensors, energy collectors and biological materials. The poly(vinylidene fluoride) (PVDF)-based polymers have the best piezoelectric properties in the piezoelectric polymer, but they still have a large room for improvement compared with the piezoelectric ceramics. Improving their content of the polar β phase has become a consensus to polish up the piezoelectric performance. Most available studies construct hydrogen bonds or coulomb interactions between the surface of the dopant and molecular chains by doping, which promotes the molecular chains arrangement and thus facilitates the formation of the polar β phase. Recent studies show that the ordered arrangement of molecular chains is also important for piezoelectric properties. At present, the main way to improve the piezoelectric performance of PVDF is through doping or complex heat treatment process. Here, the poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) film was treated by directional heat treatment which used a heating table. Compared with uniform heat treatment like muffle furnace heat treatment, this simple vertical temperature gradient has many advantages for the content of the β phase and the crystallinity of P(VDF-TrFE). The results of the experiment showed that the content of the β phase of films remained at about 88%. When the film thickness was limited to 100 μm and the heat treatment temperature was limited to 200 °C, its crystallinity could reach 75% and the highest piezoelectric coefficient could reach 33.5 ± 0.7 pC/N. P(VDF-TrFE) films based on the experimental methods described above that show great potential for future applications in electronic devices and biomedical applications.

1. Introduction

Piezoelectric materials can directly convert mechanical energy and electrical energy in both directions, so they have great application prospects in wearable electronic devices [1,2,3,4], sensors [5,6], energy harvester [7,8], biomedical materials [9,10,11,12] and other fields. Among them, piezoelectric polymers have been extensively studied due to lightness and flexibility. Poly(vinylidene fluoride) (PVDF), as the polymer with the most excellent piezoelectric properties [13], has a small piezoelectric response (e.g., |d33| < 30 pC/N) [14] compared with common piezoelectric ceramic materials (|d33| > 100 pC/N) [15,16] such as barium titanate (BaTiO3) and lead zirconate titanate (Pb(Zr,Ti)O3), which hinders its further application, so it is important to improve its piezoelectric performance.
In the case of PVDF applications, high piezoelectric properties can not only improve the sensitivity of its detection and energy harvesting efficiency [17,18] but also provide the adequate electrical stimulation required for biomedicine [19]. Therefore, in order to improve the piezoelectric properties of PVDF-based polymers, various methods have been adopted, such as heat treatment process [20,21,22,23], doping [17], stretching [24], etc. At present, the widely used method is to introduce a trifluoroethylene (TrFE) monomer to prepare a P(VDF-TrFE) copolymer. Due to the introduction of structural distortion, the TTT conformation shifts to the (TG)3 conformation [25], and the resulting structural defects are more conducive to polarization with the help of external fields and increase the piezoelectric response.
The content of the polar β phase is crucial for piezoelectricity. Most studies have used doping to increase the content of the polar β phase [26,27]. Liu et al. [1] incorporated sonicated Galn nanodroplets into P(VDF-TrFE) film, and the charge on the surface of the droplets had a coulombic effect with the P(VDF-TrFE) molecular chains, which in turn contributed to the formation of polar β phases in P(VDF-TrFE) film. Min et al. [28] doped BaTiO3 particles in P(VDF-TrFE); the content of polar β phases was improved by the electrostatic interaction between the nanostructures and the dipoles of polymer chains.
However, the methods mentioned above reduce the original flexibility of the polymer film. Therefore, exploring how to improve the piezoelectric properties of pure PVDF-based polymers has also become a research hotspot. It was found that appropriate adjustment of the preparation process can effectively promote β phase generation. Electrospinning or 3D printing technology causes the dipoles to align with the direction of the electric field by applying an electric field during the preparation process [21,29]. Huang et al. [24] obtained a pure β-phase PVDF film by applying biaxial stretching and high electric field polarization to the PVDF film, which exhibited better piezoelectric properties.
In addition, the piezoelectric properties of PVDF-based polymers are also affected by the degree of the order of the molecular chains. On the one hand, the order of the molecular chains helps the alignment of the dipoles, thus granting the film higher piezoelectric properties; on the other hand, the arrangement during the crystallization process is helpful to reduce the crystallization barrier, thereby promoting crystallization. Common methods include the application of an electric field to induce crystal orientation [30] and the construction of nanoconfinement by the template method [31,32]. At present, it has been found that good orderliness can be obtained more effectively by epitaxial growth of PVDF-based polymers on specific substrates. P(VDF-TrFE) growing on shape-tailored semiconductor rubrene single crystals [33] and graphene/high-index copper [34] exhibited a highly ordered arrangement, and they both showed good ferroelectric properties. In order to obtain PVDF-based polymers with high piezoelectric properties, it is necessary to design a reasonable preparation scheme from the aspects of the β phase and the order of the molecular chain.
Here, P(VDF-TrFE) was selected as the research object in pursuit of more efficient piezoelectric performance. Controlling the annealing process of P(VDF-TrFE) is essential for the generation of polar β phase crystals. A heating table heat treatment was used to replace the traditional muffle furnace heat treatment. The properties of the film were characterized by a series of morphology, composition and crystallization tests. The piezoelectric performance was visually demonstrated by piezoelectric coefficients, surface potentials, etc. In this study, a P(VDF-TrFE) film with a high piezoelectric coefficient (33.5 ± 0.7 pC/N) was obtained, which is a new choice for preparing electronic devices and medical materials with excellent performance.

2. Materials and Methods

2.1. Materials

The P(VDF-TrFE) (70/30) powders were purchased from Piezotech (Paris, France). The N,N-dimethylformamide (DMF) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Preparation of PVTF Films

P(VDF-TrFE) films were prepared by the tape-casting method. First of all, 1.82 g P(VDF-TrFE) powders were dissolved in 10 mL of DMF solution with magnetic stirring at room temperature. After the solution was clear and transparent, the solution was applied to the surface of the clean and dry glass plate using a film applicator, and the thickness of the film was pre-adjusted by adjusting the height of the squeegee of the film applicator. The glass plate was then placed in a vacuum drying oven and dried at 37 °C for 24 h to fully volatilize the DMF solvent. The glass plate was then immersed in deionized water to separate the film, and the resulting film was cut to obtain a sample of 1 × 1 cm2 for subsequent processing.
The annealed P(VDF-TrFE) film was obtained by heating table (JF-976A, JFTOOIS, Dongguan, China) heat treatment. The cut film was placed on a quartz substrate and pressed tightly. The effects of the film thickness including 50 μm, 100 μm and 150 μm (the preparation error was controlled within ± 5 μm) and the annealing temperature including 160 °C, 180 °C and 200 °C on the piezoelectric properties of the films were studied. The annealing time was uniformly set at 5 min to ensure the complete melting of the films. The cooling process was set to cool along with the heating table. The obtained film was contact-polarized, and finally the silicone oil was washed away by detergent, deionized water and cleaning alcohol. Finally, P(VDF-TrFE) films with different thicknesses and different piezoelectric responses were obtained.
The comparison samples were obtained as previously studied [35] in the muffle furnace. The dried P(VDF-TrFE) film was obtained using the same drying method and drying parameters as described above. Then the films were put into the muffle furnace with setting the annealing temperature to 210 °C and the annealing time to 1 h. The films were immersed in deionized water for separation, then were cut and polarized, and finally P(VDF-TrFE) films whose thickness were about 100 μm were obtained.

2.3. Characterization of Materials

By characterizing the morphology, structure and properties of different materials, the changes in the piezoelectric response of materials under different heat treatment conditions and the reasons behind them were studied. Field emission scanning electron microscopy (FESEM, SU-70, Hitachi, Tokyo, Japan) was used to observe the surface and cross-sectional topography of the films. X-ray diffractoscopy (XRD, X-Pert Powder-17005730, PANalytical B.V., Almelo, The Netherlands) with Cu Kα radiation was used to analyze the phase composition of films. At the same time, the Debye–Scherrer formula [36] was used to calculate the grain size,
D = K γ B cos ⁡ θ
where K is the Scherrer constant (half-width was calculated, so K = 0.89), γ is the X-ray wavelength (γ = 1.54 Å under experimental conditions), B is the half-width of the sample diffraction peak (the characteristic peak of β phase located in 19.9° for the diffraction peak), and θ is the Bragg diffraction angle.
The content of the polar β phase was measured using a Fourier transform infrared spectrometer (FTIR, Vertex 70, Bruker, Ettlingen, Germany). The content of the β phase was calculated by using the following formula [22].
F ( β ) = A β K 840 c m − 1 K 763 c m − 1 A α + A β
where A α represents the absorbance of α phase at 763 cm−1, A β represents the absorbance of β phase at 840 cm−1, and K 763 c m − 1 and K 840 c m − 1 represent the absorption coefficients at 763 cm−1 and 840 cm−1 (where K 840 c m − 1 = 7.7 × 104 cm2/mol, K 763 c m − 1 = 6.1 × 104 cm2/mol).
Differential scanning calorimetry (DSC, 2500, TA Instruments, New Castle, DE, USA) was used to measure the crystallinity of the films. The crystallinity was calculated by using the following formula [37],
X c = ∆ H m ∆ H m 0
where ∆ H m 0 is the standard melting enthalpy of the crystallization of P(VDF-TrFE) (70/30) ( ∆ H m 0   = 38 J/g [38]), and   ∆ H m represents the actual measured melting enthalpy.
A piezoresponse force microscope (PFM, NX 10, Park, Suwon, Korea) was used to measure the piezoelectric coefficient of the sample. Voltage of 1 ~ 10 V was applied to the tip at a frequency of 17 kHz, thus, in exchange for a local piezoelectric deformation in the z-direction.
The piezoelectric coefficient (d33) of the film was tested at a low-frequency constant force (0.25 N) using a quasi-static meter (ZJ-3AN, Institute of Acoustics, Chinese Academy of Science, Beijing, China).
The contact angle instrument (WCA, OCA 20, Dataphysics, Filderstadt, Germany) was used to measure the wettability of the sample.
A multifunctional nano-infrared spectrometer was used (NanoTA, nanoIR2-fs, Anasys Instruments, PA, USA) to characterize the crystallization differences between the upper and bottom surface of the film.
Kelvin probe force microscopy (KPFM, Dimension icon XR, Bruker, Billerica, MA, USA) was used to measure the surface potential of the film.

2.4. Statistical Analysis

All data were expressed as mean ± standard deviation. Statistical analysis was determined via Tukey’s test and one-way analysis of variance. The criterion for statistical significance was * p < 0.05, ** p < 0.01 and *** p < 0.001.

3. Results and Discussion

3.1. Adjustment of Thickness to Improve the Directional Heating

A heating table was used instead of a muffle furnace to treat the P(VDF-TrFE) film. The specific schematic diagram of heat treatment is shown in Figure 1a. The metal plate on the surface of the device provides the required heat. In order to prevent the film from sticking to the metal surface, the quartz substrate was placed at the bottom of the film for heat treatment. The heat treatment temperature was first set to 200 °C and the heat treatment time was 5 min. The films with the thicknesses of 50 μm, 100 μm and 150 μm were investigated successively, aiming to study the effects of the temperature gradient. The SEM images of the plane and section of different films are shown in Figure 1b–d. With the increase in the film thickness, the morphology of the cross-section remained unchanged, showing a sheet structure. At the same time, the fiber morphology of the surface of the films was more obvious with the increase in thickness. The possible reason is that the increased thickness makes the crystallization of the air surface of the film faster, which in turn leads to more nucleation sites. This results in a more dense change in fiber morphology.
Then the composition and crystallization of the film were analyzed, and their influence on the piezoelectric properties was studied. Among the five polymorphs of P(VDF-TrFE), the β phase shows the largest electric dipole moment [39], so effectively increasing the content of the β phase can improve the piezoelectric performance. Therefore, the influence of heat treatment process on the content of the β phase was first explored. As shown in Figure 2a, the XRD image can well prove that the β phase was obtained in a high level by the heating table heat treatment process, and the characteristic peak of the β phase is more obvious with increasing thickness. The most pronounced characteristic peak at 19.9° is attributed to the (110)/(200) composite plane of the β phase [18]. The characteristic peaks at 35.2° and 40.8° are attributed to the (001) and (201) planes of the β phase, respectively. In addition, the grain size was calculated according to the Debye–Scherrer formula. The grain sizes of 50 μm, 100 μm and 150 μm films are 8.5 nm, 13.1 nm and 11.7 nm, respectively, which shows a trend of increasing first and then decreasing. The 50 μm film was less affected by the temperature gradient control, so the grain is minimal. The 100 μm film may be affected by temperature gradient control for a long time, so the crystal growth time is long and the nucleation number is low, which makes the grain size maximized [40]. Due to the increase in thickness, the crystallization speed of the air surface of the film is faster, so there are more nucleation sites. This results in smaller grains. To further obtain the content of the β phase, Fourier infrared measurement was performed, as shown in Figure 2b. The absorption peaks belonging to the α phase were also found to be very weak. With the help of the formula, the films of different thicknesses were measured to obtain the content of the β phase. The average content of the β phase of 50 μm, 100 μm and 150 μm films reached more than 88%, indicating that the P(VDF-TrFE) film with high content of the β phase could be obtained by heating table heat treatment process.
Then, the crystallinity of the films was analyzed. The differential scanning calorimeter analysis of the films was performed. Figure 2c shows that the melting point of the films increases as the thickness increases, indicating that the crystallinity of the films increased. Similarly, with the help of the formula, we measured films of different thicknesses to obtain the crystallinity. The mean crystallinity of 50 μm, 100 μm and 150 μm films was 70.9%, 74.5% and 73.0%, respectively, showing a trend of rising first and then decreasing. The possible reason is the facilitating effect of the vertical temperature gradient. The effect of horizontal temperature gradient on directional crystallization of polymers has been proved to be useful [41]. In the same way, this vertical temperature gradient also promotes the arrangement of polymer molecular chains. As shown in Figure 2d, the temperature gradient was mainly constructed by the change in the thickness. With the increase in the thickness, the air surface and the glass surface of the film form a temperature difference. If the thickness is too small, the whole film may be greatly affected by the residual heat of the heating table, leading to the high temperature at the air interface. However, excessively high temperature increases the probability of random orientation of molecular chains, which is not conducive to subsequent crystallization. Therefore, the crystallinity of the film is reduced. When the thickness increases to a certain extent and the cooling speed of the air surface of the film is too large, the effect of the temperature gradient weakens. The molecular chains on the air surface of the film are more disordered, which hinders the crystallization of the film. By choosing the right thickness to construct a suitable temperature gradient, it helps the film to crystallize from the upper surface to the lower surface in an orderly manner. This vertical temperature gradient helps in the arrangement of the molecular chain in the vertical direction, and then promotes its crystallinity.
In order to compare the piezoelectric properties of films of different thicknesses more intuitively, the piezoelectric properties of different films were evaluated by the static piezoelectric coefficient meter. As shown in Figure 3a, the 100 μm film showed the highest piezoelectric coefficient of about 33.55 ± 0.7 pC/N, followed by 150 μm and 50 μm films, corresponding to the previous analysis. To further explore the piezoelectric output performance of films of different thicknesses, piezoresponse force microscope analysis was performed. The reverse piezoelectric effect of different films was characterized by applying different voltages to the tip. As shown in Figure 3b, the 100 μm film also exhibited the highest piezoelectric response, at 15.31 pm/V. It is the highest value in the thickness group and is consistent with the previous analysis.
Since the hydrophilicity of the material determines the degree of affinity with cells, which in turn determines its potential for medical applications [42], the hydrophilicity of the film was also analyzed. The degree of hydrophilicity of the film is mainly affected by the order of the molecular chains on the surface and the magnitude of the surface potential. It can be found that the 100 μm film has the weakest hydrophilicity among the samples in the thickness group. It may be that the accumulation of surface charge reduces the hydrophilicity of the film.

3.2. Adjustment of Temperature to Improve the Directional Heating

Based on the above research, the 100 μm film was used as the research object to amplify the influence of the temperature gradient by changing the annealing temperature of the heating table. Sample properties were investigated at heat treatment temperatures of 160 °C, 180 °C and 200 °C, respectively. Firstly, the effect of the annealing temperature change was analyzed by morphology, as shown in Figure 4. There was also no significant difference in sectional morphology. However, it can be found that the quantity of holes on the surface of the film increased with the decrease in the heat treatment temperature. The possible reason is that the melting point of P(VDF-TrFE) film is about 153 °C, so the high temperature can quickly discharge the gas inside the film. If the heat treatment temperature is not enough, the gas is slowly released, thus leaving dense holes on the surface of the film.
The composition and crystallinity of the films at different heat treatment temperatures were also analyzed, as shown in Figure 5a–c. XRD image shows that most of the phases of the film are still in the β phase. At the same time, it can be found that there is no significant change in grain size. The content of the β phase of the films treated at different temperatures was also calculated, which was also kept above 88%.
Similarly, the crystallinity of films treated at different temperatures was calculated. With the increase in annealing temperature, the crystallinity showed an upward trend, indicating that a large temperature gradient is favorable for crystallization. The higher the temperature, the longer the cooling time of the film, thus promoting the arrangement of molecular chains. At the same time, the hole structure on the surface observed by the electron microscope also hinders the orderly arrangement of molecular chains, and then reduces crystallinity. As shown in Figure 5d, the temperature gradient was mainly constructed by the change in heating temperature. As the temperature rises, the temperature gradient gradually increases. If the temperature is too low, the small temperature gradient is difficult to arrange the molecular chains, which in turn reduces the magnitude of crystallinity.
Macroscopic d33 of different samples was also measured as shown in Figure 6a. It can also be found that the piezoelectric coefficient shows an upward trend, which is consistent with the trend of crystallinity. The microscopic d33 of the sample was measured as shown in Figure 6b, and it was found that it remained roughly similar to the macroscopic pattern. But films treated at 180 °C and 200 °C were abnormal. This may be due to the fact that when the processing temperature reaches 180 °C, the piezoelectric properties of the film are already good enough. However, the local differences of the film are large, and the piezoelectric properties of the film can be made more uniform by appropriately adjusting the temperature gradient.
The hydrophilicity of the samples was also compared in Figure 6c. Among them, the difference in hydrophilicity of samples with heat treatment temperature of 160 °C and 200 °C is small. The accumulation of surface charge may reduce the hydrophilicity on the one hand, the ordered arrangement of molecular chains by the temperature gradient may increase the hydrophilicity on the other hand.

3.3. Mechanism Analysis

In order to explore the difference between heating table heat treatment and muffle furnace heat treatment, a 100 μm film obtained by muffle furnace heat treatment was also prepared. The specific data are shown in Figure 7a,b. It is found that the content of the β phase of the film obtained by the muffle furnace is 86.5%, which is slightly lower than that of the heating table treated film. Its crystallinity is 71.4%, which is also lower than that of a 100 μm film obtained by heating table heat treatment. Its piezoelectric coefficient is 29. 9 ± 1.29 pC/N, which is also inferior to the heating table sample.
In order to find out the crystallization mechanism behind it, the multi-function nano infrared spectrometer was also performed, as shown in Figure 7c. Facts confirm that the melting temperature of the glass surface of the film is higher than that of the air surface. This is mainly because the upper surface is exposed to air, so it cools faster and the kinetic conditions for crystallization are not met. The closer it is to the lower surface, the more it is affected by the temperature gradient. As a result, the molecular chain is subjected to temperature gradient manipulation for a longer time, which helps in the arrangement of the molecular chains. P(VDF-TrFE) has a higher degree of crystallinity, which leads to a higher surface potential [28]. KPFM images were used to provide a more visible view of the differences between two heat treatments and the effects of amplified temperature gradients. Compared with uniform heating, the surface of the film obtained by directional heating has a higher surface potential, and enlarging this directional temperature gradient is beneficial to increase the surface potential of the film.
The schematic diagram of the crystallization behavior mechanism of the vertical temperature gradient of the heating table is shown in Figure 7g. Compared with the maver furnace, the upper surface of the heating table sample contacts the air during the cooling crystallization process, thus forming a temperature difference with the temperature of the bottom surface, so a vertical downward temperature field is built. Due to the good thermal insulation performance of the cavity, the center of the film is cooled first, showing a trend of increasing from the center area to surrounding. Thus, the heating table sample first crystallizes from the air surface and then slowly extends down to the bottom. The maver furnace sample crystallization starts from the center and then extends to the periphery, lacking the facilitation for molecular chain arrangement.
Therefore, this vertical temperature gradient pre-sequences the molecular chains horizontally at the beginning of crystallization, which facilitates the crystallization of P(VDF-TrFE) in this initial order, thus showing the advantage of increased crystallinity, thereby improving piezoelectric properties. Uniform heating makes the molecular chains more susceptible to entanglement, making crystallization difficult. Moreover, the existence of this temperature gradient can make the polymer molecular chain oriented in the configuration of β phases, thereby increasing the content of the β phase.
The potential of films based on the above heat treatment methods for biomedical applications is enormous. A certain hydrophobicity of the surface of the material contributes to the adhesion and proliferation of cells [43]. The high piezoelectric coefficient also contributes to the adsorption of proteins and the adhesion of cells and promotes tissue repair [44,45]. It also provides some feasible solutions for improving the sensitivity of wearable healthcare testing [46]. Furthermore, this directional heat treatment method for constructing vertical temperature gradients has broad application prospects for P(VDF-TrFE) and many other piezoelectric polymers such as odd-numbered Nylons and poly-l-lactic acid (PLLA)). Choosing the right heat treatment temperature according to the melting point of different polymers can help improve the applicability of the method, and it can help avoid the influence of the addition of other substances on the material properties. While constructing the temperature gradient, the difficulty in controlling the external environmental temperature and the gradient itself also needs to be further solved.

4. Conclusions

In this work, the P(VDF-TrFE) film is easily obtained through a heating table. It is found that using this vertically downward temperature gradient which regulates P(VDF-TrFE) crystallization can effectively obtain P(VDF-TrFE) films with a high content of the β phase. By adjusting the film thickness as well as the heat treatment temperature, it is found that the 100 μm film and heat treatment temperature of 200 °C has the highest crystallinity, which can reach 75%, and the piezoelectric coefficient d33 = 33.5 ± 0.7 pC/N. A series of piezoelectric response tests also show that the P(VDF-TrFE) film with a high content of the β phase and high crystallinity has the best performance, and the high content of the β phase alone is far from enough. The orderly arrangement of molecular chains has a huge impact on the piezoelectric properties. The constructed vertical temperature gradient facilitates the orderly arrangement of molecular chains from top to bottom, and then offers the film excellent piezoelectric properties. Therefore, the construction of this directional heating method to obtain excellent piezoelectric properties of P(VDF-TrFE) films has important engineering significance in the field of sensors and biomedicine.

Author Contributions

Conceptualization, K.C. and X.X.; methodology, X.X and Z.Z.; investigation, X.X. and A.Y.; data curation, X.X., X.H.; writing—original draft preparation, X.X. and C.W.; writing—review and editing, K.C., C.W. and W.W.; visualization, X.X. and A.Y.; funding acquisition, K.C., C.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Key Research and Development Program of China (No. 2021YFC2400400), the National Natural Science Foundation of China (Grant No. 52271252, 32271373 and 82201009), the Postdoctoral Fellowship Program of China Postdoctoral Science Foundation (GZC20232243) and Key Research and Development Program of Zhejiang Province (2021C03061).

Data Availability Statement

Data will be made available on request.

Acknowledgments

Authors thank Xiaoyi Chen at Stomatology Hospital, Zhejiang University School of Medicine for providing valuable advice funding support and Jing He at State Key Laboratory of Chemical Engineering in Zhejiang University for performing KPFM test.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Schematic diagram of P(VDF-TrFE) obtained by the heating table heat treatment and the SEM images of P(VDF-TrFE) of different thicknesses including (b) 50 μm, (c) 100 μm and (d) 150 μm.
Figure 1. (a) Schematic diagram of P(VDF-TrFE) obtained by the heating table heat treatment and the SEM images of P(VDF-TrFE) of different thicknesses including (b) 50 μm, (c) 100 μm and (d) 150 μm.
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Figure 2. Composition and crystallization analysis of films of different thicknesses. (a) XRD image, (b) FTIR image, (c) DSC image, (d) schematic diagram of crystallization of films of different thicknesses.
Figure 2. Composition and crystallization analysis of films of different thicknesses. (a) XRD image, (b) FTIR image, (c) DSC image, (d) schematic diagram of crystallization of films of different thicknesses.
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Figure 3. (a,b) d33 analysis of films of different thicknesses and (c) contact angle images.
Figure 3. (a,b) d33 analysis of films of different thicknesses and (c) contact angle images.
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Figure 4. SEM images of different temperature: (a) 160 °C, (b) 180 °C.
Figure 4. SEM images of different temperature: (a) 160 °C, (b) 180 °C.
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Figure 5. Composition and crystallization analysis of films of different temperature. (a) XRD image, (b) FTIR image, (c) DSC image (the sample data for the heat treatment temperature of 200 °C here is the same as above), (d) schematic diagram of crystallization of films of different temperature.
Figure 5. Composition and crystallization analysis of films of different temperature. (a) XRD image, (b) FTIR image, (c) DSC image (the sample data for the heat treatment temperature of 200 °C here is the same as above), (d) schematic diagram of crystallization of films of different temperature.
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Figure 6. (a,b) d33 analysis of films of different annealing temperatures and (c) contact angle images (the sample data for the heat treatment temperature of 200 °C here are the same as above).
Figure 6. (a,b) d33 analysis of films of different annealing temperatures and (c) contact angle images (the sample data for the heat treatment temperature of 200 °C here are the same as above).
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Figure 7. (a) FTIR image and (b) DSC image of a 100 μm film obtained by the muffle furnace, (c) melting point between the upper and lower surfaces of a 100 μm film obtained by heating table treatment, KPFM images of (d) 100 μm film obtained by the muffle furnace, (e) the worst-performing sample made from a heating table and (f) the best-performing sample made from a heating table; (g) schematic diagram of crystallization of films obtained from muffle furnace and heating table.
Figure 7. (a) FTIR image and (b) DSC image of a 100 μm film obtained by the muffle furnace, (c) melting point between the upper and lower surfaces of a 100 μm film obtained by heating table treatment, KPFM images of (d) 100 μm film obtained by the muffle furnace, (e) the worst-performing sample made from a heating table and (f) the best-performing sample made from a heating table; (g) schematic diagram of crystallization of films obtained from muffle furnace and heating table.
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MDPI and ACS Style

Xin, X.; Yee, A.; Zhou, Z.; He, X.; Weng, W.; Wu, C.; Cheng, K. Fabrication of PVTF Films with High Piezoelectric Properties Through Directional Heat Treatment. J. Compos. Sci. 2024, 8, 512. https://doi.org/10.3390/jcs8120512

AMA Style

Xin X, Yee A, Zhou Z, He X, Weng W, Wu C, Cheng K. Fabrication of PVTF Films with High Piezoelectric Properties Through Directional Heat Treatment. Journal of Composites Science. 2024; 8(12):512. https://doi.org/10.3390/jcs8120512

Chicago/Turabian Style

Xin, Xin, Aotian Yee, Zhiyuan Zhou, Xuzhao He, Wenjian Weng, Chengwei Wu, and Kui Cheng. 2024. "Fabrication of PVTF Films with High Piezoelectric Properties Through Directional Heat Treatment" Journal of Composites Science 8, no. 12: 512. https://doi.org/10.3390/jcs8120512

APA Style

Xin, X., Yee, A., Zhou, Z., He, X., Weng, W., Wu, C., & Cheng, K. (2024). Fabrication of PVTF Films with High Piezoelectric Properties Through Directional Heat Treatment. Journal of Composites Science, 8(12), 512. https://doi.org/10.3390/jcs8120512

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