Introduction

Piezoelectric polymers have become one of the most promising piezoelectric materials owing to their merits of ease of processing, low density, excellent flexibility and biocompatibility toward applications in wearable electronics and energy devices1,2,3. Piezoelectric polymers such as polyvinylidene fluoride (PVDF) and its copolymers have shown great application potential in the field of flexible electronics owing to their flexibility and excellent electromechanical properties4,5,6,7,8,9,10,11,12,13. Notably, the piezoelectric coefficient d33 of fluorinated alkyne modified tetramer (poly(vinylidene fluoride-trifluoroethylene-chloro fluoroethylene-fluorinated alkyne)) has been reported to be comparable to that of piezoelectric ceramics14,15,16, which is superior to the d33 of PVDF and poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE). However, it still needs more explorations to realize large-scale synthesis of such a promising tetrapolymer. Therefore, currently it is of great significance to achieve large intrinsic piezoelectric coefficients of PVDF and its copolymers for commercial applications via optimizing the processing methods.

In general, the presence of -TrFE makes PVDF-TrFE easier to crystallize into polar β-phase crystals than PVDF8. As PVDF-TrFE films with β-phase crystals have the highest dipole moment due to their special all-trans (TTT) conformation, they exhibit excellent piezoelectric and ferroelectric properties10,16,17. Among a large number of reported fabrication methods, electrospinning enables achieving satisfactory piezoelectric properties more efficiently owing to the coupling of mechanical stretching and electric field polarization18,19,20,21,22,23. However, it is still challenging to achieve a very high crystallinity for wet-processed PVDF-TrFE fibers or films. As such, annealing is frequently used to further induce the formation of more β-phase crystals to improve their piezoelectric properties24,25,26,27.

Notably, current annealing treatments for polymers are more inclined to take a few hours to guarantee efficient time for the nucleation and growth of crystals in PVDF-TrFE, which increases the cost and energy consumption substantially. In order to pursue a higher production efficiency, it is necessary to shorten annealing time by understanding the underlying kinetics for molecular chain conformation changes and crystal transformation of PVDF-TrFE. To date, ultra-fast annealing is generally applied to achieve instantaneous heat treatment to reduce the overall thermal budget and avoid substrate damage during semiconductor manufacturing. Besides, flash differential scanning calorimetry (DSC) has already been used to track the rapid melting and crystallization behavior of some polymers28, hence the transition of molecular chain conformation is likely to be done in a very short time. However, this kind of heat shock treatment has not been extended to the field of polymer processing yet. Unlike flash DSC, flash annealing provides sufficient energy for the molecular chain of PVDF-TrFE in a very short time to transform it into a more stable ferroelectric β phase structure without causing material performance degradation.

In this study, flash annealing technology was innovatively applied to the post-treatment of wet-processed ferroelectric fluoropolymers. Electrospun and spin-coated PVDF-TrFE films were annealed slightly above their Curie temperature (TC) in an ultra-short time to boost their intrinsic piezoelectric coefficients thanks to increased β-phase crystal contents and remained oriented dipoles. Ferroelectric polarization, reverse piezoelectric properties and direct piezoelectric output data demonstrate that such a post processing significantly boosts the intrinsic piezoelectric coefficient and ferroelectric properties of PVDF-TrFE films. The underlying mechanisms for the formation of more β-phase crystals and conformation evolution during flash annealing were proposed based on the data from in-situ Raman and Fourier transform infrared (FTIR) spectra, differential scanning calorimeter (DSC), wide-angle X-ray diffraction (WAXD) and molecular dynamic simulation. Besides, PVDF-TrFE films subjected to flash annealing demonstrate superior efficiencies in pressure sensing and micromechanical energy harvesting by comparison with PVDF-TrFE films without annealing or with long-time annealing, as well as commercial piezoelectric PVDF films. Therefore, it unveils a simple, intriguing and cost-effective method to enable large-scale production of piezoelectric PVDF-TrFE films for fabricating high-performance sensors and transducers to advance the development of flexible electronics and artificial intelligence.

Results and discussions

Design concept

As seen in the design concept of this work shown in Fig. 1, PVDF-TrFE films fabricated by wet processing, e.g., electrospinning and spin-coating, were subjected to 60 s annealing called as flash annealing, aiming to improve their piezoelectricity. Notably, wet-processing is frequently used to fabricate ferroelectric and piezoelectric PVDF-TrFE films owing to the super high cost of PVDF-TrFE resins. Specifically, β-phase crystals in the PVDF-TrFE films were converted into α-phase ones during flash annealing at a temperature above Curie temperature (TC) of PVDF-TrFE, and they turned back into β-phase crystals upon being cooled down to room temperature after the end of flash annealing (Fig. 1). Such a simple post-treatment leads to a significant enhancement in both the content of β-phase crystals and piezoelectric coefficient d33. Piezoelectric pressure sensors made by the as-treated PVDF-TrFE films showed potential applications in the detection of acoustic vibrations and motions of human body joints, as well as harvesting micromechanical energy from the surrounding. It uncovers a simple, efficient and scalable methodology for the fabrication of highly piezoelectric PVDF-TrFE films via a simple post-treatment denoted as flash annealing.

Fig. 1: Preparation and application of PVDF-TrFE with high piezoelectricity properties.
Fig. 1: Preparation and application of PVDF-TrFE with high piezoelectricity properties.
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Design concept for the flash annealing of wet-processed PVDF-TrFE films with boosted piezoelectricity, as well as the assembly of piezoelectric sensors for potential applications in sensing and energy harvesting.

Conformation and transformation of molecular chains induced by annealing

Electrospun PVDF-TrFE fiber mats with aligned fiber structure were annealed at different temperatures for a very short time to screen the optimal annealing temperature. As revealed by DSC and FTIR spectra, there are obvious changes in Curie transition enthalpy and macromolecular conformation (Fig. 2a and S1a), which indicate the highest relative content of β-phase crystals in the aligned fiber mats when annealed at 130 °C29,30,31. Intriguingly, this temperature is almost equal to the crystallization peak temperature of PVDF-TrFE, as revealed by DSC data (129.3 °C, Fig. S1b). Therefore, here 130 °C is preferred for flash annealing of PVDF-TrFE films.

Fig. 2: Characterization of the aggregated structure of electrospun PVDF-TrFE fiber mats annealed for different time.
Fig. 2: Characterization of the aggregated structure of electrospun PVDF-TrFE fiber mats annealed for different time.
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a The Curie transition enthalpy of electrospun PVDF-TrFE fiber mats annealed at different temperatures for 60 s. b The Curie transition enthalpy and the normalized characteristic peak area of the β-phase crystals of electrospun PVDF-TrFE fiber mats with annealing (WA) at 130 °C for different time and without annealing (WOA), as well as melt processed films. Data in (a), (b) are presented as mean values and error bars represent the standard deviation (SD) of three independent measurements (n  =  3, where n is the number of samples that are used to generate statistical data). c 2D-SAXS image and 1D IQ curve of electrospun PVDF-TrFE fiber mats obtained by SAXS image integration after flash annealing. d On-line heating and cooling X-ray diffraction (XRD) and (e) room temperature XRD patterns of electrospun PVDF-TrFE fiber mats. On-line XRD tests were carried out in the temperature range of 25 ~ 130 °C with a heating rate of 300 °C/min. f DSC data of electrospun PVDF-TrFE fiber mats with two heating and cooling cycles.

Both the Curie transition enthalpy and the relative content of β-phase crystals of PVDF-TrFE films are significantly improved after flash annealing at 130 °C (Fig. 2b and S2a, b), which are much higher than those of melt processed PVDF-TrFE (Fig. 2b). However, a further increase of annealing time is not seemingly to change the Curie transition enthalpy and molecular chain conformation much (Fig. S3 and Table S1). Therefore, long-time annealing of PVDF-TrFE cannot improve the content of β-phase crystals as much as expected, and the β-phase crystal content approaches to a plateau after flash annealing, i.e., the formation of β-phase crystals is almost done within 60 s. Besides, both the increase of characteristic peak height at 845 cm-1 in Raman spectroscopy and the diffraction brightness of 2D wide-angle X-ray diffraction (WAXD) attested the above conclusion (Fig. S2c and S4a–c)32,33. One-dimensional intensity distribution curve was obtained by an azimuthal integration of 2D-WAXD curve (Fig. S4d, e, f). The Hermans orientation factor (HOF) is calculated by eq.134,

$$f=\,\frac{3\left\langle {\cos }^{2}\varphi \right\rangle \,-\,1}{2}$$
(1)

where \(\left\langle {\cos }^{2}\varphi \right\rangle\) was defined as:

$$\left\langle {\cos }^{2}\varphi \right\rangle=\,\frac{{\int }_{0}^{{{{\rm{\pi }}}}}{{{\rm{I}}}}\left(\varphi \right){\cos }^{2}\varphi \sin \varphi {{{\rm{d}}}}\varphi }{{\int }_{0}^{{{{\rm{\pi }}}}}{{{\rm{I}}}}\left(\varphi \right)\sin \varphi {{{\rm{d}}}}\varphi }$$
(2)

where φ is the azimuth angle, and the reference direction (fiber orientation direction) is aligned with the 2D-WAXD meridian direction during calculation, and \({{{\rm{I}}}}\left(\varphi \right)\) is the corresponding strength34. According to the definition of Riemann integral, it is approximated as a discrete summation formula:

$$\left\langle {\cos }^{2}\varphi \right\rangle \,\approx \,\frac{{\sum }_{{{{\rm{i}}}}}^{{{{\rm{n}}}}}{{{\rm{I}}}}\left({\varphi }_{i}\right){\cos }^{2}{\varphi }_{i}\sin {\varphi }_{i}}{{\sum }_{{{{\rm{i}}}}}^{{{{\rm{n}}}}}{{{\rm{I}}}}\left({\varphi }_{i}\right)\sin {\varphi }_{i}}$$
(3)

The unannealed sample has a HOF of −0.46 (very close to −0.5), namely, the diffraction spot direction of the (110)/(200) crystal plane of the β-phase crystal is perpendicular to the fiber length direction, indicating a high degree of crystal structure orientation. Intriguingly, the HOF of PVDF-TrFE fiber mats after flash annealing stays almost unchanged (−0.47), while it turns to −0.37 after long-term annealing. Therefore, flash annealing does not destroy the orientation structure, while long-term annealing induces significant molecular chain relaxation.

To validate this assumption, both PVDF-TrFE films obtained by spin coating and hot pressing were also annealed with the same method. Intriguingly, spin-coated films exhibited the same characteristics as that of electrospun films (Fig. S5). However, there were few increases in both the Curie transition enthalpy and β-phase crystal content of the melt-processed film after flash annealing, and even after two-hour annealing (Fig. S6). Hence, the further crystallization at a temperature above TC accounts for a significant increase in the percentage of all-trans conformation of wet-processed PVDF-TrFE films after flash annealing, which boosts the Curie transition enthalpy and β-phase crystal content. Besides, this strategy is also applicable to PVDF-TrFE with other ratios of VDF/TrFE, as revealed by the Curie transition enthalpy of electrospun PVDF-TrFE fiber mats with 20 mol% and 30 mol% TrFE (Fig. S7).

The crystalline structure of PVDF-TrFE films treated under different annealing conditions was further investigated by two-dimensional small angle X-ray scattering (2D-SAXS). To obtain the lamellar spacing, the 1D IQ curve was obtained by integrating the 2D-SAXS image (Fig. 2c and S8)35. The lamellar spacing d was calculated according to eq. 4:

$$d=\frac{2{{{\rm{\pi }}}}}{q}$$
(4)

in which q is intensity of X-ray scattering and π is a constant. As seen in Fig. 2c, there is no change in the crystal size of PVDF-TrFE after flash annealing, as revealed by the lamellar space of PVDF-TrFE before and after annealing36. Given the significant increase in Curie transition enthalpy and a slight rise in melting enthalpy, the increase of β-phase crystal content induced by flash annealing probably comes from the increase of β-phase crystal number rather than the further growth of preexisting crystals. To validate this hypothesis, on-line XRD and Raman spectra were applied to investigate the evolution of conformation changes and transformation of PVDF-TrFE during flash annealing. Figure 2d shows that the β-phase crystals in the PVDF-TrFE films are gradually decayed, accompanied by the growth of α-phase crystals with increasing temperatures37. Besides, there is almost no β-phase crystal when the temperature reaches the TC (110 °C), while α-phase crystal becomes more and more until the temperature gets close to the target annealing temperature of 130 °C. Intriguingly, the newly formed α-phase crystals are completely converted into β-phase crystals upon cooling down to room temperature (Fig. 2e)38. In-situ Raman spectroscopy shows that the change in macromolecular conformation is completely consistent with the crystal structure change investigated by in-situ Raman spectra (Fig. S9). Besides, such a flash annealing treatment significantly improves the β-phase crystal content or the crystallinity owing to the thermal treatment, as reveled by DSC data shown in Fig. 2f. In-situ Raman spectrum data for multiple heating and cooling cycles also support this conclusion. Besides, β-phase crystal contents only increase significantly after the first cycle and they show no further increase in following cycles (Fig. S10). Besides, both the Young’s modulus and tensile strength of the fiber mats after flash annealing treatment are significantly improved owing to the increased crystallinity, while they still maintain a certain mechanical flexibility as revealed by the elongation at break (Fig. S11). However, long-time annealing leads to a further improved Young’s modulus and deteriorated elongation at break (Fig. S11), which is probably ascribed to the increased crystallinity, crystal size and defects during long time annealing. Intriguingly, spin-coated PVDF-TrFE film also shows the same phenomenon, which demonstrates the universality of flash annealing for tuning mechanical properties of PVDF-TrFE.

Molecular dynamics simulation

The conformational evolution of PVDF-TrFE was explored by molecular dynamics simulation (Supplementary Movie 1). The Curie transition temperature of the ideally oriented PVDF-TrFE fibers was determined at first. The dihedral angle energy (Proper Dih.), short-range LJ energy (LJ (SR)), short-range electrostatic coulomb energy (Coulomb (SR)), and total potential energy (Potential) of PVDF-TrFE will change accordingly when it undergoes the ferroelectric to paraelectric phase transition. Apparently, the Curie transition of PVDF-TrFE fibers is in the range of 300-500 K under an axial strain of ~1% (Fig. 3a, and S12), and the conformational parameters change obviously when temperature is below TC. Furthermore, the conformation of -TrFE hardly changes with temperature, as revealed by the calculated TTT and TGTG’ conformation contents in the -VDF and -TrFE units at different temperatures (Fig. 3b), that is, the conformational transitions are all from the -VDF part. The -TrFE unit does not contribute to the increase of TTT conformation content. However, it plays a crucial role in inhibiting the shrinkage of molecular chain at temperature above the conformation transition temperature. Specifically, molecular chain rotation facilitates the transition of TTT conformation to the TGTG’ during annealing37. In this case, the central symmetry of the crystals is enhanced, which makes the ferroelectric crystal lose its spontaneous polarization ability and then changes into the paraelectric phase from the ferroelectric one39,40,41. Conformation switching and Curie transition occur simultaneously in a very short time (~400 ns)42. Molecular chains of PVDF-TrFE are rearranged into α-phase crystals as much as possible once temperature reaches TC. Vice versa, the TGTG’ conformation with high potential energy releases energy during cooling, which helps it return to a more stable TTT conformation, rendering the dipole with spontaneous polarization ability (Fig. 3c, d and S13). The change of dipole polarization during this transition can be attested by in-situ piezoelectric output test and in-situ broadband dielectric spectroscopy test (Fig. S14 and S15)36,37,43. Besides, significant increment in dielectric constant of electrospun PVDF-TrFE fiber mats with increasing temperature reflects the dipole moment change of PVDF-TrFE molecular chains, the loses of spontaneous polarization ability as well, which corresponds to the ferroelectric-paraelectric phase transition.

Fig. 3: Molecular dynamics simulation of PVDF-TrFE upon cooling and heating.
Fig. 3: Molecular dynamics simulation of PVDF-TrFE upon cooling and heating.
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a The variation of dihedral angle energy (Proper Dih.) upon cooling and heating. b Statistics of conformational content of β and α crystals in -TrFE and -VDF units under variable temperatures. c, d are enlarged local 3D structural diagrams of molecular chains at low (300 K) and high temperatures (500 K).

Molecular dynamics simulation confirmed that the β-phase crystal content increase during flash annealing was achieved through conformational switching rather than the cumulative growth of new β-phase crystals. Specifically, the TrFE part restrains the shrinkage of molecular chains in the oriented amorphous fraction like a buckle, which enables the whole molecular chains maintaining a certain degree of order at high temperature. In principle, aligned molecular chains are easier to form TTT conformations through phase transition than those are disordered, they thus form new β-phase crystals during flash annealing eventually with the aid of thermal energy driving30,36. Notably, the Curie temperature is also increased significantly after flash annealing owing to the formation of more ferroelectric phase crystals, which is consistent with other reports14.

Ferroelectric and piezoelectric properties

As the pores in electrospun PVDF-TrFE fiber mats make them easy to be broken down under high electric field, here spin-coated PVDF-TrFE films were selected for ferroelectric polarization test (P-E loop) to demonstrate its ferroelectricity10,30,40,44. The spin-coated PVDF-TrFE film shows an obvious hysteresis loop, which verifies its good intrinsic ferroelectric properties (Fig. 4a and b). Apparently, the film annealed only for 60 s shows a much larger polarization response to electric field than that of the unannealed one (Fig. S16), indicating the contribution of flash annealing to the ferroelectricity improvement of wet-processed PVDF-TrFE films. Besides, the phase hysteresis loops obtained by piezoelectric force microscopy (PFM) indicate that electrospun PVDF-TrFE fiber mats have obvious ferroelectric properties (Fig. 4c and S17a)45, which is consistent with foregoing crystalline structure characterizations of PVDF-TrFE films before and after flash annealing.

Fig. 4: Ferroelectric and piezoelectric properties of wet-processed PVDF-TrFE annealed for different time.
Fig. 4: Ferroelectric and piezoelectric properties of wet-processed PVDF-TrFE annealed for different time.
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P-E loops of spin-coated PVDF-TrFE films (a) before and (b) after flash annealing. (c) Phase and (d) amplitude versus tip bias for the PFM measurement of electrospun PVDF-TrFE fiber mats without annealing and with flash annealing. e A comparison of the intrinsic piezoelectric coefficients of this work with others reported in literatures. f Piezoelectric output properties of electrospun PVDF-TrFE fiber mats treated by different annealing time, as well as commercial PVDF films.

Intriguingly, the films treated by flash annealing show a better butterfly curve than that of the unannealed, as revealed by the PFM results shown in Fig. 4d. Besides, the film undergone a flash annealing has an absolute reverse piezoelectric coefficient d33 of −70.89 pm/V, while those for the unannealed and long-time annealed PVDF-TrFE films are −44.91 and −53.85 pm/V (Fig. 4d and S17b), respectively. Notably, after flash annealing neat PVDF-TrFE film has a much larger d33 than that of the others with much longer annealing time, as reported in literatures (Fig. 4e)24,26,46,47. Besides, as seen in Fig. S18, the d33 measured by a quasi-static d33 instrument also attests that the PVDF-TrFE electrospun fiber mats treated by flash annealing have a much higher d33 value (−68 pC/N) by comparison with those annealed for 2 h and without annealing (−45 pC/N and −33 pC/N, respectively). Therefore, both direct and indirect piezoelectric coefficient measurements indicate the higher efficiency of flash annealing for boosting piezoelectricity of PVDF-TrFE films by comparison with long-time annealing. The d33 value of the long-time annealed sample is highly correlated with the decay of the crystal orientation degree, which highlights the advantages of flash annealing. Notably, the d33 of PVDF-TrFE films obtained by flash annealing is superior to that of other PVDF and PVDF-TrFE materials loaded with fillers or complex architecture (Table S3)16,22,36,39,46,47,48,49,50,51,52,53,54,55,56. In addition, as characterized by PFM, flash annealing makes spin-coated PVDF-TrFE films have a d33 of −57.29 pm/V, which is much larger than that of unannealed films (−33.92 pm/V) (Fig. S19a, b). In contrast, for hot-pressed PVDF-TrFE films, the d33 values calculated by PFM are 20.74 pm/V and 14.49 pm/V before and after annealing (Fig. S19c, d), respectively, which highlights the advantages of flash annealing on wet-processed PVDF-TrFE films. Such an interesting phenomenon is probably ascribed to 1) the incomplete crystallization of PVDF-TrFE during rapid solvent evaporation57; 2) the formation of microstructures like chain pre-alignment and solvent-evacuated porosity in wet-processed films which help the rapid β-phase nucleation during flash annealing27.

Moreover, the contribution of flash annealing to the improvement of piezoelectric properties of PVDF-TrFE thin films can also be verified by piezoelectric force sensitivity when they are assembled to be a film force sensor (Fig. 4f)23. Specifically, electrospun PVDF-TrFE fiber mats composed of aligned nanofibers show a higher piezoelectric output coefficient (28.1 mV/N) than that of the commercial piezoelectric PVDF film sensor (18.2 mV/N), while that for the film annealed for 60 s is 96.9 mV/N under the same testing condition. In contrast, a further increase of annealing time does not improve the force sensitivity of electrospun PVDF-TrFE films by comparison with that of the film subjected to flash annealing, which is revealed by the force sensitivity of 72.1 mV/N for the films annealed for 2 h. All of these are consistent with the foregoing crystalline structure characterization and piezoelectric coefficient data, indicating the high efficiency of flash annealing in boosting the piezoelectricity of wet-processed PVDF-TrFE films.

Applications of electrospun PVDF-TrFE films treated by flash annealing

Piezoelectric materials demonstrate significant advantages in fields such as self-powered sensors, actuators, and micromechanical energy harvesters58,59,60,61,62,63.

The output power density of piezoelectric PVDF-TrFE film nanogenerators connected with different resistors was calculated by eq. 5,

$$P=\,\frac{{V}^{2}}{A{R}_{L}}$$
(5)

where V is the voltage applied to both ends of the resistance RL, A is the electrode contact area (1 cm2), and the test is carried out under an impact stimulus of 20 N and 1 Hz53. By comparison with the films without annealing and with long-time annealing, films treated by flash annealing show the highest output power density (7.53 mW/m2) at the same load resistance (680 kΩ) (Fig. 5a and S20). Besides, the piezoelectric PVDF-TrFE films undergone flash annealing can charge the capacitor (0.22 μf capacitor) better than the other two (Fig. 5b and S21).

Fig. 5: Potential applications of piezoelectric PVDF-TrFE films treated by flash annealing and conventional annealing.
Fig. 5: Potential applications of piezoelectric PVDF-TrFE films treated by flash annealing and conventional annealing.
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a The output energy density and (b) film capacitor charging of electrospun PVDF-TrFE fiber mats annealed for different time. c PVDF-TrFE films treated by different annealing time were used for monitoring the vocal cord vibration once attached to the neck. Sensing signals collected by PVDF-TrFE films (d) without annealing, with (e) flash annealing and (f) long-time annealing generated by different guitar string vibrations.

Besides, physiological activities such as coughing, swallowing, and speaking were simulated and tested with the assembled piezoelectric sensor to explore its potential applications in health monitoring. Compared with the piezoelectric PVDF-TrFE film annealed for 0 min and 2 h, piezoelectric PVDF-TrFE film annealed for 60 s can better monitor the vocal cord-like vibration during coughing, swallowing and speaking with higher resolution (Fig. 5c). Owing to the larger d33, the piezoelectric PVDF-TrFE film sensor undergone flash annealing is envisioned to monitor the vibration of the throat in real time, especially for the cough, and can be counted for clinical diagnosis64. Moreover, such a piezoelectric sensor is capable of better identifying high-frequency vibrations of the guitar generated by plucking different strings once attached to the guitar guard plate by comparison with the controls, i.e., unannealed films and those annealed for 2 h (Fig. 5d-f). Notably, it is not disturbed by noise, and is compact and easy to carry, so it can be tuned in complex environments. Besides, electrospun PVDF-TrFE fiber mats annealed for only 60 s can respond to an impact force of 124.18 mN at 1 Hz and also maintain a good response to high-frequency vibration (200 Hz, 1.18 N), which are still distinctive at an interference of the same environment noise (Figure S22, 23). In contrast, the detection limit of impact force and voltage output for electrospun PVDF-TrFE fiber mats without annealing or annealed for 2 h are apparently inferior, though they are better than that of commercial piezoelectric PVDF films (Fig. S22, 23). Hence, flash annealing helps improve the detection limit of electrospun PVDF-TrFE fiber mats owing to the boosted piezoelectric coefficients, enabling effective mechanoelectrical responding in more complex environment with an interference of noise.

An intriguing method, namely flash annealing, has been developed to significantly boost piezoelectric coefficient of wet-processed PVDF-TrFE films in a very simple and low-energy-consumption way. PVDF-TrFE films after flash annealing show an intrinsic d33 of −70.89 pm/V by PFM or −68 pC/N by a direct quasi static method, which is more than two times of that for commercial ones (−25 ~ −30 pC/N), and much higher than that of unannealed PVDF-TrFE films or annealed for 2 h. The mechanism for the improvements of intrinsic ferroelectricity and piezoelectricity of PVDF-TrFE by flash annealing is interpreted by on-line XRD and in-situ Raman spectroscopy characterizations, as well as molecular dynamics simulation. The molecular chain of PVDF-TrFE undergoes α-phase crystalline rearrangement when temperature reaches or exceeds the TC. A complete heat treatment cycle drives the conformation transformation, promoting the perfection of the main crystal region and forming a secondary β-phase crystal with high crystallinity. This study uncovers simple methods for efficient processing of piezoelectric polymers, pushing forward the advancements of flexible electronics by offering polymers with high intrinsic piezoelectricity.

Methods

Materials

PVDF-TrFE (80/20, 75/25, 70/30 Piezotech FC25) powder used in this work was purchased from Arkema (France), which has a good balance of piezoelectric coefficient and Curie temperature65. N, N-dimethylformamide (DMF) and acetone were from Tianjin Bodi Chemical Co. Ltd. and Sichuan Xilong Science Co. Ltd., respectively. All the chemicals were used without further purification.

Sample fabrication

0.75 g PVDF-TrFE powder was dissolved in a mixture of 3 ml DMF and 2 ml acetone to prepare PVDF-TrFE solution for electrospinning and spin coating. For electrospinning, the solution injection rate and the roller speed during fiber collection were set as 0.1 mm/min and 4000 rpm, respectively. The positive and negative voltages applied to the nozzle and roller were +12.5 kV and −2.5 kV, respectively. The fiber mats were placed in a fume hood for 12 h to fully volatilize the residual solvent after 4 h spinning, and dried mats with a thickness of 80 μm were obtained. Besides, PVDF-TrFE sheets (size of 5 cm × 5 cm, thickness of 10 μm) were spin-coated at a speed of 800 r/min for 90 s by using the same solution. For comparison, PVDF-TrFE films with a thickness of 110 μm were prepared by hot pressing by using as-purchased powder at 200 °C with a pressure of 4 MPa for 10 min. All PVDF-TrFE films were annealed in an oven by attaching them in an aluminum foil to suppress film shrinkage without additional tensile stress. Flash annealing is realized in a high temperature oven with an actual temperature ramp rate of 118.63 K/min (Fig. S24), and the sample is kept for 60 s after rising to the target temperature (130 °C). The samples are naturally cooled to room temperature after annealing and immersed in liquid nitrogen to freeze its oriented molecular chain structure. PVDF-TrFE films were annealed under the same condition as flash annealing except changing the holding time. Finally, they were stored at low temperature (−28 °C) to weaken the relaxation behavior of macromolecular chain. The PVDF-TrFE film prepared by electrospinning was cut into a square of 1 × 1 cm2, and copper foils (thickness of 12 μm) were attached to both sides of the sensing film as electrodes to transmit piezoelectric charges. Finally, polyimide tape was used to package the film sensor and to squeeze out air for sensing experiments.

Characterization

The Curie transition and crystallization of the samples were analyzed by a differential scanning calorimetry (DCS, Q20, TA) under atmosphere of nitrogen gas flow at a heating and cooling rate of 10 °C/min. On-line XRD tests were carried out with a Panalytical x-ray diffractometer (EMPYREAN) in the temperature range of 25 ~ 130 °C with a temperature rise rate of 300 °C/min. The Fourier transform infrared spectra of the samples were obtained by a Thermo Nicolet 6700 FTIR spectrometer. In-situ Raman measurements of PVDF-TrFE films were performed by inVia-Qontor (Renishaw, UK) between25 °C and 130 °C at a heating rate of 150 °C/min, which is the heating limitation of the equipment.

SAXS and WAXS data of films were obtained at beamline BL16B1 of Shanghai Synchrotron Radiation Facility (SSRF). The monochromatic of the light source was 1.24 Å. The sample-to-detector distance was 2200 mm and 165 mm by calibration for SAXS and WAXS, respectively.

The AC voltage generated by the impact force exerted by a vibration exciter (TJZ20, Taisite Instrument, China) was recorded with an oscilloscope (PicoScope 3403D, Pico Technology)23. During the test, the vibration exciter provides stress to make the sample produce corresponding electrical signal, which will be collected by the oscilloscope. The direct piezoelectric output performance of the sample was measured at different temperature. Broad dielectric spectroscopy was investigated by a broadband dielectric impedance relaxation spectrometer (Concept 50, Novocontrol GmbH, Germany) at 1 Hz and a heating rate of 10 °C/min. Piezoelectric response force microscopy characterization was carried out by scanning probe microscope (Cypher VRS by Oxford, UK) at room temperature with a square DC voltage of −30 ~ 30 V at 0.5 Hz to fully polarize its dipole. Besides, a sinusoidal alternating voltage (3 V, 300 kHz) was applied to drive the fiber to undergo inverse piezoelectric response deformation. The negative values of d33 denote negative voltage output under compressive strain. In the meantime, direct piezoelectric response of the films was measured at an alternating force of 0.25 N at 100 Hz by using a quasi-static piezoelectric tester (Model ZJ-6, Beijing Jingke Zhichuang, China). The ferroelectric polarization test was performed using Premier II produced by Radiant Technologies with a frequency of 100 Hz at 25 °C. The Keithley 6514 electrometer (Tektronix Inc., US) was used to test the voltage and collect the sensing signal when performing tests such as charge-discharge and output power density. The monitoring of simulated human motion and guitar string vibration (Fig. 4c-f) was performed by the Keithley 6514 electrometer (Tektronix Inc., US). The monitoring of minimum detection pressure and high-frequency vibration (Fig. S22 and S23) was completed by vibration exciter (TJZ20, Taisite Instrument, China) and oscilloscope (PicoScope 3403D, Pico Technology). All the measurement data are based on those obtained from different samples. We have signed informed consent from the research participants and the ethics approval has been waived by the ethics committee.

Simulation

Molecular dynamics (MD) simulations with all-atom model were performed by using GROMACS-2021 package and all structures were visualized using VMD software. In the theoretical simulation of the PVDF-TrFE system, all chains are pre-oriented in the form of fibers, and the phase transition rate is very fast. Therefore, in the MD simulation, the force field reported by Byutner et al.66 and Erdtman et al.67 was used to reasonably represent the three phases of α, β, and γ. The cooling/heating rate of 5 K/s has shown reasonable evidence of phase transition. Therefore, here the results were reported at a cooling/heating rate of 1 K/s, which is believed to be sufficient to study the phase transition of the fibrous PVDF-TrFE system. To further illustrate the negligible effect of cooling/heating on phase transition of PVDF-TrFE, extended simulations with slower cooling/heating rates were carried out at the same time (Fig. S25). Here, simulations were verified by comparing the results obtained by using different systems sizes, i.e., 64 chains, 100 chains and 400 chains, which shows no significant difference. The simulation details were also presented in the Supplementary Software 1.