Synthesis and characterization of P(VDF-TrFE) with C=C double bonds
In this work, P(VDF-TrFE) with the composition of 55/45 mol% close to its MPB regions is chosen for double bond modifications. C=C double bonds are induced by dehydrofluorination reactions triggered by strongly alkaline NaOH (Fig. 1a, top panel) differing from dehydrochlorination enabled by alkalescent agents revealed in Cl-contained polymers including P(VDF-CTFE), P(VDF-TrFE-CFE), P(VDF-TrFE-CTFE) (Fig. 1a, middle and bottom panels). Indeed, the reaction scheme (Fig. S1) in PVDF-based polymers has been studied in previous works. The dehydrofluorination leads to the formation of unsaturated C=C double bond which may yield further crosslinking reactions leading to reduced C=C content. Our results show that C=C double bond with a low NaOH concentration can be stable which is supported by complete dissolution into the solvent based on swelling experiments (Fig. S2). Above 0.9 mol%, crosslinking reactions are induced by the emergence of swelling effects inherent to the presence of crosslinking (Fig. S2). Such evolution arising from the reaction scheme is explicitly supported by Fourier-transform infrared (FTIR) spectroscopy. For instance, Fig. 1b shows that the presence of C=C double bond is confirmed by the characteristic infrared peak at around 1650 cm (ref. ). As NaOH dosage increases above 0.9 mol%, the characteristic peak density remains unchanged (Fig. 1b). This suggests that no additional C=C bonds forms due to the dehydrofluorination which in turn supports the formation of crosslinked structure. The saturation of C=C bonds occur near the critical NaOH content of about 0.9 mol%. Meanwhile, we find the presence of the infrared band at 1733 cm corresponding to C=O double bonds, which is not revealed in the previous works. The reaction scheme is summarized in Fig. S1. We show that the infrared band characteristic of C=O double bonds develops notably when NaOH dosage above 0.9 mol%, in constrast with that of C=C. Different from dual-functional C=C, C=O only behaves like defects which does not involve dehydrofluorination. The role of C=C and C=O in affecting d response will be analyzed later. Moreover, X-ray photoelectron spectroscopy (XPS) results show that C-H and C-F bond content decrease considerably with increasing NaOH dosage (Fig. 1c). This result indicates that dehydrofluorination still occurs which continues to produce unsaturated C=C bonds with the chararistic peak at around 285.0 eV (ref. ). The excessive formation of unsaturated C=C double bonds may trigger the opening of C=C double bonds which generates C-C single bonds resulting in crosslinked chains. As mentioned above, the presence of crosslinked structure for NaOH dosage above 0.9 mol% is confirmed by swelling results (Fig. S2). H nuclear magnetic resonance (NMR) was used to evaluate the C=C content (Figs. 1d and S3). The results show that C=C content is 0.86 mol% with a NaOH dosage of 0.9 mol% while it corresponds to 0.5 mol% with a NaOH dosage of 0.6 mol% (Fig. 1d). Consequently, these results elucidate the presence of C=C bonds at low NaOH dosage which saturates above 0.9 mol% accompanied by formation of crosslinked structure.
Structural characterization of P(VDF-TrFE) modified by various NaOH dosage is performed by XRD and FTIR. Pristine P(VDF-TrFE) is characterized by phase coexistence of all-trans and 3/1-helix conformations (Figs. 1e and S4) where the former remains the major phase (Figs. 1e and S5) according to the relative higher intensity for the peak at 2θ = 19.3 (all-trans) than that at 18.9 characteristic of 3/1-helix. Dehydrofluorination reaction not only induces the formation of C=C double bonds but also results in the decrease in VDF content. The latter may benefit the stabilization of 3/1-helix which is evident from the growth in the intensity for the peak assigned to 3/1-helix. Meanwhile, FTIR spectra provide further evidence to support the stabilization of 3/1-helix conformation and destabilization of all-trans conformation which can be seen by the notable development in the infrared band at 504 cm and smearing of the infrared band at 1285 cm (Fig. 1f). Consequently, recalling that tuning of the phase stability between all-trans and 3/1-helix conformations by varying the VDF content is essential to enhanced piezoelectric coefficients, the simultaneous realization of C=C double bonds and modifications of VDF content may offer an alternative way to tune the energetic landscape specifically for the pristine polymer composition close to MPB, benefiting d response.
The existence of phase evolution enabled by including C=C double bonds is supported by polarization-electric field (P-E) hysteresis loops (Fig. S6). P-E loop is a key signature used to distinguish long-range and short-range ferroelectric orders. As pristine P(VDF-TrFE) is dominated by all-trans conformation (Fig. 2a), its P-E loop is typical ferroelectric with a remnant polarization P of 4.5 μC cm and a coercive field E of 44.7 MV m (Fig. 2b, c). Interestingly, P(VDF-TrFE) with C=C double bonds exhibit pinched type P-E loops (Fig. 2a) with a sharp drop in P and considerable decrease in E, reminiscent of those observed in relaxor ferroelectric polymers by changing polymer compositions. The breaking of long-range ferroelectric instability induced by C=C double bonds is attributed to the stabilization of 3/1-helix conformation which is responsible for relaxor behavior in ferroelectric polymers. Like compositionally-induced MPB, such destabilization of long-range ferroelectric distortion into short-range relaxor order is crucial to enhanced d response. This approach is completely different from initial studies on PVDF to achieve a high fraction of polar all-trans conformation with high polarization. Above the saturation point (0.9 mol%), E remains nearly unchanged with a continuous decrease in P resulting from crosslinking (Fig. 2b, c). These results indicate the dominant role of dehydrofluorination (Fig. S7) while defects such as polar C=O defects play a minor role. As the opening of C=C double bonds may trigger the formation of C-C single bond, therefore, enabling covalent bonding between adjacent polymer backbones. This yields a marked reduction in crystallinity leading to lowered remanent polarization as long-range ferroelectric instability mainly arises from crystalline regions of ferroelectric polymers. In addition, we find that the inclusion of C=C and C=O defects induce a negligible change in leakage current behavior (Fig. S8) ruling out the presence of charged defects associated with the introduction of C=C and C=O defects. Consequently, P-E loop results demonstrate that incorporation of C=C defects stabilize disordered helix conformation through breaking the long-range ferroelectric distortion, which is well consistent with structural results.
Dielectric spectra under different frequencies upon heating is used to evaluate the evolution in short-range ferroelectric order (Figs. 2d-f and S9-S11). The pristine composition P(VDF-TrFE) 55/45 mol% displays the coexisiting ferroelectric and relaxor phases with the relaxor phase as a minor phase (Figs. 1e, 2a and 2d). The emergence of relaxor behavior is characterized by a strong shift in the dielectric peak temperature T towards higher temperatures as the frequency increases (Fig. 2d). In modified P(VDF-TrFE), it is found that relaxor behavior becomes stronger in terms of a broader dielectric peak at a specific frequency and marked decrease in the dielectric peak value (Fig. 2e, f). The broad dielectric peak is fitted by modified Cuire-Weiss relationship, which yields a diffuseness factor γ indicative of strength of relaxor phase (Fig. S12). It is found that γ increases as NaOH content increases, supporting the growth of relaxor phase (Fig. 2g). In addition, the temperature difference ΔT between T at 100 Hz and 1 MHz experiences a considerable increase (Fig. 2h), collaborating with stronger relaxor characteristics. In addition, the endothermic peak at around T is smeared in differential scanning calorimetry (DSC) heating scans (Figs. 2i and S13-S19), supporting more diffused phase transition due to the inclusion of C=C double bonds. Consequently, these results further support the stabilization of 3/1-helix conformation induced by incorporating C=C double bonds which is characterized by the growth of relaxor behavior.
Structural and electrical results indicate that the energetically more favorable phase evolves from ordered all-trans to disordered helix enabled by the inclusion of C=C double bonds instead of decreasing VDF content in previous compositional approach. Moreover, the presence of C=C and C=O double bonds may provide additional contribution to the piezoelectric response offering opportunities to compete favorably with previous compositional approach. In this regard, piezoelectric measurements are performed on pristine and modified P(VDF-TrFE) by using d meter and electric field-induced strain (Figs. 3a, b and S20a). To verify our measurement results, we focus on d of pristine P(VDF-TrFE) 55/45 mol% copolymer which has been measured in previous works. Our results show a d of -41.0 pC N and -40.0 pm V (Figs. 3a, b and S20a), respectively, which is well consistent with previous results. Meanwhile, through our piezoelectric measurements, d of commercial PVDF is -26.0 pC N (Fig. S21) and -26.4 pm V (Fig. S22), respectively, which also agree with recent results. The good agreement between different measurement techniques on different polymer compositions strongly substantiates the piezoelectric results obtained by other synthesized polymers. Interestingly, it is found that as NaOH dosage increases, d exhibits an abrupt increase reaching the maximum of -90.5 pC N at a critical NaOH dosage of 0.9 mol% (or 0.86 mol% C=C double bond), which is followed by dramatic drop in the crosslinking region (Fig. 3a, Fig. S20b, Movie S1). The electric field-induced strain manifesting the converse piezoelectric effect yields a converse piezoelectric coefficient d of -91.8 pm V (Fig. 3b), which verifies the existence of substantially enhanced piezoelectric response induced by C=C double bonds (Fig. S7). This corresponds to 3 times as large as that of PVDF, which also outperforms other results in P(VDF-TrFE) (see Table S1). We note that crystallinity decreases slightly from 55% to 51% when NaOH dosage increases from 0 mol% to 0.9 mol% (Fig. S25). This result indicates a minor role in decreasing d response caused by reduced crystallinity. The markedly lowered crystallinity induced crosslinking for NaOH dosage above 0.9 mol% implies that the reduction in the enhancement in d may result from the reduced crystallinity. Consequently, these results confirm the existence of markedly enhanced d enabled by incorporating C=C defects.
In addition to d (Fig. 3d), piezoelectric voltage constant g and figure of merit d×g -key parameters to evaluate the suitability of piezoelectric materials for energy harvesting and sensing applications- are also compared. We show that g and d×g are markedly improved in modified polymers (Fig. 3e, f and Table S2). The highest |g| of 459.5 × 10V m N is achieved for the NaOH dosage of 0.9 mol% (Fig. 3c), which exceeds that (394.1 × 10V m N) obtained by compositionally driven MPB (P(VDF-TrFE) 50/50 mol%). Meanwhile, the figure of merit is 41.6 pm N in modified P(VDF-TrFE) surpasses that (25.0 pm N) of P(VDF-TrFE) with MPB composition (Table S2). These results suggest the promise of double-bond modified copolymers in developing high-performance energy harvesters and sensors.
The enhanced piezoelectric response is achieved without the requirement of a static electric field, differing from that obtained in previous work. Given that the formation of C=C double bonds also involve the compositional change in VDF content through the dehydrofluorination, it is estimated that C=C and C=O double bonds themselves might account for over 40% of total enhancement in d based on the comparison between the results obtained in this work and that achieved by compositionally-induced MPB. The more electroactive ability induced by including C=C and C=O double bonds is evident from the large increase in dielectric constant (Figs. S23 and S24). In addition, only a slight decrease in crystallinity is found by incorporating a small amount of C=C double bonds (Fig. S25), which suggests a minor role of amorphous phase in affecting d response. The presence of C=C double bonds may ease the local polarization rotation in the crystalline phase, leading to the enhanced d. Further theoretical studies are highly desired to reveal the crucial role of C=C double bond in improving d response. Moreover, it is found that the benefit of enhancement in d enabled by C=C double bond requires polymer composition with a flattened energetic landscape i.e., close to MPB. For instance, it is shown that the absence of markedly enhanced piezoelectric response is observed for P(VDF-TrFE) with a well-defined ground state (i.e., VDF = 80 mol%, Fig. S26), which indicates the importance of nearly energetically degenerate phases in generating large piezoelectric response. Further studies are highly desired to modify the energetic landscape near MPB of ferroelectric polymers by using various chemical defects which generally involve a rich source of polymers such as terpolymers and polymer blends. Consequently, these results indicate the promise of chemical defects in optimizing intrinsic functionalities of ferroelectric polymers which should be generally applicable to other polymer compositions and other properties.
Given that high d exceeding commercial PVDF has been achieved by various approaches, the scaling potential has not been addressed, limiting large-scale practical applications. For instance, ultrahigh poling field of 650 MV m close to the breakdown field can be only used to pole small-size sample which may cause breakdown failure of polymer films for large scale applications. To explore the scaling potential, we fabricate solution-processed freestanding films with a size identical to A4 paper (Fig. 4a). The process of modified ferroelectric polymers does not require mechanical stretching as required in PVDF whereas flat film surface is challenging for orientation process (Fig. 4b). By contrast, solution-processed films show a much flatter surface (Fig. 4c) than PVDF (Fig. 4b) fabricated by extrusion-orientation and hot-pressing technique. Meanwhile, PVDF film thickness by orientation process typically exceeds 10 μm while our solution-processed films do not show such limitation in thickness scaling, which is of importance for device miniaturization and integration. We also examine the thickness dependance on d as scalability. The results show that d is independent of film thickness (Fig. S27) further indicating the presence of intrinsically large piezoelectric properties in modified P(VDF-TrFE), which is like commercial stretched PVDF. We find that a uniform d response is confirmed in flexible films (Fig. 4d) with a small variation in the film thickness (Fig. 4e), indicating the good scalability. Moreover, the dielectric constant exceeds that of PVDF by about 90% (Figs. 4f and S23 and S28a), which also benefits device miniaturization and integration owing to increased capacitance. The dielectric constant of PVDF and modified P(VDF-TrFE) generally decreases as the frequency of the external electric field increases (Fig. 4f). This is because orientational polarization cannot keep up with the rapid variation in the applied electric field at high frequencies. We show that dielectric loss of modified P(VDF-TrFE) remains nearly the same as that of PVDF at 1 kHz (Fig. S28b). Meanwhile, for durability of the vibrational device, it requires piezoelectric polymers with high elongation at break. We find that our modified films exhibit improved elongation at break up to 210% (Fig. S29), outperforming that of PVDF. Consequently, these results offer a new avenue for reconciling large piezoelectric properties and scalability to enable integration into massive manufacturing of next-generation ferroelectric polymers for flexible and wearable applications.
Although C=C double bonds have been previously used to yield a giant converse piezoelectric d (ref. ), it requires applying a static electric field which is not suitable for practical devices enabling the conversion of mechanical energy to electrical energy such as pressure and sound sensors. In this regard, to demonstrate the promise in flexible and wearable devices using C=C double bond modified P(VDF-TrFE), we fabricate pressure sensors (Figs. S30 and S31). We show that the open-circuit voltage measured under different stresses (Figs. 5a, S32 and S33) describes good linear dependence, in agreement with piezoelectric nature. This result is different from the nonlinear response with increasing the pressure obtained in a recent work which reported a large d of -191.4 pC N. Through the slope of voltage-pressure curve, pressure sensitivity can be obtained. Commercial stretched PVDF is known to show good energy harvesting properties and its pressure sensitivity is 8.11 mV kPa (Figs. 5b and S33) which agrees with previous results. We show that the pressure sensitivity (Fig. 5b) of pristine P(VDF-TrFE) is about 7.55 mV kPa, which is consistent with previous sensor performance (i.e., 4.56 mV kPa in ref. and 6.10 mV kPa in ref. ) based on P(VDF-TrFE) copolymers. Moreover, via double bond modifications, we find that the deduced pressure sensitivity is improved up to 14.22 mV kPa much larger than both stretched PVDF and pristine P(VDF-TrFE), which mainly originates from greatly enhanced d. We show that the capacitance before and after device fabrication remains slightly reduced (Fig. S34). Meanwhile, we find that the induced voltage displays almost no degradation up to 10 mechanical cycles (Fig. S35), demonstrating excellent durability upon vibrational testing. In addition, we find that senor performance remains unchanged under varied temperature (Fig. S36), humidity (Fig. S37) and mechanical strength (Fig. S38), suggesting excellent device stability. We also show that our device can detect small pressure above 8 kPa (Fig. S39), which is mainly limited by the force generator in our experimental setup. Based on the pressure sensitivity results (Fig. S39), we estimate that weak signals as low as tens of Pa can be resolved by our sensor based on modified polymers (Fig. S40).
The induced voltage and pressure sensitivity are strongly dependent on intrinsic factors (such as d, dielectric constant, the Young's modulus) and extrinsic factors (i.e., film thickness, encapsulation conditions, measurement conditions among different devices). For instance, regarding the intrinsic factors, although d of modified polymers is about 2.26 times as large as that of pristine counterparts, dielectric constant is also enhanced by 33% which tends to decrease the induced piezoelectric voltage. By assuming the same modulus, thickness and extrinsic conditions, the voltage generated by modified polymers is about 1.7 times as big as that of pristine counterparts which is in line with the observed experimental results.
We further show that these flexible sensors are ideal for wearable applications. For instance, we show that a wrist-worn sensor device made of modified P(VDF-TrFE) film (Fig. 5d) can distinguish the pulse wave signals at rest and after exercise (Fig. 5e). The observed physiological differences show the high sensitivity to weak signals, suggesting broad application prospects in wearable health monitoring systems. We also attach the sensor to the throat to record the signal waveforms of different spoken words (Fig. 5f). The results clearly distinguish different pronunciations (Fig. 5g, Movie S2 and S3), which indicates the potential application in acoustic sensing and speech recognition technology.