A crystal phase transition and its effect on the dielectric properties of a hydrogenated P(VDF-co-TrFE) with low TrFE molar content

Abstract

Hydrogenated P(VDF-co-TrFE)s with low TrFE molar contents were synthesized by atom transfer radical chain transfer (ATRCT) and controllable hydrogenation reactions. By using FTIR, XRD, and DSC techniques, the crystal composition was evaluated which largely depends on the TrFE content and electric field. Accordingly to explain the ferro- to paraelectric phase transition phenomenon at different electric fields, a phase diagram of hydrogenated P(VDF-co-TrFE) was drawn. Interestingly, we found that hydrogenated P(VDF-co-TrFE) with a low TrFE molar content of 9% possesses about half of all-trans beta phase in its crystal region, which is different from early reported copolymerized P(VDF-co-TrFE) with the same composition. Meanwhile, the effect of the field induced crystal structure on its ferroelectric property was depicted by dielectric spectra and displacement-electric field curves. As a result, a favourable remnant polarization of 9 μC cm⁻² and a large piezoelectric value of −25 pC N⁻¹ were obtained in a polarized hydrogenated P(VDF-co-TrFE) 80/20 mol% film, which provides a reliable result for the structure design of this kind of copolymer aiming at piezoelectric sensors and generators.

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1 Introduction

Because of its many unique advantages including a high electric breakdown electric field, good mechanical elasticity, ease of film fabrication, low cost, and self-healing ability,^1,2 poly (vinylidene fluoride) (PVDF) based ferroelectric fluoropolymers have attracted considerable attention from physics, chemistry,³ and electronic academics. After being fabricated into films or blocks with various thicknesses, they show favorable dielectric and piezoelectric properties due to high polar C–F and C–H molecular dipoles^4,5 and could be applied to nonvolatile memory devices,⁶ integrated circuits,⁷ and stationary power generations proceedings. During past decades, PVDF has been well confirmed expressing four phases α, β, γ, and σ depending on different processing conditions and tunable crystallinity from 50% to 70%.⁸ Among them β phase is most attractive conformation possessing large polarity which favors of its excellent ferroelectric and piezoelectric properties.

Several years later, a certain proportion of new monomers such as trifluoroethylene (TrFE) or tetrafluoroethylene (TFE) were added into PVDF molecular chains by Lovinger group for fabricating the conformation of the fluoropolymer.^9–11 They found that the introduced monomer could not only improve the crystal property of PVDF but also block the stability of kinetically form of C–C chains in TGTG′ conformation. As a result, the obtained copolymer poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-co-TrFE)) possesses favorable β phase when TrFE contents were adjusted to 20–50 mol%.^12,13 However, if TrFE content reduced to below 20 mol%, P(VDF-co-TrFE) shows mixed phases of α, β, and γ, where the β phase decreases as the ratio of TrFE.¹⁴ Nevertheless, after polarized the low TrFE content P(VDF-co-TrFF) at high electric filed, they appeared the phase transition from low polar α and γ to high polar β phase, and a related crystal phase diagram in regard to the relation of crystal phase composition and TrFE molar content was proposed in literatures.¹⁵

Meanwhile, in order to improve the piezoelectric property of P(VDF-co-TrFE), many researchers devoted their efforts to the field-induced phase transition mechanism and proposed a series of ferro- and paraelectric crystal phase models including the polarizing activity of crystal domain, amorphous, and interface molecular chains.^14,16,17 They used FTIR (or Raman spectra) and XRD technologies to characterize the crystal phase and chain conformation of P(VDF–TrFE).¹⁸ However, neither the bands in FTIR spectra nor the diffraction reflection peaks in XRD spectra could definitively character the composition of α, β, and γ phases. Fortunately, differential scanning calorimeter (DSC) technology was proposed in some work to reveal the crystal phase evolution of PVDF or P(VDF-co-TrFE).^14,19–22 Nevertheless, seldom of description using unique method is able to precisely identify and calculate the exact ratio of α, β, and γ phases involved in the phase transition at a strong applied field.

More recently, we obtained a P(VDF-co-TrFE) by an environment friendly and controllable P(VDF-co-CTFE) hydrogenation route named atom transfer chain transfer reaction (ATRCT),²³ which could avoid the disadvantages of conventional copolymerized process, such as high price resulting from low productivity, the poor control ability of the copolymer composition due to varied reactivity ratios of different monomers, and the hazard and difficulty during the transportation and storage of TrFE monomer, the productivity of copolymerized P(VDF-co-TrFE) was largely restricted by the high price of TrFE monomer.²⁴ Our previous study has demonstrated that VDF and TrFE units are mostly in head–head connection in the hydrogenated copolymer, which is responsible for the significant difference in the thermal and dielectric properties between the traditional P(VDF-co-TrFE) and hydrogenated copolymer with the consistent composition.^25–27 It has also been shown that the ferro- and piezo-electric properties of the hydrogenised P(VDF-co-TrFF) bearing 20 mol% TrFE is rather close to that of direct copolymer with 25 mol% TrFE, which has been widely investigated and utilized in sensors and actuators.²⁸ Although the influence of the composition and fabrication conditions on the electric properties of the hydrogenated P(VDF-co-TrFE) has been investigated recently, the systematic investigation of electrical filed-induced crystal phase transitions as well as the electric properties of hydrogenised P(VDF-co-TrFE)s have not been reported. By using DSC multipeak fitting technique, the crystal phase structure, TrFE content, and polarized electric field strength dependence of dielectric and ferroelectric properties was illustrated accordingly, which may provide a reference for tailoring the electrical properties of this kind of copolymers.

2 Experimental section

2.1 Materials

P(VDF-co-CTFE) containing varied CTFE molar ratio (6, 9, 12, and 20 mol%) was purchased from Solvay Solex and SynQuest Laboratory (America). The other chemicals were commercial available and used as received. P(VDF-co-TrFE)s with altered compositions were synthesized by an environmentally friendly and controllable P(VDF-co-CTFE) full hydrogenation process involving the transition-metal complex mediated atom transfer radical chain transfer reaction indicated in Scheme 1. According to split C–Cl bond, free radical in C–C molecular chain was obtained by using the catalyzer CuCl/Bpy. Then the radical could be compounded with H providing by N-methyl pyrrolidone (NMP) solvent, which results in the new formed C–H bond. The ¹H NMR (in acetone-d6, shown in Fig. 1.) from a Bruker AM-300 spectrometer was used to compare the structures of hydrogenized P(VDF-co-TrFE) with different TrFE contents as follows: 5.3–5.7 (m, 1H, –CFHCF₂–), 2.6–3.6 (m, 2H, –CF₂CH₂CF₂, H–T), 2.2–2.5 (m, 2H, –CF₂CH₂CH₂CF₂–, H–H). Moreover, the increasing peak intensity at 5.3–5.7 indicates that TrFE were increased from 6 mol% to 20 mol%. The series corresponding P(VDF-co-TrFE) with TrFE molar contents 6%, 9%, 12%, and 20% were marked as A6, A9, A12, and A20, respectively.

Scheme 1 Synthetic strategy of hydrogenized P(VDF-co-TrFE).

Fig. 1 ¹H NMR spectra of different hydrogenized P(VDF-co-TrFE) samples.

2.2 Preparation and characterization of hydrogenated P(VDF-co-TrFE) film

P(VDF-co-TrFE) films about 20 μm in thickness were prepared via casting the polymer solution in dimethylformamide (DMF) onto the quartz slide by evaporating solvent completely at 100 °C under reduced pressure. The obtained films were heated to 200 °C for 2 h followed by annealed at 140 °C for 24 h. Then the samples were metallized by coating both surface with gold as electrode (thickness is 80 nm and diameter is 5 mm) for all the electric properties measurements. The dielectric property was detected using an Agilent (4294A) LCR meter. The polarization–electric field hysteresis loop (P–E loop) was obtained by a TF Analyzer 2000 ferroelectric test system, and the electric field with a triangular wave form at a frequency of 10 Hz. For obtaining the filed induced samples, a high polarized electric field (275 MV m⁻¹, 30 mA) for 10 cycles was applied to P(VDF-co-TrFE) films from A6 to A20 and marked as A6-P to A20-P, respectively. NETZSCH DSC200 in nitrogen atmosphere with a heating rate of 2.5 °C min⁻¹ for the first cycles was employed for differential scanning calorimeter (DSC) analysis, and gold electrode in both surfaces of A6-P to A20-P samples was wiped off using nitro hydrochloric acid.

3 Results and discussion

3.1 Crystal structures

The crystalline structures as well as the conformation of P(VDF-co-TrFE) were determined by FTIR as shown in Fig. 2. The absorbance peak at 1284–1290 cm⁻¹ is well recognized as the trans isomer sequence with four or more than four units (TTTT or more, characteristic of β phase structure) which is different from the spectrum of PVDF, indicating that the introduction of TrFE makes the crystalline properties of resultant copolymer more complicated.^29,30 The decreasing absorbance peak at 614 cm⁻¹ as the TrFE contents from 6% to 20% is attributed to decrease of α phase with trans–gauche sequence (TG). The band at 510 cm⁻¹ is assigned to T₃G conformation corresponding to γ phase. Moreover, α phase could be clearly identified by the FTIR absorption bands at 530 cm⁻¹, 614 cm⁻¹, 765 cm⁻¹, 795 cm⁻¹, and 976 cm⁻¹ which is decreased as the TrFE contents. However, the identification of γ and β phases on FTIR is rather difficult for their similar polymer chain conformation and close characteristic absorption bands at 510 cm⁻¹, 810–840 cm⁻¹, and 1284–1290 cm⁻¹. As a matter of fact, none of these peaks could be applied to distinguish one type of crystal phase from the other one precisely, especially when the samples have the combined crystal phases.

Fig. 2 FTIR spectra of different hydrogenized P(VDF-co-TrFE) samples.

The crystal phase structure of P(VDF-co-TrFE) film could also be illustrated from XRD pattern. As shown in Fig. 3(a), the pattern of A6 has three well defined peaks at 17.82°, 19.94°, and 26.66°, corresponding to the plane 100, 110, and 021, respectively. All of them are the characteristics of α phase. Besides, a weak peak of 18.52° (plane 020) suggests the low content of γ phase. After polarization, the diffraction intensity plane 100 is depressed, which may be attributed to the phase transition from some α phase in A6-P films to β or γ phase (plane 110/200 and 002), as presented in Fig. 3(b). Comparing to A6, the higher peaks of 18.52° and 20.01° of A9 suggest that it possesses more γ and β phase before polarization. The result is different from the phase structures of traditional copolymerized P(VDF-co-TrFE) in literatures,^14,15 which is reported that β phase could only be found in the copolymer with more TrFE molar content (12–50 mol%). As expected, the peak at 20.01° is increased after polarization (Fig. 3(b)). In addition, the XRD pattern of A12-P and A20-P shows little change in position, but the increasing of peak intensity also indicating that the β crystal phase is enhanced after polarization.

Fig. 3 XRD diffraction patterns of different P(VDF-co-TrFE) samples: (a) before polarized; (b) after polarized at 275 MV m⁻¹ for 10 cycles.

DSC curves were used to detect the crystal phase composition definitely, as shown in Fig. 4. It is well confirmed that the melting temperature (T_m) of β phase is obviously lower than that of α and γ phase in neat PVDF,³¹ while T_m of α-PVDF is slightly lower than γ-PVDF. Three high endothermic peaks observed in sample A6 to A12 at elevated temperature could be attributed to the melting temperature (T_m) of β, α, and γ phases successively (marked as T^β_m, T^α_m, and T^γ_m, respectively). The peaks observed at 140.7 °C, 149.2 °C, and 147.8 °C are assigned to T^β_m A6, A9, and A12 (Fig. 4(a–c)), respectively. It is confirmed that the addition of TrFE with low content is responsible for the formation of all-trans conformation (β phase) and for part of the TTTG conformation (γ phase).^14,15,32 As TrFE content is increased to 20 mol%, the crystal phases of the copolymer are turned from α + γ + β mixed phases (in A6, A9, and A12) to neat β phase (Fig. 4(c)). Comparing the as-casted films, the polarized A6-P, A9-P, and A12-P possess the elevated T^β_m peaks at 147.1 °C, 149.2 °C, and 147.8 °C and depressed T^γ_m of 165.2 °C, 160 °C, and 157 °C, respectively. Apparently, the phase transition from α or γ to β was completed in P(VDF-co-TrFE) with low TrFE contents which is consisted with the conclusion of XRD pattern.

Fig. 4 Comparison of DSC scan results of different P(VDF-co-TrFE) samples before polarized and after polarized at 275 MV m⁻¹ for 10 cycles (T_c is the Currie point, and T^β_m, T^α_m, and T^γ_m represent the melting point of β, α, and γ phase, respectively): (a) A6, (b) A9, (c) A20, and (d) A20.

In addition, the DSC multipeak fitting technique reported in literature was employed to estimate the crystal phase structures of P(VDF-co-TrFE) as presented in Table 1.³³ As α and γ phase reduced, the contents of β phase in A6-P, A9-P, and A12-P are increased from 9.6%, 47.6%, and 64.3% to 24.5%, 68.3%, and 90.2%, respectively. The result further shows that strong electric field polarization is responsible for the ordered dipoles and even phase transition of all trans β phase in this hydrogenated P(VDF-co-TrFE).

Table 1 Crystal phase composition of hydrogenated P(VDF-co-TrFE)

P(VDF-co-TrFE)			A6	A9	A12	A20
Phase content (%)	Un-polarized	α	72.8	27.0	19.9	0
		β	9.6	47.6	64.3	100
		γ	16.6	25.4	15.8	0
	Polarized	α	32.3	10.3	7.1	0
		β	24.5	68.3	90.2	100
		γ	43.2	21.4	2.7	0

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Besides the melting point, other endothermic peaks, Currie temperature (T_c) was also shown in Fig. 4. Before polarization, T_c of A9, A12, and A20 at about 124.7 °C, 99.8 °C, and 90.9 °C indicates the existence of ferroelectric phase (Fig. 4(b–d)). After polarized 10 cycles at 275 MV m⁻¹, both the molecular chains in β phase and γ phase are oriented and shown the characters of ferroelectric crystal domains. As a result, the T_c peaks related to β phase at 131.2 °C, 100.02 °C, and 93.1 °C are intensified, known as the enhancement of polarized ferroelectric β phase of A9-P, A12-P, and A20-P, respectively. Moreover, the emerging peak of T_c at 93.02 °C of A6-P further indicates that strong electric field could lead to the phase transition in the hydrogenated P(VDF-co-TrFE) with low TrFE content.

3.2 Phase diagram of hydrogenated P(VDF-co-TrFE)

By using the T_m and T_c presented in Fig. 4, the phase diagram of hydrogenated P(VDF-co-TrFE) could be drawn as shown in Fig. 5, where the T_m of pure PVDF with different crystal phases was obtained from our previous literature.³¹ Because of the improving of regularity of the all trans conformation (TTTT) at high polarizing electric field, T_c of A6-P to A20-P is higher than A6–A20, respectively. In order to explain the DSC results, a Gibbs free energy diagram as the function of temperature was performed as illustrated in Fig. 6. Different lines represent of Gibbs free energy of ferroelectric phase (μ_F), paraelectric phase (μ_p), and melting phase (μ_m) as a function of temperature,³⁴ where T_c is known as the intersection point of μ_F and μ_p. Apparently, the reduction of Gibbs free energy of ferroelectric phase for the increase of β phase will lead to the enhancement of T_c in polarized P(VDF-co-TrFE) as indicated in dashed line. For A20 sample, the disordered dipoles in pure β phase were decreased after polarization, which leads to the reduction of Gibbs free energy of paraelectric phase as well as the increases of T_m. When TrFE content reduced to 12 mol% and 9 mol%, respectively, P(VDF-co-TrFE) possesses multiple crystal phases, and the relationship between T_m and TrFE contents is complicated. In sample A12 and A9, T^β_m, T^α_m, and T^γ_m were decreased after polarization because of the descending μ_p as depicted in Fig. 6. One is that some β phase derived from α or γ phase, which contains many low T_m disordered dipoles. Secondly, the polarization led to the reduction of the proportion of high T_m α or γ phase in A12-P and A9-P sample, and T_m was reduced accordingly. However, in A6 and neat PVDF, although a small portion of α or γ phases turned into β phase at high electric filed, the major unconverted disordered dipoles in crystal α or γ phase turned regularity, which led to the increases of T^β_m, T^α_m, and T^γ_m.

Fig. 5 Phase diagram of P(VDF-co-TrFE): Curie temperature T_c and melting temperature T_m determined from DSC peaks in heating process are plotted. T_m of different phase are distinguished (T^β_m, T^α_m, and T^γ_m represent the melting point of β, α, and γ phase).

Fig. 6 Gibbs free energy diagrams of ferroelectric, paraelectric, and melt phases of P(VDF-co-TrFE).

It is concluded that the mixed phase of α, β, and γ could be obtained in low TrFE content copolymer (6 mol%), and even the proportion of β phase reached to 47.6% in P(VDF-co-TrFE) 91/9 mol% sample, as presented in Table 1. The results different from the phase diagram of direct P(VDF-co-TrFE) where β phase was only found in more TrFE content samples (at least 10 mol%).¹⁴ After polarization, the ratio of β phase of A6-P, A9-P, and A12-P increased from 9%, 47.6%, and 64.3 to 24.5%, 68.3%, and 90.2%, which favours of their ferroelectric and piezoelectric properties.

3.3 Dielectric properties

The dielectric constant (ε_r) and loss (tan δ) of as-casted and polarized P(VDF-co-TrFE) with different TrFE contents were presented in Fig. 6(a). ε_r of all the samples decreases continuously as a function of frequency from 100 Hz to 100 kHz with a quick drop at frequency from 100 kHz to 100 MHz corresponding to the relaxation of dipoles in large scale, which is consistent with early studies. It has been reported that α-PVDF phase possesses larger ε_r at low frequency, and ε_r of γ-PVDF phase is lower than that of β-PVDF.³⁵ Therefore, relatively low ε_r of A6 and A9 was found at the frequency below 1 MHz for their high γ phase content. As the frequency increased to 10 MHz, ε_r of A12 and A20 decreased quickly and tan δ is increased for the relaxation of dipole orientation. After polarized, some of α phase crystals in A6 and A9 are turned into γ and β phase leading to the depressed ε_r, as shown in Fig. 6(b), where ε_r of A6-P and A9-P is reduced from 9.1 and 10.2 to 7.6 and 7.7, respectively. However, little change of ε_r is observed in A20-P for its pure β phase.

Moreover, the heartbreak shape peaks (Fig. 7(b)) of tan δ detected in all the polarized samples at about 10 MHz are referred to the resonance effect of piezoelectric materials. Fig. 8 presents the magnified dielectric resonance spectrum at the frequency from 40 MHz to 100 MHz. The resonance peaks at 70 MHz represent the piezoelectric and electro-mechanical properties of polarized P(VDF-co-TrFE) films, which is discussed in ref. 28. The thickness of the films is far smaller than the diameter, if it is utilized in longitudinal thickness mode, the relation of piezoelectric resonance could be expressed by eqn (1) as expressed in literatures.^36,37

d₃₃ ∝ k_t(S₃₃^Eε₃^T)^1/2 (1)

Where S₃₃^E is the compliance under constant field, ε₃^T is the dielectric constant under constant stress, and k_t is the electromechanical coupling factor. In this work, piezoelectric value (d₃₃) of P(VDF-co-TrFE) film was obtained by measuring the overall amount of surface electric charges on a ZJ-4A static piezoelectric test system. Apparently, both k_t and d₃₃ results suggest that the ferroelectric domains are enhanced as TrFE molar content increases from 6% to 12%.

Fig. 7 Dielectric properties of different P(VDF-co-TrFE) samples: (a) before polarized and (b) after polarized at 275 MV m⁻¹ for 10 cycles.

Fig. 8 Dielectric constant and loss of A6-P to A12-P around resonance frequency after polarized at 275 MV m⁻¹ for 10 cycles.

Fig. 9 presents the dielectric properties of hydrogenised P(VDF-co-TrFE) film with different TrFE contents at a range of temperature from 40 °C to 155 °C. Apparently, ε_r of all samples decreases as the frequency increases at all temperature, which consist with Fig. 7. ε_r of A6 shows the wide peaks of dielectric relaxation (Fig. 9(a)). However, the ε_r peaks appeared in Fig. 9(b–d) were not only related to dielectric relaxation but known as the ferroelectric–paraelectric (F–P) phase transition. The peaks of A9-A20 films situated at 124 °C, 113 °C, and 109 °C, respectively, which also correspond to T_c. The descending T_c from about 125 °C of A9 to 108 °C of A20 indicates that the Gibbs free energy of paraelectric phase decreases as the TrFE content increases.

Fig. 9 Dielectric properties as a function of temperature of different P(VDF-co-TrFE) samples: (a) A6, (b) A9, (c) A12, and (d) A20.

In addition, we could observed the peaks of tan δ in A9 to A20 samples above 80 °C, which is well confirmed the dielectric relaxation due to dipolar relaxation in crystalline or crystal-amorphous interface regions.³⁸ Interestingly, the relaxation peak was found to be shifted towards lower temperature from 105 °C (A9, Fig. 9(b)) to 95 °C (A20, Fig. 9(d)) as the increasing of β phase, which may attribute to decreases of crystal–amorphous of interface region.

3.4 Ferroelectric properties

To illustrate the effect of polarized field on the ferroelectric property of the hydrogenated P(VDF-co-TrFE), polarization hysteresis loops of A12-P with various polarization cycles at 200 MV m⁻¹ were presented in Fig. 10. The remnant polarization (P_r) and saturated polarization (P_s) are increased with the applied cycles, which due to the increasing of regularity of more and more disordered ferroelectric domains both in crystalline and amorphous phase as the repeated E_p.

Fig. 10 Polarization hysteresis loops (P–E loops) of A12 at 200 MV m⁻¹ with several cycles at about 10 Hz.

Fig. 11 presented the P–E loops of A6-P to A20-P at various electric field. Apparently, P–E loops of all the samples show the characters of normal ferroelectric polymer, where both P_s and P_r are increase straight as the electric field and reach to a saturated value. In samples A6-P and A9-P with low TrFE content, the crystal domains of γ phase plays an important role in the improvement polarization value, and P_r could reach to 5.8 μC cm⁻² at 375 MV m⁻¹ and 6.0 μC cm⁻² at 325 MV m⁻¹, respectively, as presented in Table 2 and Fig. 11(a–b). As TrFE increases, P_r is mostly originated from the ferroelectric domain of high polar β phase in samples A12-P and A20-P. Consequently, higher P_r of 7.9 μC cm⁻² and 8.2 μC cm⁻² could be obtained under the polarized field of 300 MV m⁻¹ and 275 MV m⁻¹, respectively, as shown in Table 2 and Fig. 11(c and d). In addition, for the majority of high polar β phase domains, A12-P and A20-P show much larger P_s (11.3 μC cm⁻² and 12.3 μC cm⁻²) than A6-P and A9-P at 275 MV m⁻¹. As a result, the high piezoelectric values (d₃₃ = −25 pC N⁻¹ and −19 pC N⁻¹) could be obtained in A20-P and A12-P after polarized at 275 MV m⁻¹ with 10 cycles, respectively.

Fig. 11 Polarization hysteresis loops at various electric field (E_p) of P(VDF-co-TrFE) with different TrFE contents at about 10 Hz: (a) A6, (b) A9, (c) A12, and (d) A20.

Table 2 Electric properties of different P(VDF-co-TrFE) samples at the E_p of 275 MV m⁻¹

P(VDF-co-TrFE)	Ec (MV m^−1)	Ps (μC cm^−2)	Pr (μC cm^−2)	d33 (pC/N)	εir
A6	105.2	5.5	2.1	−6	28.8
A9	92.9	7.83	4.7	−13	32.3
A12	87.1	11.3	7.6	−19	45.5
A20	60.1	12.3	8.33	−25	50.4

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The relationship between ferroelectric activity and TrFE content could also be illustrated with the coercive electric field (E_c) as well, which has been well proved to be more relate to the ferroelectric domains involved in the polarization reversal.^14,15 As shown in Fig. 11 and Table 2, E_c is decreased from 105.2 MV m⁻¹ of A6-P to 60.1 MV m⁻¹ of A20-P at 275 MV m⁻¹, indicating that the ferroelectric domains in β phase tend to be easier to be oriented than low polar γ phase. The reversing γ phase crystal domain may suffer from the resistance of α phases with large crystal grain size in the mixture crystal phase P(VDF-co-TrFE) (A6-P and A9-P). As E_p increases, the added E_c illustrates that more γ phase was forced to conquer the more surrounded resistance, as indicated in Fig. 9(a and b). Interestingly, as shown in Fig. 10, if the same E_p was repeated for 10 cycles, although P_r and P_s were increased accordingly, E_c of A12-P did not appear to add up. That illustrates the ferroelectric domains was not respond to the fixed E_p any more unless the strength of E_p was added.

Besides of P–E loops, we proposed a temporary dielectric constant (ε_ir) to illustrate the effect of electric field induced phase transition on the dielectric property of hydrogenated P(VDF-co-TrFE) as calculated from eqn (2) by using the data of Fig. 11.

where ε_ir represents the ratio of the polarization (P_i) to a certain polar electric field.

As presented in Fig. 12, ε_ir of P(VDF-co-TrFE) is not a constant under varied electric field. The elevated ε_ir obtained under strong electric field may be attributed to the reversal of dipoles, interface polarization, ionic polarization, and conduction loss. Interestingly, ε_ir of all the samples shows the similar reversed V shaped as E_p increases. In addition, ε_ir of A20-P is increasing dramatically as the applied electric field for the reversal of high polar β phase. As E_p is increased to 100 MV m⁻¹ over its E_c (∼60 MV m⁻¹), the dipoles of β phase responding to electric field are saturated and a maximum ε_ir of 73 is obtained accordingly. Subsequently, no more excessive dipoles polarization could response to the elevated E_p (>100 MV m⁻¹), and ε_ir decreases and maintains a proper value. For this inference, ε_ir of A6-P, A9-P, and A12-P shows the similar curve, and it only reach to 31.1, 33, and 51.7 because of its relative low β phase contents, respectively.

Fig. 12 Corresponding temporary dielectric constant of P(VDF-co-TrFE) from P–E loops.

4 Conclusions

The hydrogenated P(VDF-co-TrFE) with various TrFE molar contents were obtained through a chemical friendly ATRCT route from P(VDF-co-CTFE). The phase structure including α, β, and γ phase as well as the field induced phase transition details were expressed by the DSC detector, which indicates that polarization could reduce the disordered molecular chains and improve the polar β phase accordingly. Interestingly, hydrogenated P(VDF-co-TrFE) 91/9 mol% possesses high proportion of all trans β phase in its crystal region, which is different from traditional P(VDF-co-TrFE) with same TrFE composition. The following dielectric and ferroelectric results show that the saturated and remnant polarization increases as the TrFE proportion and the cycles of polarized electric field. Therefore, the favorable ferroelectric property of 9 μC cm⁻², a large piezoelectric value of −25 pC N⁻¹, and a high dielectric constant of 73 were obtained in hydrogenated P(VDF-co-TrFE) 20 mol% thick film. Considering the convenient synthetic strategy and wealth of original P(VDF-co-CTFE) recourse, the hydrogenated P(VDF-co-TrFE) is expected to be accepted more broadly in piezoelectric sensors and generators areas.

Acknowledgements

This work was financially supported by National Nature Science Foundation of China-NSAF (Grant No. 51103115, 2015JM5155, 2014JK0441, and 50903065).

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