Vinylidene fluoride telomers for piezoelectric devices

Abstract

Original telomers based on vinylidene fluoride (VDF), C_nF_2n+1(VDF)_m(CH₂CH₂)_pI, where n=4 or 6, m=5–23 and p=0 or 1, were synthesized, and the piezoelectric properties of the materials were investigated. Low-molecular-weight polyvinylidene fluorides (PVDFs) were prepared in good yields by iodine transfer polymerization of VDF. The polymerization of VDF was conducted in the presence of 1-iodoperfluoroalkanes (C₄F₉I or C₆F₁₃I) and bis(4-tert-butylcyclohexyl) peroxydicarbonate, which were used as a chain transfer agent and initiator, respectively. The microstructures of VDF telomers were carefully characterized by ¹H and ¹⁹F nuclear magnetic resonance (NMR) spectroscopy to assess the average molecular weights and the percentage of CH₂CF₂I functionality (that ranged from 47 to 86%). X-ray diffraction patterns of these telomers indicated that PVDFs with <7 VDF units displayed the α-form. Alternatively, telomers containing a higher amount of VDF possessed both α- and β-forms. Ethylene endcapping of C_nF_2n+1(VDF)_mI was also successfully achieved in high yield, and the telomeric precursors were converted quantitatively. Moreover, three VDF telomers were vaporized into films, and the piezoelectric properties of the materials were assessed. The results revealed that C₆F₁₃(VDF)₂₃I and C₄F₉(VDF)₂₁I telomer films displayed piezoelectric responses (d₃₃=268 pC N⁻¹) and high remanent polarizations (Pr=98.2 and 93.4 mC m⁻², respectively). In addition, C₄F₉(VDF)₂₁C₂H₄I exhibited interesting ferroelectric properties (Pr=89.9 mC m⁻²).

Introduction

Polyvinylidene fluoride (PVDF) is an interesting polymer that exhibits remarkable properties. Thus, PVDF has been used in high-tech applications, such as^{1, 2, 3} paints and coatings, medical suture wires, mechanical substrates for fuel cell membranes, liners for containers used to carry corrosive substances (for example, bromine), separators for lithium ion batteries and backsheets for photovoltaic cells. In addition, PVDF is a piezoelectric polymer,^{4, 5} and pressure or mechanical stress can induce an electrical response.

Several poly(VDF-co-TrFE) copolymers (where TrFE represents trifluoroethylene) possess piezoelectric and ferroelectric properties similar to those of CF₃(VDF)_13.4C₂H₅^{6, 7}, CF₃(CH₂CF₂)₁₂I⁸ or CF₃(VDF)₁₇I⁹ telomers, which have been prepared by radical telomerization of VDF with CF₃I. However, data on the synthesis and nature of the end group (−CH₂CF₂I, −CF₂CH₂I, −CH₂CH₂I, −CF₂CH₃ or −CH₂CH₃) are relatively scarce. Nevertheless, in previous investigations, the radical telomerization of VDF with CF₃I has been conducted, and the products were characterized by ¹⁹F nuclear magnetic resonance (NMR) and matrix-assisted laser desorption/ionization–time of flight.¹⁰ The results revealed that CF₃(VDF)₁₁I and CF₃(VDF)₂₀I structures were produced, and the number of VDF units and the identity of the end groups were unambiguously determined. The radical telomerization of VDF with various chain transfer agents (CTAs) has been recently summarized.¹¹ Moreover, studies on the iodide transfer polymerization¹² of VDF in the presence of perfluoroalkyl iodides (R_FI)^{13, 14, 15, 16} revealed that controlled polymerization could be achieved when R_FI (CTA) possessed a CF₂I end group.

Alternatively, due to their low molecular weights and satisfactory thermal stability, VDF telomers can be evaporated without thermal decomposition. Thus, a thin film can be prepared by vacuum evaporation,⁶ which is an effective method for controlling the crystalline phases and the molecular orientation of VDF telomers. Among organic ferroelectrics, CF₃(VDF)₁₇I possesses a relatively high remanent polarization.⁹ Furthermore, previous experiments on CF₃(VDF)_13.4C₂H₅ revealed that the piezoelectric coefficient of VDF telomers was greater than that of oligo(VDF-co-TrFE) copolymers, and a thinner and more uniform film could be obtained.^{6, 7}

The objective of this study was to design various VDF-based telomers, such as C_nF_2n+1(VDF)_p-(C₂H₄)_x-X (where n=4 or 6; p=6–23; x=0, 1 and X=H or I), and to alter the crystallinity, ferroelectric and piezoelectrical properties by controlling the number of VDF units and end groups.

Experimental procedure

Materials

Perfluorohexyl iodide and perfluorobutyl iodide (purity 99%) were kindly supplied by Atofina, France, and were treated with sodium thiosulfate, dried, and distilled before use. Bis(4-tert-butylcyclohexyl) peroxydicarbonate (Perkadox 16S, purity 99%) and vinylidene fluoride (VDF) were provided by Akzo Nobel, Chalons sur Marne, France and Solvay S.A., Tavaux, France, respectively, and were used as received. Acetonitrile (99%) was obtained from Aldrich Chimie, Saint Quentin-Fallavier, France, and was used without further purification.

Characterizations

Structural analyses. The composition and structure of the telomers produced by iodine transfer polymerization of VDF were determined by ¹⁹F (400 MHz) and ¹H NMR (400 MHz) spectroscopy. NMR spectra were recorded on a Bruker AC 400 Instrument (Bruker, Wissembourg, France), and deuterated acetone and tetramethylsilane were used as the solvent and the reference, respectively. Coupling constants and chemical shifts are provided in hertz (Hz) and part per million (p.p.m.), respectively. The following experimental conditions were used to obtain the ¹H (or ¹⁹F) NMR spectra: flip angle=90° (or 30°); acquisition time=4.5 s (or 0.7 s); pulse delay=2 s (or 2 s); number of scans=128 (or 512); pulse width=5 μs (¹⁹F NMR). Infrared spectra were obtained with a Nexus-Nicolet spectrophotometer (Nexus Nicolet, Nicolet, WI, USA) between 600 and 1000 cm⁻¹, and the samples were prepared as KBr pellets. The accuracy of the measurement was ±2 cm⁻¹. Size exclusion chromatography was performed with a Varian ProStar equipped with a refractive index detector (Varian, Oxford UK). Samples (20 mg) were dissolved in N,N-dimethylformamide and filtered before measurement. N,N-dimethylformamide at a flow of 0.8 ml min⁻¹ and 25 °C was used as the eluent, and a calibration curve with five polymethylmethacrylate standards was produced. X-ray powder diffraction patterns of the products were recorded on a X’PERT PHILIPPS II diffractometer (X’PERT PHILIPPS II, Charlotte, NC, USA) equipped with Cu Kα radiation (λ=1.540 Å) at 30 kV and Braggs angles of 10–50°.

Assessment of ferroelectric properties. Electric displacement–electric field (D–E) hysteresis loops of the VDF telomer films were obtained with a system used to evaluate the ferroelectrics of a material. The apparatus consisted of a current/charge-to-voltage converter (Toyo Corporation, Tokyo, Japan, Model 6252), an arbitrary waveform generator (Biomation, San Diego, CA, USA, 2414B) and an analog-to-digital converter (WaveBook 516, IOtech, Cleveland, OH, USA). All measurements were carried out under a nitrogen atmosphere at room temperature.

Determination of piezoelectric effect. The experimental apparatus used to evaluate the piezoelectricity of the samples is illustrated in Figure 1, and is identical to the device used in previous studies.^{6, 7} The sample was clamped between the upper and lower plates of the sample holder by four screws and an O-ring. To determine the piezoelectric coefficient along the thickness of the film, the sample was supported by the lower plate, which was made of rigid metal. A cavity was positioned between the upper plate and the sample, and a pressure sensor (Keyence Corp., Osaka, Japan, AP-43, AP-C40) and an inflator were coupled to the apparatus through the upper plate. Using an inflator, the pressure of the air in the cavity was manually varied, and the stress on the sample film was uniform. The electrodes were connected to a preamplifier (gain: 40 dB; NF Corp., Yokohama, Japan, LI-75A), and the output voltages and pressure of the sample were acquired with an oscilloscope and a pressure sensor, respectively. All measurements were carried out under air at room temperature.

Figure 1

Schematic depiction of the experimental apparatus used to determine the piezoelectricity of the vinylidene fluoride telomers.

Autoclave

Iodine transfer polymerizations were performed in a 160-ml Hastelloy Parr autoclave system (HC-276, Molines, IL, USA) equipped with a manometer, mechanical Hastelloy anchor, rupture disk (3000 psi), and inlet and outlet valves. An electronic device was used to regulate the stirring rate and temperature of the autoclave. Before the reaction, the autoclave was pressurized with 30 bars (430 psi) of nitrogen to identify leaks. The autoclave was conditioned for the reaction by conducting several nitrogen/vacuum cycles (10⁻² mbar) to remove oxygen. The liquid and dissolved solid phases were introduced via a funnel, and the gas (VDF or ethylene) was introduced by double weighing (that is, the difference between the weight of the system before and after the autoclave was filled with gas).

Iodine transfer polymerization of VDF and ethylene endcapping

Example of iodine transfer polymerization of VDF in the presence of perfluorobutyl iodide at 55 °C. To the aforementioned 160-ml Hastelloy (HC-276) autoclave, 1.24 g (3 × 10⁻³ mol) of bis(4-tert-butylcyclohexyl) peroxydicarbonate, 10.82 g (3 × 10⁻² mol) of perfluorobutyl iodide and 50 ml of dried acetonitrile were added. Next, 21 g (3 × 10⁻¹ mol) of VDF was inserted into the reaction mixture. The autoclave was progressively heated to 55 °C, and an exotherm of ∼10 °C was observed, followed by an increase in pressure from 5 to 12 bars. Subsequently, a sharp decrease in pressure to 1 bar was observed. When the reaction was complete, the autoclave was placed in an ice bath for 60 min, and unreacted VDF was progressively released (VDF conversion was ∼100%). When the autoclave was opened, ∼62 g of a brown liquid was obtained. The crude mixture was evaporated to remove acetonitrile and was redissolved in acetone. Finally, the telomers were precipitated from pentane. A yellow powder (crude sample, 30 g) was obtained, and ¹⁹F NMR (Figure 2) spectroscopy indicated that the number-average degree of polymerization (DPn) value was 12. The sample was further characterized by ¹H NMR and Fourier transform infrared (FT-IR) (Figure 5) spectroscopy. Next, the crude sample was dissolved in a minimal amount of acetone and was precipitated from methanol. The precipitated powder (high molecular weight) was dried under vacuum for 4 h, and a yellow powder (12 g, 96% yield) was obtained. The methanol filtrate was evaporated, and 18 g of a brown wax (low molecular weight, Supplementary Figure S1) was isolated. Both fractions were characterized by ¹⁹F NMR spectroscopy (Figure 3) and size exclusion chromatography, and the results revealed that the fractions displayed different DPn values (Figure 4): low-molecular-weight (L) DPn=9; high-molecular-weight (H) DPn=22.

Figure 2

¹⁹F nuclear magnetic resonance (NMR) (upper spectrum) and ¹H (lower spectrum) NMR spectra of C₄F₉(VDF)₂₁-I (obtained in deuterated acetone, 400 MHz, 298K) (T1-H in Table 6).

Figure 5

Fourier transform infrared (FT-IR) spectrum (between 4000 and 650 cm⁻¹) of C₄F₉-(VDF)₂₁-I, which was obtained by the iodine transfer polymerization of vinylidene fluoride (VDF) with C₄F₉I (from T1-H in Table 6).

Figure 3

¹⁹F nuclear magnetic resonance (NMR) spectra of C₄F₉(VDF)_n-I (obtained in deuterated acetone, 400 MHz, 298 K) from T1 in Table 6 (upper spectrum, C₄F₉(VDF)₁₂-I), T1-L in Table 6 (middle spectrum, C₄F₉(VDF)₉-I), and T1-H in Table 6 (lower spectrum, C₄F₉(VDF)₂₁-I).

Figure 4

Size exclusion chromatograms of C₄F₉(VDF)_n-I (obtained in N,N-dimethylformamide (DMF), 298 K) from T1 (——, C₄F₉(VDF)₁₂-I), T1-H (– –, C₄F₉(VDF)₂₁-I) and T1-L (— —, C₄F₉(VDF)₉-I) in Table 6.

Size exclusion chromatography was conducted in N,N-dimethylformamide, and the results confirmed that the two populations were completely separated. As a result, the polydispersity index of the material decreased (Figure 4). The same synthetic procedure was used for the iodine transfer polymerization of VDF with perfluorohexyl iodide. The NMR characteristics of the samples in deuterated acetone are reported in Tables 1 and 2 .

Table 1 Assignments of ¹⁹F NMR chemical shifts in poly(VDF)-I obtained by iodine transfer polymerization of VDF in the presence of C₄F₉I ([VDF]_o/[C₄F₉I]_o/[initiator]_o=100/10/1), T=55 °C; t=4 h; initiator: bis(4-tert-butylcyclohexyl) peroxydicarbonate

Table 2 Assignments of ¹H NMR chemical shifts in poly(VDF)-I obtained by iodine transfer polymerization of VDF in the presence of C₄F₉I ([VDF]_o/[C₄F₉I]_o/[initiator]_o=100/10/1), T=55 °C; t=4 h; initiator: bis(4-tert-butylcyclohexyl) peroxydicarbonate

The FT-IR characteristics of the samples (as KBr pellets) are reported in Table 3 .

Table 3 Assignments of FT-IR frequencies in R_F(VDF)_m-I obtained by iodine transfer polymerization of VDF in the presence of C₄F₉I ([VDF]_o/[C₄F₉I]_o/[initiator]_o=100/10/1), T=55 °C; t=4 h; initiator: bis(4-tert-butylcyclohexyl) peroxydicarbonate

Example of ethylenation of PVDF-I at 55 °C. The same autoclave used in the iodine transfer polymerization of VDF was used to ethylenate the telomers. Under vacuum, 1.220 g (3 × 10⁻³ mol) of bis(4-tert-butylcyclohexyl) peroxydicarbonate, 6.12 g (4 × 10⁻³ mol) of C₄F₉-(VDF)₂₁-I and 50 ml of dried acetonitrile were transferred into the autoclave. Next, 1 g (3 × 10⁻² mol) of CH₂=CH₂ was introduced into the reaction mixture, and the pressure was decreased from 10 to 4 bars. Subsequently, the pressure remained constant over time, and the mixture was stirred at 55 °C for 14 h. Upon completion, the sample was evaporated to remove acetonitrile and was dissolved in acetone. The crude mixture was precipitated from cold pentane to obtain a yellow powder in 81% yield (6.05 g). Quantitative conversion of C₄F₉-(VDF)₂₁-I was achieved, as evidenced by ¹H and ¹⁹F NMR spectroscopy (Supplementary Figures S2 and S3). The NMR characteristics of the samples in deuterated acetone are reported in Tables 4 and 5 .

Table 4 Assignments of ¹⁹F NMR chemical shifts in R_F(VDF)_m–CH₂CH₂–I obtained by ethylene endcapping of C₄F₉(VDF)₂₁I ([ethylene]_o/[C₄F₉(VDF)₂₁I]_o/[initiator]_o=10/1.33/1), T=55 °C; t=24 h

Table 5 Assignments of ¹H NMR chemical shifts in R_F(VDF)_m–CH₂CH₂–I obtained by ethylene endcapping of C₄F₉(VDF)₂₁I ([ethylene]_o/[C₄F₉(VDF)₂₁I]_o/[initiator]_o=10/1.33/1), T=55 °C; t=24 h

Preparation of thin films. VDF telomers and the top and bottom aluminum (Al) electrodes were evaporated through a shadow mask onto a quartz substrate to determine the ferroelectric and piezoelectric properties of the materials. The shape of the sample was identical to that described in previous studies.^{6, 7} During the deposition process, the temperature of the substrate was maintained at 123 K under a vacuum of 10⁻⁵ Pa to ensure that the VDF telomers were in a ferroelectric state (form I) and that the axis of the molecular chain was oriented parallel to the substrate.^{8, 9} The thickness of the VDF telomer and the Al electrodes was controlled at 300 and 50 nm, respectively. Stacks of Al/VDF telomer/Al acted as the capacitor, and the charge was proportional to the stress applied to the film.

Results and Discussion

Mechanism of iodine transfer polymerization

Iodine transfer polymerization¹² of VDF is a degenerative transfer process^{13, 14, 15, 16} that requires an alkyl iodide as the CTA. In this study, 1-iodoperfluoroalkane (perfluorobutyl or perfluorohexyl iodide) was used as the CTA. As shown in previous investigations, macromolecular chains that undergo a transfer reaction can participate in further reactions because –CF₂I bonds are rather weak (its dissociation energy is about 45 kJ mol⁻¹). The overall reaction sequence is shown in Scheme 1.

Scheme 2 depicts the iodine transfer polymerization of the monomer (M) with a 1-iodoperfluoroalkane and bis(4-tert-butylcyclohexyl) peroxydicarbonate, which was used as an initiator. To our knowledge, bis(4-tert-butylcyclohexyl) peroxydicarbonate has not been previously used as an initiator in the iodine transfer polymerization of VDF.

In the initiation step, radicals are generated by the thermal decomposition^{17, 18, 19} of a conventional initiator, such as bis(4-tert-butylcyclohexyl) peroxydicarbonate (step a). For a temperature below 90 °C (in this article), the carbonyloxy intermediate radical does not undergo any decarboxylation.^{17, 18, 19} Next, the generated radicals react with the CTA to form R_F°+ Am-I (step b), which can undergo propagation, as shown in step b. The exchange of iodine from the transfer agent (R_FI) to the propagating radical (P_n°) results in the formation of polymeric alkyl iodides with an oligo(VDF) chain (P_n-I) and a new initiating radical (R°, step c). Significant differences in the stability of the reactants and products involved in step d can shift the equilibrium to the right or the left. Therefore, when the structure of R_F closely resembles the structure of the propagating radical, the transfer step is thermodynamically neutral. Thus, the exchange process described in step d is thermodynamically neutral because propagating P_n and P_m chains are identical. In step d, dormant and living species are in equilibrium. As a result, the reaction may display a pseudoliving behavior.^{12, 13, 14, 15, 16} As in any radical-based process, termination occurs through the coupling of VDF macroradicals (step e), as reported by Timmerman and Greyson.²⁰ However, to preserve living or controlled behavior, termination must be negligible compared with chain transfer. Thus, two key parameters must be considered:

• The concentration of the initiator must be minimized to avoid the accumulation of dormant (or dead) chains (by direct oligomerization of the radical initiator onto VDF). However, a sufficient amount of initiator must be present to initiate the polymerization.

• The concentration of CTA can be used to control the molar mass of the polymers. If complete conversion of the CTA is achieved, the theoretical DPn can be determined from Equation (1):

  

where [VDF]₀, [CTA]₀ and α_VDF are the monomer and CTA concentration in the feed, and the conversion of VDF, respectively. However, if the transfer constant to C_nF_2n+1I is high (C_T=7 at 74 °C),¹⁴ the direct initiation of radicals onto VDF (oligomerization of VDF) does not occur. For instance, the transfer constant to methanol in the telomerization of VDF with methanol was 8 × 10⁻³ at 140 °C,²¹ and thus direct initiation onto VDF was observed.

Results

Iodine transfer polymerization of VDF and ethylenation were initiated by bis(4-tert-butylcyclohexyl) peroxydicarbonate at 55 °C for 4 and 24 h, respectively. The purified telomers were characterized by ¹⁹F, ¹H NMR and FT-IR spectroscopy.

Characterization of telomers produced by iodine transfer polymerization of VDF with C₄F₉I or C₆F₁₃I. As expected, a multiplet centered at −60.0 p.p.m. was not observed in the ¹⁹F NMR spectrum (Figure 2, upper spectrum) of the telomer, indicating that the −CF₂CF₂I group of R_FI underwent a high field shift to −112.0 p.p.m. Moreover, the characteristic signals of C₃F₇ (or C₅F₁₁) end groups^{13, 14, 15, 16, 22} were observed in the spectra. Namely, the peaks centered at −82.0, −124.0, −126.0 (and −128.0 p.p.m.) were assigned to CF₃ and CF₂ groups, respectively. Alternatively, the major signal centered at −92.0 p.p.m. was attributed to difluoromethylene groups –CH₂CF₂−CH₂CF₂−, which were formed via normal head-to-tail VDF linkages.^{10, 13, 14, 15, 16, 21, 22, 23}

The multiplet centered at −38 p.p.m. was indicative of –CH₂CF₂I end groups,^{10, 13, 14, 15, 16, 22} and the signal at −108.0 p.p.m. was attributed to difluoromethylene in –CH₂CF₂−CF₂CH₂I, which arise from reverse addition followed by a chain transfer.

Surprisingly, multiplets centered at −113.0 and −116.0 p.p.m., which correspond to the difluoromethylene groups in –CH₂CF₂−CF₂CH₂−CH₂CF₂− and are derived from reverse head-to-head addition, were not observed in the ¹⁹F NMR spectrum. However, peaks derived from reverse head-to-head linkages are usually observed in VDF-based telomers,²¹ oligomers²³ and copolymers.^{2, 3, 24} Nevertheless, in the iodine transfer polymerization of VDF in the presence of C₆F₁₃I or C₆F₁₃CH₂CF₂I, reverse head-to-head additions were not observed.^{13, 14}

The ¹H NMR spectrum of the telomer (Figure 2, lower spectrum) was in agreement with the results of ¹⁹F NMR spectroscopy. For instance, the methylene groups located between the difluoromethylenes produced a quintet centered at 2.80 p.p.m.,^{10, 13, 14, 21, 22, 23} and the signals centered at 3.20, 3.50 and 3.80 p.p.m. were attributed to the methylene groups in R_FCH₂−, −(VDF)_n−CH₂CF₂I and –CF₂CH₂I, respectively.¹³ The absence of signals centered at 2.3–2.4 p.p.m. (which can be attributed to –CF₂CH₂−CH₂CF₂− reverse tail-to-tail additions) suggested that the iodine transfer polymerization of VDF was a well-controlled process.^{13, 14, 15, 16}

DPn values can be determined from the ¹⁹F NMR spectrum (Figure 2) by calculating the ratio of the integrals of the CF₂ signals of PVDF, which are centered at −91.0, −38.0 and −109.0 p.p.m., to the integral of the CF₃ end group, which is centered at −82.0 p.p.m. (Equation 2).

According to Equation (3), the percentage of –CF₂I and –CF₂CH₂I end groups can be determined via NMR from the integrals of the characteristic signals in NMR spectra:

The DPn values and polydispersity indices were deduced by size exclusion chromatography (Figure 4). The chromatograms were assigned to the entire product mixture and the separation of the crude sample into two fractions with high and low molecular weights. Narrow polydispersity indices (approximately 1.5–1.7) were obtained, indicating that the reaction was a controlled process.

The telomers were characterized by NMR, and the results indicated that the CTA (R_FI) did not have an effect on the DPn. Moreover, satisfactory agreement between the experimental and theoretical DPn values was obtained. However, in this study, the percentage of –CH₂CF₂I functional groups was lower than those obtained in previous studies,^{13, 14, 15, 16} which suggested that superior control can be achieved by applying tertiobutylperoxy pivalate as an initiator. Moreover, the cyclohexyl ring of bis(4-tert-butylcyclohexyl) peroxydicarbonate was not altered during the reaction; thus, the decomposition of the initiator may affect the polymerization results.

The results of FT-IR spectroscopy confirmed that C–C (2900 cm⁻¹), C–F (830 cm⁻¹) and C–H (1400 cm⁻¹) bonds (Figure 5) were the main functional groups present in the telomer. However, C–I bonds could not be observed in the spectra because the wavelength of the C–I bond is located at 500 cm⁻¹.

Korlacki et al.²⁵ characterized VDF telomers by IR at wavenumbers between 1200 and 600 cm⁻¹, and determined the crystallinity of the material by X-ray diffraction (Figure 6). Namely, two crystalline forms (α and β)^{25, 26, 27} can be identified via FT-IR.

Figure 6

X-ray diffraction patterns of C₆F₁₃(VDF)₂₃I (T2-H in Table 6, upper spectrum) and C₆F₁₃(VDF)₆-I (T2-L in Table 6, lower spectrum).

The characteristics of VDF telomers produced by iodine transfer polymerization are summarized in Table 6 . The results suggest that the CTA has a minor effect on the DPn, the functional end group ratio and the crystalline form of the material. Alternatively, the chain length of the telomer has a significant effect on the crystallinity of the polymer. Namely, when the DPn ⩽7, low-molecular-weight telomers only displayed the α-form. However, when the DPn was >7, low-molecular-weight telomers displayed both the α- and β-form (Figure 6), which is in agreement with the results of Korlacki et al.²⁵

Table 6 Experimental conditions of iodine transfer polymerization of VDF in the presence C₆F₁₃I and C₄F₉I and results on DPn, microstructure, end groups and crystalline form

Ethylenation. Ethylenation of 1-iodoperfluoroalkanes has been studied extensively¹¹ and has been achieved under a variety of conditions. For instance, ethylenation has been conducted thermally,^{28, 29, 30, 31, 32} photochemically,³³ in the presence of copper salts or other metallic salts, ^{28, 32, 33, 34, 35, 36, 37, 38, 39, 40} and with radical initiators.^{29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43} In this study, ethylenation was conducted according to a radical pathway, and a peroxide initiator was used (Scheme 3). In addition, ethylene-containing PVDF-Is were characterized by ¹⁹F and ¹H NMR spectroscopy (Figures 7 and 8).

Figure 7

¹⁹F nuclear magnetic resonance spectra of C₄F₉(VDF)₂₁I (T1-H in Table 6, lower spectrum) and C₄F₉(VDF)₂₁–CH₂CH₂I (E1 in Table 7, upper spectrum) (recorded in deuterated acetone, 400 MHz, 298 K).

Figure 8

1H nuclear magnetic resonance spectra of C₄F₉(VDF)₂₁I (T1-H in Table 6, lower spectrum) and C₄F₉(VDF)₂₁–CH₂CH₂I (E1 in Table 7, upper spectrum) (recorded in deuterated acetone, 400 MHz, 298 K).

To achieve ethylenation, the reaction time was extended to 15 h to obtain an optimal yield, and the product was isolated by precipitation. A 10% initial (initiator)_o/(ethylene)_o molar ratio was necessary to favor the quantitative conversion of R_F(VDF)_mI. As a result, the conversion rate obtained in this study was higher than the rates obtained by Balague et al.²² and Manseri et al.²⁸ Table 7 summarizes the experimental conditions and the results of the reactions.

Table 7 Experimental conditions and results of the ethylenation of R_F(VDF)_m-I, conversions of R_F(VDF)_m-I (α_RF(VDF)m-I) and yields of R_F(VDF)_m–C₂H₄–I

After ethylenation and purification, R_F(VDF)_nCH₂CH₂I endcapped telomers were characterized by ¹H and ¹⁹F NMR spectroscopy. The ¹⁹F NMR spectrum of the telomers indicated that –CF₂I end groups (−39 p.p.m.) were not present in the product. Moreover, the peak at −39 p.p.m., which was assigned to the difluoromethylene groups in –CH₂CF₂I shifted to −92.0 p.p.m., assigned to –CH₂CF₂CH₂CH₂I. In addition, the signals centered at −92, −82 and −112 to −126 p.p.m. were attributed to VDF units and C_nF_2n+1 end groups, respectively.

C–I bonds in –CF₂CH₂I are usually less reactive than the C–I bonds in –CH₂CF₂I toward alkenes or unsaturated reactants.¹³ Consequently, ethylenation should occur selectively on –CH₂CF₂I. Interestingly, in this study, a large excess of ethylene and high initiator concentrations were used, and the successful ethylene endcapping of R_F(VDF)_m-1CF₂CH₂I isomer was achieved. Ethylene endcapping of R_F-(VDF)_m-I enables the material to be used for the synthesis of fluorinated alcohols, amines, mercaptans and carboxylic acids.¹¹

Remanent polarization of evaporated films of VDF-based telomers. The ferroelectric properties of Al/VDF telomers/Al capacitors with C₆F₁₃(VDF)₂₃I, C₄F₉(VDF)₂₁I and C₄F₉(VDF)₂₁CH₂CH₂I films were determined via D–E hysteresis loops, which were obtained by measuring the electric displacement versus the electric field. All of the VDF telomer films showed clear hysteresis loops under a triangular electric field of 100 Hz. Figure 9 displays the D–E hysteresis loop of the Al/C₆F₁₃(VDF)₂₃I/Al capacitor, and Table 8 summarizes the remanent polarization values (P_r), which were estimated from the D–E hysteresis loop of the samples. The results indicated that the P_r values of all of the samples were similar (∼100 mC m⁻²); however, P_r values obtained in this study were lower than those observed in a previous study on CF₃(VDF)₁₇I (130 mC m⁻²).⁶ The relatively low P_r values observed in the current investigation may be due to the differences in the terminal chain length of R_F (C_nF_2n+1) groups, which are bulkier than CF₃ and do not contribute to polarization reversal.

Figure 9

D–E hysteresis curves of C₆F₁₃(VDF)₂₃I telomer films with a 300 nm thickness.

Table 8 Values of the remanent polarizations (P_r) of VDF telomers calculated from electric measurements

Piezoelectric properties of evaporated films of VDF-based telomers. The piezoelectric properties of the evaporated films of C₆F₁₃(VDF)₂₃I were determined, and the piezoelectric response of the telomer film is shown in Figure 10. The data clearly indicate that the material produced in this study displays a piezoelectric response. The piezoelectric coefficient of the material determines the relationship between the mechanical input and the electrical output. Moreover, the film displays different piezoelectric coefficients, depending on the direction of the material. Thus, because the VDF telomers were glued to a glass plate, the output charge of the sample was a combination of piezoelectric coefficients along each direction of the film. However, because the sample deforms on a rigid substrate, the contribution of sample bending (namely, elongation and compression along the surface) to the piezoelectric response can be neglected. Therefore, the output current of a ferroelectric material (I) can be written as follows:^{6, 8}

where A, d₃₃ and σ are the area of overlap between the two electrodes, the piezoelectric coefficient of the material in the direction of the thickness and the applied stress, respectively. The output current is proportional to the applied stress rate (dσ/dt), and the relationship between the stress rate (dσ/dt) and the output current (I) is shown in Figure 11, along with the linear least square fit of the experimental data to Equation (4). As shown in the figure, the stress rate and output current of the samples were measured and averaged every 16 ms. The fit of Equation (4) to the experimental data formed a straight line, the slope of which corresponds to d₃₃, and it was 268 pC.N⁻¹. The d₃₃ values obtained in this study were higher than those of CF₃(VDF)_13.4C₂H₅ (181 pC.N⁻¹),⁸ PVDF (d₃₃=−30 pC.N⁻¹) and poly(VDF-co-TrFE) (d₃₃=−38 pC.N⁻¹).⁴⁴ However, in this study, the piezoelectric coefficient measured by the apparatus (Figure 1) may be higher than the actual value due to the pyroelectric effect. Nevertheless, the absence of defects in the VDF chains (or the absence of reverse head-to-head or tail-to-tail dyads) should induce VDF unit poling, which improves the piezoelectric properties of the material.

Figure 10

Representative sample output or piezoelectric response (black line) of C₆F₁₃(VDF)₂₃I telomers when cyclic pressure was applied (gray line).

Figure 11

The current (I) versus the output stress rate (dσ/dt), which was used to calculate the piezoelectric coefficient of C₆F₁₃(VDF)₂₃I telomer.

Conclusions

Various telomers based on VDFs bearing an ω-iodo atom have been synthesized by iodine transfer polymerization (ITP). ITP of VDF was achieved with two different R_FI and was initiated by bis(4-tert-butylcyclohexyl) peroxydicarbonate. Thus, for the first time, bis(4-tert-butylcyclohexyl) peroxydicarbonate has been successfully used for the iodine transfer polymerization of VDF with 1-iodoperfluoroalkyl CTAs. The microstructure of the telomers did not display head-to-head or tail-to-tail VDF chains. However, a few reverse addition linkages adjacent to the iodine atom were observed. The telomers were successfully endcapped with ethylene, and the quantitative conversion of C_nF_2n+1(VDF)_mI was attained. Although a large excess of ethylene was used, only one ethylene unit was inserted into the polymer, indicating that the transfer efficiency of R_F(VDF)_nI telomers was high. Three of the proposed telomers were processed into thin films under vacuum, and their ferroelectric and piezoelectric properties were investigated. C₄F₉(VDF)₂₁I, C₆F₁₃(VDF)₂₃I and C₄F₉(VDF)₂₁C₂H₄I telomer films displayed ferroelectric responses and remanent polarization values of 110 (10 Hz), 107(10 Hz) and 101 (100 Hz) mC m⁻², respectively. Thus, the ferroelectric responses of the telomers were slightly lower than that of CF₃(CH₂CF₂)₁₇I. Preliminary piezoelectric properties of the telomeric films were successfully obtained, and the results revealed that the piezoelectric coefficient of the material was high due to an absence of defects in the microstructure of the telomers. Further investigations on the piezoelectric properties of the remaining telomers and ethylenated polymers are currently underway. In addition, recent structural thermal and crystalline characterizations have been supplied.⁴⁵

Iodine transfer polymerization of vinylidene fluoride (VDF) with C_nF_2n+1I (n=4 or 6).

Elementary steps in the iodine transfer polymerization of vinylidene fluoride. R_FI and t-butylcyclohexylperoxydicarbonate were used as the chain transfer agent and initiator, respectively.

Radical ethylenation of R_F(VDF)_m-I telomers.