Enhanced Switchable Dielectric Performance of Î²-Phase-Dominated

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Enhanced Switchable Dielectric Performance of #-phase Dominated PVDF Composite Films Modified with Singleprotonated 1,4-diazabicyclo[2.2.2]octane Fluoborate Yan Sui, Dongsheng Liu, Wentong Chen, Ge Zhao, and Ming-Ming Xing J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04695 • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 17, 2017

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The Journal of Physical Chemistry

Enhanced Switchable Dielectric Performance of β-phase Dominated PVDF Composite Films Modified with SingleProtonated 1,4-diazabicyclo[2.2.2]octane Fluoborate Yan Sui1*, Dong-Sheng Liu1*, Wen-Tong Chen1, Ge Zhao2, Ming-Ming Xing1 1

School of Chemistry and Chemical Engineering, The Key Laboratory of Coordination

Chemistry of Jiangxi Province, Jinggangshan University, Ji'an, Jiangxi, P. R. China 2

School of Materials Science and Engineering, Jingdezhen Ceramic University, Jingdezhen,

Jiangxi, P. R. China *Corresponding author. E-mail address: [email protected] (Y. Sui), [email protected] (D.S. Liu)

ABSTRACT: We report a new flexible switchable dielectric composite film toward practical application in devices based on β-phase dominated PVDF modified with singleprotonated 1,4-diazabicyclo[2.2.2]octane fluoborate (DabcoHBF4). Our study suggested that incorporating polar molecular filler (DabcoHBF4) into β-phase dominated PVDF is an effective method to fabricate switchable dielectric thin film, which could avoid sophisticated operation in cultivating bulk crystals of molecular materials. The composite film with 40% of DabcoHBF4 (0.40BF) could exhibit large dielectric change between high and low dielectric states comparable to that of pure DabcoHBF4 at phase transition temperature. We postulated that the excellent switchable dielectric property of 0.40BF was attributed to the induced orientation alignment of β-phase dominated PVDF and dielectric confinement effect.

1. INTRODUCTION Switchable dielectric materials, which can undergo transitions between high and low dielectric states at a phase transition temperature (Tc), are important materials applicable in data communication, signal processing and rewriteable optical data storage, etc. 1 , 2 For miniaturization and integration, most dielectric materials are utilized in the form of thin film in applications.3-6 But it is very rare that the widely studied molecular switchable dielectrics are provided in thin films,7-16 which may be related to the facts that most molecular switchable dielectrics are uniaxial with only 1

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two opposite polarization directions, so randomly distributed polarization directions of microcrystalline grains in thin films will lead to reducing of the sum of the spontaneous polarization vector, and the performance of the thin films is usually not as good as that of the bulk crystals.17-19 Because the growth of bulk crystals with enough strength and stability is usually not easy, so finding an useful way to practical application for molecular switchable dielectrics is of great importance. As a semi-crystalline polar polymer poly(vinylidene fluoride) (PVDF) is of excellent film-forming ability, which has been widely used as dielectric materials, due to its versatile properties, especially its outstanding piezo- and pyroelectric properties.20-23 As known, PVDF has at least four possible types of crystal phase (α, β, γ and δ).

24 , 25

The non-polar α-phase, belonging to trans-gauche (TGTG’)

conformation, is the most common phase. The polar β-phase with all-trans planar zigzag conformation (TTTT) with all the fluorine atoms located on the same side of the polymer chains is a more attractive crystal type. This alignment of polymer chains endows β-phase PVDF with a much higher polarity than other phases, accordingly the highest piezo and pyroelectric properties as well as ferroelectric activity.26,27 PVDF is not a switchable dielectric material in itself whether in polar or non-polar phase, because its dielectric constant is usually temperature-insensitive,28 but it may be a good candidate as supporter to incorporate molecular switchable dielectrics. The polar β-phase PVDF with uniform alignment may be helpful for the orientation of polar guest molecules. Although some inorganic ferroelectric materials like BaTiO3, 29 PZT,30 (Na,K)NbO331 have been used as the fillers to improve the dielectric property, insoluble inorganic ceramics even in nano-scale cannot be homogeneously dispersed, the induced orientation is restricted. If soluble molecular dielectrics are used instead, the homogeneous dispersion and interaction between PVDF and guest molecules in composite films will be possible. In

this

paper,

single-protonated

1,4-diazabicyclo[2.2.2]octane

fluoborate

(DabcoHBF4) is selected as the polar guest molecule to prepare flexible PVDF composite films. DabcoHBF4 is a NH…N hydrogen bonded crystal with ferroelectric and switchable dielectric properties.32 DabcoHBF4 has also good solubility in many solvents, so it can be easily dispersed into PVDF matrix. The influence of loading content of DabcoHBF4 upon the structure, crystallinity, thermal stability, switchable dielectric property is investigated. The composite film with 40% of DabcoHBF4 2


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exhibits the best switchable dielectric property, which is comparable with that of powder-pressed pellet of pure DabcoHBF4. 2. EXPERIMENTS AND METHOD 2.1 Materials and methods DabcoHBF4 was obtained from the aqueous solution of equimolar 1,4diazabicyclo[2.2.2]octane and fluoboric acid. All other chemicals were analytical grade and purchased from Shanghai Adamas Reagent Co., Ltd (Shanghai, China). The average molecular weight of PVDF(-CH2-CF2-)n was 534000. X-ray diffraction (XRD; Bruker D8 Advance System, Germany) was performed at room temperature with Cu-target Kα radiation (λ=0.154nm). Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, thermoscientific USA) was carried out over a range of 500-4000 cm-1.The melting and crystallization behaviours of PVDF composite films were carried out on a differential scanning calorimeter DSC Q2000 TA Instruments. The sample was heated to 200°C at a rate of 10°C/min and held at 200°C for 5 min, and then cooled to 30°C at a rate of 10°C/min to record the non-isothermal melting and crystallization behaviour. Thermogravimetric analysis (TGA) was performed using a NETZSCH TG 209 F3 thermogravimetric analyzer. Complex dielectric permittivity was performed using automatic impedance TongHui 2828 Analyzer. The measuring AC voltage was 1 V. For the dielectric measurement, the composite films were deposited with silver conducting glue on two opposite sides and extended by copper wires. The morphologies were observed with field emission scanning electron microscope (FE-SEM) performed on JEOL JSM-6700F. 2.2 Preparation of DabcoHBF4/PVDF composite films Powdered PVDF was dissolved in DMF by magnetic stirring and heating at 60 °C to yield a clear solution (10 w.t.%). Certain amount of DabcoHBF4 was added into above PVDF solution. The mixture solution was magnetic agitated in a 60 °C water bath for 30 min and ultrasonicated for 30 min to obtain a clear solution. Before casting, the mixture was degassed under vacuum overnight to eliminate the air bubbles. Subsequently, the solution was cast on quartz glass 3



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substrates and incubated in an oven at 70 °C for 2 h to ensure the removal of solvent traces. The obtained films were denoted as 0.10BF, 0.20BF, 0.30BF, 0.40BF and 0.50BF according to the mass content of DabcoHBF4 in composite films.

3. RESULTS AND DISCUSSION 3.1 X-ray diffraction analysis

Figure 1.XRD patterns of DabcoHBF4 and DabcoHBF4/PVDF composite films. As shown in Figure 1, the broad peak at c.a. 2θ=20.6° assigned to polar β-phase PVDF 33 - 35 can be found when DabcoHBF4 contents are lower than 30% (0.10BF, 0.20BF and 0.30BF), which indicates that these films are β-phase dominated. Further increases the salt content, this characteristic broad peak will be unrecognized due to the strong diffraction peaks of DabcoHBF4. Closer examination of XRD patterns will find that the positions and shapes for the new diffraction peaks in PVDF composite films with the incorporation of DabcoHBF4 are a little different with those of pure DabcoHBF4 (shown as dash 4


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line), which indicates that there exists obvious interactions between DabcoHBF4 and PVDF. 3.2 FTIR spectra

Figure 2. (A) FTIR spectra of DabcoHBF4, PVDF and DabcoHBF4/PVDF composite films; (B)The fraction of β-phase in DabcoHBF4/PVDF composite films. According to the literature, 36 , 37 the vibration band at 840 cm-1 should be assigned to β-phase PVDF, whereas the vibration band at 763 cm-1 should be ascribed to α-phase PVDF. As shown in Figure 2(A), the intensity of vibration band at 840 cm-1 representing β-phase is very high, whereas the vibration band at 763 cm-1 assigned for α-phase is almost invisible, which indicates that all the films are β-phase dominated. This is consistent with the X-ray diffraction analysis results. With the addition of DabcoHBF4, the vibration bands belonging to DabcoHBF4 become more and more obvious (shown as dash line), which further confirms the existence of DabcoHBF4. The relative amounts of β-phase in crystalline PVDF matrix can be quantified with equation (1)38.

β . α β

(1)

Here F(β) represents the relative mass fraction of the β-phase, Aα and Aβ are the absorbance at 763 cm-1 and 840 cm-1 corresponding to α- and β-phases, respectively. As shown in Figure 2(B), the calculated result shows that F(β) value is gradually increased from 0.75 to 0.81 when the mass content is up to 5



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40%, and then it begins to decline. FTIR analysis result indicates that polar DabcoHBF4 may be used as nuclei centre for the crystallization of β-phase PVDF through hydrogen bonds and/or dipole interactions, but too much nuclei centers may influence the ordered alignment of PVDF by quenching the free chain movements of PVDF matrix.39

3.3 Morphology characterization

Figure 3. SEM images of 0.40BF with 500 (A), 1000 (B), 3000 (C) and 5000 (D) times magnification. The superior flexibility is demonstrated by a macroscopic image shown in the inset of (A); Obvious crystallization phenomenon with 60% of salt content is illustrated in the inset of (D). SEM technology was used to investigate the dispersion state of DabcoHBF4 in PVDF composite film. Figure 3(A, B, C and D) were SEM images of 0.40BF with different magnification times. The superior flexibility of 0.40BF film was demonstrated by a macroscopic image shown in the inset of (Figure 3A). The composite films were mainly comprised of irregular spherulites with varied diameter from 3 to 8 µm without obvious agglomeration, which was also the characteristic of β-phase PVDF, because that of α- and γ-phase PVDF is usually 6


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larger than 10 µm. 40 , 41 There existed small quantity of cubic microcrystals adhered on the surface of spherulites, which should be the precipitation of DabcoHBF4. If further increases the salt content up to 60%, this crystallization phenomenon will become more obvious, even the precipitation can be found inside the film, as illustrated in the inset of Figure 3D. So the salt loading content cannot be ultimately increased.

3.4 Thermal analysis

Figure 4. DSC curves of pure PVDF, DabcoHBF4 and PVDF composite films in the warming(left) and cooling(right) runs. The phase transition behaviors of pure PVDF, DabcoHBF4 and PVDF composite films were analyzed by differential scanning calorimetry and shown in Figure 4. For DabcoHBF4, an endothermic peak at 105.3°C and an exothermic peak at 101.0 °C were observed in the warming and cooling runs, respectively. The sharp peaks and thermal hysteresis of 4.3 °C reveal the discontinuous characteristic of the transition, being indicative of a first-order phase transition. In the warming run, the transition enthalpy ∆H1=4.73 kJ mol-1 and entropy ∆S1=12.49 J mol-1K-1; while the transition enthalpy ∆H2=5.22 kJ mol-1 and entropy ∆S2=13.97 J mol-1K-1 in the cooling run. Average transition entropy ∆S=(∆S1+∆S2)/2=13.23 J mol-1K-1. According to the Boltzmann equation ∆S=RlnN (R is the gas constant), the calculated N value is 4.90, which confirms the order-disorder phase transition mechanism. Pure PVDF exhibits one couple of revisable phase transition peaks corresponding to the melting and crystallization process at 162.51°C and 132.92°C, respectively. With the incorporation of DabcoHBF4, both melting and crystallization peaks are basically shifted to lower temperature, suggesting the nucleation of electroactive β7



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polymorph in composite films. Meanwhile, besides PVDF phase transition peaks, the peaks corresponding to the order-disorder structural phase transition of DabcoHBF4 become more and more obvious with the increasing of salt content. For 0.50BF, the tested transition enthalpy ∆H1=2.20 kJ mol-1 and entropy ∆S1=5.84 J mol-1K-1 in the warming run at 104.63°C; meanwhile the transition enthalpy ∆H2=2.40 kJ mol-1 and entropy ∆S2=6.41 J mol-1K-1 in the cooling run at 101.31°C. The average transition entropy should be ∆S=(∆S1+∆S2)/2=6.13 J mol-1K-1. If normalized to pure DabcoHBF4, the transition entropy should be ∆S =6.13/0.50= 12.26 J mol-1K-1. The calculated result is roughly equal to that of pure DabcoHBF4 sample, suggesting that the structure and phase transition mechanism of DabcoHBF4 is retained after doping into PVDF matrix.

Figure 5. The relationship between salt content and crystallinity (Xc). The degree of crystallinity (Xc) of the composite films is evaluated using the following equation (1):42 ∆

% ∅∆ 100%

(1)

Where ∆Hc is the melting heat of samples, ø is the weight percentage of DabcoHBF4 in the PVDF matrix, ∆Hc0 is the melting heat of 100% crystalline PVDF, which is 93.07 and 130.40 J/g for pure α- and β-phase PVDF, respectively. 8


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∆Hc0=93.07×F(α)+103.40×F(β) is used for the calculation (F(α)=1- F(β)). The calculated result (Figure 5) indicates that the crystallinity (Xc) is increased up to maximum value (48.40%) with 40% of salt content (i.e. 0.40BF), further increase of the salt content will lead to the reduce of crystallinity (Xc), which may be related to the restriction of free chain movements of PVDF matrix. 43 Adequate amount of DabcoHBF4 is advantageous in improving the crystallinity of PVDF composite films, which may be related to the heterogeneous nuclei effect with DabcoHBF4 as nuclei centers to promote the crystallization of PVDF. Thermal stability of the PVDF composite films were also investigated by TGA thermographs and illustrated in Figure 6. Doping DabcoHBF4 into β-phase dominated PVDF matrix will slightly lower the thermal stability of composite films, which should be related to the thermal decomposition of DabcoHBF4. But TGA results reveal that these films are still stable up to 200°C.

Figure 6. TGA curves of PVDF and DabcoHBF4/PVDF composite films. 3.5 Dielectric properties

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Figure 7. Variable-temperature dielectric constants of DabcoHReO4 and its PVDF composite films at 1MHz (For clarity, 0.10BF and 0.20BF are shown in the inset).

Figure 8. Variable-temperature dielectric constants of PVDF composite films with 120°C of solution casting temperature at 1MHz. 10


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Figure 9. Variable-temperature dielectric losses of DabcoHBF4 and its PVDF composite films at 1MHz (For clarity, 0.10BF and 0.20BF are shown in the inset). As shown in Figure 7, the temperature-dependent dielectric constant test indicates that DabcoHBF4 is of typical switchable dielectric property because it exhibits two plateaus before and after phase transition temperature (at about 105°C). Below 105 °C in the warming run, the dielectric constant at 1 MHz is about 11-12 and remains almost stable at low dielectric state. After that, the dielectric constant suddenly jumps up to about 80. Further increases the temperature, the dielectric constant remains almost unchanged at high dielectric state. In the cooling run, revisable switchable dielectric behavior is observed at about 101 °C. For the composite films, when the salt content is over 20% (i.e. 0.20BF), similar switchable dielectric properties can be found at the phase transition temperature of DabcoHBF4. With the increase of salt content, the dielectric change between high and low dielectric states becomes larger and larger at first, and then begins to decrease. 0.40BF exhibits the largest dielectric change, which is almost in the same level as that of pure DabcoHBF4. The switchable dielectric properties of composite films should be resulted from the order-disorder structural phase transition of DabcoHBF4, which can also be concluded from DSC results. But using only 40% of DabcoHBF4 could 11



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obtain a composite film with so large dielectric change between high and low dielectric state, indicating that PVDF does not just function as the supporter to form flexible composite film, but play an important role in improving the switchable dielectric property. We think that this should be ascribed to the induced orientation alignment of β-phase dominated PVDF and dielectric confinement effect. For powder-pressed pellet of pure DabcoHBF4, the agglomerate of microcrystalline grains is disordered, the sum of the spontaneous polarization vector of all the grains will be greatly reduced due to the randomly distributed polarization directions. When DabcoHBF4 is doping into β-phase dominated PVDF matrix, the uniform alignment of polar β-phase PVDF can induce the orientation arrangement of polar guest to form sandwich-like layered structure through hydrogen bonds and/or electrostatic interactions, so the spontaneous polarization can be strengthened. The phenomenon that 0.50BF exhibits smaller dielectric change than that of 0.40BF may be related to its high level of salt content, which will influence the orientation alignment or quench the free chain movements of PVDF matrix. This is in accordance with FTIR analysis results. In order to verify the rationality of above analysis about β-phase dominated PVDF inducing orientation alignment, α-phase dominated PVDF composite films 0.30BF* and 0.40BF* are prepared by treating the casting solution at 120°C. It is known that when the solution casting temperature is higher than 80°C, PVDF favors the formation of non-polar α-phase.44 Although the switchable dielectric property is still able to be detected in 0.30BF* and 0.40BF*, the dielectric change between high and low dielectric states is greatly reduced (Figure 8), which indicates that non-polar αphase dominated PVDF does not have function to induce the orientation alignment of polar guest as its counterpart β-phase dominated PVDF. In fact, DabcoHBF4 doped β-phase dominated PVDF matrix forms the layered structure of dielectric quantum well, in which PVDF is the barrier layer and DabcoHBF4 is the well layer, just like the widely studied PbI4-based layered semiconductors with organic alklyammonium spacers.45,46 The dielectric confinement effect will be greatly increased near the phase transition temperature, because the dielectric constant difference between barrier layer and well layer is enlarged, which can further increase the binding energy of excitons in the well. So, the dielectric change between high and low dielectric states is amplified. 12


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The variable-temperature dielectric losses of DabcoHBF4 and its PVDF composite films are given in Figure 9. The changing trends are similar with those of dielectric constants.

CONCLUSIONS In this paper, we reported a useful way towards flexible PVDF composite film with switchable dielectric properties by incorporating molecular switchable dielectric DabcoHBF4 into β-phase dominated PVDF matrix. The composite film with 40% of DabcoHBF4 could exhibit best switchable dielectric properties comparable with that of pure DabcoHBF4. Incorporating molecular switchable dielectric into β-phase dominated PVDF will pave the way toward practical application in devices for molecular switchable materials.

ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (21361012, 21661016, 21461012 and 21461013), Department of Science and Technology of Jiangxi Province (20144BCB23038 and 2133ACB20010 ), Science and Technology Research Project of Jiangxi Provincial Department of Education (GJJ160734)

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TOC Graphic

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Figure 1.XRD patterns of DabcoHBF4 and DabcoHBF4/PVDF composite films. 130x99mm (300 x 300 DPI)



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Figure 2. (A) FTIR spectra of DabcoHBF4, PVDF and DabcoHBF4/PVDF composite films; (B)The fraction of βphase in DabcoHBF4/PVDF composite films. 254x110mm (300 x 300 DPI)


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Figure 3. SEM images of 0.40BF with 500 (A), 1000 (B), 3000 (C) and 5000 (D) times magnification. The superior flexibility is demonstrated by a macroscopic image shown in the inset of (A); Obvious crystallization phenomenon with 60% of salt content is illustrated in the inset of (D). 106x80mm (300 x 300 DPI)



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Figure 4. DSC curves of pure PVDF, DabcoHBF4 and PVDF composite films in the warming(left) and cooling(right) runs. 254x99mm (300 x 300 DPI)


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Figure 5. The relationship between salt content and crystallinity (Xc). 143x99mm (300 x 300 DPI)



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Figure 6. TGA curves of PVDF and DabcoHBF4/PVDF composite films. 130x99mm (300 x 300 DPI)


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Figure 7. Variable-temperature dielectric constants of DabcoHReO4 and its PVDF composite films at 1MHz (For clarity, 0.10BF and 0.20BF are shown in the inset). 130x99mm (300 x 300 DPI)



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Figure 8. Variable-temperature dielectric constants of PVDF composite films at 1MHz at 120°C of solution casting temperature. 130x99mm (300 x 300 DPI)


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Figure 9. Variable-temperature dielectric losses of DabcoHBF4 and its PVDF composite films at 1MHz (For clarity, 0.10BF and 0.20BF are shown in the inset). 130x99mm (300 x 300 DPI)


Enhanced Switchable Dielectric Performance of Î²-Phase-Dominated

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