Characterization of PVdF(HFP) Gel Electrolytes Based on 1-(2

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J. Phys. Chem. B 2005, 109, 17928-17935

Characterization of PVdF(HFP) Gel Electrolytes Based on 1-(2-Hydroxyethyl)-3-methyl Imidazolium Ionic Liquids Sun-Hwa Yeon, Ki-Sub Kim, Sukjeong Choi, Jong-Ho Cha, and Huen Lee* Department of Chemical and Biomolecular Engineering, KAIST, Daejeon 305-701, Republic of Korea ReceiVed: June 16, 2005; In Final Form: August 3, 2005

Poly(vinylidenefluoride)-hexafluoropropylene (PVdF(HFP))-ionic liquid gel electrolytes were prepared using ionic liquids based on 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate and 1-(2-hydroxyethyl)3-methyl imidazolium hexafluorophosphate. A conventional metathesis reaction was used to prepare these ionic liquids, which have high purity and exhibit a liquid state at room temperature. The prepared polymerionic liquid gel proved to be a free-standing and rubbery film in which the degree of transparency differed according to the ratio and type of ionic liquid used. TGA and FTIR analyses confirmed that the solvent, N,N-Dimethylacetamide (DMAC), used for mixing PVdF(HFP) polymer with ionic liquid was almost totally removed during the gelling and drying processes. SEM photographs were taken of the surface structure of the PVdF(HFP)-ionic liquid gel in order to evaluate the morphology of the film’s surface according to the mixing ratio and the nature of the ionic liquid. The thermal behaviors of PVdF(HFP)-ionic liquid gels were observed to be similar to those of neat ionic liquids through DSC analysis, and the compatibility between the polymer and ionic liquid was investigated by XRD analysis. The ionic conductivities of all the gels were 10-3-10-5 S cm-1 in a temperature range of 20-70 °C.

Introduction There are many types of polymer gel electrolytes that can be used in diverse applications, including in secondary batteries, electrochromic displays, sensors, and various ionic devices.1,2 Traditional polymer electrolytes can easily be prepared from solutions consisting of a polar organic solvent and electrolyte salts in a polymer matrix. Generally, the terms “gel electrolyte”, “plasticized electrolyte”, and “hybrid film” are used to describe these materials, reflecting that either the polymer or the solvent can form the major component of the system.3 In this case, the conductivity of the electrolyte mainly depends on the physical properties of the solvent used, such as its viscosity and dielectric constant, and on the concentration of salt in the electrolyte.3 However, there are several critical problems, such as volatility and flammability, when organic solvents are used in a battery system in a severe temperature range. On the other hand, the novel use of ionic liquids as a substitute for conventional organic solvents has recently been of great interest in research on electrochemical devices. These ionic liquids have certain unique characteristics, including very low melting points, nonvolatility, thermal stability, nonflammability, and extremely high ionic conductivity.4-7,18 In particular, the ionic liquids of the imidazolium family have been widely developed since 1-ethyl-3-methyl imidazolium chloroaluminate was discovered by Wilkes and Zaworotko.8 This ionic liquid has a wide liquid range, an electrochemical window of more than 3 V, and a high ionic conductivity (∼8 × 10-3 S cm-1) at room temperature. On the basis of this imidazolium cation, our laboratory has recently conducted research on newly developed room-temperature ionic liquids (RTILs) in which a hydroxyl group (-OH) is introduced in the imidazolium cation as a functional group for electrolyte applications. Past investigations * Corresponding author. Tel.: 82-42-869-3917. Fax: 82-42-869-3910. E-mail address: [email protected].

revealed that RTILs based on a 1-(2-hydroxyethyl)-3-methyl imidazolium cation with three different anions ([BF4]-, [TFSI]-, and [PF6]-) possessed the desired physical and electrochemical properties for electrolyte applications.17 It was demonstrated that one of the valuable characteristics of the 1-(2-hydroxyethyl)3-methyl imidazolium tetrafluoroborate is the outstanding electrochemical window of about 6.4 V. In particular, an improved value in cathodic stability relative to that of N,Nalkyl functionalized imidazolium tetrafluoroborate and a high ionic conductivity of 0.0046 S cm-1 at room temperature were observed.17 Traditionally, the majority of polymers used in polymer electrolyte systems have been largely based on high-molecularweight poly(ethylene oxide) (PEO).2 More recently, various materials such as PAN (poly(acrylonitrile)), PMMA (poly(methyl methacrylate)), PVdF (poly(vinylidene fluoride)), PVP (poly(1-vinyl pyrrolidone)), PDMAA (poly(N-methyl acrylamide)), and P(VP-c-VA) (poly(1-vinyl pyrrolidone-co-vinyl acetate)) have been used as the main polymer matrix for gel polymer electrolytes.10,11 Among these materials, poly(vinylidene fluoride-hexafluoropropylene), PVdF(HFP), copolymer is well-known for having good mechanical stability and easy film formation, and it is also known to be a good supporter of liquid electrolytes, because the ionic conductivity of this polymer gel electrolyte is about 10-3 S cm-1 when a proper electrolyte is used to swell it.12 Also, in terms of optics, PVdF-based gels are optically transparent. They are expected to have a high anodic stability due to the presence of strong electron-withdrawing functional groups (-C-F-) and are resistant to UV radiation.13 Therefore, the PVdF(HFP) copolymer can be a viable alternative to the solid polymer electrolyte in rechargeable lithium batteries or electrochemical photovoltaic devices. However, despite the various advantages of this polymer PVdF(HFP), there has been little investigation into gel polymer electrolytes based on ionic liquids. Accordingly, the investigation on its

10.1021/jp053237w CCC: $30.25 © 2005 American Chemical Society Published on Web 09/03/2005

Characterization of PVdF(HFP) Gel Electrolytes

J. Phys. Chem. B, Vol. 109, No. 38, 2005 17929

Figure 1. Synthesis of ionic liquids [HEMIm][BF4] and [HEMIm][PF6].

morphology, physical properties, or ionic conductivities as an electrolyte film will affect the development of a solid electrolyte. In this study, we prepare several PVdF(HFP)-ionic liquid gels with different mixing ratios and investigate their structural surface morphologies, thermal properties, and ionic conductivities according to various compositions for applications in electronic device fields. Two kinds of ionic liquids introduced in our previous publication,17 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate ([HEMIm][BF4]) and 1-(2-hydroxyethyl)-3-methyl imidazolium hexafluorophosphate ([HEMIm][PF6]), were used as polymer-ionic liquid gels. We report here new findings pertaining to PVdF(HFP)-ionic liquid gels produced by incorporating room-temperature ionic liquids, [HEMIm][BF4] and [HEMIm][PF6], into a PVdF(HFP) polymer for practical applications as solid polymer electrolytes. Experimental Section Materials. The chemicals (source, grade, and purification) used in the synthesis of ionic liquids are as follows: 1-methyl imidazole (Aldrich, 99%, used without purification), 2-chloroethanol (Aldrich, 99.5%, used without purification), sodium tetrafluoroborate (Aldrich, 98%, used without purification), and potassium hexafluorophosphate (Aldrich, 98%, used without purification). Solvents used include deionized water from a Millipore purification unit, dichloromethane (Merck, 99.9%), acetone (Merck, 99.9%), and acetonitrile (Merck, 99.9%). Poly(vinylidenefluoride)-hexafluoropropylene copolymer (PVdF(HFP), Atofina, Kynar flex 2801-00, MW ) 477 000) was used as received. N,N-Dimethylacetamide (Aldrich, 99.9% HPLC grade, used without purification) was used as the solvent for mixing the PVdF(HFP) polymer and the prepared ionic liquids. Preparation of Ionic Liquids. The two ionic liquids used in this study were synthesized along with their corresponding chloride or bromide precursors. Figure 1 shows the synthesis scheme of the two prepared ionic liquids. To produce 1-(2-hydroxyethyl)-3-methyl imidazolium chloride ([HEMIm][Cl]), 1-methyl-imidazole (0.14 mol) was reacted with an excess of hydroxyethyl chloride (2-chloroethanol, 0.2 mol) in a round-bottom flask in a nitrogen atmosphere (70 °C, 48 h), using 200 mL of acetonitrile as a solvent. Molten salt of the white crystalline solids was obtained by recrystallization in a freezer at -40 °C (yield 89%). For 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate ([HEMIm][BF4]), the 1-(2-hydroxyethyl)-3-methyl imidazolium chloride was reacted with an equimolar amount of sodium tetrafluoroborate in acetone (25 °C, 24 h), which resulted in the formation of the ionic liquid [HEMIm][BF4]. Sodium chloride was removed by filter paper, and any residual sodium chloride in this ionic liquid was removed by low-temperature

TABLE 1: Mixing Ratio and Film Thickness of PVdF(HFP)-Ionic Liquid Gels PVdF(HFP) mass ratio (wt %)

film thickness (mm)

appearance

[HEMIm][BF4] 0 33.3 47.4 66.7

0.32 0.1 0.1

clear colorless liquid white rubbery film white rubbery film opaque film

[HEMIm][PF6] 0 33.3 47.4 66.7

0.185 0.016 0.028

clear colorless liquid white rubbery film white rubbery film opaque film

filtration using Celite.17 The organic liquid obtained from the filtration was tested for residual chloride salt with a concentrated AgNO3 solution, and some slight precipitation of AgCl was confirmed with the naked eye. For 1-(2-hydroxyethyl)-3-methyl imidazolium hexafluorophosphate ([HEMIm][PF6]), the ionic liquid was synthesized in the same manner as for [HEMIm][BF4]; [HEMIm][Cl] and potassium hexafluorophosphate were used to form the 1-(2hydroxyethyl)-3-methyl imidazolium hexafluorophosphate, and acetone was used as a solvent. The potassium chloride was removed by filter paper, and any residual potassium chloride was cleaned in an aqueous-dichloromethane biphase solution under an ice bath for 24 h. A concentrated AgNO3 solution test was conducted, and some slight precipitation of AgCl was confirmed with the naked eye. Preparation of PVdF(HFP)-Ionic Liquid (ILs) Gel. The polymer-ionic liquid gel films were prepared according to procedures outlined in the literature.14 For example, 0.528 g of PVdF(HFP), 0.587 g of ionic liquid, and 2.5 mL of DMAC (N,N-dimethylacetamide) were mixed to produce a transparent solution of 47.4 wt % PVdF(HFP) polymer, under ambient conditions, in which two kinds of ionic liquid, [HEMIm][BF4] and [HEMIm][PF6], were used. The final eight gel samples on the two prepared ILs were made to contain 33.3, 47.4, 66.7, and 100 wt % PVdF(HFP) polymer, respectively. To facilitate the miscibility of the polymer or the ionic liquids, a DMAC with polar properties was chosen despite its high boiling temperature of ca. 165 °C. The transparent solution gelled in 5 min when deposited in glass Petri dishes that were placed on a hot plate preheated to ca. 80 °C. In terms of appearance, the films with a PVdF(HFP) of more than 66.7 wt % appeared opaque, whereas those with a PVdF(HFP) of less than 47.4 wt % exhibited rubbery white gels. To remove residual solvent (i.e., DMAC) in the gels, the films were placed in ambient air for a sufficient amount of time. Table 1 shows the film thickness and appearance according to the various mixing ratios of PVDF(HFP) and ILs at room temperature.

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Figure 2. Scanning electron micrographs ((a)-(c), ×500; (d), ×500 (×4000)) of PVdF(HFP) gels in [HEMIm][BF4] system. Grain size (diameter): (a) 12.35, (b) 7.716, and (c) 4.63 µm.

Thermal Measurement (DSC and TGA). The differential scanning (DSC Q1000 V7.0 Build 244) data were obtained in a sealed aluminum pan with a cooling and heating rate of 10 °C min-1 under He purge, 50 cm3 min-1. The thermogravimetric analysis (TGA Q500 V5.0 Build 164) data were taken in air and at a heating rate of 10 °C min-1 under N2 purge, 100 cm3 min-1. Fourier Transform Infrared Spectroscopy (FTIR). The infrared spectra were recorded under a nitrogen atmosphere on a JASCO 470 PLUS spectrometer, covering a range of 4004000 cm-1. Scanning Electron Micrographs (SEM). Scanning electron microscopy (SEM) was performed on an XL-30S FEG instrument (Philips Company). The samples were sputter-coated with approximately 10 nm of gold before analysis. X-ray Diffraction (XRD). XRD patterns were obtained from wide-angle X-ray diffractometry (model D/MAX IIIB; Rigaku) with a scintillation counter detector using Cu KR radiation as a source at a generator voltage of 40 kV and a generator current of 40 mA. The scanning speed and the step were 2° min-1 and 0.02°, respectively. Angles (2θ) ranged from 2 to 60°. Ionic Conductivity. The specific ionic conductivity was measured with the Solartron 1260A frequency response analyzer (FRA). This apparatus was connected to a sealed cell containing a pair of SUS plate electrodes. To take the temperature dependency into account, this cell was placed in an oven well with a controlled temperature. Results and Discussion Properties of PVdF(HFP)-Ionic Liquid Gel Films. The morphology of the prepared PVdF(HFP)-ionic liquid gels was investigated by scanning electron micrographs (Figures 2 and 3). The percent weight ratios of polymer to ionic liquid were (a) 0:100, (b) 66.7:33.3, (c) 47.4:52.6, and (d) 33.3:66.7. The 100 wt % PVdF(HFP) gel made from only PVdF(HFP) polymer

was a free-standing, translucent film and showed a morphology composed of many spherical grains. In the case of the PVdF(HFP)-[HEMIm][BF4] gel (Figure 2), the 66.7 wt % PVdF(HFP) film in [HEMIm][BF4] appeared to have a more tightly packed structure of grains than that observed in the 100 wt % PVdF(HFP) film, and the 47.4 wt % PVdF(HFP) film, in which the weight ratio of polymer to ionic liquid is 1:1.1, showed a porous morphology having a smaller grain size (4.63 µm) than either (a) or (b) without the phase separation phenomenon. The 33.3 wt % PVdF(HFP) film, which had more [HEMIm][BF4] than did the PVdF(HFP) polymer, displayed a surface morphology consisting of a matrix and domain and showed definite phase separation. In the case of PVdF(HFP)-[HEMIm][PF6] gels, the film with 66.7 wt % PVdF(HFP), unlike the 66.7 wt % PVdF(HFP)[HEMIm][BF4] film, exhibited a very fine grain size (3.086 µm) and a tightly packed structure. However, in film (c), which had nearly equivalent PVdF(HFP) to [HEMIm][PF6] ratios, there was a sharp increase in grain size (15.43 µm), and some grains were aggregated sparsely. When the two films, (b) and (c), were considered in terms of mechanical strength, it was found that film (c) was weaker and more fragile than film (b), which had a small grain size and a packed structure. However, in the 33.3 wt % PVdF(HFP) film in the [HEMIm][PF6] system, the grain size again decreased to 6.17 µm, and the surface became more porous. The resulting structure of the PVdF(HFP) gels, including [HEMIm][PF6], indicates that changes to grain size did not occur relative to any increase in the amount of polymer. However, it was observed that increasing the amount of polymer resulted in a more porous structure in which the phase separation did not occur at 66.7, 47.4, and 33.3 wt % PVdF(HFP) compositions. Thermal Properties of PVdF(HFP)-Ionic Liquid Gels. To study the thermal behaviors and stabilities of PVdF(HFP)-ionic liquid gels, we investigated DSC and TGA. Figures 4 and 5



Figure 3. Scanning electron micrographs ((a)-(d): ×500) of PVdF(HFP) gels in [HEMIm][PF6] system. Grain size (diameter): (a) 12.35, (b) 3.086, (c) 15.43, and (d) 6.17 µm.

Figure 4. Differential scanning calorimeter thermograms of PVdF(HFP) powder and PVdF(HFP)-[HEMIm][BF4] gels on warming at 10 °C min-1 (all data: second run state).

show the DSC data for the variance of PVdF(HFP) composition in the PVdF(HFP)-[HEMIm][BF4] and PVdF(HFP)-[HEMIm][PF6] gels, respectively. The neat [HEMIm][BF4] showed a liquid state at room temperature, a glass transition temperature (Tg) of -73 °C, and no melting temperature from -150 to 150 °C.17 Thermal behaviors of 33.3, 47.4, and 66.7 wt % PVdF(HFP) in the [HEMIm][BF4] system were similar with those of neat [HEMIm][BF4]. At the same time, Tg, according to variation of PVdF(HFP) components, was in the vicinity of -70 to -77 °C, except for that of the 100 wt % PVdF(HFP) film, as shown in Table 2. However, when the amount of [HEMIm][BF4] was decreased, the magnitude of the endothermic peak on Tg tended to decrease and, consequently, disappeared completely in the 100 wt % PVdF(HFP) film. To reconfirm the thermal behavior of the 100 wt % PVdF(HFP) film, the DSC data of the neat PVdF(HFP) powder is shown in Figure 4 in a temperature range

Figure 5. Differential scanning calorimeter thermograms of PVdF(HFP)-[HEMIm][PF6] gels on warming at 10 °C min-1 (all data: second run state).

from -150 to 200 °C. The final melting point of the PVdF(HFP) polymer powder was observed to be 163 °C. A small peak presumed to be Tg in the vicinity of -50 °C (Table 2) was observed, but further investigation is required to determine the existence of contamination or essential thermal behavior in neat PVdF(HFP) powder. In the case of the PVdF(HFP)-[HEMIm][PF6] system (Figure 5), neat [HEMIm][PF6] exhibited one exotherm peak at -37 °C, at which point crystallization occurs, and another endotherm peak at 23 °C, which is the final melting point. As indicated in Figure 3, the thermal behaviors in the small PVdF(HFP) component films were similar to those of the neat [HEMIm][PF6]. In the 66.7 wt % PVdF(HFP) film, the exotherm and endotherm peaks, except for Tg, disappeared, and in the 100 wt


Yeon et al.

TABLE 2: Thermal Data for PVdF(HFP)-Ionic Liquid Gels PVdF(HFP) mass ratio (wt %) 0 33.3 47.4 66.7 100 PVdF(HFP) powder

Tg (°C)

Tm (°C)

[HEMIm][BF4] -73 -77.65 -71.54 -77.41 163 [HEMIm][PF6]

0 33.3 47.4 66.7

-62.22 -59.45 -62.27

measured temperature range (°C) -150 to150 -150 to 150 -150 to 150 -150 to 150 -150 to 150 -150 to 200 -70 to 150 -150 to 150 -150 to 150 -150 to 150

% PVdF(HFP) film, all thermal peaks disappeared. In all prepared films, except for the 100 wt % PVdF(HFP) film, Tg was measured in the vicinity of -60 °C, according to the variation of the PVdF(HFP) component. The TGA data of Figure 6 show the thermal stabilities representative of two films, i.e., in 47.4 wt % PVdF(HFP)[HEMIm][BF4] and 47.4 wt % PVdF(HFP)-[HEMIm][PF6] gels. The thermal decomposition temperatures for these two neat ionic liquids, [HEMIm][BF4] and [HEMIm][PF6], were 380 and 310 °C, respectively.17 The two films, PVdF(HFP)-[HEMIm][BF4] and PVdF(HFP)-[HEMIm][PF6], started to decompose at the original temperature of the neat ionic liquid. A TGA analysis was conducted on all of the prepared films, and the same results were observed in all samples. Therefore, we can assert that the prepared films are composed of only polymer and ionic liquid with little residual solvent. FTIR Study. Infrared spectroscopy measurements were conducted in order to identify the residual solvent of DMAC in prepared gels by observing the spectra showing the band originating from vibrations of the amide group (OdCsN) in DMAC. Figure 7 shows the FTIR data of 47.4 wt % PVDF(HFP) films, including both [HEMIm][BF4] and [HEMIm][PF6]. Typically, the important band of the amide functional group appears from 1690 to 1650 cm-1.16 Because FTIR data showed that the band of the amide functional group was not observed in its vibrational frequency bands, it was inferred that the residual solvents were almost totally removed from both films. In all samples ranging from 1690 to 1650 cm-1, no bands of the amide functional group were detected. XRD Study. Figure 8(a) shows the XRD pattern of the 100 wt % PVdF(HFP) film. There are four peaks that appear at 2θ ) 8.753, 18.368, 19.980, and 39.041°, respectively. With the addition of [HEMIm][BF4] into PVdF(HFP), these peaks shift slightly toward high 2θ. At the same time it is found that as the area of 2θ ) 19.980° is increased according to an increase in the amount of [HEMIm][BF4], the crystal morphology around the peak tends to be amorphous, indicating that the d spacing around 2θ ) 20° of the 66.7 wt % PVdF(HFP)-[HEMIm][BF4] gel and 33.3 wt % PVdF(HFP)-[HEMIm][BF4] gel is increased from 4.3710 to 4.4056 Å, respectively. Also, it is shown that the crystal peak around 2θ ) 18.368° of the 100 wt % PVdF(HFP) gel disappeared with the addition of [HEMIm][BF4]. From these results, it is noted that PVdF(HFP) and [HEMIm][BF4] must possess good reciprocal miscibility in order to form a stable film. In addition, a similar trend can be predicted from the result that the d spacing around 2θ ) 8° of the 100 wt % PVdF(HFP) film and the 33.3 wt % PVdF(HFP)-[HEMIm][BF4] film is increased from 10.0946 to 10.956 Å, respectively.

Figure 6. TGA data of 47.4 wt % PVdF(HFP)-ionic liquid gels based on [HEMIm][BF4] and [HEMIm][PF6].

Figure 7. FTIR of 47.4 wt % PVdF(HFP)-ionic liquid gels based on [HEMIm][BF4] and [HEMIm][PF6].

Figure 8. XRD patterns of PVdF(HFP)-[HEMIm][BF4] gels: (a) 100 wt % PVdF(HFP), (b) 66.7 wt % PVdF(HFP), (c) 47.4 wt % PVdF(HFP), and (d) 33.3 wt % PVdF(HFP).

Figure 9 shows the XRD pattern of PVdF(HFP)-[HEMIm][PF6] gels. Like the XRD peak patterns of Figure 8, four peaks shown at 100 wt % PVdF(HFP) film shifted slightly toward high 2θ with addition of [HEMIm][PF6] to PVdF(HFP). The crystal morphology around 2θ ) 20° was shown to be amorphous as the d spacing around 2θ ) 20° of the 66.7 wt %


Figure 9. XRD patterns of PVdF(HFP)-[HEMIm][PF6] gels: (a) 100 wt % PVdF(HFP), (b) 66.7 wt % PVdF(HFP), (c) 47.4 wt % PVdF(HFP), and (d) 33.3 wt % PVdF(HFP).

Figure 10. Ionic conductivity of the PVdF(HFP) gel in the [HEMIm][BF4] system as a function of temperature: 0 wt % PVdF(HFP) (9), 33.3 wt % PVDF(HFP) (≤), 47.4 wt % PVdF (HFP) ([), and 66.7 wt % PVdF(HFP) (0).

PVdF(HFP)-[HEMIm][BF4] gel and the 33.3 wt % PVdF(HFP)-[HEMIm][BF4] gel was increased from 4.3794 to 4.4183 Å, respectively. However, in local locations such as around 2θ ) 8°-9°, it was observed that the d spacing of the 66.7 wt % PVdF(HFP) film and the 33.3 wt % PVdF(HFP) film was reduced from 10.0407 to 9.5 Å, respectively. Therefore, in the case of PVdF(HFP)-[HEMIm][PF6] gels, although the amorphous range is generally dominant, the crystallization appears locally. In the range of 2θ ) 39-40°, the peak of 39.041° shown in the 100 wt % PVdF(HFP) film became increasingly amorphous according to the increase in the amount of [HEMIm][PF6], and was barely observed in the 33.3 wt % PVdF(HFP) gels. Ionic Conductivity of PVDF(HFP)-Ionic Liquid Gels. The prepared high-conducting liquid electrolytes, 1-(2-hydroxyethyl)3-methyl imidazolium derivatives, were incorporated with various amounts of PVdF(HFP) copolymer. Figures 10 and 11 depict the temperature dependences of the ionic conductivities according to the PVDF(HFP) composition (0-66.7 wt %) in [HEMIm][BF4] and [HEMIm][PF6] systems, respectively. All curves in Figures 10 and 11 follow an Arrhenius relation over the temperature range (20-70 °C). Both the neat [HEMIm][BF4] and the neat [HEMIm][PF6] showed a relatively high ionic conductivity of 10-3-10-4 S cm-1 over the aforementioned temperature range. Generally, the ionic conductivity of a polymer-ionic liquid gel is lower than that of a neat ionic liquid,


Figure 11. Ionic conductivity of the PVdF(HFP) gel in the [HEMIm][PF6] system as a function of temperature: 0 wt % PVdF(HFP) (9), 33.3 wt % PVdF(HFP) (≤), 47.4 wt % PVdF (HFP) ([), and 66.7 wt % PVdF(HFP) (0).

because the ionic motion of the polymer gel is connected with the local segmental motions of the polymer chain matrix, and improvements in carrier-ion density or mobility are difficult to accomplish.15 In the case of the PVdF(HFP)-[HEMIm][BF4] system, when the PVdF(HFP) content is higher, the ionic conductivities of the gels are lower. A similar trend was also observed in the PVdF(HFP)-[HEMIm][PF6] system of Figure 11. However, although the ionic conductivity of the neat [HEMIm][BF4] was about twice as high as that of the neat [HEMIm][PF6], the ionic conductivity of the 66.7 wt % PVdF(HFP)-[HEMIm][BF4] system was about 10-1 lower than that of the 66.7 wt % PVdF(HFP)-[HEMIm][PF6] system at room temperature (Table 2). This might account for the distinct differences in grain size shown in the surface morphology of the 66.7 wt % PVdF(HFP)-[HEMIm][BF4] gel and the 66.7 wt % PVdF(HFP)[HEMIm][PF6] gel; in the latter film, we can assume that the smaller grains allow for improved ionic conductivity due to the increased contact area. When compared with literature data18 taken at room temperature, for a PVdF(HFP)-[BMIm][PF6] (1-butyl-3-methyl imidazolium hexafluorophosphate) system that uses methyl-2pentanone as a solvent, the ionic conductivity of the prepared 47.4 wt % PVdF(HFP)-[HEMIm][PF6] gel is 5 times higher than that of 47.4 wt % PVdF(HFP)-[BMIm][PF6] gel. However, the ionic conductivity of the prepared 33.3 wt % PVdF(HFP)[HEMIm][PF6] gel is 4 times lower than that of the 33.3 wt % PVdF(HFP)-[BMIm][PF6] gel. The room-temperature ionic conductivities and activation energies calculated from the slope of the linear Arrhenius region (Figures 10 and 11) around 25 °C are listed in Table 3. To compare the measured ionic conductivities of the films with different mixing ratios, the film must be prepared to a uniform thickness that is as thin as possible. However, when the polymer gel films are prepared in glass Petri dishes, it is difficult to maintain a uniform thickness among the various gels, as seen in Table 1. Figure 12, which uses 66.7 wt % PVdF(HFP)-[HEMIm][BF4] and 33.7 wt % PVdF(HFP)-[HEMIm][BF4] gels, shows the degree of change that occurs in the ionic conductivities of two samples with different thicknesses. In the case of the 66.7 wt % PVdF(HFP) gels, in all the temperature ranges, the ionic conductivity of the 0.105 mm film strongly agrees with that of the 0.04 mm film. At the same time, in the case of the 33.3 wt % PVdF(HFP) gels, the ionic conductivities


Yeon et al.

TABLE 3: Ionic Conductivities and Activation Energies of PVdF(HFP)-Ionic Liquid Gels PVDF(HFP) mass ratio (wt %)

σ (S cm-1, 25 °C)

Ea (kJ mol-1)

0 33.3 47.4 66.7

[HEMIm][BF4] 4.6 × 10-3 1.48 × 10-4 7.53 × 10-5 4.34 × 10-6

29.48 38.52 31.86 36.13

0 33.3 47.4 66.7

[HEMIm][PF6] 2.1 × 10-3 1.48 × 10-4 5.06 × 10-5 2.7 × 10-5

39.29 36.03 40.82 36.18

of the two films, 0.32and 0.2 mm, showed values similar to those of the 66.7 wt % PVdF(HFP) gels. Hence, the effects of ionic conductivity on different film thicknesses were not considered in this study. Conclusions PVdF(HFP)-ionic liquid gels based on room-temperature ionic liquids (1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate and 1-(2-hydroxyethyl)-3-methyl imidazolium hexafluorophosphate) were prepared with various mixing ratios. The gels, which included prepared ionic liquids, were freestanding, rubbery films. They were prepared with increasing amounts of ionic liquid, and their surface morphology consisted of numerous spherical grains with small pores distributed throughout the gel. The thermal behaviors of the gels, including glass transition temperature, crystallization temperature, and melting temperature, were similar to those shown by neat ionic liquids. However, the heat flow magnitude of the thermal peak decreased with increasing amounts of PVdF(HFP), and in the 100 wt % PVdF(HFP) film, it was difficult to see any of the thermal peaks at temperatures ranging from -150 to 150 °C. The XRD pattern of two PVdF(HFP)-ionic liquid gels generally followed that of the neat PVdF(HFP) film. However, it can be expected from the XRD patterns that, although the PVdF(HFP)-[HEMIm][BF4] gels showed amorphous morphology caused by the high compatibility between the two materials with the addition of [HEMIm][BF4] into PVdF(HFP), the crystallization appears locally in PVdF(HFP)-[HEMIm][PF6] gels as a result of the low compatibility between [HEMIm][PF6] and PVdF(HFP). The ionic conductivity of the gels based on [HEMIm][BF4] and [HEMIm][PF6] was measured as 10-4-10-5 S cm-1 in a temperature range of 20-70 °C, whereas the ionic conductivity of neat [HEMIm][BF4] and neat [HEMIm][PF6] indicated high values of 4.6 × 10-3 and 2.1 × 10-3 S cm-1, respectively, at room temperature.17 Particularly, in the 66.6 wt % PVdF(HFP), the ionic conductivity of the PVdF(HFP)-[HEMIm][BF4] gel was 10-1 lower than that of the PVdF(HFP)-[HEMIm][PF6] gel, even though the ionic conductivity of the neat [HEMIm][BF4] is higher than that of the neat [HEMIm][PF6]. From this result it was estimated that the ionic conductivity was likely influenced by the surface morphology of the film; in the 66.6 wt % PVdF(HFP)-[HEMIm][PF6] gel, an improvement of ion mobility is expected from the increase of contact area among grain boundaries caused by the small grain size. From a mechanical strength perspective, it was found that the film having a large grain size and various pores was weaker and more fragile than that having a small grain size and packed structure. Herein, polymer gels including prepared ionic liquids, in which the volatile solvent is completely removed, will be

Figure 12. Ionic conductivity of the PVdF(HFP) gel in the [HEMIm][BF4] system at different film thickness: 66.7 wt % PVdF(HFP) (9) of 0.105 mm, 66.7 wt % PVdF(HFP) (0) of 0.04 mm, 33.3 wt % PVdF (HFP) (2) of 0.32 mm, and 33.3 wt % PVdF(HFP) (4) of 0.2 mm.

important for future developments of various devices such as batteries and sensors stably operated in a wide range of temperaturse without any solvent or plasticizer. Purity of Ionic Liquids. The evaluation of the purity of the prepared ILs was conducted by 1H NMR, FAB mass, ionic chromatography, and measurement of water contents. The 1H NMR and FAB mass spectra were recorded on a Bruker DMX 300 MHz NMR spectrometer and FAB mass JMS-HX110A spectrometer, respectively. The possible presence of residual Cl- was examined by a precipitation test of AgNO3 and ionic chromatography (Bio-LC DX-300 (Dionex, Sunnyvale, CA), Detector: Suppressed Conductivity (PED2), Column: ICSep AN 300 with ICSep ANSC guard). All of the ionic liquids were rigorously dried at 50 °C under 0.03 Torr for 5 days. The water contents of all prepared ILs were measured by a Karl Fischer titration (756 KF coulometer, Metrohm) in a dry atmosphere. [HEMIm][Cl]. The 1H NMR(DMSO, δ, relative to TMS) spectrum consists of the following peaks: 3.72(q, 2H), 3.88(s, 3H), 4.21(t, 2H), 5.29(t, 1H), 7.71(d, 1H), 7.75(d, 1H), 9.18(s, 1H). FAB mass showed m/z ) 127.17 [HEMIm]+. [HEMIm][BF4]. The 1H NMR(DMSO, δ, relative to TMS) spectrum consists of the following peaks: 3.72(q, 2H), 3.88(s, 3H), 4.21(t, 2H), 5.29(t, 1H), 7.71(d, 1H), 7.75(d, 1H), 9.18(s, 1H). FAB mass showed m/z ) 127.17 [HEMIm]+. The content of chloride anion was 24 ppm. The water content was 266 ppm. [HEMIm][PF6]. The 1H NMR (DMSO, δ, relative to TMS) spectrum consists of the following peaks: 3.72(q, 2H), 3.88(s, 3H), 4.21(t, 2H), 5.29(t, 1H), 7.71(d, 1H), 7.75(d, 1H), 9.18(s, 1H). FAB mass showed m/z ) 127.17 [HEMIm]+. The content of chloride anion was 48 ppm. The water content was 183 ppm. Acknowledgment. This work was supported by Grant R012003-000-10300-0 from the Basic Research Program of the Korea Science & Engineering Foundation, and was partially funded by the Brain Korea 21 Project. The authors thank KBSI (Korea Basic Science Institute) for assistance with NMR, FAB mass, and ion chromatography, and Reliability Assessment Center of KRICT (Korea Research Institute of Chemical Technology) for assistance with DSC and TGA. References and Notes (1) Yoshizawa, M.; Hirao, M.; Akita, K. I.; Ohno, H. J. Mater. Chem. 2001, 11, 1057-1062. (2) Gray, F. M.; MacCallum, J. R.; Vincent, C. A.; Giles, J. R. M. Macromolecules 1988, 21, 392-397.

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J. Phys. Chem. B, Vol. 109, No. 38, 2005 17935 (12) Capiglia, C.; Saito, Y.; Kataoka, H.; Kodama, T.; Quartarone, E.; Mustarelli, P. Ionics 2000, 131, 291-299. (13) Renard, I.; Li, H.; Marsan, B. Electrochim. Acta 2003, 48, 831844. (14) Fuller, J.; Breda, A. C.; Carlin, R. T. J. Electrochem. Soc. 1997, 144 (4), L67-L69. (15) Tiyapiboonchaiya, C.; Macfarlane, D. R.; Sun, J.; Forsyth, M. Macromol. Chem. Phys. 2002, 203, 1906-1911. (16) Seyhan, E. Organic Chemistry: Structure and ReactiVity, 3rd ed.; D. C. Heath and Company: Lexington, MA, 1994. (17) Yeon, S. H.; Kim, K. S.; Choi, S.; Lee, H.; Kim, H. S.; Kim, H. Electrochim. Acta 2005, doi no. 10.1016/j.electacta. (18) Marsh, K. N.; Boxall, J. A.; Lichtenthaler, R. Fluid Phase Equilib. 2004, 219, 93-98.

Characterization of PVdF(HFP) Gel Electrolytes Based on 1-(2

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