ISSN 0965-545X, Polymer Science, Ser. A, 2007, Vol. 49, No. 3, pp. 256–261. © Pleiades Publishing, Ltd., 2007. Original Russian Text © Ya.S. Vygodskii, O.A. Mel’nik, A.S. Shaplov, E.I. Lozinskaya, I.A. Malyshkina, N.D. Gavrilova, 2007, published in Vysokomolekulyarnye Soedineniya, Ser. A, 2007, Vol. 49, No. 3, pp. 413–420.
SYNTHESIS, POLYMERIZATION
Synthesis and Ionic Conductivity of Polymer Ionic Liquids1 Ya. S. Vygodskiia, O. A. Mel’nika, A. S. Shaplova, E. I. Lozinskayaa, I. A. Malyshkinab, and N. D. Gavrilovab a
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia b Faculty of Physics, Moscow State University, Leninskie gory, Moscow, 119992 Russia e-mail:
[email protected] Received July 06, 2006; Revised Manuscript Received October 7, 2006
Abstract—Four vinyl monomers containing a covalently bonded cation ethylimidazolium and various –
anions—Br–, (CF3SO2)2N–, (CN)2N–, and CF3 SO 3 —have been synthesized. High-molecular-mass polymers (Mw up to 1.84 × 106) having the structure of ionic liquids have been prepared via the free-radical polymerization of 1-vinyl-3-ethylimidazolium in bulk and molecular and ionic solvents. The thermal stability and heat resistance of the resulting polymer salts have been estimated. It has been demonstrated that the thermal characteristics of these salts significantly depend on the nature of anions. The glass-transition temperatures of the polymers range from 19 to 235°C. The ionic conductivity of the polymer salts and their compositions with individual ionic liquids has been studied in the frequency range 50–106 Hz. The highest conductivity (1.5 × 10–5 S/cm) is exhibited by the polymer containing the (CN)2N– anion. DOI: 10.1134/S0965545X07030042 1
The necessity of designing novel conducting polymer materials is associated with the intense development of modern technologies, creation of diverse portable devices powered by safe and compact current sources capable of long-life performance. A polymer electrolyte traditionally consists of a polymer matrix filled with a salt solution in a polar solvent. However, the operability of such composites is limited by a rather narrow temperature range. This situation is related to the volatility, inflammability, and explosive hazard of the organic solvents used. The employment of ionic liquids (solvents of the new class) makes it possible to overcome engineering difficulties and to enhance the environmental safety. The first polymer electrolytes based on ionic liquids were composites prepared from a preformed polymer and a liquid organic salt [1, 2]. Later on, Watanabe [3, 4] and Forsyth et al. [5, 6] developed a new approach to the synthesis of conducting gels that was based on the use of ionic liquids both as reaction media for the free-radical polymerization of vinyl monomers and electrolytes. The ionic conductivity of currently available composites reaches 10–6–10–3 S/cm regardless of their synthetic procedure (mechanical blending or in situ polymerization). 1 This
work was supported by the Russian Foundation for Basic Research, project no. 05-03-08073 ofi-a.
Over the last five years, polymer ionic liquids with imidazolium, pyridinium, and ammonium cations have been synthesized in which the moiety of an ionic liquid is covalently bonded to a polymer chain [7–12]. The electric conductivity of such polymer salts does not exceed 4 × 10–4 S/cm. These salts are usually prepared through the free-radical polymerization of corresponding vinyl monomers in organic solvents (ethyl alcohol, tetrachloroethane, or chloroform) but the molecular mass of the polymers thus prepared is very low. Previously, we synthesized high-molecular-mass PMMA [13] and PAN [14] via polymerization and polyimides, polyamides, and other polyheteroarylenes [15–17] via polycondensation in imidazolium ionic liquids of diverse natures. The goal of this study was to design conducting polymers on the basis of polymer ionic liquids or their composites with individual ionic liquids. EXPERIMENTAL Alkyl bromides (Aldrich) and N-methylimidazole (99%, Acros Organics) were distilled before use over CaH2 in a flow of argon. 1-Vinylimidazole, MMA (99%, Aldrich), and acrylonitrile (AN) (99%, Aldrich) were distilled under reduced pressure. Imidazole (99%, Merck), silver trifluoromethyl sulfonate (99%, Aldrich), lithium bis(trifluoromethylsulfonyl)imide (99%,
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Fluka), silver nitrate (99%, Fluka), and sodium dicyanoamide (97%, Acros Organics) were used as received. AIBN (98%, Aldrich) was recrystallized from methanol. Synthesis of Ionic Monomers Synthesis of 1-vinyl-3-ethylimidazolium bromide (ViEtIm)+Br–. In order to prepare this monomer, we developed a method involving the quarternization of 1-vinylimidazole. This method is distinguished from those described in [18–20] by mild reaction conditions and yields a pure colorless crystalline product. A round-bottomed flask equipped with a magnetic stirrer, a condenser, and a calcium chloride tube was loaded with 1-vinylimidazole (24.1 ml, 0.265 mol) and anhydrous methanol (35 ml) in a flow of argon. The stirred reaction mixture was cooled from 0 to 5°C in an ice bath. After 30 min, an excess of ethyl bromide (40 ml, 0.531 mol) was added very slowly, and the resulting mixture was stirred for another 24 h at 0–5°C. The temperature was gradually raised successively to 20, 40, and 55°C at 24-h intervals. When the alkylation reaction was completed, methanol and excess of ethyl bromide were removed under reduced pressure at 55°C. The residue (viscous liquid) was allowed to stay under high vacuum at 55°C until its full crystallization; Tm = 116.5°C; Yield, 52.8 g (98%). For C7H11N2Br anal. calcd. (%): C, 41.43; H, 5.42; N, 13.80; Br, 39.36. Found (%): C, 40.82; H, 5.45; N, 13.91; Br, 39.81. IR (KBr): ν 3432 (m), 3132 (w), 3061 (s), 2991 (s), 2925 (s), 2852 (w), 1661 (s), 1582 (s), 1546 (s), 1460 (m), 1376 (m), 1331 (m), 1304 (m), 1259 (m), 1186 (s), 1170 (s), 980 (w), 928 (w), 857 (m), 784 (m), 619 (m), 597 (w) cm–1. 1H NMR (400 MHz, CDCl ; δ ): 1.58 (t, 3H, 3 H NCH2CH3, J(H,H) = 7.3 Hz), 4.43 (m, 2H, NCH2CH3, J(H,H) = 7.3 Hz), 5.36 (m, 1H, NCH=CH2, HA), 6.00 (m, 1H, NCH=CH2, HB), 7.38 (m, 1H, NCH=CH2, J(H,H) = 8.7 Hz), 7.74 (s, 1H, H5 (Im)), 7.92 (s, 1H, H4 (Im)), 10.77 (s, 1H, H2 (Im)) ppm. Synthesis of 1-vinyl-3-ethylimidazolium dicyanoamide (ViEtIm)+(CN)2N–. The monomer was prepared according to techniques used for the synthesis of dicyanoamide ionic liquids [21, 22]. The excess of silver dicyanoamide (8.38 g, 0.048 mol) that was freshly prepared from the aqueous solution of sodium dicyanoamide and silver nitrate was added to the aqueous solution of 1-vinyl-3-ethylimidazolium bromide (8.0 g, 0.039 mol), and the mixture thus prepared was heated under stirring to 40°C for 1 h. The solid residue (silver bromide and excess of silver dicyanoamide) was filtered off, and water was removed from the filtrate on a rotor evaporator. The product isolated as a light yellow liquid was vacuum dried over P2O5 at 55°C. Yield, 7.1 g (96%). POLYMER SCIENCE
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For C9H11N5 anal. calcd. (%): C, 57.13; H, 5.86; N, 37.01. Found (%): C, 56.98; H, 5.68; N, 37.17. IR (KBr): ν 3436 (m), 3139 (w), 3095 (w), 3061 (w), 3018 (w), 2990 (m), 2234 (w, –CN), 2195 (m, −CN), 2134 (s, –CN), 1655 (w), 1573 (w), 1552 (w), 1466 (m), 1449 (m), 1372 (m), 1311 (s), 1171 (s), 958 (w), 914 (w), 847 (m), 754 (w), 648 (w), 599 (w), 524 (w) cm–1. NMR (400 MHz, DMSO-d6; δH): 1.46 (t, 3H, NCH2CH3 , J(H,H) = 7.5 Hz), 4.22 (m, 2H, NCH2CH3 , J(H,H) = 7.2 Hz), 5.39 (m, 1H, NCH=CH2, HA), 5.91 (m, 1H, NCH=CH2, HB), 7.25 (m, 1H, NCH=CH2 , J(H,H) = 8.8 Hz), 7.89 (s, 1H, H5 (Im)), 8.14 (s, 1H, H4 (Im)), 9.46 (s, 1H, H2 (Im)) ppm. Synthesis of 1-vinyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide (ViEtIm)+(CF3SO2)2N–. The monomer was synthesized as described in [8, 20]. To a solution of 1-vinyl-3-ethylimidazolium bromide (10.0 g, 0.049 mol) in distilled water (15 ml), lithium bis(trifluoromethylsulfonyl)imide (12.86 g, 0.045 mol) was slowly added in portions. The resulting suspension was stirred at 55°C for 2 h, the upper layer was decanted, and the oily precipitate was washed several times with water. The product (colorless liquid) was dried in vacuum over P2O5 at 55°C. Yield, 15.5 g (78%). 1H
For C9H11N3F6S2O4 anal. calcd. (%): C, 26.80; H, 2.75; F, 28.26. Found (%): C, 26.61; H, 2.76; F, 28.39. IR (KBr): ν 3466 (m), 3154 (w), 3108 (m), 2994 (m), 2951 (m), 1661 (w, SO2), 1575 (w), 1554 (w, SO2), 1471 (m), 1454 (m), 1352 (s, CF), 1194 (s, CF), 1141 (w), 1057 (s), 956 (m), 919 (m), 846 (m), 790 (m), 741 (m), 650 (m), 616 (w), 517 (w), 515 (w) cm–1. NMR (400 MHz, CDCl3; δH): 1.45 (t, 3H, NCH2CH3, J(H,H) = 7.3 Hz), 4.18 (m, 2H, NCH2CH3, J(H,H) = 7.3 Hz), 5.29 (m, 1H, NCH=CH2, HA), 5.69 (m, 1H, NCH=CH2, HB), 6.98 (m, 1H, NCH=CH2, J(H,H) = 6.9 Hz), 7.39 (s, 1H, H5 (Im)), 7.57 (s, 1H, H4 (Im)), 8.76 (s, 1H, H2 (Im)) ppm. Synthesis of 1-vinyl-3-ethylimidazolium trifluo– romethyl sulfonate (ViEtIm)+CF3 SO 3 . The monomer was prepared as described in [23]. To a solution of 1-vinyl-3-ethylimidazolium bromide (7.11 g, 0.035 mol) in distilled water (30 ml), an aqueous solution of lithium trifluoromethyl sulfonate (9.0 g, 0.035 mol) was added at room temperature. The resulting mixture was stirred at 55°C for 1 h, the silver bromide precipitate was filtered off, and the reaction solution was evaporated under reduced pressure. The product isolated as a light yellow liquid (slowly crystallizes during storage) was vacuum dried over P2O5 at 55°C; Tm = 26°C; Yield, 9.45 g (99%). 1H
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For C8H11N2F3SO3 anal. calcd. (%): C, 35.30; H, 4.07; N, 10.29. Found (%): C, 35.09; H, 4.08; N, 10.28. IR (KBr): ν 3544 (m), 3150 (w), 3109 (w), 3012 (m), 2995 (m), 1659 (w), 1577 (w), 1554 (w), 1471 (m), 1268 (s, SO3), 1223 (s), 1158 (s, SO3), 1031 (s, CF), 960 (m), 924 (m), 843 (m), 757 (w), 687 (m), 638 (s), 599 (m), 574 (w), 518 (w), 484 (m) cm–1. 1H NMR (400 MHz, DMSO-d ; δ ): 1.46 (t, 3H, 6 H NCH2CH3 , J(H,H) = 7.5 Hz), 4.23 (m, 2H, NCH2CH3 , J(H,H) = 7.3 Hz), 5.41 (m, 1H, NCH=CH2, HA), 5.95 (m, 1H, NCH=CH2, HB), 7.28 (m, 1H, NCH=CH2 , J(H,H) = 8.8 Hz), 7.94 (s, 1H, H5 (Im)), 8.18 (s, 1H, H4 (Im)), 9.49 (s, 1H, H2 (Im)) ppm. Ionic liquids based on the 1,3-dialkyl-substituted imidazole that were used as solvents in free-radical polymerization and preparation of compositions were prepared as described in [13, 16, 20, 23]. The products were characterized by elemental analysis and 1H NMR and IR spectroscopic measurements. Polymerization For the bulk polymerization, the ionic monomer of the formula (ViEtIm)+Y– containing the dissolved AIBN was placed in a glass ampoule and degassed under vacuum at room temperature for 20 min. The ampoule was sealed under vacuum and placed in a thermostat. For the polymerization of (ViEtIm)+Y– in the common organic solvent or ionic liquid, the monomer solution containing the dissolved AIBN was placed in the glass ampoule. After the freeze–pump–thaw cycle was repeated three times, the ampoule was sealed under vacuum and placed in the thermostat. Polymerization was carried out at 60°C. Polymer ionic liquids were reprecipitated from acetone solutions into methanol (Y− = (CF3SO2)2N–), from methanol solutions into –
chloroform (Y– = CF3 SO 3 , Br–), or from DMF solutions into ethyl acetate (Y– = (CN)2N–), washed many times with corresponding precipitants, and vacuum dried under heating. The copolymers of 1-vinyl-3-ethylimidazolium salts with MMA and AN (6 : 4, wt %) were synthesized in the presence of AIBN (0.5 and 0.2 wt %, respectively) at 60°C in a manner similar to that described above and were used for film casting without additional purification. Polymer films were cast from 2–3% solutions in the selected solvent onto a glass, Teflon, or Cellophane support followed by slow evaporation of the solvent at the preset temperature (from 20 to 70°C). The films thus prepared were vacuum dried at room temperature or 70°C. Measurements The intrinsic viscosity was measured at 25°C on an Ostwald viscometer (0.05 g of polymer salts in 10.0 ml
of solvent). The molecular mass of polymers was estimated by the light-scattering method. Measurements were performed on a Fica photogoniodiffusometer (France) in vertical polarized light (λ = 546 nm) in the range of scattering angles 30°–150° at 25 ± 0.1°C in MEK or methanol. Refractive index increment dn/dc was calculated from the results of refractometric measurements carried out on a refractometer equipped with a differential cuvette. The molecular mass of [(ViEtIm)+(CN)2N–]n was estimated by sedimentation measurements on a MOM 3180 analytical centrifuge (Hungary) at a wavelength of 546 nm and a temperature of 25 ± 0.1°C in DMF with the use of the Filpot–Svensson optics. The 1H NMR spectra were measured on a Bruker AMX-400 spectrometer; the IR spectra were taken on a Nicolet Magna 750 spectrophotometer. Thermomechanical studies were performed on a UIP-70M instrument at a load of 0.08 MPa in the temperature range from –100 to +350°C. The heating rate of the test sample was 2.5 K/min. Dynamic TGA testing was conducted in air on a MOM Q-1500 derivatograph (Hungary) at a heating rate of 5 K/min. Dielectric measurements were performed on a Novocontrol broadband dielectric spectrometer equipped with an Alpha analyzer and Quatro temperature controller. Polymer films were placed between gold-coated brass electrodes and examined at 20°C in the frequency range 50–106 Hz. RESULTS AND DISCUSSION The synthesis of 1-vinyl-3-alkylimidazolium polycations with various anions was reported in [8, 11, 12]. However, these polyelectrolytes have not been studied in detail. To our knowledge, no data is available on their preparation via free-radical polymerization in ionic media. In this study, we are concerned with the synthesis of four ionic vinyl monomers, namely, 1-vinyl-3alkylimidazolium salts with Br–, (CF3SO2)2N–, –
(CN)2N–, and CF3 SO 3 anions. The latter two salts were synthesized for the first time. We studied the free-radical polymerization of these monomers in bulk and solution in the presence of AIBN (0.2–1.0 wt %) at 60°C (Table 1). As reaction media, we employed both conventional organic solvents (ethanol, chloroform, acetone, and DMF) and ionic solvents carrying the same anions as the above-mentioned monomers. When polymerization was carried out in chloroform and ethanol, the polymer salts being formed precipitated from solution (in the case of alcohol, this effect was observed only for polymer ionic liquids). At the same time, in acetone, DMF, and ionic liquids, polymerization proceeded under homogeneous conditions throughout the entire polymerization process. It should be noted that the bulk polymerization yields polymers with a very high viscosity (ηinh up to POLYMER SCIENCE
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CH CH2 N Table 1. Free-radical polymerization of ionic monomers with formula + Y– (T = 60°C, monomer : solvent = 1 : 1 (g/ml), N C2H5 and the reaction time is 4 h (for experiment 3, the reaction time is 8 h, and for experiments 7, 15, and 19, the reaction time is 6 h) Polymer ionic liquid
Experiment
Solvent
[AIBN], wt % Y– = (CF3SO2)2N– 0.2
1
–
2 3 4 5 6 7 8
– Ethanol Ethanol Chloroform Chloroform Acetone Acetone
0.5 0.5 1.0 0.5 1.0 0.5 1.0
9
(MeBuIm)+ (CF3SO2)2N–
0.5 Y– =
yield, %
51 62 39 – 58 66 48 57 55
η *inh , dl/g (Mw × 10–3) 3.75 (58.8) 3.65 0.09 – 0.78 0.49 0.45 0.41 2.29 (51.3)
– CF3S O 3
10 11
Chloroform Chloroform
0.5 1.0
89 91
12
Acetone
1.0
83
1.43 2.11 (1840) 2.27
13
– (MeBuIm)+ CF3S O 3
0.5
85
2.85
61 10 60
6.50 1.24 1.50 (21.0) 5.0 (1130**)
Y– = (CN)2N– 0.2 0.5 1.0
14 15 16
DMF DMF
17
(MeBuIm)+ (CN)2N–
0.5
59
Chloroform Ethanol Ethanol
Y– = Br– 0.5 0.5 1.0
Gel 63 32
18 19 20
–
– 1.32 0.83
* In experiments 1–9, ηinh was measured in acetone; in experiments 10–13, 19, and 20, ηinh was measured in methanol; and in experiments 14–17 ηinh was measured in DMF. ** Mz was measured by the sedimentation method.
6.50 dl/g) (Table 1, experiments 1, 2, and 14). When free-radical polymerization is conducted in solution, the yield and ηinh of polymer ionic liquids noticeably depend on the nature of the reaction medium. Thus, the POLYMER SCIENCE
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polymer salt with the (CF3SO2)2N– anion prepared in ethanol in the presence of AIBN (0.5 wt %) for 8 h is characterized by a very low molecular mass (ηinh = 0.09 dl/g). This result suggests the occurrence of chain
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decrease in activation energy) and chain termination (a reduction in the viscosity of the reaction system, that is, the gel effect) [24]. It is suggested that the above factors hold true for the polymerization of ionic monomers. We examined the heat resistance and thermal stability of the polymer salts. Our studies showed that the nature of an anion strongly affects their thermal characteristics. As evidenced by TMA measurements of polymer ionic liquids, the glass-transition temperature of [(ViEtIm)+Y–]n is 19 (Y– = (CN)2N–), 60 (Y– =
Residue weight, % 100
75
3
50
4 2 1
–
25
0 250
500
750
1000 T, °C
TGA curves in air for samples of [(ViEtIm)+Y–]n with –
Y– = (1) Br–, (2) (CF3SO2)2N–, (3) CF3 SO 3 , and (4) (CN)2N–.
transfer to this solvent (Table 1, experiment 3). No polymer was formed when the quantity of the initiator is increased to 1 wt % (experiment 4). Polymer ionic liquids were prepared in chloroform with high yields (58 and 66%) and ηinh = 0.78 and 0.49 dl/g, respectively) (Table 1, experiments 5, 6). In the case of polymers synthesized in acetone for a longer time, these values were lower (Table 1, experiments 7, 8). Polymer – salts with the CF3 SO 3 anion were formed in chloroform and acetone with a high yield (83–91%) and a very high molecular mass (Mw = 1840000) (Table 1, experiments 10–12). A comparison of the efficiency of solvents of different natures demonstrated that, in contrast to reactions in conventional solvents, the molecular mass of the polymers prepared in ionic solvents is close to that of corresponding polymers synthesized in the bulk. For example, the polymer salt [(ViEtIm)+(CF3SO2)2N–]n synthesized in the ionic liquid (MeBuIm)+(CF3SO2)2N– is characterized by ηinh = 2.29 dl/g and Mw = 51300 (Table 1, experiment 9). Relatively low molecular masses of such polymer ionic liquids may be attributed to steric hindrances created by a bulky anion during polymerization. The inherent viscosity of [(ViEtIm)+(CN)2N–]n prepared in (MeBuIm)+(CN)2N– achieves 5.0 dl/g. When the synthesis is performed in DMF, the inherent viscosity is as low as 1.50 dl/g. The corresponding Mw values of the polymers are 113 × 104 and 2.1 × 104, respectively (Table 1, experiments 16, 17). According to the published data, high molecular masses and increased rates of free-radical polymerization in ionic liquids are associated with the strong effect of ionic media on reactions of chain propagation (a
(CF3SO2)2N–), 173 (Y– = CF3 SO 3 ), and 235°C (Y– = Br–). With respect to an increase in thermal stability (TGA in air), the polymers may be arranged in the – sequence (CN)2N– < Br– < CF3 SO 3 < (CF3SO2)2N– depending on the nature of anions (Fig. 1). For the most thermally stable polymer with the (CF3SO2)2N– anion, the onset temperature of degradation is 360°C (Fig. 1, curve 2). Solid polymer electrolytes based on imidazolium salts offer a number of advantages compared to liquid electrolytes; namely, they are safe, they are stable, and they operate over a wide temperature range, including elevated temperatures [8]. The ionic conductivity of the polymer salts and their compositions with ionic liquids was investigated at various frequencies (Table 2). It should be emphasized that polymer ionic liquids with – CF3 SO 3 and (CF3SO2)2N– anions form transparent solid films, whereas the polymer containing the (CN)2N– anion produce a rubberlike material characterized by a much higher σ value (Table 2, experiments 1–3). As is known, electric conductivity grows with an increase in the current frequency. We showed that the highest conductivity of polymer ionic liquids is 1.5 × 10–5 S/cm at 106 Hz (Table 2, experiment 3). To enhance conductivity, the ionic liquid of the similar structure that is well compatible with such polymers was added to the polymer salts. The resulting composites may be used for the formation of polymer gels in which the polymer ionic liquid plays the role of a polymer matrix, while the ionic liquid serves as a liquid electrolyte. Our experiments revealed that the combination of polymer ionic liquids and individual liquid salts affords polymer gels with a very high ionic conductivity. In this case, the amount of the ionic liquid being introduced does not exceed 25 wt %. If the content of the ionic liquid in the polymer is high, sticky films arise. In order to improve the mechanical characteristics of the films, we synthesized copolymers of 1-vinyl-3ethylimidazolium salts with acrylic monomers (MMA and AN) with the optimal weight ratio of monomer units, 6 : 4 (Table 2, experiments 4, 7). Interestingly, the copolymer of (ViEtIm)+(CF3SO2)2N– and MMA with ηinh = 2.77 dl/g (acetone) shows the same conductivity as the corresponding homopolymer of 1-vinyl-3-ethylimidazolium (Table 2, experiments 2, 4). In this case, POLYMER SCIENCE
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Table 2. Ionic conductivity of polymer ionic liquids and their compositions at 20°C Experiment 1 2
Ionic liquid* cation
anion
5
(CN)2N–
8
103
106
–
1.27 × 10–10 4.60 × 10–10 7.65 × 10–8
–
6.52 × 10–11 3.47 × 10–10 1.83 × 10–8
–
2.09 × 10–6 5.30 × 10–6 1.51 × 10–5
Copolymer of 1-vinyl-3- (CF3SO2)2N– – 4.84 × 10–11 2.39 × 10–10 6.23 × 10–8 ethylimidazolinium (MeBuIm)+(CF3SO2)2N– (25) 1.43 × 10–7 1.78 × 10–7 1.34 × 10–6 and MMA (6 : 4) –
1.72 × 10–7 2.13 × 10–7 1.64 × 10–6
–
2.55 × 10–11 8.32 × 10–11 1.24 × 10–8
(MeEtIm)+B F 4 (25)
6 7
50
Homopolymer of 1-vinyl- (CF3SO2)2N– 3-ethylimidazolinium – CF3S O 3
3 4
σ (S/cm) at a frequency, Hz
Polymer ionic liquid
Copolymer of 1-vinyl-3- (CN)2N– ethylimidazolinium and AN (6 : 4)
(MeEtIm)+(CN)2N– (15)
3.23 × 10–10 5.34 × 10–10 1.95 × 10–6
* The content of ionic liquid (wt %) is shown in parentheses.
the copolymer of (ViEtIm)+(CN)2N– and AN with ηinh = 3.35 dl/g (DMF) is characterized by a much lower conductivity than the polymer salt (Table 2, experiments 3, 7). When ionic liquids with a high value of σ are added to the copolymers, transparent elastic films containing up to 25 wt % salts with (CF3SO2)2N–, –
BF 4 , and (CN)2N– anions are formed (Table 2, experiments 5, 6, 8). As anticipated, the ionic conductivity of these compositions appreciably increases and achieves 10–6 S/cm. Since the conductivity of compositions is significantly affected by the electric characteristics of electrolytes, the targeted search for new ionic liquids with enhanced ionic conductivity calls for further investigations. ACKNOWLEDGMENTS We are grateful to L.V. Dubrovina and G.I. Timofeeva for the molecular-mass measurements of polymer ionic liquids, L.I. Komarova for the IR analysis, and M.I. Buzin for the TGA study of polymers. REFERENCES 1. R. T. Carlin and J. Fuller, Chem. Commun., No. 15, 1345 (1997). 2. J. Fuller, A. C. Breda, and R. T. Carlin, J. Electroanal. Chem. 459, 29 (1998). 3. A. Noda and M. Watanabe, Electrochim. Acta 45, 1265 (2000). 4. Md. H. Susan, T. Kaneko, A. Noda, and M. Watanabe, J. Am. Chem. Soc. 127, 4976 (2005). 5. C. Tiyapiboonchaiya, D. R. MacFarlane, J. Sun, and M. Forsyth, Macromol. Chem. Phys. 203, 1906 (2002). 6. D. Z. Zhou, G. M. Spinks, G. G. Wallace, et al., Electrochim. Acta 48, 2355 (2003). POLYMER SCIENCE
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