Article pubs.acs.org/Macromolecules
Effect of Pressure on Decoupling of Ionic Conductivity from Segmental Dynamics in Polymerized Ionic Liquids Z. Wojnarowska,*,†,‡ J. Knapik,†,‡ J. Jacquemin,§ S. Berdzinski,∥ V. Strehmel,∥ J. R. Sangoro,⊥ and M. Paluch†,‡ †
Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland Silesian Center for Education and Interdisciplinary Research, 75 Pulku Piechoty 1A, 41-500 Chorzow, Poland § The School of Chemistry and Chemical Engineering/QUILL Research Centre, Queen’s University of Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG, Northern Ireland, U.K. ∥ Department of Chemistry and Institute for Coatings and Surface Chemistry, Hochschule Niederrhein University of Applied Sciences, Adlerstrasse 32, D-47798 Krefeld, Germany ⊥ Department of Chemical and Biomolecular Engineering, University of Tennessee, 1512 Middle Drive, Knoxville, Tennessee 37996, United States ‡
ABSTRACT: We examine, for the first time, the relation between charge transport and segmental dynamics in polymerized imidazolium-based ionic liquid [PBuVIm][NTf2] in the temperature and pressure thermodynamic space. The results of ambient pressure dielectric experiments combined with temperature-modulated differential scanning calorimetry measurements have revealed a fundamental difference between the conducting properties of the examined poly-IL and its low-molecular-weight counterpart. While the dc conductivity is practically coupled to structural relaxation in aprotic ionic liquid, a significant separation between the time scale of charge and mass transport is found for the polymerized system. However, squeezing of the studied macromolecular system is found to reduce the decoupling between τσ and τα that is attributed to significant slowing down in anions mobility under conditions of high compression. Thereby, our studies provide a fundamental understanding of the relationships between chemical structure, morphology, and ion transport properties in polymerized ionic systems used for diverse emerging technologies.
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INTRODUCTION Polymerized ionic liquids (poly-ILs) are a novel class of materials that combine the unique physicochemical properties of molecular ionic liquids with the outstanding mechanical characteristics of polymers.1,2 Thereby, they share the unique properties of both macromolecular systems and ionic liquids, like nonflammability, mechanical and thermal stability, durability, and high ionic conductivity.3 In addition, their polymeric architecture offers opportunities for production of conducting materials with different shape. These features together make poly-ILs attractive candidates for potential applications in electrochemical devices such as batteries, supercapacitors, or solar cells, among others.4,5 It has been demonstrated many times that switching from simple ionic liquids into IL-based polymeric membrane brings about an increase in the glass transition temperature and decrease in ionic conductivity at constant temperature.6 However, in addition to these rather obvious changes in physicochemical properties, polymerization of low-molecularweight aprotic electrolytes can result in some not fully understood physical effects. Namely, for several poly-ILs © XXXX American Chemical Society
containing various vinylimidazolium cations the specific separation between the time scale of charge diffusion and segmental relaxation has been found in the vicinity of the glass transition temperature.7−11 Such decoupling of ionic conductivity from the segmental dynamics was manifested by the characteristic crossover of the temperature dependence of electric conductivity (σdc) from the Vogel−Fulcher−Tammann-like12−14 (T > Tg) to Arrhenius behavior (T < Tg) that occurs at conductivity higher than 10−15 S/cm. Interestingly, this abrupt change in the σdc(T) dependence observed at Tg of poly-ILs has never been reported in their low-molecular-weight counterparts.15 A physical insight into nature of decoupling phenomenon in polymerized electrolytes was recently given by Sangoro et al.10 Based on the results of pulsed field gradient (PFG) NMR measurements of polymerized bis(trifluoromethylsulfonyl)imide-based ionic liquid, a strong decoupling of ionic Received: September 27, 2015 Revised: November 9, 2015
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DOI: 10.1021/acs.macromol.5b02130 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
limit by AgNO3 testing (i.e., < 50 ppm). CHNS calcd C: 30.63%, H: 3.50%, N: 9.74%, S: 14.87%. Found C: 30.92%, H: 3.47%, N: 9.84%, S: 14.79%. 1H NMR (300 MHz, DMSO) δ: 9.50 (s, 1H), 8.21 (s, 1H), 7.95 (s, 1H), 7.30 (dd, J = 15.6, 8.8 Hz, 1H), 5.97 (dd, J = 15.6, 1.7 Hz, 1H), 5.44 (d, J = 8.7 Hz, 1H), 4.22 (t, J = 7.2 Hz, 2H), 3.37 (s, 3H), 1.93−1.67 (m, 2H), 1.44−1.17 (m, 2H), 0.94 (t, J = 7.3 Hz, 3H). 13 C NMR (75 MHz, DMSO) δ: 135.70 (s), 129.27 (s), 123.62 (s), 119.51 (s), 117.72 (s), 108.97 (s), 49.32 (s), 31.42 (s), 19.17 (s), 13.63 (s). Synthesis of Polymer. Chemicals. Starting materials, such as 1iodobutane, 1-vinylimidazole, and azobis(isobutyronitrile) (AIBN), were purchased from Sigma-Aldrich. 1-Iodobutane was freshly distilled before used for synthesis (bp 47 °C, 60 mbar). The inhibitor containing in 1-vinylimidazole was removed using a column filled with basic aluminum oxide before distillation of the 1-vinylimidazole (bp 51 °C, 3 mbar) was carried out. Basic aluminum oxide, tert-butyl methyl ether, ethyl acetate, dichloromethane, 1-propanole, and acetone were purchased from Carl Roth. All solvents used for manufacture and purification of the ionic liquid monomer (1-butyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imide) were distilled before use. Lithium bis(trifluoromethylsulfonyl)imide was purchased from IoLiTec (Ionic Liquids Technologies GmbH) and used as received. Deuterated solvents (CD3CN and CDCl3) were purchased from ARMAR (Europa) GmbH. Synthesis of 1-Butyl-3-vinylimidazolium Bis(trifluoromethylsulfonyl)imide. Synthesis of 1-butyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imide required two steps.16 The first step required to add 1-iodobutane (138.0 g, 750 mmol) dropwise to 70.6 g (750 mmol) of N-vinylimidazole dissolved in 75 mL of tert-butyl methyl ether at room temperature under argon while stirring occurred during the entire reaction. After completion, stirring continued at room temperature for an additional 13 days. The precipitate obtained was isolated, washed with ethyl acetate, and finally dried under vacuum at 50 °C, resulting in 83% yield on 1-butyl-3-vinylimidazolium iodide (mp: 48−50 °C). The 1-butyl-3-vinylimidazolium iodide was analyzed by FTIR spectroscopy (νmax/cm−1: 593, 748, 915, 1168, 1370, 1458, 1547, 1649 (CC), 2958 (C−H)), 1H NMR spectroscopy (300 MHz, CD3CN, TMS, δH/ppm: 10.60 (1H, s, N−CH−N), 7.86 (1H, t, 3 J 1.8 Hz, N−CH), 7.65 (1H, t, 3J 1,8 Hz, N−CH), 7.45 (1H, dd, 3J 15.6, 8.7 Hz, CHvinyl), 6.03 (1H, dd, 3J 15.6, 3.0 Hz, CHvinyl), 5.46 (1H, dd, 3J 8.7, 3.0 Hz, CHvinyl), 4.47 (3H, t, 3J 7.5 Hz, N−CH2), 2.04−1.94 (2H, m, N−CH2), 1.44 (2H, sextet, 3J 7.5 Hz, CH2), 1.00 (3H, t, 3J 7.5 Hz, CH3), and 13C NMR spectroscopy (75 MHz, CDCl3, 25 °C, TMS δC/ppm: 135.1 (N−CH−N), 128.1 (N−CH), 123.0 (N−CH), 119.5 (CHvinyl), 110.5 (CH2vinyl), 50.3 (N−CH2), 32.1 (CH2), 19.5 (CH2), 13.5 (CH3)). In a second step, anion exchange was carried out using lithium bis(trifluoromethylsulfonyl)imide (73.9 g, 258 mmol) dissolved in 122 mL of water and a solution of 68.2 g (245 mmol) of 1-butyl-3-vinylimidazolium iodide dissolved in 122 mL of water. The lithium bis(trifluoromethylsulfonyl)imide solution was slowly dropped during stirring into the monomer solution, which was kept under argon. After formation of two phases, 50 mL of dichloromethane was added. Then, the organic phase was separated, washed with water (7 × 50 mL), and dried with anhydrous sodium sulfate for at least 12 h. Finally, the solvent was evaporated under vacuum. The residue obtained was additionally dried under vacuum at room temperature for 1 day, resulting in 89% yield of 1-butyl-3vinylimidazolium bis(trifluoromethylsulfonyl)imide. Water content determined by Karl Fischer titration was 302 ppm in the ionic liquid monomer, and the glass transition temperature of the monomer was −76 °C (Netzsch Phoenix DSC 204 using an aluminum pan in a temperature range between −120 and 120 °C). The ionic liquid monomer was analyzed by FTIR spectroscopy (νmax/cm−1: 508, 568, 789, 1051 (SO), 1132, 1174, 1361 (SO), 1463, 1552 (CC), 1655 (CC), 2879 (C−H), 2968 (C−H), 3150 (C−H)), 1H NMR spectroscopy (300 MHz, CD3CN, TMS, δH/ppm: 9.00 (1H, s, N− CH−N), 7.68 (1H, s, N−CH), 7.49 (1H, s, N−CH), 7.14 (1H, dd, 3J 15.6, 8.7 Hz, CHvinyl), 5.81 (1H, dd, 3J 15.6, 3.0 Hz, CHvinyl), 5.42 (1H, dd, 3J 8.7, 3.0 Hz, CHvinyl), 4.25 (3H, t, 3J 7.5 Hz, N−CH2), 1.95−1.85 (2H, m, CH2), 1.40 (2H, sextet, 3J 7.5 Hz, CH2), 0.98 (3H, t, 3J 7.2
conductivity from segmental relaxation in this system was ascribed to the different contributions of cations and anions in these two processes. While cations, covalently bonded to the polymer backbone, were found to dominate segmental dynamics, the motions of smaller and more mobile [NTf2]− anions were considered as a source of efficient charge diffusion. If such an explanation is correct, then even small space limitation in polymer matrix should result in slowing down of anions diffusion and consequently decrease in the time scale separation of charge transport and segmental relaxation. Highpressure measurements of poly-ILs have a potential to verify this hypothesis. In this paper we examine the conducting properties of aprotic polycationic system poly-1-vinyl-3-butylimidazolium bis(trifuoromethylsulfonyl)imide [PBuVIm][NTf2] and its low-molecular-weight counterpart as a reference (see Figure 1). The dielectric measurements performed over a wide
Figure 1. Chemical structure of examined [PBuVIm][NTf2].
temperature range supplemented by temperature-modulated differential scanning calorimetry experiments (TM DSC) clearly indicate a strong decoupling of charge diffusion from segmental dynamics in the polymerized ionic liquid, while dc conductivity follows structural relaxation well in monomer. On the other hand, the compression of the studied macromolecular system was found to markedly suppress the anion’s motions that are manifested by the decrease in decoupling index under conditions of high compression.
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EXPERIMENTAL SECTION
Synthesis of Monomer. Chemicals. 1-Vinylimidazole (99%) and 1-bromobutane (99%) were purchased from Sigma-Aldrich. All solvents used were HPLC grade purchased from Riedel de Haën. Lithium bis(trifluoromethylsulfonyl)imide was purchased from 3M (>98%). Microanalysis and lithium content were performed by Analytical Services (ASEP and QUB). 1H and 13C NMR spectra were recorded at 293.15 K on a Bruker Avance DPX spectrometer at 300 and 75 MHz, respectively. Synthesis of 1-Butyl-3-vinylimidazolium Bromide. A mixture of Nvinylimidazole (4.71 g, 0.05 mol) and 1-bromobutane (6.85 g, 0.05 mol) was placed in a Parr autoclave sealed reaction vessel along with 30 cm3 of HPLC grade acetonitrile. The reaction mixture was heated up to 343 K and then held at this temperature for 24 h with stirring at 500 rpm. After cooling of the reaction mixture, the acetonitrile was removed under vacuum. The highly viscous straw colored liquid was then characterized by 1H NMR spectroscopy. 1H NMR (300 MHz, DMSO) δ: 9.58 (s, 1H), 8.24 (s, 1H), 7.98 (s, 1H), 7.31 (dt, J = 21.4, 10.7 Hz, 1H), 5.99 (d, J = 15.7 Hz, 1H), 5.46 (t, J = 7.9 Hz, 1H), 4.23 (t, J = 7.2 Hz, 2H), 1.96−1.68 (m, 2H), 1.31 (dq, J = 14.5, 7.1 Hz, 2H), 0.93 (t, J = 7.3 Hz, 3H). Synthesis of 1-Butyl-3-vinylimidazolium Bis(trifluoromethylsulfonyl)imide. A solution of lithium bis(trifluoromethylsulfonyl)imide (14.35 g, 0.05 mol) in distilled water (100 cm3) was added dropwise to a rapidly stirred solution of 1-butyl-3-vinyllimidazolium bromide (10.40 g, 0.045 mol) in dichloromethane (50 cm3) and allowed to stir under ambient conditions overnight. The organic layer was then extracted and washed with distilled water (100 cm3) repeatedly five times. The organic layer was then dried in vacuo to give the product as an off-white liquid in
