Molecular Dynamics and Charge Transport in Polymeric

and temperature (200–400 K) range by means of broadband dielectric spectroscopy (BDS); additionally, differential scanning calorimetry (DSC) is ...
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Molecular Dynamics and Charge Transport in Polymeric Polyisobutylene-Based Ionic Liquids Falk Frenzel,*,† Makafui Y. Folikumah,‡ Matthias Schulz,‡ A. Markus Anton,† Wolfgang H. Binder,‡ and Friedrich Kremer† †

Institute of Experimental Physics I, Leipzig University, Linnéstrasse 5, 04103 Leipzig, Germany Institute of Chemistry, Macromolecular Chemistry, Martin-Luther-University Halle-Wittenberg, Von-Danckelmann-Platz 4, 06120 Halle, Germany

‡

S Supporting Information *

ABSTRACT: A homologous series of 16 polymeric ionic liquids (PIL) are investigated based on monovalent and bivalent telechelic polyisobutylene (PIB) carrying the ionic liquid (IL)-like cationic headgroup (N,N,N-triethylammonium or 1-methylpyrrolidinium) and Br, NTf2, OTf, or pTOS as anions. Molecular dynamics, charge transport, and polarization at mesoscopic scales are analyzed over a wide frequency (10−2−107 Hz) and temperature (200−400 K) range by means of broadband dielectric spectroscopy (BDS); additionally, differential scanning calorimetry (DSC) is employed. In detail, (i) three molecular relaxation processes are observed including the dynamic glass transition of the polymeric matrix, (ii) a conductivity relaxation originating from charge transport in the IL-like moieties, and (iii) a weak electrode polarization caused by the accumulation of mobile charge carriers at the metal interfaces. The net conductivity of the PIL as a whole is quantitatively described by an effective-medium approximation (EMA) reflecting the microphase-separated character of the PIL under study.

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lithography.13−19 Although poly-ILs have shown remarkable performance when employed in electrochemical devices17,20 (e.g., in dye-sensitized solar cells, lithium batteries, actuators, field-effect transistors, light-emitting electrochemical cells, and electrochromic devices), the transport phenomena and the interplay between ion transport and structural21−23 (segmental)24 dynamics in poly-ILs are basically not understood, since many different dynamics (ion complexation to the main polymer chain, segmental dynamics, charge transport, microphase segregation of immiscible polymers, cluster formation) do overlap. This is of particular importance considering e.g. a newly emerging field of application of low molecular weight ionic liquids as a dielectric gating interface within functional FET devices10,12,25,26 or within organic thin film transistors (OFETs).27 In such devices, a thin layer of an ionic liquid is placed on top of the gate: this often very thin layer (usually in the range of tens of nanometers (nm)) of an ionic liquid significantly alters the gating properties of an FET, leading to significantly improved switching of the gate, thus dramatically accelerating the charge transport properties within the FET. In the present study the synthesis of a homologous series of 16 polymeric ionic liquids (PILs) is described based on

INTRODUCTION Since 1914, when Paul Walden synthesized “ethylammonium nitrate”,1 the first salt with a melting point lower than 100 °C, this exceptional class of materials known as ionic liquids (ILs) has passed through a remarkable evolution to become an essential player in a wide variety of application areas due to its unique properties like a low melting point, broad liquidity range, negligible vapor pressure, high ion conductivity, thermal stability, and wide electrochemical window.2−5 Apart from these outstanding features, the property of an often low viscosity excludes neat ILs to be employed in macroscopic stable components, e.g., as battery electrolytes6,7 or gas separator membranes.8,9 One way to satisfy the processing industry in this particular aspect is to incorporate IL-like moieties into polymers and to synthesize polymeric ionic liquids (PILs). This new class of materials combines the characteristics of neat ILs with the advantages of a polymeric system: high mechanical stability, flexibility, durability, spatial controllability, and generally better processing features. In comparison to low molecular weight ILs, poly-ILs exhibit a more defined morphology (meso (nano)-scale ordering, phase separation, ionic interaction, crystallinity) and tend to self-assemble in the bulk, causing an even richer variety of structures that can occur at interfaces/surfaces, making them attractive for applications in different fields such as in nanoelectronics e.g. in gating,10−12 optoelectronics, or nano© XXXX American Chemical Society

Received: January 4, 2016 Revised: February 17, 2016

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DOI: 10.1021/acs.macromol.6b00011 Macromolecules XXXX, XXX, XXX−XXX

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pyrrolidine and N,N,N-triethylamine were distilled over CaH2 prior to use. The solvents were dried under the following conditions: dichloromethane (DCM), N,N-dimethylformamide (DMF), and methanol (MeOH) were predried and freshly distilled over CaH2 under a nitrogen atmosphere. Toluene was dried over sodium and benzophenone and freshly distilled before usage. Synthesis of Monovalent PILs. The preparation of monovalent N,N,N-triethylammonuim telechelic PIB (NEt3-Br) and monovalent 1methylpyrrolidinium telechelic PIB (Py-Br) with 1 and 3 kDa was accomplished by a combination of living carbocationic polymerization (LCCP) and the azide−alkyne Cu(I)-mediated click reaction according to a procedure already described by Binder et al.29−31 The different ions were subsequently introduced via anion-exchange reactions. Anion-Exchange Reaction. Metathesis with the Silver Trifluoromethansulfonate Anion OTf−. 2 mol equiv of silver salt of trifluoromethanesulfonate was dissolved in a small amount of a water−methanol (1:2) solvent system and added to a solution the starting PIL in chloroform. The mixture was then stirred vigorously at room temperature for 72 h. The water phase was separated, and the organic phase was concentrated on a rotary evaporator. The crude product was then washed several times with deionized water and finally precipitated in to an excess methanol three times to obtain the pure product (yield of 90.3%). Analysis of NEt3-OTf. 1H NMR (500 MHz, CDCl3, δ in ppm): 8.50 (s, 1H), 7.26 (d, 3JH,H = 8.4 Hz, 2H), 6.80 (d, 3JH,H = 8.8 Hz, 2H), 4.68 (s, 2H), 4.65 (t, 3JH,H = 7.2 Hz, 2H), 4.01 (t, 3JH,H = 5.7 Hz, 2H), 3.40 (q, 3JH,H = 7.2 Hz, 6H), 2.44 (m, 2H). 13C NMR (500 MHz, CDCl3, δ ppm): 156.0, 143.2, 134.8, 128.4, 127.1, 113.7, 64.3, 59.5, 58.2, 53.4, 32.6, 7.8. Analysis of Py-OTf. 1H NMR (500 MHz, CDCl3, δ in ppm): 8.49 (s, 1H), 7.27 (d, 3JH,H = 7.2 Hz, 2H), 6.80 (d, 3JH,H = 8.7 Hz, 2H), 4.91 (s, 2H), 4.73 (t, 3JH,H = 7.1 Hz, 2H), 3.97 (t, 3JH,H = 5.4 Hz, 2H), 3.71 (m, 2H), 3.44 (t, 3JH,H = 5.6 Hz, 2H), 3.00 (s, 3H), 2.43 (m, 2H), 2.23 (m, 2H), 2.10 (m, 2H). 13C NMR (500 MHz, CDCl3, δ in ppm): 155.9, 143.4, 136.9, 129.1, 127.1, 121.6, 119.1, 113.6, 64.0, 59.5, 58.2, 49.0, 48.5, 21.4. Metathesis with Lithium Bis(trifluoromethanesulfonyl)Imide (NTf2−). 2 mol equiv of lithium bis(trifluoromethanesulfonyl)imide salt was dissolved in a small amount of methanol add to 1 mol equiv of starting PILs dissolved in CHCl3. The mixture was stirred at room temperature for 72 h. The solvent was removed after the complete exchange, the PIL was dissolved in a minimal amount of CHCl3, washed with an excess water to remove the inorganic byproduct LiBr, and finally precipitated into methanol to remove the excess lithium bis(trifluoromethanesulfonyl)imide (yield of 93.5%). Analysis of NEt3-NTf2. 1H NMR (500 MHz, CDCl3, δ in ppm): 8.31 (s, 1H), 7.26 (d, 3JH,H = 8.4 Hz, 2H), 6.80 (d, 3JH,H = 8.8 Hz, 2H), 4.66 (t, 3JH,H = 7.2 Hz, 2H), 4.55 (s, 2H), 4.00 (t, 3JH,H = 5.7 Hz, 2H), 3.40 (q, 3JH,H = 7.2 Hz, 6H), 2.44 (m, 2H). 13C NMR (500 MHz, CDCl3, δ in ppm): 156.0, 143.2, 134.4, 128.0, 127.1, 113.7, 64.2, 59.5, 58.2, 53.4, 32.4, 7.7. Analysis of Py-NTf2. 1H NMR (500 MHz, CDCl3, δ in ppm): 8.34 (s, 1H), 7.27 (d, 3JH,H = 8.8 Hz, 2H), 6.80 (d, 3JH,H = 8.8 Hz, 2H), 4.71−6.65 (m, 4H), 4.01 (t, 3JH,H = 5.6 Hz, 2H), 3.88 (m, 2H), 3.46 (m, 2H), 3.11 (s, 3H), 2.44 (m, 2H), 2.33 (m, 2H), 2.25 (m, 2H). 13C NMR (500 MHz, CDCl3, δ in ppm): 156.0, 143.2, 135.6, 128.0, 127.1, 121.1, 118.5, 113.7, 64.2, 59.5, 58.2, 49.5, 48.0, 21.8. Metathesis with Silver p-Toluenesulfonate (pTOS−). 2 mol equiv of silver p-toluenesulfonate was dissolved in a small amount of a water−methanol (1:2) solvent mixture and added to a solution of the starting PIL in chloroform. The mixture was then stirred vigorously at room temperature for 72 h. The water phase was separated, and the organic phase was concentrated on a rotary evaporator. The crude product was then washed several times with deionized water and finally precipitated thrice in excess methanol three times to obtain the pure product (yield of 88.4%). Analysis of NEt3-pTOS. 1H NMR (500 MHz, CDCl3, δ in ppm): 8.87 (s, 1H), 7.81 (d, 3JH,H = 8.1 Hz, 2H), 7.25 (d, 3JH,H = 8.4 Hz, 2H), 7.15 (d, 3JH,H = 7.9 Hz, 2H), 6.80 (d, 3JH,H = 8.8 Hz, 2H), 4.87

monovalent and bivalent telechelic polyisobutylene (PIB) carrying the ionic liquid (IL)-like cationic headgroup (N,N,Ntriethylammonium or 1-methylpyrrolidinium) and Br, NTf2, OTf, or pTOS as anions. Charge transport, dielectric relaxations, and electrode polarization are analyzed by employing broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). Additionally, transmission electron microscopy (TEM) is used to visualize the microphaseseparated character of the samples with highly conductive ILlike micelles embedded in a strongly insulating PIB matrix. The net conductivity of the PILs under study is quantitatively described by an effective medium approximation using the standard Bruggemann formula.28

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EXPERIMENTAL DETAILS

Measurement Techniques. Six different techniques are employed in this study. Broadband Dielectric Spectroscopy (BDS). BDS measurements were carried out using a high-resolution Alpha analyzer combined with a Quatro temperature controller (both from NOVOCONTROL Technologies GmbH & Co. KG), ensuring absolute thermal stability of ≤1 K (≤0.1 K relative thermal stability) within a temperature range of 200−400 K and a frequency window of 10−2−107 Hz. The sample cells for this measurements consist of two brass electrodes (upper: d = 10 mm; lower: spectrometer ground plate (d = 40 mm)) which are separated by 3−5 50 μm thick glasfiber spacers ordered in parallel. The arrangement of lower electrode, spacers, and sample material is afterwords heated out at 150 °C under a vacuum of 10−6 mbar for about 24 h. While the sample material is in a liquid state, the upper electrode is placed on top of this droplet. The annealing process and all the dielectric measurements are carried out in an inert atmosphere of dry nitrogen or argon gas. Differential Scanning Calorimetry (DSC). DSC was conducted using a Netzsch DSC 204 F1 Phoenix 240-120-0142-L instrument. The glass transition temperatures were determined by heating the samples to 120 °C and cooling to −120 °C, with a heating ramp rate of 10 °C/min. The glass transition temperature was taken as a midpoint of a small heat capacity change upon heating from the amorphous glass state to the liquid state. Transmission Electron Microscopy (TEM). TEM images were recorded using a LEO 912 transmission electron microscope with 120 kV. To this endeavor the sample material was ultrathin (80 nm) cryosliced at −60 °C. The images were carried out without using contrast agents. 1 H and 13C Nuclear Magnetic Resonance (NMR). NMR spectra were recorded on a Varian Gemini 2000 spectrometer (400 MHz). MestRec-C software was used for data interpretation. Deuterated chloroform (CDCl3) and dimethyl sulfoxide (DMSO-d6) were used as solvents. All chemical shifts (δ) are reported in parts per million (ppm) using tetramethylsilane (TMS) as internal standard. Coupling constants (J) are given in hertz (Hz). Attenuated Total Reflection Infrared Spectroscopy (ATR-IR). ATR-IR measurements were performed on a Bruker Tensor VERTEX 70 equipped with a Golden Gate Heated Diamond ATR top plate. Opus 6.5 software was used for analyzing the data. Gel Permeation Chromatography (GPC). GPC was carried out on a Viscotek GPCmax VE2001 system with a refractive index detector using polyisobutylene standards (Mn = 340−87 600 g/mol) for external calibration and THF as solvent. The polystyrene− divinylbenzene based column set consists of a HHR-HGuard-17,369 precolumn followed by a GMHHR-N-Mixed Bed 18,055 (1000 to 4 · 105 Da) and a G2500HHR-17,354 (1000 to 2 · 104 Da) column. The concentration of all samples prepared was 3 mg/mL, with the flow rate of the instrument set to 1 mL/min. Data were analyzed with OmniSec (4.5.6) software. Synthesis. Chemicals. All chemicals used for the synthesis and polymerizations were purchased from Sigma-Aldrich and were used without further purification unless stated otherwise. 1-MethylB

DOI: 10.1021/acs.macromol.6b00011 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Chemical Structure of the Examined Homologous PIL Series as Well as the Neat IL-like Moiety Py-IL and Polymeric Unit PIB-N3

(s, 2H), 4.61 (t, 3JH,H = 7.2 Hz, 2H), 3.98 (t, 3JH,H = 5.7 Hz, 2H), 3.39 (q, 3JH,H = 7.2 Hz, 6H), 2.44 (m, 2H), 2.34 (s, 3H). 13C NMR (500 MHz, CDCl3, δ in ppm): 156.1, 143.8, 143.0, 139.2, 135.3, 129.1, 128.6, 127.1, 125.9, 113.7, 64.4, 59.5, 58.2, 53.6, 32.4, 21.3, 8.0. Analysis of Py-pTOS. 1H NMR (500 MHz, CDCl3, δ in ppm): 8.79 (s, 1H), 7.83 (d, 3JH,H = 8.1 Hz, 2H), 7.18 (d, 3JH,H = 7.9 Hz, 2H), 6.80 (d, 3JH,H = 8.8 Hz, 2H), 5.06 (s, 2H), 4.60 ((t, 3JH,H = 7.4 Hz, 2H), 4.07 (m, 2H), 4.04 (t, 3JH,H = 5.7 Hz, 2H), 3.51 (m, 2H), 3.21 (s, 3H), 2.41 (m, 2H), 2.35 (s, 3H), 2.30 (m, 2H), 2.20 (m, 2H). Synthesis of Bivalent PILs. Bivalent N,N,N-triethylammonuim telechelic PIB (BVNEt3-Br) and bivalent 1-methylpyrrolidinium telechelic PIB (BVPy-Br) with 3 kDa were synthesized by a combination of living carbocationic polymerization (LCCP) and azide−alkyne Cu(I)-mediated click-reaction as described. The synthesis of bivalent bromo-telechelic PIB was carried out according to the literature,20 and the synthesis of bivalent azido-telechelic PIB was accomplished according to a procedure adapted from Morgan et al.32 Synthesis of Bivalent N,N,N-Triethylammonium Telechelic PIB Bromide. 4 equiv of N,N,N-triethylammonium bromide (0.9 g, 4 mmol) and azido-functionalized PIB (3.10 g, 1 mmol) were dissolved in a solvent mixture of toluene/water/isopropanol (40 mL:20 mL:20 mL) in a one-neck round-bottom flask. N,N-diisopropylethylamine (DIPEA) (2.6 g, 20 mmol) and copper(I) iodide (CuI) (0.08 g, 0.4 mmol) were then introduced into the flask, sealed with rubber septum, and purged for 1 h to get rid of oxygen. The reaction was then stirred continuously at 80 °C for 4 days. Monitoring of the reaction via TLC was carried out intermittently to check the extent of conversion. After 4 days of complete reaction, the solvent was removed in vacuo, and the crude product washed with a small amount of distilled water. Purification of the crude product was carried out via column chromatography (stationary phase: SiO2). Removal of the unreacted azido-functionalized PIB was achieved using chloroform as eluent (Rf ∼ 1). Upon changing the eluent to chloroform/methanol mixture (15:1), the desired product with an Rf value of 0.25 was obtained. The obtained product was finally precipitated in methanol to remove unreacted/excess N,N,N-triethylammonium bromide. Removal of the solvent and drying of the product under high vacuum yielded the desired compound. BVNEt3-Br (Yield: 1.9 g; 63%). 1H NMR (500 MHz, CDCl3, δ in ppm): 8.72 (s, 2H), 7.26 (d, 3JH,H = 8.4 Hz, 4H), 7.18 (s, 3H) 6.80 (d, 3JH,H = 8.7 Hz, 4H), 4.87 (s, 4H), 4.64 (t, 3JH,H = 7.2 Hz, 4H),

4.00 (t, 3JH,H = 5.7 Hz, 4H), 3.40 (q, 3JH,H = 7.2 Hz, 12H), 2.43 (m, 4H). Synthesis of Bivalent 1-Methylpyrrolidinium Telechelic PIB Bromide. 1 equiv of azido-functionalized PIB (2.97 g, 1 mmol) and 4 equiv of 1-propargyl-1-methylpyrrolidinium bromide (0.82 g, 4 mmol) were dissolved in a solvent mixture of toluene/water/ isopropanol (40 mL: 20 mL: 20 mL) in a one-neck round-bottom flask. N,N-diisopropylethylamine (DIPEA) (2.6 g, 20 mmol) and copper(I) iodide (CuI) (0.08 g, 0.4 mmol) were then introduced into the flask, sealed with a rubber septum, and purged for 1 h to get rid of oxygen. The reaction was then stirred continuously at 80 °C for 4 days. Monitoring of the reaction via TLC was carried intermittently to check the extent of conversion. After 4 days, which are proven to provide a complete reaction, the solvent was removed in vacuo, and the crude product was washed with a small amount of distilled water. Purification of the obtained product was carried out via column chromatography (stationary phase: SiO2). Removal of the unreacted azido-functionalized PIB was achieved using chloroform as eluent (Rf ∼ 1). Upon changing the eluent to chloroform/methanol mixture (15:1), the desired product with an Rf value of 0.2 was obtained. The obtained product was finally precipitated in methanol to the remove excess 1-methylpyrrolidinium bromide. Removal of the solvent and drying of the product under high vacuum yielded the desired compound. BVPy-Br (Yield: 2.0 g; 67%). 1H NMR (500 MHz, CDCl3, δ in ppm): 8.77 (s, 2H), 7.27 (d, 3JH,H = 8.8 Hz, 4H), 7.18 (s, 3H), 6.81 (d, 3JH,H = 8.8 Hz, 4H), 5.23 (s, 4H), 4.65 (t, 3JH,H = 7.2 Hz, 4H), 4.15 (m, 4H), 4.02 (t, 3JH,H = 5.6 Hz, 4H), 3.55 (m, 4H), 3.27 (s, 6H), 2.44 (m, 4H), 2.38 (m, 4H), 2.22 (m, 4H). Anion-Exchange Reactions. Anion-exchange reactions with silver trifluoromethanesulfonate (anion: OTf−), lithium bis(trifluoromethanesulfonyl)imide (anion: NTf2−), and silver p-toluenesulfonate (anion: pTOS−) were performed as stated above. Analysis of BVNEt3-OTf (Yield 85.2%). 1H NMR (500 MHz, CDCl3, δ in ppm): 8.50 (s, 2H), 7.26 (d, 3JH,H = 8.4 Hz, 4H), 7.18 (s, 3H) 6.81 (d, 3JH,H = 8.8 Hz, 4H), 4.70 (s, 4H), 4.67 (t, 3JH,H = 7.2 Hz, 4H), 4.00 (t, 3JH,H = 5.7 Hz, 4H), 3.45 (q, 3JH,H = 7.2 Hz, 12H), 2.44 (m, 4H). Analysis of BVPy-OTf (Yield 84.8%). 1H NMR (500 MHz, CDCl3, δ in ppm): 8.51 (s, 2H), 7.27 (d, 3JH,H = 8.8 Hz, 4H), 7.18 (s, 3H), 6.81 (d, 3JH,H = 8.4 Hz, 4H), 4.94 (s, 4H), 4.65 (t, 3JH,H = 7.2 Hz, C

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Figure 1. Real and imaginary part of the complex dielectric function (ε* = ε′ − iε″) and complex conductivity (σ* = σ′ + iσ″) vs frequency of the sample BVPy-pTOS (A) and, in addition, σ′( f) and ε″(f) for the polymeric backbone PIB-N3 (B) and the neat IL-like functionalization Py-IL (C) at four different temperatures as indicated. A1: The increase on the low-frequency side is caused by electrode polarization. A2: the dielectric spectrum exhibits a conductivity contribution (straight line) and three separated molecular relaxation processes. The latter are presented by Havriliak−Negami fits (dotted, dashed-dotted, dashed line) with relaxation times τ1, τ2, and τ3. A3: subtracting the relaxation processes exposes the dc conductivity plateaus σ0 as well as the critical frequencies ωc, which mark the onset of the power law dependence σ′ ∼ ωx (derivations from plateaus are caused by EP). B1 and C1: comparison between the polymeric- and IL-like moieties demonstrates the investigated PILs as a composition of two contrasting components with widely varying conductivities. B2: the relaxation process proves to be identical with τ1 in A2. The error bars are smaller than the symbols, unless otherwise indicated. The logarithm is to base 10.

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4H), 3.96 (m, 4H), 3.68 (m, 4H), 3.43 (m, 4H), 2.98 (s, 6H), 2.44 (m, 4H), 2.38 (m, 4H), 2.22 (m, 4H). Analysis of BVNEt3-NTf2 (Yield 88.4%). 1H NMR (500 MHz, CDCl3, δ in ppm): 8.29 (s, 2H), 7.26 (d, 3JH,H = 8.4 Hz, 4H), 7.18 (s, 3H) 6.81 (d, 3JH,H = 8.8 Hz, 4H), 4.68−4.65 (m, 8H), 4.01 (t, 3JH,H = 5.7 Hz, 4H), 3.45 (q, 3JH,H = 7.2 Hz, 12H), 2.44 (m, 4H). Analysis of BVPy-NTf2 (Yield 82.0%). 1H NMR (500 MHz, CDCl3, δ in ppm): 8.29 (s, 2H), 7.27 (d, 3JH,H = 8.8 Hz, 4H), 7.18 (s, 3H), 6.80 (d, 3JH,H = 8.8 Hz, 4H), 4.68−4.65 (s, t, 8H), 3.85 (m, 4H), 3.56 (m, 4H), 3.44 (m, 4H), 3.09 (s, 6H), 2.44 (m, 4H), 2.34 (m, 4H), 2.25 (m, 4H). Analysis of BVNEt3-pTOS (Yield 81.6%). 1H NMR (500 MHz, CDCl3, δ in ppm): 8.89 (s, 2H), 7.82 (d, 3JH,H = 8.1 Hz, 4H), 7.25 (d, 3 JH,H = 8.8 Hz, 4H), 7.18−7.15 (s, d, 7H), 6.80 (d, 3JH,H = 8.7 Hz, 4H), 4.91 (s, 4H), 4.61 (t, 3JH,H = 7.2 Hz, 4H), 3.99 (t, 3JH,H = 5.7 Hz, 4H), 3.45 (q, 3JH,H = 7.2 Hz, 12H), 2.44 (m, 4H), 2.34 (s, 6H). Analysis of BVPy-pTOS (Yield 89.2%). 1H NMR (500 MHz, CDCl3, δ in ppm): 8.81 (s, 2H), 7.83 (d, 3JH,H = 8.1 Hz, 4H), 7.27 (d, 3 JH,H = 8.8 Hz, 4H), 7.19−7.17 (s, d, 7H), 6.81 (d, 3JH,H = 8.8 Hz, 4H), 5.08 (s, 4H), 4.62 (t, 3JH,H = 7.3 Hz, 4H), 4.09 (m, 4H), 4.01 (t, 3 JH,H = 5.6 Hz, 4H), 3.51 (m, 4H), 3.23 (s, 6H), 2.42 (m, 4H), 2.36 (s, 6H), 2.32 (m, 4H), 2.18 (m, 4H).

RESULTS AND DISCUSSION

The dielectric properties of the PIL’s under study (see Scheme 1 for chemical formulas and abbreviations) are characterized by the superposition of (i) electrode polarization (EP), (ii) charge transport, and (iii) dielectrically active molecular fluctuations.33 Electrode polarization (i) is caused by the accumulation of ionic charges at the metal interfaces; it shows up as a steep increase in ε′(ω,T) vs frequency (Figure 1 A1) at elevated temperatures and on the low-frequency side (Figure 1 A4) of σ″(ω,T) vs frequency.34 Charge transport (ii) can be recognized at the strong increase in ε″(ω,T) vs frequency at elevated temperatures (Figure 1 A2) and in σ′(ω,T) vs frequency (Figure 1 A3) as plateaus on the low-frequency side. Critical frequencies ωc are observed, which mark the onset of the power law dependence σ′ ∼ ωx. Dielectric relaxation processes (iii) contribute as well and are best analyzed in ε″(ω,T) vs frequency representation (Figure 1 A2). In total, three wellseparated processes are found which can be quantitatively described by use of Havriliak−Negami fits. For comparison, also the dielectric properties of the PIL without the IL-like D

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Macromolecules group (Figures 1 B1 and 1 B2), sample PIB-N3, and without the polymeric moiety (Figures 1 C1 and 1 C2), sample Py-IL, are measured. The former shows one dielectric τ1 relaxation, which is assigned to the dynamic glass transition (DGT) of the polymer as discussed below, the latter is characterized by the typical signature of an ionic liquid, with levels of the dc conductivity more than 4 orders of magnitude higher than observed in the PIL BVPy-pTOS. The charge transport as observed in the samples BVNEt3pTOS, BVPy-pTOS, and the neat Py-IL (Figure 2) obeys the

Figure 3. Activation plot of the structural fluctuations of the samples BVNEt3-pTOS (red), BVPy-pTOS (black), and the polymeric counterpart without IL-like functionaliziation (PIL-N3: blue) (A) as well as the conductivity relaxations of the former and the neat IL Py-IL (green) (B). Compared are the α-relaxations τ1 of the two PILs with the sole structural relaxation of the PIB-N3 (triangles) and the DSCmeasured Tg; the fluctuations of the IL-like moieties τ2 (circles) with the charge transport relaxation of the Py-IL ωc‑IL as well as the latter with conductivity relaxation of both PILs ωc‑PIL (stars). Figure 2. Barton−Namikawa−Nakajima relation σ0 ∼ ωc between the dc conductivity σ0 and the charge carrier relaxation ωc for the samples BVNEt3-pTOS and BVPy-pTOS, as well as for Py-IL. The inset shows the correlation of σ0 with the structural fluctuation τ2‑max of transient dipole dynamics in the IL-like PIL moiety. The straight line describes a linear fit, and the dotted line indicates slope 1.

dependence for the low molecular weight IL in accordance with the literature.38−43 In contrast to this, shows the conductivity relaxation in all PILs an Arrhenius-like thermal activation; it is by about 6 orders of magnitude smaller than for the neat IL. Its frequency and temperature position is similar to the τ3 relaxation supporting the above assignment. While in neat IL the dynamic glass transition and the conductivity relaxation coincide,39,44 both processes are strongly decoupled in PILs. This is observed in a variety of chemically strongly different PILs41,42,45−47 and takes place also in the systems under study. Because of the fact that in the latter the polymer fraction of the PIL (93%) is large compared to that of the IL-like moieties (7%), the decoupling between conductivity relaxation and the dynamic glass transition of the polymeric part is especially wide corresponding to about 8 decades. It is remarkable that the former (ωc‑PIL) shows an Arrhenius-like thermal activation while the latter (α-relaxation) follows the Vogel−Fulcher−Tammann law (Figure 2). Thus, both processes are fully independent from of each other reflecting the phase separation between high conductive micelles made out of IL-like groups and the insulating polymeric matrix. In order to describe the net conductivity of the PILs, one has to employ an effective medium approximation (EMA). This is fulfilled by using the Bruggemann equation (ε*EMA − ε*IL)/(ε*pol − ε*IL)(ε*EMA/εIL)1/3 = 1 − ϕ, where εEMA* is the complex dielectric function of the effective medium, εpol * is the complex dielectric function of the polymer matrix, εIL * is that of the ILlike micelles, and ϕ is the micelle volume fraction.28 This equation converted and solved for the complex conductivity reads as

Barton−Namikawa−Nakajima (BNN) relation σ0 ∼ ωc over many orders of magnitude.35−37 This is well-known for low molecular weight IL’s38,39 and proves its character as a conductivity relaxation. The thermal activation of the dielectric and electrical relaxation processes is compared in Figure 3 A,B. The τ1 relaxation is found in all PILs under study and in the sample PIB-N3; this demonstrates its assignment to the dynamic glass transition (DGT) of the polymer resp. the polymeric part of the PILs. It shows a Vogel−Fulcher− Tammann (VFT -temperature dependence and scales correctly with the calorimetrically determined glass transition temperature Tgcal (Figure 3 A). The τ2 relaxation has a VFT dependence as well but is slowed down compared to the DGT by typically 6 orders of magnitude. It is assigned to fluctuations of transient dipoles being formed by the cationic moiety in the terminal position of the polymeric main chain and the low molecular weight counteranion. This is also indicated by the fact that the τ2 relaxation depends on its spectral position strongly on the cation−anion composition as shown in the Supporting Information. Additionally, a further dielectric active process, the τ3 relaxation, is found; it is tentatively interpreted as fluctuation of assemblies of IL-like moieties. It is remarkable that this process has an Arrhenius-like thermal activation. Comparing the conductivity relaxations of the neat and the polymeric IL, one finds (Figure 3 B) as expected a VFT-

E

DOI: 10.1021/acs.macromol.6b00011 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules ⎡ ⎛ (1 − ϕ)(σ * − σ *) ⎞3⎤1/3 pol IL ⎟ ⎥ ⎢2⎜ 1/3 σpol ⎠ ⎥⎦ ⎢⎣ 3 ⎝

* σEMA =

1/3 ⎡ 9 ⎞1/2 ⎤ ⎡ ⎛ (1 − ϕ)(σ * − σ *) ⎞6 * * * * ⎛ ⎞ ⎛ ⎞ (1 )( ) (1 )( ) ϕ σ σ ϕ σ σ − − − − pol IL pol IL pol IL ⎥ ⎢9⎜ ⎟σ * + 31/2 ⎢27⎜ ⎟ σ *2 − 4⎜ ⎟ ⎟ *1/3 *1/3 *1/3 ⎢ ⎝ σpol σpol σpol ⎠ IL ⎠ IL ⎝ ⎠ ⎟⎠ ⎥ ⎢⎣ ⎝ ⎦ ⎣

+

1/3 ⎡ 9 1/2 ⎤ ⎡ ⎛ (1 − ϕ)(σ * − σ *) ⎞6 * − σIL *) ⎞ * − σIL *) ⎞ ⎞ ⎛ ⎛ (1 )( (1 )( ϕ σ ϕ σ − − pol pol IL pol 1/2 2 ⎢9⎜ ⎥ ⎟σ * + 3 ⎟ σ * − 4⎜ ⎟ ⎟ ⎢27⎜ *1/3 *1/3 *1/3 ⎢ ⎝ σpol σpol σpol ⎠ IL ⎠ IL ⎝ ⎠ ⎟⎠ ⎥ ⎢⎣ ⎝ ⎣ ⎦

21/332/3

It can be approximated for σ*IL ≫ σ*pol as

* = σEMA

(1)

caused by electrode polarization; experimental scatter in the frequency range at 103−106 Hz was smoothed using the Savitzky−Golay algorithm with a range of 20 data points. In addition to the homologous PIL series, the sample NEt3Br was investigated for four different IL volume fractions by varying the molecular weight between Mn = 1000 g/mol (ϕV‑IL = 20%) and Mn = 8000 g/mol (ϕV‑IL = 2.5%). For these samples the EMA can be approved for ϕV‑IL = 2.5%, 6.5%, and 13%, while for ϕV‑IL = 20% the conductivity of the PIL is higher than predicted. It is assumed that the latter forms conductivity paths instead of isolated micelles, which might explain the higher conductivity values compared to the former three. Considering the microphase-separated morphology of our PILs, the length scale, on which charge transport takes place, can be estimated. With the assumption that the IL-like micelles behave as an assembly of low molecular weight IL’s, it is possible to calculate their diffusion length S = (6Dt)1/2 through the polymer using well-known diffusion constants D of neat IL’s from the literature39,40 and the time span t of the applied electrical field. This delivers for S values of S ≈ 7−25 nm being in the range of the micelle diameter (S ≈ 13 nm), but much smaller than their mean separation (S ≈ 40 nm). Additionally, one has to take into account that the cationic headgroups in the micelles are covalently bond to the embedding polymeric units, which will decrease the mobility and S further. In summary it is indicated, that the charge transport takes primarily place within the IL-like micelles. Comparing different PILs (Table 1), one finds, that the ratio of the molecular weight of the IL-like and the polymeric moieties plays an essential role for the resulting conductivity at T = Tg + 50 K. In contrast, the glass transition temperature has no systematic effect.41,42,45,46 Pronounced deviations from this dependency occur for the side chain PILs studied in ref 46. The observed discrepancies demonstrate that the interplay between

* σpol (1 − ϕ)3

* + σIL

(2)

where the variables are specified analogue. This effective medium approximation displays a good agreement with the measured data both for the real and imaginary parts as shown in Figure 4. Deviations especially in the low frequency range are

Figure 4. Plot of log σ′ vs frequency for BVNEt3-Br at three different temperatures as indicated. The straight lines represent fits within the effective medium approximation (EMA) using the Bruggemann fomula. The upper inset describes ε′ vs log f with the EMA fits. In the lower insets, ε′ and σ′ vs the ion content for the four different samples with identical chemical structure is shown.

Table 1. Comparison of Molecular Weight Fraction from IL-like Moieties to Polymeric Moieties of PILs and Their Influence to the DC Conductivity sample

Tg [K]

anion

Mn,IL‑like moiety/Mn,polymeric moiety

NEt3-NTf2 NEt3-OTf PBuVIm-NTf242 HSO3 BVIM-OTf41 poly(PVIM)-NTf245 short-NTf246 intermediate-NTf246 long-NTf246

208 208 319 225 328 329 288 230

NTf2 OTf NTf2 OTf NTf2 NTf2 NTf2 NTf2

0.159 0.111 14.960 13.180 15.480 13.930 4.920 1.420 F

log σ0 (Tg + 50 K) [S/cm] −16.0 −15.0 −6.0 −6.0 −5.0 −8.0 −5.0 −6.0

± ± ± ± ± ± ± ±

1.0 1.0 1.0 0.5 0.5 0.5 1.0 1.0

DOI: 10.1021/acs.macromol.6b00011 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules molecular architecture, molecular dynamics, and charge transport in PILs is by no means fully understood.

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relaxation process for all PILs and precise TEM-images various resolutions (ZIP) are provided.

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +49 (0) 3419732560 (F.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.F. and F.K. are deeply grateful for the financial support from the Deutsche Forschungsgesellschaft under the DFG project “Neue Polymermaterialien auf der Basis von funktionalisierten ionischen Flüssigkeiten für Anwendungen in Membranen ‘Erkenntnistransfer-Projekt’” (KR 1138/24-1). W.H.B., M.Y.F., and M.S. thank the SFB TR 102, project A3; and F.K. within the project B08, for financial support. The authors acknowledge grateful Sylvia Goerlitz for transmission electron microscopy investigations and Clement Appiah for differential scanning calorimetry measurements.

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Figure 5. Scheme presenting electrode polarization, charge transport, and molecular fluctuations in polymeric ionic liquids (PIL). Top: schematic display of the sample cell, upon which an electric field is applied. The area between the electrodes and the dashed lines shows the EP-zone. Center left: representative TEM micrograph. A pronounced phase separation takes place between high-conductive IL-like (volume fraction: 7%) micelles imbedded in a insulating polymeric (volume fraction: 93%) matrix. Center right: the diffusion length S of the micelles (diameter ≈ 5−20 nm) through the polymer matrix is indicated: S ≈ 3−10 nm. Hence, the charge transport occurs primarily within the IL-like micelles. Bottom: representation of the dynamics within a PIL molecule. The process which determines the glassy behavior is characterized through the α-relaxation of the benzene−oxygen unit within the polymeric main chain, whereas the dynamics of the IL-like functionalization is given by the fluctuation of transient dipoles in micelle interior. The latter gives rise to the resulting charge transport.

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CONCLUSION In summary, charge transport, molecular dynamics, and electrode polarization are analyzed for a set of 16 novel monovalent and bivalent telechelic polyisobutylene (PIB) carrying the ionic liquid (IL)-like cationic headgroup (N,N,Ntriethylammonium or 1-methylpyrrolidinium) and Br, NTf2, OTf, or pTOS as anion. Compared to the neat IL, the conductivity for the PIL’s is by about 6−7 orders of magnitude lower; this is attributed to microphase separation between the IL-like moieties and the polymeric part leading to a twocomponent system, which can be quantitatively described by an effective medium approximation.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00011. More detailed dielectric presentations of the sample BVPy-pTOS and the PIL-subunits Py-IL and PIL-N3 as well as the spreading of the second dielectrically G

DOI: 10.1021/acs.macromol.6b00011 Macromolecules XXXX, XXX, XXX−XXX

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