Effect of Poly(ethylene glycol) Crystallization on Ionic Conduction and

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Effect of Poly(ethylene glycol) Crystallization on Ionic Conduction and Dielectric Response of Imidazolium-Based Copolyester Ionomers Minjae Lee,† Yong Ku Kwon,‡ Jehan Kim,§ and U Hyeok Choi*,∥ †

Department of Chemistry, Kunsan National University, Gunsan, 55150, Korea Department of Polymer Science and Engineering, Inha University, Incheon 22212, Korea § Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 37673, Korea ∥ Department of Polymer Engineering, Pukyong National University, Busan 48513, Korea

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‡

S Supporting Information *

ABSTRACT: A molecular-level understanding of ion and polymer dynamics in imidazolium−poly(ethylene glycol) (PEG) copolyester ionomers having different counteranions [PF6− vs (CF3SO2)2N−], alkylene spacer lengths [(CH2)6 vs (CH2)11], and PEG segment lengths [(CH2CH2O)22 vs (CH2CH2O)44] is investigated for the development of fast single-ion conducting materials, using dielectric relaxation spectroscopy and X-ray scattering. The variations of the counteranion size and PEG segment length lead to substantial changes in simultaneously conducting counteranion content (p), their mobility (μ), ion (α2), and segmental (α) relaxations, with consequences for ion transport. However, there is no significant influence of the alkylene spacer length on the counteranion conductivity (σDC). Imidazolium−PEG copolyester ionomers with larger (CF3SO2)2N− (= Tf2N) counterions show higher p with lower activation energy and higher μ with lower Vogel temperature as well as fast α2 and α motions with lower glass transition temperature, resulting in higher σDC. For the PEG segment length, the longer PEG segment leads to a copolyester crystallization, reflected by the observation of distinct X-ray diffraction peaks and a Maxwell− Wagner−Sillars interfacial polarization, bringing out an abrupt decrease in σDC at the phase transition. On the other hand, imidazolium−PEG copolyester with the shorter PEG segment has higher conducting counteranion fraction (p/p0), μ, and static dielectric constant (εs) compared to the similar polyester analogue with no PEG segment. Incorporation of the PEG segments with an optimized length into the main chain polymer plays an important role in directly boosting counteranion conductivity of the single-ion conducting copolyester ionomers. a controllable mechanical durability and processability.8−11 From these advantages, PIL single-ion conductors (so-called ionomers) may be ideal and safe polymer electrolytes for advanced energy storage devices (batteries and supercapacitors), energy conversion devices (solar cells and fuel cells), and electroactive devices (actuators and sensors).12−14 Since the pioneering works by Ohno and Ito in 1998,15 these studies have greatly focused on PILs employing pendant imidazolium groups via free radical polymerization of

1. INTRODUCTION For the past few decades, much effort has been devoted to the development of ion-conducting polymers that contain ionic functionality,1,2 allowing to modify structural, thermal, mechanical, and charge transport properties of the parent polymer for a variety of applications, such as water purification separators, packing materials, gene delivery systems, and ion conducting energy materials.3−6 In particular, polymerized ionic liquids (PILs), carrying an ionic liquid (IL) species in each repeating unit through polymerization of IL monomers, allow for a combination of the unique physical properties of ILs, such as high thermal and chemical stability and high ionic conductivity,7 and advantages in a macromolecular design, like © XXXX American Chemical Society

Received: October 31, 2018 Revised: February 23, 2019

A

DOI: 10.1021/acs.macromol.8b02332 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Chemical structures of five imidazolium−PEG copolyester ionomers (C6-P22-PF6, C6-P22-Tf2N, C6-P44-Tf2N, C11-P22-Tf2N, and C11-P44-Tf2N) having different alkylene spacer lengths [(CH2)6 vs (CH2)11], PEG segment lengths [(CH2CH2O)22 vs (CH2CH2O)44], and counteranion types (PF6− vs Tf2N−).29

increased ionic conductivity by 400 times compared with those without a well-defined microstructure. Recently, a series of copolyester ionenes containing the equivalent ratio of imidazolium units and poly(ethylene glycol) (PEG) segments were synthesized with different counteranions, alkylene spacer lengths, and PEG segment lengths.29 The counteranion type and alkylene spacer length significantly affected the molecular arrangement of the PEG segments (i.e., crystallization of PEG segments and microphase separation between the imidazolium units and PEG segments), which were characterized by thermal and X-ray scattering measurements. Moreover, the introduction of PEG segments to the PILs, where the ethylene oxide units are known for improving charge transport properties,30 led to an increase in ionic conductivity, but the role of the PEG segment in the conduction mechanism was not fully understood. In this paper we present the influence of the PEG segments on polymer and ion dynamics of the imidazolium−PEG copolyester ionomers having different counteranions (PF6− vs Tf2N−), alkylene spacers [(CH2)6 vs (CH2)11], and PEG segments [(CH2CH2O)22 vs (CH2CH2O)44], with the structure shown in Figure 1. These investigations are quantitatively reported with ionic conductivity, number density of simultaneous counteranions, their mobilities, dielectric relaxations, and dielectric constant, using dielectric relaxation spectroscopy, which are also correlated to the previous thermal and morphological observations.

imidazolium monomers containing either a vinyl, acrylate, or methacrylate functionality.16−22 These PILs exhibited promising ionic conductivities upon the utilization of a large and polarizable (CF3SO2)2N− (= Tf2N−) anion, whose negative charges delocalize to minimize ion pairing with the imidazolium cation, resulting in a weak interaction (310 kJ/ mol) for 1-butyl-3-methylimidazolium cation with Tf2N− at 0 K in a vacuum.20 Elabd and co-workers16 reported ion conduction of PIL random copolymers of imidazolium methacrylate and hexyl methacrylate. Long and co-workers17 studied thermal properties and ionic conductivities of a series of alkyl-substituted vinylimidazolium PILs with varying counteranion type and alkyl chain length. Extensive PIL morphological characterization was also conducted by Winey and co-workers18 and showed X-ray scattering features, indicating characteristic distances between backbones, anions, or pendants. Moreover, Colby and co-workers19−21 studied ion and polymer dynamics of imidazolium (meth)acrylate and their PILs with different pendant structures, particularly tail length, counteranion type, side chain type, and side chain length using dielectric and viscoelastic measurements. They also reported the dependence of glass transition temperature and dielectric constant on molecular volume of the repeat unit for PILs.22 In addition to these efforts, new attention has been paid to the development of PIL ionenes that possess regular IL units (e.g., ammonium, imidazolium, and phosphonium) along the main chains, not only enabling an easy control of the reactivity and total ion concentration but also allowing for uniform microphase separation, via a step growth polymerization.23−25 Wagener and co-workers26 reported the synthesis of imidazolium-based ionenes with precisely functionalized imidazolium moieties in the polymer backbones via acyclic diene metathesis (ADMET) polymerization. The synthesized ionenes exhibited microphase-separated morphologies, where most of ion dipoles were immobilized or had net antiparallel arrangements in ion aggregates, yielding a strong polymer film with a rubbery plateau, but with a rather low ionic conductivity.27 To achieve high ionic conductivity with substantial mechanical modulus, Gibson and co-workers28 developed main-chain imidazolium polyesters from bis(ωhydroxyalkyl)imidazolium salts and sebacoyl chloride. One of the PIL polyesters with PF6− anions containing long alkylene spacers led to semicrystalline morphology with a good mechanical property. Although the presence of crystalline phase in such PIL polyesters led to a slowing down of polymer segmental motion, thereby significantly dropping ionic conductivity upon crystallization, the microphase-separated, lamellar morphology often found in this series of polyesters

2. EXPERIMENTAL SECTION Figure 1 shows the molecular structures of five imidazolium-PEG copolyester ionomers (termed Cn-Pm-X, where n is an alkylene spacer length, m is a PEG segment length, and X is a counteranion type) with the equivalent ratio of imidazolium and PEG units, but with different alkylene spacer lengths [n = 6; (CH2)6 vs n = 11; (CH2)11], PEG segment lengths [m = 22; (CH2CH2O)22 (= PEG1000) vs m = 44; (CH2CH2O)44 (= PEG2000)], and counteranion types (X = PF6− vs X = Tf2N−). The detail preparations of these copolyesters were described in our earlier study.29 Thermal Characterization. Thermal results were obtained on a TA Instruments Q200 differential scanning calorimeter (DSC) with scan heating and cooling rates of 10 K min−1 under N2. TA Instruments Universal Analysis software was used to identify the phase transitions. Wide-Angle X-ray Diffraction (XRD) and Small-Angle X-ray Scattering (SAXS). Two-dimensional-XRD (2D-XRD) data were collected for the powdered specimens at room temperature. The intensity measurements were performed using a Bruker 2D-GADDS diffractometer with Cu Kα radiation (λ = 1.5418 Å), which was calibrated with silicon powder. The 2D-XRD patterns were obtained in transmission mode and recorded on a Hi-Star 2D detector system with a pixel size of 100 × 100 μm2. Synchrotron small-angle X-ray scattering (SAXS) measurements on the copolyester ionomers were B


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Figure 2. (a) Temperature dependence of ionic conductivity σDC for all the copolyester ionomers.29 (b−f) σDC comparison for the ionomers having different (b) counteranions (PF6− vs Tf2N−), (c, d) alkylene spacer lengths [(CH2)6 vs (CH2)11], and (e, f) PEG segment lengths [1000 Da vs 2000 Da]. DRS Tgs are indicated by arrows. conducted using the 3C-SAXSI beamline at the Pohang Accelerator Laboratory (PAL) with a monochromatic X-ray radiation source of 10.22 keV (λ = 1.213 Å) and a 2D X-ray detector. The resulting 2D scattering patterns were azimuthally integrated to obtain intensity versus scattering wave vector. Dielectric Relaxation Spectroscopy (DRS). Ionic conductivity and dielectric properties were measured using a Novocontrol Technologies GmBH & Co. Concept 40 broadband dielectric spectroscopy, combined with a Quatro Cryosystem high-quality temperature control system having vacuum-isolated cryostat and nitrogen lines. These measurements were conducted on samples that were prepared by allowing them to flow to cover a 30 mm diameter freshly polished brass electrode and dried in a vacuum oven at 100 °C for 24 h. To control the sample thickness at 100 μm, silica spacers were placed on top of the sample after it flowed to cover the electrode. Then, a 10 mm diameter freshly polished brass electrode was placed on top and gravity formed a 100 μm parallel plate capacitor cell as the extra sample was squeezed away (with precise thickness verified after dielectric measurements were complete). The sandwiched samples between two electrodes were placed in the Quatro Cryosystem sample chamber and then annealed at 100 °C for 1 h prior to the measurements to remove any water acquired during sample loading. The dielectric permittivity was measured using an AC voltage amplitude of 0.1 V for all experiments. Frequency sweeps were performed isothermally from 10 MHz to 0.1 Hz in the temperature range from 120 to −100 °C (cooling cycles).

on ion conduction, each ionomer is directly compared with its corresponding analogue, but having either different counteranions (PF6− vs Tf2N−, Figure 2b), alkylene spacer lengths (C6 vs C11, Figures 2c,d), or PEG segment length (1000 Da vs 2000 Da, Figures 2e,f). For the copolyester ionomers having the same main chain structure [i.e., C6 [= −(CH2)6−] spacers and P22 (= PEG1000) segments], the ionomer with the relatively larger Tf2N− counteranion (C6-P22-Tf2N) has higher σDC than that with the smaller PF6− counteranion (C6-P22-PF6), and the anion size effect on the ionic conductivity is more dominant at lower temperatures (see Figure 2b). This is consistent with the previous results: imidazoliumbased ionomers with the larger counteranions show much higher conductivity than those with the smaller counteranions.20,21,28 The larger Tf2N− counteranion acts as a plasticizer, lowering the glass transition temperature (Tg), while the smaller PF6− anion is more strongly associated with the imidazolium cation, imparting higher Tg.29,31 Although the Tgs for the copolyester ionomers with the equivalent ratio of imidazolium units and crystalline PEG segments are more difficult to detect by DSC,29 probably resulting from increased breadth and relatively small heat capacity change at Tg, their segmental relaxations allow us to determine dynamic glass transition (termed DRS Tg, listed in Table 1), showing that C6-P22-Tf2N has ∼16 K lower Tg than C6-P22-PF6. Unlike the counteranion impact, the alkyl spacer length does not significantly affect the ionic conductivity of the copolyester ionomers (see Figures 2c,d). The σDC of the ionomer with the shorter −(CH2)6− spacer (C6-P22-Tf2N) is similar to that of the ionomer with the longer −(CH2)11− spacer (C11-P22-

3. RESULTS AND DISCUSSION Ionic Conductivity. Figure 2a shows the temperature dependence of the ionic conductivity σDC for all the imidazolium−PEG copolyester ionomers. Their σDCs were evaluated from the frequency ω independent value of the inphase part of the conductivity σ′(ω) as shown in Figure S1. To better understand the effect of the ionomer molecular structure C


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shorter PEG1000 segments and the high crystalline ionomers with longer PEG2000 segments (see Figures 2e,f). Before the crystallization, both C6-P22-Tf2N and C6-P44-Tf2N (or C11P22-Tf2N and C11-P44-Tf2N) have almost identical conductivities, but upon crystallization, the conductivities of the PEG2000 ionomers significantly decrease by nearly 2 orders of magnitude, compared to those of the PEG1000 ones. To have further understanding of these conductivity results in terms of the influence of the counteranion, alky spacer length, and PEG segment length, a physical model of electrode polarization (EP) for single-ion conductors21,30,36−41 is next applied to estimate how many counteranions simultaneously participate in the ion conduction (i.e., number density of simultaneously conducting anions p) and how fast the counteranions diffuse (i.e., conducting anion mobility μ). Conducting Ion Content and Mobility. To assess simultaneously conducting ion content p and mobility μ, the EP model (Supporting Information explains how to do the EP analysis in detail, with Figures S1−S3) was used to separate σDC into p and μ because of σDC = epμ, where e is the elementary charge, for single-ion conductors.21,30,36−41 The temperature dependence of the number density p(T) of counteranions, simultaneously participating in conduction, is displayed in Figure 3, and p(T) is well described by an Arrhenius equation:

Table 1. Melting Points (Tm), Heats of Fusion (ΔHm), Degrees of Crystallinity (χc), and Dynamic Glass Transition Temperatures (DRS Tg) of the Imidazolium−PEG Copolyester Ionomers C6-P22-PF6 C6-P22-Tf2N C6-P44-Tf2N C11-P22-Tf2N C11-P44-Tf2N C6-Tf2N

ΔHm (J/g)

χ ca

295c 306c 298c 310c

3.5c 60c 34c 66c

0.02 0.30 0.17 0.33

DRS Tgb (K) 231 215 213 223d

χc = (with J/g as the “perfect crystal” Tg determined from dielectric enthalpy of fusion of PEG). relaxation spectroscopy (defined at ωα(Tg) = 10−2 rad/s). cValues suggested in the literature.29 dThe DRS Tg value is identical to the Tg determined by DSC.28 a

ΔHm/ΔH0f

Tm (K)

ΔH0f = 203 32,33 b

Tf2N) over the entire temperature range studied (Figure 2c), in spite of the former having higher ion content, and the same result is also observed in the σDC comparison of the ionomers between C6-P44-Tf2N and C11-P44-Tf2N (Figure 2d). The semicrystalline ionomers with longer PEG2000 segments (C6P44-Tf2N and C11-P44-Tf2N showing a high crystallinity χc ≥ 0.3, summarized in Table 1) reveal ionic conductivities with discontinuities and changes in slope at the phase transition temperature (Figure 2d).34,35 At temperatures above crystallization temperature (Tc), the ionomers are a melting state, leading to a continuous decrease in σDC with decreasing temperature, whereas at Tc, their phase changes from an amorphous to a semicrystalline state with the 30% crystallinity, resulting in a sudden drop in the σDC (see Figure 2d and Figure S2). Such a crystallization effect is more clearly observed from the σDC comparison between the low crystalline ionomers with

i E y p(T ) = p∞ expjjj− a zzz k RT {

(1)

wherein p∞ is the conducting ion concentration as T → ∞ and Ea is the activation energy for conducting ions, and both are fitting parameters (listed in Table 2). For the p(T) comparison between the ionomer with PF6− and Tf2N− counteranion (see Figure 3b), the Tf2N− ionomer (C6-P22-Tf2N) has ∼2 times

Figure 3. (a−d) Temperature dependence of simultaneously conducting ion concentration for imidazolium−PEG copolyester ionomers (C6-P22PF6, C6-P22-Tf2N, C6-P44-Tf2N, C11-P22-Tf2N, and C11-P44-Tf2N); solid lines are Arrhenius fits to eq 1 with two fitting parameters (p∞ and Ea, listed in Table 2). The inset shows the fraction of counteranions simultaneously participating in conduction (p normalized by the total anion concentration p0, listed in Table 2). D


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Table 2. Fitting Parameters of the Arrhenius Equation for Conducting Ion Content (Eq 1) and the VFT Equation for Conducting Ion Mobility (Eq 2) conducting ion content T > Tt

conducting ion mobility T < Tt

T > Tt

T < Tt

sample

p0 (nm−3)

p∞ (nm−3)

Ea (kJ/mol)

p∞ (nm−3)

Ea (kJ/mol)

μ∞ (cm2 V−1 s−1)

D

T0 (K)

μ∞ (cm2 V−1 s−1)

D

T0 (K)

C6-P22-PF6 C6-P22-Tf2N C6-P44-Tf2N C11-P22-Tf2N C11-P44-Tf2N C6-Tf2N

0.50 0.47 0.32 0.43 0.31 0.99

0.13 0.07 0.18 0.06 0.01 0.20

13.5 10.4 13.3 9.6 9.5 12.2

a a 0.26 a 0.04 a

a a 14.9 a 12.4 a

0.49 0.17 0.06 0.35 0.17 0.08

4.2 3.6 2.1 4.5 2.1 3.6

196 187 206 180 207 193

a a 5.20 a 0.23 a

a a 7.2 a 4.6 a

a a 179 a 187 a

a

C6-P22-PF6, C6-P22-Tf2N, C11-P22-Tf2N, and C6-Tf2N do not exhibit a transition in conducting ion content and mobility.

Figure 4. (a−d) Temperature dependence of the EP mobility (filled symbols) and the extended mobility (open symbols) of the simultaneously conducting ions.20 Both mobilities are fit to eq 2 as solid curves. The inset shows μ(T) with respect to inverse temperature normalized by either T0 or Tg.

−(CH2)6− spacers (C6-P22-Tf2N and C6-P44-Tf2N), at the melting state (T > Tt), both ionomers have comparable p, while upon the PEG2000 crystallization (T < Tt), the ionomer with the longer PEG2000 segments (C6-P44-Tf2N) has lower p than that with the shorter PEG1000 segments (C6-P22Tf2N) as shown in Figure 3c. Such a negative effect of crystallization on p is more clearly observed in the ionomers with the longer −(CH2)11− spacers (C11-P22-Tf2N and C11P44-Tf2N) (see Figure 3d). C11-P44-Tf2N has a number density of ions participating in conduction p, almost 5 times lower than C11-P22-Tf2N over the entire temperature range, and exhibits the lowest p among all the imidazolium−PEG copolyester ionomers. Therefore, for the semicrystalline ionomers with high crystallinity, their ion transport at temperatures below Tt was dominated by the only counterions in the environment of the amorphous phase, whereas the other counterions that are trapped at the interface between the crystalline and amorphous domains may not be mobile. Figure 4a shows the temperature dependence of the simultaneously conducting ion mobility μ(T), determined from the EP model (termed EP mobility, filled symbols) and from dividing σ(T) by the product of the elementary charge e and the Arrhenius fit to eq 1 of p(T) (termed extended mobility, μ(T) = σ(T)/{ep∞ exp[−Ea/(RT)]}, open symbols).

more counterions, simultaneously taking part in conduction, with lower activation energy (Ea = 10.4 kJ/mol), compared to the PF6− ionomer (C6-P22-PF6) with higher Ea = 13.5 kJ/mol. The lower Ea in the Tf2N− ionomers tells us the lower binding energy of the larger Tf2N− anions to the imidazolium cations compared to the smaller PF6− anions.30 This is consistent with − 6 the higher quadrupole energy for imidazolium−PF6− (EPF quad ∼ −

2N 732 kJ/mol) than for imidazolium−Tf2N− (ETf quad ∼ 680 kJ/ 20 mol) from the ab initio calculations, implying that more aggregation in imidazolium−PF6− salts occurred than that of imidazolium−Tf2N−.21 In Figures 3c,d, on the other hand, the semicrystalline ionomers having the longer PEG2000 segments (C6-P44-Tf2N and C11-P44-Tf2N) underwent an abrupt change in p(T) at a transition temperature Tt = 294 K, 1000/Tt = 3.4 K−1 for C6P44-Tf2N and Tt = 299 K, 1000/Tt = 3.35 K−1 for C11-P44Tf2N, which are close to the melting temperature of the corresponding ionomer (see Table 1). As a result, these PEG2000 ionomers have two sets of p∞ and Ea values; i.e., one set above and the other below Tt, listed in Table 2. The crystallization leads to a significant decrease in p with an increase of Ea for conducting counterions: Ea changing from 13.3 to 14.9 kJ/mol for C6-P44-Tf2N and from 9.5 to 12.4 kJ/ mol for C11-P44-Tf2N. For the ionomers with the shorter

E


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Macromolecules For all the imidazolium−PEG copolyester ionomers, both the EP and extended mobilities are collectively fit to a Vogel− Fulcher−Tammann (VFT) equation: ij DT0 yzz μ(T ) = μ∞ expjjj− z j T − T0 zz k {

(2)

wherein μ∞ is the high-temperature limit of the mobility, D is reciprocally related to fragility (the so-called strength parameter), and T0 is the Vogel temperature, and these three fitting parameters are listed in Table 2. In Figure 4b, the Tf2N− anion mobility of C6-P22-Tf2N is higher than the PF6− anion mobility of C6-P22-PF6 because the larger Tf2N− anion imparts ∼10 K lower Vogel temperature (T0, Table 2) as well as ∼16 K lower glass transition temperature (Tg, Table 1) compared to the smaller PF6− anion. This is supported by the mobilities plotted against T0/T or Tg/T (see the inset of Figure 4b), where the data merge into a single curve, telling us that these anion mobilities of the ionomers with short −(CH2)6− spacers and PEG1000 segments are coupled with the polymer segmental motion, directly correlated to the Vogel or glass transition temperature. Furthermore, for the low crystalline PEG1000 ionomers with Tf2N− counteranions, an effect from the −(CH2)6− vs −(CH2)11− spacers on the anionic mobility is subtle; i.e., both C6-P22-Tf2N and C11-P22-Tf2N have almost identical μ(T) over the whole temperature range studied, regardless of the spacer length (see Figure 4c). However, the high crystalline PEG2000 ionomers (C6-P44Tf2N and C11-P44-Tf2N), after crystallization, exhibit a significant drop in ion mobility at around Tt by ∼100× (Figure 4d). From the conduction mechanism proposed in the literature,42,43 ion transport in polymer electrolytes takes places by segmental motions, creating suitable coordination sites adjacent to counterions and hence allowing ions to hop. Therefore, in the semicrystalline ionomers, where their conducting species are in the amorphous phase confined by the crystalline PEG phases, the crystallization presumably leads to an increase in the anion hopping distance, thereby raising energy barrier for anion hopping. Consequently, at temperatures below Tt, both C6-P44-Tf2N and C11-P44-Tf2N show a decrease in ion mobility (slow dynamics) and an increase in the strength parameter D (less fragile) (Table 2). The lower fragility is probably due to the restricted segmental motions by the barrier of the PEG crystalline phases.44 Dielectric Relaxations. The dielectric derivative spectra45 εder(ω) = −

π ∂ε′(ω) 2 ∂[ln ω]

Figure 5. Dielectric derivative spectra εder at 268 K shifted by horizontal shift factor, X [C6-P22-PF6 (X = 1); C6-P22-Tf2N (X = 0.07); C6-P44-Tf2N (X = 2.12); C11-P22-Tf2N (X = 0.09); C11-P44Tf2N (X = 1.61)]. The solid curves are fits of the sum of a power law for EP and derivative forms of the HN function for either interfacial polarization (MWS) or orientation polarization (α2 and α) to the εder data (individual contributions shown as dashed curves).

exchanging ion states between isolated pairs and aggregates of pairs), while the faster segmental α relaxation involves the typical characteristics of the glass transition dynamics. For the high crystalline ionomers (C6-P44-Tf2N and C11P44-Tf2N), on the contrary, their εder show only one relaxation designated as Maxwell−Wagner−Sillars (MWS), whose intensity is much larger than the dipolar α2 and α relaxations observed in the amorphous and low crystalline ionomers (see Figure 5). The MWS interfacial polarization arises from ionic charge accumulation near the interfaces between the various phases in heterogeneous characteristic materials, where there are differences in conductivity and dielectric permittivity of the phases.27,47 Such a polarization was previously reported for multiphase blends, block copolymers, and semicrystalline polymers.47,48 Because both C6-P44-Tf2N and C11-P44-Tf2N exhibit semicrystalline behavior, as observed in DSC (Table 1), ionic conductivity (Figure 2), and X-ray scattering (which will be discussed in detail later) measurements, it is expected that their MWS relaxation process, observed in Figures 5 and 6c,e, originates from charge buildup at the interfacial boundary between the imidazolium ion domains and the PEG crystalline segmentsthe ionic domain having higher conductivity and permittivity than the PEG domain. The fact that the MWS process is activated by the PEG crystallization is further supported in the frequency and temperature dependence of the dielectric derivative spectra εder and dielectric permittivity spectra ε′ as shown in Figure 7, where the remarkable changes of both spectra below and above the phase transition temperature (Tt ∼ 273 K for C6-P44-Tf2N and Tt ∼ 283 K for C11-P44-Tf2N) are clearly observed, and Figure S4, showing the comparison between permittivity, conductivity, and electric modulus.49 To determine the peak relaxation frequency ωmax and relaxation strength Δε of the α, α2, and MWS processes, we then fit εder using a sum of a power law for EP and derivative forms of the Havriliak−Negami (HN) function for the three relaxation peaks (Figure 6):47

(3)

elucidating relaxation processes (such as ion rearrangement, segmental relaxation, or interfacial polarization) by removing the pure-loss conductivity contribution, were used to investigate the effect of anion type, alkylene spacer length, and PEG segment length on polymer chain or ion dynamics of the imidazolium−PEG copolyester ionomers. Figure 5 shows εder(Xω) at 268 K, horizontally shifted by a shift factor X, to display the relaxation process difference among the ionomers, and their original εder(ω) at 268 K are also shown in Figure 6. The derivative curves of the amorphous and low crystalline ionomers (C6-P22-PF6, C6-P22-Tf2N, and C11-P22-Tf2N) exhibit two relaxation processes, assigned to α2 and α in order of increasing frequency (Figures 5 and 6a,b,d), like usual single-ion conducting ionomers.19,21,46 The slower dipolar relaxation α2 corresponds to ion rearrangement19 (an act of F


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Figure 6. Dielectric derivative spectra εder at 268 K and their fits (solid lines) of the sum of a power law for EP and derivative forms of the HN function for MWS, α2, and α processes (eq 4) for (a) C6-P22-PF6, (b) C6-P22-Tf2N, (c) C6-P44-Tf2N, (d) C11-P22-Tf2N, and (e) C11-P44Tf2N.

Figure 7. Frequency dependence of dielectric derivative spectra εder (left, a and c) and dielectric permittivity spectra ε′ (right, b and d) of semicrystalline ionomers C6-P44-Tf2N (top, a and b) and C11-P44-Tf2N (bottom, c and d) as a function of temperature in the region of the dielectric spectrum where the crystallization effect on ion and polymer dynamics is clearly observed. G


Macromolecules εder = Aω−s −

π 2

∑ i = α , α2

ÄÅ ÉÑ ÅÅ ∂εHN Ñ ÅÅ ′ (ω) ÑÑÑ ÅÅ Ñ ÅÅÇ ∂ ln ω ÑÑÑÖi

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The larger Tf2N− counteranions (C6-P22-Tf2N) accelerate both the ion rearrangements (α2, open symbols in Figure 8) and the segmental motions (α, filled symbols in Figure 8) compared to the smaller PF6− counteranions (C6-P22-PF6). However, the ionomers with the same Tf2N− counterion have 2N almost identical α2 and α relaxation frequencies (ωC6‑P22‑Tf ≈ α2 C6‑P22‑Tf2N C11‑P22‑Tf2N C11‑P22‑Tf2N and ωα ≈ ωα ) over the whole ωα 2 temperature range, as shown in Figure 8, despite having the different alkyl spacer length [−(CH2)6− vs −(CH2)11−]. This can be explained by the dynamic glass transition temperature (DRS Tg, listed in Table 3),51 determined by extrapolating the VFT fit of the segmental α peak relaxation time to τα (= 1/ωα) = 100 s, which is clearly visible in derivative spectra (Figure 6). The PF6− ionomer C6-P22-PF6 has ∼16 K higher DRS Tg than the Tf2N− ionomer C6-P22-Tf2N, while both C6-P22-Tf2N and C11-P22-Tf2N, having the different alkyl spacer length, exhibit almost identical DRS Tg ∼ 214 K, listed in Table 3. The effect of Tg on the relaxation frequency maxima ωmax of the α2 and α processes is more clearly seen in the inset of Figure 8, where the temperature has been normalized by Tg. Each relaxation frequency maxima ωmax merges into its corresponding curve (see solid line in the inset of Figure 8), demonstrating that ωα2 and ωα are strongly coupled with the glass transition temperature. For the semicrystalline ionomers having longer PEG2000 segments (C6-P44-Tf2N and C11-P44-Tf2N), Figure 9a shows the temperature dependence of the relaxation frequency maxima of the MWS process from DRS (ωDRS MWS, filled symbols) compared with the DC conductivity (σDC, open symbols). As noted earlier, upon the PEG crystallization, the ionic conductivity σDC of these ionomers drops significantly at the phase transition temperature, and at the same time the MWS interfacial polarization of these semicrystalline ionomers appears. This is why ωMWS displays a similar temperature dependence as σDC (see Figure 9a). In addition, the theoretical relaxation time of the MWS process for C6-P44-Tf2N and C11-P44-Tf2N can be estimated using the following Debyetype equation:47,52,53

| l o o Δε o ′ (ω) = Realo with εHN m o [1 + (iω/ω )a ]b } o o o (4) HN n ~ where A and s are constants, a and b are shape parameters, Δε is a relaxation strength, and ωHN is a characteristic frequency related to ωmax given by47,50 or MWS

i aπ zy ji abπ zy zz zz jjsin ωmax = ωHNjjjsin (5) k 2 + 2b { k 2 + 2b { For the amorphous ionomers (C6-P22-PF6, C6-P22-Tf2N, and C11-P22-Tf2N), Figure 8 displays the temperature 1/ a

−1/ a

Figure 8. Temperature dependence of relaxation frequency maxima ωmax of the α2 (ωα2, open symbols) and α (ωα, filled symbols) for the copolyester ionomers having shorter PEG segments (C6-P22-PF6, C6-P22-Tf2N, and C11-P22-Tf2N) (ωα2 and ωα vs Tg/T in the inset). Solid curves are fits of the VFT equation (eq 6). The arrows indicate the DRS Tgs, listed in Table 3.

dependence of the peak relaxation frequency maxima ωmax of the α2 process (ωα2, open symbols) and the α process (ωα, filled symbols). As shown in μ(T) (Figure 4), these ωα2 and ωα also follow the VFT temperature dependence:

τMWS = ε0

(1 − n)εPEG + nεion + n(εPEG − εion)φion (1 − n)σPEG + nσion + n(σPEG − σion)φion (7)

where ε0 is the vacuum permittivity, n is a shape factor, εPEG and εion are the dielectric constants of PEG and ion domains, respectively, φion is the volume fraction of ions, and σPEG and σion are the conductivities of PEG and ions, respectively. The theoretical predicted MWS relaxation time τMWS was then calculated for the semicrystalline copolyester ionomers by assuming that the ionic components are spherical (n = 0.33), the volume fraction of ions is φion = 0.33 for C6-P44-Tf2N and

ij DT0 yzz ωmax (T ) = ω∞ expjjj− z j T − T0 zz (6) k { The solid curves in Figure 8 are fit to eq 6 using the same T0 from the VFT mobility fit (Figure 4), high-temperature limiting frequency ω∞, and D, listed in Table 3.

Table 3. Fitting Parameters of the VFT Temperature Dependence (Eq 6) of the α2 and α Processes and DRS Glass Transition Temperatures (DRS Tg) α2 process

α process

sample

log(ω∞) (rad/s)

D

T0 (K)

log(ω∞) (rad/s)

D

T0 (K)

DRS Tga ± 5 (K)

C6-P22-PF6 C6-P22-Tf2N C11-P22-Tf2N C6-Tf2N

9.7 8.9 8.4 8.0

5.1 4.1 4.4 4.0

196 187 181 193

11.1 10.6 10.8 9.8

5.4 4.4 5.2 4.1

196 187 181 193

231 215 213 223

Tg determined from dielectric relaxation spectroscopy (defined at ωα(Tg) = 10−2 rad/s).

a

H


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Figure 9. (a) Temperature dependence of relaxation frequency ωmax for the MWS process from experimental DRS measurement (ωDRS MWS, filled symbols, left axis) and from theoretical prediction via eq 7 (ωeq7 MWS, × symbols, left axis) and of ionic conductivity σDC (open symbols, right axis) for the copolyester ionomers having longer PEG segments. The inset displays MWS relaxation strength ΔεMWS as a function of temperature. (b) Wideangle X-ray scattering intensity as a function of scattering wavevector q for the imidazolium−PEG copolyester ionomers with the longer PEG segments, compared with PEG9900 oligomer from the literature data,55 at room temperature. The X-ray scattering peaks labeled by letters and indicated by arrows corresponding to crystalline PEO reflections. The data were shifted on the intensity scale for clarity. (c) Small-angle X-ray scattering data as a function of scattering vector q for C6-P44-Tf2N and C11-P44-Tf2N copolyester ionomers at room temperature. The scattering peaks (indicated by arrows) at 1q*, 2q*, 3q*, and 4q* (q* = 2π/L) correspond to the lamellar spacing (L). (d) Schematic of the lamellar morphology of imidazolium−PEG copolyester ionomers with longer PEG segments (C6-P44-Tf2N and C11-P44-Tf2N) showing the PEG crystalline lamellae (green) with a lamellar thickness (lc), the amorphous domains (blue) containing the imidazolium cations with Tf2N counteranions (orange), and a lamellae-lamellae spacing (L). Some of the imidazolium moieties are polarized at the interface between the crystalline and amorphous domains to activate the MWS process.

φion = 0.31 for C11-P44-Tf2N, obtained from group contribution method54 based on structure, the crystallized PEG has a dielectric constant of εPEG ∼ 5.4 and a negligible conductivity (σPEG ∼ 0) in the temperature range of the current experiments, and for the ionic components their dielectric constant εion and conductivity σion are the measured dielectric constant and DC conductivity of the copolyester ionomers, respectively. In Figure 9a, the predicted MWS relaxation times calculated from eq 7 are plotted as a function of temperature. For C6-P44-Tf2N and C11-P44-Tf2N, their eq7 calculated τMWS’s (τeq7 MWS = 1/ωMWS, × symbols in Figure 9a) are nearly identical to the experimental relaxation time of the DRS MWS process from DRS (τDRS MWS = 1/ωMWS, filled symbols in Figure 9a). This quantitative agreement further supports that the origin of this process shown in the semicrystalline copolyester ionomers arises from MWS interfacial polarization. The MWS relaxation strength ΔεMWS in the inset of Figure 9a increases with decreasing temperature and then approaches ΔεMWS > 100, which is far too large to be attributed to dipolar relaxations. Moreover, C6-P44-Tf2N with relatively higher ion content has higher ΔεMWS than C11-P44-Tf2N with lower ion content (see the inset of Figure 9a). This is another evidence for the assignment of this relaxation process as MWS interfacial polarization, originating from the PEG crystallization. This is also in agreement with findings from the wide-angle X-ray scattering (WAXS) measurements as shown in Figure 9b, where the Bragg peaks from the crystallized PEG-based

copolyester ionomers of C6-P44-Tf2N and C11-P44-Tf2N are compared to literature data55 of the PEG oligomer with Mn = 9900 g/mol (PEG9900). For the PEG9900 oligomer with 84% crystallinity,55 its multiple scattering peaks (labeled A−F in Figure 9b and Table 4) are related to the crystalline reflections of typical poly(ethylene oxide) (PEO) crystals with a helical chain conformation and a monoclinic lattice.55 Table 4. Observed d-Spacings of the Bragg Peaks in the XRD Data of C6-P44-Tf2N and C11-P44-Tf2N

a

peaks

q (nm−1)

d-spacinga (nm)

(hkl)

A B C D E F

9.5 10.6 13.5 16.4 18.4 18.8

0.66 0.59 0.47 0.38 0.34 0.33

(021) (110) (120) (112) (024) or (024) (224) or (224)

The d-spacings were calculated from 2π/q.

Although the diffracted intensities of some of the Bragg peaks of C6-P44-Tf2N and C11-P44-Tf2N were weak because of their low crystallinity (∼30%), their positions (see arrows in Figure 9b) are identical to those measured in the PEG9900 oligomer with no ions, indicating that the PEG segments crystallize into the monoclinic lattice structure as pure PEO, and the lattice parameters remain unchanged in the PEG-based I


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Figure 10. (a, b) Temperature comparison of dielectric permittivity spectra ε′(Xω) for (a) C6-P44-Tf2N at 268 and 283 K and (b) C11-P44-Tf2N at 273 and 293 K. (c, d) Copolyester comparison of ε′(Xω) at 268 K for (c) C6-P22-Tf2N vs C6-P44-Tf2N and (d) C11-P22-Tf2N vs C11-P44Tf2N. Their ε′(Xω) were shifted by horizontal shift factor X, allowing to compare the static dielectric constant εs before the onset of electrode polarization (indicated by solid lines), and the inset shows the original dielectric permittivity spectra ε′(ω).

(C6-P44-Tf2N and C11-P44-Tf2N) is related to the MWS interfacial polarization, Figure 10 compares the static dielectric constant εs, determined from the sum of the dielectric constant of all the observed relaxations (Figure S5),19 of semicrystalline copolyesters at temperature above vs below Tt (Figures 10a,b) and of semicrystalline versus amorphous copolyester (Figures 10c,d). For the semicrystalline ionomers (Figures 10a,b), the crystallization results in a considerable increase in εs (from εs ∼ 81 at 283 K to εs ∼ 157 at 268 K for C6-P44-Tf2N and from εs ∼ 66 at 293 K to εs ∼ 123 at 273 K for C11-P44-Tf2N). The 2 times enhanced εs cannot be explained by the sum of the two relaxation processes (α2 and α) observed in the melt state, but this is due to an additional source of polarization that is the MWS interfacial polarization, corresponding to charge accumulation near the interface between amorphous and crystalline phases (see Figure 9d). Moreover, from the comparison between the amorphous and semicrystalline copolyesters (Figures 10c,d), since the amorphous C6-P22Tf2N (p0 = 0.47 nm−3, listed in Table 2) has higher total ion content p0 than the semicrystalline C6-P44-Tf2N (p0 = 0.32 nm−3, listed in Table 2), it is expected that the more ion containing C6-P22-Tf2N has higher εs than the less ion containing C6-P44-Tf2N because of εs ∼ p0m2, where m is the dipole moment and both ionomers have the same m of an imidazolium−Tf2N ion pair, predicted by the Onsager theory.56 However, Figure 10c exhibits that εs of C6-P44Tf2N (εs ∼ 157) is 2 times higher than that of C6-P22-Tf2N (εs ∼ 74). The same result is also observed in the comparison between C11-P44-Tf2N and C11-P22-Tf2N; the semicrystalline C11-P44-Tf2N with lower ion content (εs ∼ 127) exhibits higher εs than the amorphous C11-P22-Tf2N with higher ion content (εs ∼ 68). Consequently, the Tf2N− anions are presumably accumulated along the PEG crystallites, resulting

copolyester ionomers. This suggests that the imidazolium− Tf2N ions are excluded from the PEG crystal lamella and reside in the hydrocarbon amorphous phase as shown schematically in Figure 9d.55 In addition to the WAXS profiles, Figure 9c displays the room temperature small-angle X-ray scattering (SAXS) data of the investigated copolyester ionomers. For C6-P44-Tf2N, the SAXS data consist of a weak and single peak at a 1q* of about 0.40 nm−1, whereas a series of scattering peaks with a major peak at a 1q* of about 0.35 nm−1 were observed for C11-P44Tf2N. The SAXS peaks measured in this q range was originated from the phase-separated lamellar morphology of the PEG crystal. Based on the lq* position of both specimens, the lamellar spacings (L, Figure 9d) of ∼15.6 and ∼18.3 nm were determined for C6-P44-Tf2N and C11-P44-Tf2N, respectively, as schematically drawn in Figure 9d. The absence of the higher order scattering peaks for C6-P44-Tf2N indicates the structural imperfection of the lamellar morphology, whereas a series of the SAXS peaks were measured from C11-P44-Tf2N, indicative of a well-defined lamellar morphology.55 Furthermore, in Figure 9b, showing typical WAXS data of both samples, measured at room temperature, the sharpness and higher intensity of the Bragg peaks for C11-P44-Tf2N reveals a higher crystallinity. The lamellar thicknesses (lc, see Figure 9d) were ∼10.6 and ∼11.3 nm for C6-P44-Tf2N and C11-P44-Tf2N specimens, respectively, estimated based on the half-width of the (120) Bragg peak (marked as C in Figure 9b). These data also indicate that the structural perfection of the lamellar morphology of C11-P44-Tf2N causes more crystallization, consistent with the DSC result (Table 1), and larger lamellar thickness of the PEG crystal. To further prove that upon crystallization the relaxation process observed in the semicrystalline copolyester ionomers J


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Figure 11. Temperature dependence of (a) ionic conductivity σDC, (b) simultaneously conducting ion EP mobilities (filled symbols) and extended mobilities (open symbols), and (c) simultaneously conducting ion content p (p/p0, where p0 is total ion concentration, in the inset) for the ionomers with and without PEG1000 segments (C6-P22-Tf2N and C6-Tf2N).

Figure 12. (a, c) Angular frequency dependence of (a) dielectric derivative spectra εder at 268 K and (c) dielectric permittivity spectra ε′ at 293 K, shifted by a shift factor X, for C6-P22-Tf2N, C6-Tf2N, and PEG600.57 (b, d) Temperature dependence of (b) relaxation frequency maxima ωmax of the α2 and α processes and (d) static dielectric constant εs (solid lines are Onsager56 fits for εs).

in the large increase in ΔεMWS, attributed to MWS polarization, as well as an abrupt drop in the ionic conductivity (Figure 2), simultaneously ion content (Figure 3), and mobility (Figure 4). Comparison of Copolyester and Polyester Ionomers. For further understanding an influence of the PEG segments on ionic conduction and dielectric response of the imidazolium−PEG copolyester ionomers, a previously reported imidazolium−Tf2N-based polyester ionomer without

PEG segment28 (termed C6-Tf2N with the structure shown in Figure 11) was directly compared to the corresponding copolyester ionomer with the PEG1000 segments (C6-P22Tf2N). Note that both C6-Tf2N and C6-P22-Tf2N ionomers exhibit amorphous morphologies, allowing to eliminate the crystalline effect. Ion Conduction. Figure 11a compares ionic conductivities between the imidazolium polyester ionomer (C6-Tf2N) and the imidazolium−PEG copolyester ionomer (C6-P22-Tf2N). K


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Macromolecules 2N The ionomer with the PEG segment (σC6‑P22‑Tf ∼ 7.5 × 10−5 DC S/cm at 298 K) exhibits ∼3 times higher room temperature 2N conductivity than that without the PEG segment (σC6‑Tf ∼ DC −5 2.3 × 10 S/cm at 298 K) due to the PEG segment lowering Tg by ∼8 K (see Table 3). This is consistent with the observation of simultaneously conducting anion mobility, following the VFT (eq 2) temperature dependence (see solid curves in Figure 11b). The EP (filled symbols in Figure 11b) and extended (open symbols in Figure 11b) mobilities increase by ∼8× upon incorporating the PEG segments into the polyester over the entire temperature range studied, which goes with the ∼6 K decrease in T0 (Table 2). For simultaneously conducting anion concentration p (see Figure 11c), although C6-Tf2N exhibits higher p than C6-P22-Tf2N, when p is normalized by the total anion number density p0 (listed in Table 2), meaning the fraction of anions simultaneously participating in conduction, the ionomer with the PEG segments (having lower p0) exhibits ∼1.5 times higher fraction of anions simultaneously participating in conduction (p/p0) compared to the ionomer without the PEG (having higher p0). After applying the Arrhenius equation (eq 1) to p (see solid lines in Figure 11c), the PEG segment 2N reduces the conducting anion activation energy from EC6‑Tf ∼ a C6‑P22‑Tf2N 12.2 kJ/mol to Ea ∼ 10.4 kJ/mol (Table 2). All these results suggest that the relatively short PEG segments allow more counteranions to participate in conduction with lower activation energy and higher mobility with lower Tg, thereby the PEG incorporation boosting ionic conductivity. Dielectric Properties. Figure 12a compares the dielectric derivative spectra ε der between the imidazolium-based polyester ionomer (C6-Tf2N) and imidazolium−PEG copolyester ionomer (C6-P22-Tf2N) at 268 K, and both the ionomers exhibit the two α2 and α processes, regardless of the PEG segments. For the relaxation frequency maxima ωmax of the two relaxations, following the VFT temperature dependence (see solid curves in Figure 12b), however, the incorporation of PEG segment into the polyester accelerates 2N 2N the α2 ion rearrangement (ωαC6‑P22‑Tf > ωαC6‑Tf , open 2 2 symbols in Figure 12b) and the α segmental motion 2N 2N (ωC6‑P22‑Tf > ωC6‑Tf , filled symbols in Figure 12b) up to α α ∼20×. This is why C6-P22-Tf2N has a lower Tg than C6-Tf2N (Table 3). To investigate the difference of the static dielectric constant εs in terms of the PEG segment, the horizontally shifted dielectric permittivity spectra ε′ of the ionomer without the PEG (C6-Tf2N) was compared to those of the ionomer with the PEG (C6-P22-Tf2N) and the PEG oligomer with Mn = 600 g/mol (PEG600) at 293 K.57 The nonionic PEG600 has the lowest room temperature static dielectric constant εs = 12, defined as the low-frequency plateau of ε′(ω) before the onset of EP (see purple horizontal line in Figure 12c). For the ionomers, their εs values were obtained from the sum of the dielectric constant of all the observed relaxations (see Figure S5).19 The PEG-containing ionomer (C6-P22-Tf2N, εs = 68) exhibits higher εs than the PEG-free one (C6-Tf2N, εs = 56) at 298 K. The increase in the static dielectric constant εs is presumably due to the enhanced dipole moment. The interaction between the hydrogen atoms of the imidazolium ring and the ether oxygen of the PEG may allow the contact ion pairs of imidazolium−Tf2N or their quadrupoles to form the separated ion pairsthe latter having higher dipole moment than the former. Moreover, since the static dielectric constant is in the denominator of the Coulomb energy, the

obtained εs of the ionomers allows us to divide the 0 K/ vacuum quadrupole energy for imidazolium−Tf2N− (Equad ∼ 2N 2N 680 kJ/mol) by εC6‑P22‑Tf = 68 and εC6‑Tf = 56 and then s s estimates activation energy for an ion pair to dissociate from 2N the quadrupoles; i.e., Equad/εC6‑P22‑Tf ∼ 10 kJ/mol for C6s C6‑Tf2N ∼ 12 kJ/mol for C6P22-Tf2N is lower than Equad/εs Tf2N. These estimated activation energies are almost identical to the activation energies for conducting ion (Ea = 10.4 kJ/mol for C6-P22-Tf2N and Ea = 12.2 kJ/mol for C6-Tf2N, listed in Table 2). The temperature dependence of the static dielectric constant for C6-P22-Tf2N, C6-Tf2N, and PEG600 is also displayed in Figure 12d with the solid line Onsager predictions (εs ∼ 1/T).56 For the nonionic PEG600, εs ∼ 1/T holds at temperatures before crystallization temperature, which is consistent with the previous result from nonionic semicrystalline polymers.58 On the other hand, εs of the ionomers without the PEG segments (C6-Tf2N) follows the Onsager equation across the entire temperature range studied. The ionomer with the PEG (C6-P22-Tf2N) first follows εs ∼ 1/T with decreasing T, but then εs begins to decrease as T is lowered toward Tg. At temperatures near Tg, the dipole motion can be restricted by neighboring ones, limiting rotation and alignment of dipoles under an electric field, so that εs decreases on cooling near Tg. Such a nonmonotonic temperature dependence is previously observed in the other imidazolium-based ionomers.21 Consequently, this result tells us that introducing PEG segments into a polyester backbone as making a copolyester ionomer is one of the ways to boost ionic conductivity of the single-ion conductor. However, there still exists the question of whether the PEG segments in the backbone is better for ionic conductivity than (i) the blending of PEG with the polyester C6-Tf2N or (ii) the addition of PEG as a pendant group to the polyester. For the mixing of the imidazolium polyester with PEG, it turns out to be macroscopically phase-separated into the polyester phases and PEG phases (see Figure S6a). This is presumably due to the long hydrophobic hydrocarbon chains [(CH2)n] which is not able to be mixed with the hydrophilic PEG segments. On the other hand, the copolyester C6-P22Tf2N shows a transparent single phase (Figure S6b). This tells us that our copolymerization is a proper method to allow the effective phase mixing of the two immiscible components. For the copolymer combining the ionic group and pendant PEG segment, Colby and co-workers59,60 have reported ionic conductivities of polyanion ionomers; one is for random copolymers with sodium sulfonated styrene and PEG on the side chain, and the other is for polyesters with sodium sulfonated styrene and PEG spacer on the backbone. The random copolymers with PEG side chains exhibited 40 K lower Tg than the polyester with PEG in the backbone, so that the copolymers showed much higher ionic conductivity than the polyesters. For polycation ionomers, it has been shown that for imidazolium acrylate ionomers the introduction of diethyleneoxy units on the imidazolium cation boosts ionic conductivity due to a reduction of imidazolium−Tf2N pair dissociation energy by interactions between the ether oxygen atoms of the diethyleneoxy units and hydrogen atoms of the imidazolium ring.30 However, directly attaching pendant PEG segments to imidazolium ionic groups has rarely been reported, so designing PEG as a pendant group, likely providing more degree of freedom and thereby helping anion dissociation, might be another solution to develop fast singleion conductors. L


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4. CONCLUSIONS The ion conduction and dielectric response of the imidazolium−PEG copolyester ionomers were carefully investigated from dielectric relaxation spectroscopy and X-ray scattering. The effects of counteranion size, alkylene spacer length, and PEG segment length were clearly observed in counteranion conductivity, simultaneously conducting ion concentration, ion mobility, and dielectric relaxations such as ion rearrangement (α2), segmental motion (α), and MWS interfacial polarization. The copolyester ionomers with larger Tf2N− have a higher number density of counteranions simultaneously participating in σDC, due to the lower activation energy, compared to those with the smaller PF6−. The Tf2N− anions also impart lower T0 and Tg to the copolyesters than the PF6− anions, consistent with the acceleration of the α2 and α processes, thereby boosting counteranion mobility and conductivity. The PEG segment incorporated to the crystallization of the copolyesters; the imidazolium−PEG copolymers with longer PEG segments exhibit higher crystallinity than those with shorter PEG segment. The higher crystallinity is clearly reflected by the observation of the MWS interfacial polarization and of the phase-separated lamellar morphology, indicating counteranion accumulation at the interfaces of the imidazolium domains and the PEG crystalline segments, so that the σDC drops significantly at the phase transition temperature. However, the introduction of shorter PEG segment on the main-chain imidazolium polymer not only allows more counterions to participate in conduction with lower activation energy due to the εs enhancement but also provides higher counteranion mobility due to the Tg reduction, compared to the polyester without the PEG segment, thereby the PEG incorporation boosting ionic conductivity of the single-ion conducting copolyesters.

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ACKNOWLEDGMENTS

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REFERENCES

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grants 2016R1D1A1B03932055 and 2017R1A2B4012669), by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT(2018M3D1A1058624), and by the BB21+Project in 2018.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02332. Application of electrode polarization model, dielectric response of C11-P22-Tf2N to applied AC field at 273 K, frequency dependence of in-phase part of conductivity for C6-P44-Tf2N, C11-P44-Tf2N, and C6-P22-PF6, angular frequency dependence of loss tangent for C11P22-Tf2N and C11-P44-Tf2N at 273 K, complex permittivity, conductivity, and electric modulus for C11-P44-Tf2N at 273 K, dielectric permittivity of C6P22-Tf2N at 293 K, and photographs for mixture of C6P22-Tf2N and PEG1000 and C6-P22-Tf2N (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yong Ku Kwon: 0000-0003-1839-9593 U Hyeok Choi: 0000-0002-0048-9550 Notes

The authors declare no competing financial interest. M


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