Modulating Ion Transport and Self-Assembly of Polymer Electrolytes

Apr 5, 2017 - In a 50 mL round-bottom flask, SEO-ene (85 mg, 0.0061 mmol), mercaptosuccinic acid (36.6 mg, 0.244 mmol), and AIBN (4 mg, 0.0244 mmol) w...
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Modulating Ion Transport and Self-Assembly of Polymer Electrolytes via End-Group Chemistry Ha Young Jung,† Prithwiraj Mandal,†,‡ Gyuha Jo,† Onnuri Kim,† Minju Kim,§,∥ Kyungwon Kwak,§,∥ and Moon Jeong Park*,†,‡ †

Department of Chemistry and ‡Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang, Korea 790-784 § Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS), Korea University, Seoul 02841, Korea ∥ Department of Chemistry, Korea University, Seoul 136-701, Republic of Korea S Supporting Information *

ABSTRACT: We report a rational design of solid-state dry polymer electrolytes with high conductivity, high mechanical strength, and improved cation transference number. Thiol− ene click chemistry provided orthogonal control over the type and number of end groups in poly(styrene-b-ethylene oxide) (PS-b-PEO) block copolymers. This approach permitted the synthesis of PEO chains with reduced crystallinity, reminiscent of PEO oligomers, thereby playing a key role in improving the room temperature conductivity. Intriguingly, the incorporation of diol or dicarboxylic acid end groups in PS-b-PEO produced a well-defined gyroid structure, leading to order of magnitude improvements in the storage modulus. Out of the various samples examined, the electrolytes bearing terminal diol displayed the highest ionic conductivity and a 2-fold increase in lithium transference number. The improvements in performance are attributed to the reduced interchain aggregation and the anion stabilization mediated by the terminal diol group. The fact that the dramatic changes in ion transport and mechanical properties of PS-b-PEO samples were brought about solely by the modification of single terminal group of the PEO unit confirmed end-group chemistry as a powerful tool for the design of efficient solid-state polymer electrolytes. This work should find applications in various emerging electrochemical technologies, namely those employed in energy storage and conversion.

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INTRODUCTION

framework, the insulating nature of most hard polymers has resulted in polymer electrolytes with low ionic conductivity.14,15 Attempts to produce materials with mixed PEO and hard polymer chains have shown that the resulting products exhibit decreased degree of salt dissociation and increased Tg of PEO chains, thereby leading to ionic conductivity that was more notably deteriorated than would be expected based on predictions.16 These outcomes prompted researchers to investigate microphase-separated polymer electrolytes with sharply defined interfaces14,17,18ones where the individual components could play their specific roles, i.e., PEO would impart ion conductivity and hard polymers would impart mechanical strength, without diminishing the overall performance. Poly(styrene-b-ethylene oxide) (PS-b-PEO) block copolymer electrolytes, pioneered by Balsara and co-workers,14,19−21 have shown potential as such polymer electrolytes where PS and PEO chains remain immiscible. Numerous variations of the core polymer architecture have also been

In recent years, large-scale rechargeable batteries have generated significant interest owing to their potential to meet our increasing energy storage requirements.1−3 Lithium batteries, comprising a lithium metal anode and solid-state dry polymer electrolyte, exhibit several advantages that make them excellent candidates for this application, viz. high energy density, improved thermal stability, and low-cost fabrication.4,5 The most widely studied dry polymer electrolytes to date are based on a combination of poly(ethylene oxide) (PEO) and lithium salts, where PEO can effectively solvate and conduct lithium ions.6,7 In the endeavor to develop competent dry polymer electrolytes, extensive efforts have been devoted to the molecular design of polymer backbones that would possess inherently high dielectric environment8,9 and low glass transition temperature (Tg),10 which would facilitate fast ion transport. Unfortunately, such an approach often resulted in the loss of mechanical integritya key feature for developing safe lithium batteries.11−13 While several research groups have attempted to incorporate hard polymers into the PEO © 2017 American Chemical Society

Received: February 1, 2017 Revised: March 29, 2017 Published: April 5, 2017 3224

DOI: 10.1021/acs.macromol.7b00249 Macromolecules 2017, 50, 3224−3233

Article

Macromolecules reported to date, e.g., multiblock copolymers,22 graft copolymers,23−25 dendrimers,26 and star-shaped polymers.27,28 Nevertheless, the major shortcoming of PEO-based dry solid electrolytes is their low room-temperature conductivity, arising from the inherently crystalline nature of PEO, which has a melting temperature (Tm) of 65 °C.7,29−31 The crystallinity of PEO can reduce conductivity by orders of magnitude, thereby significantly degrading the battery performance in cold weather. The majority of strategies directed at reducing the PEO crystallinity have focused on the preparation of graft polymers with branched PEO chains.32,33 While this method has certainly achieved improvements in room-temperature conductivity, the overall ionic conductivity was found to decrease as a result of limited Li+ hopping across branched PEO chains. It is clear that viable solid-state dry polymer electrolytes require three critical characteristics: (i) a reasonable ionic conductivity in the range of 10−5−10−3 S/cm at ambient temperature, (ii) intact mechanical strength, and (iii) low crystallinity of PEO chains in order to ensure functionality at low temperatures. The challenge, therefore is the design of such polymer electrolytes, ideally through simple synthetic steps. Herein, we report the preparation of PS-b-PEO block copolymer electrolytes whose ion transport and self-assembly behavior are tailored through end-group chemistry. Endmodified PEO was first introduced by Ohno and co-workers in 1995 as a way of modulating the value of Tg of PEO.34,35 By Frielinghaus and co-workers the order−disorder phase transition of PS-b-PEO terminated by either a hydroxyl or a methoxy group was also investigated in 2001.36 This end-group chemistry was revisited by our group in 2013 as a facile and robust method for controlling the thermodynamics of PS-bPEO block copolymers.37 The presented work demonstrates orthogonal control over the type and density of end-functional groups in PEO-based polymers via thiol−ene “click” chemistry, which has not been explored to date. Notably, the attachment of terminal diol group to the PEO chains of PS-b-PEO yielded a well-defined gyroid structure that lead to an order of magnitude enhanced storage modulus, ample reduction in PEO crystallinity, and improved ion transport efficiency over the entire temperature range of interest. On the basis of a comprehensive understanding of inter- and intramolecular interactions in the endfunctionalized polymer electrolytes, we propose a simple but powerful approach for the design of advanced polymer electrolytes as future materials in battery technologies.

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methane)sulfonimide (LiTFSI, >99%) were purchased from SigmaAldrich. Characterization of Molecular Weights. All synthesized polymers were purified by repeated precipitations in ether and dried under vacuum for a week at room temperature. 1H nuclear magnetic resonance (1H NMR) experiments were carried out using 300 and 500 MHz Bruker (topspin 3.1 software) spectrometers at room temperature. CDCl3 and MeOD were used as internal reference. Molecular weight distributions of synthesized polymers were characterized by size exclusion chromatography (SEC, Waters Breeze 2 HPLC) using PS standards in THF. The SEC system was equipped by three PS/DVB columns (Shodex KF-801, KF-802, and KF-803, 300 × 8.0 mm) and two detectors (Waters 2489 UV/vis detector, Waters 2414 refractive index detector). Eluent was THF delivered by a Waters 1515 isocratic HPLC pump at a flow rate of 1 mL/min. Synthesis of Allyl-Terminated PS-b-PEO (SEO-ene). In a 50 mL round-bottom flask, NaH (3.4 mg, 0.14 mmol) was added to a solution of PS-b-PEO (200 mg, 0.014 mmol) in 4 mL of anhydrous benzene. The mixtures were stirred at room temperature for 3 h, followed by dropwise addition of allyl bromide (87 mg, 0.72 mmol). After reaction for overnight, unreacted NaH was removed by filtration. 1 H NMR (500 MHz, CDCl3) δ ppm: 7.10−6.40 (b, n × 5H, −CH2CH(C6H5)), 5.95−5.87 (m, 1H, CHCH2), 5.29−5.16 (m, 2H, CHCH2), 4.0 (d, 2H, OCH2CHCH2), 3.64 (b, n × 4H, −OCH2CH2O−), 2.21−1.20 (b, n × 3H, −CH2CH(C6H5)). Synthesis of Thioglycolic Acid-Modified PS-b-PEO (SEO-c). In a 50 mL of round-bottom flask, SEO-ene (80 mg, 0.0057 mmol), thioglycolic acid (10.57 mg, 0.1147 mmol), and AIBN (1.9 mg, 0.0114 mmol) were taken and dissolved in 1.6 mL of anhydrous toluene under Ar conditions. The reaction was carried out at 80 °C for 2.5 h. 1 H NMR (500 MHz, CDCl3) δ ppm: 7.10−6.30 (b, n × 5H, −CH2CH(C6H5)), 3.64 (b, n × 4H, −OCH2CH2O−), 3.23 (s, 2H, −SCH2COOH), 2.78−2.75 (t, 2H, −CH2SCH2COOH), 2.21−1.20 (b, n × 3H, −CH2CH(C6H5)). Synthesis of Mercaptosuccinic Acid-Modified PS-b-PEO (SEO-2c). In a 50 mL round-bottom flask, SEO-ene (85 mg, 0.0061 mmol), mercaptosuccinic acid (36.6 mg, 0.244 mmol), and AIBN (4 mg, 0.0244 mmol) were taken and dissolved in 1.7 mL of anhydrous dioxane under an Ar atmosphere. The reaction was carried out at 80 °C for 1.5 h. 1H NMR (500 MHz, CDCl3 and MeOD (5:1)) δ ppm: 7.10−6.30 (b, n × 5H, −CH2CH(C6H5)), 3.56 (b, n × 4H of −OCH2CH2O- and 1H of −C(H)COOH), 2.90−2.70 (m, 2H of −CH2COOH and 2H of −CH2S−), 2.20−1.20 (b, n × 3H, −CH2CH(C6H5)). Synthesis of Thioglycerol-Modified PS-b-PEO (SEO-2h). In a 50 mL round-bottom flask, SEO-ene (85 mg, 0.0061 mmol), thioglycerol (26.4 mg, 0.244 mmol), and AIBN (4 mg, 0.0244 mmol) were dissolved in 1.7 mL of anhydrous toluene under an Ar atmosphere. The reaction was carried out at 80 °C for 1.5 h. 1H NMR (500 MHz, CDCl3) δ ppm: 7.10−6.30 (b, n × 5H, −CH2CH(C6H5)), 3.64 (b, n × 4H of −OCH2CH2O− and 3H of thioglycerol), 2.70− 2.60 (m, 4H, −CH 2 SCH 2 −), 2.20−1.20 (b, n × 3H, −CH2CH(C6H5)). Preparation of Salt-Doped Polymers. Predetermined amounts of LiTFSI and polymers were mixed using methanol/benzene (50/50 vol %) mixtures and stirred overnight at room temperature. Solvents in the mixtures were slowly evaporated under an argon atmosphere until the quasi-dried samples were obtained, which were further exposed to vacuum for a week. In order to avoid water contamination of hygroscopic samples, all sample preparation and drying procedures were carried out in an argon-filled glovebox, equipped with oxygen sensor, moisture sensor, and vacuum oven. Small-Angle X-ray Scattering (SAXS) Experiments. Morphologies of end-functionalized polymers were investigated by synchrotron SAXS experiments using the 9A beamline at the Pohang Light Source (PLS). The wavelength (λ) of the incident X-ray beam was 0.118 nm (Δλ/λ = 10−4). An airtight aluminum sample cell having two Kapton windows was designed to prevent water contamination during the measurements. Two different sample-to-detector distances of 0.5 and

EXPERIMENTAL SECTION

Materials. A poly(styrene-b-ethylene oxide) (PS-b-PEO) block copolymer (7.4−6.5 kg/mol) was prepared by sequential anionic polymerization, following the procedures given elsewhere.18 In brief, the PS block was first synthesized in benzene at 40 °C with secbutyllithium (sec-BuLi) initiator. After 4 h of reaction, EO monomer was added to form hydroxyl-terminated PS. The tert-butylphosphazene base (tert-BuP4) was then added to the reaction mixtures as a catalyst under argon-filled glovebox, and the predetermined amount of EO monomer was transferred into the reactor. After 40 h of reaction, once the color of the reaction mixtures turns dark blue, a mixture of methanol and HCl was added to the reactor to terminate the reaction. The resultant polymer was purified by filtration and precipitation in methanol. Poly(ethylene glycol) methyl ether (PEG, Mn = 5 kg/mol), sodium hydride (NaH, 95%), allyl bromide (99%), thioglycolic acid (99%), mercaptosuccinic acid (97%), 1-thioglycerol (97%), 2,2′azobis(2-methylpropionitrile) (AIBN, 99%), and lithium bis(trifluoro3225

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prepared. As shown in Figure 1, the terminal −OH group in PEO of PS-b-PEO (7.4−6.5 kg/mol) was first modified by reaction with allyl bromide in the presence of NaH. Subsequent thiol−ene coupling reaction employing different thiolating agents, i.e., thioglycolic acid, mercaptosuccinic acid, or thioglycerol, completed the library of polymers bearing hydroxyl, allyl, carboxylic acid, diol, or dicarboxylic acid terminal groups, referred to as SEO-h (pristine polymer), SEO-ene, SEO-c, SEO-2h, and SEO-2c, respectively. Analogous reaction steps were also conducted for a PEO homopolymer (5.0 kg/mol) to prepare polymers PEO-h, PEO-ene, PEO-c, PEO-2h, and PEO-2c. It should be noted that the molar mass increments for PEO as a result of end-group modification were less than 0.19 kg/mol for all samples. Figure 2a shows 1H NMR spectra of SEO-ene, SEO-c, SEO2h, and SEO-2c, with the characteristic peaks assigned within. The disappearance of resonances at 5.94−5.88 and 5.29−5.16 ppm and the appearance of new resonances at around 3.30− 2.50 ppm for SEO-c, SEO-2h, and SEO-2c confirmed the successful thiol−ene reactions with various thiolating agents. The extent of end-group modification was determined by comparing the integral areas of peaks of sec-butyl group with those of the thiolated groups, which was >95% for all samples. For example, for SEO-c, the integral areas of peaks at 3.23 ppm (−SCH2COOH) and 2.78−2.75 ppm (−CH2SCH2COOH) were compared with those at 0.75−0.55 ppm (−CH(CH3)CH2CH3), yielding the extent of end-functionalization of 98%. The synthesis of end-functionalized SEO samples, without noticeable side reactions such as cross-linked polymer, was further confirmed using SEC, as presented in Figure 2b. The polydispersity index of pristine SEO (SEO-h) was 1.05, which was slightly increased to 1.06 with the attachment of terminal groups (SEO-c, SEO-2h, and SEO-2c). The molecular weights of SEO-h, SEO-c, SEO-2h, and SEO-2c determined with PS standards in THF were 14.0, 14.4, 14.2, and 14.6 kg/mol, respectively, which do not completely match with the actual molecular weight increase by terminal functional group. Alterations in polymer−polymer and polymer−column interactions as a result of end-group modification should be responsible for the observation. FT-IR spectra (Figure 2c) acquired at 22 °C in the 3700− 3100 cm−1 region showed that the IR peak intensity arising from O−H stretching in each sample is roughly in proportion to the number of terminal groups present. The CO peak in the 1750−1700 cm−1 range was also observed for SEO-c and SEO-2c samples, and its intensity was again found to be closely related to the number of −COOH terminal groups. FT-IR analysis will be discussed in more detail in a later section examining molecular interactions in end-functionalized polymers. Morphology and Viscoelastic Properties of EndFunctionalized SEO Block Copolymers. In the next step, we investigated the morphology of end-modified SEO samples. Figure 3 shows SAXS profiles of the prepared end-modified SEO samples, measured at 60 °C. These profiles were found to be essentially independent of temperature. The pristine polymer SEO-h, bearing a single −OH terminal group, exhibited a Bragg peak at q* = 0.363 nm−1, with no detectable long-range order, in good agreement with literature.36 Modification of the PEO terminal group in SEO with a −COOH group produced SEO-c with Bragg peaks in a 1q*:2q* ratio and a qualitatively similar q* value (domain spacing, d100 = 17.3 nm). These results indicate the develop-

1.5 m were used to cover wide range of scattering wave vector q (q = 4π sin(θ/2)/λ, where θ is the scattering angle). Differential Scanning Calorimetry (DSC). DSC thermograms of end-functionalized polymers were measured using a TA Instruments (model Q20). Approximately 5 mg of the samples was loaded in standard aluminum pan inside an argon-filled glovebox. An empty pan was used as a reference. Two different heating and cooling rates of 10 and 5 °C/min were employed to measure the melting and glass transition temperatures of the samples in the temperature range of −65 to 120 °C. Rheology. An Anton Paar MCR 302 (Graz, Austria) rheometer equipped with a parallel plate (8 mm diameter and 0.5 mm gap) was used to measure the dynamic storage and loss moduli of endfunctionalized polymers in strain-controlled mode. All measurements were performed in the linear viscoelastic regime with a small strain of 0.1% under a nitrogen atmosphere. Temperature scans were carried out at a rate of 1 °C/min and at a frequency of 0.5 rad/s. Frequency sweeps in the range of 0.1−100 rad/s were performed at 50 °C. Conductivity Measurements. In an argon-filled glovebox, the through-plane conductivities of LiTFSI-doped samples were measured using a potentiostat (VersaSTAT 3, Princeton Applied Research). A home-built two-electrode cell was employed, which consists of stainless steel blocking electrodes and 1 cm × 1 cm Pt working/ counter electrodes. The sample thickness was fixed at 200 μm by designing engraved counter electrode. Polarization Experiments. The lithium salt-doped samples were sandwiched between two lithium metal foils, and electrode polarization tests were carried out. Sample temperature was set at 60 °C, and current profiles were monitored for 1 h after applying ΔV of 0.1 V. All procedures were performed inside the argon-filled glovebox. Fourier Transform Infrared (FT-IR) Spectroscopy. FT-IR spectra of the samples were recorded using a Bruker Vertex 70 FTIR spectrometer at a constant temperature of 22 °C. For powder samples (high molecular weight), the measurments were conducted in attenuated total reflection mode where 32 accumulations were signalaveraged with a frequency resolution of 1 cm−1. For liquid samples (low molecular weight), transmission mode was used where 16 accumulations were signal-averaged with a frequency resolution of 4 cm−1. Self-Diffusion Coefficient Measurements. Self-diffusion coefficients of TFSI− anion in end-functionalized samples were measured using 19F pulsed gradient spin-echo NMR (Bruker AVB-300) in a 7.049 T superconducting magnet. The magnetogyric ratio was 4258 Hz/G. Time interval of the field gradient was 1−10 ms, and duration time between the leading edges of the two gradient pulses was in the range 100−300 ms. Sample loading was carried out inside an argonfilled glovebox and sealed with a lid to exclude the issue of water contamination.

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RESULTS AND DISCUSSION Synthesis of End-Functionalized PS-b-PEO Block Copolymers. End-functionalized PS-b-PEO block copolymers with different types and numbers of terminal groups were

Figure 1. Synthesis of end-functionalized PS-b-PEO block copolymers via thiol−ene click chemistry. 3226

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Figure 2. (a) 1H NMR spectra of SEO-ene, SEO-c, SEO-2h, and SEO-2c. (b) SEC traces and (c) FT-IR spectra of SEO-h (pristine SEO), SEO-c, SEO-2h, and SEO-2c. IR peaks of O−H stretching and CO stretching are assigned in (c).

results are interesting because the end-group concentration in the PEO employed in this study is less than 1 mol %. The modification of PEO chain terminal groups in SEO produced significant changes in the linear viscoelastic properties. Figure 4a shows the storage (G′) and loss moduli (G″) of the end-modified SEO samples, measured during cooling from 80 °C and subsequent heating at a fixed rate of 1 °C/min. The observed hysteresis indicates dissimilar crystallization kinetics of the end-modified PEO chains. By comparing the moduli determined over the heating and cooling cycles, it is possible to observe strongly enhanced storage and loss moduli at temperature above the melting of PEO (marked with dotted lines) in samples with attached terminal groups, i.e., G′ = 17 MPa (SEO-h), 35 MPa (SEO-c), 122 MPa (SEO-2h), and 121 MPa (SEO-2c). Samples SEO-2h and SEO-2c exhibited the highest moduli, and this result can be attributed to the advantages of their gyroid structure having cubic symmetry. It should be also noted here that the end-group modification of PEO homopolymer resulted in reduced G′ value regardless of the type of terminal group employed (Figure S1 of Supporting Information). Overall, these results lead us to conclude that the number of terminal groups in SEO exerts a tremendous influence on the mechanical properties. In Figure 4b, we show a direct comparison of moduli and viscoelastic properties for block copolymer SEO-2c and homopolymer PEO-2c, both possessing the same dicarboxylic acid terminal group. For PEO-2c, frequency sweeps at T = 323 K (the temperature above the melting of PEO) showed an interesting viscoelastic response of moduli, G′(ω) ∼ G″(ω) ∼

ment of ordered lamellar morphology. Compared to SEO-h, a considerable increase in scattering intensity at low q values was observed for SEO-c, indicating a heterogeneous distribution of ionic clusters or formation of aggregated morphology driven by terminal −COOH. Nevertheless, comprehensive understanding of such structure is not available at present. Incorporation of two end-functional groups in the PEO of SEO produced a well-defined gyroid structure for both SEO-2h and SEO-2c, with characteristic scattering peaks at √6q*, √8q*, √14q*, √16q*, √20q*, and √22q*. Such intriguing morphological transition was accompanied by noticeable increase in domain spacing (d211) to 18.4 nm for SEO-2h and 18.8 nm for SEO-2c. These changes indicate that the free volume in PEO in polymers bearing diol and dicarboxylic acid terminal groups increased. Overall, we were able to infer from these results that the crystallinity of PEO chains decreased considerably as a result of the terminal group incorporation, thereby increasing the free volume. It should be noted that the density of crystalline PEO is 1.21 g/cm3 while that of amorphous PEO is 1.12 g/cm3. DSC experiments confirmed that the terminal units in PEO chains act as crystal defects. The DSC thermograms (Figure 3 inset) display significantly lower values of latent heat of melting (ΔHm) at lower Tm for PEO in SEO-c, SEO-2h, and SEO-2c than that for SEO-h. The percentage crystallinities determined for PEO samples based on a value of ΔHm = 215.6 J/g for 100% crystalline PEO were 60.3%, 36.0%, 27.9%, and 31.8% for SEO-h, SEO-c, SEO-2h, and SEO-2c, respectively. These 3227

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dicarboxylic acid terminal group. The static dielectric constant (εS) was determined to be 34, 41, and 28 for PEO-c, PEO-2c, and PEO-2h, respectively. Given that the dielectric constant of PEO is known to be around 10, it is inferred that the end-group modification increases the dielectric constant of PEO. Ion Transport in End-Functionalized Polymer Electrolytes. In the next step, we investigated the ion transport properties of end-functionalized SEO samples containing lithium salt. Figure 5a shows the temperature-dependent ionic conductivity obtained using ac impedance spectroscopy after LiTFSI doping at r = 0.02 (r ≡ [Li+]/[EO]). The results clearly showed that end-group modification improves room temperature conductivity by orders of magnitude. The degree to which the PEO crystallinity was reduced was most pronounced for the materials incorporating the carboxylic acid terminal groups. Heating the samples resulted in qualitatively similar ionic conductivities for all four samples. This outcome is interesting because attachment of terminal groups increased the Tg value, i.e., −65 °C (SEO-h), −45 °C (SEO-c), −44 °C (SEO-2h), and −37 °C (SEO-2c) (DSC data are shown in Figure S3 of the Supporting Information). These results (Figure 5a) are quite remarkable, in particular, taking into consideration the fact that the storage moduli of SEO-2h and SEO-2c samples (Figure 4a) were found to be 3−7-fold higher compared to those of SEO-h. We noted that the morphologies of SEO-c, SEO-2h, and SEO-2c remained intact, while that of SEO-h adopted an ordered lamellar morphology at r = 0.02 as a result of the increased segregation strength between the salt-containing PEO and ionophobic PS phases.39,40 While the conductivity data for all samples converged to qualitatively similar values at high temperatures, a noticeable enhancement in lithium transference number (TLi+) was observed for the sample having terminal diol group. Figure 5b shows the TLi+ values of all samples determined at r = 0.02, quantified using electrode polarization experiments at 60 °C and a polarization voltage (ΔV) of 0.1 V using two Li metal electrodes. TLi+ value of 0.25 was determined for the pristine sample (terminal group: −OH), which is in good agreement with the values of conventional PEO−lithium salt complexes reported in the literature.41 The presence of carboxylic acid terminal groups did not enhance TLi+. In contrast, addition of the diol end-group almost doubled the value of TLi+ (0.48). The representative current profiles determined for samples bearing

Figure 3. SAXS profiles of SEO-h (pristine SEO), SEO-c, SEO-2h, and SEO-2c obtained at 60 °C. The inverted filled triangles (▼) in SEO-c indicate Bragg peaks at q* and 2q*. The inverted open triangles (▽) in SEO-2h and SEO-2c represent Bragg peaks at √6q*,√8q*,√14q*,√16q*,√20q*, and √22q*. The curvature modifications of SEO with attached terminal groups are schematically depicted in the figure. Inset DSC thermograms represent different degrees of PEO crystallinity in the end-modified SEO samples.

ω1/4, in the wide frequency regime. It is probably owing to the hydrogen-bonding interactions of the terminal dicarboxylic acid groups with ether oxygen atoms in melted PEO, yielding physical cross-links.38 At the same temperature, SEO-2c exhibited over 3 orders of magnitude higher moduli than PEO-2c, with relatively weak dependence on frequency (G′(ω) ∼ ω0.12, G″(ω) ∼ ω0.03). Such results are indicative of elastic behavior that is characteristic for cubic phases and the glassy state of the PS block. It should be noted here that the dielectric permittivity spectra of end-modified PEO (Figure S2 in Supporting Information) indicated that the strength of α-relaxation (PEO chain segmental motion) increases from diol to carboxylic acid and

Figure 4. (a) Storage (G′, filled symbols) and loss (G″, open symbols) moduli during cooling (blue) and subsequent heating (red) with a fixed rate of 1 °C/min at a frequency of 0.5 rad/s and a strain of 0.1% for SEO-h (pristine SEO), SEO-c, SEO-2h, and SEO-2c. Storage moduli of the samples after melting of PEO are marked with dotted lines. The inset schematics represent the type of self-assembled morphology of each sample. (b) G′ and G″ for SEO-2c and PEO-2c during frequency sweep at 50 °C. 3228

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Figure 5. (a) Temperature-dependent ionic conductivity of end-functionalized SEO electrolytes at r = 0.02. (b) Lithium transference number obtained using electrode polarization for end-functionalized samples at r = 0.02, T = 60 °C, and ΔV = 0.1 V. Representative current profiles are shown in the inset. (c) Temperature-dependent ionic conductivity of end-functionalized SEO electrolytes at r = 0.06. Solid lines represent the results obtained by fitting of conductivity data to the Vogel−Tammann−Fulcher (VTF) equation.

data for PEO-2h revealed an interesting red-shift (41 cm−1) and intensity enhancement in the band arising from O−H stretching. This shift was smaller than the values commonly reported for hydrogen-bonding interactions between interchain O−H groups, and therefore, it can be most likely attributed to intrachain hydrogen-bonding interactions.43 These results demonstrated that simply increasing the number of −OH terminal groups can produce dramatic changes in the chain conformations of the prepared samples (e.g., PEO-h vs PEO2h). These changes, in turn, played a crucial role in reducing PEO crystallinity, as confirmed by both DSC and rheology measurements. PEO-c and PEO-2c samples also exhibited IR peaks arising from O−H stretching. However, very broad and less intense peaks were observed in the 3000−3700 cm−1 region, indicating that the terminal carboxylic acid groups in PEO-c and PEO-2c actively participate in hydrogen-bonding interactions with the ether oxygen atoms present in the PEO backbone. Notably, we found that the FT-IR spectra of PEO-c and PEO-2c (Figure 6b) differ significantly in the region of 1850− 1600 cm−1, corresponding to CO stretching. While only a single peak was evident for PEO-2c, three peaks were observed for PEO-c. These differences indicate that the −COOH terminal group in PEO-c is involved in dimer formation mediated by hydrogen-bonding interactions and quadrupole interactions with neighboring chains. Such interactions, in contrast, are disfavored for PEO-2c, presumably due to the steric hindrance present at the end of the chains.44 Examination of the changes in the CO stretching in PEO-c and PEO-2c containing LiTFSI revealed the presence of a new peak in the low-frequency region of PEO-2c (Figure S6 in Supporting Information), analogous to that of neat PEO-c. This suggests that the pairing of terminal −COOH groups in PEO2c is mediated by their interactions with Li+ and is tied to low ionic conductivity at increased salt doping (Figure 5c). In the presence of lithium salt, the most pronounced changes in FT-IR spectra were associated with molecular interactions formed between the ether oxygen atoms and Li+ for all samples. The representative FT-IR spectrum obtained for PEO-c at r = 0.06 in the range of 1220−1040 cm−1 is shown in Figure 6c. A red-shift in the C−O peak (over 20 cm−1) arising as a consequence of the Li+ coordination is marked by arrows.45 Peaks associated with TFSI− anion are marked with inverted filled triangles.

−OH and −(OH)2 end groups over 1 h are shown in the inset of Figure 5b. The mechanism underlying these intriguing results will be discussed further in the following sections. Figure 5c shows conductivity data obtained at r = 0.06, revealing that all four samples adopt well-defined lamellar morphology regardless of the type of terminal group employed (SAXS profiles are shown in Figure S4 of the Supporting Information). DSC thermograms confirmed that the PEO in all four samples is amorphous at r = 0.06 (data not shown here). The samples equipped with carboxylic acid terminal groups exhibited the lowest ionic conductivity, indicating the slow segmental motion of PEO chains by internal dipole−dipole interactions. It is intriguing to note that the high conductivity observed for SEO-2h surpassed that of SEO-h at high temperatures. Further, the 2-fold enhancement in the TLi+ value (0.39) determined for samples with attached diol group was observed even at increased salt-doping (r = 0.06). This is markedly different from the low TLi+ value of ∼0.2 determined for other samples (data not shown here). Fitting of the conductivity data to the Vogel−Tammann− Fulcher (VTF) equation (the fitted lines are shown as solid lines in Figure 5c) allowed us to determine the potential barriers to ion conduction as 974, 1181, 1380, and 1227 K for SEO-h, SEO-c, SEO-2h, and SEO-2c, respectively. The increased activation energy seen for end-functionalized samples can be attributed to the Tg elevation by strong ion−dipole interactions, which lower ion mobility coupled to reduced chain mobility.42 Terminal-Group-Driven Inter- and Intramolecular Interactions in PEO Phases. In-depth study of the interand intramolecular interactions in PEO substituted with terminal groups was conducted using FT-IR spectroscopy. In order to amplify the signals from terminal groups, we synthesized a new set of end-modified PEO samples based on low molecular weight PEO (0.55 kg/mol) with increased end-group concentration of 8 mol %. The synthesized polymers were produced as clear liquids, and FT-IR spectra were recorded by sandwiching them between CaF2 windows. C−H stretching peak at around 2900 cm−1 was used as internal standard. We first investigated the influence of the type and number of terminal groups on the molecular interactions in PEO samples in the absence of lithium salt. Figure 6a shows the FT-IR spectra of the samples acquired at 22 °C in the 3700−2600 cm−1 region. In comparison to the spectrum of PEO-h, the IR 3229

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Figure 6. FT-IR spectra of (a) end-functionalized PEO in the frequency range of 3700−2600 cm−1 and (b) PEO-c and PEO-2c in the frequency range of 1850−1625 cm−1. Inverted arrows in (b) indicate dimer formation in PEO-c. (c) Comparison of FT-IR spectra obtained for PEO-c with and without LiTFSI-doping, highlighting Li+ coordination with ether oxygen atoms in the PEO backbone (inverted arrows). The inverted filled triangles in (c) show the IR peaks arising from the TFSI− anion. (d) FT-IR spectra of neat PEO-2h and PEO-2h with LiTFSI-doping (r = 0.06), indicating hydrogen-bonding interactions mediated by TFSI− anion and dipolar interaction between Li+ and the −OH terminal group. Relevant peaks in the wavenumber range 3700−2500 cm−1 are assigned in the figure. (e) Representative 19F NMR spectra of PEO-h and PEO-2h with LiTFSI-doping (r = 0.02), measured at 303 K. (f) I/I0 plots as a function of γ2g2δ2(Δ − δ/3) for PEO-h and PEO-2h at r = 0.02 and r = 0.06, measured at 303 K.

Table 1. Diffusion Coefficients of TFSI− Anion (DTFSI−) for PEO-h and PEO-2h at r = 0.02 and r = 0.06, Measured at 303 and 323 K

hydroxyl end groups than carboxylic acid-terminated analogues. Representative results obtained for PEO-2h are shown in Figure 6d, which reveals the broad and red-shifted character of the IR band arising from O−H stretching. Background subtraction using the spectrum of neat PEO-2h allowed the changes in the O−H stretching to be identified in both low(3332 cm−1) and high-frequency (3542 cm−1) regions. These results can be attributed to the hydrogen-bonding interactions between the TFSI− anion and the O−H group and the coordination of Li+ to the oxygen atom of the O−H group, respectively. The blue-shift in the OH band observed as a consequence of Li+ coordination was in good agreement with the estimated results obtained using ab initio calculations based on density functional theory performed with B3LYP exchange-correlation

DTFSI− (m2/s) 303 K r = 0.02 r = 0.06

PEO-h PEO-2h PEO-h PEO-2h

7.18 1.39 4.63 8.84

× × × ×

10−11 10−11 10−11 10−12

323 K 1.68 8.96 2.21 6.11

× × × ×

10−10 10−11 10−10 10−11

In the presence of LiTFSI, all samples exhibited, in addition to the C−O−C vibrations, hydrogen-bonding interactions between the TFSI− anion and the PEO terminal group. These interactions were more noticeable in samples incorporating 3230

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Figure 7. (a) Schematic drawings of the molecular interactions and PEO chain conformations of samples with attached terminal groups. (b) Molecular interactions in PEO-2c and PEO-2h in the presence of added lithium salt, displaying Li+ coordination with ether oxygen atoms and hydrogen-bonding interactions between the terminal groups and TFSI− anion for both samples. Dimer formation observed for PEO-2c is also depicted.

gradient pulse, and Δ is the duration of the time between the leading edges of gradient pulses. The I/I0 plots as a function of γ2g2δ2(Δ − δ/3) for PEO-h and PEO-2h at r = 0.02 and r = 0.06, measured at 303 K, are shown in Figure 6f. We see radically different slopes of the plots depending on the number of terminal group. The obtained DTFSI− values of the samples at 303 and 323 K are summarized in Table 1. It is evident that the diffusion of TFSI− anion in PEO-2h is 2−5 times slower than that in PEO-h, suggesting anion-stabilizing effects in diol-terminated samples through hydrogen-bonding interactions. These results suggest that the increased TLi+ observed with diol end group (Figure 5b) stems from the anion-stabilizing effect of the terminal group mediated by hydrogen-bonding interactions. This leads us to conclude that increasing the number of terminal group in PEO may be an effective way of achieving both high conductivity and high cation transference number in salt-doped PEO electrolytes. Work directed at examining whether SEO block copolymers bearing dendritic terminal groups can exhibit further improvements in conductivity and cation transference number is currently underway in our laboratory.

functional, as shown in Table S1 and Figure S5 of Supporting Information. Regarding hydrogen-bonding interactions of TFSI− anion and −OH terminal group, it is not yet clear whether O−H interacts with F atom of TFSI− or O atom of TFSI−. Given that SO in TFSI− anion is in form of S−O−, it is predictable to have favorable interactions between S−O− and partially positive H atom of O−H. In that case, O−H vibration should red-shift as reported for water/sulfate systems.46 This implies that the increase in the number of end functional group can contribute to the effective stabilization of anion of lithium salts, which can be confirmed by 19F NMR experiments. Figure 6e shows representative 19F NMR spectra of PEO-h and PEO-2h containing LiTFSI (r = 0.02), measured at 303 K. Diffusion coefficients of TFSI− anion (DTFSI−) at different temperatures were determined by 19F pulsed−field gradient spin echo NMR experiments by eq 1: 2 2 2 I = e−Dγ g δ (Δ− δ /3) I0

(1)

where I is the spin-echo signal intensity, I0 is the signal intensity with zero gradient, γ is the magnetogyric ratio, g is the magnitude of the gradient pulses, δ is the duration of the 3231

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ACKNOWLEDGMENTS This work was financially supported by the Global Frontier R&D program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea (2012-054173).

Representative images of chain conformations and molecular interactions in PEO samples with attached terminal groups are shown in Figure 7a, illustrating high crystallinity (PEO-h), dimer formation (PEO-c), and intramolecular hydrogenbonding interactions (PEO-2h). In the presence of lithium salt (Figure 7b), Li+ coordinates primarily with the ether oxygen atoms present in the PEO backbone, accompanied by hydrogen-bonding interactions between the lithium salt anion and the terminal group (regardless of the type of terminal group). The sample bearing diol terminal group exhibited higher conductivity and lithium transference number compared to those with dicarboxylic acid group, which disfavored quadrupole interactions.

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CONCLUSIONS Terminal group-driven modulation of self-assembly, linear viscoelastic properties, and ion transport properties of PS-bPEO block copolymers was investigated. Two important findings can be summarized from the present study. First, the inclusion of multiple end groups in the PEO chains of PS-bPEO generated a considerable increase in the PEO free volume and changes in the PEO chain conformation, thereby yielding cocontinuous and noncrystalline PEO phases. These changes exerted a tremendous influence on the room temperature conductivity (∼30-fold increase in ionic conductivity) and linear viscoelastic properties (3−7-fold higher storage moduli) compared to those of the pristine PS-b-PEO. The PS-b-PEO bearing a terminal diol group exhibited the highest ion transport efficiency over the entire temperature range of interest and, thus, demonstrated a notable potential for application as a viable dry polymer electrolyte. Second, while Li+ coordinated primarily with the ether oxygen atoms of the PEO backbone, irrespective of the type of terminal group attached, the hydrogen-bonding interactions formed between the lithium salt anion and the terminal groups improved the lithium transference number remarkably. Given that the low lithium transference number has been a fundamental shortcoming of PEO/salt electrolytes, the end-group chemistry with controlled modification density presents a desirable tool for the fabrication of fast rechargeable solid-state polymer electrolytes. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00249. Synthetic details, characterization of materials by rheology, dielectric permittivity spectra, DSC, SAXS, and DFT calculations, including Figures S1−S6 and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (M.J.P.). ORCID

Moon Jeong Park: 0000-0003-3280-6714 Author Contributions

H.Y.J. and P.M. contributed equally. Notes

The authors declare no competing financial interest. 3232

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