Ionic Liquid-Blended Films

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Cite This: Langmuir 2019, 35, 3733−3747

Control of Phase Separation in Polystyrene/Ionic Liquid-Blended Films by Polymer Brush-Grafted Particles Yoshikazu Yahata,† Keiji Kimura,† Yohei Nakanishi,† Shoko Marukane,‡ Takaya Sato,‡ Yoshinobu Tsujii,† and Kohji Ohno*,† †

Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Department of Material Engineering, Tsuruoka National College of Technology, 104 Sawada, Inooka, Tsuruoka 997-8511, Japan

‡

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S Supporting Information *

ABSTRACT: Immiscible composite materials with controlled phase-separated structures are important in areas ranging from catalysis to battery. We succeeded in controlling the phaseseparated structures of immiscible blends of polystyrene (PS) and two ionic liquids (ILs), namely, N,N-diethyl-N-(2methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI) and 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide, by adding precisely designed concentrated polymer brush-grafted (CPB-grafted) silica nanoparticles (CPB-SiPs) prepared by surface-initiated atom-transfer radical polymerization. We discuss relationships between chemical species and molecular weights of the CPB and phase-separated structures. When the CPB was composed of a PS homopolymer of an appropriate molecular weight, the IL phase formed a continuous structure and a quasi-solid-blended film was successfully fabricated because the CPB-SiPs were adsorbed at the PS/IL interface and prevented macroscopic phase separation. We propose that CPB-SiP adsorption and the fabrication of quasi-solid films are governed by the degree of penetration of the matrix PS chains into the CPB and deformability of the CPB-SiPs. We found that the DEME-TFSI domain size can be controlled by the CPB-SiP content and that only 1 wt % of the CPB-SiPs was needed to fabricate a quasisolid film. In addition, we investigated the ionic properties of the quasi-solid PS/DEME-TFSI-blended film. Owing to continuous ion channels composed only of DEME-TFSI, the film exhibited an ionic conductivity of 0.1 mS/cm, which is relatively high compared to previously reported quasi-solid electrolytes. Finally, we demonstrated that an electric double-layer capacitor fabricated using this film as the electrolyte exhibited high charge/discharge cycling stability and reversibility.

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INTRODUCTION Composites of different materials have attracted much attention because of their pivotal properties that combine the advantages of the constituents, while compensating their weaknesses. Controlling the phase-separated structures of composites of immiscible substances that include fluids is a significant challenge; these structures determine material properties such as chemical transport capabilities,1−3 mechanical properties,4,5 catalytic activities,6 electric conductivities,7−11 photovoltaics,12 and molecular encapsulation abilities.13 Phase-separated structures have been controlled using a variety of strategies, including the use of spinodal decomposition,1,2 particles,3,4,14−16 and templates formed by the selfassemblies of block copolymers5,10,12 and liquid crystals.7,8,13,17 In this study, we focused on controlling the phase-separated structure of an immiscible blend through the use of particles, and the control mechanism is described below. In an immiscible blend devoid of particles, the separated phases expand to eventually produce a macroscopically phaseseparated structure to reduce its interfacial area and surface free energy in the system. On the other hand, the addition of © 2019 American Chemical Society

particles with appropriate interfacial energies to an immiscible blend results in the adsorption of the particles at the interface between the separated phases, which arrests the phaseseparated structure once the interface is occupied by the particles. Therefore, the addition of particles to an immiscible blend prevents macroscopic phase separation. When particles are added to a phase-A/phase-B phase-separated system, particles with interfacial energies that satisfy eq 1 are adsorbed at the A/B interface. In addition, when a particle dispersed in phase-A is adsorbed at the A/B interface, the interfacial energy gained, ΔEA, is described by eq 2, which is derived by considering that parts of the interfaces between phase-A and the particle and between A and B disappear and a new interface between phase-B and the particle appears |σA/P − σB/P| < σA/B

(1)

Received: November 21, 2018 Revised: January 10, 2019 Published: January 24, 2019 3733

DOI: 10.1021/acs.langmuir.8b03891 Langmuir 2019, 35, 3733−3747

Article

Langmuir ΔEA = πRP 2/σA/B(σA/P − σB/P + σA/B)2

derivatives of poly(ethylene oxide),37−39,42,43 polycarbonate,39−41 polysiloxane,42,43 polyimine,44 polysulfide,45 and polyphosphazene46 led to increases in glass-transition temperature (Tg), resulting in decreases in ion mobility. Therefore, increase in the ionic conductivity is limited even when salt concentration is increased. Solutions or gels composed of polymers and ionic liquids (ILs), which are room-temperature molten salts, that are categorized in “quasi-solid” electrolytes in terms of their solid/liquid mixtures solve this problem because Tg decreases with increasing IL concentration. For example, mixtures of polypyridinium/pyridinium halides,47 polypyridinium/aluminum halides,48 poly(vinylidene fluoride-co-hexafluoropropylene), (PVFH)/1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI-CF3SO3),49 PVFH/1-ethyl-3methylimidazolium tetrafluoroborate (EMI-BF4),49 PVFH/1butyl-3-methylimidazolium hexafluorophosphate (BMI-PF6),50 PVFH/1-ethyl-3-n-propylimidazolium (or butyl-3-n-propylimidazolium) sulfonate,51 poly(2-hydroxyethyl methacrylate) (PHEMA)/EMI-BF4, PHEMA/1-butylpyridinium-BF4,52 and poly(methyl methacrylate) (PMMA) gel/EMI-bis(trifluoromethylsulfonyl)imide (TFSI)53 have been reported. However, it is difficult to enhance both ionic conductivities and mechanical strengths of these polymer/IL-miscible blends because of decrease in ionic carrier mobility that is coupled to the local mobility of site that interacts with the carrier in the polymer chain matrix. The introduction of IL-immiscible reinforcing fillers to these blends provides a solution to this problem. In immiscible polymer/IL blends, optimal carrierpath structures, including bicontinuous and columnar structures, are required for high electric conductivities and mechanical strengths. To date, a variety of approaches for optimizing path structures have been reported. For example, Hoarfrost and Segalman reported that quasi-solid electrolytes that have continuous ion-conducting channels were fabricated by selectively incorporating an IL into one phase of a microphase-separated block copolymer composed of IL-philic and IL-phobic blocks.10 Kato et al. fabricated one-dimensional ion-conductive films containing columnar ionic channels that were obtained through polymerization of an acrylate groupbearing IL that self-assembled into a columnar structure with the ionic components arranged inside the column.7,8 They also obtained continuous ionic channels through the self-assembly of ILs bearing ion-phobic alkyl chains.11 In addition to optimizing carrier-path structures, carrier mobilities require enhancing to achieve high ionic conductivities. Reducing the viscosity of the conductive phase is one strategy for improving carrier mobility, which is achieved through the introduction of a pure IL solely to the conductive phase. To introduce a pure IL into only the conductive phase, we adopted a new method for controlling the phase-separated structure of an immiscible polymer and the IL, which involves the addition of CPB-grafted particles prepared by surfaceinitiated atom-transfer radical polymerization (SI-ATRP). This method is advantageous since it introduces a conductive phase composed of a pure IL while allowing various combinations of the matrix polymer and IL to be used; these advantages are derived from the versatile design of the CPB. As immiscible polymer/IL combinations, we focused on polystyrene (PS)/ EMI-TFSI54,55 and PS/N,N-diethyl-N-(2-methoxyethyl)-Nmethylammonium bis(trifluoromethylsulfonyl)imide (DEMETFSI).56,57 DEME-TFSI has a wide electrochemical potential window and should be useful for electrical device applications.58 In addition, with the aim of developing particle design

(2)

where σA/P, σB/P, and σA/B represent the interfacial energies between the particle and phase-A, the particle and phase-B, and the two phases (A/B), respectively, and RP is the diameter of the particle.4,18−21 Therefore, precise control of these interfacial energies is required for particle adsorption at the interface and for control of the phase-separated structure. The particle addition gives more stable phase-separated structures than is achieved by the addition of a common surfactant.22 Control of the phase-separated structure of an immiscible blend with particles has attracted much attention for a long time; examples include the formation of Pickering emulsions23−26 and bicontinuous jammed emulsions, referred to as “bijels”,3 in liquid/liquid systems, and stabilized liquid droplets, referred to as “liquid marbles”, in liquid/air systems.27,28 In recent years, increasing attention has been paid to controlling the phase-separated structures of immiscible polymer systems with particles;15,16,29−31 in these studies, control was achieved by adjusting the quantity of the particles15,29 or the interactions between the particles and the matrix polymer.16,20,30,31 Concentrated polymer brush-grafted (CPB-grafted) particles, in other words, particles onto which polymers are densely grafted by surface-initiated living radical polymerization (SI-LRP), not only provide precise control of the chemical species and molecular weights of the CPB,32,33 but also impart steric repulsion between the CPB and other macromolecules (the size-exclusion effect); this repulsion is derived from a decrease in entropy resulting from graft polymer chain stretching associated with the penetration of the macromolecules into the CPB.34,35 Recently, Composto et al. have reported that the phase-separated structure of an immiscible polymer blend, namely, deuterated poly(methyl methacrylate)/poly(styrene-ran-acrylonitrile), was controlled through the addition of CPB-grafted particles; the effect of the molecular weight and the chain-end functional groups of the graft polymer on the phase-separated structure was also revealed.21 The size-exclusion effect was suggested to play an important role in controlling the phase-separated structure. In CPB-grafted spherical particles, the degree of interpenetration between the CPB and polymer chains of the matrix increases with the distance from the surface of the particle “core” because the effective graft density at the surface of the polymer brush is reduced owing to the curvature of the core surface.36 This means that the interfacial energy between the particles and the polymer matrix can be controlled by the molecular weight of the graft polymer. Therefore, the phase-separated structures of immiscible polymer blends with CPB-grafted particles are expected to be controlled by the chemical species, as well as the graft densities or molecular weights of the graft polymer. In spite of the above-mentioned advantages, few studies have systematically focused on the addition of CPBgrafted particles to immiscible polymer blends to reveal the relationship between graft polymer design and the phaseseparated structure. Meanwhile, solid electrolytes have attracted much attention due to their applications to all-solid batteries, which are expected to be safe in terms of spillability, burstability, and flammability. In particular, polymer-based solid electrolytes are important due to features that include flexibility, lightness, transparency, thin film-forming properties, and ease of molding. In early reported polymer-based solid electrolytes, high salt densities in miscible blends composed of salts and 3734


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Alternating current (AC)-impedance measurements were made using a multifrequency LCR meter (E4980A Precision LCR meter, Agilent Technology). The sample film (thickness = 55 μm) was cut into a circle (diameter = 18 mm) and loaded between two polished stainless steel disks (diameter = 15 mm) acting as ion-blocking electrodes using the Teflon sheet spacer (thickness = 105 μm). The measurements were carried out in an electrode cell in an oven chamber (SU-241, Espec) at various frequencies ranging from 20 Hz to 2 MHz and at various temperatures ranging from −30 to 120 °C. A 2032-type coin cell60 equipped with circular electrodes (diameter = 16 mm) was used to investigate the charge/discharge performance of an electric double-layer capacitor (EDLC), following a previous report.61 The electrodes were built up by the following method. The carbon paint, comprising carbonized palm shell charcoal (surface area = 2000 m2/g, average pore diameter = 20 nm, average particle size = 8 mm), acetylene black, and poly(vinylidene fluoride) (PVDF; Mw = 534 000, Sigma-Aldrich) in N-methyl-2-pyrrolidone (NMP), was coated by the screen-printing method using an HP-320 hand printer (Newlong Seimitsu Kogyo Co. Ltd.) with a thin metal mask (Bon Mark Co. Ltd.) onto one side of an aluminum oxide foil (thickness = 30 mm). The active electrode layer was dried at 80 °C for 1 h in an oven to remove NMP and moisture. The dried electrode was subsequently pressed in a roll press at 50 MPa and further dried at 100 °C in vacuum for 15 h. The electrode obtained in this way was composed of 90 wt % palm shell charcoal, 5 wt % acetylene black, and 5 wt % PVDF; the printed electrode was 65 mm thick. The activated carbon layers of both the electrodes were wetted with a small amount (5 μL) of DEME-TFSI. Synthesis of NBD-Labeled Silica Nanoparticles (SiPs). We chose 4-nitro-7-(3-(triethoxysilyl)propylamino)-2,1,3-benzoxadiazole (NBD-APTS) as the silane coupling agent, since it contains the fluorescent NBD moiety that is compatible with the 488 nm wavelength Ar laser used for excitation in the CLSM studies; this compound was synthesized by stirring a mixture of NBD-Cl (2.0 g) and APTS (4.4 g) in dry ethanol (100 mL) at room temperature for 24 h, following a previously reported procedure.62 The product was purified by silica gel flash chromatography with 2:1 hexane/ethyl acetate as the eluent. The NBD-labeled SiPs were prepared according to the method developed independently by Yokoi et al.63 and Tsapatsis et al.64,65 In a typical run, L-lysine (1.2 g) was dissolved in a mixture of water (1.1 L) and n-octane (58.5 g) at 60 °C. A solution of NBD-APTS (0.32 g) in TEOS (83.2 g) was added to this mixture, after which it was stirred at 60 °C for 20 h and allowed to stand at 100 °C for 20 h. The reaction mixture was subjected to dialysis against water, using a regenerated cellulose tube (Spectrum Laboratories Ltd., Spectra/Por6, molecular cutoff: 8000), for 1 week to remove unreacted compounds. Ethylene glycol (45 g) was added to the dialyzed solution, and the mixture was rotary-evaporated at 55 °C to remove water. Fixation of ATRP Initiator on the SiPs. (2-Bromo-2-methyl)propionyloxypropyltriethoxysilane (BPE), a silane coupling agent containing an ATRP initiating site, was synthesized following a previously reported procedure33 and used to immobilize ATRP initiator groups on the particle surfaces. A mixture of ethanol (250 mL) and BPE (7.5 g) was added dropwise to the NBD-labeled SiPs (solid, 5 g) in ethylene glycol over 2 h with vigorous stirring, to which a mixture of ethanol (250 mL) and ammonium hydroxide solution (16.5 g) was added dropwise over 2 h with vigorous stirring to prevent particle aggregation. The reaction mixture was continuously stirred at room temperature for 10 days. The ATRP initiator-fixed SiPs were collected by centrifugation (20 000 rpm, 2 h) and subsequently washed by consecutive centrifugation (20 000 rpm, 2 h) and redispersion in ethanol three times. Finally, N,Ndimethylformamide (DMF) (10 g) was added to the dispersion, and the mixture was rotary-evaporated to obtain an ATRP initiatorfixed SiP suspension in DMF to stock. SI-ATRP on the Initiator-Fixed SiPs. A mixture of the ATRP initiator-fixed SiPs in DMF containing a prescribed concentration of monomers, EBiB as a free ATRP initiator, and dNbpy was quickly added to a glass tube charged with a predetermined amount of CuCl.

guidelines for the fabrication of quasi-solid electrolytes, we elucidated relationships between the phase-separated structures and the chemical species and molecular weights of the graft polymer. Finally, we investigated the ionic conductivities of the resulting quasi-solid films and demonstrated their potential applications in electrical devices.

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EXPERIMENTAL SECTION

Materials. Tetraethyl orthosilicate (TEOS, 99%), ammonium hydroxide solution (NH3, 28%), methyl methacrylate (MMA, 98%), copper(I) chloride (CuCl, 99.9%), copper(II) dichloride (CuCl2, 99.9%), 4,4′-dinonyl-2,2′-dipyridyl (dNbpy, 97%), and 2,2-azobis(isobutyronitrile) (AIBN, 98%) were purchased from Wako Pure Chemicals, Osaka, Japan. 4-Chloro-7-nitro-2,1,3-benzoxadiazole (NBD-Cl, 98%), L-(+)-lysine, and ethyl 2-bromoisobutyrate (EBiB, 98%) were obtained from Tokyo Chemical Industry, Tokyo, Japan. 3Aminopropyltriethoxysilane (APTS, 98%), styrene (S, 99%), and ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na, 99%) were received from Nacalai Tesque Inc., Osaka, Japan. EMITFSI (98%) and DEME-TFSI were obtained from Kanto Chemical Inc., Tokyo, Japan. PS (weight-average molecular weight, Mw = 280 000 g/mol) was obtained from Sigma-Aldrich, St. Louis. Methacryloxyethyl thiocarbamoyl rhodamine B (PolyFluor 570) was purchased from Polysciences, Warrington. Water was purified by a Milli-Q system (Nihon Millipore Ltd.) to a specific resistivity of ca. 18 MΩ cm. MMA and S were purified by flash chromatography over activated neutral alumina. All other reagents were used as received from commercial sources. Measurements. Gel permeation chromatography (GPC) analyses using tetrahydrofuran (THF) as eluent at a flow rate of 0.8 mL/min were carried out at 40 °C on a Shodex GPC-101 high-speed liquid chromatography system equipped with a guard column (Shodex GPC KF-G), two 30 cm mixed columns (Shodex GPC LF-806, exclusion limit = 1.5 × 107), and a differential refractometer (Shodex RI-71S). PS standards were used to calibrate the GPC system. This system was used for characterization of PS and P(S-co-MMA). GPC analysis for characterization of poly(N,N-diethyl-N-(2-methacryloylethyl)-Nmethylammonium bis(trifluoromethylsulfonyl)imide) (PDEMMTFSI) was carried out using water/acetonitrile mixed solvent at a volume ratio of 1:1 containing 0.1 M sodium nitrate and 0.25 M acetic acid as eluent at a flow rate of 0.8 mL/min at 40 °C on a Shodex GPC-101 high-speed liquid chromatography system equipped with a guard column (Shodex GPC SB-G), two 30 cm mixed columns (Shodex GPC SB-804 HQ, exclusion limit = 1.0 × 106), a differential refractometer (Shodex RI-71S), and a multiangle light-scattering detector (MALLS, Wyatt Technology DAWN HELEOS).59 PMMA standards were used to calibrate the GPC system. For the determination of absolute molecular weights by the MALLS, the differential refractive index (dn/dc) of PDEMM-TFSI was measured to be 0.197 in water/acetonitrile mixed solvent at a volume ratio of 1:1 containing 0.1 M sodium nitrate and 0.25 M acetic acid using five PDEMM-TFSI concentrations of 0.36, 0.71, 1.1, 1.4, and 1.8 g/mL at a laser wavelength of 658 nm by the MALLS. 1H NMR (300 MHz) spectra were obtained on a JEOL/AL300 spectrometer. Transmission electron microscopy (TEM) observations were made on a JEOL transmission electron microscope, JEM-2100, operated at 200 kV. Thermogravimetric analysis (TGA) was conducted on a Shimadzu TGA-50 under argon (Ar) flow. Differential scanning calorimetric (DSC) analyses were made on a PerkinElmer Diamond DSC under Ar flow. Confocal laser scanning microscopy (CLSM) observations were made on an inverted-type microscope (LSM 5 PASCAL, Carl Zeiss) with 488 nm wavelength Ar laser, 543 nm wavelength He−Ne laser, and 20× objective (Plan Apochromat, Carl Zeiss) in fluorescence mode. The distance of the focal plane from interface between the sample and cover glass was about 30 μm. Scanning electron microscopy (SEM) was carried out on a Hitachi SEM (S3400N) operated at an accelerating voltage of 5.0 kV. Samples were mounted on an aluminum stub and sputter-coated with gold and palladium to minimize charging using a Hitachi ion sputter E-1010. 3735


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Langmuir Oxygen in the solution was removed by three freeze−pump−thaw cycles, and the tube was sealed under vacuum. Polymerization was carried out in a shaking oil bath (Personal H-10, TAITEC Corp.) thermostated at 100 °C and, after a prescribed time, quenched to room temperature. An aliquot of the reaction mixture was removed to determine the monomer conversion by 1H NMR spectroscopy and for the GPC determination of the molecular weight and the distribution of the EBiB-initiated polymer, which can be regarded as the same as that of the graft polymer.33 The remainder of the reaction mixture was diluted with chloroform and washed once with 2 wt % aqueous EDTA-2Na to remove the copper catalyst and subsequently three times with water. The chloroform solution was diluted with 1:1 THF/pentane to settle the CPB-grafted SiPs (CPB-SiPs) by lowering the density of the solution, after which they were centrifuged (20 000 rpm, 2 h) and redispersed in 1:1 THF/pentane; this procedure was repeated three times. Finally, the THF/pentane CPB-SiP dispersion was solvent-exchanged with THF by repeated redispersion/ centrifugation (20 000 rpm, 2 h, three times) to obtain the THF stock suspension. The amount of graft polymer was determined by subjecting the CPB-SiPs to TGA following evaporation of the solvent. In a typical run, copolymerization of S and MMA was carried out at 100 °C for 48 h with the starting materials S (0.911 g, 8.75 mmol), MMA (0.125 g, 1.25 mmol), EBiB (1.95 mg, 0.01 mmol), CuCl (4.95 mg, 0.05 mmol), dNbpy (40.9 mg, 0.1 mmol), initiator-fixed SiPs (20.0 mg), and DMF (22.8 mg). This gave a monomer conversion of 53% and a free polymer with Mn = 49 200 g/mol and Mw/Mn = 1.28, where Mn and Mw/Mn are the number-average molecular weights and polydispersity index, respectively. The ratio of S and MMA units in the graft polymer was estimated to be 94:6 from the peak area ratio of 1 H NMR spectrum. The graft density was calculated to be 0.38 chains/nm2 from the amount of the graft polymer measured by the TGA and its Mn. A list of CPB-SiPs used in this report is shown in Table 1. Preparation of the Matrix PS. The matrix PS was prepared by 9:1 (w/w) mixture of the commercially available PS (Mw = 280 000 g/mol) and rhodamine B-labeled PS (Mw = 305 000 g/mol). The rhodamine B-labeled PS was synthesized by free-radical copolymerization of S (3.12 g, 30 mmol) and PolyFluor 570 (18.9 mg, 0.03 mmol) using AIBN (2.46 mg, 0.015 mmol) as the radical initiator in dimethyl sulfoxide (0.244 g) at 60 °C for 18 h and then purified through reprecipitation in methanol three times. Rhodamine B is compatible with the 543 nm wavelength He−Ne laser used for the excitation in the CLSM studies. Preparation of PS/IL-Blended Films. The blended films were prepared by drop-casting homogeneous mixtures of prescribed amounts of the matrix PS, IL (EMI-TFSI or DEME-TFSI), CPBSiPs, and THF (90 wt %) onto circular glass substrates. The matrix PS and the IL phase-separated during THF evaporation. For ACimpedance measurements and capacitance testing, 76.2 mm-circular films with thickness of 55 μm were formed using 5.0 g of the mixture and appropriately sized (76.2 mm diameter) glass slides. Circular films with diameter of 15 mm and thickness of 100 μm were used for SEM, CLSM, and DSC; these films were formed using 0.20 g of the mixture and 15.0 mm diameter circular glass slides. Preparation of Amphipathic Block Copolymer. A polymerizable IL DEMM-TFSI59 was used to prepare a block copolymer PSb-P(DEMM-TFSI) composed of a PS block immiscible in DEMETFSI and a P(DEME-TFSI) block miscible in DEME-TFSI, through ATRP as described below. P(DEMM-TFSI) homopolymer was prepared by ATRP at 70 °C for 18.5 h with the starting materials DEMM-TFSI (150 g, 0.31 mol), EBiB (1.10 g, 5.68 mmol), CuCl (0.450 g, 4.54 mmol), CuCl2 (0.153 g, 1.14 mmol), dNbpy (1.95 mg, 12.5 mmol), and acetonitrile (300 g). The resulting polymer was purified by washing with EDTA-Na once and water three times, followed by precipitation in hexane three times. This gave P(DEMMTFSI) with Mn = 31 400 g/mol and Mw/Mn = 1.05. The P(DEMMTFSI) precursor was used for the subsequent block copolymerization with the starting materials S (1.04 g, 10 mmol), PDEMM-TFSI (0.628 g, 0.02 mmol), CuCl (19.8 mg, 0.2 mmol), dNbpy (0.164 g, 0.4 mmol), and DMF (1.12 g) at 100 °C for 18.5 h. The resultant

Table 1. List of CPB-SiPs Prepared in this Report sample name

copolymer ratio (S/MMA)

Mn (g/mol)

Mw/Mn

graft density (chains/nm2)

A1a A2b AB3c B1d B2e B4f B5g

75:25 94:6 100:0 100:0 100:0 100:0 100:0

39 800 48 500 42 300 3100 16 100 66 800 93 900

1.41 1.28 1.24 1.40 1.30 1.32 1.39

0.32 0.38 0.30 0.26 0.28 0.32 0.30

a Copolymerization was carried out at 100 °C for 48 h with the starting materials: [S]0/[MMA]0/[free ATRP initiator EBiB]0/ [CuCl]0/[dNbpy]0 = 750:250:1:5:10 with ATRP initiator-fixed SiPs (2 wt %) and DMF (2 wt %). This gave a monomer conversion of 43%. bCopolymerization was carried out at 100 °C for 48 h with the starting materials: [S]0/[MMA]0/[free ATRP initiator EBiB]0/ [CuCl]0/[dNbpy]0 = 875:125:1:5:10 with ATRP initiator-fixed SiPs (2 wt %) and DMF (2 wt %). This gave a monomer conversion of 53%. cPolymerization was carried out at 100 °C for 10 h with the starting materials: [S]0/[free ATRP initiator EBiB]0/[CuCl]0/ [dNbpy]0 = 200:1:1:2 with ATRP initiator-fixed SiPs (2 wt %) and DMF (10 wt %). This gave a monomer conversion of 17%. d Polymerization was carried out at 100 °C for 36 h with the starting materials: [S]0/[free ATRP initiator EBiB]0/[CuCl]0/[dNbpy]0 = 200:1:1:2 with ATRP initiator-fixed SiPs (2 wt %) and DMF (10 wt %). This gave a monomer conversion of 89%. ePolymerization was carried out at 100 °C for 18 h with the starting materials: [S]0/[free ATRP initiator EBiB]0/[CuCl]0/[dNbpy]0 = 1000:1:5:10 with ATRP initiator-fixed SiPs (2 wt %) and DMF (10 wt %). This gave a monomer conversion of 48%. fPolymerization was carried out at 100 °C for 33 h with the starting materials: [S]0/[free ATRP initiator EBiB]0/[CuCl]0/[dNbpy]0 = 1000:1:5:10 with ATRP initiator-fixed SiPs (2 wt %) and DMF (10 wt %). This gave a monomer conversion of 77%. gPolymerization was carried out at 100 °C for 33 h with the starting materials: [S]0/[free ATRP initiator EBiB]0/[CuCl]0/ [dNbpy]0 = 2000:1:10:20 with ATRP initiator-fixed SiPs (2 wt %) and DMF (10 wt %). This gave a monomer conversion of 58%. The reactivity ratio of S and MMA is almost the same so that the copolymers produced here do not suffer from compositional drift.

polymer was purified in the same way as PDEMM-TFSI. The Mn and monomer composition of the PS-b-P(DEMM-TFSI) were determined to be 53 100 g/mol and 76:24 of PS/P(DEMM-TFSI), respectively, by 1H NMR spectroscopy.

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RESULTS AND DISCUSSION CPB-SiP Design. In this study, we used seven CPB-SiPs composed of graft polymers with different chemical species (A1, A2, and AB3) and Mn (B1, B2, AB3, B4, and B5), as summarized in Table 1. To investigate the relationship between graft polymer chemical species and the phaseseparated structure, we introduced random S/MMA copolymers (PS-r-PMMA) with a variety of S/MMA copolymer ratios and almost identical Mn’s as graft polymers (A1, A2, and AB3) to tune the interfacial energies between the matrix PS and the CPB-SiPs and between the ILs and the CPB-SiPs (σA/P and σB/P in eqs 1 and 2, respectively), since PS is insoluble in the EMI-TFSI and DEME-TFSI ILs, while PMMA is soluble.56 To investigate the relationship between the Mn’s of the graft polymer and phase-separated structure, we introduced PS homopolymers with a variety of Mn’s as graft polymers (B1, B2, AB3, B4, and B5). To determine the location of the CPB-SiPs in the blended films by CLSM, we used an Ar laser operating at an excitation wavelength of 488 nm, which required the core SiPs to be labeled with the NBD fluorescent dye. We 3736


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confirmed whether the CPB-SiPs prevent macroscopic phase separation by comparing the appearance of a PS (45 wt %)/DEME-TFSI (50 wt %) + AB3 (5 wt %)-blended film to that of a PS (50 wt %)/DEME-TFSI (50 wt %)-blended film devoid of AB3, photographic images of which are shown in Figure 2. The addition of AB3 led to a solidlike film devoid of holes, cracks, and DEME-TFSI spillage. On the other hand, the film exhibited holes and DEME-TFSI spillage, a result of macroscopic phase separation, in the absence of AB3, which indicates that addition of the CPB-SiPs is required to fabricate a quasi-solid film. To confirm briefly whether additives other than particles can prevent macroscopic phase separation, an amphipathic polymer (5 wt %), as an example of an additive that acts as a surfactant and prevents macroscopic phase separation in immiscible liquid/liquid systems, was added to PS (45 wt %)/DEME-TFSI (50 wt %) to form a blended film. As an amphipathic polymer, we chose PS-b-P(DEMM-TFSI), which consists of an IL-phobic PS block and an IL-philic P(DEMMTFSI) block. Figure 2c shows a photographic image of the resulting film. As was observed for the film devoid of AB3, this film macroscopically separated into solid and liquid phases, which indicates that it is not easy to control phase-separated structures in the present system using amphipathic polymers. Effect of THF Evaporation Rate on the PhaseSeparated Structure. In this study, phase separation occurred during the evaporation of THF; therefore, the effect of the THF evaporation rate requires consideration. In this method, the viscosity of the system increases as the solvent evaporates, which decreases the diffusion rate of the matrix PS, IL, and CPB-SiPs. Therefore, if the evaporation rate is too high, the phase-separated structure and the CPB-SiP locations are determined kinetically. To discuss the relationship between CPB-SiP design and the phase-separated structure, the evaporation rate should be sufficiently low to allow time for the matrix PS, ILs, and CPB-SiPs to migrate to their thermodynamically most stable locations. To optimize the evaporation rate, we used SEM to compare the phaseseparated structures of a PS (45 wt %)/DEME-TFSI (50 wt %) + AB3 (5 wt %)-blended film formed by fast (5 min) evaporation in open air with that formed by the slow (over 48 h) evaporation of THF in a Petri dish filled with THF vapor. Figure 3 displays cross-sectional and surface SEM images of the resulting films following freeze-fracturing. Prior to SEM, each film was immersed in hydrochloric acid to selectively remove the DEME-TFSI phase; consequently, the locations of the DEME-TFSI phase are observed as holes in these images.

confirmed sizes, shapes, and uniformity of the core NBDlabeled SiPs by TEM. To prepare a monolayer of SiPs onto a TEM grid, a droplet of a SiP suspension in DMF was deposited onto the grid, followed by DMF evaporation in a vacuum oven at 60 °C (note that DMF was used to ensure the high dispersibility of the particles). Figure 1 shows a TEM image of

Figure 1. Transmission electron micrograph of core NBD-labeled silica nanoparticles (scale bar: 50 nm).

the prepared SiP film, which reveals that the core SiPs, shown as dark circles, have uniform spherical shapes and sizes, with an average diameter of 15 nm. All CPB-SiPs had graft densities (σ’s) in the 0.26−0.38 chains/nm2 range, as calculated by eq 3 σ = (w/M n)A v /((πdc 2))

(3)

where Av is Avogadro’s number, dc is the diameter of the SiP core (15 nm), and w is the mass of the polymer determined by TGA of the CPB-SiPs. For this calculation, the SiP density was set to 1.9 g/cm3. Preparation of the PS/IL-Blended Films Containing CPB-SiPs. As described in Experimental Section, the blended films were prepared by drop-casting homogeneous mixtures of THF, matrix PS, IL (EMI-TFSI or DEME-TFSI), and CPBSiPs onto circular glass slides. To observe the phase-separated structure in the blended films by CLSM, the matrix PS was labeled with the rhodamine B fluorescent dye, which was excited using a He−Ne laser operating at 543 nm. Effect of the Addition of CPB-SiPs on the Blended Films. As mentioned in Introduction, an immiscible blend devoid of additives phase-separates macroscopically. We first

Figure 2. Photographic images of blended films of PS (45 wt %)/DEME-TFSI (50 wt %) (a) with AB3 (5 wt %), (b) without AB3, and (c) with the PS-b-P(DEMM-TFSI) amphipathic polymer (5 wt %) (scale bar: 5 mm). The image in (a) is the same as the images shown in Figures 4c, 7c, and 10a. 3737


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Figure 3. Cross-sectional SEM images of the surface, and SEM images of the air and glass interfaces of freeze-fractured blended films of PS (45 wt %)/DEME-TFSI (50 wt %) with AB3 (5 wt %) formed by (a) fast THF evaporation (over 5 min) in open air and (b) slow THF evaporation (over 48 h) of a mixture of the matrix PS (4.5 wt %), DEME-TFSI (5 wt %), AB3 (0.5 wt %), and THF (90 wt %) dropped on a glass slide. The DEMETFSI phase was selectively removed by hydrochloric acid (scale bars: 50 μm (cross-sectional images) and 30 μm (interfaces)).

Figure 4. Photographic images of blended films of PS (45 wt %)/DEME-TFSI (50 wt %) with (a) A1 (5 wt %), (b) A2 (5 wt %), and (c) AB3 (5 wt %) (scale bar: 5 mm). The image in (c) is the same as the images shown in Figures 2a, 7c, and 10a.

toward both the air and glass interfaces. This homogeneous structure suggests that the rate of evaporation of THF is approximately the same throughout the film during the slow evaporation process. In addition, this structure is expected to be useful for electrolytes in electrical devices, including capacitors and batteries because ionic carriers can pass through the film. Therefore, we adopted this slow evaporation procedure during the preparation of the films used in the following discussion. Relationship between Graft Polymer Chemical Species and the Phase-Separated Structure. Regulating the

The results reveal that fast evaporation provided a heterogeneous phase-separated structure in which a skin layer and small DEME-TFSI domains were formed close to the air interface, with large domains formed near the glass interface. The differences in the sizes of the domains near the air and glass interfaces seem to be due to difference in evaporation rates at these interfaces. Therefore, this heterogeneous structure is consistent with a kinetically determined phase-separated structure. On the other hand, slow evaporation provided a homogeneous phase-separated structure in which the matrix PS and the DEME-TFSI phases appeared to be continuous 3738


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Figure 5. CLSM images of blended films of PS (45 wt %)/DEME-TFSI (50 wt %) with (a) A1 (5 wt %), (b) A2 (5 wt %), and (c) AB3 (5 wt %) (scale bar: 100 μm). Red, black, and green correspond to the matrix PS, the DEME-TFSI phase, and the CPB-SiPs. The image in (c) is the same as the images shown in Figures 8c and 11a.

Figure 6. CLSM images of mixtures of PS, DEME-TFSI, and THF with (a) A2 and (b) AB3 during solvent evaporation; the weight ratio of PS/ DEME-TFSI/THF/(A2 or AB3) of the starting mixture was 4:5:90:1 (scale bar: 100 μm). The left and right images in each panel were obtained using Ar and He−Ne lasers, respectively. Red, black, and green correspond to the matrix PSt/THF phase, the DEME-TFSI/THF phase, and CPBSiPs. The image in (b) is the same as that shown in Figure 9c.

chemical species in the CPB-SiP graft polymer is one of the most promising methods for modifying the interfacial energies between the matrix PS and CPB-SiPs and between the DEMETFSI and CPB-SiPs to satisfy the conditions for CPB-SiP adsorption at the PS/DEME-TFSI interface, as described by eq 1. Therefore, we introduced PS-r-PMMAs as graft polymers with a variety of S/MMA copolymer ratios and investigated the relationship between these ratios and the phase-separated structures in the PS/DEME-TFSI system. Figure 4 shows photographic images of blended films composed of PS (45 wt %)/DEME-TFSI (50 wt %) and A1 (5 wt %), A2 (5 wt %), and AB3 (5 wt %). These reveal that a solidlike film devoid of holes, cracks, and DEME-TFSI spillage was only formed when the PS homopolymer was used as the graft polymer (AB3 was added); when the S/MMA copolymer ratios were 75:25 and 94:6 (A1 or A2 were added, respectively), holes and cracks were observed in the formed films, accompanied by DEMETFSI spillage. Figure 5 displays CLSM images of the same films, which reveal that the CPB-SiPs (shown as green dots) are located at the interface between the matrix PS phase (shown in red) and the DEME-TFSI domains (shown in black) only when the PS homopolymer was used as the graft polymer (AB3 was added). In addition, the matrix PS phase formed a continuous structure, and the spherical DEME-TFSI domains interconnected each other (open-cell structure). On the other hand, the images reveal flocculated CPB-SiPs in the DEME-TFSI phase (aggregated green dots located in the black region) with large DEME-TFSI domains, when the S/MMA copolymer ratios were 75:25 and 94:6 (A1 and A2 were added, respectively). On the basis of these results, we proposed that the size-exclusion effect of the CPB against the matrix PS

chains enhances the interfacial energy between the matrix PS and the CPB-SiPs strongly enough to drive the CPB-SiPs to the PS/IL interface, even though the graft and the matrix polymer chemical species are the same. Additional increases in interfacial energy facilitated through the introduction of MMA units into the graft polymer should result in particles located in the DEME-TFSI phase. To evaluate this hypothesis, we confirmed differences in the compatibilities of A2 and AB3 in the matrix PS and DEMETFSI phases throughout film formation using a PS/DEMETFSI system and determined the locations of A2 and AB3 by CLSM during solvent evaporation. We simultaneously obtained two images from a single location in each sample through the use of both Ar and He−Ne lasers, and the resulting images are displayed in Figure 6. In the Ar laser images, the PS/THF (shown in red) and DEME-TFSI/THF (shown in black) phases appear to form “sea−island” structures, in which PS/THF and DEME-TFSI/THF phases correspond to seas and islands, although accurate phase shapes were not observed due to system drift. In the images obtained using the He−Ne laser, the CPB-SiPs (shown as green dots) occupied the seas when AB3 was added, but islands when A2 was added. This result suggests that AB3 migrated to the interface from the PS/THF phase through the size-exclusion effect of the CPB against the matrix PS chains. On the other hand, A2 is also proposed to be located in the DEME-TFSI/ THF phase during the evaporation process because the introduction of the MMA units in the graft polymer increased the interfacial energies between the matrix PS and the CPBSiPs too much for CPB-SiPs to be adsorbed at the PS/DEMETFSI interface and allowed CPB-SiPs to be located in the 3739


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Figure 7. Photographic images of blended films of PS (45 wt %)/DEME-TFSI (50 wt %) with (a) B1 (5 wt %), (b) B2 (5 wt %), (c) AB3 (5 wt %), (d) B4 (5 wt %), and (e) B5 (5 wt %) (scale bar: 5 mm). The image in (c) is the same as the images shown in Figures 2a, 4c, and 10a.

Figure 8. CLSM images of blended films of PS (45 wt %)/DEME-TFSI (50 wt %) with (a) B1 (5 wt %), (b) B2 (5 wt %), (c) AB3 (5 wt %), (d) B4 (5 wt %), and (e) B5 (5 wt %) (scale bar: 100 μm). Red, black, and green correspond to the matrix PS, the DEME-TFSI phase, and the CPBSiPs. The image in (c) is the same as the images shown in Figures 5c and 11a.

the CPB-SiPs can be controlled by the Mn of the graft polymer. In addition, adsorption of the CPB-SiPs at the PS/IL interface and the formation of a quasi-solid film were achieved through the use of a PS homopolymer as the graft polymer. Hence, we introduced PS homopolymers with varying Mn’s, as graft polymers, to investigate the relationship between the Mn and the phase-separated structures in the PS/DEME-TFSI system. Figure 7 displays the photographic images of blended films of PS (45 wt %)/DEME-TFSI (50 wt %) with B1 (5 wt %), B2 (5 wt %), AB3 (5 wt %), B4 (5 wt %), and B5 (5 wt %); these images reveal that systems with Mn’s of 42 300 and 66 800 g/

DEME-TFSI phase rather than at the PS/DEME-TFSI interface. These results suggest that CPB-SiPs composed of a PS homopolymer, as the graft polymer, are required for adsorption at the PS/DEME-TFSI interface and the fabrication of a quasi-solid film. Relationship between Graft Polymer Mn’s and PhaseSeparated Structure. As described in Introduction, owing to curvature of the core-SiP surface, the “effective” graft density and the size-exclusion effect of the CPB against the matrix polymer decrease as the Mn of the graft polymer increases.36 Therefore, the interfacial energy between the matrix PS and 3740


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Figure 9. CLSM images of mixtures of PS, DEME-TFSI, and THF with (a) B1, (b) B2, (c) AB3, (d) B4, and (e) B5 during solvent evaporation; the weight ratio of PS/DEME-TFSI/THF/(B1, B2, AB3, B4, or B5) of the starting mixture was 4:5:90:1 (scale bar: 100 μm). The left and right images in each panel were obtained using Ar and He−Ne lasers, respectively. Red, black, and green correspond to the matrix PS/THF phase, the DEME-TFSI/THF phase, and the CPB-SiPs. The image in (c) is the same as that shown in Figure 6b.

mol (when AB3 or B4 was added) formed solidlike films devoid of holes, cracks, and DEME-TFSI spillage; however, when the Mn’s were too low or too high (i.e., B1, B2, or B5 was added), holes, cracks, and DEME-TFSI spillages from the films were observed. Figure 8 displays the CLSM images of the same films, which reveal that the highest number of CPB-SiPs were located at the PS/DEME-TFSI interface (green dots located at the interface between the red and black regions) and the smallest DEME-TFSI domains (shown in black) were formed when the Mn was 42 300 g/mol (AB3 was added). On the other hand, Mn’s lower or higher than 42 300 g/mol (when B1, B2, B4, or B5 was added) led to fewer CPB-SiPs located at the PS/DEME-TFSI interface and larger DEME-TFSI domains. The CPB-SiPs that did not adsorb at the interface aggregated in the DEME-TFSI domain (aggregated green dots in the black regions rather than at the red/black interfaces) at lower Mn’s, but they were located in the PS phase (green dots located in the red regions rather than at the red/black interfaces and appeared to be yellow dots through overlap with the red region) when the Mn’s were higher. As a result of these observations, the following hypothesis is proposed. When the Mn is 42 300 g/mol, the interfacial energy between the matrix

PS and the CPB-SiPs is optimal and meets the conditions required for CPB-SiP adsorption at the PS/IL interface, as described by eq 1, resulting in a large gain in the free energy of the system as the CPB-SiPs become adsorbed at the interface, as described by eq 2. When the Mn is too low, the interfacial energy is too high to satisfy these conditions, resulting in a small gain in interfacial energy due to excessive effective graft density at the surface of the polymer brush owing to the curvature of surface of the core SiPs; consequently, the sizeexclusion effect of the polymer brush against the matrix PS chains is too strong. This drives the CPB-SiPs into the DEMETFSI phase rather than to the PS/IL interface. It is remarkable that despite the CPB and the matrix polymer being composed of the same chemical species (PS), the CPB-SiPs aggregate in the PS-immiscible DEME-TFSI phase. When the Mn is too high, the interfacial energy is too low to meet the abovementioned conditions, leading to a small gain in interfacial energy because of the ineffective size-exclusion effect of the polymer brush against the matrix PS chains, a result of an effective graft density that is too low. This drives the CPB-SiPs into the matrix PS phase rather than to the PS/IL interface. 3741


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Figure 10. Photographic images of blended films of (a) PS (45 wt %)/DEME-TFSI (50 wt %) with AB3 (5 wt %), (b) PS (47 wt %)/DEME-TFSI (50 wt %) with AB3 (3 wt %), and (c) PS (49 wt %)/DEME-TFSI (50 wt %) with AB3 (1 wt %) (scale bar: 5 mm). The image in (a) is the same as the images shown in Figures 2a, 4c, and 7c.

Figure 11. CLSM images of blended films of (a) PS (45 wt %)/DEME-TFSI (50 wt %) with AB3 (5 wt %), (b) PS (47 wt %)/DEME-TFSI (50 wt %) with AB3 (3 wt %), and (c) PS (49 wt %)/DEME-TFSI (50 wt %) with AB3 (1 wt %) (scale bar; 100 μm). Red, black, and green correspond to the matrix PS, the DEME-TFSI phase, and the CPB-SiPs. The image in panel (a) is the same as the images shown in Figures 5c and 8c.

IL interface; therefore, they have a strong tendency to adsorb at the interface. Not only do the sizes of the CPB-SiPs play important roles in positioning the CPB-SiPs but their deformabilities are also important. Richtering et al. reported that microgel adsorption at the oil/water interface of an oil/ water system with poly(N-isopropylacrylamide) microgels not only depends on the oil/microgel and water/microgel interfacial energies but also on the microgel swelling, elasticity, and deformability.66 Compared with rigid particles, deformable microgels have a strong tendency to adsorb at the interface because they can spread themselves along the interface and cover a large interfacial area. By analogy, the CPB-SiPs with long graft polymer chains in the current study are assumed to be more deformable because of their low effective graft densities; therefore, they have strong tendencies to adsorb at interfaces compared to short graft polymer CPB-SiPs. Consequently, we infer that the optimal Mn’s required to form quasi-solid blended films in the PS/DEME-TFSI system are determined by a balance between the size-exclusion and deformability effects of the polymer brush and the overall size of the CPB-SiPs. Relationship between CPB-SiP Quantity and the Phase-Separated Structure. When the interface between the separated phases is occupied by particles, the width of the interfacial area is determined by the quantity of particles. In other words, the separated domains become larger with decreasing amounts of interface-adsorbed particles. Therefore, we investigated the minimum amount of AB3 required to produce a quasi-solid film. We compared the appearances of blended films of PS (45 wt %)/DEME-TFSI (50 wt %) with AB3 (5 wt %), PS (47 wt %)/DEME-TFSI (50 wt %) with AB3 (3 wt %), and PS (49 wt %)/DEME-TFSI (50 wt %) with AB3 (1 wt %). Figures 10 and 11 show photographic and

We used a PS/DEME-TFSI system to evaluate this hypothesis by confirming differences in the compatibilities of B1, B2, AB3, B4, and B5 in the matrix PS and DEME-TFSI phases during film preparation by solvent evaporation using the method described above; we examined the locations of B1, B2, AB3, B4, and B5 by CLSM during solvent evaporation, the results of which are displayed in Figure 9. In the Ar laser images, the PS/THF phase (shown in red) and the DEMETFSI/THF phase (shown in black) form sea−island structures, in which PS/THF and DEME-TFSI/THF phases correspond to seas and islands. In the He−Ne laser images, the CPB-SiPs (shown as green dots) occupy the islands when B1 or B2 was added, but occupy the seas when AB3, B4, or B5 was added. These results suggest that AB3, B4, and B5 were driven to the PS/DEME-TFSI interface from the PS/THF phase by the sizeexclusion effect of the CPB against the matrix PS chains. However, the interfacial energies between B4 and B5 and the matrix PS are proposed to be too low for their adsorption at the PS/DEME-TFSI interface, and they were highly compatible with the PS phase owing to an ineffective sizeexclusion effect between the polymer brush and matrix PS chains. On the other hand, B1 and B2 were likely to have been located in the DEME-TFSI phase during solvent evaporation because interfacial energies between B1 and B2 and the matrix PS were higher than those with DEME-TFSI, owing to the strong size-exclusion effect of the polymer brush against the matrix PS chains. We propose that they eventually aggregated in the DEME-TFSI phase owing to the immiscibility of the grafted PS and DEME-TFSI. In addition, the effect of RP in eq 2 on the CPB-SiP location needs to be considered. CPB-SiPs with long graft polymer chains have large RP values. According to eq 2, these CPB-SiPs facilitate large interfacial energy gain when adsorbed at the PS/ 3742


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Langmuir CLSM images of the resulting films, respectively; the former reveals that all blends formed quasi-solid films devoid of holes or cracks, while the latter reveals that the size of the DEMETFSI domain (shown in black) increases with decreasing amounts of AB3. These results indicate that only 1 wt % of AB3 is required for the fabrication of a quasi-solid film, but the addition of more AB3 is needed to obtain smaller DEME-TFSI domains. Ionic Conductivities of the PS and DEME-TFSI Phases. The ionic conductivities of each PS and the DEME-TFSI phase in CPB-SiP blended films are expected to significantly affect the overall ionic conductivities of these films. To investigate ionic conductivities of the PS and DEME-TFSI phases, we evaluated the purities of the phases in the quasisolid PS (45 wt %)/DEME-TFSI (50 wt %) + AB3 (5 wt %)-blended film by comparing the thermal behavior of the film and the neat matrix PS by DSC. To destroy any thermal history, each sample was heated at 200 °C for 10 min prior to DSC, after which the sample was heated at a rate of 10 °C/ min. Figure 12 displays the heat-flow traces of the film and

is about the same as that of previously reported quasi-solid films of immiscible blends containing ILs. For example, a film with continuous ionic channels formed by employing microemulsion of BMI-TFSI and TiO2 was determined to have a conductivity of 0.5 mS/cm,68 whereas a film with continuous ionic channels formed by filling the spaces between CPB-SiPs that were three-dimensionally self-assembled in a face-centered cubic structure by DEME-TFSI exhibited a value of 0.17 mS/ cm (at 30 °C).9 In addition, to determine the number of effective carriers and the activation energy, Ea, we measured ionic conductivities (σc) over the −30 to 120 °C temperature (T) range and attempted to fit the data to the Arrhenius eq 4 σc = A exp( −B /T )

(4)

where A and B are the fitting parameters; A reflects the number of effective carriers and B is related to Ea by B = Ea/R, where R is the gas constant. However, as shown in Figure 13a, the line

Figure 12. DSC heating scans of a blended film of PS (45 wt %)/DEME-TFSI (50 wt %) with AB3 (5 wt %) (solid line) and neat matrix PS (broken line) at 10 °C/min, following initial heating at 200 °C for 10 min to destroy any thermal history.

neat matrix PS, which are shown as solid and broken lines, respectively. Both traces exhibit baseline shifts at the same temperature, which indicates that the film and the neat matrix PS have the same Tg, which is defined as the temperature of the inflection point between the two flat regions of the heatflow curve. This result suggests that the phase-separated PS and DEME-TFSI phases are of high purity and, therefore, can act as ionic nonconductive and conductive phases, respectively. Ionic Conductivity of the Blended Film. As shown in the SEM image in Figure 3b and the CLSM images in Figures 5c, 8c, and 11a, a quasi-solid PS (45 wt %)/DEME-TFSI (50 wt %) + AB3 (5 wt %)-blended film contains interconnected spherical DEME-TFSI domains that lead to both air and glass interfaces. In addition, the matrix PS had not mixed to any extent into the phase-separated DEME-TFSI phase, as suggested by DSC. This DEME-TFSI phase structure is expected to work effectively as an electrolyte in electrical device because the ionic carriers can pass through channels without being blocked by nonconductive PS phases, and pure DEME-TFSI facilitates high ionic carrier mobility. To confirm whether this film is useful as a quasi-solid electrolyte, we investigated its ionic properties by AC-impedance measurements. The ionic conductivity of this film was measured to be 0.1 mS/cm at 25 °C, which is 1 order of magnitude lower than that of neat DEME-TFSI (2.6 mS/cm).67 However, this value

Figure 13. (a) Arrhenius and (b) VFT plots of ionic conductivities (σc) of a blended film of PS (45 wt %)/DEME-TFSI (50 wt %) with AB3 (5 wt %) over the −30 to 120 °C temperature (T) range. The solid lines are lines of best fit, which in (b) fits the VFT eq 5 with A = 0.030 S/cm, Ea = 6.0 kJ/mol, and T0 = 174 K.

of best fit was curved; consequently, the data are not described well by eq 4, which we ascribe to the glass-transition effect of the ion-conductive DEME-TFSI phase. Therefore, we analyzed the relationship between σc and T using the Vogel−Fulcher− Tammann (VFT) equation (eq 5),69−72 which considers the glass-transition effect σc = A exp{−B /(T − T0)}

(5)

where T0 is the temperature at which the configurational entropy of the carriers becomes 0.73 As shown in Figure 13b, the line of best fit was linear at a T0 value of 174 K. The best-fit values of A and Ea were determined to be 0.030 S/cm and 6.0 kJ/mol, respectively. Hayamizu et al. analyzed the temperature 3743


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Langmuir dependence of the ionic conductivity of neat DEME-TFSI using the VFT equation, which provided best-fit values for A and Ea of 0.62 S/cm and 5.6 kJ/mol, respectively, when T0 was 174 K.74 The lower value of A for the film (0.030 S/cm) compared to that of neat DEME-TFSI (0.62 S/cm) reflects the continuous structure of the PS phase, which effectively reinforces the film, as shown in the SEM and CLSM images in Figures 3b, 5c, 8c, and 11a. The almost identical value of Ea for the film (6.0 kJ/mol) and neat DEME-TFSI (5.6 kJ/mol) suggests high carrier mobility in the film, compared to neat DEME-TFSI. High ionic mobility is ascribed to the pure PS and DEME-TFSI phases in the film. This hypothesis is supported by the results of the above-mentioned DSC analyses. Application of the Blended Film to Electric Devices. An electric double-layer capacitor (EDLC), sometimes also referred to as a “supercapacitor” or “ultracapacitor”, is an electrical energy storage device that stores electrical charge in an electrochemical double layer formed at the interfaces between the electrodes and the electrolyte. EDLCs are regarded as promising power sources for electric vehicles75,76 and portable electronic devices76,77 because of their long cyclic lives, high charge/discharge rates, high power capabilities, and charging/discharging reversibilities. To demonstrate the potential of the quasi-solid PS (45 wt %)/DEME-TFSI (50 wt %) + AB3 (5 wt %)-blended film in electrical devices, the charge/discharge performance of an EDLC fabricated using this film as the electrolyte was measured and compared to that of an EDLC constructed with a neat DEME-TFSI electrolyte and cellulose-based porous membrane (thickness = 40 μm) as the separator. Figure 14a displays the charge/discharge curves of these two EDLCs at constant currents of 0.15 and −0.15 mA during charging and discharging, respectively. The curves were closely coincident over 10 charge/discharge cycles and displayed linear voltage−time (V−t) dependences during both the charging and discharging processes. This result indicates that the EDLC with our blended film exhibits ideal capacitive behavior, as does the EDLC containing neat DEME-TFSI because capacitance (C) is described by eq 6 C = I dt /dV

(6)

where I is the constant current (0.15 or −0.15 mA) and C should be constant in an ideal capacitor. The deteriorations in C, as determined by the slopes of the charge/discharge curves in Figure 14a for the EDLCs containing the blended film and neat DEME-TFSI, are displayed in Figure 14b as functions of charge/discharge cycle number. The blended film-containing EDLC retained over 86% of its initial C value, which was as high as that of the neat DEME-TFSI-containing EDLC over the 10 cycles; this result demonstrates that the blended filmcontaining EDLC has good cycling stability. Figure 14c shows variations in the coulombic efficiencies (η) of the EDLCs as functions of cycle number, as defined by eq 7 η = Q D/Q C × 100

Figure 14. (a) Charge/discharge curves for EDLCs containing a blended film of PS (45 wt %)/DEME-TFSI (50 wt %) with AB3 (5 wt %) (solid line) and neat DEME-TFSI (broken line); (b) deteriorations in the capacitances of EDLCs containing a blended film of PS (45 wt %)/DEME-TFSI (50 wt %) with AB3 (5 wt %) (closed circles) and neat DEME-TFSI (open circles) as functions of charge/discharge cycle number; and (c) coulombic efficiencies of EDLCs containing a blended film of PS (45 wt %)/DEME-TFSI (50 wt %) with AB3 (5 wt %) (closed circles) and neat DEME-TFSI (open circles) as functions of charge/discharge cycle number at constant currents of 0.15 and −0.15 mA during charging and discharging, respectively.

(7)

cycling of the blended film-containing EDLC is reversible at the present current. Although we focused on an EDLC as an example of an application of the film in this study, these results reveal that the blended film can be potentially applied to other electrical devices, including all-solid batteries. Generality of the Relationship between Graft Polymer Chemical Species, Mn, and Phase-Separated Structure. To determine whether the relationships between the CPB chemical species, their Mn’s, and phase-separated

where QD and QC are the electrical charges during discharging and charging, respectively. The values of η were determined by the ratios of the discharging and charging times (tD/tC) in each of the cycles shown in Figure 14a, since I was constant. As shown in Figure 14c, the coulombic efficiency of the blended film-containing EDLC maintained a higher η value than the EDLC containing the neat DEME-TFSI and exceeded 90% over 10 cycles. This result indicates that charge/discharge 3744


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Langmuir structures observed in PS/DEME-TFSI-blended films with CPB-SiPs are applicable to other polymer/IL-blended films, we compared the appearances and phase-separated structures of immiscible blended films composed of PS and EMI-TFSI, another IL, with A1, A2, AB3, B1, B2, B4, or B5. Figures S1 and S2 (Supporting Information) show photographic and CLSM images of a blended film of PS (50 wt %)/EMI-TFSI (45 wt %) with CPB-SiPs (5 wt %), which contains PS-rPMMA in varying S/MMA copolymer ratios and with almost the same Mn’s as those of the graft polymers (A1, A2, and AB3). As shown in Figure S1, only when the PS homopolymer was used as the graft polymer, a quasi-solid film devoid of holes, cracks, and EMI-TFSI spillage was formed. In addition, Figure S2 reveals that when PS homopolymer was used as the graft polymer, the CPB-SiPs (shown as green dots) were located at the interface between the PS phase (shown in red) and the EMI-TFSI phase (shown in black) and formed continuous ionic channels of pure EMI-TFSI. The introduction of a small number of MMA units in the graft polymer drove the CPB-SiPs into the EMI-TFSI phase rather than to the PS/EMI-TFSI interface. We next compared the photographic and CLSM images of blended films of PS (50 wt %)/EMI-TFSI (45 wt %) with CPB-SiPs (5 wt %) that contain PS homopolymers with appropriate Mn’s (B1, B2, AB3, B4, and B5) as graft polymers, as shown in Figures S3 and S4, respectively. Figure S3 reveals that quasi-solid films devoid of holes, cracks, and EMI-TFSI spillage were only formed at Mn values of 42 300 and 66 800 g/ mol (when AB3 or B4 was added). In addition, Figure S4 shows that the highest number of CPB-SiPs were located at the PS/EMI-TFSI interface when the Mn was 42 300 g/mol (AB3 was added), while the smallest EMI-TFSI domains were observed. On the other hand, when the Mn was lower or higher than 42 300 g/mol (when B1, B2, B4, or B5 was added), fewer CPB-SiPs were located at the PS/EMI-TFSI interface and the EMI-TFSI domains were larger. At lower Mn’s, the CPB-SiPs that did not adsorb at the interface were aggregated in the EMI-TFSI domain, but they were located in the PS phase at higher Mn’s. The relationships between the CPB chemical species, their Mn’s, and the phase-separated structures revealed here are similar to those elucidated for blended films of PS/DEMETFSI with CPB-SiPs, although DEME-TFSI and EMI-TFSI have different surface energies and viscosities, which suggests that these relationships are very general. This study has developed guidelines for designing particles to control phaseseparated structures that are universally applicable to various combinations of polymer/IL blends.

adjusting the CPB-SiP quantity, and only 1 wt % of the CPBSiPs was required to fabricate a quasi-solid film. In addition, we investigated the ionic properties of the quasi-solid PS/DEMETFSI-blended film. Owing to continuous ionic channels composed of pure DEME-TFSI, the film exhibited an ionic conductivity of 0.1 mS/cm, which is relatively high compared to previously reported quasi-solid electrolytes. Finally, we fabricated an EDLC using the film as the electrolyte; this EDLC exhibited high charge/discharge cycling stability and reversibility. The film showed high potential for application to other electrical devices, including all-solid batteries. We found that similar relationships between CPB chemical species, Mn, and the phase-separated structure existed when EMI-TFSI, another IL, was used, demonstrating the generality of these relationships. Control of phase-separated structures through the addition of CPB-SiPs prepared by SI-LRP is expected to be applicable to a variety of immiscible blends, in addition to PS/ DEME-TFSI and PS/EMI-TFSI, since various types of graft polymers can be introduced by SI-LRP. This study provides new strategies for the fabrication of immiscible composite materials with controlled phase-separated structures.

CONCLUSIONS We elucidated relationships between the CPB chemical species, their Mn’s, and the phase-separated structure of immiscible PS/DEME-TFSI blend with CPB-SiPs. When CPB-SiPs that contain a PS homopolymer with an appropriate Mn (=42 300 g/mol) as the graft polymer were added, quasisolid films with continuous ionic channels composed of pure DEME-TFSI were successfully fabricated because the CPBSiPs were adsorbed at the PS/DEME-TFSI interface and prevented macroscopic phase separation. We found that the balance between the size-exclusion effect of the CPB against the matrix PS chains and the deformability of the CPB plays an important role during the adsorption and fabrication of quasisolid films. The DEME-TFSI domain size was controlled by

ACKNOWLEDGMENTS This work was supported by the Japan Science and Technology Agency (JST) “Precursory Research for Embryonic Science and Technology (PRESTO)” for a project of “Molecular technology and creation of new function”.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b03891.

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Photographic and CLSM images of a blended film of PS (50 wt %)/EMI-TFSI (45 wt %) with CPB-SiPs (5 wt %), which contains PS-r-PMMA in varying S/MMA copolymer ratios and with almost the same Mn’s as those of the graft polymers (A1, A2, and AB3) (Figures S1 and S2); photographic and CLSM images of blended films of PS (50 wt %)/EMI-TFSI (45 wt %) with CPB-SiPs (5 wt %) that contain PS homopolymers with appropriate Mn’s (B1, B2, AB3, B4, and B5) as graft polymers (Figures S3 and S4) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kohji Ohno: 0000-0002-1812-3354 Notes

The authors declare no competing financial interest.

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REFERENCES

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