Ionic Liquid-Blended Films

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

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 Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03891 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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Control of Phase Separation in Polystyrene/IonicLiquid 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.

‡

Tsuruoka National College of Technology, Department of Material Engineering, 104 Sawada, Inooka,

Tsuruoka, 997-8511, Japan.

*

To whom correspondence should be addressed. E-mail: [email protected]

KEYWORDS: Colloid, Surface, Interface, Composite, Polymerization, Ion, Electrolyte

1 ACS Paragon Plus Environment

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ABSTRACT: Immiscible composite materials with controlled phase-separated structures are important in area ranging from catalysis to battery. We succeeded in controlling the phase-separated structures of immiscible blends of polystyrene (PS) and two ionic-liquids (ILs), namely, N,N-diethylN-(2-methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI) and 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI), 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 the 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 CPBSiP 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 only 1 wt% of the CPB-SiPs was needed to fabricate a quasi-solid 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 when 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 photovoltaicities,12 and molecular-encapsulation abilities.13 Phase-separated structures have been controlled using a variety of strategies; examples include the use of spinodal decomposition,1,2 particles,3,4,14-16, and templates formed by the self-assemblies 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, the control mechanism is described below. In an immiscible blend devoid of particles, the separated phases expand to eventually produce a macroscopically phase-separated structure in order to reduce its interfacial area and surface free energy in the system. On the other hand, the addition of 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 phaseA/phase-B phase-separated system, particles with interfacial energies that satisfy equation (1) adsorb at the A/B interface. In addition, when a particle dispersed in phase-A is adsorbed at the A/B interface,


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the interfacial energy gained, ∆EA, is described by equation (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)

∆EA = πRP2 ⁄ σA/B (σA/P - σB/P + σA/B)2

(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.4,1821

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 over 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, growing 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


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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. reported that the phase-separated structure of an immiscible polymer blend, namely deuterated poly(methyl methacrylate) (dPMMA)/poly(styrene-ran-acrylonitrile) (dPMMA/SAN), 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 sizeexclusion 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 the polymer chains of the matrix increases with the distance from 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 abovementioned advantages, few studies have systematically focused on the addition of CPB-grafted particles to immiscible polymer blends in order to reveal the relationship between graft-polymer design and the phase-separated structure.


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Meanwhile, solid electrolytes have attracted much attention due to their applications to allsolid 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 polymerbased solid electrolytes, high salt densities in miscible blends composed of salts and derivatives of poly(ethylene oxide),37-39,42,43 polycarbonate,39-41 polysiloxane,42,43 polyimine,44 polysulfide,45 and polyphosphazene,46 led to increases in glass-transition temperature (Tg), resulting in decreases in ion mobility. Therefore, increase in the ion-conductivity is limited even when salt concentration is increased. Solutions or gels composed of polymers and ionic-liquids (ILs), which are roomtemperature 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

poly(vinylidene

halides,47

fluoride-co-hexafluoropropylene)

polypyridinium/aluminium

halides,48

(PVFH)/1-ethyl-3-methylimidazolium

trifluoromethanesulfonate (EMI-CF3SO3),49 PVFH/EMI-BF4,49 PVFH/1-butyl-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/1butylpyridinium-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


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decreasing 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 carrier-path structures, including bicontinuous and columnar structures, are required for high electric conductivities and mechanical strengths. Until now, 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 the polymerization of an acrylate-group-bearing 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 As well as optimizing carrier-path structures, carrier mobilities require enhancing in order 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 surface-initiated atom-transfer radical polymerization (SI-ATRP). This method is advantageous since it introduces a conductive phase composed of a pure


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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-(2methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI).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 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.

EXPERIMENTAL SECTION Materials. Tetraethyl orthosilicate (TEOS, 99%), ammonium hydroxide solution (28% NH3), 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. 3-Aminopropyltriethoxysilane (APTS, 98%), styrene (S, 99%), and ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na, 99%) were received from Nacalai Tesque Inc., Osaka, Japan. EMI-TFSI (98%) and DEME-TFSI were obtained from Kanto


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Chemical Inc., Tokyo, Japan. PS (weight-average molecular weight, Mw = 280,000 g/mol) was obtained from Sigma-Aldrich, St. Louis, Missouri, USA. Methacryloxyethyl thiocarbamoyl rhodamine B (PolyFluorTM 570) was purchased from Polysciences, Warrington, PA, USA. Water was purified by a Milli-Q system (Nihon Millipore Ltd.) to a specific resistivity of ca. 18 MΩcm. 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 an eluent at a flow rate of 0.8 mL/min were carried out at 40 °C on a Shodex GPC-101 highspeed 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)-N-methylammonium bis(trifluoromethylsulfonyl)imide) (PDEMM-TFSI) was carried out using water/acetonitrile mixed solvent at volume ratio of 1/1 containing 0.1 M sodium nitrate and 0.25 M acetic acid as an eluent at a flow rate of 0.8 mL/min at 40 °C on a Shodex GPC101 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 multi-angle light scattering detector (MALLS, Wyatt Technology DAWN HELEOS).59 PMMA standards were used to calibrate the GPC system. For the


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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 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 the laser wavelength of 658 nm by the MALLS. 1H NMR (300 MHz) spectra were obtained on a JEOL/AL300 spectrometer. Transmission electron microscopic (TEM) observations were made on a JEOL transmission electron microscope, JEM-2100, operated at 200 kV. Thermal gravimetric analyses (TGA) were made 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 microscopic (CLSM) observations were made on an invertedtype microscope (LSM 5 PASCAL, Carl Zeiss) with 488-nm-wavelength Ar laser, 543-nmwavelength He-Ne laser, and ×20 objective (Plan Apochromat, Carl Zeiss) in fluorescence mode. The distance of the focal plane from interface between sample and cover glass was about 30 μm. Scanning electron microscopic (SEM) observations were carried out on a Hitachi SEM (S-3400N) 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. AC-impedance measurements were made using a multi-frequency 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 discs (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 10 ACS Paragon Plus Environment

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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 a screen printing method using an HP-320 hand printer (Newlong Seimitsukogyo 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 layer of the both 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 the 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


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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/Por6, molecular cut off: 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), 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, and 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,N-dimethylformamide (DMF) (10 g) was


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added to the dispersion, and the mixture was rotary evaporated to obtain an ATRP initiator-fixed 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. Oxygen in the solution was removed by three freeze−pump−thaw cycles, and the tube was sealed under vacuum. The 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 in order 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 to be 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 in order 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 solventexchanged 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


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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), the initiator-fixed SiPs (20.0 mg) and DMF (22.8 mg). This gave a monomer conversion of 53%, 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 the 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 1H 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-labelled PS (Mw = 305,000 g/mol). The rhodamine B-labelled PS was synthesized by free radical copolymerization of S (3.12 g, 30 mmol) and PolyFluorTM 570 (18.9 mg, 0.03 mmol) using AIBN (2.46 mg, 0.015 mmol) as the radical initiator in dimethyl sulfoxide (DMSO) (0.244 g) at 60 °C for 18 h, after which by purification through reprecipitation in methanol three times. The 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, the IL (EMI-TFSI or DEME-TFSI), the CPB-SiPs, and THF (90 wt%) onto circular glass substrates. The matrix PS and the IL phaseseparated during THF evaporation. For AC-impedance measurements and capacitance testing, 76.2-


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Table 1. A list of CPB-SiPs prepared in this report. Sample name Copolymer ratio (S/MMA) Mn (g/mol) Mw/Mn Graft density (chains/nm2)

a

A1a

75/25

39,800

1.41

0.32

A2b

94/6

48,500

1.28

0.38

AB3c

100/0

42,300

1.24

0.30

B1d

100/0

3,100

1.40

0.26

B2e

100/0

16,100

1.30

0.28

B4f

100/0

66,800

1.32

0.32

B5g

100/0

93,900

1.39

0.30

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 initiatorfixed SiPs (2 wt%) and DMF (2 wt%). This gave a monomer conversion of 43%. b 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 = 875/125/1/5/10 with ATRP initiator-fixed SiPs (2 wt%) and DMF (2 wt%). This gave a monomer conversion of 53%. c Polymerization 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%. e Polymerization 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%. f Polymerization 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%.

g

Polymerization 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.

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 15 ACS Paragon Plus Environment

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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 PS-b-P(DEMM-TFSI) composed of a PS block immiscible in DEME-TFSI and a P(DEME-TFSI) block miscible in DEME-TFSI, thorough 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(DEMM-TFSI) with Mn = 31,400 g/mol and Mw/Mn = 1.05. The P(DEMM-TFSI) precursor was used for the subsequent block copolymerization with the starting materials S (1.04 g, 10 mmol), the 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 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.

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


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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 Mns as the graft polymers, (A1, A2, and AB3) in order 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 equations (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 of the graft polymer and the phase-separated structure, we introduced PS homopolymers with a variety of Mns as the 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 confirmed sizes, shapes, and uniformity of the core NBD-labeled 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 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 (σs) in the 0.26–0.38 chains/nm2 range, as calculated by equation (3): σ = (w⁄Mn)Av⁄((πdc2))

(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.


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Figure 1. A transmission electron micrograph of core NBD-labeled silica nanoparticles; scale bar: 50 nm.

Preparation of the PS/IL Blended Films Containing the CPB-SiPs. As described in the Experimental Section, the blended films were prepared by drop-casting homogeneous mixtures of THF, the matrix PS, the IL (EMI-TFSI or DEME-TFSI), and the CPB-SiPs 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 the introduction, an immiscible blend devoid of additives phase-separates macroscopically. We first confirmed whether or not 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 with 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 18 ACS Paragon Plus Environment

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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 bars: 5 mm. The image in panel 2a is the same as those shown in Figures 4c, 7c, and 10a.

addition of AB3 led to a solid-like film devoid of holes, cracks, and DEME-TFSI spillage. On the other hand, the film exhibited holes and DEME-TFSI spillage from the film, 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 or not 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(DEMM-TFSI) 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. 19 ACS Paragon Plus Environment

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Effect of THF Evaporation Rate on the Phase-Separated 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, the IL, and the 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 slow to allow time for the matrix PS, the ILs, and the CPB-SiPs to migrate to their thermodynamically most stable locations. To optimize the evaporation rate, we used SEM to compare the phase-separated 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. 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


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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 DEME-TFSI phase was selectively removed by hydrochloric acid. Scale bars: 50 µm (cross-sectional images), 30 µm (interfaces).

evaporation provided a homogeneous phase-separated structure in which the matrix PS and the DEMETFSI phases appeared to be continuous toward both the air and the 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. 21 ACS Paragon Plus Environment

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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 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 the CPB-SiPs, and the DEME-TFSI and the CPB-SiPs, in order to satisfy the conditions for CPB-SiP adsorption at the PS/DEME-TFSI interface, as described by equation (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 solid-like film devoid of holes, cracks, and DEME-TFSI spillage was only formed when the PS homopolymer was used as the graft polymer (AB3

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 bars: 5 mm. The image in panel 4c is the same as those shown in Figures 2a, 7c, and 10a.


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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 bars: 100 µm. Red, black, and green correspond to the matrix PS, the DEME-TFSI phase, and CPB-SiPs, respectively. The image in panel 5c is the same as those shown in Figures 8c and 11a.

was added); when the S/MMA copolymer ratios were 75/25 or 94/6 (A1 or A2 were added), holes and cracks were observed in the formed films, accompanied by DEME-TFSI 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 CPBSiPs in the DEME-TFSI phase (aggregated green dots located in the black region) with large DEMETFSI domains, when the S/MMA copolymer ratios were 75/25 and 94/6 (A1 and A2 were added). Based on 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


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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 bars: 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, DEME-TFSI/THF phase, and the CPB-SiPs, respectively. The image in panel 6b is the same as that shown in Figure 9c.

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 DEME-TFSI phases throughout film formation using a PS/DEME-TFSI 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, 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”, respectively, although accurate phase shapes were not observed due to system drift. In the images obtained using the He-Ne laser, the 24 ACS Paragon Plus Environment

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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 CPB-SiPs too much for CPB-SiPs to be adsorbed at the PS/DEME-TFSI interface and allowed CPB-SiPs to be located in 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 the Graft-Polymer Mns and the Phase-Separated Structure. As described in the 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 decreases as the Mn of the graft polymer increases.36 Therefore, the interfacial energy between the matrix PS and 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 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 Mns, as graft polymers, in order to investigate the relationship between the Mn and the phase-separated structures in PS/DEME-TFSI system. Figure 7 displays 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


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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 bars: 5 mm. The image in panel 7c is the same as those shown in Figures 2a, 4c, and 10a.

that systems with Mns of 42,300 g/mol and 66,800 g/mol (when AB3 or B4 were added) formed solidlike films devoid of holes, cracks, and DEME-TFSI spillage; however, when the Mns were too low or too high (i.e., B1, B2, or B5 were added), holes and cracks, and DEME-TFSI spillages from the films were observed. Figure 8 displays 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 the 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, Mns lower or higher than 26 ACS Paragon Plus Environment

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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 bars: 100 µm. Red, black, and green correspond to the matrix PS, the DEME-TFSI phase, and the CPB-SiPs, respectively. The image in panel 8c is the same as those shown in Figures 5c and 11a.

42,300 g/mol (when B1, B2, B4, or B5 were added), led to fewer CPB-SiPs located at the PS/DEMETFSI 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 Mns, 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 Mns 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-


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SiPs is optimal and meets the conditions required for CPB-SiP adsorption at the PS/IL interface, as described by equation (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 equation (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 size-exclusion effect of the polymer brush against the matrix-PS chains is too strong. This drives the CPB-SiPs into the DEME-TFSI 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 above-mentioned 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 CPBSiPs into the matrix-PS phase rather than to the PS/IL interface. 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 DEME-TFSI/THF phase (shown in black) form “sea-island” structures, in which PS/THF and DEME-TFSI/THF phases


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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 bars: 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, respectively. The image in panel 9c is the same as that shown in Figure 6b.

correspond to “seas” and “islands”, respectively. In the He-Ne-laser images, the CPB-SiPs (shown as green dots) occupy the “islands” when B1 or B2 were added, but occupy the “seas” when AB3, B4, or B5 were added. These results suggest that AB3, B4, and B5 were driven to the PS/DEME-TFSI interface from the PS/THF phase by the size-exclusion effect of the CPB against the matrix-PS chains.


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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 size-exclusion effect between the polymer brush and the 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 equation (2) on the CPB-SiP location needs to be considered. CPB-SiPs with long graft-polymer chains have large RP values. According to equation (2), these CPBSiPs facilitate a large interfacial-energy gain when adsorbed at the PS/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


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densities; therefore they have strong tendencies to adsorb at interfaces compared with short-graftpolymer CPB-SiPs. Consequently, we infer that the optimal Mns 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 CLSM images of the resulting films, respectively; the former reveals that all blends formed quasi-solid films devoid of holes or cracks, although Figure 11 reveals that the size of the DEME-TFSI 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 in order to obtain smaller DEME-TFSI domains. Ionic Conductivities of the PS and DEME-TFSI Phases. The ionic conductivity of each PS and 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,


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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 bars: 5 mm. The image in panel 10a is the same as those 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%)/DEMETFSI (50 wt%) with AB3 (1 wt%); scale bars; 100 µm. Red, black, and green correspond to the matrix PS, the DEME-TFSI phase, and the CPB- SiPs, respectively. The image in panel 11a is the same as those shown in Figures 5c and 8c.

we evaluated the purities of the phases in the quasi-solid 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


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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.

the sample was heated at a rate of 10 °C/min. Figure 12 displays the heat-flow traces of the film and the 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 heat-flow curve. This result suggests that the phase-separated PS and DEME-TFSI phases are of high purity, and, therefore, can act as ionic non-conductive 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


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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 non-conductive PS phases, and pure DEME-TFSI facilitates high ioniccarrier mobility. To confirm whether or not 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 one order of magnitude lower than that of neat DEME-TFSI (2.6 mS/cm).67 However this value is about the same as those 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 while 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 (fcc) structure by DEME-TFSI exhibited a value of 0.17 mS/cm (at 30 °C).9 In addition, in order 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 equation (4): σc = Aexp(-B⁄T)

(4)


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where A and B are the fitting parameters; A reflects number of the 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 of best fit was curved; consequently the data are not described well by equation (4), which we ascribe to the glasstransition effect of the ion-conductive DEME-TFSI phase. Therefore, we analyzed the relationship

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. Solid lines are lines of best fit, which in panel (b) fits the VFT equation (5) with A = 0.030 S/cm, Ea= 6.0 kJ/mol, and T0 = 174 K. 35 ACS Paragon Plus Environment

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between σc and T using the Vogel-Fulcher-Tamman (VFT) equation (equation (5))69-72 that considers the glass-transition effect: σc = Aexp{-B⁄(T - T0)}

(5)

where T0 is the temperature at which the configurational entropy of the carriers becomes zero.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 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, comparable to that of 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 to be promising power sources for electric vehicles,75,76 and portable electronic devices76,77 because of their long cyclic lives, high


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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 with that of an EDLC constructed with a neat DEME-TFSI electrolyte and a 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 mA and −0.15 mA during charging and discharging, respectively. The curves were closely coincident over ten 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 equation (6): C = Idt⁄dV

(6)

where I is the constant current (0.15 mA 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 ten cycles; this result demonstrates that the blended film-containing EDLC has good cycling stability. Figure 14c


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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%)/DEMETFSI (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 mA and −0.15 mA during charging and discharging, respectively.

shows variations in the coulombic efficiencies (η) of the EDLCs as functions of cycle number, as defined by equation (7): 38 ACS Paragon Plus Environment

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η = QD⁄QC × 100

(7)

where QD and QC are the electrical charge 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 that of the EDLC containing the neat DEME-TFSI, and exceeded 90% over ten cycles. This result indicates that charge/discharge 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 the Phase-Separated Structure. To determine whether or not the relationships between the CPB chemical species, their Mns, and the phase-separated structures observed in PS/DEME-TFSI blended films with CPB-SiPs are applicable to other polymer/IL blended films, we compared the appearances and phaseseparated 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-r-PMMA in varying S/MMA copolymer ratios, and with almost the same Mns 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


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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 photographic and CLSM images of blended films of PS (50 wt%)/EMITFSI (45 wt%) with CPB-SiPs (5 wt%) that contain PS homopolymers with appropriate Mns (B1, B2, AB3, B4, and B5) as graft polymers, as shown in Figures S3 and S4, respectively. Figure S3 reveal that quasi-solid films devoid of holes, cracks, and EMI-TFSI spillage were only formed at Mn values of 42,300 g/mol and 66,800 g/mol (when AB3 or B4 were 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 were added), fewer CPB-SiPs were located at the PS/EMI-TFSI interface and the EMI-TFSI domains were larger. At lower Mns, 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 Mns. The relationships between the CPB chemical species, their Mns, and the phase-separated structures revealed here are similar to those elucidated for blended films of PS/DEME-TFSI with CPBSiPs, although DEME-TFSI and EMI-TFSI have different surface energies and viscosities, which


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suggests that these relationships are very general. This study has developed guidelines for designing particles to control phase-separated structures that are universally applicable to various combinations of polymer/IL blends.

CONCLUSIONS We elucidated relationships between the CPB chemical species, their Mn, 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, quasi-solid films with continuous ionic channels composed of pure DEME-TFSI were successfully fabricated because the CPB-SiPs 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 quasi-solid films. The DEME-TFSI domain size was controlled by adjusting the CPBSiP quantity, and only 1 wt% of the CPB-SiPs was required to fabricate a quasi-solid film. In addition, we investigated the ionic properties of the quasi-solid PS/DEME-TFSI 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 when 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


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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 polymer can be introduced by SI-LRP. This study provides new strategies for the fabrication of immiscible composite materials with controlled phase-separated structures.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:???. Additional photographic and CLSM images of blended films (PDF)

ACKNOWLEDGEMENTS

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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For Table of Contents Use Only

Control of Phase Separation in Polystyrene/Ionic-Liquid Blended Films by Polymer-BrushGrafted Particles

Yoshikazu Yahata, Keiji Kimura, Yohei Nakanishi, Shoko Marukane, Takaya Sato, Yoshinobu Tsujii, and Kohji Ohno*


Ionic Liquid-Blended Films

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