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Thermo-sensitive Phase Separation Behavior of Poly(benzyl methacrylate)/Solvate Ionic Liquid Solutions Yumi Kobayashi, Yuzo Kitazawa, Kei Hashimoto, Takeshi Ueki, Hisashi Kokubo, and Masayoshi Watanabe Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03378 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 25, 2017

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Thermo-sensitive Phase Separation Behavior of Poly(benzyl methacrylate)/Solvate Ionic Liquid Solutions Yumi Kobayashi,a Yuzo Kitazawa,a Kei Hashimoto,a Ueki Takeshi,b Hisashi Kokubo,a and Masayoshi Watanabe a* ––––––––– a

Department of Chemistry & Biotechnology, Yokohama National University 79-5

Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan b

National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

E-mail: [email protected] –––––––––

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Abstract We report a lower critical solution temperature (LCST) behavior of binary systems consisting of poly(benzyl methacrylate) (PBnMA) and solvate ionic liquids (SILs): equimolar

mixtures

of

triglyme

(G3)

or

tetraglyme

(G4)

and

lithium

bis(trifluoromethanesulfonyl)amide (Li[TFSA]). We evaluated the critical temperatures (Tcs) using transmittance measurements. The stability of the glyme–Li+ complex ([Li(G3 or G4)]+) in the presence of PBnMA was confirmed using Raman spectroscopy, pulsed-field gradient spin-echo NMR (PGSE-NMR), and thermogravimetric analysis (TGA) to demonstrate that the complex was not disrupted. The interaction between glyme–Li+ complex and PBnMA was investigated via 7Li-NMR chemical shifts. Upfield shifts originating from the ring-current effect of the aromatic ring within PBnMA were observed with the addition of PBnMA, indicating localization of the glyme–Li+ complex above and below the benzyl group of PBnMA, which may be a reason for negative mixing entropy; a key requirement of the LCST.


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INTRODUCTION Ionic liquids (ILs) are molten salts at ambient temperatures and have attracted considerable attention as solvents, electrolytes, and materials due to their unique physicochemical properties such as negligible volatility, low flammability, chemical and thermal stabilities, wide electrochemical window, and high ionic conductivity.1,2 Studies on these fundamental properties of ILs have promoted and expanded applications of ILs in various fields, for example, in separation technologies, as reaction media and catalysts for organic syntheses, and as electrolytes for electrochemical devices.3–5 In particular, ILs are considered new candidates for electrolytes in lithium secondary batteries in place of organic electrolyte solutions.6 Several groups, including our group, have reported the properties of ILs as Li+-conducting electrolytes through the dissolution of Li salts in ILs.7–11 However, dissolution of salts induces a large increase in viscosity, resulting in a low optimal concentration of Li salts, although sufficiently high concentration (> 0.5 M) is generally required for electrolytes of lithium batteries.6 Moreover, the system possesses a low Li+ transference number due to the presence of multiple ionic species (at least Li+, IL cation, and the common anion), resulting in concentration polarization during charging and


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discharging.12 Such issues result in large overpotential and low power density, deteriorating the performances of batteries. Previously, we reported equimolar mixtures of triglyme (G3) or tetraglyme (G4) and lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) as a new class of ILs, “solvate ionic liquids (SILs)”, which have similar physicochemical properties to those of conventional ILs (Figure 1).13,14 These properties originate from the formation of stable glyme–Li+ complex cations ([Li(G3)]+ or [Li(G4)]+) as discrete cations. The complex cations possess an oxidative stability higher than 4 V vs. Li/Li+, because lone pair electrons from the ether oxygen atoms are donated to the Li+ cation, resulting in decrease of the highest occupied molecular orbital (HOMO) energies of the glymes.15,16 SILs also possess relatively high Li+ transference numbers (~ 0.5)13 and high Li+ cation concentrations (~ 3 M).11 Thus, we proposed SIL as a promising electrolyte for lithium ion, lithium-sulfur, and lithium-air (O2) batteries.17–19 ILs have been combined with polymers to form polymer electrolytes.20–23 The use of stimulus-responsive polymers enables control of transport properties in solvents.24 This feature—especially thermo-sensitive behavior—is not limited to aqueous or organic solutions; thermo-sensitive phase separation behaviors of polymer/IL solutions have been 5 ACS Paragon Plus Environment

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reported in the last decade, beginning with our studies.25 In imidazolium-based ILs, polyethers, poly(benzyl methacrylate) (PBnMA), and poly(n-butyl methacrylate) showed a lower

critical

solution

temperature

(LCST)

phase

separation

behavior,

while

poly(N-isopropyl acrylamide) (PNIPAm) showed an upper critical solution temperature (UCST) phase separation behavior.25–30 Similar to aqueous solution systems, mechanisms for LCST behaviors have been discussed from the viewpoints of thermodynamics (macroscopic) and molecular interactions (microscopic). In terms of thermodynamics, negative enthalpy and negative entropy of mixing (∆Hmix and ∆Smix, respectively) are essential for LCST behaviors.31 Around the critical temperature (Tc) of an LCST behavior, the Gibbs energy of mixing (∆Gmix) inverts from negative to positive with increasing temperature, resulting in phase separation during the heating process. Negative ∆Hmix and ∆Smix may originate from the “structure-forming” solvation of ILs toward polymers. For instance, polyethers interact with imidazolium ILs through hydrogen bonds between the ether

oxygens

and

the

acidic

protons

in

the

imidazolium

cations

(1-ethyl-3-methylimidazolium: [C2mim]+); PBnMA interacts through cation–π interactions between the imidazolium cations and the aromatic rings in the PBnMA side chain.32–35 These interactions are accompanied by enthalpic stabilization with the entropic penalty of 6 ACS Paragon Plus Environment

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solvation, leading to negative ∆Hmix and ∆Smix, respectively. At the present stage, LCST behaviors has been reported mostly for imidazolium ILs solutions.36 However, the designability of ILs may enable us to control their physicochemical properties, including the thermo-sensitive properties.1 Particularly, designing thermo-sensitive IL-polymer systems that conduct Li+ cations has a large potential in electrochemical applications through switching of the transport properties in lithium batteries by thermal stimulation.37– 42

We report herein an LCST phase separation behavior of a PBnMA/SIL binary system. We emphasize the importance of the absence of the “free glyme”—which is a glyme uncoordinated with the Li+ cation in the mixture of a lithium salt and a glyme—to support IL-like properties, efficient conductivity, and high oxidative stability.14,43,44 Further, the addition of polymers to SILs also affects the amount of free glyme, i.e., the stability of the glyme–Li+ complex cation.45,46 Herein, we explore the stability of the glyme–Li+ complex cation in the PBnMA/SIL binary system by means of Raman spectroscopy, pulsed-field gradient spin-echo nuclear magnetic resonance (PGSE-NMR), and thermogravimetric analysis (TGA). In addition, we deal with the origin of an LCST phase separation in terms of the solvation structure at the molecular level, using NMR. 7 ACS Paragon Plus Environment

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Figure 1. Chemical structures of PBnMA and SILs.

EXPERIMENTAL SECTION Materials. Distilled G3 and G4 with water content < 50 ppm were kindly provided by Nippon Nyukazai. Li[TFSA] was purchased from Morita Chemical Industries. The glymes and dried Li[TFSA] were directly mixed in a vial in a glovebox filled with Ar gas ([H2O] < 0.5 ppm). The mixtures were stirred for 24 h at 60 °C, and homogeneous liquids were


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obtained. The resulting SILs ([Li(G3)][TFSA] and [Li(G4)][TFSA]) were stored and handled in the glovebox. BnMA monomer, dehydrated solvents and other chemicals were purchased from Tokyo Chemical Industry, Wako Pure Chemical Industries, and Kanto Chemical, Inc. All chemical reagents were used as received, unless otherwise noted. Polymerization and Characterization of PBnMA. PBnMA was synthesized by atom transfer radical polymerization (ATRP) using ethylene bis(2-bromoisobutyrate) (2f-BiB) as initiator and purified according to the previously reported procedure.47,48 To obtain polymers with various number-average molecular weights (Mns), the ratio of the initiator to the monomer was varied. Polymerizations was carried out with CuBr, CuBr2 (0.25 mol% Cu

based

on

2f-BiB,

CuBr:CuBr2

=

1:0.01)

and

N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA, 0.25 mol% based on 2f-BiB) in anisole at 50 °C for 4–24 h. When the monomer conversions (calculated from 1H-NMR spectra (Bruker, DRX-500)) reached 50%, the reaction was quenched by liquid nitrogen. The products were dissolved again in ethyl acetate and purified by column chromatography (alumina) and repetitive reprecipitation (ethyl acetate/methanol). The obtained polymers were dried under vacuum and stored in the glovebox to prevent moisture effects. 9 ACS Paragon Plus Environment

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The obtained polymers were characterized by

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1

H-NMR and gel permeation

chromatography (GPC) equipped with two TSK-gel columns (Tosoh, G3000HXL and G4000HXL) and a refractive index (RI) detector (Shimadzu, RID-20A), with tetrahydrofuran (THF) as the eluent. Mn was calculated from the conversion of the monomer (monitored by 1H-NMR) and the amounts of initiator, and monomer in the feed. The polydispersity index (Mw/Mn) was determined by GPC calibrated against polystyrene (PSt) standards using THF as the eluent (Figure S1). The PBnMA with low Mn was analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS, Bruker Daltonics, Autoflex speed TOF/TOF) with dithranol as matrix and sodium trifluoroacetate as cationizing agent (Figure S2) . The characterization results of the polymers are summarized in Table 1.


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Table 1. Molecular weight (Mn) and polydispersitiy index (Mw/Mn) of synthesized PBnMA Mn / kDa

Mw / Mn

4.0 a (4.9 c)

1.10 c (1.21 b)

19.0 a

1.14 b

30.7 a

1.15 b

43.7 a

1.17 b

59.0 a

1.19 b

107.3 a

1.33 b

137.2 a

1.29 b

163.9 a

1.31 b

a

Estimated from the monomer conversion measured by 1H-NMR. b Determined from GPC analysis using PSt standards with THF as eluent. c Determined by MALDI-TOFMS.


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Preparation of PBnMA/SIL Solutions. All samples were prepared in a glovebox by the co-solvent evaporation method. PBnMA was dissolved in a co-solvent that does not disrupt the

glyme–Li+

complex

(dichloromethane),

followed

by

[Li(G4)][TFSA]

or

[Li(G3)][TFSA].43 The homogeneous solution was evacuated at room temperature to completely remove the co-solvent. The obtained PBnMA/SIL solutions were stored in the glovebox. Transmittance Measurement. The prepared polymer solution was poured into a UV-vis cell (GL Science, thickness: 2 mm) in the glovebox and sealed with Teflon tape. The cell was placed on a hot stage (Imoto, Japan) that allowed precise temperature control. The sample was heated from 25 to 200 °C at 1 °C min-1. The transmittance of the solution was monitored at 500 nm using a USB 2000 fiber optic spectrometer (Ocean Optics). The phase behavior during cooling was also observed using the same sample at the same rate. Raman Spectroscopy. Raman spectra were recorded at 30 °C using a JASCO RMP-330 laser Raman spectrometer (50 mW laser power) with resolution of 1 cm−1. The wavelength of the excitation beam was 532 nm. The PBnMA/SIL solution was placed in a glass capillary and sealed with epoxy resin. Spectral data were collected and are averages of 8 scans. 12 ACS Paragon Plus Environment

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Pulsed-field Gradient Spin−Echo NMR (PGSE-NMR) Measurement. Self-diffusion coefficients (D) were measured by PGSE-NMR. A JEOL-AL 400 NMR spectrometer (with a 9.4 T narrow-bore superconducting magnet equipped with a pulsed-field gradient probe, current amplifier, and the sine gradient pulse providing gradient strength up to 12 T m−1) was used for this purpose. The sample was poured into an NMR microtube (BMS-005J, Shigemi) to a height of 5 mm and sealed with Teflon tape (in the glovebox). PGSE-NMR measurements were performed at 30 and 60 °C. Detailed measurement procedures for PGSE-NMR are described elsewhere.49 The diffusivity data were obtained using a simple Hahn spin-echo sequence and Stejskal equation as follows: ln(E) = ln(S/S0) = −γ2g2Dδ2(4∆ – δ)/π2 where E is the free diffusion echo signal attenuation, S is the spin-echo signal intensity, δ is the duration of the field gradient with magnitude g, γ is the gyromagnetic ratio, and ∆ is the interval between two gradient pulses. In this experiment, the value of ∆ was set at 50 ms. 1

H, 7Li, and

19

F spectra were measured for glymes, Li+ cations, and [TFSA]− anions,

respectively. Thermogravimetric Analysis (TGA). Thermal stabilities of the PBnMA/SIL solutions were evaluated using a Seiko Instruments thermogravimetry/differential thermal analyzer 13 ACS Paragon Plus Environment

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(TG/DTA 6200) in an N2 atmosphere. Temperature dependence of the weight loss was measured by increasing the temperature from room temperature to 550 °C at the heating rate of 10 °C min−1. Thermal decomposition (weight-loss) temperature was defined as the temperature for 5 % weight loss. 7

Li-NMR Chemical Shift Measurement. 7Li-NMR chemical shifts were measured using a

JEOL-AL 400 NMR spectrometer with a double tube (SC-008, Shigemi). The PBnMA/SIL solution was poured into the outer tube and 1.0 M lithium chloride (LiCl)/D2O solution (external reference) was poured into the inner tube. All measurements were performed at 30 °C.


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RESULTS AND DISCUSSION LCST Phase Behavior of PBnMA/SIL Binary Systems.

Figure 2 shows the

temperature dependencies of the transmittances of 3 wt% PBnMA/[Li(G3)][TFSA] and PBnMA/[Li(G4)][TFSA] solutions, where Mn of PBnMA was 59.0 kDa. The transmittances decreased suddenly during heating, indicating phase separation between the polymer-rich and SIL-rich phases. Tcs of the LCST for [Li(G3)][TFSA] and [Li(G4)][TFSA] solutions were evaluated to be 158 and 130 °C, respectively, by extrapolation of the decay curves at the inflection point to 100 % transmittance in the heating process. The higher Tc in [Li(G3)][TFSA] solution could be ascribed to higher interaction energy between G3–Li+ complex and PBnMA. In the SILs, the positive charge of Li+ cation was stabilized by (solvating) oxygen atoms of the glyme molecules whereby the interaction between the Li+ cation and the [TFSA] anion was weakened.15,16 Further, the solvation energy increased with increasing number of O atoms in glyme. It was plausible that PBnMA shows stronger mutual interaction with G3–Li+ complex that was less stabilized than the G4–Li+ complex. Certainly the stronger interaction between PBnMA and the complex cation increases compatibility of PBnMA in SIL as well as induces highly ordered solvation structure. The Tcs in ILs are thus greatly dominated by the balance of 15 ACS Paragon Plus Environment

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both enthalpic stabilization and entropic penalty. We previously found that Tcs of PBnMA solutions in imidazolium ILs increased with longer alkyl chain on the cation.50 It was concluded

that

entropic

stabilization—induced

by

increase

in

alkyl

chain

length—contributed to the increase in Tcs. Nano-phase separation between polar imidazolium and nonpolar alkyl chain groups within the cation is likely to contribute to entropic stabilization. Such a domain is not likely to be formed in SILs since the SILs do not have amphiphilic nature like the alkylimidazolium ILs. Therefore, it appeared that the higher Tc in [Li(G3)][TFSA] solution was originated from enthalpic stabilization rather than entropic stabilization. Solution turbidities disappeared gradually during cooling, and the solutions completely returned to transparency. Hysteresis between desolvation (heating process) and resolvation (cooling process) was observed in both solutions. The beginning temperature for resolvation was slightly higher than that during the heating process. Since Tcs of the solutions were higher than the glass transition temperature (Tg) of PBnMA (58 °C, Table S1), aggregated PBnMA melts to be a transparent liquid phase, as shown in Figure S3. Thus, a gradual progress of macro-phase separation over Tc induced “apparent” increase of the transmittance as shown in Figure S4. 16 ACS Paragon Plus Environment

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However, it was clearly observed that [Li(G4)][TFSA] solution showed slower recovery of transmittance than [Li(G3)][TFSA] solution during cooling. We reported that the rate of the resolvation process strongly depended on the difference between Tc and the Tg of bulk PBnMA, as well as the viscosity of the solvent.51 (Tc – Tg) in [Li(G3)][TFSA] and [Li(G4)][TFSA] solutions were 100 and 72 °C, respectively. (Figure 2, Table S1) The viscosity of neat SIL at Tc (ηTc) of [Li(G3)][TFSA] (10.4 mPa s) was higher than that of [Li(G4)][TFSA] (4.5 mPa s).52 The mobility of polymer chains at Tc strongly dominated resolvation rather than the diffusion of solvent into the phase separated polymer particles, resulting in wide hysteresis of the [Li(G4)][TFSA] solution. Figure

3

(a)

shows

the

concentration-dependent

phase

diagram

for

PBnMA/[Li(G3)][TFSA] and PBnMA/[Li(G4)][TFSA] solutions. Both curves were downwardly convex, and the dynamic range of Tcs ranged over ~ 30 °C. We investigated the Mn-dependent phase diagram for the solutions. (Figure 3 (b)) Tcs decreased with increase in Mn, and the curvature became gentle over 59.0 kDa. Both PBnMA/SIL solutions with 4.9 kDa polymer did not show Tcs, even at 200 °C (upper limit of measured temperature). The two phase diagrams were typical of an LCST phase separation behavior,


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implying that the LCST phenomena in SILs are based on the thermodynamics of the polymer solutions, analogous to conventional imidazolium-based IL systems.


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Figure 2. Temperature dependence of transmittance at 500 nm for (a) 3 wt% PBnMA/[Li(G3)][TFSA] and (b) 3 wt% PBnMA/[Li(G4)][TFSA] solutions measured at heating and cooling rates of 1 °C min−1. Mn of PBnMA in both solutions was 59.0 kDa.

Figure 3. Phase diagrams of PBnMA/[Li(G3)][TFSA] and PBnMA/[Li(G4)][TFSA] solutions depending on (a) the concentration and (b) Mn of PBnMA. Tcs were determined as extrapolated temperatures at which the opaqueness appeared during the heating process as shown in Figure 2. The Mn of PBnMA in (a) is 59.0 kDa.


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Stability of Glyme–Li+ Complex in the Presence of PBnMA. To analyze competitive interactions between Li+, [TFSA]–, glyme, and PBnMA, the structures of the glyme–Li+ complexes in the presence of PBnMA were investigated by Raman spectroscopy. It was reported14,43,44 that the formation of SILs between a lithium salt and either G3 or G4 is very sensitive to the anion’s Lewis basicity; weaker Lewis basicity led to increased formation of glyme–Li+ complexes. Complex formation in combination of Li[TFSA] and either G3 or G4 was predominant and the amounts of free glymes negligible. Figure 4 (a) and (b) show the Raman spectra normalized by the height of the anion peak at around 740 cm−1. The peak at around 870 cm−1 is known as the breathing mode, which characterizes the formation of the glyme–Li+ complex having crown-ether-like coordination.53–55 Unfortunately, the estimation of the amount of uncoordinated glymes in the solutions by deconvolution of the Raman spectra was not possible due to overlap of the broad scattering peak from PBnMA with that from the breathing mode.43 However, even at high polymer concentration (up to 20 wt%), the breathing mode remained almost unchanged and peaks based on “free” glyme molecules at ~809, 828, 851 cm−1 were negligible in both ([Li(G3)][TFSA] and [Li(G4)][TFSA]) solutions, indicating that nearly all the glyme


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molecules coordinated with the Li+ cations at room temperature, even in the presence of PBnMA. By increasing the polymer concentration, an anionic peak assigned to the S–N stretching mode (740 cm−1) slightly shifted to lower wavenumber. (Figure 4 (c)) The S–N stretching band reflects the distance between Li+ cations and [TFSA]– anions in SILs;56 the wavenumber for the solvent shared ion pair (SSIP) was lower than that for the contact ion pair (CIP) of Li+ and [TFSA]– anion. Therefore, the result indicates that the addition of PBnMA promoted dissociation of the solvate cation and the anion (SSIP), which implies that PBnMA weakened the interaction between the cation and the anion through competitive solvation. This tendency was more apparent in PBnMA/[Li(G4)][TFSA] solutions, as judged from lower scattering wave numbers for this system than in the G3 system.


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Figure 4. Raman spectra of (a) PBnMA/[Li(G3)][TFSA] and (b) PBnMA/[Li(G4)][TFSA] solutions at different polymer concentrations at 30 °C and (c) the wavenumbers of anionic peaks, which are assigned to the S–N stretching mode. Mn of PBnMA in both solutions was 59.0 kDa.

Transport Dynamics in PBnMA/SIL Solutions. To evaluate the interactions between the components—PBnMA, Li+, the glyme, and the anion—through transport dynamics, the self-diffusion coefficients were determined using PGSE-NMR. Figure 5 shows the ratios 22 ACS Paragon Plus Environment

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of diffusion coefficients of glyme and the anion against that of the cation (Dglyme/Dlithium and Danion/Dlithium, respectively) at 30 and 60 °C. The absolute D values are shown in Figure S5. In the [Li(G3)][TFSA] system with high polymer concentrations (15 and 20 wt%) at 30 °C, the reproducible data cannot be obtained due to the high viscosity at a low temperature. Except them, the data were well fitted by Stejskal equation. The Dglyme/Dlithium ratios were nearly equal to unity in all systems examined here. In previous studies, we found that the Dglyme/Dlithium ratio can be a metric used to determine the stability of the glyme–Li+ complex and that a value close to unity indicates the formation of a long-lived stable complex, i.e., Li+ diffuses together with a glyme molecule in the form of [Li(G3)]+ or [Li(G4)]+. In contrast, Dglyme/Dlithium > 1 suggests the existence of uncoordinated glymes that can diffuse faster than Li+.45 Therefore, we concluded that the interaction between PBnMA and Li+ cation was not strongly competitive with those between the glymes and Li+ cation; hence there was no disruption in the solvate structure of the glyme–Li+ complex. The [Li(G4)][TFSA] solutions showed a decrease in the Li+ transference number (t+), which is defined as t+ = Dlithium/(Dlithium + Danion)


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PBnMA appears to interact with cations rather than anions, and thereby relatively free anions diffuse faster than cations. This can be rationalized using the Raman results, which indicated the tendency to change from CIP to SSIP with increasing polymer concentrations. Previously

we

have

discussed

the

ionic

motion

in

poly(methyl

methacrylate)/[Li(G4)][TFSA] solution by ionic conductivity at Tg (σ(Tg)), which was decoupled systems from the structural relaxation; ionic motion is not strongly coupled with the segmental motion.45,46 Similarly, the ionic motion in PBnMA/[Li(G4)][TFSA] solution resulted in the decoupled behavior. (Figure S6, Table S2) In short, PBnMA interacts with G4–Li+ complexes, but it is not so strong that polymer chain interrupts the ionic motion. In the [Li(G3)][TFSA] system, it is assumed that PBnMA interacted more strongly with the complex cations compared to [Li(G4)][TFSA] system. This decreases the exchange rate of bulk cation and bound cation with polymer chain. Thus the bulk cation diffused preferentially, resulting in the little change to the t+.


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Figure 5. Concentration dependence of Dglyme/Dlithium, Danion/Dlithium, and t+ for (a) PBnMA/[Li(G3)][TFSA] and (b) PBnMA/[Li(G4)][TFSA] solutions at 30 and 60 °C. Mn of PBnMA in both solutions was 59.0 kDa.

Thermal Stability of PBnMA/SIL Solutions. An equimolar mixture of Li[TFSA] and either G3 or G4 maintains a stable liquid state until 200 °C due to coordination of the glyme molecule with Li+ cation.14 It was also revealed that the dissolution of polymers greatly affects the stability of the glyme–Li+ complex (SILs); polymers with high donor number (e.g. poly(ethylene oxide): PEO) compete with the glymes to coordinate with the Li+ cations and disrupt the glyme–Li+ complex, spoiling the thermal stability of the solution.45 Therefore, the thermal decomposition temperatures (Td) of the PBnMA solutions


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were explored to confirm the stability of the complex around Tc. Figure 6 shows the TGA curves for PBnMA, G3, G4, [Li(G3)][TFSA], [Li(G4)][TFSA], and solutions of PBnMA/[Li(G3)][TFSA] and PBnMA/[Li(G4)][TFSA]. Pure G3 and G4 evaporated at ~100 °C and the thermal decomposition of PBnMA started at 307 °C. Decreases in weight were not observed at the vaporization temperatures of pure glymes in any of the solutions examined. The addition of PBnMA slightly decreased the thermal stabilities of the SILs, indicating that PBnMA changed the solvate states of the glymes around Li+ to make them slightly unstable. However, Td did not greatly decrease with increasing concentration of PBnMA in either SIL system, which is in contrast to the addition of PEO reported earlier,45 indicating that the interaction between PBnMA and Li+ was not strong enough to disrupt the glyme–Li+ complexes. The glyme–Li+ complexes maintained their structures around Tc (Figure 3).

This result indicates that the decomposition of the glyme–Li+ complex did not

trigger off the phase separation behavior.


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Figure 6. TGA curves and Td of (a) PBnMA/[Li(G3)][TFSA] and (b) PBnMA/[Li(G4)][TFSA] solutions at several polymer concentrations at a heating rate of 10 °C min−1. Mn of PBnMA in both solutions was 59.0 kDa.

Solvation Structure of PBnMA/SIL Solutions.

In the SIL system, the interaction

between the complex cation and the polymer was studied by Raman, PGSE-NMR and TGA measurements. Judging from persistence of the glyme–Li+ complex (even in the presence of PBnMA), the interaction is not very strong. However, the LCST phase behaviors of PBnMA/SIL binary systems should be caused by structure-forming solvation of the SILs toward PBnMA. Here, we used 7Li-NMR chemical shifts to clarify the solvation structure around Li+. In addition to PBnMA, we used model compounds of PBnMA: methyl


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isobutyrate (MB), toluene, and benzyl isobutyrate (BnB), which are models of the main chain, aromatic ring, and monomer, respectively. Figure 7 shows the 7Li-NMR chemical shifts of [Li(G4)][TFSA] solutions against the chemical shift of Li+ in 1.0 M LiCl/D2O solution (δ = 0, in the inner tube of a double tube) at 30 °C. The Li+ peak of neat [Li(G4)][TFSA] appeared at δ = −0.92. It was confirmed by PGSE-NMR from solutions containing model compounds that the glyme–Li+ complex was stable as Dglyme/Dlithium ≃ 1 (Figure S7). We previously reported that glyme molecules preferentially coordinated to Li+ cation in less polar solvents (for example, in toluene, diethyl carbonate, and hydrofluoroethers) even at concentrations of less than 1 mol dm−3, while the complex cations became unstable in the presence of polar solvents such as propylene carbonate and water.43 Polarities of the model compounds appeared to be relatively low judging from relative dielectric constants (εr)—5.6 for methyl n-butyrate57 and 2.38 for toluene—which did not result in disruption of the glyme–Li+ complex. In Figure 7 (a), with increasing molar ratio (x) of MB, the chemical shift of Li+ shifted downfield. The anion peaks of S–N stretching around 740 cm−1 in the Raman spectrum of MB solution shifted to lower wavenumber than that in PBnMA solution because the cation was further solvated by ester group in MB (Figure S8).58 The downfield shift means less 28 ACS Paragon Plus Environment

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electron density on the Li+ cation, which together with the Raman result indicates release of the Li+ cation from coordination with the [TFSA]– anion. On the other hand, as shown in Figure 7 (b), the peak of Li+ was shifted upfield with increasing x of toluene. Since alkali metal cations interact with aromatic rings perpendicularly, it was suggested that Li+ was shielded due to ring-current effect.59–63 In Figure 7 (c) and (d), with increasing x of BnB or concentration of PBnMA, the peak of Li+ shifted upfield, which is the same behavior as in the toluene system. This result indicates that the interaction between the aromatic rings and Li+ cation is larger than the interaction between the ester group and Li+ cation. We summarize the chemical shifts of Li+ in Figure 8. The shifts of the BnB solutions may reflect an average of the results of the MB and toluene solutions, indicating the coexistence of two types of interactions: Li+–ester group and Li+–aromatic ring. Notably, the chemical shifts of PBnMA solutions were much closer to those of toluene solutions. The ester groups of PBnMA close to the main chain are very crowded, which seems to interrupt the approach of Li+ to the ester group. The chemical shifts of Li+ PBnMA in [Li(G3)][TFSA] (Figure S9) also showed behavior similar to the corresponding toluene solution, which is consistent with the results in the [Li(G4)][TFSA] system. Therefore, we


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conclude that Li+ selectively interacted with aromatic rings of the PBnMA side chain, resulting in the larger ring-current effect seen in the NMR results. The negative ∆Hmix and ∆Smix of PBnMA/[C2mim][TFSA] solutions were confirmed by highly sensitive DSC measurements in our previous report.27 The complexation energy (∆Eion–π) between Li+ cations and benzene, calculated using DFT calculations, is surprisingly high (−150.69 kJ mol−1).61 In the actual PBnMA/SIL binary system, Li+ cations were stabilized by strong coordination with the glymes. However, the cation−π interaction still persisted. Structure-forming solvation between the aromatic ring of BnMA and the [C2mim] cation was also revealed by small angle neutron scattering (SANS), high energy X-ray total scattering (HEXTS), and molecular dynamics (MD) simulations.64–66 In the present system, the [Li(G4)]+ cation was preferentially solvated by the aromatic ring of PBnMA to form structure-forming solvation, leading to negative ∆Smix.


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Figure 7. 7Li-NMR spectra of Li+ in (a) MB/[Li(G4)][TFSA], (b) Toluene/[Li(G4)][TFSA], (c) BnB/[Li(G4)][TFSA], and (d) PBnMA/[Li(G4)][TFSA] solutions at 30 °C. The samples were measured with double tube where the inner tube contained 1.0 M LiCl/D2O solution (reference) and the outer tube is the sample. The molar ratio (x) of additives in the solutions of (a)−(c) are 0 (black), 0.125, 0.25, 0.375, and 0.5 (from pale to deep). The concentrations of PBnMA in the solutions (d) are 0 (black), 5, 10, 15, and 20 wt% (from pale to deep).


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Figure 8. Summary of chemical shift of Li+ signal in each solution at 30 °C. The molar ratio x of the PBnMA solution is calculated based on the BnMA monomer unit.


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CONCLUSIONS We investigated the LCST phase separation behaviors of PBnMA in SILs, [Li(G3)][TFSA] and [Li(G4)][TFSA]. The glyme–Li+ complex structure was carefully investigated by Raman spectroscopy, PGSE-NMR, and TGA measurements, where Li+ ions were coordinated by glymes even in the presence of PBnMA to form stable glyme–Li+ complexes. Thermal stability of glyme–Li+ complexes were higher than the LCST phase separation temperatures, suggesting that the SILs did not decompose during the phase separation process. The LCST phase separation behavior was explained in terms of structure-forming solvation (negative ∆Hmix and ∆Smix). Upfield shift of the Li+ peak in PBnMA/[Li(G4)][TFSA] solutions (in 7Li-NMR spectra) suggested cation–π interactions between [Li(G4)]+ and the aromatic ring of the PBnMA side chain, where the aromatic rings perpendicularly coordinate to the [Li(G4)]+, which appears to be the origin of an LCST behavior.


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ASSOCIATED CONTENT Supporting information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M.W.) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by the Grant-in-Aid for Scientific Research for Basic Research (A-23245046 and S-15H05758) to M.W., and in part by the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (13J00192) to Y.K.

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