Gelation Mechanism of Tetra-armed Poly(ethylene glycol) in Aprotic

Mar 13, 2015 - Kei Hashimoto , Kenta Fujii , Kengo Nishi , Takamasa Sakai , and Mitsuhiro Shibayama. Macromolecules 2016 49 (1), 344-352. Abstract | F...
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Gelation Mechanism of Tetra-armed Poly(ethylene glycol) in Aprotic Ionic Liquid Containing Nonvolatile Proton Source, Protic Ionic Liquid Kei Hashimoto,† Kenta Fujii,*,‡ Kengo Nishi,† Takamasa Sakai,§ Nobuko Yoshimoto,‡ Masayuki Morita,‡ and Mitsuhiro Shibayama*,† †

Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8581, Japan Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan § Department of Bioengineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan ‡

ABSTRACT: We report the gelation mechanism of tetraarmed prepolymer chains in typical aprotic ionic liquid (aIL), i.e., A-B type cross-end coupling reaction of tetra-armed poly(ethylene glycol)s with amine and activated ester terminals (TetraPEG−NH2 and TetraPEG−NHS, respectively) in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mIm][TFSA]). In the ion gel system, we focused on the pH (or H+ concentration) dependence of the gelation reaction. We thus applied the protic ionic liquid (pIL), 1-ethylimidazolium TFSA ([C2ImH][TFSA]), as a nonvolatile H+ source, and added it into the solvent aIL. It was found that the gelation time of TetraPEG ion gel can be successfully controlled from 1 min to 3 h depending on the concentration of pIL (cpIL = 0−3 mM). This suggests that the acid−base properties of TetraPEG−NH2 showing acid− base equilibrium (−NH2 + H+ ⇆ −NH3+) in the solutions play a key role in the gelation process. The acid dissociation constants, pKa’s of TetraPEG−NH3+ and C2ImH+ (cation of pIL) in aIL were directly determined by potentiometric titration to be 16.4 and 13.7, respectively. This indicates that most of the H+ ions bind to TetraPEG−NH2 and then C2ImH+ exists as neutral C2Im. The reaction efficiency of amide bond (cross-linked point) systematically decreased with increasing cpIL, which was reflected to the mechanical strength of the ion gels. From these results, we discuss the gelation mechanism of TetraPEG in aIL to point out the relationship between polymer network structure and [H+] in the solutions.

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INTRODUCTION

swelling/shrinking behavior, that is, a volume phase transition.24 In the development of ion gels as polymer electrolytes and gas separation membranes, one of the requirements for the applications is mechanical toughness gel at low-polymer and high-solvent IL contents. Lodge and co-workers reported hightoughness ion gels composed of ABA-type triblock copolymers in conventional aILs with relatively low polymer concentrations (10−20 wt %), which show higher ionic conductivity and high performance as CO2 separation membranes.25,26 Recently, they have also reported high toughness ion gel with 10 wt % triblock copolymer based on poly(styrene-b-ethylene oxide-b-styrene), which is prepared by chemical cross-linking in IL.27,28 Kamio et al. proposed the thin membranes for CO2 separations using poly(vinylpyrrolidone)-based ion gels containing 50−70 wt % amino acid-based IL.29−31 The free-standing ion gel containing 70 wt % amino acid-based IL exhibits the compression breaking stress of 1 MPa for 60% strain, resulting in good CO2

Polymer networks combined with ionic liquids (ILs); i.e., ion gels are investigated and applied as polymer electrolytes for electrochemical devices,1−6 polymer actuators,7−11 and membranes for gas separations.12−17 Ion gels are well-known as environmentally green materials18,19 because ILs have negligible volatility, nonflammability, and thermal and chemical stabilities.20−22 Physicochemical and electrochemical properties of ion gels strongly depend on solubility of polymers in ILs, and they can be controlled by combination of polymers and ILs. That is, the ion gels composed of polymers which have good compatibility with ILs can contain a large amount of solvent IL, resulting in their high ion conductivity and gas absorption selectivity. Ueki and Watanabe reviewed the solubility of several polymers in typical aprotic ILs (aILs).23 They pointed out that certain combinations of polymers and ILs show phase separations by varying temperature. Both lower and upper critical solution temperature type phase separations occur in the ILs, and their phase separation temperatures easily change by the polymer−IL combinations. Furthermore, the cross-linked ion gels obtained by their combinations show reversible © 2015 American Chemical Society

Received: January 10, 2015 Revised: March 13, 2015 Published: March 13, 2015 4795

DOI: 10.1021/acs.jpcb.5b00274 J. Phys. Chem. B 2015, 119, 4795−4801

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The Journal of Physical Chemistry B

lar amounts of HTFSA and 1-ethylimidazole (C2Im) as a base are mixed in acetonitrile. The acetonitrile solution was dried in a vacuum at room temperature for 1 day to obtain [C2ImH][TFSA]. The C2Im was dried with molecular sieves (3A) and purified by distillation. The HTFSA was kept in a glovebox and used without further purification. Similarly, equimolar amounts of HBF4 and C2Im were mixed in water, and then the aqueous solution was dried in a vacuum at room temperature for 1 day to obtain [C2ImH][BF4]. Water contents in the pILs were also checked by Karl Fisher titration to be less than 100 ppm for both [C2ImH][TFSA] and [C2ImH][BF4]. Density measurements were performed for the [C2ImH][TFSA] and [C2ImH][BF4]. The values were 1.577 and 1.343 g mL−1, respectively. Rheological Measurement. Rheological measurements were performed using a stress control rheometer (MCR-501, Anton Paar, Austria) with cone plate geometry to obtain the gelation time. Ion gel samples were prepared as follows: First, mixed IL solvent was prepared by adding a small amount of pIL to aIL in both the TFSA− and BF4− system. The pIL concentrations ranged from 1.2 to 3.1 mM. Second, TetraPEG−NH2 and TetraPEG−NHS macromer solutions (50 mg/mL, Mw = 20 kg mol−1) were prepared by dissolving TetraPEG macromer in mixed IL solvent. Finally, the mixture of equivalent amounts of macromer solutions was stirred for 1 min and set on a thermostatic plate (25.0 °C). The sample (570 μL) was held between two plates and the time dependencies of the storage modulus (G′) and loss modulus (G″) for the gelation process were monitored under shearing by plates. The strain and angular frequency were set to 2% and 1 Hz, respectively. Potentiometric Titration. HTFSA (acid) and TetraPEG− NH2 (base) in [C2mIm][TFSA] were prepared in a glovebox. The TetraPEG−NH2 solution (concentration of terminal NH2 group, cNH2 = 9.4 mM, 2.8 mL) was titrated with the HTFSA solution (cH = 31 mM) in a vessel with a water jacket at 298 K. The electromotive force (emf), Ei at each titration point i, was measured by using an ion-selective field-effect transistor (ISFET) electrode (HORIBA 0040-10D) coupled with an Ag/ AgCl reference electrode that was separated from the sample by a porous ceramic separator with a double junction (HORIBA 2565A-10T). This electrode system is well established to show an excellent Nernstian response to [H+] and quick response with 3 min in ILs and nonaqueous solvents.45−47 The titration experiments were performed three times and the observed data were analyzed according to Gran’s method48 to determine an equilibrium constant, Ka_NH3+, given by the following acid−base reaction (eq 1)

permeability and separation performance. We also proposed high-toughness ion gel using tetra-armed poly(ethylene glycol) (TetraPEG) as a network polymer.32,33 The TetraPEG ion gel can be prepared by cross-end coupling reaction of two different amine- and N-hydroxysuccinimide-terminated TetraPEGs (TetraPEG−NH2 and TetraPEG−NHS, respectively) in aILs to give a free-standing ion gel even with low polymer content (3−6 wt %). The mechanical properties of the TetraPEG ion gel includes, for example, the maximum breaking compression modulus which reaches 18 MPa (83.5% strain) at 6 wt % polymer content. The excellent mechanical toughness is ascribed to a homogeneous polymer network structure according to our structural study.34−37 More recently, we tried to apply the TetraPEG ion gel to CO2 separation membranes.38 It is found that the 6 wt % TetraPEG ion gel acts as a CO2 separation membrane over the temperature range up to 398 K and shows excellent permselectivities as well as the corresponding neat IL. According to our systematic study on the TetraPEG hydrogel system, the gelation reaction of TetraPEGs strongly depends on the pH in aqueous solutions. It has been established in our previous work that Tetra-PEG gelation undergoes as a simple second-order reaction kinetics, −d[−NH2]/dt = kgel[−NH2][−NHS], in an aqueous system where kgel is the reaction rate constant.39−41 Furthermore, we have pointed out that the gelation time is directly related to the acid−base reaction of the terminal NH2 group within TetraPEG−NH2 (TetraPEG−NH2 + H+ ⇆ TetraPEG−NH3+). The concentration of the NH2 group decreases in a lower pH (or higher [H+]), resulting in a longer gelation time. It is thus expected in an aIL system that has no dissociative H+ within either the cation or the anion the addition of H+ sources is thus required for gelation control. We recently proposed the addition of protic IL (pIL) as a nonvolatile H+ source into solvent aIL.38 The gelation time of TetraPEG in [C2mIm][TFSA] is evidently prolonged in the presence of 1-ethyllimidazolium TFSA, [C2ImH][TFSA] (pIL). This implies that the acid−base properties of TetraPEG−NH2 in [C2mIm][TFSA] play a key role in the gelation process as well as the hydrogel system. However, knowledge of acid−base properties of solutes in solvent aILs is still limited at the present stage.42,43 Particularly, those in polymer/aIL systems have never been reported. In this work, we report the pIL effects on the gelation mechanism of TetraPEG in aIL systems. We mainly focus on the acid−base equilibrium of the terminal NH2 groups within TetraPEG and discuss the relationship between gelation mechanism and mechanical properties of TetraPEG ion gel.

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EXPERIMENTAL SECTION Materials. NH2-terminated and NHS-terminated TetraPEGs (TetraPEG−NH2 and TetraPEG−NHS, respectively) were prepared from tetrahydroxyl-terminated PEG, which is described in elsewhere in detail.32 The molecular weight (Mw) of TetraPEG−NH2 and TetraPEG−NHS were 20 kg mol−1. The aILs, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mIm][TFSA]) and 1ethyl-3-methylimidazolium tetrafluoroborate ([C2mIm][BF4]), were synthesized according to conventional methods.44 Water contents in aILs were checked by Karl Fisher titration to be less than 50 ppm. The pILs used in this work are 1-ethylimidazolium bis(trifluoromethanesulfonyl)amide ([C2ImH][TFSA]) and 1ethylimidazolium tetrafluoroborate ([C2ImH][BF4]). Equimo-

TetraPEG−NH3+ ⇆ TetraPEG−NH 2 + H+ (or [HTFSA])

K a_NH3 + =

[ − NH 2][H+] [ − NH+3 ]

(1)

From a Nernstian equation, the observed Ei at the titration point is represented as Ei = E°′ + (2.303RT /F ) log[H+]i

(2)

where E°′, R, T, and F denote the standard electrode potential, the gas constant, temperature, and Faraday constant, respectively. The observed Ei’s were analyzed based on Gran’s method. Here, total concentrations of H+ (HTFSA) 4796

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The Journal of Physical Chemistry B and base (TetraPEG−NH2), CH,i and Cbase,i at each titration point i, are C H, i =

Vi [H+]0 = [−NH3+]i + [H+]i V0 + Vi

C base, i =

V0[ − NH 2]0 = [−NH 2]i + [−NH3+]i V0 + Vi

(3)

(4)

where V0, Vi, [−NH2]0, and [H+]0 are the initial volume for base solution, the titrated volume at titration point i, and the initial concentrations of base and acid (titrant) solutions, respectively. According to Gran’s method, the left-hand side of the following equation plotted against Vi to give a straight line in CH > Cbase region and the slope and the intercept essentially correspond to the [H+]0 and E°′, respectively.

Figure 1. pIL concentration dependencies of dynamic moduli, G′ (solid line) and G″ (broken line) for the gelation of TetraPEG in C2mIm-based ILs with (a) TFSA− and (b) BF4− anions plotted against the reaction time (tr). The concentration of polymer was set to be 3 wt %.

(V0 + Vi )10 Ei /(2.303RT / F ) = 10 E °′ /(2.303RT / F )(Vi [H+]0 − V0[−NH 2]0 )

(5)

In CH < Cbase region, the following equation including the Ei is obtained: Vi [H+]0 10−Ei /(2.303RT / F ) = 10−[E°′/(2.303RT / F ) + logKa_NH3+](V0[−NH 2]0 − Vi [H+]0 ) (6)

From the slope and the intercept on the left-hand side vs Vi plot of eq 6, the pKa_NH3+ can be estimated. The details of the theory and analysis are described in our previous work.43,49 The acid−base reaction of neutral C2Im in [C2mIm][TFSA] was also investigated by similar potentiometric titration; i.e., the HTFSA in [C2mIm][TFSA] solution (cH = 0.086 M, 3.5 mL) was titrated with the C2Im in [C2mIm][TFSA] solution (cC2Im = 0.66 M), resulting in the determination of Ka_C2ImH+ given by eq 7 as follows:

Figure 2. Gelation time corresponding to the intersection of G′ and G″ (tgel) plotted against the pIL concentrations (cpIL).

C2ImH+ ⇆ C2Im +H+ (or [HTFSA])

K a_C2ImH + =

[C2Im][H+] [C2Im H+]

(7)

Stretching Measurement. The rectangular [C2mIm][TFSA] ion gel films (5.0 × 30.0 × 1.0 mm) were prepared at room temperature in the same way described in the Rheological Measurement section. Concentrations of TetraPEG−NH2 and TetraPEG−NHS were set to be 50 mg/mL (3 wt %). The pIL concentrations were 1.4 and 2.3 mM, corresponding to be 29% and 48% of the −NH2 group concentration (4.8 mM), respectively. The stretching measurement was carried out using a mechanical testing apparatus (EZTEST, SHIMADZU, Japan) at a velocity of 10 mm min−1.

Figure 3. (a) Potentiometric titration curves for base TetraPEG−NH2 titrated with acid HTFSA in a [C2mIm][TFSA] solution. The solid line corresponds to the theoretical curve calculated by using the Ka obtained from Gran’s plot. (b) Gran’s plot for the observed titration data. The straight line (solid) shows theoretical line calculated by a linear squared fitting method.

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Figure 1a shows typical results of the storage (G′) and loss (G″) moduli for the gelation of 3 wt % TetraPEG in aprotic [C2mIm][TFSA] with varying protic [C2ImH][TFSA] concentrations, cpIL. Both G′ and G″ gradually increased with increasing the reaction time, tr. The intersection of the G′ and G″ profiles was clearly observed for all the pIL concentrations, which corresponds to the gelation point.50,51 We also investigated the G′ and G″ in aprotic [C2mIm][BF4] containing protic [C2ImH][BF4], which is shown in Figure 1b. Similar to the TFSA system, both G′ and G″ increased with tr, and then the gelation points were clearly observed for all the cpILs examined here. Figure 2 shows the reaction time at each

RESULTS AND DISCUSSION Protic IL Effect on the Gelation Time. As mentioned in the Introduction, reaction time on TetraPEG gelation strongly depends on the concentration of H+, [H+] in ILs. Therefore, we focused on protic ILs (pILs) as nonvolatile H+ sources. In this work, [C2ImH][TFSA] and [C2ImH][BF4] that are analogous pILs of [C2mIm][TFSA] and [C2mIm][BF4] aILs, respectively, were added into the corresponding TetraPEG/aIL solutions, and we first confirmed experimentally the relationship between gelation time and pIL concentration using rheological measurements. 4797

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quantitatively evaluated by a direct pH measurement in solvent aIL to determine their acid dissociation constants, pKa values. Acid−Base Reaction of Prepolymer in Aprotic IL. Figure 3a shows a typical potentiometric titration curve observed for the TetraPEG−NH2 in [C2mIm][TFSA] solution. The value of Ei in the TetraPEG−NH2 (base) solution slightly and gradually increased with the addition of the titrant HTFSA (acid) solution. The value sharply increased at the equivalent neutral point, indicating concentration of TetraPEG−NH2, [−NH2], is equal to [HTFSA] at the point. The large Ei jump (approximately 800 mV) was observed in the TetraPEG−NH2/ [C2mIm][TFSA] system. This suggests that the protonated TetraPEG−NH2 (i.e., TetraPEG−NH3+) has a large acid dissociation constant, pKa_NH3+ value in [C2mIm][TFSA]. To estimate the apparent pKa_NH3+ value, the observed titration data were analyzed using Gran’s method, which is shown in Figure 3b. The experimental data fell on a straight line in both the acidic and basic regions to lead to the quantitative evaluation of the apparent pKa_NH3+. The pKa_NH3+ value was estimated to be 16.4(5) mol dm−3. The calculated curve using the pKa_NH3+ value is shown by the solid line colored with red in Figure 3. As can be seen in Figure 3, the calculated curve successfully reproduced the experimental titration points. Here, we note that the pKa_NH3+ value of the TetraPEG−NH2 obtained here is almost similar to the corresponding value of analogous model molecule, butylamine (BuNH2) in [C2mIm][TFSA].43 This suggests that there are essentially no molecularweight dependence of the acid−base property of the NH2 group in aILs. Figure 4a shows a typical potentiometric titration curve observed for the C2Im in [C2mIm][TFSA] solution. Similar to the TetraPEG−NH2 system, the Ei in the HTFSA (acid) gradually decreased with the addition of the C2Im (base) solution titrant solution, followed by the sharp increase in the Ei at the neutralization point, cH − cC2Im = 0. It was found that the extent of Ei jump (600 mV) is appreciably small relative to that for the TetraPEG−NH2 system. This is because apparent pKa is lower for the C2Im system than for the TetraPEG−NH2 one. By applying Gran’s plot analysis to the observed data, the apparent pKa_C2ImH+ for the protonated C2Im was successfully estimated to be 13.7(9) mol dm−3. The result of the Gran’s plot is shown in Figure 4b. The calculated potentiometric curve using the pKa_C2ImH+ value is shown as a solid red line in Figure 4a. It seems that the calculated curves slightly deviated from the experimental data relative to the TetraPEG−NH2 system, particularly in the range of cH − cC2Im < 0. However, we finally concluded that the pKa_C2ImH+ value obtained here is reasonable because a pKa value is directly related to the extent of Ei jump. In [C2mIm][TFSA] (aIL), the pKa value for the acid−base reaction of C2Im is 2 orders smaller than that of TetraPEG− NH2. This means that the Brønsted basicity of TetraPEG−NH2 is significantly stronger than that of C2Im in aIL. That is, most of H+ in the aIL solution preferentially protonated the TetraPEG−NH2 to form the TetraPEG−NH3+, whereas cation of pIL exists as neutral C2Im. In the present TetraPEG ion gel system, a small amount of pIL with a dissociative H+ is added into the TetraPEG in aIL solution to control the gelation time. On the basis of the facts obtained here, the gelation mechanism may be proposed as follows: (1) When pIL ([C2ImH][TFSA] in this work) is added into TetraPEG/aIL solution, the H+ dissociated from the C2ImH+ cation binds to the TetraPEG−NH2 to form the TetraPEG−NH3+. (2) The concentration of the TetraPEG−

Figure 4. Potentiometric titration curves for acid HTFSA titrated with base C2Im in [C2mIm][TFSA] solution. The solid line corresponds to the theoretical curve calculated by using the Ka value obtained from Gran’s plot. (b) Gran’s plot for the observed titration data. The straight line (solid) shows theoretical line calculated by a linear squared fitting method.

Figure 5. (a) Stress−strain curve for 3 wt % TetraPEG ion gels with varying pIL concentration, cpIL. The times sign ( × ) shows the breaking point. (b) Elastic modulus, G, plotted against cpIL.

Figure 6. pIL concentration dependence on reaction efficiency, p.

gelation point (here, we call gelation time, tgel) plotted against cpIL. It was found that the tgel is gradually and systematically prolonged with increasing the cpIL for both TFSA and BF4 systems. The tgel at a given cpIL was appreciably longer in the TFSA system than in the BF4 one. At the present stage, it is difficult to clearly interpret the anion effect on the tgel. However, at least, we can propose that the tgel can be successfully controlled from seconds to hours by the addition of pILs (proton source) into aIL (solvent). To understand the effect of cpIL or H+ concentration on the tgel in detail, we investigated the acid−base reaction of terminal NH2 group within TetraPEG−NH2 in aIL. It might be expected that the acid−base equilibrium of pIL adding as a proton source in solvent aIL plays an essential role in the tgel. In this work, the acid−base properties of TetraPEG−NH2 and pIL were 4798

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Figure 7. Possible gelation mechanism in TetraPEG ion gel system.

NH3+ in aIL is much larger than that of the C2ImH+ because of the 2 orders larger pKa_NH3+ relative to the pKa_C2ImH+. (3) The TetraPEG−NH3+ cannot react with TetraPEG−NHS. Hence, the concentration of reactive NH2 group decreases with increasing cpIL (or [H+]) to lead to a slow gelation time. Finally, we note the acid−base properties of TetraPEG−NH2 and C2Im in aqueous solutions. The pKa values of TetraPEG− NH2 and C2Im in water were reported to be 9.351 and 7.3,52 respectively. Both pKa’s in [C2mIm][TFSA] (aIL) were around 7 orders larger than the corresponding values in water. This indicates the solutes in aIL act as a much stronger Brønsted base than those in water. The large difference of pKa between aIL and aqueous solutions may originate from the solvation of solutes (both neutral and protonated species), which should be investigated in terms of thermodynamic and structural aspects as our future work. Reaction Efficiency. Figure 5 shows stretching stress− strain curves obtained for TetraPEG ion gel prepared in [C2mIm][TFSA] containing [C2ImH][TFSA] (cpIL = 0, 1.4, and 2.3 mM). The stretching stress (σ) gradually increased by elongation (λ) and reached the breaking stress at λmax = 4.8, 3.0, and 2.5 for cpIL = 0, 1.4, and 2.3 mM, respectively. The breaking stress decreased with increasing cpIL. From the initial slope of the observed stress−strain curve, we can estimate the elastic modulus G for TetraPEG ion gel. The G’s were estimated to be 2.8, 1.7, and 0.9 kPa for the ion gels with cpIL = 0, 1.4, and 2.3 mM, respectively. The linear relationship between G and cpIL was found in Figure 5b, which implies that the mechanical strength of TetraPEG ion gel can be controlled by adding pIL, in other words, acid−base reaction of TetraPEG−NH2. According to our systematic study for TetraPEG hydrogel system, the decrease in the G value is directly related to incompleteness of gelation reaction or reaction efficiency of amide bond (cross-linking point).40 The same might be applied to the present ion gel system; i.e., cpIL or [H+] in solvent aIL affects the reaction efficiency of TetraPEG ion gel. We thus evaluated reaction efficiency, p, in this system based on a treelike theory and phantom network model.53−57 The relationship between G and p is described as

1/2 ⎛1 3⎞ 1 P(F ) = ⎜ − ⎟ − 4⎠ 2 ⎝p out

(9)

where c, R, T, and P(Fout) correspond to the preparation concentration of bonds, gas constant, temperature, and probability that 1 of 4 arms leads out to polymer network, respectively. Figure 6 shows the p’s obtained for cpIL = 0, 1.4, and 2.3 mM. The p was 72% for cpIL = 0 mM (neat aIL), and the value was linearly decreased with increased cpIL. This suggests that cross-end coupling reaction (formation of amide bond) is appreciably suppressed by a small amount of H+ in the solution. We can propose that the acid−base reaction of TetraPEG−NH2 in aIL is predominant in this gelation system. Finally, we conclude TetraPEG gelation mechanism in ILs (Figure. 7). In TetraPEG−NH2/[C2mIm][TFSA] (aIL) solutions containing [C2ImH][TFSA] (pIL), most of the H+ ions within the C2ImH+ cation of pIL are dissociated and then bind to TetraPEG−NH2 to form TetraPEG−NH3+ (step 1). This is because the pKa_NH3+ is 2 orders larger than the pKa_C2ImH+. The TetraPEG coexists as protonated TetraPEG− NH3+ and neutral TetraPEG−NH2 in the solution, and then the latter only reacts with TetraPEG−NHS to form the amide bond (HN−CO). The TetraPEG−NH3+ cannot form the amide bond, resulting in a residual unreactive group. The gelation time, tgel, thus depends on the distribution ratio of TetraPEG−NH3+:TetraPEG−NH2, which is controlled by [H+] or cpIL. After amide bond formation, the H+ and NHS− are dissociated from NH2- and NHS-terminated polymers, respectively. The dissociated H+ preferentially binds to the reactive NH2 group due to its strong Brønsted basicity (large pKa) (step 2). Thus, the reactive NH2 group within TetraPEG gradually decreases with proceeding of the gelation reaction (step 3). When all the reactive NH2 groups are protonated completely by the dissociated H+, the gelation reaction is stopped to give defects in polymer network (step 4). The [H+] in the solution (aIL, in this work) thus predominates the reaction efficiency, p, of TetraPEG ion gel, which can be quantitatively controlled by initial pIL concentration and its acid−base property (pKa of pIL).

G = c( 4C3P(F out)[1 − P(F out)]3 /2 + [1 − P(F out)]4 )RT (8) 4799

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CONCLUSION The gelation reaction of TetraPEG in aprotic [C2mIm][TFSA] was investigated in terms of gelation time, acid−base reaction of terminal NH2 group of TetraPEG, and reaction efficiency. The gelation behavior of TetraPEG strongly depends on the [H+] in the solution. We thus control the gelation reaction by adding protic [C2ImH][TFSA] as a proton source, in this work. The gelation time of TetraPEG in aprotic [C2mIm][TFSA] can be successfully controlled by the pIL concentration, cpIL, suggesting that the acid−base properties of the terminal group of TetraPEG and pIL play a key role in the gelation reaction. The acid dissociation constant, pKa, of protonated TetraPEG− NH3+ and C2ImH+ (pIL cation) are quantitatively determined in aprotic [C2mIm][TFSA]. The pKa value for the TetraPEG− NH3+ (16.4) is significantly larger than that of the C2ImH+, which indicates that most of H+ in the system preferentially binds to the NH2 group within TetraPEG. The TetraPEG− NH3+ cannot react with TetraPEG−NHS to give the unreactive terminal groups. We thus conclude that reaction efficiency or extent of defect polymer network in TetraPEG ion gel is essentially dominated by the concentration of TetraPEG− NH3+ depending on [H+] (or cpIL) in the solvent [C2mIm][TFSA].

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.F.). *E-mail: [email protected] (M.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (no. 24750066 to K.F. and no. 25248027 to M.S.).

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