Tetra-PEG Network Containing Ionic Liquid Synthesized via Michael

Mar 23, 2017 - Thus, other strategies to fabricate tetra-PEG ion gels are required to circumvent these drawbacks. Here, we propose a novel method to p...
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Tetra-PEG Network Containing Ionic Liquid Synthesized via Michael Addition Reaction and Its Application to Polymer Actuator Shunta Ishii, Hisashi Kokubo, Kei Hashimoto, Satoru Imaizumi, and Masayoshi Watanabe* Department of Chemistry & Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan S Supporting Information *

ABSTRACT: Ion gels consisting of poly(ethylene glycol) (PEG) network and ionic liquids were synthesized via Michael addition reaction using tetra-arm PEG (tetra-PEG) precursors with amino groups and maleimide groups at the chain ends. The use of the addition reaction to synthesize the tetra-PEG networks ensures that any byproducts, which may influence the electrochemical properties of the obtained gel, are not released in the reaction system. Fourier transform infrared (FT-IR) spectra, gel fraction, and rheological measurements indicated the progress of the addition reaction. A polymer network started to be formed after 2 h when the two tetra-PEG precursors were mixed in an ionic liquid at polymer concentrations above overlap concentration (= 7.2 wt %). From tensile test, the elastic modulus of the ion gel was estimated to be lower than that of conventional hydrogel, indicating some flaws in the network. Compared with the theoretical elastic modulus for tetra-PEG network, the reaction efficiency of the tetra-PEG ion gel (10 wt %) using the Michael addition reaction was ca. 80%, which was lower than that of conventional hydrogels using condensation reaction (ca. 90%). However, a Mooney−Rivlin plot of the ion gel indicates that the polymer network has few loop chains and entanglements and relatively homogeneous structure. The fracture energy of the tetra-PEG ion gels (10 wt %) was more than 30 times higher than that of a 30 wt % PMMA ion gel prepared by conventional free radical polymerization. The improved strength of the tetra-PEG ion gel was caused by relatively few structural defects. Polymer actuators were fabricated using the tetra-PEG ion gel as an electrolyte layer by sandwiching the gel between two carbon electrodes. The tetra-PEG ion gel actuators showed greater durability than a PMMA ion gel actuator.

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INTRODUCTION Polymer actuators are promising candidate components of smart devices, including biomimetic robots,1−5 micropumps,6 and braille displays,7,8 owing to their soft motion like artificial muscles, lightweight composition, flexibility, and processability. They show deformation in response to against stimuli such as voltage,5−28 light,4,29−33 and temperature.34−38 Electroactive polymers (EAPs) are of particular interest because of their response to applied voltages that can be readily controlled. EAP actuators that use ionic liquids (ILs) as an electrolyte have been reported in order to realize operation of ionic polymer actuator under open atmospheres.14−17,19−28 ILs are room temperature molten salts that consist entirely of anions and cations; they have unique properties such as low volatility, nonflammability, high ionic conductivity, and thermal and chemical stability.39−44 Mixing ILs with compatible polymers holds the ILs in the polymer network and maintains desirable properties of the ILs, and the resulting gels (ion gels) are flexible and ionconductive.45 We have developed several different methods to produce ion gels. For example, a chemically cross-linked poly(methyl methacrylate) (PMMA) ion gel was prepared by the in situ radical polymerization of methyl methacrylate with a cross-linking agent in an IL.46 Physical gels are also readily © XXXX American Chemical Society

fabricated from amphiphilic ABA-type triblock copolymer with a solvatophobic A-segment and solvatophilic B-segment by mixing the polymer with IL. For example, an ion gel can be obtained by mixing a triblock copolymer (PSt−PMMA−PSt (PSt: polystyrene)) and 1-methyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mim][NTf2]), in which the PSt segments aggregate and act as physical cross-linking points.26,47 Changing both the polymer weight fraction in the ion gel and the molecular weight of each segment, resulting in changes in the balance of solvatophobic and solvatophilic segments, affects the gel’s physicochemical properties. Thereby, the ionic conductivity and mechanical properties of the ion gels can be controlled, and these properties affect the performance of ion gel actuators. The mechanical strength of ion gels is one of the important factors for developing durable actuators. Mechanically tough gels have been recently developed, such as double network gel,48 slide-ring gel,49 nanocomposite gel,50 and tetra-PEG gel (PEG: poly(ethylene glycol)).51 Especially, tetra-PEG gels, Received: December 21, 2016 Revised: March 16, 2017

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scanning calorimetry (DSC) measurements were conducted using a Seiko Instruments DSC 6220 under ambient atmosphere. The samples were first heated to 80 °C at a heating rate of 10 °C min−1 to remove the thermal history of the materials. Then, the samples were cooled at a rate of 5 °C min−1 to 10 °C and were held at 10 °C for 10 min. A second scan was carried out from 10 to 80 °C at a heating rate of 1 °C min−1. The ionic conductivity of the tetra-PEG ion gels was determined from complex impedance measurements using a Hewlett-Packard 4192A LF impedance analyzer over a frequency range of 5 Hz−13 MHz at an amplitude of 10 mV. The ion gel samples were sandwiched between two stainless steel disk electrodes separated by a silicone spacer with an inner diameter of 10 mm and thickness of 1 mm. The cell constant was determined from the geometry of tetra-PEG ion gel. The tensile properties of the ion gels were measured using a Shimadzu EX-X at a cross-head speed of 6 mm min−1. The ion gels were cut into a simple rectangular piece (20 mm length, 10 mm width, 1 mm thickness) and were clamped them with 2 mm gap by two chucks. The Young’s modulus and fracture energy of the ion gels were calculated from the initial slope of the stress−strain curve and the area under stress−strain curves, respectively. The selfdiffusion coefficients for the ions were measured using a pulsed-field gradient spin-echo (PGSE)-NMR technique, with a method similar to that described in our previous report.56 A sine gradient pulse with a gradient strength of up to 9 T m−1 was used throughout the measurement. The samples were inserted into an NMR microtube (BMS-005J, Shigemi, Tokyo) with an outer diameter of 5 mm in the glovebox. Estimation of Melting Point and Degree of Crystallinity. The melting point (Tm) of the tetra-PEG and cross-linked tetra-PEG was evaluated from the peak top of the DSC curve. The degree of crystallinity (Xc) for the cross-linked tetra-PEG was estimated using the equation

which are prepared from two end-modified tetra-arm PEG precursors, are interesting materials focusing on the homogeneity of the polymer network structure; therefore, the gels have few structural defects such as loops and dangling chains and show desirable mechanical properties even at low polymer concentrations. A tetra-PEG ion gel has already been reported and shows the characteristics of ILs and favorable mechanical properties.52−55 Applying tetra-PEG ion gel to polymer actuators will be expected to enable not only high durability due to favorable mechanical properties of the ion gel but also high response speed due to higher ionic conductivity at lower polymer concentrations. Previously reported tetra-PEG gels are synthesized by a condensation reaction using two tetra-PEG precursors that have amino groups and active ester groups at the polymer ends. If condensation reaction is utilized, the leaving groups of the reaction remain as byproducts, which influence the electrochemical stability of the system. Additionally, the speed of the condensation reactions is too quick (occurring within tens of seconds) to form appropriately shaped materials for actuator devices. Addition of buffer solution during the formation of tetra-PEG hydrogels51 and the use of a protic ionic liquid to form the tetra-PEG ion gel53−55 allow the gelation speed to be easily adjusted. However, these methods restrict the available selection of materials. Thus, other strategies to fabricate tetra-PEG ion gels are required to circumvent these drawbacks. Here, we propose a novel method to prepare the ion gels using Michael addition reaction between two tetra-PEG precursors, which have amino groups and maleimide groups at the polymer ends, in an IL. This addition reaction generates no byproducts. In this article, we discuss the fabrication of the ion gels and their fundamental properties based on the application of polymer actuator. Furthermore, the performance of a polymer actuator using the tetra-PEG ion gel is compared to a conventional PMMA ion gel actuator.

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Xc =

ΔHm × 100 ΔHm0

(1)

where ΔHm is the melting enthalpy of the dried tetra-PEG gel as evaluated from the peak area in the DSC curve and ΔH0m is the equilibrium melting enthalpy of fully crystallized poly(ethylene oxide) (ΔH0m = 203 J g−1).58 Synthesis of Tetra-PEG(mal). Tetra-PEG(OH) (10.0 g, 1.00 mmol) was dissolved in anhydrous dichloromethane (100 mL) under an Ar atmosphere. DCC (2.47 g, 12.0 mmol), 4-DMAP (48.8 mg, 0.400 mmol), and MHA (2.53 g, 12.0 mmol) were added to the solution and stirred for 17 h at 25 °C. The reaction mixture was filtered, and the filtrate was washed with brine. After removing the dichloromethane, a pink solid was obtained. The crude polymer was purified by reprecipitation using THF and methanol as a good solvent and a poor solvent, respectively. After vacuum drying at room temperature, tetra-PEG(mal) was obtained as a pink powder (yield: 8.00 g, 80%). 1H NMR (500 MHz): δ 1.32 (m, maleimide−CH2− CH2−, 8H), 1.62 (m, maleimide−CH2CH2CH2CH2−, 16H), 2.32 (t, PEG−O(CO)−CH2−CH2−), 3.64 (m, PEG, 800H), 3.68 (m, maleimide−CH2−), 4.22 (t, −COO−CH2−, 8H), 6.69 (s, maleimide, 8H). 13C NMR (125 MHz): δ 24.2 (maleimide−CCCCC(CO)−), 26.07 and 28.3 (maleimide−CCCCC−COO−), 33.8 (maleimide− CCCCC−COO−), 37.5 (maleimide−CCCCC−COO−), 63.3 (−(CO)O−CC−), 69.0 (−(CO)O−CC−), 70.4 (PEG), 134.0 (CCmaleimide), 170.7 (COmaleimide), 173.2 (−(CO)O−). Synthesis of Tetra-PEG(Ms). Tetra-PEG(OH) (10.0 g, 1.00 mmol) was dissolved in anhydrous dichloromethane (50 mL) under an Ar atmosphere. Triethylamine (1.39 mL, 10.0 mmol) was added to the solution, and the reaction mixture was cooled to 0 °C. Then MsCl (930 μL, 12.0 mmol) dissolved in dichloromethane (5.0 mL) was added to the solution dropwise over 1 h. After addition was complete, the reaction solution was stirred for 12 h at 25 °C. After the reaction, the solution was washed with brine and the solvent was evaporated. The crude white solid was dissolved in toluene, and residual salts were removed by adsorption on KYOWAAD (Mg0.7Al0.3O1.15). After filtration using Celite, the toluene was removed by vacuum drying.

EXPERIMENTAL SECTION

Materials. Tetra-PEG with terminal hydroxyl groups (tetraPEG(OH)) was provided by NOF Corp. The number-average molecular weight (Mn) was estimated from titration of the hydroxyl groups (Mn = 10 240). N,N′-Dicyclohexylcarbodiimide (DCC), N,Ndimethyl-4-aminopyridine (4-DMAP), anhydrous dichloromethane, methanesulfonyl chloride (MsCl), ammonia solution (28%), and MMA were purchased from Wako Chemical. Maleimidehexanoic acid (MHA) was purchased from Aldrich. Ethylene glycol dimethacrylate (EGDMA) and triethylamine were purchased from Tokyo Chemical Industry. MMA was dried with calcium hydride overnight, and MsCl was vacuum distilled prior to use. All other chemicals were used without further purification. 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mim][NTf2])56 and 1-butyl-3methylimidazolium hexafluorophosphate ([C4mim]PF6)57 were prepared in a similar manner to previous reports. Characterization and Measurements. 1H NMR spectra (500 MHz) were recorded on a Bruker DRX500 spectrometer using CDCl3 as the solvent. Fourier transform infrared (FT-IR) spectroscopy measurements were recorded on a Nicolet iS50 FT-IR spectrometer (Thermo Scientific) using the attenuated total reflectance (ATR) method at room temperature. Calcium fluoride was used as a window material for measurements with temperature control. Dynamic viscoelastic measurements were conducted using a Physica MCR301 rotary rheometer (Anton Paar) with a temperature controller (HPTD200). A parallel plate (50 mm diameter) was installed, and a gap spacing of approximately 0.5 mm was used for all measurements. The elastic moduli (storage elastic modulus (G′) and loss elastic modulus (G″)) were examined in the linear viscoelastic region and measured at a strain of γ = 1% and a frequency of ω = 1 rad s−1. Differential B

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Macromolecules Again, the white solid was dissolved in THF, and the solution was poured into a large excess amount of diethyl ether. The precipitated solid was isolated by filtration and dried under vacuum at room temperature. The purified tetra-PEG(Ms) with terminal mesyl group was obtained as a white powder (yield: 8.40 g, 84%). 1H NMR (500 MHz): δ 3.08 (s, CH3−SO3−, 12H), 3.64 (m, PEG, 960H), 3.77 (m, CH3SO3−CH2CH2−, 8H), 4.38 (m, CH3SO3−CH2CH2−, 8H). 13C NMR (125 MHz): δ 37.6 (CH3SO3−), 68.9 and 69.18 (CH3SO3− CC−), 70.46 (PEG). Synthesis of Tetra-PEG(NH2). Tetra-PEG(Ms) (8.00 g, 0.800 mmol) was dissolved in deionized water. The aqueous solution was added to an aqueous ammonia (28%, 300 mL) dropwise, and the mixture was stirred for 4 days at 25 °C. The reaction mixture was dialyzed against deionized water, and the dialysis was repeated until the solution became neutral. After removing the water by evaporation, the obtained white solid was dissolved in a small amount of THF, and this solution was poured into a large excess of diethyl ether. The precipitate was filtered and dried under vacuum at room temperature. The purified tetra-PEG(NH2) was obtained as white powder (yield: 7.20 g, 90%). 1H NMR (500 MHz): δ 2.92 (m, NH2−CH2CH2−, 8H), 3.58 (m, NH2−CH2CH2−, 8H), 3.64 (m, PEG, 780H). 13C NMR (125 MHz): δ 41.48 (NH2−CC−PEG), 70.46 (PEG), 72.29 (NH2−CC−PEG). Preparation of Ion Gels and Dried Ion Gels. Tetra-PEG(mal) and tetra-PEG(NH2) were separately dissolved in dichloromethane. The polymer solutions were each added in a given amount of IL ([C2mim][NTf2]) or [C4mim]PF6)). After evaporating the dichloromethane, the polymer/IL solutions were mixed in an equimolar ratio with respect to tetra-PEG(mal) and tetra-PEG(NH2). The mixed solution was cast in a Teflon mold, and the Michael addition reaction between tetra-PEG(mal) and tetra-PEG(NH2) was performed for 4 days in an oven maintained at a constant temperature of 60 °C. Transparent, flexible, and self-standing tetra-PEG ion gel membranes were obtained after the reaction was complete. The notation used hereafter to denote the composition of ion gels is, for example, “10 wt % ion gel”, which refers to an ion gel consisting of 10 wt % polymer and 90 wt % IL. Dried gels with eliminated ILs were prepared as follows. A diskshaped ion gel film was immersed in excess acetone and stirred for 1 day at room temperature. The acetone was evaporated from the swollen tetra-PEG gels in air at room temperature before the remaining acetone was removed by vacuum drying. A 30 wt % PMMA ion gel was prepared in a similar manner to a previously reported procedure.46 Preparation and Measurement of Ion Gel Actuator. Ion gel actuators with three-layered structure (carbon electrode|ion gel|carbon electrode) were prepared by hot-pressing the three layers at 130 °C. Tetra-PEG ion gels were prepared for actuators by mixing two tetraPEG precursors with terminal maleimide and amino group (tetraPEG(mal) and tetra-PEG(NH2), respectively) and then placing the solutions between two glass plates separated by a Teflon spacer having 100 μm thickness. Carbon electrodes were prepared by mixing acetylene black (12 wt %, DENKA Co. Ltd.), activated carbon (20 wt %, Kuraray Co. Ltd.), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP; 8 wt %, Arkema), and an IL (60 wt %) and hotpressed at 130 °C with a Teflon spacer having 50 μm thickness. The obtained three layered films were cut into rectangular pieces (2 mm width and 7 mm length), and the strips were clipped between Cu tapes connected to a potentiostat (HA-301, Hokuto Denko) and a function generator (HB-104, Hokuto Denko) (Figure 1). Displacement of the actuator was measured at room temperature by a laser displacement meter (LC-2440, KEYENCE) placed 4 mm from the clamped end.

Figure 1. Schematic configuration of polymer actuator in this study.

Scheme 1. Synthetic Route of Two Tetra-PEG Precursors for the Preparation of Tetra-PEG Ion Gels via a Michael Addition Reaction

ammonia and tetra-PEG(Ms), which was synthesized from tetra-PEG(OH). Ion gels were prepared by Michael addition reaction between the terminal maleimide and amine groups of tetra-PEG(mal) and tetra-PEG(NH2) in an ionic liquid ([C2mim][NTf2] or [C4mim]PF6). Comparing the 1H NMR spectra of the resultant tetraPEG(mal) (Figure 2a,a′) with those of the starting material, tetra-PEG(OH) (Figure 2b,b′), shows the clear disappearance of the signals of the two methylene protons next to the hydroxyl group (δ 3.61 and 3.72)59 in the spectrum of tetraPEG(mal) (Figure 2a′). New signals corresponding to the esterified MHA appeared. The carbon signal next to the hydroxyl group (δ 61.7) also disappeared, and new carbon signals corresponding to esterified MHA appeared in Figure 3a. These results indicate that the hydroxyl group of tetra-PEG was totally substituted by the maleimide group. The 1H NMR spectrum of tetra-PEG(Ms) (Figure 2c′) shows that the triplet signals of the methylene next to the hydroxyl group disappeared. The 13C NMR spectrum (Figure 3c) also shows that the reaction proceeded to completion. The proton signals of the mesyl group (δ 3.08, 3.77, and 4.38) (Figure 2c,c′) disappeared from the spectra of tetra-PEG(NH2) (Figure 2d,d′) as the signals corresponding to the methylene protons next to the amino group (δ 2.92 and 3.58) appeared. Moreover, the signals of the mesyl group also disappeared in the 13C NMR spectra of tetra-PEG(NH2) (Figure 3d). Therefore, the amination reaction of tetra-PEG(Ms) proceeded to completion. Tracing the Gelation Process. The progress of the gelation reaction via Michael addition in IL (Scheme 2) was confirmed by the FT-IR spectra of tetra-PEG(mal) and the dried gel, where IL was removed from the tetra-PEG network, as shown in Figure 4. As the Michael addition reaction proceeded over 4 days, the CO stretching band in the maleimide of tetra-PEG(mal) at 1706 cm−1 was slightly shifted to a lower wavenumber region (1702 cm−1).60 Moreover, the ring deformation band of the maleimide group at 696 cm−1 61 was not present in the spectrum of the dried gel. Although these IR spectra did not give us quantitative information on the progress of the gelation due to difficulty in deconvolution of the peaks, these results indicate that significant part of the maleimide structure reacted to form a succinimide structure

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RESULTS AND DISCUSSION Synthesis of Tetra-PEG Precursors. Tetra-PEG derivatives were prepared according to Scheme 1. Tetra-PEG(mal) was synthesized from the reaction of tetra-PEG(OH), MHA, and DCC activated by 4-DMAP. Tetra-PEG(NH2) was synthesized by a nucleophilic substitution reaction between C

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Figure 2. 1H NMR spectra of (a) tetra-PEG(mal), (b) tetra-PEG(OH) (c) tetra-PEG(Ms), and (d) tetra-PEG(NH2) in CDCl3. The spectra in (a′), (b′), (c′), and (d′) are magnified view of the corresponding spectra between δ 3.0 and δ 4.0. The symbols▲, ◆, and ★ are assigned to the satellite signals (δ 3.51 and 3.78) of PEG proton (δ 3.64), methylene protons at the center of tetra-PEG (δ 3.41 (s), δ 3.53 (t), δ 3.59 (t)), and methylene protons at the polymer end of tetra-PEG(OH) (δ 3.61 (t), δ 3.72 (t)), respectively.

gel fraction =

wgel × 100 wsol

(2)

where wgel and wsol are the weight of tetra-PEG gel after washing with acetone and of tetra-PEG precursors in the feed, respectively. Figure S1 shows the time dependence of the gel fraction for a 10 wt % tetra-PEG/[C2mim][NTf2] reaction mixture. The gel fraction increased during the reaction and reached 100% after 30 h. This result indicates that all tetra-PEG macromonomers in the system participated in forming the polymer network structure. The reaction ratio at gelation (αg) is estimated by the equation αg =

1 f−1

(3)

where f (= 4) is the number of functional group per reactant. Therefore, αg is 0.33 in the present case. Thus, the system is expected to become a gel when the reaction ratio exceeds 33%. The gel fraction reached 59% at 2 h after initiation of the reaction. Thus, the Michael addition reaction was expected to lead to gel formation from the sol state within 2 h. The storage and loss moduli G′ and G″ and loss tangent (tan δ) of a 10 wt % tetra-PEG/[C2mim][NTf2] mixture are shown as a function of reaction time in Figure 5. G′ became larger than G″ at 2 h after mixing the two tetra-PEG precursor/ [C2mim][NTf2] solutions, which corresponds to the expected gelation time predicted by the gel fraction measurement. G′ increased and reached a value greater than 104 Pa when the reaction reached to a stationary state. A tetra-PEG gel synthesized using a conventional reaction between amino and active ester groups formed a gel within 30 s.51 In contrast, the

Figure 3. 13C NMR spectra of (a) tetra-PEG(mal), (b) tetraPEG(OH) (c) tetra-PEG(Ms), and (d) tetra-PEG(NH2) in CDCl3.

and the Michael addition reaction between tetra-PEG(mal) and tetra-PEG(NH2) proceeded well. The gel fraction was measured to estimate the amount of the tetra-PEG macromonomer participating in the polymer network. The gel fraction of the tetra-PEG ion gels was evaluated by equation

Scheme 2. Synthetic Route of Tetra-PEG Ion Gels via a Michael Addition Reaction

D

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Figure 4. FT-IR spectra of tetra-PEG(mal) (gray) and dried tetra-PEG gel (black) in the 1600−1750 cm−1 region (left) and the 650−800 cm−1 region (right).

Figure 5. G′, G″, and tan δ of a 10 wt % tetra-PEG/[C2mim][NTf2] ion gel as a function of time at 60 °C.

Figure 6. Dependence of the gel fraction (red, left axis) and density (blue, right axis) of the dried tetra-PEG gels on the polymer concentration. The vertical dashed line indicates C* (= 7.2 wt %) of the tetra-PEG ion gel in [C2mim][NTf2].

Michael addition used to form the tetra-PEG ion gel proceeded more moderately, thereby allowing enough time for the fabrication of the actuator sample. Network Structure of the Tetra-PEG Ion Gel. The tan δ values of the ion gels reflect the homogeneity of the polymer network, including the presence of loops and dangling chains. The presence of a small number of loops and dangling chains in a homogeneous network is associated with a low value of tan δ. The tan δ values of 10 wt % tetra-PEG and 30 wt % PMMA ion gel46 were 2.7 × 10−3 (Figure 5) and 1.2 × 10−2, respectively. These results indicate that the tetra-PEG ion gel had greater network homogeneity than the PMMA ion gel. A previously reported conventional tetra-PEG hydrogel prepared from tetraPEG precursors with amino group and active ester groups had a tan δ value of the order of 10−4.62 Thus, structural defects may exist in the polymer network of the tetra-PEG ion gel using Michael addition reaction compared to the conventional tetraPEG hydrogels. The defects probably arise from unreacted polymer ends that cannot be detected by the FT-IR measurement. To estimate the amount of the unreacted polymer ends, we measured the gel fraction and density of dried tetra-PEG gels with several polymer concentrations (Figure 6). Tetra-PEG ion gels with polymer concentration of 3, 7.2, 10, 20, and 30 wt % were prepared. The overlap concentration of macromonomers (C*) in [C2mim][NTf2] was estimated to be 118 mg/mL (= 7.2 wt %) by viscosity measurement (Figure S2). Gel fractions of tetra-PEG ion gels above C* (7.2, 10, 20, and 30 wt %) reached 100%, and the density of these dried tetra-PEG gels remained approximately constant. However, the gel fraction of tetra-PEG ion gel below C* (3 wt %) did not reach 100%, and the density of its dried tetra-PEG gel was lower than that of the

others. These results imply that the tetra-PEG macromonomers gelled during the addition reaction between the terminal groups when the polymer concentration was greater than C*. Therefore, it can be assumed that the dried sample of tetraPEG gels that were prepared above C* had approximately the same density. The density of the dried tetra-PEG gels prepared from the hydrogel was also 1.18 g cm−3.63 This indicates that the reaction efficiency of the polymer end of the ion gels is rather high after sufficient reaction time (∼4 days). Furthermore, this result indicates that the excluded volume in the tetra-PEG macromonomers in the IL decreases with increasing polymer concentrations since there are fewer entanglements of the PEG chain. For tetra-PEG ion gels prepared below C*, there may be unreacted polymer terminal groups and structural defects in the system because of their lower gel fraction. Therefore, the density of the dried tetra-PEG gels prepared below C* was lower than for the other gels. The thermal properties of the polymer network change depending on the cross-linking density of the polymer network. For example, the melting point (Tm) and degree of crystallinity (Xc) of low/high-density polyethylene decrease with increasing cross-linking density.64 If dried gels are prepared with identical network structures but with different polymer concentrations, the dried gels would have the same values of Tm and Xc. However, the existence of chain entanglements in the polymer network may influence the thermal properties of the gels because the entanglements can act as pseudo-cross-linking points. The dried ion gels showed very similar Tm (Figure S3) and Xc regardless of the polymer concentration used in their E

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where c, R, T, nCr, and P(Fout) are the concentration of the tetra-PEG precursor, gas constant, absolute temperature, combination formula, and the probability that one of four arms does not lead out to an infinite polymer network as the function of p, respectively. Based on the G value, the reaction efficiency in this study was evaluated to be ca. 80%, which was lower than the conventional tetra-PEG hydrogel system using condensation reaction (more than 90%).65 The lower reaction efficiency may result from higher viscosity of the tetra-PEG/IL solution and lower reactivity of Michael addition reaction. Further, the large clusters of tetra-PEG, which is formed in the course of the reaction, cannot diffuse due to its large excluded volume and the branched structures, leading to a decrease in the probability for collision between tetra-PEG(mal) and tetraPEG(NH2). This results in topological defects, in other words, incompleteness of the reaction. It is assumed that use of small cross-linkers can improve the reaction efficiency as reported in the literature.66 However, the σ−λ curve showed a good agreement with the fitting line using eq 4 even at relatively long λ region (∼3). The Mooney−Rivlin equation, which is the experimental rule and represents rubber elasticity, is as follows:

preparation (Figure S4). Therefore, topological defects such as entanglements were less dominant in the tetra-PEG ion gels. Mechanical Properties. The Young’s moduli (E) of tetraPEG ion gels with polymer concentrations of 3, 7.2, and 10 wt % were 2.82, 26.5, and 54.2 kPa, respectively, as evaluated by measuring their stress−strain curves (Figure 7). The E values

Figure 7. Stress−strain curves of 3, 7.2, and 10 wt % tetra-PEG/ [C2mim][NTf2] ion gels, a 30 wt % PMMA/[C2mim][NTf2] ion gel, and a 10 wt % tetra-PEG/[C4mim]PF6 ion gel.

σ* =

increased with increasing polymer concentrations owing to an increase in the density of cross-linking points. Tensile tests were performed on a 30 wt % PMMA ion gel prepared by a conventional in situ radical polymerization to compare with a series of tetra-PEG ion gels. The fracture energy of a 10 wt % tetra-PEG ion gel was 592 kJ m−3, while that of a 30 wt % PMMA ion gel was 17.7 kJ m−3, less than 1/30 of fracture energy of the tetra-PEG ion gel. Therefore, the tetra-PEG ion gels are tough compared with PMMA ion gel despite their low polymer concentration. We also carried out repetitive tensile tests on a 10 wt % tetraPEG ion gel (Figure S5). Similar Young’s moduli and almost similar stress−strain curves were observed for every increase in the applied strain. Thus, the tetra-PEG/[C2mim][NTf2] ion gel deforms ideally without breaking polymer bonds, i.e., without significant change in the cross-linking density. The shear modulus (G) of the 10 wt % tetra-PEG ion gel in this study was also calculated from the stress−elongation (σ−λ) curve based on Figure 7 (Figure S6a). G values of polymer networks can be calculated from σ−λ curves at small deformation, where they behave as imcompressive neoHookean materials, by using eq 4.65 σ G= (4) λ − λ −2 where σ and λ are the stress and the elongation corresponding to the strain, respectively. The G value of the ion gel was evaluated from the σ−λ curve at small deformation (λ < 1.2) to be 19.6 kPa. A reaction efficiency of the tetra-PEG precursor (p) can be evaluated from the G value by using eqs 5 and 6 in both water and IL systems.55,65

Table 1. Dependence of the Gel Fraction, Physicochemical Properties, and Mechanical Properties of Ion Gels on the IL Structure

(5)

⎛1 3⎞ P(Fout) = ⎜ − ⎟ 4⎠ ⎝p

−

1 2

(7)

where C1 and C2 are constants, and in the case of C2 = 0, the material is called neo-Hookean material. The Mooney−Rivlin plot (Figure S6b) showed a flat region around a wide λ−1 range. These results, including the proper correlation of the σ−λ curve with the fitting line, indicate that the tetra-PEG ion gels have few loop chains and entanglements and relatively homogeneous polymer network structure, although the reaction efficiency is not sufficiently high. Therefore, the tetra-PEG ion gels prepared by Michael addition reaction have a great potential if suitable conditions for high reaction efficiency will be found. Dependence of Tetra-PEG Ion Gels Properties on IL. Tetra-PEG ion gels were prepared using [C2mim][NTf2] and [C4mim]PF6. The physicochemical and mechanical properties of the tetra-PEG ion gels and their dried gels are summarized in Table 1. Both gel fractions approached 100%, and the densities of their dried gels were similar. The values of Tm and Xc of these dried tetra-PEG gels were determined by DSC analysis and found to be almost equal for both ILs. The mechanical properties, Young’s moduli, fracture stress, and strain of the ion gels were also the same. Thus, the tetra-PEG macromonomers

⎛ C P(F )[1 − P(F )]3 ⎞ 4 out out G = cRT ⎜⎜ 3 + [1 − P(Fout)]4 ⎟⎟ 2 ⎝ ⎠ 1/2

2C2 σ = 2C1 + −2 λ λ−λ

(6)

a

F

IL

[C2mim][NTf2]

[C4mim]PF6

gel fraction/% densitya/g cm−1 Tma/°C Xca/% Young’s modulus/kPa fracture stress/kPa fracture strain/% fracture energy/kJ m−3 tan δ

100 1.08 39.0 55.1 54.2 136 737 592 2.7 × 10−3

100 1.13 38.9 56.7 58.3 134 775 621 2.3 × 10−3

Dried gel. DOI: 10.1021/acs.macromol.6b02750 Macromolecules XXXX, XXX, XXX−XXX

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displacement at the same time as the polarity of the voltage changes and to maintain its displacement at the constant voltage between polarity changes. The displacement of the tetra-PEG/[C2mim][NTf2] actuator closely matched the waveform of the applied voltage. Therefore, the tetra-PEG/ [C2mim][NTf2] actuator shows a high response rate. This high responsivity results from high ionic conductivity of tetraPEG/[C2mim][NTf2] ion gel at 25 °C (5.60 mS cm−1). In contrast, tetra-PEG/[C4mim]PF6 actuator took longer to reach the maximum displacement because of its lower ionic conductivity. However, the tetra-PEG/[C4mim]PF6 actuator showed a larger displacement than the tetra-PEG/[C2mim][NTf2] actuator. We have previously proposed a model equation for the displacement of the polymer actuator as follows:

formed the similar network structures regardless of the choice of IL. Ionic Conductivity. Arrhenius plots showing the ionic conductivity of tetra-PEG ion gels and their component ionic liquids are shown in Figure 8. Compared with pure ILs, ionic

d=

L2 Q t+v+ − t −v− 3h V0 q

(8)

where L and h are the length and thickness of the actuator, respectively; Q is the ionic charge stored at the electric double layer; V0 is the initial volume of the electrode; t+ and t− are the cationic and anionic transference numbers, respectively; v+ and v− are the volume of the cation and anion, respectively; and q is the elementary charge.66 In this experiment, the Q values of the tetra-PEG/[C2mim][NTf2] and the tetra-PEG/[C4mim]PF6 actuators were obtained by integrating the current−time curve (not shown) were found to be same (∼21 mC). The other factors in eq 8, other than the (t+v+ − t−v−) term, are constants. Therefore, the key factor in the different displacements seen for the difference actuators is the (t+v+ − t−v−) term. The ion volume,68 diffusion coefficients (as estimated by PGSE NMR), and the calculated (t+v+ − t−v−) values of the 10 wt % ion gels are shown in Table 2. The value of (t+v+ − t−v−)

Figure 8. Temperature dependence of the ionic conductivity for 3, 7.2, and 10 wt % tetra-PEG/[C2mim][NTf2] ion gels, a 10 wt % tetraPEG/[C4mim]PF6 ion gel, and neat [C2mim][NTf2], and [C4mim]PF6.

conductivity of tetra-PEG ion gels decreased as the polymer concentration increased. Differences were observed between ionic conductivities of the tetra-PEG/[C2mim][NTf2] ion gel and the tetra-PEG/[C4mim]PF6 ion gel despite their identical polymer concentrations. These results arise from the intrinsic ionic conductivity difference of pure the ILs. The ionic conductivity of the 10 wt % tetra-PEG/[C2mim][NTf2] ion gel was 4 times higher than 10 wt % tetra-PEG/[C4mim]PF6 ion gel at 25 °C. However, these tetra-PEG ion gels had sufficiently high ionic conductivities (>1 mS cm−1) for application as the electrolyte in polymer actuators. Displacement Response of Tetra-PEG Ion Gel Actuators. The displacement responses of tetra-PEG actuators to an applied ±2 V rectangular voltage are shown in Figure 9. The ideal displacement response to the application of a rectangular voltage to a polymer actuator is the displacement to match the waveform of the applied voltage; i.e., to quickly reach maximum

Table 2. Ion Volume, Diffusion Coefficients, and (t+v+ − t−v−) of 10 wt % Tetra-PEG/IL Ion Gels

ion volume (v)/Å3

a

transference number (t) in 10 wt % tetraPEG networka

IL

cation (v+)

anion (v−)

t+

t−

t+v+ − t−v−

[C2mim][NTf2] [C4mim]PF6

156 196

232 109

0.63 0.56

0.37 0.44

12.4 61.8

Evaluated by PGSE NMR.

for the [C4mim]PF6-based ion gel was larger than that for the [C2mim][NTf2]-based system. Therefore, the model eq 8 predicts a larger displacement for the tetra-PEG/[C4mim]PF6 ion gel actuator than for the tetra-PEG/[C2mim][NTf2] ion gel actuator, which agrees with the experimental results. The dependence of the calculated and experimental displacements of the tetra-PEG actuators on the number of ions in the electrodes are shown in Figure 10. The experimental displacements increased linearly with increasing ionic charge and showed good agreement with the theoretical displacements. Durability of Tetra-PEG Ion Gel Actuator. Figure 11 shows the durability of the tetra-PEG/[C2mim][NTf2] actuator and a conventional polymer actuator using PMMA/[C2mim][NTf2] ion gel when a ±1.5 V triangular waveform voltage was applied at a scan rate of 0.5 V s−1. The vertical axis dn/d1 (d1: initial displacement; dn: displacement of nth cycles) indicates

Figure 9. Displacement response of tetra-PEG/[C2mim][NTf2] (red) and tetra-PEG/[C4mim]PF6 (green) actuators to an applied ±2 V rectangular wave over a cycle of 100 s. G

DOI: 10.1021/acs.macromol.6b02750 Macromolecules XXXX, XXX, XXX−XXX

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IL used ([C2mim][NTf2] and [C4mim]PF6). The displacement behavior of an ion gel actuator based on a tetra-PEG ion gel electrolyte layer could be modeled by a previously proposed equation based on ionic conductivity, ion size, and transference numbers of the ILs and ion gels. The tetra-PEG ion gel actuator showed improved durability compared to a conventional PMMA ion gel actuator because of the high fracture energy of the ion gel. The high fracture energy resulted from the lack of structural defects in the tetra-PEG ion gel, such as loops chains in the polymers. Therefore, these tough tetra-PEG ion gels are good electrolyte materials for both polymer actuators and other electrochemical devices.

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Figure 10. Displacement of tetra-PEG/[C2mim][NTf2] (red) and tetra-PEG/[C4mim]PF6 (green) actuators as a function of the number of ions charged in the electrodes.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02750. Figures S1−S7 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.W.). ORCID

Masayoshi Watanabe: 0000-0003-4092-6150 Author Contributions

S.I. and H.K. contributed equally to this work. Notes

The authors declare no competing financial interest.

■

Figure 11. Normalized peak-to-peak displacement of tetra-PEG/ [C2mim][NTf2] and PMMA/[C2mim][NTf2] actuators as a function of the cycle number with the application of a ± 1.5 V triangular waveform voltage at a scan rate of 0.5 V s−1.

ACKNOWLEDGMENTS This work was financially supported by the Grants-in-Aid for Scientific Research S (15H05495) and for Specially Promoted Research on “Iontronics” from Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

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the displacement with respect to the initial displacement in the first cycle. The tetra-PEG/[C2mim][NTf2] actuator exhibited more than 90% retention of its initial displacement and maintained its initial shape even after 5000 cycles, with good reproducibility. In contrast, while the PMMA/[C2mim][NTf2] actuator exhibited similar tendency of tetra-PEG actuator and more than 90% retention of its initial displacement after 1000 cycles, the displacement behavior was not stable and showed poor reproducibility. Damage was observed at the clamped point of the PMMA ion gel actuator after 1000 cycles (Figure S7). We conclude that the durability of the tetra-PEG ion gel actuator results from the high fracture energy of the ion gel, which arise from the defect-free network structure. The tetraPEG ion gels are sufficiently durable for continuous use. Therefore, tetra-PEG ion gels are a suitable candidate for use the electrolyte in ionic polymer actuators.

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CONCLUSIONS Tetra-PEG ion gels were synthesized via Michael addition reactions between tetra-PEG precursors with terminal amino and maleimide groups, which were conducted in ionic liquids. The Michael addition used to form the tetra-PEG ion gels proceeded steadily at a moderate rate without any byproduct formation, allowing enough time to fabricate samples for use in actuator. Tetra-PEG ion gels prepared with polymer concentrations greater than C* (7.2 wt %) had a relatively homogeneous network structure that was independent of the H

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