Thermoreversible Ion Gel with Tunable Modulus Self-Assembled by a

Article pubs.acs.org/Macromolecules

Thermoreversible Ion Gel with Tunable Modulus Self-Assembled by a Liquid Crystalline Triblock Copolymer in Ionic Liquid Yu-Dong Zhang, Xing-He Fan, Zhihao Shen,* and Qi-Feng Zhou Beijing National Laboratory for Molecular Sciences, Department of Polymer Science and Engineering, and Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center for Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: Ion gels with tunable storage moduli are prepared through the self-assembly of an ABA triblock copolymer AOA-12 comprised of a thermotropic liquid crystalline (LC) polymer as the end-block A and poly(ethylene oxide) (PEO) as the mid-block B in a room-temperature ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethylsufonyl)imide ([EMIM][TFSI]). The PEO mid-block is well-solvated by this ionic liquid, whereas the LC polymer end-blocks aggregate and serve as the physical crosslinkers. For comparison, a triblock copolymer AOA-0 containing a non-LC side-chain polymer with the same mesogen is also synthesized, and its corresponding ion gel is prepared. The ion gels with relatively high concentrations of the LC triblock copolymer have hierarchical structures with different microphase-separated nanostructures and the LC arrangement of the LC blocks. By incorporating the azobenzene mesogen in the side chains, transparent AOA-n/[EMIM][TFSI] (where n is the number of carbon atoms in the spacer between the azobenzene mesogen and the polymethacrylate backbone, and n = 0, 12), ion gels are obtained with concentrations of the polymer as low as around 2 wt %. The ion gel obtained has a storage modulus as high as ∼10 kPa, while its conductivity is close to that of the pure IL mainly because of the high IL concentration of the ion gel. Furthermore, the storage modulus of the AOA-12/IL ion gel can be tuned by temperature because of the thermotropic phase behavior of the LC block. These ion gels are potentially useful as high-temperature ionic membranes or thermal-responsive soft actuators.

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INTRODUCTION Recently ion gels which are made from polymeric networks swollen with ionic liquids (ILs) have attracted a great deal of attention because of their diverse applications in electronics,1,2 actuators,3 and gas separation.4 Various methods of making ion gels have been reported, including in situ thermal- or UVinitiated radical polymerization,5 direct cross-end-coupling of two macromonomers,6 and physical cross-linking methods,7,8 Among these methods, self-assembly of ABA triblock copolymers9−11 proves to be simple, universal, and efficient. Lodge and co-workers reported an ion gel comprised of polystyrene-b-poly(ethylene oxide)-b-polystyrene (PS-b-PEOb-PS, SOS, as low as 5 wt %) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]).9 The storage modulus of the ion gel was around 1 kPa, while the conductivity of ion gel was close to that of the pure ionic liquid because of the relatively low concentration of SOS in the ion gel. By incorporating poly(N-isopropylacrylamide) (PNIPAm), which exhibits the upper critical solution temperature (UCST) phase behavior in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([EMIM][TFSI]), into the triblock copolymer as the end-blocks, thermoreversible ion gels were obtained.12 Furthermore, thermoreversible ion gels with tunable gel−sol transition temperatures were achieved by introducing a second IL-phobic block, PS, into the PNIPAm-b-PEO-b-PNIPAm © XXXX American Chemical Society

triblock copolymer to produce a PNIPAm-b-PS-b-PEO-b-PS-bPNIPAm pentablock copolymer, and the gel−sol transition temperature of the resulting ion gel could be tuned over a range of 17−48 °C simply by varying the molecular weight (MW) of the PS block.13 Similarly, the ion gel with a reversible lowtemperature-sol−high-temperature-gel transition was obtained by incorporating poly(benzyl methacrylate), which exhibits a lower critical solution temperature (LCST) phase behavior in ILs, into the triblock copolymer.14 Thermoreversible ion gels can also be obtained via supramolecular chemistry, such as hydrogen bonding15,16 and metal−ligand interactions.17 In addition, by introducing a partially chemical cross-linking inside the physical cross-linkers, ion gels with a high toughness and a high conductivity were prepared.18 For the above-mentioned ion gels, conductivities, thermal and chemical stabilities, and tunable moduli are important properties. Lodge and co-workers have systematically studied the factors influencing the conductivities and moduli.10,11 They found that a higher modulus could be obtained with a larger amount of polymer or by using a mid-block having a smaller entanglement MW,11 and higher conductivities could be obtained by using a mid-block having a lower glass transition Received: May 21, 2015

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DOI: 10.1021/acs.macromol.5b01103 Macromolecules XXXX, XXX, XXX−XXX

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compensate for the thermal expansion of the tool set. The temperature was controlled to within ±0.2 °C of the set point with an environmental control circulator. The temperature dependences of storage modulus G′ and loss modulus G″ were measured with a strain amplitude of 2%, a frequency of 1 rad s−1, and a heating rate of 0.5 °C min−1. The ionic conductivities of the ion gels were measured by impedance spectroscopy over a frequency range of 100−1000 kHz with an AC amplitude of 10 mV using an IVIUMSTAT electrochemistry workstation. The membrane electrode assemblies (MEAs) were prepared by ion gels and two carbon papers (electrodes) separated by a Teflon spacer. The MEA was sandwiched between two graphite plates, and the resistance measurements were carried out within a temperature range of 30−150 °C. Synthetic Procedures. Synthesis of Macroinitiator and Monomers. The synthesis of the macroinitiator is shown in Scheme 1. Detailed synthetic procedures and characterization data of the macroinitiator and the monomers are shown in the Supporting Information.

temperature (Tg) or shorter end-blocks. However, a larger amount of polymer would lead to lower conductivities, and end-blocks should be long enough to maintain mechanical integrity at a certain polymer concentration. Liquid crystalline (LC) polymers combine the properties of liquid crystals and polymers. The polymer aspect contributes to the rheological integrity to the system, while the LC moieties can be designed to exhibit reversible responses to external stimuli. LC transitions can be tuned by the external stimuli and will affect the rheological properties of the LC polymer (LCP). Hammond and co-workers reported that with the transition from the smectic LC phase to the isotropic state in an azobenzene-containing siloxane-based LCP a dramatic decrease in the storage and loss moduli was observed owing to the disruption of the long-range order of the mesogens.19 Herein, we incorporate a thermotropic LCP into a triblock copolymer as end-blocks which serve as the physical cross-linkers. The phase of the LC block can be adjusted by temperature, and then the properties of the ion gel can be tuned.

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Scheme 1. Synthesis of the AOA-n (n = 0, 12) Triblock Copolymer

EXPERIMENTAL SECTION

Materials. PEO precursor (number-average MW, Mn = 20.0 × 103 g mol−1, PDI = 1.07) was purchased from Sigma-Aldrich and precipitated in hexanes 4 times. N,N,N′,N″,N″-Pentamethyldiethylenetriamine (PMDETA, TCI, 98%), dimethylformamide (DMF, Beijing Chemical Co., A.R.), 4-methoxyaniline (J&K, 99%), and ethyl 2bromoisobutyrate (EB, TCI, 98%) were used as received. Tetrahydrofuran (THF, Beijing Chemical Co., A.R.) was refluxed over sodium and distilled before use. Anisole was washed with NaOH and distilled water, dried with anhydrous calcium chloride, and finally distilled. CuBr (Beijing Chemical Reagents Co., A.R.) was purified by washing with acetic acid, followed by washing with methanol, and then dried for use. Dichloromethane (CH2Cl2, Beijing Chemical Co., A.R.) was dried over anhydrous magnesium sulfate. All other reagents and solvents were used as received from commercial sources. Measurements. Elemental analyses, gel permeation chromatographic (GPC) measurements, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), polarized light microscopy (PLM), one-/two-dimensional (1D/2D) wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS) experiments were performed according to the procedures described previously.20,21 For 1D WAXD experiments, the film sample was cast from the homogeneous THF solution onto an aluminum foil substrate, and the solvent was evaporated at ambient temperature. For 2D WAXD experiments, the oriented samples prepared by mechanically drawing the fibers or mechanically shearing the films were mounted on the sample stage with the point-focused X-ray incident beam perpendicular to the shear direction (X direction). Synchrotron-radiation SAXS experiments were conducted at the synchrotron X-ray beamline BL16B1 in Shanghai Synchrotron Radiation Facility (SSRF) and beamline 1W2A in Beijing Synchrotron Radiation Facility (BSRF). The wavelength of the X-ray beam was 1.24 Å, and a MarCCD detector was used in SSRF. The wavelength of the X-ray beam was 0.154 Å, and a Pilatus 1M detector was used in BSRF. The reflection peak positions are calibrated with silver behenate in both SSRF and BSRF. The samples were annealed at 150 °C under vacuum for 24 h and then characterized by synchrotron-radiation SAXS under ambient conditions. 1H NMR (400 MHz) spectra were obtained with a Bruker ARX400 spectrometer using tetramethylsilane as the internal standard at ambient temperature and deuterated chloroform as the solvent. The chemical shifts were reported in the ppm scale. Mass spectra were recorded on a Bruker Apex IV Fourier transform ion cyclotron resonance mass spectrometer by electrospray ionization (ESI). Shear rheological experiments were performed on a rheometer (MCR 301, Anton Paar) equipped with a heat exchanger using a 25 mm plate− plate geometry. A gap spacing of approximately 1 mm was used for all measurements. The gap was adjusted at each temperature to

Polymerization. The azobenzene-containing polymethacrylate P12 (with 12 methylene units between the azobenzene mesogen and the polymethacrylate backbone) and the triblock copolymers AOA-n (where n is the number of carbon atoms in the spacer between the azobenzene mesogen and the polymethacrylate backbone, and n = 0, 12) are synthesized by atom transfer radical polymerization (ATRP) in solution (Scheme 1). The details are shown in the Supporting Information. Preparation of the Ionic Liquid and Ion Gels. The IL [EMIM][TFSI] was prepared following the procedure reported in the literature.5,22 Yield: 70%. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 9.10 (s,1H), 7.77 (t, 1H), 7.68 (t, 1H) 4.19 (q, 2H), 3.85 (s,3H), 1.42 (t, 3H). HRMS (ESI): calcd (EMIM)+/z, 111.091 67; found (EMIM)+/z, 111.091 25; calcd (TFSI)−/z, 279.917 84; found (TFSI)−/z, 279.916 82. The ion gels were made by mixing weighed amounts of the AOA-0 or AOA-12 triblock copolymer and the IL [EMIM][TFSI] in a cosolvent CH2Cl2. After the mixture was stirred for 2 h, the cosolvent was removed by evaporation at ambient temperature for 24 h, and then it was placed in a vacuum oven at 70 °C until a constant weight was achieved.

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RESULTS AND DISCUSSION Synthesis and Characterization of the Polymers. The azobenzene-containing LC polymethacrylate homopolymer P12 and the amphiphilic triblock copolymers AOA-0 and AOA-12 were synthesized in satisfactory yields using ATRP (Scheme 1). The P12 block renders IL-phobicity and LC properties and serves as the physical cross-linker, while the PEO block provides IL-philicity. The molecular characteristics of the polymers are summarized in Table 1. GPC analysis (Figure 1) of the polymers shows that the Mn values of P12, B


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Macromolecules Table 1. Molecular Characteristics and Thermal Properties of P12, AOA-0, and AOA-12

a

polymer

Mn (×103 g mol−1)a

Mn (×103 g mol−1)b

Mw/Mna

Tdc (°C)

P12 AOA-0 AOA-12

17.0 47.0 49.0

34.5 46.3

1.17 1.21 1.20

329 311 348

b

1

Ttransitiond (°C) and corresponding enthalpy change (J g−1) 83 (4.6)

98 (0.3)

134 (11.1)

83 (4.6)

102 (0.07)

137 (4.2)

c

Determined by GPC in THF using polystyrene standards. Determined by H NMR. 5% weight loss temperature evaluated by TGA under a nitrogen atmosphere at a heating rate of 10 °C min−1. dEvaluated by DSC during the second heating cycle at a rate of 20 °C min−1.

Figure 2. DSC thermogram of P12 at a heating rate of 20 °C min−1 under a nitrogen atmosphere following a cooling process at 5 °C min−1.

atmosphere following a cooling procedure at a rate of 5 °C min−1. The DSC thermogram shows two well-defined endothermic peak at 83 and 134 °C and a small transition at around 98 °C, indicating three transitions. Birefringence of P12 was observed by PLM with film samples at 30, 70, 90, 110, and 125 °C (Figure 3), and the textures indicate LC phases. And P12 goes into the isotropic (Iso) state at 145 °C as the birefringence disappears.

Figure 1. GPC trace of P12 (a) and those of AOA-0 and AOA-12 (b).

AOA-0, and AOA-12 are 17.0 × 103, 47.0 × 103, and 49.0 × 103 g mol−1 with the polydispersity indexes (PDIs) of 1.17, 1.21, and 1.20, respectively. The Mn values of AOA-0 and AOA-12 were further calculated on the basis of the Mn of PEO calculated from 1H NMR and the ratio of the integration of the peaks of hydrogen on azobenzene (7.86−7.88 ppm) of the azobenzene-containing block to that of the methylene (3.47− 3.58 ppm) of the PEO block in the 1H NMR spectra of AOA-0 and AOA-12 (Figure S1 in Supporting Information). The calculated degrees of polymerization of P0 (without any spacer between the azobenzene mesogen and the polymethacrylate backbone) and P12 are 38 and 46, respectively, and the Mn values of AOA-0 and AOA-12 are 34.5 × 103 and 46.3 × 103 g mol−1, respectively. Ion gels with different weight fractions of AOA-0 or AOA-12 in [EMIM][TFSI] were prepared. Liquid Crystalline Properties of P12, AOA-12, and AOA-12/IL. Liquid Crystalline Properties of P12. The thermal and LC properties of P12 were investigated by TGA, DSC, PLM, and WAXD. The thermal stability of P12 is excellent as its temperature at 5% weight loss is above 320 °C in nitrogen. Figure 2 shows the DSC thermogram of P12 on the second heating process at a rate of 20 °C min−1 under a nitrogen

Figure 3. PLM micrographs of P12 at different temperatures.

However, the PLM textures were ambiguous for the phase structure identification. Therefore, temperature-dependent 1D and 2D WAXD experiments were conducted. One-dimensional WAXD profiles of P12 during the first heating process are shown in Figure 4. At ambient temperature, the profile of the sample shows two diffraction peaks at scattering vectors (q’s) q1 = 1.87 nm−1 and q2 = 3.79 nm−1 with a ratio of 1:2 in the lowangle region, indicating a smectic phase according to the literature,23,24 and a diffraction peak at q = 14.6 nm−1 attributed C


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at 90 °C is similar (Figure 5b). For both patterns, the sharp diffraction peak at q ≈ 14.6 nm−1 in 1D WAXD indicates a strong lateral correlation among the mesogens, which suggests high-order titled smectic phases. When the temperature is 110 °C, two pairs of arcs, correlating to the smectic layer diffractions, appear on the equator, while a pair of amorphous halos are located on the meridian, indicating a smectic A (SmA) phase. Hence, the phase sequence of P12 can be summarized as SmX1-SmX2-SmA-Iso. Liquid Crystalline Properties of AOA-0/IL and AOA-12/IL. The thermal stability of AOA-12 was investigated by TGA, and the result shows that it is also excellent, with a temperature at 5% weight loss of above 320 °C in nitrogen. DSC and PLM were used to confirm the existence of the LC phases in the triblock copolymer AOA-12 and the corresponding ion gel. Figure 6a shows the comparison of the DSC thermograms of the homopolymer P12, the triblock copolymer AOA-12, and the ion gel AOA-12/IL. The curve of AOA-12/IL displays two broadened transitions with temperatures similar to those of the LC-to-LC (TSmX1→SmX2) and LC-to-isotropic (Ti) transitions of P12 and AOA-12. The small transition at ∼98 °C in Figure 2 was not observed due to the low concentration of AOA-12 in the ion gel. The DSC results indicate that the LC phases are retained in the ion gel. Meanwhile, the endothermic peak corresponding to the melting of the PEO crystal in the DSC curve of AOA-12 disappears in the curve of the AOA-12/IL system, indicating the solubility of the PEO block in the IL [EMIM][TFSI]. Figure 6b shows the PLM micrographs of AOA-12/IL (10 wt %) at different temperatures which are recorded during the seconding heating and cooling processes. The ion gel is clearly birefringent at low temperatures when the P12 block is liquid crystalline, and the birefringence disappears when the temperature is higher than the Ti of the P12 block. When the temperature is lowered to a range in which the P12 block is liquid crystalline, the birefringence reappears, consistent with the LC transitions of the P12 block from DSC results. The structures of AOA-12 and AOA-12/IL were investigated by wide-angle X-ray scattering (WAXS) with the SAXS instrument (Figure 6c). For AOA-12, a diffraction peak attributed to the lateral distance of the azobenzene mesogens appears at q = 14.5 nm−1, and the diffraction peaks at q = 13.1 and 16.1 nm−1, corresponding to the (120) and the overlapped (1̅32), (032), (112), (2̅12), (1̅24), (2̅04), and (004) diffractions, are the characteristic peaks of the PEO crystal.25,26 The low-angle

Figure 4. 1D WAXD profiles of P12 during the heating process.

to the lateral arrangement of the mesogens. The d-spacing (d) of the smectic layers is calculated to be 3.36 nm, which is a little smaller than the molecular length of the monomer assuming a fully extended conformation (3.48 nm calculated with ChemBio 3D software), thus suggesting a single layer of tilted mesogens. When the temperature is increased to 90 °C, the profile of the sample is similar to that at ambient temperature except that the diffraction peak at q = 14.6 nm−1 is slightly shifted to the lower-angle region, indicating that the mesogens are packed a little more loosely. Upon heating to 110 °C, q1 and q2 are shifted to 1.67 and 3.33 nm−1, respectively, and the diffraction peak at q = 14.6 nm−1 becomes a halo which is attributed to a loss of the lateral correlation among the mesogens. The d-spacing of the smectic layers becomes significantly larger, which may suggest an LC-related transition. When the temperature is increased to 145 °C, the diffraction peaks of the LC layers also disappear, which indicates the Iso state. The 2D WAXD patterns of P12 recorded at different temperatures are shown in Figure 5. At ambient temperature, two pairs of sharp arcs, correlating to the smectic layer diffractions, appear on the equator in the low-angle region (Figure 5a). On the other hand, two pairs of high-angle diffraction arcs with the scattering maximum at around 20.5° (14.5 nm−1) are located in the quadrants. Because the polymer main chain is usually oriented parallel to the fiber direction, this diffraction pattern indicates that the smectic layers are parallel to the polymer chain, while the mesogens in the side chains are tilted with respect to the smectic layers. The diffraction pattern

Figure 5. 2D WAXD patterns of P12 at ambient temperature (a), 90 °C (b), and 110 °C (c) with the X-ray beam perpendicular to the fiber direction. D


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the 10 wt % AOA-12/IL was also conducted. However, the WAXS profile (Figure S3 in Supporting Information) only shows the halos related to the lateral arrangement of the mesogens and anion-to-anion correlations, and the diffraction peak from the LC phase is not observed, which can be attributed to the low concentration of the polymer. In addition, DSC and PLM experiments were also conducted on AOA-0 and its corresponding ion gels for comparison. The DSC curves (Figure S4 in Supporting Information) of AOA-0 and AOA-0/IL do not show any transition peaks corresponding to LC transitions in a temperature range of 0−160 °C. The DSC results indicate that no LC phases are formed in the AOA0 triblock copolymer and its ion gel. Meanwhile, the disappearance of the endothermic peak corresponding to the melting of the PEO crystal in the DSC curve of AOA-0/IL also indicates the solubility of the PEO block in the IL [EMIM][TFSI]. PLM micrographs (Figure S5 in Supporting Information) of the ion gel AOA-0/IL (10 wt %) do not show birefringence during the seconding heating and cooling processes because no LC phases are formed in the AOA-0/IL system. The PLM results of the ion gel AOA-0/IL also prove that the birefringence of the AOA-12/IL is caused by the formation of LC phases. Hierarchical Structure of the AOA-12/IL Ion Gel. In order to investigate the nanostructure of the AOA-12/IL system, synchrotron-radiation SAXS experiments were conducted on the AOA-12 triblock copolymer and its corresponding ion gels containing 20, 30, 40, and 50 wt % of AOA-12. The results are shown in Figure 7a. The SAXS profile of the AOA12 sample has two diffraction peaks with a scattering vector ratio of 1:31/2, characteristic of hexagonally packed cylinders (HEX) in which the PEO cylinders are dispersed in the LC matrix. The primary reflection is at q = 0.221 nm−1, corresponding to a d-spacing of 28.4 nm. For the HEX structure, the distance between two PEO cylinders (a) is equal to 2d/31/2, and thus a = 32.8 nm. The hierarchical structure was further studied by the combination of 2D WAXD and SAXS (Figure S2 in Supporting Information). The shear direction is along the long axis of the PEO cylinders, in agreement with results in the literature.29 In these two diffraction patterns, the diffractions attributed to the block copolymer (BCP) microphase separation are along the equatorial direction, while the smectic layer diffractions are present in the meridian direction, indicating that the smectic layers are arranged perpendicular to the long axis of the PEO cylinders. Similar to the homopolymer P12, the mesogens in the P12 side chains are also tilted with respect to the smectic layers because the diffractions originating from the lateral arrangement of the mesogens are located in the quadrants. For the diffractions from the PEO crystal, two pairs of the (120) diffractions are observed, with the pair of a stronger intensity present in the meridian direction and the other of much weaker intensity along the equatorial direction. This is consistent with the predicted [120] uniaxial diffraction pattern and implies that the PEO chain axis is parallel to the smectic layers26,29 (as shown by the top HEX structure in Figure 7b). When the ion gels are formed, the ILs will selectively swell into the PEO domains because of the compatibility between the IL and PEO. The incorporation of the IL brings the enhanced unfavorable interactions between the blocks and/or swelling,30−32 resulting in the changes of the nanostructures of the ion gel. The SAXS profiles of the 40 and 50 wt % AOA-12/IL ion gels exhibit diffraction peaks with a scattering vector ratio of

Figure 6. (a) DSC traces of P12, AOA-12, and AOA-12/IL (10 wt %) (during the second heating). (b) PLM micrographs of AOA-12/IL (10 wt %) at different temperatures (the ion gel was first heated to 160 °C and then cooled to 30 °C at a rate of 5 °C min−1; the observations were carried out during the second heating and cooling processes). (c) WAXS profiles of AOA-12 and AOA-12/IL (20 wt %) at ambient temperature.

diffraction peak at q = 1.86 nm−1 is the first-order diffraction of the smectic layers of the P12 block which has been determined by a combination of DSC and 1D/2D WAXD (Figure S2 in Supporting Information). For AOA-12/IL (20 wt %), the similar diffraction peak with q = 1.83 nm−1 associated with the LC ordering is also observed, indicating that the smectic phase of the P12 block is retained in the ion gel, which is consistent with the DSC and PLM results. The broad halo at q = 8.9 nm−1 can be attributed to anion-to-anion correlations,27,28 and the disappearance of the diffractions from the PEO crystal confirms that the PEO block is dissolved in [EMIM][TFSI], in agreement with the DSC results. The WAXS experiment on E


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block is retained in the BCP and all the ion gels. The above results suggest that the AOA-12/IL ion gels are hierarchically structured. Figure 7b illustrates the transitions of the hierarchical nanostructures of the AOA-12 triblock copolymer and its corresponding ion gels as the IL swells into the PEO domains. In addition, temperature-dependent SAXS measurements during heating from 30 to 150 °C (Figure S6 in Supporting Information) were also conducted on the AOA-12/ IL samples, and the SAXS profiles are barely changed throughout the entire experimental temperature range, indicating that the nanostructures of the ion gels remain unchanged. Synchrotron-radiation SAXS experiments were also conducted on the AOA-0 triblock copolymer and the AOA-0/IL ion gels with different concentrations of AOA-0 for comparison (Figure S7 in the Supporting Information). No diffraction peaks are observed in the SAXS profile of the AOA-0 triblock copolymer, indicating that it is not a strongly microphaseseparated system. With the addition of the IL, a diffraction peak appears, suggesting that the AOA-0/IL ion gels are microphaseseparated owing to the enhanced unfavorable interactions between the blocks with the selective swelling of the IL into the PEO domains. However, no higher-order diffractions are observed in the SAXS profiles, indicating poorly ordered structures in the AOA-0/IL ion gels. In addition, no diffraction peak at q = 1.83 nm−1 corresponding to the LC ordering is observed, implying that no LC phases are formed in the AOA0/IL ion gels, as expected. The weaker microphase separation of AOA-0 and AOA-0/IL compared with AOA-12 and AOA12/IL can be directly associated with the lack of LC phases. The rod−rod interactions in LC phases contribute to the increase of incompatibility. Effect of the LC Domains on the Rheological Behavior of the Ion Gel. In order to investigate the effect of the LC domains on the rheological behavior of the ion gel, a series of shear rheological experiments were performed. The gel points of the AOA-0/[EMIM][TFSI] and AOA-12/[EMIM][TFSI] binary systems were investigated by dynamic viscoelastic measurements. Representative data obtained at 25 °C are shown in Figure 8. For AOA-12/IL (Figure 8a), the 10 wt % sample is a gel because the G′ is clearly higher than the G″ over the entire frequency range and is independent of frequency at low frequencies. The 1 wt % sample is a viscous liquid and shows a complicated rheological response. The moduli have a crossover at a frequency (ω0) of around 11 rad s−1. When the frequency is lower than ω0, G″ is higher than G′, indicating a liquid-like behavior. When the frequency is higher than ω0, G′ becomes higher than G″, indicative of a solid-like behavior. The solid-like behavior at a high frequency can be attributed to the interchain associations which lead to the transient formation of a physical network.33,34 The moduli of the 2 wt % sample also exhibit a crossover at around 11 rad s−1. However, G′ and G″ are almost equal. The rheological behavior of the AOA-0/IL ion gel (Figure 8b) is similar to that of the AOA-12/IL system, except that the 2 wt % sample is a viscous liquid. All these results suggest that the gel point of AOA-n/IL is approximately 2 wt %. Lodge et al.9 reported a series of ion gels formed by PS-bPEO-b-PS (SOS) triblock copolymer (in which the Mn of the PEO block was 25.5 × 103 g mol−1, the Mn of each PS block was 4.7 × 103 g mol−1, the PDI value was 1.23, and the weight fraction of PEO was 72%) in the IL [BMIM][PF6]. The gel point of the binary system was 4 wt %, which is higher than that

Figure 7. Synchrotron-radiation SAXS profiles of the AOA-12 triblock copolymer and the AOA-12/IL ion gels (20, 30, 40, and 50 wt %) (a) and the schematic illustration of the transitions of the hierarchical nanostructures in AOA-12 triblock copolymer and its corresponding ion gels with increasing content of the IL swelling into the PEO domains (b).

1:2:4:5, characteristic of lamellar (LAM) structures (Figure 7b). The primary reflections of the two samples are at q values of 0.176 and 0.170 nm−1, corresponding to d-spacing values of 35.7 and 36.9 nm, respectively. On the other hand, the SAXS profile of the 30 wt % AOA-12/IL shows diffraction peaks with a scattering vector ratio of 1:31/2:71/2:131/2, characteristic of an inverse HEX structure in which the LC cylinders are dispersed in the PEO/IL matrix because of the swelling of the PEO domains by the IL (as shown by the bottom HEX structure in Figure 7b). The primary reflection is at q = 0.175 nm−1, corresponding to a d-spacing of 35.9 nm and a cylinder-tocylinder distance of 41.4 nm. However, for the 20 wt % AOA12/IL sample, no sharp diffraction peaks are observed for q values below 1.8 nm−1, indicating a poorly ordered structure. Meanwhile, the diffraction peak with q = 1.83 nm −1 corresponding to the LC ordering is always present in all the above samples, implying that the smectic LC phase of the P12 F


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Figure 8. Dynamic shear measurements of AOA-n/IL systems with various weight fractions of AOA-n at 25 °C (a: n = 12; b: n = 0).

Figure 9. Storage moduli and loss moduli as a function of strain for ion gels with 10 wt % AOA-0 (a) and AOA-12 (b) at different temperatures.

of the AOA-n/IL (n = 0, 12) system. The lower gel point of AOA-n/IL than that of SOS/IL can be attributed to the more IL-phobicity of the azobenzene-containing block and the strong interactions between the LC mesogens, with the consideration that the structure (PEO) and the Mn (25.5 × 103 g mol−1 vs 23.3 × 103 g mol−1) of the IL-philic segments are basically the same. On the other hand, the two systems have similar weight fractions of the IL-phobic blocks (1.1−1.4 wt % of SOS vs 0.5− 1.0 wt % of AOA-12), which may be another reason that the system in this work has a lower gel point. Dynamic strain sweeps at different temperatures were conducted with a frequency of 5 rad s−1. The results are shown in Figure 9. For the AOA-0/IL system, a linear regime is maintained below the critical value (γc) of approximately 10% where G′ is independent of the strain (Figure 9a). When a larger strain is applied, the polymer networks are broken, which is attributed to the slipping-off of the P0 chains from the physical cross-linkers. In addition, γc is invariant with the change in temperature. However, for the AOA-12/IL system, γc clearly decreases with increasing temperature, indicating that the aggregate strength of the physical cross-linkers is related to the state of the segments in the physical cross-linkers (Figure 9b).35,36 At ambient temperature, the LC interchain interactions and the long-range order of the mesogens contribute to the aggregate strength of the physical cross-linkers. Thus, γc is larger. When the temperature is 100 °C, P12 goes into the SmA phase, and the long-range lateral order of the mesogens disappears, leading to a lower aggregate strength of the physical cross-linkers. Thus, γc is decreased. When the temperature is higher than the Ti of the P12 block, the smectic LC phase is

destroyed, and thus γc is even smaller due to the further decrease in the aggregate strength of the physical cross-linkers. In addition, the plateau G′ values of the AOA-0/IL and the AOA-12/IL ion gels are 5.8 and 10 kPa at ambient temperature, respectively. According to the classical rubber elastic theory,37 the modulus is determined by the number density of the crosslinkers, and it can be expressed as G′ = νkBT, where ν is the number density of cross-linkers and kB the Boltzmann constant. For an ideal ion gel, all the mid-block chains bridge two micelles instead of looping back into the same core, and ν = cNA/Mx, where c is the concentration of the triblock copolymer in w/v, NA is the Avogadro’s number, and Mx is the MW between the cross-linkers. Thus, the modulus can be expressed as G′ = cf RT/Mx, where f is the fraction of bridging molecules inside the triblock copolymer. For the AOA-n/IL ion gels, Mx is the MW between entanglements of the PEO chains and can be estimated by Mx = Me,PEO/wAOA‑n, where Me,PEO is the entanglement MW of the molten PEO, which is 1.6 kDa at 140 °C, and wAOA‑n is the weight fraction of the AOA-n in the gel.11,38 Assuming that all the mid-block chains are effective (f = 1), the G′ values of 10 wt % AOA-0/IL and AOA-12/IL are 22.7 kPa, and the bridging fractions inside AOA-0 and AOA-12 are 1/4 and 1/2, respectively. This implies that more looped chains are present in the AOA-0/IL system than those in the AOA-12/IL ion gels, resulting in a lower plateau storage modulus. Dynamic temperature ramp tests from 25 to 170 °C at a rate of 1 °C min−1 in the second heating and seconding cooling procedures following a 5 °C min−1 cooling process were conducted with a strain of 2% and a frequency of 1 rad s−1. The G


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Macromolecules

still keeps its gel properties. Because of the temperature limitation of the apparatus, the dynamic temperature sweep data at temperatures higher than 170 °C were not obtained. Alternatively, we placed the 10 wt % AOA-0/IL and AOA-12/ IL ion gels on the hot stage and investigated the transitions with increasing temperature at a rate of 10 °C min−1. The AOA-12/IL system still kept as an ion gel up to 300 °C, while the AOA-0/IL ion gel underwent a gel−sol transition at approximately 200 °C, which can be attributed to the different solubilities of the two blocks in the IL. Representative photos are shown in Figure S9 of the Supporting Information. Ion Conductivity of the Ion Gel. The ionic conductivities (σ’s) of the 10 and 20 wt % AOA-12/IL ion gels were investigated as a function of temperature (Figure 11). These

results are shown in Figure 10. The storage modulus of the 10 wt % AOA-0/IL ion gel shows a continuous and monotonic

Figure 11. Temperature dependence of ionic conductivity for [EMIM][TFSI] and ion gels with 10 and 20 wt % of AOA-12. Dashed lines are best fits using the VTF equation.

Figure 10. Dynamic temperature sweep of 10 wt % AOA-0 (a) and AOA-12 (b) in IL.

ion gels are used for testing for the sake of easy handling during measurements. The temperature dependency of σ for the ion gels is represented by the Vogel−Fulcher−Tammann (VFT) equation, and the dashed lines in Figure 11 demonstrate that the temperature dependencies of the σ values follow the equation well (see Table S1 in the Supporting Information for fitting parameters). Both ion gels show high conductivities (about 0.7 × 10−2 S cm−1 at 30 °C) which are comparable to those of the pure IL. The reduction in σ can be attributed to the addition of the nonconductive polymer, especially the P12 domains. The ion gels with 2−4 wt % of AOA-12 are expected to have even higher σ values which may approach those of the pure IL. In addition, it should be noted that the ionic conductivities of the ion gels are continuous as expressed by the VFT equation from 30 to 150 °C even though the ion gels undergo two sharp decreases in the storage moduli during the LC transitions in the physical cross-linkers. This indicates that the ionic conductivities of the ion gels are not affected by the changes in the storage moduli of the ion gels following such LC transitions. This phenomenon is similar to that of the polystyrene-b-poly(methyl methacrylate)-b-polystyrene/ [EMIM][TFSI] ion gel3 because the motion of [EMIM][TFSI] is relatively decoupled from that of the PEO segments owing to the fact that [EMIM][TFSI] is composed of an extremely weak Lewis acidic cation and an extremely weak Lewis basic anion.

decrease with increasing temperature in the range of 110−150 °C (Figure 10a). However, for the 10 wt % AOA-12/IL ion gel, two sharp decreases in the storage modulus are observed in temperature ranges where the transitions of the LC phases of the P12 block occur (Figure 10b) according to the DSC and the PLM results. All these results suggest that the changes in the storage modulus are associated with the LC phase transitions of the P12 block with increasing temperature because the temperature-dependent SAXS results do not show any changes in the nanostructures of the ion gels (Figure S6 in Supporting Information). As mentioned before, the aggregate strength of the physical cross-linkers is related to the state of the segments in the physical cross-linkers, and it decreases following the transitions of the LC phases of the P12 block during heating. A lower aggregate strength means the faster exchange of the end-blocks in and out of the aggregates which allows the cross-linkers to become mobile and rearrange to reduce the stress. During the cooling process, the storage and loss moduli of the AOA-0/IL and AOA-12/IL ion gels show reversible transitions corresponding to those during heating. For AOA-12/IL ion gels, two hysteresis loops are observed in temperature ranges of the LC transitions of the P12 block, consistent with the DSC results (Figure S8). In addition, the loss modulus is lower than the storage modulus in the temperature range investigated, indicating that a solid-like behavior is retained. Interestingly, the storage modulus and loss modulus of the AOA-0/IL ion gel tend to cross each other with further increase in temperature, while the AOA-12/IL ion gel

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CONCLUSION In conclusion, we successfully prepared an ion gel by the selfassembly of a triblock copolymer containing a thermotropic LC H


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Macromolecules

(9) He, Y.; Boswell, P. G.; Bühlmann, P.; Lodge, T. P. J. Phys. Chem. B 2007, 111, 4645−4652. (10) Zhang, S.; Lee, K. H.; Frisbie, C. D.; Lodge, T. P. Macromolecules 2011, 44, 940−949. (11) Zhang, S.; Lee, K. H.; Sun, J.; Frisbie, C. D.; Lodge, T. P. Macromolecules 2011, 44, 8981−8989. (12) He, Y.; Lodge, T. P. Chem. Commun. 2007, 2732−2734. (13) He, Y.; Lodge, T. P. Macromolecules 2008, 41, 167−174. (14) Kitazawa, Y.; Ueki, T.; Niitsuma, K.; Imaizumi, S.; Lodge, T. P.; Watanabe, M. Soft Matter 2012, 8, 8067−8074. (15) Noro, A.; Matsushita, Y.; Lodge, T. P. Macromolecules 2008, 41, 5839−5844. (16) Noro, A.; Matsushita, Y.; Lodge, T. P. Macromolecules 2009, 42, 5802−5810. (17) Noro, A.; Matsushima, S.; He, X.; Hayashi, M.; Matsushita, Y. Macromolecules 2013, 46, 8304−8310. (18) Gu, Y.; Zhang, S.; Martinetti, L.; Lee, K. H.; McIntosh, L. D.; Frisbie, C. D.; Lodge, T. P. J. Am. Chem. Soc. 2013, 135, 9652−9655. (19) Verploegen, E.; Soulages, J.; Kozberg, M.; Zhang, T.; McKinley, G.; Hammond, P. Angew. Chem., Int. Ed. 2009, 48, 3494−3498. (20) Xu, Y.; Yang, Q.; Shen, Z.; Chen, X.; Fan, X.-H.; Zhou, Q.-F. Macromolecules 2009, 42, 2542−2550. (21) Shi, L.-Y.; Pan, Y.; Zhang, Q.-K.; Zhou, Y.; Fan, X.-H.; Shen, Z.H. Chin. J. Polym. Sci. 2014, 32, 1524−1534. (22) Bonhôte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Grätzel, M. Inorg. Chem. 1996, 35, 1168−1178. (23) Wolff, D.; Cackovic, H.; Krüger, H.; Rübner, J.; Springer, J. Liq. Cryst. 1993, 14, 917−928. (24) Zhu, X.-Q.; Liu, J.-H.; Liu, Y.-X.; Chen, E.-Q. Polymer 2008, 49, 3103−3110. (25) Zhou, Y.; Ahn, S.-k.; Lakhman, R. K.; Gopinadhan, M.; Osuji, C. O.; Kasi, R. M. Macromolecules 2011, 44, 3924−3934. (26) Zhu, L.; Cheng, S. Z. D.; Calhoun, B. H.; Ge, Q.; Quirk, R. P.; Thomas, E. L.; Hsiao, B. S.; Yeh, F.; Lotz, B. J. Am. Chem. Soc. 2000, 122, 5957−5967. (27) Triolo, A.; Russina, O.; Fazio, B.; Triolo, R.; Di Cola, E. Chem. Phys. Lett. 2008, 457, 362−365. (28) Triolo, A.; Russina, O.; Fazio, B.; Appetecchi, G. B.; Carewska, M.; Passerini, S. J. Chem. Phys. 2009, 130, 164521. (29) Huang, P.; Zhu, L.; Cheng, S. Z. D.; Ge, Q.; Quirk, R. P.; Thomas, E. L.; Lotz, B.; Hsiao, B. S.; Liu, L.; Yeh, F. Macromolecules 2001, 34, 6649−6657. (30) Parveen, N.; Schönhoff, M. Macromolecules 2013, 46, 7880− 7888. (31) Noro, A.; Tomita, Y.; Shinohara, Y.; Sageshima, Y.; Walish, J. J.; Matsushita, Y.; Thomas, E. L. Macromolecules 2014, 47, 4103−4109. (32) Zardalidis, G.; Ioannou, E. F.; Gatsouli, K. D.; Pispas, S.; Kamitsos, E. I.; Floudas, G. Macromolecules 2015, 48, 1473−1482. (33) Witten, T. A.; Cohen, M. H. Macromolecules 1985, 18, 1915− 1918. (34) Winnik, M. A.; Yekta, A. Curr. Opin. Colloid Interface Sci. 1997, 2, 424−436. (35) Sato, T.; Watanabe, H.; Osaki, K. Macromolecules 2000, 33, 1686−1691. (36) Seitz, M. E.; Burghardt, W. R.; Faber, K.; Shull, K. R. Macromolecules 2007, 40, 1218−1226. (37) Hiemenz, P. C.; Lodge, T. P. Polymer Chemistry; CRC Press: New York, 2007. (38) Fetters, L.; Lohse, D.; Richter, D.; Witten, T.; Zirkel, A. Macromolecules 1994, 27, 4639−4647.

block in a room-temperature IL [EMIM][TFSI]. The ion gel obtained has a storage modulus as high as approximately 10 kPa, while its conductivity is close to that of the pure IL mainly because of the low content of the triblock copolymer. The ion gels with relatively high contents of the triblock copolymer have hierarchical structures including the LC arrangement of the P12 blocks and the nanostructure owing to the microphase separation of the LC domains and the PEO/IL domains. The LC domains serve as the physical cross-linkers. The storage modulus of the ion gel can be tuned by temperature because of the thermotropic phase behavior of the LC block. In this respect, the ion gels are potentially useful as high-temperature ionic membranes or thermal-responsive soft actuators.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedures, 2D WAXD pattern of AOA-12 at ambient temperature, WAXS profiles of AOA-12, AOA-12/IL (10 wt %), and AOA-12/IL (20 wt %), DSC traces of P0, AOA0, and AOA-0/IL (10 wt %), PLM micrographs of AOA-0/IL (10 wt %) at different temperatures, SAXS profiles of the 10, 20, 30, 40, and 50 wt % AOA-12/IL at different temperatures, and complex viscosity vs angular frequency for shear thickening effects of a 1 wt % AOA-12/IL solution. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01103.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Beijing Natural Science Foundation (Grant 2142016). The authors thank Prof. Longcheng Gao at Beihang University, Prof. Chenyang Liu at the Chinese Academy of Sciences, and our colleague Prof. Shuang Yang for helpful discussions. We also thank Prof. Shanfu Lu at Beihang University for access to the impedance spectroscopy equipment and our colleague Prof. Dehai Liang for access to the rheological equipment. The authors gratefully acknowledge the scientists at beamline 1W2A at BSRF and at beamline BL16B1 at SSRF for their assistance on the synchrotron-radiation SAXS experiments. The authors are also thankful for assistance during SAXS experiments.

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REFERENCES

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Thermoreversible Ion Gel with Tunable Modulus Self-Assembled by a

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