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Hierarchical Sol−Gel Transition Induced by Thermosensitive SelfAssembly of an ABC Triblock Polymer in an Ionic Liquid Yuzo Kitazawa,† Takeshi Ueki,‡ Lucas D. McIntosh,§ Saki Tamura,† Kazuyuki Niitsuma,† Satoru Imaizumi,† Timothy P. Lodge,§,∥ and Masayoshi Watanabe*,† †

Department of Chemistry & Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ‡ Department of Materials Engineering Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan § Department of Chemical Engineering and Materials Science and ∥Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States S Supporting Information *

ABSTRACT: We investigated a hierarchical morphology change and accompanying sol−gel transition using a doubly thermosensitive ABC-triblock copolymer in an ionic liquid (IL). The triblock copolymer contains two different lower critical solution temperature (LCST) thermosensitive polymers, poly(benzyl methacrylate) (PBnMA) and poly(2-phenylethyl methacrylate) (PPhEtMA), as the end blocks and poly(methyl methacrylate) (PMMA) as the middle block (PBnMA-b-PMMA-b-PPhEtMA: BMP). BMP undergoes a hierarchical phase transition corresponding to the selfassembly of each of the thermosensitive blocks in the IL, and a sol−gel transition was observed in concentrated, above 10 wt %, polymer solutions. The gelation behavior was affected by polymer concentration, and at 20 wt %, the BMP/IL composite showed a phase transition, with increasing temperature, from solution through a jammed micelle suspension to a physically cross-linked gel. Each phase was formed reversibly and rapidly over the corresponding temperature range. The jammed micelle and cross-linked gel states were characterized using viscoelastic measurements and small-angle X-ray scattering (SAXS).

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INTRODUCTION Stimuli-responsive polymers change their solubility in response to stimuli, such as temperature, light, pH, electric field, and solvent composition. For example, poly(N-isopropylacrylamide) (PNIPAm), a well-known thermosensitive polymer, has a lower critical solution temperature (LCST) phase transition at ca. 32 °C in water.1,2 Because this phase transition temperature (Tc) is close to human body temperature, PNIPAm has been widely studied for applications in drug delivery,3 surface property switching,4 optical property control,5 and gelation.6 However, a serious problem of these materials is the solvent evaporation that occurs if they are used even under atmospheric pressure. Recently, room-temperature ionic liquids (RTILs) have received much attention due to their unique properties such as negligible volatility, nonflammability, high thermal stability, and high ionic conductivity. As RTILs consist entirely of ions, they show unique solvation toward low-molecular-weight compounds and polymers when compared to conventional molecular solvents.7−11 Previously, we found that certain stimuli-responsive polymers in RTILs; for example, both poly(benzyl methacrylate) (PBnMA) and poly(2-phenylethyl © XXXX American Chemical Society

methacrylate) (PPhEtMA) show LCST phase transitions in the hydrophobic RTIL 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mim][NTf2]) at 105 and 42 °C, respectively.12−15 The large difference in phase transition temperature, Tc, between these two polymers is caused by the presence of an extra methylene spacer between the aromatic ring and the carbonyl group in the monomer unit. In addition to RTILs, ion gels, i.e., three-dimensional networks swollen by RTILs, are also of interest as soft−solid materials with properties derived from RTILs. Many methods have been proposed to form such networks in RTILs, such as in situ polymerization of monomers and cross-linkers,16 cross-end coupling of multifunctional macromers,17,18 supramolecular interactions,19−22 blending with nanomaterials,23−25 and selfassembly of block copolymers.26−28 Because of their remarkable properties, such ion gels have been studied as solid electrolytes for electrochemical devices, 29−31 gas separation membranes,32,33 and light reflective materials.23,34 Among these Received: December 1, 2015 Revised: January 23, 2016

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DOI: 10.1021/acs.macromol.5b02616 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthetic Procedure for a Thermosensitive Triblock Copolymer PBnMA-b-PMMA-b-PPhEtMA Using RAFT Polymerization

the polymer network inevitably has a significant fraction of loop chains in the B-blocks. Because the loops do not contribute to network elasticity, the amount of polymer needed for gelation increases in these copolymers. Furthermore, the ABC-triblock copolymer has a very sharp phase transition at low polymer concentrations due to stepwise self-assembly, from micellization to gelation. In addition, each end block forms separate cores. A decrease in structural defects in the polymer networks formed by self-assembly of the ABC-triblock copolymer has been supported by dissipative particle dynamics simulations.40 Here we report the thermoresponsive micellization and gelation behavior of an ABC-triblock copolymer in an RTIL and the microphase separation structure of the polymer network.

methods, we have focused on the formation of polymer networks by the self-assembly of block copolymers. For example, the amphiphilic ABA-triblock copolymer polystyrene-b-poly(ethylene oxide)-b-polystyrene (PSt-b-PEO-b-PSt) can gel RTILs by forming cross-links; these cross-linkages arise because of the self-assembly of RTIL-phobic PSt blocks and bridging chains of RTIL-philic PEO, where RTILs are preferentially dissolved.26 In addition, block copolymers containing stimuli-responsive polymer segments have also been the subject of intense research.35,36 For example, an ABA-triblock copolymer consisting of thermosensitive A blocks and an RTIL-philic B block exhibits thermoreversible sol−gel transitions due to the thermosensitive self-assembly of the A block.37 The thermoresponsive phase transition temperature of block copolymers is readily controlled by tuning of chemical structures of RTILs or thermosensitive polymers.37,38 In this work, we demonstrate the hierarchical thermoreversible phase transitions of an ABC-triblock copolymer that has a different thermosensitive polymer at each end (A and C blocks) and an RTIL-philic B block. The three blocks are PBnMA, poly(methyl methacrylate) (PMMA), and PPhEtMA. [C2mim][NTf2] was chosen as the solvent. Compared with the ABA- and CBC-triblock copolymer, the ABC-triblock copolymer shows three different states over three different temperature ranges. First, the three blocks are compatible with [C2mim][NTf2] at low temperature; consequently, the copolymers dissolve as unimers. Next, when the temperature is raised between the lower Tc (for PPhEtMA) and higher Tc (for PBnMA) of the two thermosensitive blocks, the copolymers form micelles with PPhEtMA cores and PMMAb-PBnMA coronas. Finally, at temperatures above the Tc of both thermosensitive blocks, the PBnMA end blocks in the corona self-aggregate. Under these conditions and above the micellar overlap concentration, we expected that a network structure would form due to cross-linking of the micelles with each other. Such hierarchical self-assembly using an ABC-triblock copolymer reduces structural defects in the polymer network compared with those formed using an ABA-triblock copolymer. For example, Zhou et al. reported a distinct difference in gelation behavior for ABA- and ABC-triblock copolymers with an LCST A-block in water.39 In an ABA-triblock copolymer,

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EXPERIMENTAL SECTION

Chemicals. BnMA and MMA monomers were purchased from Wako Chemicals, and PhEtMA monomer was synthesized and characterized according to a previous report.38 Prior to use, these monomers were purified by distillation under reduced pressure over CaH2. The ionic liquid, [C2mim][NTf2],41 and a chain transfer agent (CTA), 2-cyanoprop-2yl 1-dithionaphthalate (CPDN),42 were synthesized and characterized according to literature procedures with slight modifications. 2,2′-Azobis(isobutyronitrile) (AIBN) was purchased from Sigma-Aldrich and was recrystallized from methanol prior to use. All other chemical reagents were used as received unless otherwise noted. Synthesis of PBnMA Macro-CTA. The ABC-triblock copolymer, PBnMA-b-PMMA-b-PPhEtMA (BMP), was synthesized using reversible addition−fragmentation chain-transfer (RAFT) polymerization, following the procedure in Scheme 1. In the first step, a PBnMA macro-chain-transfer agent (macro-CTA) was prepared. CPDN (0.26 g, 0.96 mmol) was placed in a 50 mL Schlenk flask, and the atmosphere was replaced with nitrogen gas. Then, dry anisole (56 mL) and BnMA (58 mL, 0.34 mol) were added to the flask, and nitrogen gas was bubbled through the stirred solution for 15 min. AIBN (54 mg, 0.33 mmol) was dissolved in dry anisole (2 mL) in a second round-bottom flask and deaerated by bubbling nitrogen gas through the solution for 15 min. After deaerating both flasks, the AIBN solution was added to the monomer solution. RAFT polymerization was carried out at 60 °C for 10 h and was terminated by quenching the solution with dry ice/methanol. The product was purified by two rounds of reprecipitation using ethyl acetate and methanol as the “good” and “poor” solvents, respectively. The product was dried for 24 h at room temperature under vacuum. The PBnMA macroinitiator was characterized using 1H NMR spectroscopy and size exclusion B


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temperature for 2 days and then dried under vacuum to evaporate the cosolvent for 24 h. Dynamic Light Scattering (DLS). DLS measurements were performed using a DLS-6500 (Otsuka Electronics Co., Ltd.) equipped with an ALV correlator and an Ar laser (488 nm). Experiments were conducted in the temperature range from 25 to 140 °C within an accuracy of ±0.1 °C. The intensity autocorrelation functions g2(q,t) were recorded at scattering angles of 40°, 60°, 80°, 90°, and 120° for 100 s. The electric field correlation function g1(q,t) was converted from g2(q,t) using the Siegert relation (g2(q,t) (= fg12(q,t) + 1; f ≤ 1 is an instrument coherence factor)).43 The mean hydrodynamic radius (Rh) of BMP in [C2mim][NTf2] was estimated using cumulant fitting, and the size distribution functions were obtained by the CONTIN program, as in a previous report.44 Sample solutions were passed through 0.20 μm filters to eliminate dust prior to use. Rheological Measurement. Dynamic viscoelastic measurements were performed on an Anton Paar Physica MCR 301 rheometer using the parallel plate geometry with 50 mm diameter plates. A gap spacing of approximately 0.5 mm was used for all measurements. The elastic moduli (G′ and G″) were examined in the linear viscoelastic regime. The frequency dependence of the elastic moduli over the range of 0.1−100 rad s−1 was obtained with a strain amplitude γ = 1% from 0 to 140 °C with an accuracy of ±0.1 °C. The temperature was changed in increments of 10 °C, and the measurements were performed after waiting for 60 min so that BMP/[C2mim][NTf2] composites could achieve equilibrium. Steady flow measurements were performed using cone-and-plate geometry with 25 mm (cone angle = 2°) and 50 mm (cone angle = 1°) diameters. The samples were annealed at 70 °C for 1 h before the measurement. The flow curves were obtained over the shear rate range of 10−4−103 s−1. Small-Angle X-ray Scattering (SAXS). SAXS measurements were performed at the Argonne National Laboratory Advanced Photon Source. Samples with 10 and 20 wt % BMP in [C2mim][NTf2] were used for the measurements. Samples were prepared by heating at 50 °C under dynamic vacuum overnight to release dissolved air. Then, the samples were sealed in aluminum TA Instruments hermetic differential scanning calorimetry (DSC) pans. Samples were exposed at 30, 70, and 140 °C with a 5 min annealing period at each temperature. Scattering data were fit to a hard-sphere model, which included terms for the self-correlation of the cores and the cross-correlation of different cores. The form factor, P, describing the self-correlation of a sphere with radius Rc is given by eq 1

chromatography (SEC). The number-average molecular weight (Mn) was calculated from the monomer conversion in reaction solution determined by 1H NMR, and the dispersity Đ (Mw/Mn, where Mw is the weight-average molecular weight) was determined by SEC calibrated with PMMA standards using tetrahydrofuran (THF) as the eluent. Synthesis of PBnMA-b-PMMA Macro-CTA. In the second step, the PBnMA-b-PMMA macro-CTA was prepared using the PBnMA macro-CTA. PBnMA macro-CTA (6.6 g, 0.19 mmol) was placed in a 100 mL three-necked round-bottom flask, and the atmosphere was replaced with nitrogen gas. Then, dry anisole (23 mL) and MMA (25 mL, 0.24 mol) were added to the flask and stirred with nitrogen gas bubbling for 15 min. AIBN (11 mg, 6.7 × 10−2 mmol) was dissolved in dry anisole (2 mL) in second round-bottom flask and deaerated by bubbling nitrogen gas through the solution for 15 min. After bubbling nitrogen through both flasks, the AIBN solution was added to the monomer solution. RAFT polymerization was carried out at 60 °C for 20 h and was terminated by quenching the solution with dry ice/ methanol. The product was purified twice by reprecipitation twice using ethyl acetate and methanol as the “good” and “poor” solvents, respectively. The product was dried for 24 h at room temperature under vacuum. The Mn of the PMMA block was calculated from the results of 1H NMR using the integral intensity of PBnMA. The dispersity of the PBnMA-b-PMMA macroinitiator was determined by SEC calibrated with PMMA standards using THF as the eluent. Synthesis of PBnMA-b-PMMA-b-PPhEtMA and Sample Preparation. In the third step, PBnMA-b-PMMA-b-PPhEtMA macro-CTA was prepared using the PBnMA-b-PMMA macro-CTA and a similar procedure to that of the second step. PBnMA-b-PMMA macro-CTA (6.0 g, 5.1 × 10−2 mmol) in dry anisole (30 mL) and PhEtMA (10 mL, 51 mmol) were mixed and deaerated. AIBN (3.6 mg, 2.2 × 10−2 mmol) in dry anisole (3 mL) was deaerated and added to the monomer solution. RAFT polymerization was carried out at 60 °C for 4 h, terminated by quenching, and twice purified by reprecipitation using ethyl acetate and methanol as the “good” and “poor” solvents, respectively. Finally, the dithionaphthalate group attached to the terminus of the triblock copolymer was removed by the following procedure. The triblock copolymer with CTA (7.1 g, 4.9 × 10−2 mmol) and AIBN (0.33 g, 2.0 mmol) were placed in a 200 mL three-necked roundbottom flask, and the atmosphere was replaced with nitrogen gas. Then, dry anisole (45 mL) was added to the flask and stirred to dissolve all agents. The reaction was performed at 80 °C for 18 h and was terminated by quenching the solution with dry ice/methanol. The product was twice purified by reprecipitation using ethyl acetate and methanol as the “good” and “poor” solvents, respectively. The product was dried for 24 h at room temperature under vacuum. After the removal reaction, the product changed color from pastel red to white. The Mn of the PPhEtMA block was calculated from the results of 1H NMR relative to the integral intensity of PBnMA. The dispersity of BMP was determined by SEC calibrated with PMMA standards using THF as the eluent. The triblock copolymer showed a relatively narrow dispersity, and the SEC profile was unimodal (see Figure S1). The characterization results of the synthesized polymers are summarized in Table 1 and Figure S2. The BMP/[C2mim][NTf2] composites were prepared by a cosolvent method. BMP was mixed with the cosolvent (THF) and [C2mim][NTf2]. The homogeneous solution was stirring at ambient

Pcore(q) = Nagg 2βcore 2Acore 2 (q)

where Acore is the scattering amplitude from a uniform hard sphere and is given by Acore(q) = 3[sin(qR c) − qR c cos(qR c)]/(qR c)3

PBnMA PBnMA-b-PMMA PBnMA-b-PMMA-b-PPhEtMA

Đ

35 35−83 35−83−27

1.24 1.33 1.51

(2)

Nagg is the aggregation number of chains in a core, β is the excess scattering length density of the core relative to the IL and corona matrix, and q is the scattering wave vector. The SAXS data presented here were not reduced to absolute intensity, so the prefactor Nagg2β2 was ignored and was replaced by an arbitrary multiplicative fitting parameter. The structure factor for low-q scattering was described using the Percus−Yevick closure relation,45 which gives the direct correlation function, c(r), as a function of interparticle spacing, r, for particles with excluded volume interaction potential; that is, scatterers experience infinite interaction potential at a center-to-center distance less than or equal to 2Rc and have zero interaction potential with particles further than 2Rc away. The direct correlation function given in eq 3.

Table 1. Characterization of Macro-Chain-Transfer Agents and ABC Triblock Copolymer Mn/kDa

(1)

xBnMA, xPhEtMAa

c(r ) = 0

for r > 2R c 3

0.17, 0.13

c(r ) = − λ1 − 6ϕλ 2

a

The mole fraction of the PBnMA and PPhEtMA repeat units was estimated from Mn of the PBnMA and PPhEtMA blocks, respectively.

ϕ r r − λ1 for r ≤ 2R c 2R c 2 (2R c)3

(3)

ϕ is the volume fraction of spheres and is calculated as C


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Macromolecules ϕ=

4 3 πNR ̅ c 3

(4)

N̅ is the number volume density of scattering particles. The parameters λ1 and λ2 in eq 3 are defined as

λ1 = λ2 =

(1 + 2ϕ)2 (1 − ϕ)4

− (1 + ϕ/2)2 (1 − ϕ)4

(5)

In Fourier space, the direct correlation function, C(q), is 3 NC ̅ (q) = − 24ϕ{λ1[(sin(x) − x cos(x))/x ]

− 6ϕλ 2[(x 2 cos(x) − 2x sin(x) − 2 cos(x) + 2)/x 4] λ − ϕ 1 [(x 4 cos(x) − 4x 3 sin(x) − 12x 2 cos(x) + 24x sin(x) 2 + 24 cos(x) − 24)/x 6]} (6)

Figure 1. Temperature dependence of Rh for BMP in [C2mim][NTf2] (0.1 wt %). The inset shows the temperature dependence of the normalized scattering intensity.

where x = 2qRc. The structure factor, S(q), is given by eq 7. S(q) =

1 1 − NC ̅ (q)

(7)

The total scattered intensity from an isotropic solution of uniform spheres is given by eq 8.

Iuniform(q) ∼ Pcore(q)S(q)

(8)

To account for dispersity in core radius, a Gaussian distribution was assumed, given by D(R c) =

⎡ − (R − ⟨R ⟩)2 ⎤ 1 c c ⎥ exp⎢ 2π σR 2σR 2 ⎣ ⎦

for R c ≥ 0

(9)

Figure 2. Distribution function of Rh for a 0.1 wt % BMP in [C2mim][NTf2] at low, middle, and high temperature. The functions were calculated by CONTIN using the autocorrelation function at a scattering angle of 90°.

where σR is the standard deviation of core radii about the mean, ⟨Rc⟩. The scattered intensity from a solution of disperse spheres is then given by

∫ D(Rc)Iuniform(q) dRc ∼ S(q) ∫ D(Rc)Pcore(q) dRc = C1[S(q) ∫ D(R c)Pcore(q) dR c] + C 2

I(q) ∼

the Rh distribution function of BMP analyzed by the CONTIN program at selected temperatures. The disappearance of small scatterers and the appearance of large assemblies are observed over the mid-temperature region. The narrow size distribution of the scatterers indicates polymer micelles with one predominant morphology. The system was heated continuously, and when the temperature reached 120 °C, the Rh value decreased to ca. 70 nm. This temperature is also comparable to the Tc of the PBnMA block in block copolymer,36,44 supporting a structural transition of the PBnMA blocks at the end of the corona. We hypothesize that the changes in Rh above 120 °C are due to the deformation of the micelles from spherical “hairy micelles” toward more “flower-like micelles”. Zhou et al. investigated the phase transition behavior of micelles using an analogous ABC-triblock copolymer, consisting of a hydrophobic block (poly(ethylene-alt-propylene), PEP), a hydrophilic block (PEO), and an LCST thermosensitive block (PNIPAm) as the A, B, and C blocks, respectively, in water.46 The triblock copolymer formed micelles with PEP cores and coronas of PEO-b-PNIPAm below Tc of PNIPAm. They reported that if the PNIPAm blocks collapse into hydrophobic cores or form a thin layer or “sticky” PNIPAm patches around the hydrophilic shell, the micellar morphology at temperatures above the phase transition of PNIPAm may be a flower-like. The experiments showed an increase in Rh of the micelle due to the phase transition of PNIPAm, and thus, they suggested that

(10)

C1 and C2 are arbitrary scaling parameters.

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RESULTS AND DISCUSSION Thermosensitivity of BMP in [C2mim][NTf2]. The thermosensitive self-assembly of BMP in [C2mim][NTf2] was investigated by DLS. Figure 1 shows the temperature dependence of the mean Rh for BMP in [C2mim][NTf2] at dilute concentration (0.1 wt %), and the inset shows the temperature dependence of the normalized scattering intensity. The normalized scattering intensity was defined as the intensity at each temperature divided by that at 30 °C. These results show three different Rh in the measured temperature range. At low temperatures (≤60 °C), littler scattering was observed and Rh was approximately constant at ≈10 nm, indicating unimolecular dissolution in [C2mim][NTf2]. When the temperature reached 70 °C, Rh abruptly increased. The temperature is slightly higher than the Tc of the PPhEtMA homopolymer but is consistent with the previously reported Tc of PPhEtMA in a block copolymer;38 thus, BMP micelle formation due to the LCST phase behavior of the PPhEtMA block is implied. Judging from the molar fraction of the blocks, the morphology of these micelles is expected to be spherical. However, in the mid-temperature region (80−110 °C), the micelle size was almost constant (ca. 85 nm). Figure 2 shows D


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BMP/[C2mim][NTf2] system, we investigated the frequency dependence of the moduli. Figure 4 shows the frequency

it is likely that the ABC-triblock copolymers form micellar aggregates as a consequence of interactions between micelles with sticky patches; however, they are unlikely to form flowerlike micelles. They speculated that the formation of flower-like micelles is entropically unfavorable because the collapsed PNIPAm blocks cannot penetrate the long hydrophilic PEO shell at temperatures greater than the Tc of PNIPAm. Furthermore, the collapsed PNIPAm blocks cannot cover the PEP core, even at high temperatures. In contrast, the Rh of the micelle with a PPhEtMA core decreased at temperatures greater than the Tc of PBnMA. The temperature-dependent scattered intensity also showed a decrease upon aggregation of PBnMA block (see the inset in Figure 1), and the Rh distribution function at 140 °C was unimodal. These results suggest that the PBnMA block of the micelles underwent an intramicellar transition. This difference in the structural transitions of the micelles may be dependent on the low polymer concentration of the IL (0.1 wt %). Indeed, for a 1 wt % polymer system, the solution became cloudy at temperatures greater than the Tc of PBnMA, indicating the formation of micellar aggregates in solution, as suggested by Zhou et al.46 Thus, we concluded that the temperature dependence of the ABC-triblock copolymer is due to a unimer to hairy micelle to flower-like micelle transition due to the double thermosensitivity of the PPhEtMA and PBnMA blocks. Concerning this transition, we also considered the possibility of a change in Nagg in the high temperature region, but we could neither determine nor compare Nagg. Gelation Behavior of BMP/[C2mim][NTf2] Composites. Based on the observation of the LCST phase transition of each end block in the ABC-triblock copolymer by DLS, a sol-to-gel transition of BMP/[C2mim][NTf2] mixtures is expected above some critical concentration. Figure 3 shows the temperature

Figure 4. Frequency dependence of the dynamic moduli (G′ and G″) for 20% BMP in [C2mim][NTf2] at 25, 70, and 140 °C. The measurement was performed at a strain γ = 1%.

dependence of the dynamic moduli for a 20 wt % BMP/ [C2mim][NTf2] mixture at selected temperatures. The frequency dependence indicates power laws typical of viscous liquids (G′ ∼ ω2, G″ ∼ ω) at low temperatures. Furthermore, at high temperatures (above the Tc of PBnMA), the moduli did not depend on the frequency, consistent with gelation. However, at temperatures ranging from 40 to 100 °C (intermediate temperatures), G′ and G″ showed similar power laws (G′, G″ ∼ ω0.5), corresponding to an intermediate situation between a sol and a gel.47 Therefore, we assume that the apparent solid-like state at intermediate temperatures was not due to the formation of a polymer network by the ABCtriblock copolymer, but rather to jamming of the micelles. Sugihara et al. reported similar behavior using micellization of a thermosensitive diblock copolymer in aqueous solution,48 and Gkermpoura et al. reported the apparent gelation of a micelle suspension in RTIL.49 Above the overlap concentration of the polymer micelles (c*), the closely packed micelles had solid-like rheological properties, even though there is no polymer network present. We therefore conclude that the BMP triblock copolymer also shows a sol-to-jamming micelle transition due to the LCST phase transition of the PPhEtMA block. The jammed micelle system was also observed as an equilibrium state over the mid-temperature region on the heating and cooling temperature-sweep measurement (see Figure S3). The profile also shows the jammed micelle state over the same temperature interval on both heating and cooling, and this remained constant over multiple thermal cycles (see Figure S4). In addition, the profiles showed only modest hysteresis during the phase transition of the PPhEtMA block. This phenomenon is consistent with a previous report.38 When Tc − Tg is large, hysteresis is not observed (e.g., PBnMAb-PMMA-b-PBnMA in [C2mim][NTf2], Tc − Tg = 105 − 54 °C = 51 °C). When Tc − Tg is small, hysteresis is observed (e.g., PPhEtMA-b-PMMA-b-PPhEtMA in [C2mim][NTf2], Tc − Tg = 40 − 38 °C = 2 °C). This is because redissolution of the thermosensitive block is accompanied by a rubber-to-glass transition upon cooling. Nevertheless, these results indicate rapid phase transitions and good thermoreversibility of BMP/ [C2mim][NTf2] mixtures. On the other hand, for a 10 wt % BMP solution, gelling behavior was observed on heating (see Figure 5). However, the mixture was liquid-like at intermediate temperatures, and the

Figure 3. Temperature dependence of the dynamic moduli (G′ and G″) for 20 wt % BMP in [C2mim][NTf2]. The measurement was performed at a frequency ω = 1 rad s−1 and strain γ = 1%.

dependence of the dynamic moduli (G′ and G″) for 20 wt % BMP in [C2mim][NTf2]. Figure 3 suggests a solution (G′ < G″) at low temperatures and a gel (G′ > G″) at high temperatures. Judging from the micellar transitions of BMP, we expected that the system exists in a sol state below the Tc of the PBnMA block, due to unimers or micelles with PPhEtMA cores, and a gel state above the Tc of the PBnMA block, due to the formation of a polymer network. However, contrary to expectation, the apparent gel point (G′ = G″) is located at a much lower temperature than the Tc of the PBnMA block. The apparent gelation temperature was close to the Tc of the PPhEtMA block. To clarify the gelation mechanism of the E


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jammed micelle system had lower mechanical strength than the gel state at high temperature and flowed at strains greater than γ = 20%. Therefore, these results are consistent with the conclusion that that the apparent gel formed at mid-range temperatures is formed by weak interactions between micelles, such as the interpenetration of the corona chains, and the resulting impaired mobility of the micelles. We also performed steady state shear flow measurements using various concentration solutions at the intermediate temperature (70 °C). As indicated from DLS, BMP forms micelles in [C2mim][NTf2] at the selected temperature. It is well-known for colloidal suspensions that the relationship of the relative viscosity (ηr) and the effective volume fraction of colloids (ϕeff) can be described using the Batchelor equation:50 η ηr = 0 = 1 + 2.5ϕeff + 5.9ϕeff 2 ηs (11)

Figure 5. Temperature dependence of the elastic moduli (G′ and G″) for BMP (10 wt %) in [C2mim][NTf2] solution. The measurement was performed at frequency ω = 1 rad s−1 and strain γ = 1%.

sol−gel transition occurred at 130 °C, near to the Tc of the PBnMA block. Thus, the c* of the micelle apparently lies between 10 and 20 wt %. By the experiments at lower BMP concentrations, the critical gelation concentration by the phase transition of PBnMA was found to be 8 wt % (see Figure S5). We consider that the high critical concentration is caused by low glass transition temperature (Tg) of the physical cross-linking points. The Tg of PBnMA (ca. 54 °C) is much lower than the phase transition temperature. The rubbery state of aggregated PBnMA domains may decrease toughness of the cross-linking points and limit the retention capability of the IL in the polymer network. Similar high critical gelation concentration of the ABA-triblock copolymer is observed for PBnMA-b-PMMA-b-PBnMA in the IL.44 Jamming Micelle System. To further explore the jamming of the micelles, we investigated the mechanical properties of the gel-like states. Figure 6 shows the strain dependence of the

η0 and ηs are the zero-shear viscosity of the colloidal suspensions and the viscosity of [C2mim][NTf2]; the latter was calculated using the reported VTF equation and its related parameters.51 ϕeff indicates the effective volume fraction of colloidal particles. This equation has been applied to microgels,52 polymer-grafted nanoparticles,23 core−shell latexes,53,54 and micellar suspensions.55,56 Thus, we also employed the relationship for the ABC-triblock copolymer micelle dispersion system. For dilute solutions of BMP in [C2mim][NTf2] at 70 °C, the solutions showed a proportional relation between shear rate and shear stress; furthermore, the solutions exhibited Newtonian fluid behavior in the measured shear rate region up to 4 wt % polymer concentration (see Figure S6). Figure 7

Figure 7. Relationship between relative viscosity and polymer concentration for dilute concentration of BMP in [C2mim][NTf2] at 70 °C. The solid line represents the best fitting results according to the Batchelor equation.

Figure 6. Strain dependence of G′ and G″ for 20 wt % BMP in [C2mim][NTf2] at 70 and 140 °C. The frequency was held constant at 1 rad s−1.

shows the concentration dependence of the relative viscosity for dilute solutions and the fitting curve using the Batchelor equation. The relationship of ϕeff and weight fraction of particle was expressed using shift factor k and weight fraction c as shown in eq 12.

moduli (G′ and G″) for the 20 wt % BMP solution at 70 and 140 °C. At 140 °C, the ion gel showed a linear response, up to at least 30% strain. Then, the ion gel transitioned to liquid-like behavior (G″ > G′) at about 150% strain. As previously suggested, the change in elastic moduli may be due to the removal of the thermosensitive blocks from the cross-linking points.26 The reported LCST-type thermosensitive ion gels using ABA-triblock copolymers also showed similar resistance to strain, and the formation of polymer network by selfassembly has been suggested.44 In contrast, at 70 °C, the

ϕeff = kc

(12)

The experimental data fit well to the equation, and the shift factor k was determined to be 22.1. k is the parameter for converting the weight fraction to the volume fraction, and the large value implies the solvent-swollen state of the micelle. F


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Macromolecules −[η]ϕeff ⎛ ϕ ⎞ ηr = ⎜⎜1 − eff ⎟⎟ ϕmax ⎠ ⎝

While higher concentration polymer solutions, above 10 wt %, exhibited shear thinning behavior with a Newtonian plateau (see Figure S7). The zero-shear viscosity for the shear thinning system was determined by fitting to the Cross model (eq 13). η − η∞ 1 = 1 + (κγ )̇ m η0 − η∞ (13)

(14)

[η] and ϕmax are set at 2.5 (the Einstein value) and 0.64 (random close-packing limit) for hard spheres, respectively.49 At low ϕeff, the data were expressed well by the hard-sphere model. At higher values of ϕeff, the experimental data deviated from the hard sphere model and ϕeff exceeded the random close-packing limit for hard spheres (ϕmax) at above 3 wt % BMP. The ηr for the BMP solution increased with increasing ϕeff above ϕmax; however, the change in behavior was not sudden, like that seen with the hard-sphere model; instead, a moderate increase in ηr was observed. This phenomenon may occur for soft segments surrounding the PPhEtMA core, in contrast to those of hard spheres. Below ϕmax, the experimental value is identical to that of the hard-sphere model, and the micelles are isolated particles. In contrast, above ϕmax, the micelles weakly interact with each other by interpenetration or deformation of the PBnMA-b-PMMA corona segment that has become swollen and plasticized by [C2mim][NTf2]. Thus, the micellar solution can have a greater value of ϕeff than ϕmax. The divergent behavior from a hard-sphere model on loading of soft dispersions at high concentrations was also observed for soft shell particles.58−62 In BMP/[C2mim][NTf2] system, it has been suggested that micelles consisting of a PPhEtMA core interact each other by overlap of the corona segment, and with increasing temperature, a polymer network is formed by the Tc of thermosensitive PBnMA in the end block of the corona. However, from the rheological measurement, the jamming system was not observed below 10 wt % polymer concentrations regardless of a higher value of ϕeff than ϕmax. The crossover concentration for micelle dispersion between sol and gel has been reported to be around 10 wt %,48,49 and in our case the loosely packed micellar structure affected their ability to flow (vide inf ra). Microphase Separation in Thermosensitive Ion Gel. To observe the mutual interference of micelles in the jammed state and microphase separation in the ion gel, we conducted SAXS measurements for BMP/[C2mim][NTf2] composites.

Using the shift factor, the relative viscosity was plotted as a function of ϕeff. Figure 8 shows the relative viscosity as a

Figure 8. Effective volume fraction (ϕeff) dependence of the relative viscosity for BMP in [C2mim][NTf2] at 70 °C. The broken line represents the Krieger−Dougherty expression with [η] = 2.5 and ϕmax = 0.64.

function of ϕeff. As a comparison with experimental data, the Krieger−Dougherty equation with a close-random packed hard sphere volume fraction (ϕmax) and the intrinsic viscosity ([η]) is also drawn in the same plot.57

Figure 9. SAXS profiles for BMP in [C2mim][NTf2] at selected temperatures: (a) 10 wt % and (b) 20 wt %. G


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were fitted according to eq 10. Figure 11 compares the best fit results to the experimental data, and Table 2 summarizes the

Figure 9 shows SAXS profiles for 10 and 20 wt % BMP in [C2mim][NTf2] at 30, 70, and 140 °C, and Figure 10 shows the

Table 2. Fitting Parameters for 20 wt % BMP in [C2mim][NTf2] at 70 and 140 °C parameter

70 °C

140 °C

⟨Rc⟩/nm σR/nm Rhs/nm ϕ

15.6 ± 0.007 1.74 ± 0.006 32.0 ± 0.018 0.405 ± 0.001

20.1 ± 0.018 2.20 ± 0.014 33.7 ± 0.046 0.463 ± 0.001

fitting parameters. The fits are good and corroborate the physical picture of closely packed spherical cores bridged by middle blocks. At 70 °C, only one block (PPhEtMA) aggregates into cores, so Rc can be used to estimate the aggregation number, Nagg. Inserting the bulk density of PPhEtMA (1.18 g cm−3) and a core radius of 15.6 nm, we find Nagg ≈ 418, and this value may be too large to reflect a polymer-pure core. More reasonably, the core may be swollen with [C2mim][NTf2]. Similar behavior has been reported using PNIPAm aqueous solution: the polymer-rich phase is wet even above Tc of PNIPAm.63 Thus, we should not expect polymerpure cores for PPhEtMA aggregation. At 70 °C, the volume fraction of equivalent hard spheres was found to be 40.5%, which is physically reasonable in light of the pronounced structure factor at low q. Given the simplicity of the model used, we can only make general comments on the results of the Rhs (hard-sphere radius), the radius of the hypothetical core and corona. Rhs at 70 °C was estimated as 32 nm, which is about twice the value of Rc and smaller than the Rh obtained from DLS. This suggests that overlap of the coronas occurs. When the concentration of micelles increases, the interparticle distance was decreased below Rhs of the micelle due to overlap of the corona regions to certain degree. The jamming state was formed by the repulsion between micelles due to osmotic pressure caused by overlapping of the corona chains. An increase in the temperature from 70 to 140 °C enhances the average core radius from 15.6 to 20.1 nm. It is not clear whether this reflects a population of new micelle cores formed by PBnMA or the PBnMA chains associated with the already formed PPhEtMA cores. Because there is no significant increase in the number of micellar cores if Rhs stays roughly constant,

Figure 10. Comparison of appearance for 10 and 20 wt % BMP in [C2mim][NTf2] at 25, 70, and 140 °C.

appearance of the systems at each temperature. For 20 wt % polymer, we observed clear structure factor peaks at 70 and 140 °C, but no particular lattice reflections. The appearance of peaks in the profiles at 70 °C indicates the presence of a closepacked structure of the aggregates due to the soft repulsive interactions between micelles with PPhEtMA cores, implying overlap of the micelle coronas. The reversibility of the SAXS profiles also indicates that each phase is at equilibrium, and the phase transition occurred quickly (see Figure S8). Note that the profile for 10 wt % BMP solution does not show clear structure factor peaks in the scattering profile at either 70 or 140 °C, despite the fact that the thermosensitive blocks showed an LCST phase transition. Thus, the aggregates formed by PPhEtMA or PBnMA blocks seemed to disperse randomly with a relatively low density of the aggregates in the 10 wt % BMP system. This is consistent with the sol state of the system at 70 °C and was also confirmed by the hazy appearance of the sample. The 10 wt % BMP solution turns a pale blue color above Tc of PPhEtMA. Even with the low density of the aggregates and less overlapping of corona segment, the solution showed gelation at 140 °C, and thus we speculated the PPhEtMA and PBnMA segments formed separate aggregates. In contrast, the 20 wt % BMP ion gel was consistently transparent, suggesting homogeneity on length scales greater than 100 nm. To evaluate the microphase-separated structure, the scattering profiles for the 20 wt % solution at 70 and 140 °C

Figure 11. Fitting results using the hard-sphere model (red line) to experimental scattering data (black open circles) for 20 wt % BMP in [C2mim][NTf2] at (a) 70 and (b) 140 °C. H


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Argonne National Laboratory, was supported by the U.S. DOE under Contract DE-AC02-06CH11357.

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CONCLUSION In this paper, we have demonstrated thermoreversible sol−gel transition in [C2mim][NTf2] using an ABC-triblock copolymer consisting of PBnMA and PPhEtMA as LCST thermosensitive end blocks and PMMA as an RTIL-philic middle block. The structural transition of PBnMA-b-PMMA-b-PPhEtMA/ [C2mim][NTf2] was observed in two steps, corresponding with the LCST phase transition of each end block: unimers, spherical micelles with PPhEtMA cores, and polymer networks physically cross-linked by PBnMA and PPhEtMA aggregates. With increasing polymer concentration up to 20 wt %, even at intermediate temperatures (higher than Tc of PPhEtMA but lower than that of PBnMA), the binary system showed a gellike state, from a rheological point of view, without formation of the network structure due to the formation of jammed micelles with PPhEtMA cores. The relationship between the effective volume fraction of polymer micelles and the relative viscosity of the system suggests the overlapping of PBnMA-b-PMMA corona chains, and this is consistent with weaker strain resistance of the jammed micelles, compared to that of the gel state temperatures greater than the phase transition temperatures of both thermosensitive blocks, i.e., where polymer network structure was formed. The rheological changes at 20% polymer concentration were also explored using SAXS, and closely packed micelles were observed. At high temperatures the PPhEtMA and PBnMA blocks associate by physical cross-linking; thus, the system shows gelation behavior.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02616. Figures S1−S8 (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

T.U.: Polymer Materials Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Grants-in-Aid for Scientific Research of #A-23245046 (M.W.), 15H05495 and 26620164 (T.U.), the National Science Foundation Polymers Program through Award DMR-1206459 (T.P.L.), and in part from the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (No. 13J00192 to Y.K.). SAXS measurements were performed at the DuPont− Northwestern−Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by E.I. DuPont de Nemours & Co., The Dow Chemical Company, and Northwestern University. Use of the APS, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by I


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