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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Design and Regulation of Lower Disorder-to-Order Transition Behavior in the Strongly Interacting Block Copolymers Rui-Yang Wang,† Xiao-Shuai Guo,† Bin Fan,† Shu-Fen Zou,† Xiao-Han Cao,† Zai-Zai Tong,‡ Jun-Ting Xu,*,† Bin-Yang Du,*,† and Zhi-Qiang Fan† †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China ‡ Key Laboratory of Advanced Textile Materials and Manufacturing Technology (ATMT), Ministry of Education, Department of Materials Science and Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, China S Supporting Information *

ABSTRACT: Lower disorder-to-order transition (LDOT) phase behavior is seldom observed in block copolymers (BCPs). Design of LDOT BCPs is important for broadening the applications and improving the high temperature properties of BCPs. In this work, the LDOT phase behavior was first achieved in the strongly interacting BCPs consisting of poly(ethylene oxide) (PEO) and poly(ionic liquid) (PIL) blocks (EOm-b(IL-X)n, X: counterion) by introducing two extra strong forces (hydrogen-bonding and Coulombic interaction) with different temperature dependences. It is also found that the LDOT phase behavior of the EOm-b-(IL-X)n BCPs can be regulated by molecular weight (related to mixing entropy), counterion, and salt doping. Increasing counterion size and salt content shifts the disorder-to-order transition temperature (TDOT) to higher temperature, whereas a higher molecular weight leads to a lower TDOT. Based on our findings, some general rules for design of LDOT phase behavior in the strongly interacting BCPs were proposed. Moreover, the conductivity of the EOm-b-(IL-X)n BCPs was correlated with the LDOT phase behavior. A remarkable increase in conductivity after LDOT, i.e., a thermo-activated transition, is observed for the EOm-b-(IL-X)n BCPs, which can be attributed to the cooperative effects of temperature rising and LDOT.

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INTRODUCTION Charged polymers, including salt-doped polymers and polyelectrolytes, have aroused great attention in the fields of energy storage, catalyst, separation, antimicrobial, and stimuli-responsive materials.1−6 Block copolymers (BCPs) containing two or more covalently connected segments with different chemical properties can self-assemble into various nanoscale morphologies under suitable conditions.7−14 The aggregation state (disordered and various ordered structures) of BCPs greatly affects the material properties, including the modulus, conductivity, and dielectric property, which provides good opportunities to tune the material performance. For example, the conductivity of charged BCPs is sensitive to the morphology (especially, there is a great difference in conductivity between the disordered and ordered morphology).14−19 Basically, two kinds of phase transition behaviors, known as upper critical solution temperature (UCST) and lower critical solution temperature (LCST) for polymer mixtures or upper order-to-disorder transition (UODT) and lower disorder-toorder transition (LDOT) for BCPs, can be categorized.20−29 For UCST or UODT, the polymers change from homogeneously mixed state into phase-separated state with decreasing temperature. This is quite common because the mixing entropy at low temperatures cannot overcome the unfavorable enthalpy © XXXX American Chemical Society

interaction between the unlike segments. By contrast, for LCST or LDOT the disordered phase exists at low temperatures and phase separation occurs at high temperatures. LCST phase behavior happens in many weakly30 or strongly31,32 interacting polymer mixtures due to negative mixing entropy or weakening of the favorable interaction between two components with increasing temperature. However, as compared with polymer blends, LDOT seldom occurs in BCPs because of the large interfacial energy and the entropic loss during phase transition,33 which are related to the nanoscale domain size and covalent linkage of the blocks, respectively. So far, LDOT has been reported in only three kinds of weakly interacting BCPs: polystyrene-b-poly(n-alkyl methacrylate), poly(ethylene oxide)-b-poly(2-vinylpyridine), and poly(n-hexylnorbornene)b-poly(cyclohexylnorbornene),20−29 in which only van der Waals force exists. The LDOT in the first two BCPs was attributed to the large mismatch of compressibility between the two blocks, while it was ascribed to the distinct temperature dependences of the solubility parameters for the two blocks in the last BCP. BCPs with LDOT phase behavior have some unique characteristics, especially for charged BCPs. For Received: January 30, 2018 Revised: March 7, 2018

A

DOI: 10.1021/acs.macromol.8b00227 Macromolecules XXXX, XXX, XXX−XXX

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and distilled from CaH2. Toluene was refluxed and distilled from sodium. N,N-Dimethylformamide (DMF) was distilled from magnesium sulfate under vacuum. Poly(ethylene oxide)s (PEOs) with different molecular weights were purchased from Sigma-Aldrich, dissolved in CH2Cl2, washed by alkaline aluminum oxide, then filtered and dried with a rotary evaporator, and finally azeotropically distilled with dry toluene before use. Synthesis of EOm-b-(IL-X)n BCPs. Four kinds of strongly interacting BCPs denoted as poly(ethylene oxide)-b-poly(1-((2acryloyloxy)ethyl)-3-methylimidazolium) with X counterion (EOm-b(IL-X)n) (Figure 1), where m and n are the numbers of repeating units

instance, many studies have revealed that well-organized nanostructures are advantageous to both conductivity and modulus;14−19 therefore, LDOT BCPs can maintain both good mechanical and conductive properties at higher temperatures as compared with UODT BCPs.14 Moreover, around the LDOT temperature of charged BCPs, the cooperative effects of temperature increase and morphological transition may result in a dramatic increase of conductivity; thus, an evident thermoactivated transition in conductive property is expected. By contrast, for charged BCPs with UODT, the increase of conductivity due to temperature increase may be counteracted by the effect of order-to-disorder morphological transition, leading to unobvious change in conductivity. Therefore, exploring the LDOT behavior in the charged BCPs is of great importance for material applications. In the strongly interacting BCPs, it is difficult to design LDOT behavior based on compressibility of polymers. Herein we design LDOT BCPs from the viewpoint of interaction between the two blocks, i.e., introduction of two interaction forces with different temperature dependences. The first is the hydrogen (H)-bonding interaction between the blocks, which enhances the miscibility of components at low temperatures. However, the H-bonding interaction is strongly dependent on temperature and may be weakened at higher temperatures.34 The second force is the Coulombic interaction. We notice that most of the neutral-poly(ionic liquid) (PIL) BCPs reported in the literature usually keep ordered structures at very high temperatures.35−42 Even the order-to-disorder transition temperature (TODT) was not reported for most neutral-PIL BCPs, probably exceeding the decomposition temperature of BCPs. This can be ascribed to the high dissimilarity between the neutral and charged blocks from the viewpoint of Coulombic interaction. As a result, Coulombic interaction is also a strong force but less temperature-sensitive than Hbonding. It should be noted that both H-bonding and Coulombic interaction are strong forces, so that they will not be covered by the van der Waals force widely existing in common BCPs. In the present work, we synthesized a series of BCPs composed of poly(ethylene oxide) (PEO) neutral block and poly(ionic liquid) (PIL) block bearing imidazolium cation in the side groups and various anions as counterions. There exists H-bonding interaction between the PEO and PIL blocks, like that reported for the PEO solution in 1,3-dialkylimidazolium ionic liquid.43−45 At lower temperatures, the H-bonding prevails over the Coulombic interaction among the PIL blocks; thus, the BCPs are disordered. At elevated temperatures, the Hbonding is weakened and the PEO and PIL blocks become immiscible due to the strong and less temperature-dependent Coulombic interaction; thus LDOT is achieved. We also found that the LDOT phase behavior of the PEO-b-PIL BCPs could be tuned by molecular weight, counterion, and salt-doping.

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Figure 1. (a) Chemical structure of EOm-b-(IL-X)n. (b) Scheme for the fundamental interactions in the EOm-b-(IL-X)n strongly interacting BCPs: the van der Waals force (magenta), hydrogen bonding (green), and Coulombic interaction (cyan). and X is the counterion (X = Br, PF6, TFO, and TFSI to stand for the bromide ion, hexafluorophosphate, trifluoromethanesulfonate, and bis(trifluoromethane)sulfonimide), were synthesized. The details for synthesis of EOm-b-(IL-X)n BCPs are shown in the Supporting Information. The hydroxyl end group of the PEO was first functionalized with a chain transfer agent, and then the reversible addition−fragmentation termination (RAFT) living radical polymerization of 2-bromoethyl acrylate was carried out with the functionalized PEO as the macromolecular chain transfer agent. Finally, the neutral BCPs were charged by quaternization reaction. Ionic exchange reactions were performed to yield the BCPs with various counterions (Figure S1). Table 1 lists the synthesized EOm-b-(IL-TFO)n together with their molecular weights, volume fractions, morphologies, and phase transition temperatures. Characterization of BCPs. Since the molecular weight distribution (Mw/Mn) of the neutral-charged BCP is hard to characterize, here we just characterize the molecular weight distribution of the unquaternized samples. It was measured by gel permeation chromatography (GPC) on a Waters system calibrated with standard polystyrenes. Tetrahydrofuran (THF) was used as the eluent at a flow rate of 1.0 mL/min at 40 °C. 1H NMR spectra were recorded on a Bruker DMX-400 instrument. CDCl3, D2O, DMSO, and CD3OD were used as solvents, depending on the solubility of the products. The Fourier-transform infrared (FTIR) spectra were recorded on a Thermo Fisher Scientific LLC Nicolet 6700 spectrometer. OPUS spectroscopic software was used for data analysis. The samples were pressed into thin films at 70 °C for FTIR experiments. The thermal behavior was characterized by differential scanning calorimetry (DSC) on a TA Q200 instrument. About 3−5 mg of the sample was sealed in an aluminum pan. The samples were first heated from 40 to 100 °C, kept for 5 min to eliminate the thermal history, then cooled to −80 °C, and reheated to 100 °C to obtain DSC curves. Both the heating and cooling rates are 10 °C/min. Preparation of Salt-Doped BCPs. Prescribed amounts of EOm-b(IL-TFO)n and LiTFO were first dissolved in methanol at the concentration of 30 mg/mL and 2.5 wt %, respectively. These two solutions were mixed quantitatively according to the molar ratio of

EXPERIMENTAL SECTION

Materials. Acryloyl chloride (97%, contains 97%, HPLC), methanol (99.8%), 1-methylimidazole (98%), nhexane (≥98.5%), potassium hexafluorophosphate (KPF6, 98%), lithium trifluoromethanesulfonate (LiTFO, 98%), and lithium bis(trifluoromethane) sulfonimide (LiTFSI, 98%) were used as received. Azobis(isobutyronitrile) (AIBN, 98%, Sigma-Aldrich) was purified by recrystallization twice from methanol. Dichloromethane was refluxed B

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before measurement. The impedance was measured with a voltage of 30 mV in frequency range from 1 Hz to 1 MHz at nitrogen atmosphere. The bulk resistance (R) was determined by extrapolating the high-frequency plateau value of the real part impedance. The conductivity (σ) was calculated by the equation σ = L/(SR), where L is the distance between two electrodes and S is the area of the film. The average conductivity and the error were taken from the three parallel tests. The Arrhenius equation was used to describe the relationship between conductivity σ and temperature T in disordered morphology:

Table 1. Molecular Weights, Volume Fractions, Phase Structures, and Phase Transition Temperatures of EOm-b(IL-TFO)n

sample

Mna (total) (g/mol)

Mw/Mnb

f EOc

EO45-b-(IL-TFO)10 EO45-b-(IL-TFO)33 EO45-b-(IL-TFO)59 EO114-b-(IL-TFO)9 EO114-b-(IL-TFO)15 EO114-b-(IL-TFO)33 EO227-b-(IL-TFO)7

5300 12900 21500 8000 10000 15900 12300

1.03 1.05 1.07 1.03 1.03 1.07 1.03

0.422 0.183 0.111 0.675 0.555 0.361 0.842

EO227-b-(IL-TFO)17

15600

1.04

0.686

EO227-b-(IL-TFO)25 EO227-b-(IL-TFO)44

18300 24500

1.04 1.08

0.598 0.458

morphology change upon heating Dis Dis → Bcc Dis → Bcc Dis Dis → Lam Hex (PEO) Dis → Orderedd Hex (PIL) → Lam Lam Hex (PEO)

TDOT or TOOT (°C) 180 160

⎛ E ⎞ σ = A exp⎜ − a ⎟ ⎝ T⎠

80

(1)

where A is the prefactor and Ea is the activation energy.

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RESULTS AND DISCUSSION Effect of Molecular Weight and Volume Fraction. The phase behavior of neutral BCPs is mainly determined by χN and volume fraction (f) of the components, where χ is the Flory−Huggins parameter and N is the total polymerization degree of BCP. Similarly, the effects of molecular weight and volume fraction on the phase behavior of EOm-b-(IL-TFO)n were first investigated. The molecular characteristics and structural change upon heating of EOm-b-(IL-TFO)n BCPs are summarized in Table 1. Figure 2 is the representative

120 120

a

Mn is the number-average molecular weight of the BCPs and was determined by 1H NMR. bThe molecular weight distribution (Mw/ Mn) was determined from the unquaternized samples by GPC. cf EO is the volume fraction of PEO in the BCPs. The densities of amorphous PEO (1.066 g/cm3) and P(IL-TFO) (1.31 g/cm3, Supporting Information) were used for calculation. dThe accurate morphology is unclear because the high-order SAXS peaks are not sharp enough.

LiTFO salt to the EO repeating unit, r = [salt]/[EO]. Finally the blends were dried under dynamic vacuum for more than 24 h. All the samples were kept in the dryer prior to use. Small-Angle X-ray Scattering (SAXS). Temperature-variable small-angle X-ray scattering (SAXS) experiments were performed at BL16B1 beamline in Shanghai Synchrotron Radiation Facility (SSRF), China. The wavelength of X-ray at SSRF is 1.24 Å. The sample-todetector distance was set as 2000 or 5000 mm. Two-dimensional (2D) SAXS patterns were recorded with a Mar 165 CCD detector.46,47 Each sample was heated to 100 °C for 8 h under vacuum and slowly cooled down to room temperature before characterization. A Linkam hot stage was used to heat the samples. The SAXS experiments were performed in the temperature range 70−200 °C, which is higher than the melting temperature of PEO block but lower than the decomposition temperature of the BCPs. The samples were heated to the preset temperature at a rate of 10 °C/min and held for 3 min to reach equivalent state under nitrogen flow before acquisition of SAXS data. The average exposure time was 30 s for each scan. The chicken tendon was used as standard material for calibrating the scattering vector. The 2D SAXS patterns were converted into one-dimensional (1D) SAXS profiles using Fit2D software. The morphology of BCPs was determined in terms of the scattering pattern if the electron density contrast of the polymers is big enough. No scattering peak or a broad maximum appears in the SAXS profiles for homogeneously mixed morphology because of weak electron density fluctuation. However, the correlation hole effect will arise a broad scattering maximum in the microphase-separated but disordered BCPs.48 The ordered morphologies in BCPs were determined by the relative positions of the high-order peaks to the primary peak in the SAXS profiles. The disorder-to-order transition temperatures (TDOTs) were determined by the discontinuous change of the inverse of the primary peak intensity, I−1(q*), versus inverse of temperature, T−1.49 The broad scattering maximum in the disordered state was used to calculate the effective Flory−Huggins parameter, χeff,48 and the details are shown in the Supporting Information (Figure S20). Measurements of Conductivity. Impedance spectra were recorded on an electrochemical workstation (CHI660E, CH Instruments, Inc.). All samples were annealed at 100 °C for 8 h and dried in a dryer for 3 days before use. Polymers were sandwiched between 304 stainless steel electrodes with a Teflon washer. The sample was then heated at the rate of 10 °C/min and kept at equilibrium for 0.5 h

Figure 2. Temperature-variable SAXS profiles of EOm-b-(IL-TFO)n with similar f EO (∼0.50) but different molecular weights. The black, blue, and red colors of the lines indicate homogeneously mixed, disordered, and ordered morphologies, respectively.

temperature-variable SAXS profiles of EO45-b-(IL-TFO)10, EO114-b-(IL-TFO)15, and EO227-b-(IL-TFO)25, which have similar compositions but different molecular weights (f EO = 0.422, 0.555, and 0.598, respectively). The SAXS profiles of other EOm-b-(IL-TFO)n samples are compiled in Figure S9. One can see that no ordered structure is formed in the temperature range of 70−200 °C for EO45-b-(IL-TFO)10 (Figure 2a). However, the trend of demixing is enhanced with increasing temperature in this sample, and disordered (Dis) structure is formed at higher temperature, as evidenced by the broad SAXS maximum but disappearance of high-order peaks. It should be pointed out that the absence of scattering maximum may be caused by the poor electron density contrast27 and/or homogeneously mixing. Nevertheless, the C

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Figure 3. χeff values of EOm-b-(IL-PF6)n as the function of temperature and f EO.

enhanced upon heating, which is in accordance with the LDOT phase behavior of EOm-b-(IL-X)n BCPs. In order to further understand the effects of temperature and volume fraction on the phase behavior of the EOm-b-(IL-X)n BCPs, the “effective” Flory−Huggins parameter (χeff), which is often used in salt-doped BCPs,51−54 is plotted versus f EO and temperature (Figure 3, Figures S21 and S22). It is observed that χeff increases with temperature rising, which results from the weakening of H-bond. This can be verified by the FTIR result (Figures S29 and S30). In addition, we notice that χeff first decreases and then increases with increasing f EO, leading to the lowest χeff at f EO ≈0.5. Although χeff reflects the overall effects of H-bonding, Coulombic interaction, and van der Waals force, this phenomenon can be mainly attributed to the largest quantity of the formed H-bonds when the volume fractions of PEO and PIL blocks are comparable. Effect of Counterion. Since the mobile counterions can greatly affect the interactions and properties of the charged BCPs, it inspires us to explore the relationship between the counterion type and the LDOT behavior. Figures 2a and 4 collect the representative SAXS profiles of four EO45-b-(IL-X)10 (X = TFO, Br, PF6, TFSI) samples. One can see that although these four samples possess the same repeating units, their phase behaviors are quite different. EO45-b-(IL-Br)10 shows a well-

scattering maximum is remarkable in the whole temperature range for some high molecular weight EOm-b-(IL-X)n BCPs (Figures S8−S10). This means that the electron density contrast in all EOm-b-(IL-X)n BCPs is big enough for SAXS characterization, and the homogeneously mixed and disordered states can be distinguished from the SAXS pattern. For EO114b-(IL-TFO)15, it is disordered below 80 °C, but ordered lamellar (Lam) structure is formed above 90 °C (Figure 2b), showing a LDOT phase behavior. When the molecular weight is further increased, for example EO227-b-(IL-TFO)25, the BCPs can be always ordered in the temperature range studied (Figure 2c). The appearance of ordered structures upon heating, which corresponds to the LDOT phase behavior, in some EOm-b-(ILTFO)n BCPs confirms that our concept for design of LDOT BCPs is feasible and effective. However, the molecular weight also has a great influence on the LDOT behavior. The molecular weight is related to the mixing entropy. The larger the molecular weight, the smaller the mixing entropy; thus, a higher molecular weight is favorable to demixing. The EOm-b(IL-TFO)n BCPs with very low molecular weight may be disordered at high temperatures, but the disorder-to-order transition temperature (TDOT) may be too low to be observed when the molecular weight is very high. The molecular weight dependence of LDOT is similar to that in other weakly interacting LDOT and UODT BCPs.20,21,28 Above result reveals that the effect of mixing entropy is comparable to those of other two forces: H-bonding and Coulombic interaction, which will be deeply discussed later. As for the change of morphology with volume fraction, two dramatic features are observed. First, the phase diagram is asymmetry (Table 1 and Tables S3−S5). The PIL phase tends to form the matrix even though its volume fraction is not very large. As shown in Table 1, the PEO block in EO227-b-(ILTFO)44 with f EO = 0.458 forms a hexagonally packed cylindrical (Hex) PEO phase, while a Lam structure is usually formed at such a volume fraction in common BCPs. A similar phenomenon is more clear in EOm-b-(IL-Br)n BCPs (Table S3). The f EO values in EO227-b-(IL-Br)17 and EO227-b-(IL-Br)44 are 0.745 and 0.530, respectively, and the corresponding PEO nanostructures are Lam and Hex. By contrast, Hex and Lam structures are formed at these two volume fractions in neutral BCPs. The asymmetric phase diagram confirms the theoretical prediction for neutral-charged BCPs.50 Second, the Hex-to-Lam order-to-order transition (OOT) is observed upon heating some BCPs, for example, EO227-b-(IL-TFO)17 (Table 1) and EO227-b-(IL-PF6)17 (Table S4). Such an OOT sequence with regard to temperature is opposite to that in common BCPs. This shows that the segregation strength for demixing is

Figure 4. Temperature-variable SAXS profiles of EO45-b-(IL-X)10 (X = Br, PF6, TFSI). The black, blue, and red colors of the lines indicate homogeneously mixed, disordered, and ordered morphologies, respectively. D

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Macromolecules Table 2. Morphology Change upon Heating and TDOTs of Salt-Doped EO227-b-(IL-TFO)17 and EO227-b-(IL-TFO)25 sample

ra

wsaltb

f EO/saltc

EO227-b-(IL-TFO)17-d1 EO227-b-(IL-TFO)17-d2 EO227-b-(IL-TFO)17-d3 EO227-b-(IL-TFO)17-d4 EO227-b-(IL-TFO)25-d1 EO227-b-(IL-TFO)25-d2 EO227-b-(IL-TFO)25-d3 EO227-b-(IL-TFO)25-d4

1/120 1/60 1/40 1/30 1/16 1/12 1/10 1/8

0.019 0.036 0.054 0.070 0.108 0.139 0.162 0.195

0.689 0.693 0.696 0.699 0.623 0.631 0.637 0.645

morphology change upon heating Dis Dis Dis Dis Dis Dis Dis Dis

TDOT (°C)

→ Hex (PIL) → Hex (PIL) → Hex (PIL) + Dis

140 160 140

→ → → →

110 120 130 160

Lam Lam Lam Lam

a

The molar ratio of the salt to the EO repeating unit, r = [salt]/[EO]. bThe weight fraction of the salt in the polymer mixture. cf EO/salt is the volume fraction of the EO/salt phase. The calculation of f EO/salt is given in part 3 of the Supporting Information.

organized Hex structure in the whole temperature range (Figure 4a). EO45-b-(IL-PF6)10 transforms from homogeneously mixed state to disordered state and then to Hex structure as temperature increases with a TDOT located at ∼150 °C. EO45-b-(IL-TFO)10 only exhibits a transition from homogeneously mixed state to disordered state between 70 and 200 °C, while EO45-b-(IL-TFSI)10 keeps homogeneously mixed state in this temperature range. The phase transition behaviors of all the EOm-b-(IL-X)n samples are summarized in Table 1, Tables S3− S5, and Figures S7−S10. One can see that the threshold molecular weight (Mc) for the appearance of ordered structures varies with the type of counterion. All the EOm-b-(IL-Br)n BCPs with different molecular weights keep ordered between 70 and 200 °C (Table S3 and Figure S7), indicating a very low Mc. The Mc values of EOm-b-(IL-PF6)n, EOm-b-(IL-TFO)n, and EOm-b-(IL-TFSI)n are around 5000 g/mol (Table S4 and Figure S8), 10 000 g/mol (Table 1 and Figure S9), and 20 000 g/mol (Table S5 and Figure S10), respectively. We find that Mc is proportional to the counterion size: Mc(Br) < Mc(PF6) < Mc(TFO) < Mc(TFSI). It is reported that bigger counterion leads to smaller ion cohesion energy (Γ )50,55 and solvation energy,51−53 which is advantageous to mixing of the two blocks in neutral-charged BCPs.56 To form ordered structures, a higher molecular weight, i.e., smaller mixing entropy, is required for the BCPs with bigger counterions. The dependence of phase behavior on counterion size is also reported for polymer solutions in ILs31,57 and salt-doped BCPs.51−53 For example, the solubility of PEO in ILs varies with the counterion. PEO cannot be dissolved in 1,3-dialkylimidazolium chloride ([Cnmim][Cl]), while it exhibits a LCST phase behavior in 1,3-dialkylimidazolium tetrafluoroborate ([Cnmim][BF4]) and is soluble in 1,3-dialkylimidazolium hexafluorophosphate ([Cnmim][PF6]) and 1,3-dialkylimidazolium bis(trifluoromethane)sulfonamide [Cnmim][TFSI].31 Briefly, the bigger the counterion, the better the solubility,57 which agrees well with our result. Effect of Salt Doping. PEO can associate with salt to form solid polyelectrolytes. PEO-containing BCPs are also frequently doped with salt to regulate the phase behavior.51−53,58−60 Here we added different amounts of LiTFO salt into EOm-b-(ILTFO)n BCPs to tune the LDOT phase behavior. The salt amount, morphology and TDOT are listed in Table 2, in which we use di (i = 1, 2, 3, 4) to represent the salt-doped samples. The parameter r is the molar ratio of the salt to the EO repeating unit, i.e., r = [salt]/[EO], wsalt is the weight fraction of salt in the polymer mixture, and f EO/salt is the volume fraction of the EO/salt phase. The representative SAXS profiles of two salt-doped EO227-b-(IL-TFO)25 samples with r = 1/16 and 1/8, respectively, are shown in Figure 5. First, we find that the salt

Figure 5. Temperature-variable SAXS profiles of two representative salt-doped EO227-b-(IL-TFO)25: (a) r = 1/16; (b) r = 1/8. The blue and red colors of the lines indicate disordered and ordered morphologies, respectively.

can arouse the LDOT of EO227-b-(IL-TFO)17 and EO227-b-(ILTFO)25 dramatically. For instance, both EO227-b-(IL-TFO)17 and EO227-b-(IL-TFO)25 keep ordered between 70 and 200 °C (Table 1), indicating very low TDOTs of them. By contrast, LDOT is observed for all the LiTFO-doped samples (Table 2, Figure 5, and Figures S11 and S12). The TDOT increases as the amount of the added LiTFO increases. When only a small amount of LiTFO is added into EO227-b-(IL-TFO)17 (r = 1/ 120, wsalt = 0.019), the LDOT takes place at 140 °C (Table 2), whereas the TDOT is increased to 160 °C at r = 1/60 (Table 2). This shows that addition of LiTFO can enhance the compatibility of the two blocks in EOm-b-(IL-X)n. Comparing the SAXS profiles of LiTFO-doped EO227-b-(IL-TFO)17 and EO227-b-(IL-TFO)25 (Figures S11 and S12), one can see that a larger amount of salt is needed to raise the TDOT from a low temperature to above 70 °C for the BCPs with a higher molecular weight. We deduce that the TDOT of EO227-b-(ILTFO)25 should be lower than that of EO227-b-(IL-TFO)17, considering the higher molecular weight and more symmetric composition of EO227-b-(IL-TFO)25. Therefore, more salts are required to enhance the miscibility of EO227-b-(IL-TFO)25. Second, the morphology of the BCPs may be changed after addition of salt. EO227-b-(IL-TFO)17 exhibits a Hex-to-Lam transition at 120 °C, while the LiTFO-doped sample EO227-b(IL-TFO)17-d1 (wsalt = 0.019) keeps the Hex structure above 140 °C. To further reveal the effect of salt doping on the phase behavior of the EOm-b-(IL-TFO)n BCPs, the change of χeff with temperature and r is shown in Figure 6a and Figure S23. It can be seen that for a specific sample the χeff increases with temperature rising, indicating that the BCPs become more immiscible at elevated temperatures. This is in accordance with their LDOT phase behavior. The Bragg spacings (D = 2π/q*) of LiTFO-doped EO227-b-(IL-TFO)25 BCPs are also plotted in Figure 6b. It is observed that the value of D also increases with E

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Figure 6. Values of χeff (a) and D (b) of neat EO227-b-(IL-TFO)25, EO114-b-(IL-TFO)15, and LiTFO-doped EO227-b-(IL-TFO)25 as a function of temperature and salt-doping ratio r. Since the values of χeff EO227-b-(IL-TFO)25 are not available due to absence of disordered structure, the χeff values of neat EO114-b-(IL-TFO)15 (empty solid squares) with a similar composition were given for replacement.

where ΔHV, ΔHH, and ΔHC are the enthalpy changes upon mixing corresponding to van der Waals force, H-bonding, and Coulombic interaction, respectively, and ΔS is the mixing entropy. When ΔGmix is negative, mixing is preferred and the BCPs are disordered. Otherwise, microphase separation takes place. In the strongly interacting BCPs the H-bonding and Coulombic interaction are much stronger than the van der Waals force, thus ΔHV can be ignored. ΔHH is negative since H-bonding is favorable to mixing. By contrast, ΔHC is positive because the Coulombic interaction results in difficulty in mixing. ΔS is related to the entropy change of the blocks upon mixing and is also positive, like that in most BCPs.20−29 It should be noted that mixing or demixing affects the H-bonding and Coulombic interaction as well and alters the conformations of two blocks31,32 and freedom of the counterions accordingly.50,51,55 Therefore, H-bonding and Coulombic interaction also contribute to ΔS. However, the remarkable decrease of TDOT with increasing molecular weight (Table 1) shows that molecular weight is the dominant factor affecting ΔS. In brief, the LDOT behavior of strongly interacting EOm-b-(IL-X)n BCPs is mainly influenced by H-bonding, Coulombic interaction, and mixing entropy. As temperature increases, the absorbance of H-bonding in the FTIR spectra decreases continuously (Figures S29 and S30), meaning that the Hbonding becomes weaker. Because of the change of H-bonding with temperature, the absolute value of ΔHH decreases as temperature rises. On the other hand, ΔHC changes slower than ΔHH with temperature; thus, ΔGmix becomes more positive with increasing temperature. The different variations of ΔHH and ΔHC with temperature are the origin of LDOT in the strongly interacting EOm-b-(IL-X)n BCPs. The values of ΔHH and ΔHC can be simultaneously tuned by changing the counterion. First, the C+−H···X− interaction is replaced by the C+−H···O interaction upon formation of H bonds. It is reported that the C+−H···X− interaction is inversely proportional to the counterion size.43,44 As a result, larger counterion will reduce the absolute value of ΔHH (ΔHH < 0). Second, larger counterion leads to smaller ion cohesion energy and solvation energy50−53,55 and thus decreases the value of ΔHC (ΔHC > 0). Both effects are favorable to mixing, so the miscibility between the PEO and PIL blocks is improved when the counterion is bigger. This can be seen from the χeff values of EOm-b-(IL-X)n BCPs with similar compositions but different counterions (X = PF6, TFO, TFSI) (Figure 3, Figures S21 and S22). We find that the χeff values increase in the following order: χeff (TFSI) < χeff (TFO) < χeff (PF6). Because of absence

increasing temperature. This further supports the enhanced immiscibility at higher temperatures,21 which can be attributed to the weakening of H-bonding with increasing temperature. Note that even after LDOT, the H-bonding continues to be weakened due to increase of temperature, as evidenced by the enhanced intensities of the primary and high-order SAXS peaks with temperature rising (Figure 5). Moreover, as the amount of LiTFO added in the BCPs increases, the values of χeff and D at a specific temperature become smaller (Figure 6). This shows that salt doping enhances the miscibility of the PEO and PIL blocks at a fixed temperature, which is also in accordance with the shift of TDOT to higher temperature. The enhanced miscibility after salt doping is unique for EOm-b-(IL-X)n BCPs because salt doping usually promotes demixing in most salt-doped BCPs.61,62 We compared the FTIR spectra of the neat and LiTFO-doped samples (Figures S29 and S30). It seems that the C−H stretching vibration is barely perturbed after salt doping, indicating that the shift of TDOT to higher temperature and the enhanced miscibility are not caused by the intensification of H-bonding. Since LiTFO is mainly associated with the PEO block to form polyelectrolyte, our BCPs transform from neutral-charged type into charged−charged type after salt doping; thus, the two blocks become more similar from the viewpoint of Coulombic interaction. This may be the reason for the enhanced miscibility after salt doping in our experiments. A similar result was reported for the saltdoped poly(ε-caprolactone)-b-poly(ethylene oxide) BCPs in our previous work.58−60 We found that when salt was only located in the PEO microdomains at low salt concentrations, the PEO and PCL blocks become more repulsive, whereas the miscibility between them was improved when salt is distributed in both PCL and PEO microdomains at high salt concentrations. It should be noted that salt doping may have an opposite effect on the common neutral−neutral BCPs, since they turn into neutral−charged ones after salt doping. Therefore, the two blocks become more unlike, and the tendency of microphase separation is enhanced. Thermodynamic Explanation for the LDOT Behavior in the Strongly Interacting BCPs. In the EOm-b-(IL-X)n BCPs, there exist two strong interactions: H-bonding and Coulombic interaction, besides the weak van der Waals force in the common BCPs. Therefore, the Gibbs free energy for mixing (ΔGmix) can be expressed as ΔGmix = ΔHV + ΔHH + ΔHC − T ΔS

(2) F

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Macromolecules of disordered structure, the values χeff cannot be obtained for EOm-b-(IL-Br)n BCPs, but χeff(Br) should be the largest. Therefore, the smaller the counterion size, the larger the value of χeff. This result is accordant with the counterion size dependence of Mc. As a result of the change of χeff, the TDOT shifts to higher temperature as counterion becomes bigger. The weaker demixing ability of bigger counterions is both experimentally and theoretically verified in the Li+-doped BCPs.51−53 ΔHC can also be regulated by salt doping. The externally added salt is mainly located in the PEO microdomains, since the PEO block can associate with the metal cations of the salt. The enhanced similarity of both blocks, from the viewpoint of Coulombic interaction, leads to improved miscibility, as revealed by the reduced χeff and D (Figure 6). Accordingly, the TDOT of EOm-b-(IL-X)n BCPs shifts to higher temperature after salt doping. In the salt-doped BCPs, it is also reported that the miscibility of BCPs is enhanced when both blocks become charged.55,58−60 Although both H-bonding and Coulombic interaction are strong forces, the mixing entropy still plays an important role in the LDOT phase behavior of EOm-b-(IL-X)n BCPs. The mixing entropy ΔS varies with the molecular weight of the BCPs. The larger the molecular weight, the smaller the mixing entropy. As a consequence, the EOm-b-(IL-X)n BCPs become more immiscible as molecular weight increases, leading to shift of TDOT to lower temperature. In summary, the LDOT phase behavior of EOm-b-(IL-X)n BCPs can be regulated by three methods: counterion size, salt doping, and molecular weight (Figure 7). As the counterion

Because the charged polymers are quite unlike neutral ones, introduction of Coulombic interaction usually leads to a higher phase transition temperature in BCPs.51−53 Nevertheless, the Coulombic interaction should be restricted in the microdomain of the charged block. When both blocks are charged, the miscibility may become improved, resulting in disappearance of ordered structures at elevated temperatures. For example, in poly(ethylene oxide)-b-poly(styrene-4-sulfonyltrifluoromethylsulfonyl)imide with lithium or magnesium counterions (PEOb-PSLiTFSI or PEO-b-P[(STFSI)2Mg]), where the cations can complex with PEO, the domain size and segregation strength were found to decrease gradually with increasing charged volume fraction, and no LDOT behavior was observed.65,66 This is because an attraction force is produced when the two blocks are oppositely charged. In addition, when both blocks of BCPs are salt-doped, the miscibility can also be enhanced due to the presence of similar Coulombic interaction in two blocks.58−60 The above rules are important to the occurrence of LDOT phase behavior. The reason for few observations of LDOT phase behavior for BCPs in the literature is probably that above requirements are not met simultaneously. Conductivity of BCPs. The ion conductivity in the charged BCPs is affected by the morphology14−19 and the segmental relaxation behavior.67,68 Figure 8a collects the conductivity of P(IL-PF6) and EO45-b-(IL-PF6)n, respectively. The conductivity does not show obvious transition upon heating although the EO45-b-(IL-PF6)n samples experience the LDOT process. Note that the weight fraction of the PIL block is higher than 0.60 in these three BCPs, which may conceal the influence of the morphological transformation. The slopes of the curves in

Figure 7. Three methods to regulate the TDOT of EOm-b-(IL-X)n BCPs.

size and salt content increase, the TDOT shifts to higher temperature. On the other hand, when molecular weight increases, demixing becomes easier and LDOT takes place at lower temperature. Moreover, some general rules for design of LDOT BCPs can be inferred from our results. First, a strong attracting force between different blocks should be introduced to overwhelm the repulsive van der Waals force widely existing in common BCPs; thus, the disordered state can be achieved at low temperatures. Moreover, the attracting force for mixing should be weakened at higher temperatures to allow microphase separation and formation of ordered structures. Considering the above two requirements, H-bonding is a good choice for such a force. However, it is suggested that Hbonding should take place between the two different blocks, so its compatibilization effect can be realized. If H-bonds are formed within a single block or between a block and externally added molecules, demixing may be enhanced instead.16,17,63,64 Second, a strong and stable force had better to be introduced to enhance immiscibility at higher temperatures, since common BCPs tend to become disordered with increasing temperature.

Figure 8. (a) Conductivities of P(IL-PF6) and EO45-b-(IL-PF6)10. Some of the error bars are omitted because they are smaller than the points. (b) Conductivity and phase behavior of EO227-b-(IL-PF6)7. The blue vertical dashed line separates the Hex and Dis morphology. The black axis is the conductivity σ as the function of the inverse of temperature, T−1. The black dashed line is an extrapolation of the conductivity in the disordered morphology. The red axis is the inverse of the primary SAXS peak intensity, I−1(q*), versus T−1. G

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Macromolecules Figure 8a are related to the activation energy Ea. It is found that increase of the PEO content can greatly reduce the activation energy because of the plasticization effect of PEO.68 Such a plasticization effect is more remarkable in EOm-b-(IL-PF6)n with smaller counterion than that in EOm-b-(IL-TFO)n and EOm-b-(IL-TFSI)n with bigger counterions (Figures S32−S34). The Tgs of P(IL-PF6), P(IL-TFO), and P(IL-TFSI) are 84, 31, and 7 °C, respectively. Correspondingly, the Tgs of EO45-b-(ILPF6)10, EO45-b-(IL-TFO)10, and EO45-b-(IL-TFSI)10 are 5, −31, and −30 °C, respectively (Tables S6−S8). This indicates that the largest decrease of Tg occurs in EO45-b-(IL-PF6)10 but the smallest in EO45-b-(IL-TFSI)10. There is no doubt that the plasticization effect is the most obvious when the counterion is the smallest PF6. Figure 8b shows the conductivity and phase behavior of EO227-b-(IL-PF6)7, in which the PIL block is minor. The black dashed line is an extrapolation of the conductivity in the disordered morphology. One can see that the conductivity experiences an abrupt increase by about an order of magnitude after LDOT, and the conductivity line after LDOT evidently deviates from that in disordered state. A similar phenomenon is observed for other eight EOm-b-(IL-X)n BCPs with LDOT phase behavior (Figure S35). This result reveals that the EOmb-(IL-X)n LDOT BCPs exhibit thermo-activated transition in conductivity. Since the improvement of conductivity has been reported for many BCPs with well-organized morphologies,14−19 the thermo-activated transition behavior of EOm-b(IL-X)n LDOT BCPs can be ascribed to the cooperative effects of temperature rising and LDOT. This is a unique characteristic of the BCPs with LDOT phase behavior as compared with the UODT BCPs. For example, the increase of conductivity with increasing temperature is slowed down or the conductivity is even depressed after UODT; thus, no conductivity transition is observed.14 Moreover, since typically the modulus is improved in the materials with ordered morpologies,21 simultaneous enhancement of conductivity and mechanical property can be achieved in the strongly interacting LDOT BCPs, which is beneficial to material applications.

increase in conductivity is observed after LDOT, which can be attributed to the cooperative effects of temperature rising and formation of ordered structures. This reveals that LDOT BCPs may exhibit thermo-activated transition in properties, which is advantageous to the applications of BCPs.

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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.8b00227. Synthesis and structure characterization, phase behaviors, FTIR spectra of the BCPs, Tg values and conductivity of the BCPs, methods to calculate χeff (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-T.X.). *E-mail: [email protected] (B.-Y.D.). ORCID

Rui-Yang Wang: 0000-0001-9561-3237 Zai-Zai Tong: 0000-0001-7115-0442 Jun-Ting Xu: 0000-0002-7788-9026 Bin-Yang Du: 0000-0002-5693-0325 Zhi-Qiang Fan: 0000-0001-8565-5919 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21774111 and 21674097) for financial support and beamline BL16B1 at SSRF for providing the beam time.

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REFERENCES

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CONCLUSIONS To our best knowledge, this is the first example that the LDOT BCPs are intentionally designed. The LDOT phase behavior is realized in strongly interacting BCPs EOm-b-(IL-X)n by introducing extra H-bonding and Coulombic interaction, which have opposite effects on phase behavior and vary differently with temperature. At low temperatures, the Hbonding prevails, leading to the disordered state of BCPs. However, at high temperatures, the H-bonding is weakened and the Coulombic interaction becomes dominant; thus, microphase separation takes place. The LDOT phase behavior of EOm-b-(IL-X)n BCPs can be tuned by changing the relative strength of these two forces, such as alteration of counterion size and doped salt content. Moreover, mixing entropy also plays a vital role in the LDOT phase behavior. The TDOT shifts to lower temperature with increasing molecular weight. The LDOT phase behavior of the EOm-b-(IL-X)n BCPs can be explained from thermodynamic viewpoint. Some rules are suggested to guide the design of new strongly interacting BCPs with LDOT phase behavior. It is expected that other forces, like complexation, supramolecular interaction, and π−π interaction, can also be introduced into BCPs to create LDOT phase behavior. Moreover, the relationship between conductivity and LDOT behavior was preliminarily explored. A remarkable H

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DOI: 10.1021/acs.macromol.8b00227 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b00227 Macromolecules XXXX, XXX, XXX−XXX