Closed-Loop Phase Behavior of Block Copolymers ... - ACS Publications

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

Closed-Loop Phase Behavior of Block Copolymers in the Presence of Competitive Hydrogen-Bonding and Coulombic Interaction Rui-Yang Wang,† Jie Huang,† Xiao-Shuai Guo,† Xiao-Han Cao,† Shu-Fen Zou,† Zai-Zai Tong,‡ Jun-Ting Xu,*,† Bin-Yang Du,*,† and Zhi-Qiang Fan† †

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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: The closed-loop phase behavior, where a lower disorder-to-order transition (LDOT) takes place first, followed by an upper order-to-disorder transition (UODT) upon heating, is seldom observed in block copolymers (BCPs). In this work, we prepared a model BCP, LiClO4-doped poly(ethylene oxide)-bpoly(tert-butyl acrylate-co-acrylic acid) (PEO-b-P(tBA-co-AA)), in which the hydrogen (H)-bonding between the PEO and AA units and the Coulombic interaction in salt-doped PEO block have opposite effects on the miscibility of BCPs. The relative strength of the Hbonding and Coulombic interaction can be easily tuned by the hydrolysis degree (DH) of the PtBA block and the amount of doped salt. Various phase behaviors are observed by changing relative strength of different forces. Especially, the closed-loop phase behavior can be achieved when H-bonding, Coulombic interaction, and mixing entropy reach a delicate balance.

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INTRODUCTION Block copolymers (BCPs) containing two or more covalently connected segments with different chemical structures are useful in batteries, separation, and stimuli-responsive materials.1−9 The different blocks can mix to form disordered morphology or demix into various ordered nanostructures.10−18 Basically, the phase transition behaviors of BCPs can be categorized as upper order-to-disorder transition (UODT) and lower disorder-to-order transition (LDOT). For UODT, the BCPs form ordered structures at low temperatures and become disordered upon heating.19 By contrast, for LDOT the disordered phase exists at low temperatures, and ordered structures occur at high temperatures. UODT is quite common in BCPs because the mixing entropy at low temperatures usually cannot overcome the unfavorable enthalpy interaction between the unlike blocks. However, LDOT seldom occurs in BCPs because of the large interfacial energy and the entropic loss during phase transition at high temperatures,20 which are related to the small 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) (PS-b-PnAMA),21−28 poly(ethylene oxide)-b-poly(2-vinylpyridine),29 and poly(n-hexylnorbornene)-b-poly(cyclohexylnorbornene),30 where only the van der Waals force exists. It was believed that the large mismatch of compressibility or solubility parameter between the two blocks at higher temperatures leads to the LDOT in these BCPs.21,30 © XXXX American Chemical Society

Moreover, LDOT and UODT may coexist in a single BCP. When the disorder-to-order temperature (TDOT) is lower than the order-to-disorder temperature (TODT), the so-called closed-loop phase behavior is observed, which means that the ordered state only exists in an intermediate temperature window and the BCPs are disordered at both low and high temperatures. Up to now, the closed-loop phase behavior in BCPs under ambient pressure was only experimentally observed in polystyrene-b-poly(n-pentyl methacrylate) (PS-bPnA5MA).22 The dependence of TDOT and TODT on pressure revealed that the closed-loop phase behavior in PS-b-PnA5MA was entropically driven.25−28 At the moment, exploring BCPs with closed-loop phase behavior is still a challengeable task. The first difficulty is how to construct LDOT, since most BCPs usually exhibit UODT. The lack of the data about the temperature dependence of compressibility or solubility parameter for polymers makes it difficult to screen LDOT BCPs. The second difficulty lies in switching abnormal LDOT BCPs into normal UODT ones at higher temperatures. In our previous work, two extra strong forces (hydrogenbonding and Coulombic interaction) with different temperature dependences were simultaneously introduced into BCPs, and LDOT was achieved in the strongly interacting poly(ethylene oxide)-b-poly(ionic liquid) (PEO-b-PIL) BCPs.31 Received: March 23, 2018 Revised: May 25, 2018

A

DOI: 10.1021/acs.macromol.8b00627 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Details for LiClO4-Doped PEO-b-P(tBA-co-AA) Samples sample code sample sample sample sample sample sample sample sample sample sample sample sample sample

0 1 2 3 3-d1 3-d2 4 4-d1 4-d2 4-d3 5 5-d1 5-d2

chain structurea

rb

DHc (%)

wsaltd (%)

f EO/salte (%)

PEO113-b-PtBA135 PEO113-b-P(tBA131-co-AA4) PEO113-b-P(tBA128-co-AA7) PEO113-b-P(tBA120-co-AA15) PEO113-b-P(tBA120-co-AA15) PEO113-b-P(tBA120-co-AA15) PEO113-b-P(tBA107-co-AA28) PEO113-b-P(tBA107-co-AA28) PEO113-b-P(tBA107-co-AA28) PEO113-b-P(tBA107-co-AA28) PEO113-b-P(tBA92-co-AA43) PEO113-b-P(tBA92-co-AA43) PEO113-b-P(tBA92-co-AA43)

0 0 0 0 1/24 1/12 0 1/24 1/12 1/6 0 1/12 1/6

0 3.0 5.2 11.1 11.1 11.1 20.7 20.7 20.7 20.7 31.9 31.9 31.9

0 0 0 0 2.3 4.5 0 2.4 4.6 8.8 0 4.8 9.1

21.4 21.7 22.3 22.4 23.2 23.9 23.4 24.2 25.0 26.5 24.7 26.3 27.8

a The subscripts are the polymerization degrees of the EO, tBA, and AA repeating units and were determined by 1H NMR. bThe molar ratio of the salt to the EO repeating unit, r = [salt]/[EO]. cDH is the hydrolysis degree determined by 1H NMR. dThe weight fraction of the salt in the polymer mixture. ef EO/salt is the volume fraction of salt-doped PEO phase in the melt of the BCPs. The densities of amorphous PEO (1.066 g/cm3),48 PtBA (1.008 g/cm3),49 PAA (1.22 g/cm3),50 and LiClO4 (2.43 g/cm3)42 were used for calculation.

level. This is the advantage of salt-doped PEO-b-P(tBA-co-AA) BCPs over the PEO-b-PIL BCPs reported in our previous work,31 in which the strength of Coulombic interaction was only stepwise changed by ionic exchange and the strength of H-bonding was also fixed at room temperature for a specific BCP. This advantage makes it easier to achieve different kinds of phase behavior, especially the closed-loop phase behavior, by subtly regulating the relative strength of H-bonding and Coulombic interaction.

The hydrogen (H)-bonding between the PEO block and the imidazolium cation in the side groups of the PIL block results in disordered state at lower temperatures. At elevated temperatures, the H-bonding is weakened and the less temperature-dependent Coulombic interaction in the PIL block becomes dominant, leading to microphase separation. The Gibbs free energy for mixing (ΔGmix) in PEO-b-PIL BCPs with various interactions can be written as31 ΔGmix = ΔHV + ΔHH + ΔHC − T ΔS

(1)

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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. In eq 1, ΔHH is negative, while ΔHV, ΔHC, and ΔS are positive. It is expected that the PEO-b-PIL BCPs may also exhibit UODT at higher temperatures, since −TΔS will become more negative and prevail over ΔHV and ΔHC when temperature is high enough. However, probably due to the too strong Coulombic interaction, the TODT exceeds the decomposition temperature of PEO-b-PIL and UODT cannot be observed. Very high TODTs are also reported for other PIL-containing BCPs,32−39 which may result from the charges fixed to the polymer chains. The high charge density in the PIL may result in the absence of the UODT in experimental temperature window for the PIL-containing BCPs, which is also necessary for the closed-loop phase behavior of BCPs. Meanwhile, we notice that that the salt-doped BCPs, in which there also exists Coulombic interaction, usually exhibit an observable TODT.40−46 Therefore, we speculate that the closed-loop phase behavior of BCPs may be constructed via simultaneous introduction of H-bonding and salt doping. In order to verify this speculation, we synthesized poly(ethylene oxide)-bpoly(tert-butyl acrylate) (PEO-b-PtBA). The H-bonding interaction between the two blocks was introduced by partial hydrolysis of the tBA units into acrylic acid (AA) ones. The poly(ethylene oxide)-b-poly(tert-butyl acrylate-co-acrylic acid) (PEO-b-P(tBA-co-AA)) BCPs were then doped with LiClO4 to introduce the Coulombic interaction into the PEO block. In this system, the strengths of H-bonding and Coulombic interaction can be continuously tuned by the hydrolysis degree (DH) and the salt doping ratio (r) from a very weak initial

EXPERIMENTAL SECTION

Materials. Monomethyl poly(ethylene oxide) (PEO) with a number-average molecular weight of 5000 g/mol and a dispersity of Mw/Mn = 1.04 was purchased from Aldrich. The PEO was dried by azeotropic distillation with toluene. tert-Butyl acrylate (tBA) with a purity of 99% was purchased from Alfa Aesar. It was first washed with aqueous solution of NaOH thrice, then washed with deionized water until neutral, dried with MgSO4 overnight, and finally distilled under reduced pressure. CuBr (98% purity, Aldrich) was washed with 2% aqueous solution of acetic acid until the solution became colorless and then washed with deionized water until neutral. After being washed with ethanol and diethyl ether repeatedly, it was dried under vacuum for 3 days and stored in a glovebox. N,N,N′,N″,N″-Pentamethyldiethylenetriamine (PMDETA) (98% purity, Alfa Aesar), 2bromoisobutyryl bromide (98% purity, Aldrich), and trifluoroacetic acid (98% purity, Aldrich) were used without further purification. All the solvents, including dichloromethane (DCM), diethyl ether, pyridine, tetrahydrofuran (THF), and toluene, were distilled before use. Preparation of Polymers. PEO-b-PtBA (Mw/Mn = 1.12) was prepared by the atom transfer radical polymerization (ATRP) of tBA using the bromide end-functionalized PEO as macroinitiator. The tBA units were partially hydrolyzed with trifluoroacetic acid as catalyst to yield various PEO-b-P(tBA-co-AA)) BCPs.47 The reaction conditions are listed in Table S1. Finally, different amounts of LiClO4 were mixed with PEO-b-P(tBA-co-AA) BCPs to introduce the Coulombic interaction into the hydrolyzed BCPs. Although the repeating units of PEO and PtBA are constant, the volume fraction of PEO phase may be changed after hydrolysis of PtBA and salt doping. So we use f EO/salt to stand for the volume fraction of salt-doped phase in the melt of the BCPs, which is calculated by the following equation: B

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converted into one-dimensional data using Fit2D software.52 The samples were pretreated by annealing at 100 °C for 24 h under a N2 atmosphere, followed by slow cooling down to room temperature before experiment. During SAXS measurements, the samples were heated to the preset temperature at a rate of 10 °C/min with a Linkam hot stage and held for 3 min to reach equilibrium 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 to calibrate the scattering vector. The morphology change was determined by the SAXS pattern. Disordered and ordered morphologies have different scattering patterns if the electron density contrast is big enough. The correlation hole effect of the BCP in the disordered state will arise a broad scattering maximum.53 In the ordered morphologies, the primary scattering peak with corresponding scattering vector q* becomes sharper and more intense accompanied by the appearance of high-order peaks. The ordered morphologies were determined by the relative positions of the highorder peaks with scattering vector (q) to the primary scattering peak, i.e., q/q*. The TDOT s and TODTs were determined by the discontinuous change of the inverse of the primary scattering peak intensity, I−1(q*), versus T−1 or the discontinuous change of the full width at half-maximum (fwhm) of the scattering peak versus T−1.54

fEO/salt = NEOMEO/ρEO + rNEOMsalt /ρsalt NEOMEO/ρEO + rNEOMsalt /ρsalt + Nt BAMt BA /ρt BA + NAAMAA /ρAA

(2) where N is the number of the monomers in one polymer chain, M is the molecular weight of the corresponding monomer, r is the molar ratio of the salt to the EO repeating unit, and ρ is the density of salt or homopolymers. The densities of amorphous PEO (1.066 g/cm3),48 PtBA (1.008 g/cm3),49 PAA (1.22 g/cm3),50 and LiClO4 (2.43 g/ cm3)42 were used for calculation. As shown in Table 1, f EO/salt ranges from 21.4% to 27.8%. All these values correspond to the Hex structure in the common diblock copolymers, and thus the influence of volume change due to salt doping on the phase behavior is insignificant. The synthesis details are shown in the Supporting Information (Figure S1) as well as our previous work.51 The structure information and chemical structure of the salt-doped PEO113-b-P(tBAm-co-AAn)) BCPs are listed in Table 1 and Figure 1, respectively.

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RESULTS Effect of H-Bonding on Phase Behavior. We first investigated the phase behavior of PEO113-b-P(tBAm-co-AAn) BCPs with different hydrolysis degrees (DHs), in which only van der Waals force and H-bonding exist but Coulombic interaction is absent. The FTIR spectra in the range of 1640− 1790 cm−1 for the PEO113-b-P(tBAm-co-AAn) BCPs with different DHs are shown in Figure 2. The main band at

Figure 1. (a) Chemical structure of the salt-doped PEO113-b-P(tBAmco-AAn). (b) Scheme for the fundamental interactions in the saltdoped PEO113-b-P(tBAm-co-AAn) strongly interacting BCPs: the van der Waals force (magenta), hydrogen-bonding (green), and Coulombic interaction (cyan). Characterization of BCPs. The molecular weight distribution was measured on a Waters gel permeation chromatography (GPC) system at 40 °C. THF was used as the eluent, and the flow rate was 1.0 mL/min. Polystyrene standards were used for calibration.1H NMR spectra were collected on a Bruker DMX-400 instrument, and CDCl3 was used as solvents. The thermal behavior of the salt-doped PEO113b-P(tBAm-co-AAn) was characterized by differential scanning calorimetry (DSC) on a TA Q200 instrument. About 4−6 mg of the sample sealed in an aluminum pan was first heated from 40 to 120 °C, kept for 5 min to erase the thermal history, then cooled to −80 °C, and heated again to 120 °C. Both the heating and cooling rates were set as 10 °C/min. Thermal gravimetric analysis (TGA) was carried out on a PerkinElmer TGA instrument with the heating rate of 10 °C/min under a nitrogen atmosphere with a flow rate of 20 mL/min. The Fourier-transform infrared (FTIR) spectra were recorded on a Thermo Fisher Scientific LLC Nicolet 6700 spectrometer equipped with a hot stage. The samples were heated to 100 °C and pressed into thin films before test. The data were analyzed with OPUS software. Small-Angle X-ray Scattering (SAXS). Temperature-variable small-angle X-ray scattering (SAXS) experiments were performed at the BL16B1 beamline in Shanghai Synchrotron Radiation Facility (SSRF), China. The wavelength of X-ray at SSRF is 1.24 Å. The sample-to-detector distance was set as 2000 mm. Two-dimensional (2D) SAXS patterns were recorded with a Mar 165 CCD and then

Figure 2. FTIR spectra of PEO113-b-P(tBAm-co-AAn) BCPs with different hydrolysis degrees at 70 °C in the region 1640−1790 cm−1.

∼1730 cm−1 is assigned to the free carbonyl group. One can see that as the hydrolysis degree increases, a shoulder peak appears at ∼1710 cm−1, and its intensity becomes stronger gradually. This band is produced by the H-bonded CO,55 which confirms the formation of H-bonds between PEO and the AA units. Figure 3 shows the representative temperaturevariable SAXS profiles of PEO113-b-P(tBAm-co-AAn) BCPs with DH = 0, 3.0%, and 11.1%. The SAXS profiles of other PEO113b-P(tBAm-co-AAn) BCPs with different DHs are presented in Figure S4. It is observed that the unhydrolyzed PEO113-bPtBA135 BCP exhibits a hexagonally packed cylindrical (Hex) structure above the melting temperature of PEO (70 °C), then transforms into the body-centered cubic (Bcc) structure at 160 C

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the two blocks, which improves the miscibility of BCPs gradually. Closed-Loop Phase Behavior of Salt-Doped PEO113-bP(tBA120-co-AA15). One can see that PEO113-b-P(tBAm-coAAn) BCPs may become disordered at low temperatures when the introduced H-bonding is strong enough, which is necessary for LDOT. However, the miscibility at high temperatures is also improved by H-bonding, as evidenced by the decreased TODT. This is disadvantageous to the formation of an ordered structure at high temperatures and the occurrence of LDOT. In order to enhance the immiscibility at high temperatures, we added LiClO4 to the PEO113-b-P(tBA120-co-AA15) BCP with DH = 11.1%, which is originally disordered at 70 °C. The ether groups in the PEO block can associate with the Li+ cations in the added salt to form solid electrolyte.40−46 Figure 4 shows

Figure 3. Representative temperature-variable SAXS profiles of PEO113-b-P(tBAm-co-AAn) BCPs. (a) PEO113-b-PtBA135 (DH = 0); (b) PEO113-b-P(tBA131-co-AA4) (DH = 3.0%); (c) PEO113-b-P(tBA120co-AA15) (DH = 11.1%); (d) BCPs with different DHs at 70 °C. The dashed vertical line is drawn for easy observation.

°C, and finally becomes disordered at 200 °C (Figure 3a). The sample PEO113-b-P(tBA131-co-AA4) with DH = 3.0% also forms a Hex structure at lower temperature and directly changes into disordered state at TODT = 140 °C (Figure 3b). The TODT is further lowered to 100 °C when DH is increased to 5.2% (Figure S4a). This shows that the unhydrolyzed PEO113-bPtBA135 and the PEO113-b-P(tBAm-co-AAn) BCPs with lower DHs exhibit a typical UODT phase behavior, like most common BCPs. The TODT decreases as DH increases, which is also in accordance with our previous result.51 This reveals that when van der Waals force and H-bonding coexist in the BCPs, only UODT phase behavior can be achieved. At higher DH, such as 11.1% for PEO113-b-P(tBA120-co-AA15), a broad maximum is observed in the SAXS profile and the higherorder peaks disappear, indicating that the BCP is disordered in the whole temperature range studied (Figure 3c). When DH is up to 31.9%, the broad scattering maximum also disappears, indicating a homogeneously mixed state (Figure S4c). SAXS profiles of BCPs with different DHs at 70 °C are collected in Figure 3d. The microphase structures and the positions of the primary scattering peaks barely change at 70 °C when DH is less than 11.1%. As DH reaches 11.1%, only a broad maximum is observed. With increasing DH, the broad maximum shifts to lower q and finally disappears. The decrease of TODT and final disappearance of the ordered structure with increasing DH can be ascribed to the enhanced H-bonding interaction between

Figure 4. FTIR bands for C−O−C group (a) and CO group (b) of neat and LiClO4-doped PEO113-b-P(tBA120-co-AA15) (DH = 11.1%). The doping ratios (r) are indicated.

the FTIR spectra of PEO113-b-P(tBA120-co-AA15) doped with different amounts of LiClO4. The shoulder peak at ∼1094 cm−1 corresponds to the stretching vibration of associated C− O−C groups in PEO.56,57 One can see that the intensity of this peak increases gradually with increasing the amount of LiClO4 added (Figure 4a), whereas the vibration band of the carbonyl groups barely changes after LiClO4 is added (Figure 4b). This indicates that LiClO4 mainly interacts with the PEO block. By contrast, the interaction between LiClO4 and the P(tBAm-coD

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overwhelms the Coulombic interaction at low temperatures, leading to a disordered state. In the intermediate temperature range, the H-bonding is weakened and the less temperaturedependent Coulombic interaction prevails; thus, microphase separation takes place. However, at high temperatures, the term of TΔS will be larger than the sum of ΔHC and ΔHV, which means that the mixing entropy becomes dominant and the BCP tends to form disordered melt again. Competition between H-Bonding and Coulombic Interaction. It should be noted that the H-bonding between PEO and AA units can improve the miscibility of the PEO113b-P(tBAm-co-AAn) BCPs at both low and high temperatures, since the H-bonds will not be completely destroyed at high temperature (Figure S11). On the other hand, the Coulombic interaction may enhance the segregation strength at both low and high temperatures. As a consequence, these two forces are also competitive, which will affect the phase behavior of PEO113-b-P(tBAm-co-AAn) BCPs. To reveal such an effect, we added different amounts of LiClO4 into the PEO113-b-P(tBAmco-AAn) BCPs with various hydrolysis degrees. As shown in Figure 6a, the structure of PEO113-b-P(tBA120-co-AA15) (DH = 11.1%) with r = 1/12 is always Hex in the temperature of 40− 200 °C, as compared with the closed-loop phase behavior at r = 1/24 (Figure 5a). This shows that the Coulombic interaction overwhelms the H-bonding in the whole temperature range studied at a higher doping ratio. For the LiClO4-doped PEO113-b-P(tBA107-co-AA28) with a DH = 20.7%, it is disordered at r = 1/24 (Figure 6b) but possesses a Hex structure in 40−160 °C at r = 1/12 (Figure 6c). When DH is further increased to 31.9%, it is disordered at r = 1/12 and becomes ordered at r = 1/6 (Figures 6d,e). This reveals that more salt should be added to induce the formation of an ordered structure when the hydrolysis degree is higher. The structure and phase transition of neat and LiClO4doped PEO113-b-P(tBAm-co-AAn) BCPs are summarized in Table 2. Different phase behaviors are observed, depending on the interplay of H-bonding and Coulombic interaction, which can be further schematically depicted in Figure 7 in terms of the relative strength of these two forces. When both forces are weak (at low DH and low r), the widely existing van der Waals force and mixing entropy are dominant; the BCPs behave like most common BCPs and exhibit UODT.43−46 When DH is small but r is large, where the strength of Coulombic interaction far exceeds that of H-bonding, the BCPs have a strong tendency of microphase separation and always possess ordered structures in the temperature range investigated.32−39 On the other hand, at a high DH but a small r, the BCPs are generally disordered because of the presence of a large amount of H-bonds. Moreover, at a high doping ratio, the Coulombic interaction may still prevail over the H-bonding even though the BCP has a high hydrolysis degree. This can be seen from the ordered structures of PEO113-b-P(tBA107-co-AA28) at r = 1/ 12 or 1/6 and PEO113-b-P(tBA92-co-AA43) at r = 1/6. However, the phase behavior of PEO113-b-P(tBAm-co-AAn) at both high DH and large r may be different from that at high DH but small r. At r = 1/6, a Hex-to-Lam transition is observed for PEO113-b-P(tBA107-co-AA28) with a DH = 20.7% (Figure S5) and PEO113-b-P(tBA92-co-AA43) with a DH = 31.9% (Figure 6e) upon heating, in which the f EO is 0.265 and 0.278, respectively. Usually Lam structure cannot be formed at such highly asymmetric compositions in neutral BCPs. However, according to the theoretical prediction58 and experimental observation,31 the phase diagram is highly asymmetric in

AAn) block, if there is any, is weak and occurs at high doping ratios (Figure S13). The SAXS profiles of PEO113-b-P(tBA120-co-AA15) with a doping ratio r = 1/24 are compiled in Figure 5a. It is observed

Figure 5. Representative temperature-variable SAXS profiles of saltdoped PEO113-b-P(tBA120-co-AA15) (DH = 11.1%) with r = 1/24 (a) and plots of fwhm (b) and I−1(q*) (c) against the inverse of temperature. The temperature ranges of Hex and Dis structures are painted in pink and cyan, respectively.

that the salt-doped PEO113-b-P(tBA120-co-AA15) is disordered at 30 and 50 °C, since only a broad maximum is observed in the SAXS profiles. As temperature rises, the primary SAXS peak becomes shaper and narrower. At 70 °C, a second-order peak starts to appear at 31/2q*, and it becomes stronger gradually with increasing temperature. This clearly shows the occurrence of LDOT. When temperature exceeds 110 °C, the primary SAXS peak becomes broader with increasing temperature. The second-order peak also fades away and completely disappears above 145 °C. As a result, UODT also takes place at higher temperature in this salt-doped sample. The TDOT and TODT of salt-doped PEO113-b-P(tBA120-co-AA15) with r = 1/24 are 70 and 140 °C, respectively. The coexistence of LDOT and UODT as well as higher TODT than TDOT shows that the saltdoped PEO113-b-P(tBA120-co-AA15) with r = 1/24 exhibits a closed-loop phase behavior. This can be further confirmed by the variation of the intensity and width of the primary SAXS peak with temperature. The full width at half-maximum (fwhm) and inverse of intensity (I−1(q*)) of the primary scattering peak are illustrated in Figures 5b and 5c, respectively. It is found that both fwhm and I−1(q*) first gradually decrease with increasing temperature, implying the transformation from the disordered state into an ordered structure. In an intermediate temperature window, fwhm and I−1(q*) are less variable with temperature, agreeing with the stable ordered structure. At higher temperatures, I−1(q*) and fwhm increase rapidly as temperature rises, corresponding to the disappearance of the ordered structure. Such changes of fwhm and I−1(q*) with temperature are similar to those reported for PS-b-PnA5MA BCP with a closed-loop phase behavior.22 The SAXS result reveals that the closed-loop phase behavior is achieved for the first time in strongly interacting BCPs by simultaneous introduction of H-bonding and Coulombic interaction. The occurrence of closed-loop phase behavior results from cooperation of these two forces. H-bonding E

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Figure 6. Representative temperature-variable SAXS profiles of salt-doped BCPs. (a) PEO113-b-P(tBA120-co-AA15) (DH = 11.1%) with r = 1/12. (b) PEO113-b-P(tBA107-co-AA28) (DH = 20.7%) with r = 1/24. (c) PEO113-b-P(tBA107-co-AA28) (DH = 20.7%) with r = 1/12. (d) PEO113-b-P(tBA92-coAA43) (DH = 31.9%) with r = 1/12. (e) PEO113-b-P(tBA92-co-AA43) (DH = 31.9%) with r = 1/6.

Table 2. Structure and Phase Transition of Salt-Doped PEO113-b-P(tBAm-co-AAn) BCPs with Different Hydrolysis Degrees and Doping Ratios DH (%) r 0

1/24

1/12 1/6

0 Hex → Bcc → Disa

3.0 Hex → Disa

5.2 Hex → Disa

11.1 Dis

d

Dis → Hex → Disb Hexe

20.7 Dis

d

31.9 homogeneously mixedd

Disd Hexe Hex → Lamc

Figure 7. Scheme for the variations of phase behavior of salt-doped PEO113-b-P(tBAm-co-AAn) BCPs with the relative strength of ΔHH and ΔHV + ΔHC. Four kinds of experimentally observed phase behaviors (UODT, disordered, ordered, and closed-loop) are signed by the “check mark”, and the expected LDOT phase behavior is signed by the “question mark”.

Disd Hex → Lamc

a

The UODT behavior is written in red. bClosed-loop behavior is written in brown. cHex-to-Lam transition is written in blue. d Disordered is written in green. eOrdered is written in purple.

segregation strength at fixed composition, as compared with the Hex structure. This means that the tendency of microphase separation in PEO113-b-P(tBA107-co-AA28) and PEO113-bP(tBA92-co-AA43) at r = 1/6 is enhanced upon heating, which is in accordance with the LDOT phase behavior.31 The H-bonding between the PEO block and AA units becomes weakened with temperature rising, while the Coulombic interaction is less variable with temperature. Since the

charged BCPs due to the presence of Coulombic interaction, and Lam structure can be formed at a relatively small volume fraction. Moreover, such an order-to-order transition (OOT) with respect to the temperature sequence is opposed to that for common BCPs, in which the OOT of Lam-to-Hex is usually observed upon heating.52 Based on the phase diagram of common BCPs, the Lam structure is formed at a larger F

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Coulombic interaction is favorable to demixing, the tendency of microphase separation is enhanced at elevated temperatures, leading to the occurrence of the Hex-to-Lam transition. As a result, we deduce that salt-doped PEO113-b-P(tBAm-co-AAn) BCPs at both high DH and large r should exhibit a LDOT phase behavior. Nevertheless, the disorder-to-order temperature (TDOT) may be lower than room temperature and is unable to be experimentally observed. It is expected that the LDOT phase behavior will appear in PEO113-b-P(tBAm-coAA135−m) with r = 1/6 when DH is further increased (>31.9%). Finally, the central region of Figure 7 represents intermediately strong and comparable H-bonding and Coulombic interaction. Under such conditions, the closed-loop phase behavior may appear, for example, in PEO113-b-P(tBA120-co-AA15) with DH = 5.2% and r = 1/24. We believe that the appearance of closedloop phase behavior requires suitable strengths of H-bonding, Coulombic interaction, and mixing entropy and delicate balance of these forces. This is the reason why the closedloop phase behavior is observed in only one BCP.

CONCLUSIONS In summary, the relative strength of H-bonding and Coulombic interaction in salt-doped PEO-b-P(tBAm-co-AAn) BCPs can be tuned by the hydrolysis degree of the tBA units and the salt content. The phase behavior of salt-doped PEO-bP(tBAm-co-AAn) BCPs varies with the interplay of these two forces. Particularly, the closed-loop phase behavior is achieved for the first time in strongly interacting BCPs, which results from the suitable strengths of H-bonding, Coulombic interaction, and mixing entropy and the delicate balance among them. Other phase behaviors, such as disordered, ordered, UODT, and abnormal order-to-order transition, can be yielded by shifting the balance of these forces. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00627.

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Article

Synthesis and characterization of BCPs, SAXS profiles, FTIR spectra, and DSC and TGA curves of doped and undoped BCPs (PDF)

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. G

DOI: 10.1021/acs.macromol.8b00627 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.8b00627 Macromolecules XXXX, XXX, XXX−XXX