Influence of Ionic Species on the Microphase Separation Behavior of

Nov 24, 2014 - results showed that the microphase separation behavior of the PCL-b-PEO/salt hybrids was sensitive to the competitive interactions of t...
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Influence of Ionic Species on the Microphase Separation Behavior of PCL‑b‑PEO/Salt Hybrids Jie Huang, Rui-Yang Wang, Zai-Zai Tong, Jun-Ting Xu,* and Zhi-Qiang Fan MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: The microphase separation behavior of the hybrids of poly(ε-caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) with different inorganic salts at various doping ratios (r) was studied by temperature-variable SAXS. It was observed that the salts could induce microphase separation to form ordered structure in the originally miscible melt of PCL-b-PEO. The effects of the metal ion and anion were correlated with the competitive interactions of PEO/salt and PCL/ salt, which were characterized by FT-IR and DSC, respectively. It was found that at lower doping ratios the salts preferentially interacted with PEO. The larger association number of the metal ion and stronger association between PEO and salt led to a lower onset doping ratio for formation of ordered structure (r0). At higher doping ratios the salt interacted with PCL as well. When the metal ion exhibited a highly selective interaction toward PEO, a more ordered structure with a higher order−order transition temperature (TODT) tended to be formed. The anion in the salt also affected the interactions of PEO/salt and PCL/salt. Weaker Lewis basicity of the anion would result in a stronger interaction of PEO/salt and thus a lower r0. The results showed that the microphase separation behavior of the PCL-b-PEO/salt hybrids was sensitive to the competitive interactions of the salt with the PCL and PEO blocks.

1. INTRODUCTION In the past decade, inorganic salt-doped block copolymer (BCP) hybrid system has received great attention for its potential applications in lithium batteries, fuel cells, organic photovoltaic cells, and high density templates.1−11 This kind of system can possess the advantages of both components: ordered structure of BCP and function of inorganic salt. When microphase separation takes place in BCPs, various ordered nanostructures, including body-centered cubic spherical (BCC), hexagonally packed cylindrical (HEX), gyroid (GYR), and lamellar (LAM), may be formed, as the composition of BCPs and the segregation strength between different blocks change.12−15 The structure and properties of the BCP/salt hybrids have been intensively studied, including the phase behavior,16−36 distribution of inorganic salt among different microdomains,37,38 and electric property.39−51 Although the microphase-separated morphology of the BCP/ salt hybrids is mainly determined by the structure of BCP, salt concentration and ionic species also have a vital effect. Both the volume fractions of components and the segregation strength between the two blocks, thus the morphology of the hybrids, vary with the salt concentration. On the other hand, in the BCP/salt hybrids, the metal ions can provide empty electron orbital, and the atoms such as oxygen and nitrogen in polymer chains can provide lone pair electrons. As a consequence, there exists interaction between the BCP and salt, which will affect © 2014 American Chemical Society

the segregation strength between the two blocks in return. Such an effect is dependent on the ionic species, i.e., the nature of the cation and anion in the salt. Epps et al. used lithium salts with different anions to hybridize with a polystyrene-b-poly(ethylene oxide) (PS-b-PEO). They found that weaker Lewis basicity of the anion could lead to a larger segregation strength between PS and PEO and thus a larger microdomain spacing.52 Ho et al. investigated the effect of the association strength between the metal ion and poly(4-vinylpyridine) on the phase behavior of poly(4-vinylpyridine)-b-poly(ε-caprolactone) (P4VP-b-PCL)/ salt hybrids. It was observed that the association strength of Au3+ was stronger than that of Cu2+, resulting in a higher degree of domain swelling.38 However, so far only the selective interaction of the salt with one of the blocks is considered in the literature, which may be simplification of the real situation. In our previous work, we studied the phase behavior of poly(ε-caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO)/LiClO4 hybrids, in which the Li+ ions can competitively associate with both the PEO and PCL blocks.53−55 The competitive interactions of PEO/Li+ and PCL/Li+ may reduce the effective Flory−Huggins interaction parameter between the PCL and PEO blocks (χeff) at high salt Received: October 6, 2014 Revised: November 13, 2014 Published: November 24, 2014 8359

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1W2A beamline in Beijing Synchrotron Radiation Facility (BSRF), China. The wavelengths of X-ray at SSRF and BSRF are 1.24 and 1.54 Å, respectively. The sample-to-detector distance was set as 2000 mm. Two-dimensional (2D) SAXS patterns were recorded by time-resolved mode. The average exposure time was 300 s for each scan. The bull 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. A Linkam hot-stage was used to heat the samples. Differential Scanning Calorimetry. The thermal behavior of salt-doped PCL-b-PEO samples was characterized by differential scanning calorimetry (DSC) on a TA Q200 instrument. About 3−4 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, and then cooled to −40 °C to obtain the nonisothermal crystallization DSC curves. Subsequently, the samples were heated to 100 °C to acquire the melting traces. Both the heating and cooling rates are 10 °C/min. Fourier Transform Infrared. Fourier transformation infrared (FT-IR) spectra of the PCL-b-PEO/salt hybrids were recorded on a Nicolet 6700 spectrometer at room temperature. The scan range is from 2000 to 400 cm−1 with a resolution of 2 cm−1. The samples for FT-IR measurements were prepared by hot-pressing at 70 °C for 5 min and then cooled down to room temperature under dynamic vacuum.

concentrations and even lead to some abnormal order−order transitions.53,54 Consideration of the competitive interactions of the salt with both blocks represents a more general case, and introduction of competitive interactions into the BCP/salt hybrid system will provide more approaches to regulate the morphology and properties of the hybrids by adjusting the strengths of the competitive interactions. When the salt can interact with both blocks of the BCPs, it is imaginable that the selectivity of such competitive interactions toward different blocks, which is strongly dependent on the nature of the metal ion and anion in the salt,56−59 will be a key factor affecting the phase behavior of the BCP/salt hybrids. Moreover, the morphology of the BCP/salt hybrids can be regulated by external fields, such as electrostatic field.60 The responsivity to the external field may also vary with the salt. Therefore, it is of great importance to study the phase behavior of the hybrids of BCP with various salts. In the present work, inorganic salts with different metal ions and anions were hybridized with PCL-b-PEO BCPs. The effects of the cation and anion on the microphase separation behavior of the hybrids and the competitive interactions of PEO/salt and PCL/salt were investigated. In the literature, BCPs with strong segregation strength are usually hybridized with salts. When the salt only causes a small change in the Flory−Huggins interaction parameter (χ), as compared with the original χ between the two blocks, the phase behavior of the BCP may be just slightly altered and the effect of the salt cannot be well identified. By contrast, herein we used a BCP composed of miscible PCL and PEO blocks to hybridize with salts. A smaller change in χ due to introduction of the salts may still result in an evident change of the phase behavior; thus the effects of different ionic species can be readily compared.

3. RESULTS AND DISCUSSION Effect of Metal Ion on the Microphase Separation Behavior of Salt-Doped PCL56-b-PEO44. In order to investigate the effect of metal ion on the phase behavior of PCL-b-PEO/salt hybrids, three inorganic salts with the same anion but different metal ions (LiCl, CuCl2, and FeCl3) were doped with PCL56-b-PEO44 at four doping ratios: r = 1/24, r = 1/12, r = 1/6, and r = 1/3. The structure, order−disorder transition temperature (TODT), domain spacing, and χeff at 70 °C of salt-doped PCL56-b-PEO44 hybrids are summarized in Table 1. Because both blocks in the PCL-b-PEO BCPs can

2. EXPERIMENTAL SECTION Materials. The PCL-b-PEO BCPs with a narrow molecular weight distribution were synthesized according to our previous work.61 The number-average molecular weights of the PCL-b-PEO BCPs and the polymerization degrees of the PCL and PEO blocks are calculated from 1H NMR spectra. The molecular weight distribution (Mw/Mn) was determined by gel permeation chromatography (GPC). The PCLb-PEO BCPs are also denoted as PCLm-b-PEOn, where the subscripts m and n are the polymerization degrees of the PCL and PEO blocks, respectively. Ferric trichloride (FeCl3), lithium chloride (LiCl), and lithium trifluoromethanesulfonate (LiCF3SO3) were purchased from Alfa-Aesar, J&K Scientific Ltd., and TCI, respectively. Copper chloride (CuCl2) and lithium perchlorate (LiClO4) were purchased from Acros-Organics. Before preparing the salt-doped PCL-b-PEO samples, all the materials were dried for 24 h under dynamic vacuum. Inorganic salts were used as-received without further purification. Tetrahydrofuran (THF) was refluxed with sodium flakes and distilled prior to use. Preparation of PCL-b-PEO/Salt Hybrids. A predescribed amount of inorganic salt and PCL-b-PEO were dissolved in THF and stirred for 24 h. All the operations including weighing and mixing were conducted in a glovebox under a N2 atmosphere to avoid the possible effect of humidity. The solution was dried under dynamic vacuum for 24 h at 70 °C to remove the solvent THF completely. After drying, the hybrids were stored in a glovebox. The doping ratio, r, which is defined as the molar ratio of Mn+ ion to EO unit (r =[Mn+]/ [EO]), was used to describe the relative content of the inorganic salt in the hybrids. The volume fractions of the PCL, PEO, and the salt in the hybrids were calculated based on their densities. The data are summarized in Tables S1 and S2 of the Supporting Information. Small Angle X-ray Scattering. Temperature-variable small-angle X-ray scattering (SAXS) experiments were performed at BL16B1 beamline in Shanghai Synchrotron Radiation Facility (SSRF) and

Table 1. Structure, Domain Spacing and χeff at 70 °C, and TODT of Salt-Doped PCL56-b-PEO44 Hybrids TODT (°C)

sample PCL56-b-PEO44 LiCl/BCP

CuCl2/BCP

FeCl3/BCP

r r r r r r r r r

= = = = = = = = =

0 1/24 1/12 1/6 1/3 1/24 1/12 1/6 1/3

r r r r

= = = =

1/24 1/12 1/6 1/3

80

100 120

80 100

structure DIS DIS DIS DIS LAM DIS DIS HEX LAM → HEX DIS LAM LAM DIS

d70 °C (nm)

χeff,70 °C

33.2

0.096

20.1 21.7

0.362 0.634

16.9 20.4

0.211 0.174

crystallize at room temperature, which may alter the morphology of the hybrids in the melt, SAXS, instead of transmission electron microscopy (TEM), was used to characterize the morphology of the PCL-b-PEO/salt hybrids in the melt. Figure 1 shows the SAXS profiles of LiCl-doped PCL56-b-PEO44. For the hybrids with r = 1/24 and 1/12, no scattering peak is observed at 70 °C, and the SAXS profiles decay smoothly (Figure 1a), indicating that the hybrids at these 8360

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Figure 1. SAXS profiles of PCL56-b-PEO44/LiCl hybrids with r = 1/24, 1/12, and 1/6 at 70 °C (a) and r = 1/3 at different temperatures (b).

Figure 2. SAXS profiles of PCL56-b-PEO44/CuCl2 hybrids at r = 1/6 (a) and 1/3 (b).

two doping ratios are miscible. When the doping ratio is increased to r = 1/6 (Figure 1a), only a broad first-order peak is observed at 70 °C. This shows that microphase separation occurs in the melt, but no ordered structure is formed. For the hybrid with r = 1/3 (Figure 1b), two distinct scattering peaks appear in the SAXS profile at 70 °C, indicating the presence of an ordered and microphase-separated structure. Since the scattering vector ratio of the second-order peak to the firstorder peak is q/q* = 2 (where q* is the scattering vector of the first-order peak), the formed structure can be assigned to LAM. As temperature increases, the intensity of the scattering peaks decreases gradually and the hybrid becomes basically disordered at 80 °C. The SAXS profiles at various doping ratios show that low concentration LiCl is not enough to induce microphase separation in PCL56-b-PEO44, and microphase separation only occurs at a high doping ratio. As for the CuCl2-doped PCL56-b-PEO44 hybrids, no microphase separation was observed in the melt at r = 1/24, like the LiCl-doped PCL56-b-PEO44 at r = 1/24 and 1/12. Only a broad first-order peak is observed at r = 1/12, which is similar to the LiCl-doped PCL56-b-PEO44 at r = 1/6, indicating a microphase-separated but disordered structure (Figure S1 in Supporting Information). The SAXS profiles of CuCl2-doped PCL56-b-PEO44 at r = 1/6 and 1/3 are shown in Figure 2. The hybrid with r = 1/6 exhibits a HEX structure at 70 °C, since two scattering peaks are located at q* and 31/2q*, respectively. This means that the onset doping ratio for occurrence of ordered structure of microphase separation (r0) in the CuCl2doped PCL56-b-PEO44 hybrids is lower than that in the LiCldoped hybrids. The PCL56-b-PEO44/CuCl2 hybrid with r = 1/6 becomes disordered when the temperature is higher than 100 °C (Figure 2a). For the PCL56-b-PEO44/CuCl2 hybrid with r = 1/3 at 70 °C, two scattering peaks appear at q* and 2q*, respectively, indicating a LAM structure. The structural change from HEX at r = 1/6 to LAM at r = 1/3 is due to the increase of the CuCl2 content in the hybrid, as reported in our previous work.53 This also shows that CuCl2 is mainly enriched in the PEO microdomains, since the PEO block is the minor component in PCL56-b-PEO44. Upon heating this hybrid, the second scattering peak shifts to 31/2q*, showing that an order− order transition (OOT) of LAM-to-HEX occurs at about 100 °C. When this hybrid is further heated, the order−disorder transition is observed at 120 °C (Figure 2b).

FeCl3 was also used to dope with PCL56-b-PEO44. Figure 3 shows the SAXS profiles of the PCL56-b-PEO44/FeCl3 hybrids with r = 1/24 and 1/3 at 70 °C and with r = 1/12 and r = 1/6

Figure 3. SAXS profiles of PCL56-b-PEO44/FeCl3 hybrids with r = 1/ 24 and 1/3 at 70 °C (a) and with r = 1/12 (b) and r = 1/6 (c) at different temperatures. 8361

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at different temperatures. Weak microphase separation with a disordered structure is just observed for the hybrid with r = 1/ 24, as revealed by the weak and broad first-order SAXS peak (Figure 3a). For the hybrid with r = 1/12 at 70 °C, the firstorder scattering peak appears at q* = 0.372 nm−1 (Figure 3b). Besides, a broad second-order can be observed around 2q*. This shows that microphase separation takes place in this hybrid, but the formed structure is not so ordered. Similarly, the PCL56-b-PEO44/FeCl3 hybrid with r = 1/6 also forms a not very ordered LAM structure at 70 °C (Figure 3c). The order− disorder transition temperature (TODT) of this hybrid is about 100 °C. For the hybrid with r = 1/3, no scattering peak is observed in the melt, showing that there is no microphase separation (Figure 3a). We can see from the SAXS data that the onset doping ratios for appearance of an ordered and microphase-separated structure in the melt of PCL56-b-PEO44/salt hybrids are r = 1/3, 1/6, and 1/12 for LiCl, CuCl2, and FeCl3, respectively. Therefore, FeCl3 is easier to induce microphase separation of PCL56-b-PEO44. On the other hand, at a high doping ratio (r = 1/3), no microphase separation occurs in the PCL56-b-PEO44/ FeCl3 hybrid at 70 °C, while the PCL56-b-PEO44/CuCl2 hybrid exhibits a higher TODT than the PCL56-b-PEO44/LiCl hybrid. This shows that the ability of salts to induce microphase separation does not monotonously increase with the doping ratio. Moreover, based on the mean-field theory modified by Fredrickson and Helfand,12,54,62 the effective Flory−Huggins interaction parameters (χeff) of salt-doped PCL56-b-PEO44 hybrids can be calculated by comparing the d-spacings of the microdomains at 70 °C and TODT.53,54 The calculated data are shown in Table 1. We can see that, at low doping ratios (such as r = 1/12), the PCL56-b-PEO44/FeCl3 hybrid exhibits the largest χeff, which is in accordance with the strongest ability of FeCl3 to induce microphase separation in the melt of PCL-bPEO. By contrast, at r = 1/3, the χeff of the PCL56-b-PEO44/ CuCl2 is the largest, agreeing with the strongest microphase separation behavior of this hybrid. Competitive Interactions of Metal Ion with PEO and PCL. As pointed out in our previous work, there exist competitive interactions of Li+ ions with both the PEO and PCL blocks in the PCL-b-PEO/LiClO4 hybrids.54 Such competitive interactions may vary with the doped salt, leading to the different phase behaviors of the PCL56-b-PEO44/salt hybrids. The interaction between the metal ion and the PEO block was first studied with FT-IR. The association of the metal ions with the PEO block can result in a shift of the C−O−C stretching vibration to a lower wavenumber. However, such an association is dynamic, and there exist both associated and free C−O−C groups. Figure 4 shows the FT-IR spectra for the saltdoped PCL56-b-PEO44 hybrids at various doping ratios. One can see from Figure 4 that the addition of LiCl, CuCl2, and FeCl3 leads to a shift of the C−O−C stretching vibration around 1109 cm−1 (free C−O−C) to 1094 cm−1 (associated C−O−C), like in the PCL-b-PEO/LiClO4 hybrids.54 As the salt concentration increases, the band of the associated C−O−C groups becomes stronger. The FT-IR spectra in the wavenumber range from 1075 to 1130 cm−1 were deconvoluted using software. The parameter Ia/If, which is the ratio of the peak intensity corresponding to the associated C−O−C to the peak intensity at 1109 cm−1 of the free C−O−C, is used to evaluate the interaction between the metal ion and the PEO block (Table 2). It is found that the values of Ia/If are quite

Figure 4. FT-IR spectra for salt-doped PCL56-b-PEO44 hybrids at different doping ratios: PCL56-b-PEO44/LiCl hybrids (a); PCL56-bPEO44/CuCl2 hybrids (b); PCL56-b-PEO44/FeCl3 hybrids (c).

Table 2. Data of Ia/If for the Hybrids of PCL56-b-PEO44 with Different Salts salt Ia/If

r r r r

= = = =

1/24 1/12 1/6 1/3

LiCl

CuCl2

FeCl3

0.14 0.31 0.47 2.17

0.27 0.37 2.55 2.66

0.31 2.24 2.10 2.95

small for all the PCL56-b-PEO44/salt hybrids at r = 1/24. As the doping ratio increases, there is an abrupt increase in Ia/If for LiCl at r = 1/3, CuCl2 at r = 1/6, and FeCl3 at r = 1/12, implying an obvious enhancement in the amount of associated C−O−C. These doping ratios for the abrupt increase in Ia/If are exactly the same as the r0s for the corresponding PCL56-bPEO44/salt hybrids. This shows that a critical amount of associated PEO blocks may be the precondition for formation of an ordered structure of microphase separation. The metal ions can also interact with the PCL block. However, the association between the metal ions and the PCL block cannot lead to an obvious shift of the vibration band of CO in the FT-IR spectra. As a result, the interaction between the salts and the PCL block is evaluated with DSC, since crystallization of the PCL block is affected by the salt. Usually, more associated PCL blocks will lead to a lower crystallization temperature and smaller crystallinity of the PCL block.61 The DSC nonisothermal crystallization traces of the neat PCL56-bPEO44 and PCL56-b-PEO44/salt hybrids with different salts and doping ratios are shown in Figure S2 of the Supporting Information. The calculated crystallization enthalpies (ΔHc) are presented in Figure 5. For the purpose of comparison, the data of ΔHc for the PEO block are given in Figure 5a as well. It can be seen from Figure 5a that ΔHc of the PEO block drastically decreases with increasing r, showing a remarkable decrease in crystallinity due to the association between the metal ion and 8362

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ignored. As can be seen from Figure 5a, the PEO blocks in the PCL56-b-PEO44/FeCl3 hybrid with r = 1/12 are totally amorphous, but parts of the PEO blocks in the PCL56-bPEO44/LiCl hybrid with r = 1/3 are still crystalline. Such a difference cannot be interpreted only from the viewpoint of association number. For a given type of ligand, the association strength of a metal ion is related to its electronegativity. The electronegativity of cationic metals increases in the order Li+ < Fe3+ < Cu2+,63,64 so the association strength between the PEO block and the Cu2+ is stronger than Li+ and Fe3+. Such a sequence of electronegativity can also explain the more ordered structure in the PCL56-b-PEO44/CuCl2 hybrids than in the PCL56-b-PEO44/FeCl3 hybrids at the same salt doping ratio. On the other hand, the metal ions can interact with the PCL block as well, especially at high doping ratios, at which the interaction between the metal ion and the PEO block may be saturated. At a given concentration of the PEO block, Fe3+ ions tend to reach association saturation at a lower doping ratio than Li+ ions due to the larger association number. Therefore, at r = 1/3, more Fe3+ ions associate with the PCL block (Figure 5b). The simultaneous associations of the PEO and PCL blocks with the metal ions may make them miscible again, leading to a less ordered structure or even disappearance of microphase separation. Moreover, we notice that the values of Ia/If are similar in the PCL56-b-PEO44/CuCl2 hybrids with r = 1/6 and 1/3 (Table 2), showing that the interaction between the Cu2+ ions and the PEO blocks is nearly saturated at r = 1/6, and lots of Cu2+ ions should associate with the PCL block at r = 1/3. However, DSC result shows that the interaction between the Cu2+ ions and the PCL blocks at r = 1/3 is still weak (Figure 5b), and strong segregation between the PEO and PCL blocks still exists in the melt of this hybrid (Figure 2b). Overall, the microphase separation behavior of the PCL-b-PEO/salt hybrids is dominated by the selectivity of the PEO/salt and PCL/salt interactions. When the salt has a weak interaction with both blocks (such as LiCl) or has a strong interaction with both blocks (such as FeCl3 at r = 1/3), microphase separation is difficult to occur in the hybrids. By contrast, if the salt exhibits a preferential interaction with one of the blocks but has only a weak interaction with the other block, microphase-separated structure can be easily formed. Effect of Anion on the Microphase Separation Behavior of Salt-Doped PCL49-b-PEO44. In the PCL-bPEO/salt hybrids, the anion will compete with the PEO and PCL blocks to associate with the metal cation, thus affecting the phase behavior of the hybrids. In order to investigate the effect of anion, three salts containing the same metal ion (Li+) but different anions (Cl−, CF3SO3−, and ClO4−) were used to hybridize with PCL49-b-PEO44. The structure at 70 °C and the TODT of salt-doped PCL49-b-PEO44 hybrids are summarized in Table 3. Figure 6 shows the SAXS profiles of LiCl-doped PCL49-b-PEO44. For the hybrids with r = 1/24, 1/12, and 1/6 at 70 °C, no scattering peaks are observed, and the SAXS profiles decay smoothly (Figure 6a), indicating that the hybrids are homogeneous. When the doping ratio is increased to r = 1/ 3 (Figure 6b), three scattering peaks appear at q*, 2q*, and 3q*, respectively, in the SAXS profile at 70 °C, showing a LAM structure. The TODT of this hybrid is 80 °C. Figure 7 shows the SAXS profiles of LiCF3SO3-doped PCL49b-PEO44. At the doping ratios of r = 1/24 and 1/12, only a broad first-order peak is observed (Figure 7a,b). This shows that LiCF3SO3 can induce microphase separation in these two hybrids, but no ordered structure is formed. When r is

Figure 5. Plots of crystallization enthalpies of PEO (a) and PCL (b) versus salt doping ratio (rsalt) for the PCL56-b-PEO44/salt hybrids.

the PEO block. The PEO becomes completely amorphous when r is beyond 1/24 for FeCl3 and 1/12 for CuCl2. This shows that, at the same doping ratio, the amount of associated PEO blocks increases in the following order: LiCl < CuCl2 < FeCl3, which is in accordance with the FT-IR result. As for the ΔHc of the PCL block (Figure 5b), it slightly decreases with increase of r under most conditions. This shows that metal ions in most of the hybrids have a weaker interaction with the PCL block in the studied range of r, as compared with the PEO block. This agrees with the conclusion drawn in our previous work that Li+ ions preferentially interact with the PEO block.54 However, we also notice that the ΔHc in the PCL56-b-PEO44/ FeCl3 hybrid with r = 1/3 is remarkably low, showing that at this doping ratio FeCl3 has a strong interaction with the PCL block. The simultaneous strong interactions of FeCl3 toward PCL and PEO at r = 1/3 may be responsible for the disappearance of the microphase-separated structure. The competitive interactions of the metal ions with PCL and PEO may be correlated with the microphase separation behavior of the PCL56-b-PEO44/salt hybrids. At low doping ratios, the metal ions preferentially interact with the PEO block. Such an interaction can alienate the PEO block from the PCL block. However, only when the associated PEO blocks reach a critical level, the PCL and PEO blocks, which are originally miscible in the melt, become segregated and ordered structure of microphase separation is formed. The amount of the associated PEO blocks is mainly determined by following three factors: doping ratio, association number of the metal ion, and association strength between the PEO block and the metal ion. The amount of the associated PEO blocks increases with increasing the doping ratio, as shown in Figure 4. The association number refers to the number of ligand (such as the C−O−C group in PEO) that can associate with a single metal ion. At the same doping ratio, more associated PEO blocks should be formed for the metal ion with a larger association number (Table 2). This means that to reach a certain amount of associated PEO blocks, a smaller doping ratio is needed for the metal ion with a larger association number. When the valence state and the volume of central ion are larger, the association number will be bigger, so the association numbers of the metal ions Fe3+, Cu2+, and Li+ increase in the following order: Li+ < Cu2+ < Fe3+. As a result, the r0 is the lowest in the PCL56-b-PEO44/FeCl3 hybrids, whereas it is the largest in the PCL56-b-PEO44/LiCl hybrids. Ho et al. also observed that a higher valence state of the metal ion led to a stronger segregation between the P4VP and PCL in the P4VP-b-PCL/ salt hybrids.38 Moreover, the effect of the association strength between the PEO block and the metal ion should not be 8363

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Table 3. Structure, Domain Spacing, and χeff at 70 °C and TODT of Salt-Doped PCL49-b-PEO44 Hybrids TODT (°C)

samples PCL49-b-PEO44 LiCl/BCP

LiCF3SO3/BCP

LiClO4/BCP

r r r r r r r r r r r r r

= = = = = = = = = = = = =

0 1/24 1/12 1/6 1/3 1/24 1/12 1/6 1/3 1/24 1/12 1/6 1/3

80

80

90 130 110

structure DIS DIS DIS DIS LAM DIS DIS LAM LAM DIS HEX LAM LAM

d70 °C (nm)

χeff,70 °C

25.8

0.147

14.1

0.236

13.5 15.7 14.2

0.263 0.551 0.233

Figure 7. SAXS profiles of PCL49-b-PEO44/LiCF3SO3 hybrids with r = 1/24 (a), r = 1/12 (b), r = 1/6 (c), and r = 1/3 (d). Figure 6. SAXS profiles of PCL49-b-PEO44/LiCl hybrids with r = 1/24, 1/12, and 1/6 at 70 °C (a) and r = 1/3 at different temperatures (b).

LiClO4 hybrid with r = 1/6 is about 130 °C. A LAM structure is formed in the PCL49-b-PEO44/LiClO4 hybrid with r = 1/3 as well. However, the second-order peak is evidently weaker than that in the hybrid with r = 1/6. The TODT of the PCL49-bPEO44/LiClO4 hybrid with r = 1/3 also decreases to 110 °C. Comparing the microphase separation behaviors of the hybrids of PCL49-b-PEO44 with salts containing different anions, one can see that the r0s are 1/3, 1/6, and 1/12 for LiCl, LiCF3SO3, and LiClO4, respectively. The calculated values of χeff are shown in Table 3. It can be seen that, at the same salt doping ratio, the PCL49-b-PEO44/ LiClO4 hybrids exhibit a higher χeff than the PCL49-b-PEO44/ LiCl and PCL49-b-PEO44/LiCF3SO3 hybrids, confirming the strongest microphase separation in the PCL49-b-PEO44/LiClO4 hybrids. It is also noticed that the χeff of the PCL49-b-PEO44/ LiClO4 hybrid with r = 1/3 becomes smaller, as compared with that of the hybrid with r = 1/6. The phenomenon that χeff does not increase linearly with r was also observed in our previous work, which can be attributed to the competitive interaction of Li+ and PCL at a higher salt doping ratio.54 The SAXS results show that, among these three types of inorganic salts, LiClO4 has the strongest ability to induce microphase separation in PCL49-b-PEO44, whereas LiCl exhibits the weakest ability. On the other hand, at high doping ratios, such as r = 1/3, the tendency of microphase separation becomes weaker.

increased to 1/6, two distinct scattering peaks are located at q* and 2q*, respectively, indicating a LAM structure (Figure 7c). The microphase-separated morphology of this hybrid becomes disordered at 80 °C. For the PCL49-b-PEO44/LiCF3SO3 hybrid with r = 1/3, a LAM structure is also formed, as indicated by the positions of the two scattering peaks (Figure 7d). However, both the first- and the second-order peaks are quite broad, showing the diffused boundary between the microphaseseparated domains. The SAXS profiles of the PCL49-b-PEO44/LiClO4 hybrids at various doping ratios are illustrated in Figure 8. One can see that the hybrid with r = 1/24 exhibits only a weak scattering peak at 70 °C and no higher order peak (Figure 8a), indicating a microphase-separated but disordered structure. When the doping ratio increased to 1/12, a HEX structure is formed, since the hybrid exhibits three distinct scattering peaks at q*, 31/2q*, and 71/2q* at 70 °C, respectively (Figure 8b). At r = 1/ 6, three scattering peaks are observed at q*, 2q*, and 4q*, respectively (Figure 8c), indicating a LAM structure. The structural transformation from HEX at r = 1/12 to LAM at r = 1/6 is due to both the stronger interaction between PEO and salt and the increase of the volume fraction of the PEO/salt phase at a higher r, assuming that the salt is preferentially located in the PEO phase. The TODT of the PCL49-b-PEO44/ 8364

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Figure 9. FT-IR spectra for salt-doped PCL49-b-PEO44 hybrids at different doping ratios: PCL49-b-PEO44/LiCl hybrids (a); PCL49-bPEO44/LiCF3SO3 hybrids (b); PCL49-b-PEO44/LiClO4 hybrids (c).

Table 4. Effect of Anion on Ia/If of the PCL49-b-PEO44/Salt Hybrids salt Ia/If

Figure 8. SAXS profiles of PCL49-b-PEO44/LiClO4 hybrids with r = 1/ 24 (a), r = 1/12 (b), r = 1/6 (c), and r = 1/3 (d).

r r r r

= = = =

1/24 1/12 1/6 1/3

LiCl

LiCF3SO3

LiClO4

0.20 0.55 0.75 2.81

1.02 0.91 2.87 3.33

0.84 7.53 12.52 13.93

increase of Ia/If at r = 1/3, 1/6, and 1/12 for LiCl, LiCF3SO3, and LiClO4, respectively, which are the same as the r0s for these three types of hybrid. This shows that the anion greatly affects the association strength between the Li+ ion and the PEO block, since the metal ion and the association number of the metal ion in these three salts are the same. The FT-IR spectra reveal that, among these three salts, LiClO4 exhibits the strongest association strength with the PEO block, while the association strength between LiCl and PEO is the weakest. This result can be interpreted in terms of the Lewis basicity of different anions, which decreases in the following sequence: Cl− > CF3SO3− > ClO4−. This means that ClO4− has the weakest association with Li+, leading to the strongest association of the Li+ ion in LiClO4 with PEO. Our finding agrees with Epp’s result that weaker Lewis basicity of the anion could lead to a larger segregation strength between PS and PEO in the PS-bPEO/salt hybrids.52 In addition, we notice that the value of Ia/If for the PCL49-bPEO44/LiCl hybrid with r = 1/3 (Ia/If = 2.81, Table 4) is larger than that for PCL56-b-PEO44/LiCl hybrid at the same r (Ia/If = 2.17, Table 2). This shows that the interaction between LiCl and PEO in the PCL49-b-PEO44/LiCl hybrid with r = 1/3 is stronger than that in the PCL56-b-PEO44/LiCl hybrid with r = 1/3. The effect of anion on the interaction of Li+ ion with PCL was characterized with DSC. The DSC nonisothermal

Moreover, one can see from Tables 1 and 3 that the χeff for the PCL56-b-PEO44/LiCl hybrid with r = 1/3 (χeff,70 °C = 0.096) is a little smaller than that of the PCL49-b-PEO44 hybrid with the same doping ratio (χeff,70 °C = 0.147). Generally, the Flory− Huggins parameter χ is only dependent on the polymer− solvent or polymer−polymer pair but is independent of the chain length of polymer. The different values of χeff in these two hybrids show that the competitive interactions of PCL/salt and PEO/salt vary with the block length. This is the reason why “effective” χ is usually adopted in the BCP/salt systems. The smaller χeff in the PCL56-b-PEO44/LiCl hybrid with r = 1/3 can be attributed to the stronger interaction between PCL and LiCl due to the longer PCL block, which weakens the interaction between PEO and LiCl and thus better affinity between PCL and PEO is achieved. This will be further discussed in the next section. Effect of Anion on the Competitive Interactions of the Metal Ion with PEO and PCL. In order to understand how the anion in the salt takes effect, the interactions of various salts with the PEO and PCL blocks were investigated. Figure 9 shows the FT-IR spectra for the hybrids of PCL49-b-PEO44 with different salts. As can be seen from Figure 9, the band intensity corresponding to the associated C−O−C varies with the anion in the salts. The data of Ia/If are summarized in Table 4. It is found that, as the doping ratio increases, there is an abrupt 8365

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doped PCL-b-PEO hybrids as well as the competitive interactions of PEO/salt and PCL/salt. A higher valence state and stronger Lew acidity of the metal ion and weaker Lewis basicity of the anion will lead to more associated PEO blocks and thus a lower r0. At higher doping ratios, the interaction of PEO/salt may be saturated, and parts of the salts can also interact with the PCL block, which results in difficulty in formation of an ordered structure or lowers the TODT. CuCl2 and LiClO4 exhibit a better selectivity to interaction with PEO at higher doping ratios; therefore, the hybrids form an ordered structure of microphase separation with a higher TODT.

crystallization curves for the PCL49-b-PEO44/salt hybrids with different salts are presented in Figure S3 of the Supporting Information, and the obtained crystallization enthalpies (ΔHc) of PCL are plotted versus the doping ratio (Figure 10). It is

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ASSOCIATED CONTENT

S Supporting Information *

Volume factions of the components in the hybrids, SAXS profiles of PCL56-b-PEO44/CuCl2 hybrids with r = 1/24 and 1/ 12 at 70 °C, DSC curves of the hybrids, and crystallization enthalpy of PEO the PCL49-b-PEO44/salt hybrids. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 10. Plots of crystallization enthalpy of PCL versus the salt doping ratio (rsalt) for the PCL49-b-PEO44/salt hybrids.

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observed that addition of salt reduces the ΔHc, i.e. crystallinity, of the PCL block, irrespective of the salt type. At the same doping ratio, the crystallinity of PCL in the PCL49-b-PEO44/ LiCl hybrids is evidently higher than that in the PCL49-bPEO44/LiCF3SO3 and PCL49-b-PEO44/LiClO4 hybrids, indicating a weaker interaction between LiCl and PCL. This is in accordance with the result in the previous section. At r = 1/24 and 1/12, the PCL49-b-PEO44/LiCF3SO3 and PCL49-b-PEO44/ LiClO4 hybrids have similar crystallinity of the PCL block. However, at r = 1/6 and 1/3, the crystallinity of PCL in the PCL49-b-PEO44/LiCF3SO3 hybrids is lower than that in the PCL49-b-PEO44/LiClO4 hybrids. This shows that the interaction between LiCF3SO3 and PCL is stronger than that between LiClO4 and PCL at r = 1/6 and 1/3. On the other hand, as revealed by FT-IR, LiClO4 has a stronger interaction with the PEO blocks than LiCF3SO3. Considering that the PEO/salt interaction is stronger than the PCL/salt interaction, we can conclude that the interactions of LiCF3SO3 toward PEO and PCL are less selective. By contrast, LiClO4 interacts more selectively toward PEO, as compared with LiCF3SO3. This can explain why the PCL49-b-PEO44/LiCF3SO3 hybrids have a more diffused boundary between the microdomains and exhibit a lower TODT than the PCL49-b-PEO44/LiClO4 hybrids with the same doping ratio. On the other hand, the value of ΔHc for the PCL block in the PCL49-b-PEO44/LiCl hybrid with r = 1/3 is 70.0 J/g (Figure 10), while it is 65.6 J/g in the PCL56-b-PEO44/LiCl hybrid with r = 1/3 (Figure 5b). This shows that the PCL/LiCl interaction in the PCL56-b-PEO44/LiCl hybrid with r = 1/3 is stronger than that in the PCL49-b-PEO44/LiCl hybrid with the same doping ratio. Combining with the weaker PEO/LiCl interaction in the PCL56-b-PEO44/LiCl hybrid with r = 1/3 (as revealed by FT-IR, Tables 2 and 4), we confirm that the competitive interactions of PCL/salt and PEO/salt vary with the block length. Because of the stronger PCL/LiCl interaction but weaker PEO/LiCl interaction, the interactions of LiCl toward PCL and PEO are less selective in the PCL56-b-PEO44/ LiCl hybrid with r = 1/3, leading to a smaller χeff.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.-T.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by National Natural Science Foundation of China (21274130). The authors thank beamline BL16B1 at SSRF and beamline 1W2A at BSRF for providing the beam time.

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