Microphase Separation and High Ionic Conductivity at High

Nov 24, 2015 - ... })}(P{(St-g-PEO)-alt-(MI-g-PMPCS)}), was synthesized by alternating copolymerization ... Michael J. Maher , Haley J. Schibur , Fran...
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Microphase Separation and High Ionic Conductivity at High Temperatures of Lithium Salt-Doped Amphiphilic Alternating Copolymer Brush with Rigid Side Chains Jing Ping, Yu Pan, Hongbing Pan, Bin Wu, Henghui Zhou, Zhihao Shen,* and Xing-He Fan* Beijing National Laboratory for Molecular Sciences, Department of Polymer Science and Engineering, and Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center for Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: An amphiphilic alternating copolymer brush (AACPB), poly{(styrene-g-poly(ethylene oxide))-alt-(maleimide-g-poly{2,5-bis[(4methoxyphenyl)oxycarbonyl]styrene})}(P{(St-g-PEO)-alt-(MI-gPMPCS)}), was synthesized by alternating copolymerization of styreneterminated poly(ethylene oxide) (St-PEO) and maleimide-terminated poly{2,5-bis[(4-methoxyphenyl)-oxycarbonyl]styrene} (MI-PMPCS) macromonomers using the “grafting through” strategy. 1H NMR and gel permeation chromatography coupled with multiangle laser light scattering were used to determine the molecular characteristics of AACPBs. Although these AACPBs cannot microphase separate with thermal and solvent annealing methods, they can form lamellar structures by doping a lithium salt. This is a first report on lithium salt-induced microphase separation of AACPBs, and the lithium salt-doped AACPBs can serve as solid electrolytes for the transport of lithium ion. For the same AACPB, the ionic conductivity (σ) increases with increasing doping ratio. In addition, σ values of different AACPBs with the same doping ratio become higher for shorter PMPCS side chains. The σ value of the lithium salt-doped AACPB increases with increasing temperature in the range of 25−240 °C, and σ is 1.79 × 10−4 S/cm at 240 °C. The relatively high σ values of the lithium-doped AACPBs at high temperatures benefit from the rigid PMPCS side chain and the AACPB architecture. The lithium salt-doped AACPBs have the potential to serve as solid electrolytes in high-temperature lithium ion batteries.

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INTRODUCTION The various ordered nanostructures provided by the microphase separation of block copolymers (BCPs) have attracted great attention for their potential applications in nanoporous materials,1−3 nanophotonics,4−6 and solid electrolytes.7−9 BCPs can self-assemble into lamellae (LAM), hexagonally packed cylinders (HEX), bicontinuous gyroids, and body-centered cubic (BCC) morphology, determined by the volume fraction f of the individual block and the product of the Flory−Huggins interaction parameter χ and the total degree of polymerization (DP) of the BCP N.10−13 When different side chains are successively attached to a linear main chain, the obtained polymer brushes will act as block copolymers, namely brush block copolymers (BBCPs). Different from the microphase separation behavior of BCPs, most BBCPs studied selfassemble into LAM and cylinders morphologies because of the multidimensional architecture of BBCPs.14−17 When different side chains are randomly distributed on a linear main chain to form brush random copolymers (BRCPs), the resultant polymers are difficult to self-assemble into ordered nanostructures in bulk because they need to overcome the large entropic loss to orderly arrange the side chains. So far there are only two reports on the microphase separation of BRCPs.18,19 © XXXX American Chemical Society

Different from the above two copolymer brushes, alternating copolymer brushes (ACPBs) have alternating macromonomer units,20 which is beneficial to controlling the content of each component and the sizes of the resulting nanostructures. ACPBs with backbones constructed from styrene and maleimide have been widely investigated owing to their advantages in the easiness of preparation by alternating copolymerization21,22 and a relatively long repeat unit.23 Ishizu et al. first synthesized polystyrene/poly(methyl methacrylate) prototype brushes by alternating radical copolymerization.21 Hillmyer et al. designed well-defined m-A(BC)n “miktobrush” terpolymers using alternating copolymerization.24 Zhao et al. prepared heterografted toothbrush-like copolymers by reversible addition−fragmentation chain transfer processes.25 However, up to now, there are no reports on the selfassembling behavior of ACPBs or the synthesis of ACPBs containing rigid side chains, which may be due to the low χ between the adjacent, different side chains, and the difficulty in synthesis. Received: July 28, 2015 Revised: November 19, 2015

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DOI: 10.1021/acs.macromol.5b01678 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of the Macromonomers and the Amphiphilic Alternating Copolymer Brushes

at 240 °C. To the best of our knowledge, this is the first report on lithium salt-induced microphase separation of AACPBs and relatively high ionic conductivity of lithium salt-doped AACPBs at high temperatures.

In order to synthesize ACPBs with rigid side chains, we used the special liquid crystalline (LC) polymer, mesogen-jacketed LC polymer (MJLCP), which was first reported by Zhou et al.26 With the side-on, bulky side chains attached to the polymer backbones with a short spacer or via a single covalent bond, MJLCPs are regarded as rigid polymers and show LC phases in a broad temperature range.27,28 MJLCPs can be prepared by different living free radical polymerization methods, and their chain lengths can be easily controlled by changing the monomer:initiator ratio and the reaction time.29,30 Poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS) is a typical and frequently studied MJLCP, and its synthetic method is well established.31−33 Addition of lithium salt can alter the χ value between different blocks in BCPs containing poly(ethylene oxide) (PEO) and then affect the microphase-separated structures.34−36 As reported by Epps et al., upon LiCF3SO3 doping in poly(styrene-block-ethylene oxide) (PS-b-PEO), χ between the two blocks increased with increasing concentration of the salt added, owing to the coordination of lithium ion with the oxygen atom in PEO.37 When the BCP/salt complexes undergo microphase separation and form a continuous PEO phase, it can be used as a good pathway for the transport of lithium ion, and the material can be applied as a polymer electrolyte for solid-state lithium batteries.7,38−40 PEO-based polymer electrolytes have advantages such as low glass transition temperature (Tg), low crystallinity, and a relatively high ionic conductivity (σ).41 ACPB/salt complexes with rigid side chains may be used as solid electrolytes at high temperatures because of their high Tg’s and excellent thermal stabilities. In this work, we synthesized amphiphilic alternating copolymer brushes (AACPBs) containing the rigid PMPCS and the flexible PEO side chains by the “grafting through” strategy. The pure AACPBs do not form ordered microphaseseparated structures, but they can form lamellar structures after doping of lithium salt, as proven by the results from transmission electron microscopy (TEM) and small-angle Xray scattering (SAXS) experiments. The contents of PEO and the doped lithium salt strongly influence the ionic conductivity of the resultant lithium-doped AACPBs. The ionic conductivity of lithium salt-doped PMPCS9-alt-PEO22 is 1.79 × 10−4 S/cm

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EXPERIMENTAL SECTION

Materials. 4-Chloromethylstyrene was passed through a neutral alumina column and then purified by a silica gel column (100% petroleum ether). 2,2′-Azobis(isobutyronitrile) (AIBN) was recrystallized from ethanol. Tetrahydrofuran (THF) was purified by using the Solvent Processing System (M. Braun, Inc.). PEO (with a numberaveraged molecular weight, Mn, of 1000 g/mol, ρ of 1.09 g/cm3, Beijing HWRK Chem. Co.) was used as received. The initiator 2bromo-2-methylpropionic acid 2-(3,5-dioxo-10-oxa-4-azatricyclo[5.2.1.02,6]dec-8-en-4-yl)ethyl ester was synthesized according to the previous report,42 and the monomer 2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene (MPCS) was prepared according to the literature.31 All other reagents were used as received from commercial sources. Methods. All NMR spectra were recorded on a Bruker 400 MHz spectrometer. Gel permeation chromatographic (GPC) examination in THF was performed on a Waters 2410 instrument equipped with a Waters 2410 refractive index (RI) detector. GPC coupled with multiangle laser light scattering (GPC-MALLS) measurements were carried out on the GPCmax VE-2001 (Viscotek) equipped with a Viscotek TriSEC Model 302 triple detector array (refractive index detector, viscometer detector, and laser light scattering detector (7° and 90°)) with two I-3078 polar organic columns. Thermogravimetric analysis (TGA) examinations were conducted on a TA SDT 2960 instrument at a heating rate of 10 °C/min in a nitrogen atmosphere. Differential scanning calorimetry (DSC) was recorded on a TA DSC Q100 instrument on the first cooling at a rate of 5 °C/min and a subsequent heating process at a rate of 20 °C/min under nitrogen. One-dimensional (1D) wide-angle X-ray diffraction (WAXD) experiments were performed on a Philips X’Pert Pro diffractometer with a 3 kW ceramic tube as the X-ray source (Cu Kα) and an X’celerator detector. Silicon powder and silver behenate were used to calibrate the reflection peak positions. SAXS experiments were conducted on a Bruker Nanostar SAXS instrument using Cu Kα radiation (λ = 0.154 nm at 40 kV and 40 mA). The scattering vector q is defined as q = 4π/ λ sin θ, where the scattering angle is 2θ and the d-spacing (d) is given by 2π/q. TEM bright-field images were obtained with a JEM-100cx instrument using an accelerating voltage of 100 kV. The ionic conductivity σ of the polymer electrolyte was measured on a button cell. The polymer electrolyte was sandwiched between two stainless B

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Figure 1. GPC traces of different macromonomers (a), PMPCS9-alt-PEO22 and its macromonomers (b), and other amphiphilic alternating copolymer brushes and their macromonomers (c).

Table 1. Polymerization Conditions and Properties of MI-PMPCS entry

[M]0/[I]0

time (h)

Mna (103 g/mol)

PDIa

Mnb (103 g/mol)

DPc

Tdd (°C)

Tge (°C)

MI-PMPCS 1 MI-PMPCS 2 MI-PMPCS 3

10 30 40

0.5 1.5 4

2.5 5.8 7.7

1.06 1.07 1.08

3.8 8.8 11.7

9 21 28

360 378 378

110 121 126

Determined by GPC in THF (calibrated with polystyrene standards). bAbsolute MW calculated by Mna × 1.52 according to our previous report.44 Same as the ratio of the absolute MW of PMPCS to the MW of MPCS. d5% weight loss temperature (Td) was determined by TGA at a heating rate of 10 °C/min under nitrogen. eGlass transition temperature was obtained from the DSC thermogram on the second heating process at a rate of 20 °C/min under nitrogen. a c

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steel disk electrodes. Upon extruding the button cell, the sample formed a circular film with a diameter of 7 mm. The σ values were obtained by electrical impedance spectroscopy measurements at different temperatures, and the samples were thermally equilibrated at each temperature for 10 min before the measurement. Synthetic Procedures. Synthesis of the St-PEO and MI-PMPCS Macromonomers. The whole synthetic procedure is shown in Scheme 1. St-PEO was synthesized according to the literature.43 The detailed characterization data are illustrated in Figures S1 and S2 of the Supporting Information. The MI-PMPCS macromonomers were prepared with the protection strategy according to our previous report.42 A series of MI-PMPCS samples with different molecular weights (MWs) were obtained by changing the feeding ratio and the polymerization time. Synthesis of Amphiphilic Alternating Copolymer Brushes. The amphiphilic alternating copolymer brushes were synthesized by alternating radical copolymerization of St-PEO and MI-PMPCS with the “grafting through” strategy. When the maleimide-terminated monomer is in excess, the vinylbenzyl-terminated and maleimideterminated monomers will be copolymerized in an alternating manner. In the polymerization process, the feeding ratio of MI-PMPCS to StPEO was set as 2:1 to ensure that MI-PMPCS was in excess. As an example, MI-PMPCS (0.300 g, 200 equiv), St-PEO (100 equiv), AIBN (1 equiv), and THF (40 wt %) were charged into a glass tube, and the tube was sealed under vacuum after being degassed with three freeze− pump−thaw cycles. After polymerization at 60 °C for 72 h, the tube was immersed into liquid nitrogen for a while. The solution was diluted with THF and added to 200 mL of methanol to obtain the raw product. The raw polymer brush was reprecipitated with a THF/nhexane system to remove the unreacted St-PEO and MI-PMPCS, and then the resulting polymer was dried under vacuum at 35 °C for 24 h. LiCF3SO3 Doping. The polymer brush (20.0 mg) was dissolved in 1.5 mL of THF, and an appropriate amount of LiCF3SO3 was added to the above solution. The mixture solution was stirred for 24 h and then passed through a filter membrane with a diameter of 0.22 μm. After the solvent was removed by slow evaporation at ambient temperature, the mixture solid was annealed in vacuum at 150 °C for 24 h. The molar ratio of LiCF3SO3 to PMPCS-alt-PEO was denoted as the doping ratio r = [Li+]/[EO].

RESULTS AND DISCUSSION Synthesis of the St-PEO and MI-PMPCS Macromonomers. The GPC traces of St-PEO and MI-PMPCS with different MWs are shown in Figure 1a, which indicates that the macromonomers are all monodispersed. Because the apparent Mn values measured by GPC using polystyrene standards are not accurate, NMR was used to calculate the MWs of the polymers. In the 1H NMR spectrum of St-PEO (Figure S1 in the Supporting Information), the characteristic resonances of Ar−CH= appear at 6.65−6.80 ppm, and those at 3.45−3.85 ppm are attributed to the protons in PEO chains. By comparing the areas of these two peaks, the DP and Mn of StPEO were calculated to be 22 and 1116 g/mol, respectively. From our previous study, the ratio of the absolute MW of PMPCS to the Mn value from GPC measurement is about 1.52,44 and then the DP and Mn of MI-PMPCS can be calculated by using this ratio. Table 1 summarizes the polymerization conditions and properties of MI-PMPCS. Synthesis of Amphiphilic Alternating Copolymer Brushes. Because the side-chain length affects the properties of the polymer brushes, three amphiphilic copolymer brushes were prepared by alternating copolymerization of the same StPEO macromonomer and three MI-PMPCS macromonomers of different DPs. After reprecipitation, the polymer brush PMPCS9-alt-PEO22 without any residual macromonomers was successfully synthesized, indicated by the GPC trace (Figure 1b). In the 1H NMR spectrum of PMPCS9-alt-PEO22 (Figure 2), all characteristic resonances peaks are assigned to the different hydrogens in St-PEO and MI-PMPCS. In order to confirm the alternating copolymerization and the 1:1 ratio of St-PEO and MI-PMPCS in the AACPBs, we assumed the molar fraction of St-PEO as x, set up an equation according to the areas of the two characteristic resonance peaks at 6.05−7.95 and 3.05−3.95 ppm, and then solved the equation to obtain that x = 0.5. Because MI-PMPCS cannot be homopolymerized, C

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Figure 3. DSC second-heating thermograms of St-PEO, MI-PMPCS 1, PMPCS9-alt-PEO22, and PMPCS9-alt-PEO22/LiCF3SO3 (r = 0.6) at a rate of 20 °C/min following a cooling process at 5 °C/min. Figure 2. 1H NMR spectra of St-PEO in CDCl3 (top), MI-PMPCS 1 in CDCl3 (middle), and PMPCS9-alt-PEO22 in CD2Cl2 (bottom).

120 °C, with the first corresponding to the glass transition of the doped PEO, the middle one corresponding to the mixed phase of PMPCS and PEO, and the third corresponding to the glass transition of PMPCS. The difference in the DSC results of the neat and lithium salt-doped PMPCS9-alt-PEO22 illustrates that the addition of the lithium salt induces the mixed phase of PMPCS and PEO in PMPCS9-alt-PEO22 to partially microphase separate. On the other hand, the heat capacities of PMPCS9-alt-PEO22/LiCF3SO3 (r = 0.6) and PMPCS9-altPEO22 at the Tg of PMPCS were measured by DSC and then calibrated on the basis of the heat capacity of sapphire. The heat capacity change of PMPCS9-alt-PEO22/LiCF3SO3 (r = 0.6) at 120 °C is 0.09 J/(g °C), which is higher than that of PMPCS9-alt-PEO22 at 107 °C (0.07 J/(g °C)), consistent with the increased amount of the PMPCS phase after the Li saltinduced microphase separation. LC Properties of Amphiphilic Alternating Copolymer Brushes. Variable-temperature 1D WAXD was used to investigate the phase structures of the AACPBs. In the 1D WAXD profiles of MI-PMPCS 3 (Figure 4a), only two broad halos in the low- and high-angle regions are observed during the heating process, which indicates that the sample remains amorphous. The 1D WAXD profiles of MI-PMPCS 1 (Figure S3a), MI-PMPCS 2 (Figure S3b), PMPCS9-alt-PEO22 (Figure S3c), and PMPCS21-alt-PEO22 (Figure S3d) are similar to those of MI-PMPCS 3. However, PMPCS28-alt-PEO22 (Figure 4b) is quite different. The original scattering halo in the low-angle region which represents the disordered packing of the PMPCS side chains becomes narrower during the heating process. A narrow diffraction peak appears at 200 °C, and the intensity of the diffraction peak increases with increasing temperature. The peak at 2θ = 5.55° has a d-spacing of 1.59 nm, which can be attributed to the ordered packing of PMPCS side chains. Because the 1D WAXD results of PMPCS28-alt-PEO22 are

we can deduce that the amphiphilic copolymer brushes are alternating on the basis of the above x value. Figure 1c shows the GPC traces of PMPCS21-alt-PEO22 and PMPCS28-altPEO22, in which the PMPCS and PEO side chains are alternately arranged according to the 1H NMR results. The absolute MWs of the amphiphilic copolymer brushes were measured by GPC-MALLS, and the detailed information is summarized in Table 2. As expected, the DP of the main chain increases when the DP of the side chain becomes smaller. The DP of the main chain of PMPCS9-alt-PEO22 is 106, which is a significantly high value for polymer brushes with rigid side chains. Thermal Properties. TGA was used to study the thermal stabilities of the AACPBs. The 5% weight loss temperatures (Td’s) of all samples are all above 300 °C, suggesting that they have excellent thermal stabilities. The Td’s of the neat PMPCS9alt-PEO22, PMPCS21-alt-PEO22, and PMPCS28-alt-PEO22 are 323, 339, and 353 °C, respectively, indicating that Td increases with longer PMPCS side chain, which may be owing to the lower content of the less stable PEO that has a Td of only 317 °C. In order to measure the ionic conductivity at high temperatures, we examined the thermal stabilities of PMPCS9alt-PEO22/LiCF3SO3 (r = 0.6), which has a Td value of 326 °C. The thermal transitions were investigated by DSC, and the results of St-PEO, MI-PMPCS 1, and the neat and lithium saltdoped PMPCS9-alt-PEO22 are shown in Figure 3 as an example. St-PEO exhibits a melting peak of PEO crystal at 44 °C, but the neat PMPCS9-alt-PEO22 only shows a glass transition of PMPCS (107 °C) and a glass transition of the mixed phase of PMPCS and PEO (63 °C), indicating that the sample is partially phase mixed. After the doping of 0.6 equiv of Li+, the complex displays three distinct glass transitions at −45, 58, and

Table 2. Polymerization Conditions and Molecular Characteristics of the AACPBs entry

Mn,MI‑PMPCSa (103 g/mol)

DPMI‑PMPCSa

Mn,polymer brushb (103 g/mol)

PDIb

DPmain chainc

Tdd (°C)

Tge (°C)

PMPCS9-alt-PEO22 PMPCS21-alt-PEO22 PMPCS28-alt-PEO22

3.8 8.8 11.7

9 21 28

519.7 220.1 230.6

1.07 1.02 1.40

106 22 18

323 339 353

110 100 111

a

Obtained from Table 1. bDetermined by GPC-MALLS (with THF as the eluent). cCalculated by the ratio of Mn,polymer brush to the summation of Mn,MI‑PMPCS and Mn,St‑PEO. dDetermined by TGA at a heating rate of 10 °C/min under nitrogen. eObtained from the DSC thermogram on the second heating process at a rate of 20 °C/min under nitrogen. D

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Lithium Salt-Induced Microphase Separation of AACPBs. The neat and lithium salt-doped AACPBs were characterized by SAXS and TEM to investigate their selfassembling behaviors. All samples were treated with solvent annealing using THF and thermal annealing for 24 h and then measured at ambient temperature. For the neat PMPCS9-altPEO22 (Figure 5a), there is no diffraction peak in the low-angle

Figure 4. 1D WAXD profiles of MI-PMPCS 3 (a) and PMPCS28-altPEO22 (b) and the schematic illustration (c) and the d-spacing (d) of the columnar nematic phase of PMPCS.

similar to those of the linear PMPCS which forms a columnar nematic (Coln) phase according to our earlier report,32 we can deduce that PMPCS28-alt-PEO22 also develops into a Coln phase, although the DP of PMPCS side chain is lower than 39 required for PMPCS homopolymer to be liquid crystalline. The packing model and the d-spacing of the Coln phase of PMPCS are schematically illustrated in Figures 4c and 4d, respectively. The liquid crystallinity of PMPCS in neat PMPCS28-alt-PEO22 indicates that some PMPCS side chains form pure PMPCS domains. Such an improved liquid crystallinity in AACPBs has also been observed in the monografting PMPCS alternating copolymer brush. The reasons are the combination of the confinement effect and the increased MW effect in the brush architecture, which was described in detail in our previous report.42 However, it is worth mentioning that the diffraction peak disappears in the 1D WAXD profile of PMPCS28-altPEO22 when the temperature is decreased, indicating that the Coln phase of PMPCS28-alt-PEO22 is less stable than that of the monografting PMPCS ACPB, which may be caused by the stronger confinement effect in AACPBs compared with that in ACPBs.

Figure 5. SAXS profiles of PMPCS9-alt-PEO22 at different doping ratios at ambient temperature (a: in the low-angle region; b: in the high-angle region) and TEM micrographs of PMPCS9-alt-PEO22/ LiCF3SO3 (c: r = 0.2; d: r = 0.6).

region in the SAXS profiles, indicating that the sample is disordered. After being doped with 0.2 equiv of LiCF3SO3, the SAXS profile of PMPCS9-alt-PEO22/LiCF3SO3 (r = 0.2) exhibits a single diffraction peak at q = 0.611 nm−1 in the low-angle region, which is associated with the size (10.3 nm) of the microphase-separated domain. TEM was performed to further determine the nanostructure. Figure 5c shows that PMPCS9-alt-PEO22/LiCF3SO3 (r = 0.2) displays a lamellar structure, with a periodic size of 8 nm, which is close to that obtained from the above-mentioned SAXS result. The different self-assembling behaviors of the neat and lithium salt-doped AACPBs are attributed to the increased value of χ between the two blocks after the doping of lithium salt. Although the E

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Macromolecules lamellar structure in the TEM micrograph is not well ordered, it is the first time to observe ordered nanostructures formed by ACPBs after the doping of salt. The lithium salt-induced microphase separation of AACPBs is schematically illustrated in Figure 6. When the doping ratio r increases from 0.4 to 0.6, the

Figure 6. Schematic illustration of the lithium salt-induced microphase separation of AACPBs.

SAXS and TEM results of the resulting complexes are similar to those of PMPCS9-alt-PEO22/LiCF3SO3 (r = 0.2), but the diffraction peak shifts to the smaller q value. The lamellar spacing is expected to increase due to the increase of lithium salt when the salt is included in the AACPB. In addition, when more lithium salt is added to the AACPB, more Li+ interacts with oxygen in PEO, which forces the PEO chain to be more extended and increases the lamellar spacing. When 0.8 equiv of LiCF3SO3 is added, there are many diffraction peaks in the high-angle region (q = 5−25 nm−1). These diffraction peaks originate from pure LiCF3SO3 crystal, indicating that the lithium salt is in excess and that macrophase separation occurs between the excessive salt and the lithium salt-doped AACPB. For PMPCS21-alt-PEO22 with a doping ratio of 0.2−0.8 and PMPCS28-alt-PEO22 with a doping ratio of 0.2−0.6, the selfassembling behaviors are similar to those of PMPCS9-altPEO22/LiCF3SO3, as shown in Figure S4, but the doping ratios when macrophase separation occurs are 1.0 and 0.8, respectively. Ionic Conductivity of Lithium Salt-Doped AACPBs. Because lithium salt-doped AACPBs form lamellar structures, the lithium ion-complexed PEO domain will act as good continuous pathways for the transport of lithium ion. The ion conductivities of lithium salt-doped AACPBs were obtained by electrical impedance spectroscopy measurements. When the lithium salt is in excess and self-aggregates in the complexes, there is no point in measuring the ionic conductivity. For PMPCS9-alt-PEO22/LiCF3SO3 (r = 0.2−0.6) (Figure 7a), σ increases with increasing r. Two factors contribute to such an increase in σ. The σ value of a sample is the product of the intrinsic conductivity of the conducting phase σc(T), volume fraction of the conducting phase ϕc, and the ideal morphology factor f ideal, σ(T) = f idealϕcσc(T).45 First, when r increases, more Li+ interacts with oxygen in PEO, which increases the lamellar spacing and ϕc. In addition, more Li+ is involved in the lithium ion transport process, which leads to a larger σc(T). The resistance value of PMPCS21-alt-PEO22/LiCF3SO3 (r = 0.2− 0.4) are too large to be measured, which means that their ionic conductivities are approximately zero. From the ionic conductivities of PMPCS21-alt-PEO22/LiCF3SO3 (r = 0.4− 0.8) (Figure 7a), σ also becomes higher when r increases. For PMPCS28-alt-PEO22, when r is 0.2 and 0.4, the σ value of the complex is also too low for the sample to serve as a solid electrolyte. In addition, the ionic conductivity of PMPCSn-alt-

Figure 7. σ values of different lithium salt-doped AACPBs at ambient temperature (a), those of PMPCS9-alt-PEO22/LiCF3SO3 (r = 0.6) at different temperatures (b), and the plot of the logarithmic scale of σ vs 1/(T − T0) (c).

PEO22 with 0.6 equiv of Li+ (Figure 7a) increases when the DP of the PMPCS side chain decreases, which may be attributed to the higher content of PEO in the system. In order to find the suitable solid electrolytes, we chose PMPCS9-alt-PEO22/LiCF3SO3 (r = 0.6) to study the influence of temperature on the ionic conductivity because this sample has the highest content of PEO and σ value. As shown in Figure 7b, σ increases with increasing temperature between 25 and 240 °C, which is consistent with the general trend of higher ionic mobilities at higher temperatures. The values of σ at 40 and 240 °C are 7.32 × 10−6 and 1.79 × 10−4 S/cm, respectively. The relationship of σ and temperature can be further studied by the Vogel−Tamman−Fulcher (VTF) equation which is described as follows:46,47 σ(T ) = A exp[−B /k b(T − T0)]

where A, B, and kb are the pre-exponential factor, apparent activation energy of the ion transport, and Boltzmann constant, F

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

respectively. T0 is the ideal glass transition temperature, which is 50 K lower than Tg of PEO. The T0 value in this system was calculated to be 227 K according to the Tg of PEO. The plot of a logarithmic scale of σ vs 1/(T − T0) is shown in Figure 7c. When the temperature is below the Tg of PMPCS, all σ and 1/ (T − T0) values fit the VTF equation. However, when the temperature is higher than the Tg of PMPCS, the value of σ increases faster than the inherent trend. This phenomenon can be attributed to the higher polymer chain mobilities and more ordered packing of PMPCS side chains at higher temperatures. To the best of our knowledge, so far the highest test temperature of most PEO-based polymer electrolytes is below 150 °C.7,48,9 Therefore, the sample in this work is expected to serve as a solid electrolyte at high temperatures.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from National Natural Science Foundation of China (Grants 21134001 and 21374002) is gratefully acknowledged. The authors also thank Prof. Decheng Wu and Xing Wang at Institute of Chemistry, Chinese Academy of Sciences, for the assistance with the GPCMALLS measurements and beamline 1W2A (Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences) for the assistance with the SAXS and WAXD experiments.

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CONCLUSIONS We synthesized AACPBs containing rigid PMPCS and flexible PEO side chains using the “grafting through” approach. The DP of the main chain increases with decreasing DP of the PMPCS side chain, and the highest DP of the main chain is 106, which is relatively large for polymer brushes with rigid side chains. Only PMPCS28-alt-PEO22 develops into a Coln phase, but the Coln phase is less stable than that of the monografting PMPCS ACPB owing to the stronger confinement effect. Although all the neat AACPBs are disordered, the complexes self-assemble into lamellar structures after doping of lithium salt because of the increased value of χ between the two blocks. The lamellar spacing increases when more lithium salt is added to the same AACPB. For PMPCS9-alt-PEO22/LiCF3SO3 (r = 0.2−0.6), σ increases with increasing r because more Li+ is involved in the transport process. For the same doping ratio, when the DP of PMPCS side chain decreases, σ of lithium salt-doped PMPCSnalt-PEO22 increases owing to the higher content of PEO in the AACPB. The increase in σ of PMPCS9-alt-PEO22/LiCF3SO3 (r = 0.6) with increasing temperature between 25 and 240 °C is attributed to higher ionic mobilities at higher temperatures, and the σ value at 240 °C is 1.79 × 10−4 S/cm. When the temperature is higher than the Tg of PMPCS, the increase in σ is more rapid compared with the inherent trend due to the higher polymer chain mobilities and more ordered arrangement of PMPCS side chains at higher temperatures. This is the first report of lithium salt-induced microphase separation of AACPBs and relatively high σ values of lithium salt-doped AACPBs at high temperatures. The complexes obtained in this work are potentially useful as solid electrolytes in hightemperature lithium ion batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01678. 1 H NMR spectrum and GPC trace of St-PEO, 1D WAXD profiles of MI-PMPCS, PMPCS9-alt-PEO22, and PMPCS21-alt-PEO22, SAXS profiles and TEM micrographs of PMPCS21-alt-PEO22/LiCF3SO3, and SAXS profiles and TEM micrographs of PMPCS28-alt-PEO22 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.-H.F.). G

DOI: 10.1021/acs.macromol.5b01678 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b01678 Macromolecules XXXX, XXX, XXX−XXX