Hierarchically Self-Assembled Amphiphilic Alternating Copolymer

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Hierarchically Self-Assembled Amphiphilic Alternating Copolymer Brush Containing Side-Chain Cholesteryl Units Jing Ping, Kehua Gu, Sheng Zhou, Hongbing Pan, 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: We synthesized a novel amphiphilic alternating copolymer brush (AACPB), poly{maleimide-g-poly[10-(cholesteryloxycarbonyl)decyl methacrylate]}-alt-(styrene-g-poly(ethylene oxide)) (P(MI-g-PCholMA)-alt-(St-g-PEO)), by copolymerization of maleimide-terminated poly[10-(cholesteryloxycarbonyl)decyl methacrylate] (MI-PCholMA) and styrene-terminated poly(ethylene oxide) (St-PEO). The thermal properties of the polymer brushes were investigated by thermogravimetric analysis and differential scanning calorimetry. After solvent and thermal annealing, the AACPB self-assembles into a hierarchically ordered nanostructure. One is the microphase-separated lamellar nanostructure with a 9.66 nm scale. The other is the cholesteryl double-layer smectic A phase (SmAd) with a 5.46 nm scale. The order−disorder transition of the AACPB is associated with the SmAd−isotropic transition. It is the first report on the microphase separation of AACPBs. We can construct ordered nanostructures with a sub-10 nm length scale with AACPBs. After the doping of 0.2 equiv of LiCF3SO3, the d-spacing of the lamellar structure formed by the PCholMA8-alt-PEO25/LiCF3SO3 complex increases because the interaction between Li+ and oxygen atom makes the PEO chains more stretched. Such a structure offers lithium salt-doped PEO nanochannels which can act as pathways for the transport of lithium ion.

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However, because the χ value between the adjacent, different side chains in most ACPBs is low, the microphase separation behavior of pure ACPBs has not been reported so far, although there have been reports about self-assembled lamellar structures formed by brush random copolymers13 and Janus particles by polymer brushes with two arms per monomer.19 One way to increase the χ value between different side chains is through the introduction of liquid crystallinity because the liquid crystalline (LC) ordering can offer the additional free energy of mixing, which can be expressed by the equation χeff = χ + χLC.20 Moreover, LC ordering will influence the microphase separation of BCPs.21−23 For example, a driving force to form continuous LC domains makes LC BCPs exhibit an LCinduced order−order transition.24,25 Furthermore, LC BCPs can form hierarchically ordered nanostructures with multiple length scales: a 10−100 nm microphase separation domain and the 3−10 nm LC ordering.26,27 LC polymers comprising cholesteryl mesogens have attracted a lot of interest owing to their special properties. For example, homopolymers can reflect polarized light with a certain frequency range.28−30 BCPs containing cholesteryl mesogens can be used to construct polymersomes that can be served as drug carriers.31 And BCPs can also form hierarchical nanostructures that can be applied in nanomaterials.32−34

INTRODUCTION Nanomaterials have been intensively researched in recent decades, and they can be used in nanoporous materials,1−3 nanophotonics,4−6 and sensors.7−9 Block copolymers (BCPs) are one of the building blocks of nanomaterials. Because of the incompatibility of the different blocks, BCPs will microphase separate and form lamellae (LAM), hexagonally packed cylinders (HEX), bicontinuous gyroids, body-centered cubic (BCC) spheres, and other morphologies. The self-assembled morphologies are dependent on f (the volume fraction of the individual block) and the product of χN (χ is the Flory− Huggins interaction parameter, and N is the total degree of polymerization, DP, of the BCP).10,11 Because the molecular weights (MWs) are limited, BCPs mostly form ordered nanostructures in the range of 10−100 nm. Recently, brush block copolymers (BBCPs) are studied to solve the problem because they have many advantages, such as high molecular weights (MWs),12 less chain entanglement, and highly elongated conformation.4 As a result, BBCPs easily selfassemble into LAM and HEX morphologies with an over 100 nm length scale.13−15 Alternating copolymer brushes (ACPBs) with styrene and maleimide backbones16,17 can render microphase-separated structures on the length scale of sub-10 nm because ACPBs have alternating side chains that are distributed on both sides of the backbone, and the d-spacing values of the ordered nanostructures are only determined by the lengths of the side chains18 instead of that of the backbone and the overall MW. © XXXX American Chemical Society

Received: May 18, 2016 Revised: June 29, 2016

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DOI: 10.1021/acs.macromol.6b01043 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of the Macromonomers and the AACPB

Another way to increase the χ value between different blocks is by the addition of lithium salt in BCPs. The χ value became larger when lithium salt was doped in BCPs containing PEO.35−37 Russell et al. found that the χ value also significantly increases after adding lithium chloride into polystyrene-bpoly(methyl methacrylate).38 The increase in the χ value results from the interactions between lithium ion and the oxygen atom in PEO or poly(methyl methacrylate). In addition, the ordered nanostructures of PEO-containing BCP/salt complexes can offer a continuous PEO phase with lithium salt, resulting in solid polymer electrolytes.39−42 In our previous work, we synthesized amphiphilic alternating copolymer brush (AACPB) with LC poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS) and PEO side chains. The pure AACPB can not microphase separate with thermal and solvent annealing methods.18 Considering that the AACPB with more flexible side-chain LC polymers containing cholesteryl mesogens, which is beneficial for the self-assembly of the AACPB, in this work, we prepared a new AACPB containing LC poly[10(cholesteryloxycarbonyl)decyl methacrylate] and semicrystalline PEO side chains, namely P(MI-g-PCholMA)-alt-(St-gPEO) (PCholMA-alt-PEO). The AACPB can microphase separate into ordered nanostructures after solvent and thermal annealing. Polarized light microscopy (PLM), small-angle X-ray scattering (SAXS), two-dimensional (2D) SAXS, and trans-

mission electron microscopy (TEM) experiments were used to investigate the hierarchically ordered morphologies. The AACPB provides two different orders. One is the microphase-separated LAM nanostructure with a 9.66 nm scale. The other is the cholesteryl LC phase. After being doped with 0.2 equiv of LiCF3SO3, the AACPB/LiCF3SO3 complex also forms the LAM morphology with a larger d-spacing, which can offer a continuous PEO phase with lithium salt.

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EXPERIMENTAL SECTION

Materials. CuBr was washed with glacial acetic acid, methanol, and ether successively until it was white, and then it was dried under vacuum at 35 °C for 48 h. Chlorobenzene and tetrahydrofuran (THF) were purified through the Solvent Processing System (M. Braun, Inc.). The initiator 2-bromo-2-methylpropionic acid 2-(3,5-dioxo-10-oxa-4azatricyclo[5.2.1.02,6]dec-8-en-4-yl) ethyl ester (Nb-Br) and styreneterminated poly(ethylene oxide) (St-PEO) were prepared according to our previous reports.18,43 All other reagents were used as received without purification. Methods. 1H NMR and 13C NMR spectra were performed on a Bruker 400 MHz spectrometer. Number-averaged MW (Mn) was obtained from gel permeation chromatographic (GPC) measurements in THF by using a Waters 2410 instrument equipped with a Waters 2410 refractive index (RI) detector. GPC coupled with multiangle laser light scattering (GPC-MALLS) examination was performed on the GPCmax VE-2001 (Viscotek) equipped with a Viscotek TriSEC Model 302 triple detector array (refractive index detector, viscometer B

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Macromolecules

Anal. Calcd for C42H70O4: C, 78.94; H, 11.04. Found C, 78.91; H, 10.99. HRMS (ESI): calcd (M + H)+/z, 639.53; (M + NH4)+/z, 656.56; found (M + H)+/z, 639.5; (M + Na)+/z, 656.6. Synthesis of the Macromonomer MI-PCholMA (4). The macromonomer MI-PCholMA was prepared with a protected strategy, which has been described in our previous report.43 The maleimide was protected with furan by a Diels−Alder reaction, and then furan was removed by a retro-Diels−Alder reaction after polymerization of CholMA. The detailed information is as follows. CholMA (3.20 g, 5.00 mmol), Nb-Br (0.358 g, 1.00 mmol), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 0.346 g, 2.00 mmol), CuBr (0.286 g, 2.00 mmol), and 14.2 g of PhCl were charged into a glass tube. After being degassed with three freeze−pump−thaw cycles, the tube was sealed under vacuum, and then the polymerization was carried out at 75 °C for 4.5 h. The polymerization was stopped by dipping the tube in liquid nitrogen, and then the solution was added to 300 mL of methanol to obtain Nb-PCholMA (3) as a white solid. For the deprotection of furan, the solid was heated at 120 °C, which is the same as in our previous work.43 Synthesis of the AACPB. In the copolymerization process of maleimide-terminated and vinylbenzyl-terminated monomers, when the former is in excess, the polymerization type is alternating. Therefore, the feeding ratio of MI-PCholMA to St-PEO was controlled to be larger than 1. The detailed information is as follows. MI-PCholMA (0.400 g, 110 equiv), St-PEO (0.0675 g, 80 equiv), azodiisobutyronitrile (AIBN, 1 equiv), and THF (40 wt %) were added into a glass tube, and then the tube was degassed with three freeze−pump−thaw cycles. After the tube was sealed under vacuum, the mixture was heated at 60 °C for 72 h, and then the polymerization was quenched with liquid nitrogen. The resultant mixture was diluted with THF and then added dropwise to 300 mL of methanol to obtain a white solid. In order to remove the residual macromonomers, the raw product was reprecipitated with a THF/methanol system, and then the pure polymer brush was obtained as a white solid. LiCF3SO3 Doping. The polymer brush (20.0 mg) and 1.5 mL of THF were charged into a 5 mL bottle, and then a precalculated amount of LiCF3SO3 was added to the mixture. After being stirred for 24 h, the solution was filtered with a filter membrane (Φ = 0.22 μm), and then the solvent was slowly evaporated at ambient temperature. The remaining solid was thermally annealed in vacuum at 100 °C for 24 h. The doping ratio r = [Li+]/[EO] is defined as the molar ratio of LiCF3SO3 to the EO unit in PCholMA-alt-PEO.

detector, and laser light scattering detector (7° and 90°)) with two I3078 polar organic columns. Thermogravimetric analysis (TGA) examinations were recorded on a TA SDT 2960 instrument at a heating rate of 10 °C/min under nitrogen. Differential scanning calorimetry (DSC) was performed on a TA DSC Q100 instrument on the first cooling and a subsequent heating process at a rate of 5 °C/ min in a nitrogen atmosphere. PLM was conducted on a Leitz Laborlux 12 microscope with a Leitz 350 hot stage. SAXS experiments were carried out on a Bruker Nanostar SAXS instrument using Cu Kα radiation (λ = 0.154 nm at 40 kV and 40 mA). The d-spacing (d) is obtained by 2π/q, where the scattering vector q is defined as q = 4π/λ sin θ (2θ is the scattering angle). Two-dimensional SAXS experiments were conducted at Beamline 1W2A in Beijing Synchrotron Radiation Facility (BSRF).44 The wavelength of the X-ray beam was 0.154 nm, and a Pilatus 1M detector was used. The reflection peak positions are calibrated with silver behenate in both SAXS and 2D SAXS experiments. TEM images were performed with a Tecnai T20 TEM instrument using an accelerating voltage of 200 kV. The samples were stained with RuO4 vapor for 10 min to enhance the contrast, and the PEO phase was stained first. Synthetic Procedures. Scheme 1 shows the whole synthetic procedure. Maleimide-terminated poly[10-(cholesteryloxycarbonyl)decyl methacrylate] (MI-PCholMA) and St-PEO were prepared individually, and then they were used to synthesize the AACPB by alternating radical copolymerization. St-PEO was prepared according to our previous report.18 In order to obtain the macromonomer MIPCholMA, the monomer CholMA was prepared first. Synthesis of CholMA. Synthesis of CholBr (1). Cholesterol (CholOH, 3.86 g, 10.0 mmol), 11-bromoundecanoic acid (3.35 g, 10.0 mmol), and 40 mL of CH2Cl2 were charged into a flask with a stir bar, and then N,N-diisopropylcarbodiimide (DIC, 2.52 g, 20.0 mmol), and 4-(dimethylamino)pyridine (DMAP, 0.976 g, 8.00 mmol) were added to the mixture. After being stirred at ambient temperature for 24 h, the suspended solid was filtered, and then the solvent was evaporated under a reduced pressure. The residue was purified by a silica gel column chromatography (100% CH2Cl2) as a white powder. Yield: 60%. 1H NMR (Figure S1 in Supporting Information) (400 MHz, CDCl3, δ, ppm): 0.68 (s, 3H, −CH3 in Chol), 0.76−2.13 (m, 38H in Chol, 3H in CH3−C(COO−)CH2, 16H in methylene spacer), 2.26 (m, 2H, −CH2 in Chol, 2H, −CH2−COO− in methylene spacer), 3.40 (t, 2H, −CH2−Br in methylene spacer), 4.61 (m, 1H, −COO− CH− in Chol), 5.37 (d, 1H, −CCH− in Chol). 13C NMR (Figure S1) (100 MHz, CDCl3, δ, ppm): 10.53−58.51 (34C, in Chol and methylene spacer), 73.60 (1C, CH in −COO−CH− in Chol), 122.61 (1C, CH in −CCH− in Chol), 139.59 (1C, C in −CCH− in Chol), 173.15 (1C, COO in −COO−CH2− in methylene spacer). Synthesis of CholMA (2). The above white solid Chol-Br (3.17 g, 5.00 mmol), methacrylic acid (0.860 g, 10.0 mmol), and K2CO3 (1.00 g, 10.0 mmol) were dissolved into 100 mL of N,N-dimethylformamide (DMF), and then the mixture was stirred at 100 °C for 12 h. After DMF was evaporated under a reduced pressure, the remaining solid was extracted with 50 mL of CH2Cl2 and washed with 25 mL of H2O twice, and then the solvent was removed by vacuum rotary evaporation. The resulting raw product was purified by silica gel column chromatography with CH2Cl2/petroleum ether (v/v = 1:1) as the eluent. The product is a white solid. Yield: 80%. 1H NMR (Figure S2 in Supporting Information) (400 MHz, CDCl3, δ, ppm): 0.68 (s, 3H, −CH3 in Chol), 0.77−2.10 (m, 38H in Chol, 3H in CH3− C(COO−)CH2, 16H in methylene spacer), 2.28 (m, 2H, −CH2 in Chol, 2H, −CH2−COO− in methylene spacer), 4.13 (t, 2H, −COO− CH2− in methylene spacer), 4.60 (m, 1H, −COO−CH− in Chol), 5.37 (d, 1H, −CCH− in Chol), 5.55 and 6.09 (s, 2H, CH2 in CH3− C(COO−)CH2). 13C NMR (Figure S2) (100 MHz, CDCl3, δ, ppm): 10.50−58.89 (34C, in Chol, CH3 in CH3−C(COO−)CH2, and methylene spacer), 64.84 (1C, −COO−CH2− in methylene spacer), 73.73 (1C, CH in −COO−CH− in Chol), 122.48 (1C, CH in −CCH− in Chol), 125.15 (1C, CH2 in CH3−C(COO−)CH2), 136.46 (1C, C in CH3−C(COO−)CH2), 139.73 (1C, C in −C CH− in Chol), 167.65 (1C, COO in CH3−C(COO−)CH2), 173.25 (1C, COO in −COO−CH2− in methylene spacer). Elem.

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RESULTS AND DISCUSSION Synthesis of the Macromonomers and AACPB. St-PEO is unimodal and narrowly dispersed according to the GPC trace in Figure 1, and the absolute MW of St-PEO was calculated to be 1248 by comparing the areas of the characteristic resonance peaks of Ar−CH= at 6.65−6.80 ppm and those at 3.45−3.85 ppm in Figure S3 of the Supporting Information. The GPC

Figure 1. GPC traces of the AACPB PCholMA8-alt-PEO25 and its macromonomers. C

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Macromolecules Table 1. Properties of MI-PCholMA and the AACPB entry

Mn,MI‑PCholMAa (103 g/mol)

DPMI‑PCholMA

Mn,polymer brushb (103 g/mol)

ĐM b

DPmain chainc

Tdd (°C)

Tge (°C)

MI-PCholMA PCholMA8-alt-PEO25

5.4 5.4

8 8

1.41

66

322 325

44

441.6

Determined by GPC in THF (calibrated with polystyrene standards). bDetermined by GPC-MALLS in THF, ĐM is molar mass dispersity. Calculated by the ratio of Mn,polymer brush to the summation of Mn,MI‑PCholMA 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 5 °C/min under nitrogen. a c

trace and the detailed information on MI-PCholMA are shown in Figure 1 and Table 1. The AACPB was synthesized by alternating copolymerization of MI-PCholMA and St-PEO, and its GPC trace is also shown in Figure 1, indicating that the polymer brush does not have residue macromonomers. All characteristic resonance peaks in 1 H NMR originate from the hydrogens in PEO and PCholMA, confirming the chemical structure of the AACPB, and the composition of the polymer brush was also determined by 1H NMR. The molar ratio of PEO to PCholMA in PCholMA8-altPEO25 was calculated to be 1:1 by comparing the areas of the characteristic resonance peaks of a at 5.30−5.45 ppm and e at 3.45−3.85 ppm in Figure 2. The absolute MW of the AACPB was determined by GPC-MALLS, and the DP of the main chain was calculated to be 66, as shown in Table 1.

Figure 3. DSC second-heating thermograms of St-PEO, MIPCholMA, PCholMA 8 -alt-PEO 25 , and PCholMA 8 -alt-PEO 25 / LiCF3SO3 (r = 0.2) at a rate of 5 °C/min following a cooling process at 5 °C/min.

weak mesophase−isotropic transition of PCholMA, which may be due to the dissolution of some LiCF3SO3 into the PCholMA phase. Furthermore, the complex also shows a glass transition of the mixed phase of PCholMA and PEO at −13 °C. Both the neat and lithium salt-doped PCholMA8-alt-PEO25 show the melting or glass transition of PEO, indicating that they microphase separate. Hierarchically Ordered Nanostructures of the Neat and Lithium Salt-Doped AACPB. The LC properties of the neat and lithium salt-doped PCholMA8-alt-PEO25 were preliminarily studied by PLM. As shown in Figure 4a−c, MIPCholMA is birefringent between 30 and 140 °C, and then the birefringence disappears at 150 °C, which corresponds to the mesophase−isotropic transition of PCholMA. The PLM textures of the neat and lithium salt-doped PCholMA8-altFigure 2. 1H NMR spectra of PCholMA8-alt-PEO25 (top), MIPCholMA (middle), and St-PEO (bottom) in CDCl3.

Thermal Properties. The thermal stabilities of the macromonomers and the AACPB were studied by TGA (Figure S4). The 5% weight loss temperatures (Td’s) of all samples are all above 280 °C, suggesting that they possess good thermal stabilities. DSC was performed to measure the thermal transitions, and Figure 3 shows DSC second-heating thermograms of different samples. St-PEO exhibits a melting peak at 40 °C, while MI-PCholMA shows a glass transition at 44 °C and a possible mesophase-isotropic transition (Ti) at 132 °C. However, PCholMA8-alt-PEO25 shows a glass transition of PEO at −44 °C, a melting peak of PEO at 4 °C, and a possible mesophase−isotropic transition of PCholMA at 110 °C, suggesting that PCholMA8-alt-PEO25 microphase separates into PEO and PCholMA phases. Besides, the T i of PCholMA8-alt-PEO25 is lower than that of MI-PCholMA because of the plasticization effect of the flexible PEO chains chemically connected with the PCholMA phase. After being doped with 0.2 equiv of lithium salt, the complex displays a

Figure 4. PLM morphologies of MI-PCholMA (a−c), PCholMA8-altPEO25 (d−f), and PCholMA8-alt-PEO25/LiCF3SO3 (r = 0.2) (g−i) at different temperatures. D

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PCholMA8-alt-PEO25 with the X-ray beam perpendicular to the shear direction which is the Z direction. The sample was sheared at 130 °C and measured at ambient temperature after being quenched. A pair of low-angle diffraction arcs associated with microphase separation appears on the meridian. Because most brush random copolymers form LAM morphologies,13,45 we deduce that the AACPB forms poorly ordered lamellae with the lamellae parallel to the shear direction. There is also another pair of diffraction arcs with a higher q value on the meridian, which represents the packing of the cholesteryl mesogens. On the basis of the assumption that the main chains of the AACPB are aligned along the shear direction, the selfassembled structure of the neat AACPB can be illustrated in Figure 6. Because the d-spacing of SmAd (5.46 nm) is much larger than the length of the repeat unit (0.5 nm) in the main chain of the polymer brush, the alignment of the cholesteryl mesogens is not in the plane composed of the main chain of the AACPB and the backbone of the PCholMA side chain, but perpendicular to the above-mentioned plane, and the cholesteryl mesogens form the SmAd phase with the layer normal along direction 1. Because the microphase-separated LAM structure is poorly ordered and uniaxially oriented along the shear direction, the 2D SAXS pattern in Figure 5b can be regarded as the superposition of two SAXS patterns: one obtained when the X-ray beam is along direction 1 and the other along direction 2. Therefore, both of the diffraction arcs from the microphase-separated LAM structure and from the SmAd phase appear on the meridian. The above two ordered structures with different sizes construct a hierarchically ordered nanostructure. Figure 5d is the TEM micrograph of PCholMA8-alt-PEO25 that forms a poorly ordered LAM structure with a periodicity of 9.58 nm, which is similar to the above SAXS result of 9.66 nm. Figure 7a shows the variable-temperature SAXS profiles of PCholMA8-alt-PEO25; the intensities of both peaks decrease with increasing temperature. And both peaks disappear simultaneously at 140 °C during heating, indicating that the order−disorder transition of the whole AACPB is associated with the SmAd−isotropic transformation. When the sample is cooled to ambient temperature, both two peaks appear again, suggesting the two transitions are thermally reversible. After being doped with 0.2 equiv of lithium salt, the SAXS profile (Figure 5a), 2D SAXS pattern (Figure S5a), and the TEM micrograph (Figure S5c) of the resulting complex are similar to those of PCholMA8-alt-PEO25, indicating that the complex also forms a LAM structure. However, the intensity of the peak at q = 0.476 nm−1 (with a d-spacing of 13.2 nm) in the low-angle region of the complex is higher than that of the neat AACPB, which can be attributed to a more ordered LAM structure of the complex and/or increased electron density contrast between the PCholMA phase and the PEO phase with the addition of LiCF3SO3. The d-spacing of the resulting complex is larger than that of the neat AACPB, which can be rationalized by two factors. On the one hand, LiCF3SO3 is included in the AACPB and increases the volume of the PEO phase. On the other hand, the interaction between Li+ and oxygen atom makes the PEO chains more stretched. Furthermore, as shown in Figure 7b, a shoulder peak appears at q = 0.977 nm−1 (with a d-spacing of 6.43 nm) in addition to the main peak, and the intensity of the shoulder decreases with increasing temperature. And then the shoulder almost disappears. The combination of the SAXS and PLM results of PCholMA8-alt-PEO25/LiCF3SO3 (r = 0.2) indicates that

PEO25 (Figure 4d−i) are similar to those of the macromonomer MI-PCholMA; the only difference is the temperature at which the birefringence disappears. Comparing with MIPCholMA, PCholMA8-alt-PEO25 does not show birefringence at temperatures below 120 °C because the Ti of PCholMA8-altPEO25 is lower than that of MI-PCholMA, as indicated by the DSC result. In addition, the temperature at which the birefringence of different samples disappears is higher than Ti, which can be mainly attributed to the temperature differences in the instruments and the stability of the LC phase. The hierarchically self-assembled nanostructures of the neat and lithium salt-doped AACPB were investigated by SAXS after solvent annealing using THF and thermal annealing at 100 °C for 24 h. Figure 5a are the SAXS profiles of PCholMA and the

Figure 5. SAXS profiles of PCholMA and the neat and lithium saltdoped PCholMA8-alt-PEO25 at ambient temperature (a), 2D SAXS pattern of PCholMA8-alt-PEO25 with the X-ray beam along the Z direction (b), the shearing geometry (c), and TEM micrograph of PCholMA8-alt-PEO25 (d).

neat and lithium salt-doped PCholMA8-alt-PEO25 at ambient temperature. The macromonomer PCholMA shows two diffraction peaks in the low-angle region, representing that PCholMA forms a smectic A (SmA) phase and a double-layer SmA (SmAd) phase.33,34 For PCholMA8-alt-PEO25, it only shows one peak at q = 1.15 nm−1 (with a d-spacing of 5.46 nm) in the low-angle region, indicating that it only forms the SmAd phase.32 However, a second peak at q = 0.650 nm−1 (with a dspacing of 9.66 nm) appears in the even lower-angle region, which can only be attributed to the microphase-separated nanostructure. Figure 5b shows the 2D SAXS pattern of E

DOI: 10.1021/acs.macromol.6b01043 Macromolecules XXXX, XXX, XXX−XXX

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Figure 6. Schematic diagram of the microphase separation of the neat and lithium salt-doped AACPB.

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CONCLUSIONS In this work, we synthesized an amphiphilic alternating copolymer brush with PCholMA and PEO side chains, and the copolymer brush self-assembles into a hierarchically ordered nanostructure: the microphase-separated LAM structure with a d-spacing of 9.66 nm and the cholesteryl SmAd phase with a 5.46 nm scale. The order−disorder transition of the AACPB occurs when the cholesteryl SmAd transforms into isotropic. It is the first time to discover the self-assembled behaviors of AACPBs. We can use the microphase separation of AACPB to construct ordered nanostructures with a sub-10 nm length scale. In addition, the PCholMA8-alt-PEO25/LiCF3SO3 (r = 0.2) also forms a hierarchical structure including a microphase-separated LAM structure with a larger d-spacing. Such a complex may be used as solid polymer electrolytes.

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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.6b01043. 1 H/13C NMR spectra of CholBr and CholMA, 1H NMR spectrum of St-PEO, TGA curves of St-PEO, MIPCholMA, and PCholMA8-alt-PEO25, and 2D SAXS pattern and TEM micrograph of PCholMA8-alt-PEO25/ LiCF3SO3 (r = 0.2) (PDF)

Figure 7. SAXS profiles of PCholMA8-alt-PEO25 (a) and PCholMA8alt-PEO25/LiCF3SO3 (r = 0.2) (b) at different temperatures.

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Corresponding Authors

PCholMA forms the SmAd phase. Therefore, the hierarchically ordered nanostructure of the lithium salt-doped AACPB is also shown in Figure 6. The q and d values of different samples are summarized in Table 2.

*E-mail [email protected] (X.-H.F.). *E-mail [email protected] (Z.S.). Notes

The authors declare no competing financial interest.

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Table 2. q and d Values of the Neat and Lithium Salt-Doped AACPB microphase separation entry PCholMA8-alt-PEO25 PCholMA8-alt-PEO25/LiCF3SO3 (r = 0.2)

ACKNOWLEDGMENTS We thank Prof. Decheng Wu and Xing Wang at Institute of Chemistry, Chinese Academy of Sciences, for the assistance with the GPC-MALLS measurements. The assistance from the scientists at the Beamline 1W2A (Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences) in the SAXS experiments and the financial support from National Natural Science Foundation of China (Grants 21134001 and 21374002) and Beijing Natural

LC phase

q1 (nm−1)

d1 (nm)

q2 (nm−1)

d2 (nm)

0.650 0.476

9.66 13.2

1.15 0.977

5.46 6.43

AUTHOR INFORMATION

F

DOI: 10.1021/acs.macromol.6b01043 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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Science Foundation (Grant 2142016) are also gratefully acknowledged.

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DOI: 10.1021/acs.macromol.6b01043 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01043 Macromolecules XXXX, XXX, XXX−XXX