High-Density and Robust Charge Storage with Poly(anthraquinone

Apr 8, 2015 - A robust polymer–air secondary battery with the high capacity was .... of charge-storage density, and characterization of prepared mat...
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High-Density and Robust Charge Storage with Poly(anthraquinonesubstituted norbornene) for Organic Electrode-Active Materials in Polymer−Air Secondary Batteries Takuma Kawai, Kenichi Oyaizu,* and Hiroyuki Nishide* Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan S Supporting Information *

ABSTRACT: An excellent functional group tolerance of ruthenium complex catalysts for olefin metathesis gave rise to ring-opening polymerization of norbornene functionalized with redox-active anthraquinone (AQ) pendants, yielding a highmolecular-weight and processable polynorbornene with large redox capacity. A thin layer of the polymer cast on current collectors showed reversible redox reaction at −0.85 V vs Ag/ AgCl when immersed in basic aqueous electrolyte solutions. Good cycle performance was observed with a capacity comparable to the formula weight-based theoretical density of 212 mAh/g, which was the largest among those for the previously reported redox-active polynorbornenes. This suggested that all of the AQ units in the layer were redox-active, that electroneutralization was accomplished by successive compensation of counterions throughout the layer, and that the mechanical strength of the polymer layer prevented dissolution or exfoliation from the current collector surface. A robust polymer−air secondary battery with the high capacity was fabricated by using the polymer layer as the anode-active material. The battery showed a discharge voltage of 0.68 V and long life of over 300 cycles of charging/ discharging, maintaining the moderate energy density of 143 mWh/g.

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INTRODUCTION Lithium ion batteries have provided us this full of technology life, but it has also given us the huge amount of industrial waste that our descendants will suffer cleaning. To overcome this problem, metal-free batteries must come into the picture.1,2 Organic batteries are considered to be the candidates for the environmentally benign batteries.3−5 Reversible charge storage performance is required for organic electrode-active materials, especially for anode materials in organic batteries possessing negative redox potentials to increase the electromotive force.6,7 The same also goes for cathode materials in lithium ion batteries which accommodates the Li+ ions during discharge.8 The search for such organic materials has been taking place for a long period of time, but the reports have been limited by employing quinoids,6,9 imides,7 zwitterionic nitroxides,10,11 and various π-deficient ring molecules as the redox-active sites.12−22 Recently, we have revealed the electrochemically totally reversible 1e− redox reaction of organic robust radicals such as galvinoxyls and nitronyl nitroxides which are suitable materials for the entirely organic radical batteries.23−25 Our principal finding was that not only π-conjugated but polymers with aliphatic main chains also transport charge with large current densities, giving large redox capacities by incorporation of redox units as pendant groups in the repeating units.26,27 These characteristics are accomplished by the electrolyte counterions deeply diffusing into the wholly amorphous © XXXX American Chemical Society

polymer layer, giving rise to the redox gradient-driven electron self-exchange reaction which is a result of the highly packed population of the redox-active units.26−31 Such materials also have the possibility to be applied to solar cells,32−34 electrochromic devices,35−38 sensors,39 and other “wet-type” or electrochemical devices.40,41 For the molecule to be bound to the aliphatic main chain, we focused on anthraquinone (AQ) for several reasons: (i) the reversible 2e− reaction of quinones (AQ2−/0) which results in polymers with large theoretical charge-storage capacity,42−45 (ii) the robustness needed to withstand highly severe conditions known from the AQ process used to produce H2O2,46−49 and (iii) the negative potential (−0.5 to −1 V vs Ag/AgCl)50,51 suited for increasing electromotive force when used as the anode material for battery. We have recently reported poly(2-vinylanthraquinone) (PVAQ) as a novel anode-active material in aqueous electrolyte solutions.6 PVAQ showed excellent electrochemical characteristics by the means of the negative potential (∼−0.8 V vs Ag/AgCl) and high charge-storage capacity (∼220 mAh/g). However, attempts to further develop rechargeable organic−air battery using PVAQ have been impeded by the limited formula-weight-based Received: November 27, 2014 Revised: March 17, 2015

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DOI: 10.1021/ma502396r Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules theoretical redox density for PVAQ and unsufficient robustness to cause exfoliation from the current collector during the charge/discharge process for more than few hundred cycles. To overcome this challenge, we focused on norbornene (NB) as a hydrophobic main chain. We have previously reported NB main chain redox polymers with functional groups such as redox-active TEMPO (109 mAh/g)52 and nitronyl nitroxide (96 mAh/g)53 with ambipolar redox capability, which showed excellent processability based on their high molecular weights and affinity with electrolytes, with wide possibilities to yield graft, block, and brush type copolymers.54 Such capability is based on the living character of the ring-opening metathesis polymerization (ROMP) of norbornenes and the excellent functional group tolerance of the Grubbs catalyst. The polynorbornene chain has also been employed to bind many types of redox-active pendant groups, such as metal complexes,55,56 cholic acids,57 and quinones,58 but all of their formula weight-based redox density remained less than 50 mAh/g due to the limited synthetic method to bind the pendant groups while minimizing the formula weight of the repeating unit. Here we report the synthesis of poly(dianthraquinonesubstituted norbornene) (PQNB) with a sufficiently high molecular weight for electrode fabrication by tandem Suzuki coupling reaction from 2-iodoanthraquinone (IAQ) and 2pinacolboronanthraquinone (AQB) followed by ROMP with the Grubbs second-generation catalyst. The hydrophobic NB main chain showed excellent effort in maintaining the initial capacity after >500 cycles of charge/discharge and also preventing the exfoliation of the polymer into electrolyte solutions. The high population of the two AQ units per repeating unit accomplished the large formula-weight-based theoretical redox density (212 mAh/g) and swift electron selfexchange reaction within the polymer layer, resulting in excellent rate performance to allow full discharge only in a few seconds while maintaining the redox capacity. The polymer was applied as the anode-active material of the organic−air secondary battery, which demonstrated the desired performance with the moderate energy density of 143 mWh/g and good cyclability. The present method of polymerizing NBs functionalized with two redox-active groups, without substantial side reactions, demonstrated the capability of populating the functional groups on the high-molecular-weight aliphatic chain in large density (i.e., per repeating unit), which may also lead to those with various types of functional groups.

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Scheme 1. Synthesis of IAQ and AQB

h. This operation was repeated for four times. The black mold obtained by this operation was purified by silica gel column chromatography using CH2Cl2 as eluent to yield IAQ as yellow solid. Yield: 60%. FAB-MS (m/z): M+ 334.0. Found 334.5. 1H NMR (CDCl3, 500 MHz, ppm, TMS): δ 7.81 (t, 1H, Ph), 7.82 (t, 1H, Ph), 7.99 (d, 1H, Ph), 8.15 (d, 1H, Ph), 8.30 (m, 2H, Ph), 8.65 (d, 1H, Ph). 13 C NMR (CDCl3, 500 MHz, ppm, TMS): δ 182.8, 182.1, 143.2, 136.4, 134.5, 134.4, 134.1, 133.4, 133.1, 132.7, 128.7, 127.5, 127.4, 102.3. Synthesis of 2-Pinacolboronanthraquinone (AQB).59 2Bromoanthraquinone (1 g, 3.48 mmol), bis(pinacolato)diboron (973 mg, 3.83 mmol), and potassium acetate (1.02 g, 10.4 mmol) were added together and sealed in a round-bottom flask and was charged with nitrogen. Dry DMSO (20 mL) was added as a solvent. PdCl2(dppf) (570 mg, 0.7 mmol) was dissolved in 3.2 mL of dry DMSO and was added to the mixture. The mixture was heated to 80 °C and stirred for 24 h. After washing with brine, the crude product was purified by silica gel column chromatography using CHCl3 as eluent to yield AQB as a yellow crystal. Yield: 97%. FAB-MS (m/z): M+ 334.1. Found 334.2. 1H NMR (CDCl3, 500 MHz, ppm, TMS): δ 8.74 (s, 1H, Ph), 8.27 (m, 4H, Ph), 7.88 (m, 2H, Ph), 1.38 (s, 16H, CH3). 13C NMR (CDCl3, 500 MHz, ppm, TMS): δ 183.5, 183.2, 140.1, 135.2, 134.2, 134.1, 133.9, 133.7, 133.6, 132.6, 127.4, 127.3, 126.2, 84.7, 25.0. Synthesis of Dianthraquinone-Substituted Norbornene (AQNB).60 IAQ (1 g, 3 mmol), AQB (1.1 mg, 3.3 mmol), and potassium carbonate (1.1 g, 7.92 mmol) were added together and sealed in a round-bottom flask and was charged with nitrogen. A mixture of THF/H2O (1/1 in v/v) (10 mL) was added as a solvent. Then 2,5-norbornadiene (1.2 mL) was added and was heated to 60 °C. PdOAc (27 mg, 0.12 mmol) and PPh3 (75.4 mg, 0.29 mmol) were dissolved in 2 mL of dry THF and added to the mixture. The mixture was stirred for 24 h. After washing with brine, the product was purified by silica gel column chromatography using CHCl3 as an eluent to yield AQNB as yellow solid. Yield: 53%. FAB-MS (m/z): M+ 507.1. Found 506.5. IR (KBr, cm−1): 1672 (υ CO). Anal. Calcd for C35H22O4: C, 83.0; H, 4.35%. Found: C, 82.9; H, 4.31%. 1H NMR (CDCl3, 500 MHz, ppm, TMS): δ 8.18 (m, 4H, Ph), 7.95 (d, 2H, Ph), 7.88 (d, 2H, Ph), 7.71 (m, 4H, Ph), 7.32 (dd, 2H, Ph), 6.53 (m, 2H, HCCH), 3.48 (m, 2H, CH), 3.29 (m, 2H, CH), 2.45 (d, 1H, CH2), 2.01 (d, 1H, CH2). 13C NMR (CDCl3, 500 MHz, ppm, TMS): δ 207.0, 183.1, 182.8, 144.6, 139.8, 134.6, 134.0, 133.9, 133.5, 133.4, 132.9, 131.2, 127.2, 127.1, 127.0, 126.9, 50.3, 47.2, 46.2, 29.8. Polymerization. The monomer AQNB (100 mg, 0.2 mmol) and the Grubbs second-generation catalyst (0.168 mg, 0.001 eq, 0.2 umol) were added to a round-bottom flask which was charged with nitrogen. Dry CHCl3 (10 mL) was added and was heated to 60 °C which was kept stirring for 20 h. After removing the solvent by evaporation, the solution was centrifuged with methanol, washed with methanol, and dried to yield PQNB as yellow solid. GPC: Mw = 6.0 × 105, Mw/Mn = 1.5. The polymer was soluble in chloroform but insoluble in other conventional organic solvents such as acetone, ethanol, and water. IR (KBr, cm−1): 1675 (υ CO). 1H NMR (CDCl3, 500 MHz, ppm,

EXPERIMENTAL SECTION

For reagents, methods of electrochemical measurements, evaluation of charge-storage density, and characterization of prepared materials, see the Supporting Information. Synthesis. Poly(dianthraquinone-substituted norbornene) was synthesized by the following procedures. Incorporation of two AQ units into the repeating unit of polynorbornene was accomplished by preparing IAQ and AQB and step-by-step condensation to yield the monomer AQNB which was then polymerized to obtain PQNB, according to Schemes 1 and 2. Synthesis of 2-Iodoanthraquinone (IAQ). Sulfuric acid (20 g, 20 mmol), NaNO2 (5 g, 72 mmol), and 2-aminoanthraquinone (5 g, 22 mmol) were slowly added together and stirred for 2 h at 0 °C. The mixture was poured into ice water (2 L) and stirred for another 1 h. The resulting mixture was filtered to remove unreacted precipitate. Then, KI (2.5 g, 15 mmol) was added to the filtrate and was kept quiet for 3 h at room temperature. The residue was obtained by filtration. KI (2.5 g, 15 mmol) was added again to the filtrate and kept for another 3 B

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Macromolecules Scheme 2. Synthesis of the Monomer AQNB and the Polymer PQNB

TMS): δ 8.20−7.31 (br, 14H, Ar), 5.86−5.01 (br, 2H, −CH), 3.88−2.50 (br, 6H, aliphatic). The polymerization went through best by using CHCl3 as solvent due to the low solubility of AQNB in other organic solvents. Preparation of Polymer−Air Battery. A tailor-made twocompartment glass cell (Watanabe Kagaku Co.) was employed as the electrochemical cell. Separation of the compartments was accomplished by applying a salt bridge using a fine glass filter in between allowing only the electrolyte to surpass during the course of the measurement. PQNB layer electrode was applied as the anode, and an oxygen reduction catalyst composed of vapor-grown carbon fiber (Showa Denko Co.), MnO2, and poly(vinylidene fluoride) (90:5:5: in wt) molded on a Ni grid was applied as the cathode. The anode room was sealed and filled with a N2-saturated 10 M NaOH aqueous solution enough to cover the active area of the electrode. N2 was continuously flowed in the anode room during all electrochemical measurements to disable unfavorable redox reactions of O2 in the anode side. The cathode room was kept at open air condition and filled with a 10 M NaOH aqueous solution as made in order to keep the cathode close as possible to ambient air conditions. The voltages obtained by this battery corresponded to the potential of PQNB against that of O2.

chloroform, NMP, and DMF. Solubility of the polymer is a very important factor for use as the electrode-active materials, since they need to be soluble or swellable to be fabricated into the redox-active electrode, but insoluble or robust to survive on current collectors during charging/discharging cycles without dissolution into the electrolyte solution that should cause selfdischarge. Tuning of these characters is based on the precise design of polymer structures, such as the physical properties of backbones, molecular weights, and cross-linking density. The high hydrophobicity of norbornene improves the poor solubility of the bulky AQ in organic solvents, while it also prevents the polymer dissolution from the substrate to the aqueous electrolyte. These characters are what we need for excellent cyclic performance of the electrode materials. It may be noted that the Grubbs third-generation catalyst, characterized by the larger rate constant for the initiation than that of the second-generation catalyst, gave lower-molecular-weight PQNB (Mw = 7.8 × 104, Mw/Mn = 1.7) probably as a result of the incompatibility of the catalyst with the AQ unit in the monomer. For further experiments, polymers earned by the second catalyst were used. A CHCl3 solution of PQNB was drop-cast on a glassy carbon disk electrode and dried under solvent atmosphere to form a thin layer of PQNB. All electrochemical measurements for the PQNB layer were done by the layer obtained by this method using a 10 M NaOH aqueous electrolyte solution. As described in the Introduction, AQ-substituted polymers such as PVAQ show reversible redox behavior in basic aqueous electrolyte solutions. To examine the behavior of PQNB, the layer with 50 nm in thickness was set as the working electrode in a conventional one-compartment electrochemical cell. The peak current of the cyclic voltammogram increased for a couple of cycles during the swelling process and became constant. After that, the peak current did not show any decrease, or change after long cycles, revealing the robustness for the redox process of the PQNB layer that stably went through under the basic conditions without undesired side reactions. When AQ undergoes the charging process, the quinoid framework accept two electrons (i.e., four electrons per repeating unit in PQNB) to yield the corresponding dianion unit which made the AQsubstituted polymers, especially PVAQ, highly hydrophilic to cause exfoliation from the substrate. It should be noted that the cyclability is much improved for PQNB as a result of the balanced swellability and enhanced hydrophobicity of the main chain compared with that of PVAQ, as revealed from the cyclic voltammetry which clearly demonstrated that the reversible redox reaction went through in one step. Integration of the voltammetric peak revealed that the wave involved a transfer of two electrons per AQ, which characterized PQNB as the high charge-storage density redox polymer.

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RESULTS AND DISCUSSION Polymers with high charge-storage capability should possess redox-active sites in large density. Incorporation of the redoxactive sites, such as AQ, per repeating unit has been accomplished by the polymerization of the corresponding monomers, rather than the polymer-analogous condensation of the redox-active units with prepolymer chains.9,61 However, polymerization of the redox-active monomers requires that the propagating end should be tolerant against the functional group. Our successful attempts to prepare polynorbornenes bearing a variety of redox-active organic robust radicals25,26,52,53 prompted us to examine ROMP to prepare a high-molecularweight, densely AQ-substituted polymer. Another advantage of using norbornene as the monomer is the potential capability of incorporating two AQ units, giving rise to a large redox capacity. The monomer AQNB was synthesized by a tandem Suzuki-coupling reaction, which was originally developed for synthesis of 5,6-diarylnorbornene compounds. An interesting feature of the tandem Suzuki-coupling reaction was the selective formation of a cis-exo isomer,60 which was most likely applied in the present case of the substitution with AQ, as indicated from the almost σ-symmetric signals in the NMR spectrum (Figure S1). The monomer AQNB was polymerized to PQNB by choosing the Grubbs second-generation catalyst, resulting in a sufficiently high molecular weight of Mw = 6.0 × 105 (Mw/Mn = 1.5). The NMR spectrum suggested no side reaction or structural change in the pendant AQ group (Figure S2). The living nature of the polymerization allowed control of the molecular weight by adjusting the monomer/catalyst ratio. PQNB appeared as a powdery white-yellow solid soluble in C

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Macromolecules Cottrell plots were earned from the chronoamperogram of the PQNB layer in the potential range of 0 to −1.0 V (Figure 1c). As the reduction of the anthraquinone groups in the

Figure 2. Charge−discharge curves of PQNB layer in 10 M NaOH aqueous solution. Inset: cycle performance for charging (open circles) and discharging (closed circles). Dashed line represents the theoretical or formula weight-based calculated redox density.

aqueous solutions. While the high solubility of the reduced state is frequently the main problem for many types of redox polymers that undergo negative (or n-type) charging from the chargeless state, the present four-electron storable polymer showed no sign of exfoliation. Figure S3 shows the rate performance of the discharging process of the PQNB layer. The term “C rate” represents the rate performance for the charge/ discharge process, where 1 C corresponds to the response for 3600 s of charging or discharging and 3600 C to that for 1 s. After charging, the PQNB layer was discharged by various C rates to reveal that it was able to be fully discharged in only 6 s (i.e., 600 C) without loss of the capacity nor significant increase in the overpotential. An air battery was fabricated by employing the PQNB layer as the anode-active material and an oxygen reduction catalyst for the cathode side with a two-compartment electrochemical cell to separate the degassed and air ambient electrolytes, both of which contained 10 M NaOH aqueous solution. Among various types of oxygen reduction catalysts designed for zinc− air batteries and fuel cells,64−67 the most conventional MnO2/C catalyst widely used for the zinc−air battery68 was employed as the cathode catalyst in Figure 3.

Figure 1. (a) Reversible charging and discharging of PQNB. (b) Cyclic voltammogram recorded at a scan rate of 10 mV/s and (c) chronoamperometric Cottrell plots obtained for a 50 nm thick cast layer of PQNB in 10 M NaOH aqueous solution.

polymer layer is accompanied by the incorporation of the counterions that diffuse through the layer to accomplish electrical neutrality, it is likely that the current earned in the experiment was limited by the diffusional process. The linearity of the Cottrell plots of current vs t−1/2, found at the early stage of the electrolysis under semi-infinite diffusion, supported the diffusion-limited behavior and produced the diffusion coefficient from the slope of the plots. The diffusion coefficient of D = 1.2 × 10−12 cm2/s gave the rate constant for the electron selfexchange reaction kex = 1.4 × 105 M−1 s−1 according to D = kexδ2C/6, where C and δ were the redox site concentration in the layer and the distance at the time of the electron transfer approximated by (NAC)−1/3, respectively.26,62 These values were around the order of those previously evaluated for TEMPO-containing redox polymers for facile charge storage,28,63 indicating the swift electron transport characteristics of the PQNB layer comparable to the radical polymers. The charging/discharging property of the PQNB layer was investigated by chronopotentiometry. A distinct plateau voltage region was observed between −0.80 and −0.92 V referring to the galvanostatic electrolysis of the PQNB layer. The charge and discharge capacities amounted to 210 mAh/g with a 100% Coulomb efficiency with respect to the initial capacity of 212 mAh/g, indicating that all of the AQ groups in the repeating unit underwent the reversible 2 × 2e− redox reaction to 2AQ2− without any undesired side reactions such as dimerization, electrophilic attack, irreversible electrolyte, and cation bindings and was not limited by the “break-in” effect of the electroneutralizing ions to cause hysteretic charge/discharge response. The cycle performance is shown in Figure 2 (inset) where it maintained 95% of the initial capacity after 500 cycles. Compared with the previously reported redox polymers such as PVAQ that maintained 91% after 300 cycles, the PQNB layer is regarded as the ideal charge-storage material to operate in basic

Figure 3. Schematic image of the polymer−air battery.

The cyclic voltammogram obtained for the battery is shown in Figure 4. The voltammogram revealed a single set of redox waves that indicated the one-step four-electron redox reaction of PQNB per repeating unit, which indicated that all of the AQ groups underwent charging and discharging without unfavorable side reactions of the partially reduced radicals. Indeed, this is a critical factor for organic batteries since side reactions of D

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Macromolecules

the polymer and/or limited degree of swelling for the thick layers. Exploring the electrode configuration to increase the amount of the polymer in the anode is the future topic of research. Whether we take the robustness that leads to high cycle performance, or the swelling properties to accomplish a highly mounted electrode, has been and will be continuously the key to explore the organic electrode-active materials.

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CONCLUSION The ROMP of dianthraquinone-substituted norbornene yielded poly(dianthraquinone-substituted norbornene) with a significantly high molecular weight to allow application to organic anode-active materials that has to be insoluble, swellable, chemically stable, and equilibrated in aqueous electrolyte solutions. All AQ units in the PQNB layer underwent redox reaction in basic aqueous electrolyte solution at the negative potential around −0.85 V, and one-step four-electron redox process per repeating unit was confirmed by the voltammetric responses obtained for the PQNB layer. From chronopotentiometry, the charge/discharge capacities amounted to 210 mAh/g, and it was maintained for 500 cycles. The PQNB layer was employed as the anode-active material to fabricate a rechargeable polymer/air battery, which accomplished the large energy density of around 150 mWh/g by the large output voltage and the use of O2 as the externally supplied cathodeactive material. The research suggested that designing functional polymers such as those especially for charge-storage applications, the balance between solubility needed for both electrode fabrication and electrochemical reactivity and insolubility (or robustness) against the electrolyte has always been the most difficult problem. By applying a hydrophobic main chain to prevent physical loss of the redox polymers designed to work in aqueous electrolyte solutions, the longcycle high-power organic−air secondary battery was accomplished.

Figure 4. Cyclic voltammogram obtained for the polymer−air secondary battery fabricated with the PQNB layer as the anode and the MnO2/C oxygen reduction catalyst for cathode, sandwiching an electrolyte layer of 10 M NaOH aqueous solution, recorded at a scan rate of 5 mV/s.

intervening radicals frequently lead to the degradation of active materials and/or the battery itself. Charge/discharge performance along with its cycle performance is shown in Figure 5. The charging/discharging voltage

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Figure 5. Charge−discharge curves of air secondary battery. The electrode configuration was as in Figure 4. Inset: cycle performance for charging (open circles) and discharging (closed circles). Dashed line represents the theoretical capacity.

ASSOCIATED CONTENT

S Supporting Information *

Equations for capacity calculation, additional figures for rate performance, and NMR data for the monomer AQNB. This material is available free of charge via the Internet at http:// pubs.acs.org.

appeared near −0.78 V, which corresponded to the potential gap between the anode and the cathode. The charging curve showed a plateau voltage around −0.85 V, and the discharge curve at the same current density was observed around −0.70 V with the Coulomb efficiency of 99%, indicating the excellent reversible charge-storage property of the battery. The chronopotentiogram or the charge−discharge curve in Figure 5 revealed that the capacity earned met good value to the theoretical capacity of the PQNB layer (Figure 2), indicating that the overall capacity of the battery was determined by the anode side as a result of sufficient O2 supply at the cathode. The discharging capacity was maintained for 300 cycles (Figure 5 (inset)). This result revealed the possibility of fabricating an organic-based rechargeable air battery composed mainly of the organic anode-active materials, with a potential capability of the large theoretical capacity larger than that of the Li-ion batteries we use today. It may be added that the excellent rate performance or the high power-rate capability is the advantage of using the organic anode-active materials, while the energy density remained roughly a third of the Li-ion batteries as a result of using oxygen as the cathode-active material. When the thickness of the layer outwent 200 nm, the capacity in mAh/g started dropping. This may be due to the low hydrophilicity of

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.O.). *E-mail: [email protected] (H.N.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grants-in-Aid for Scientific Research (Nos. 24225003, 25288056, 25107733, and 26620108) from MEXT, Japan. T.K. acknowledges the Leading Graduate Program in Science and Engineering, Waseda University from MEXT, Japan.

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DOI: 10.1021/ma502396r Macromolecules XXXX, XXX, XXX−XXX