Structurally Well-Defined Anion Conductive Aromatic Copolymers

1 day ago - For improving the alkaline stability and other properties of aromatic semiblock copolymer [QPE-bl-11a(C1)] membranes containing benzyltrim...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Structurally Well-Defined Anion Conductive Aromatic Copolymers: Effect of the Side-Chain Length Ryo Akiyama,† Naoki Yokota,§ Kanji Otsuji,‡ and Kenji Miyatake*,†,‡ †

Fuel Cell Nanomaterials Center and ‡Clean Energy Research Center, University of Yamanashi, 4-4 Takeda, Kofu 400-8510, Japan § Takahata Precision Co. Ltd., 390 Maemada, Sakaigawa, Fuefuki, Yamanashi 406-0843, Japan S Supporting Information *

ABSTRACT: For improving the alkaline stability and other properties of aromatic semiblock copolymer [QPE-bl-11a(C1)] membranes containing benzyltrimethylammonium groups, several novel hydrophilic monomers with different side-chain lengths and substitution positions were designed and synthesized for the polymerization. The pendant-type preaminated copolymers PE-bl-11s were quaternized using iodomethane to obtain the target QPE-bl-11s with well-defined chemical structure. In TEM analyses, QPE-bl-11a(C3) and QPE-bl-11a(C5) membranes with propyl and pentyl sidechains, respectively, showed more developed phase-separated morphology with greater hydrophilic domains (ca. 10−20 nm in width) than that of the C1 equivalent. The phase separation was more distinct and larger for the QPE-bl-11a membranes linked with p-phenylene groups in the hydrophilic part than for the QPE-bl-11b membranes with m-phenylene groups. In particular, QPE-bl-11b(C5) membrane exhibited considerably smaller hydrophilic/hydrophobic domains compared to those of the other membranes. After the alkaline stability test in 1 M KOH aqueous solution at 60 °C for 1000 h, the remaining conductivity was better as increasing the side-chain length: 34% for QPE-bl11a(C1), 54% for QPE-bl-11a(C3), and 72% for QPE-bl-11a(C5) at 60 °C. The results suggest that the pendant alkyl chains could improve the alkaline stability and the main-chain bond position could improve morphology, water utilization, and mechanical properties of QPE-bl-11 membranes. An H2/O2 fuel cell with QPE-bl-11 membrane showed 139 mW cm−2 of the maximum power density at 0.28 A cm−2 of the current density.

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INTRODUCTION Polymer electrolyte membrane fuel cells (PEMFCs) using proton exchange membranes (PEMs) have been commercialized for automobile and residential applications. However, use of strongly acidic PEMs requires expensive platinum as electrocatalysts causing high cost of the system. Recently, anion exchange membrane fuel cells (AEMFCs) have attracted much attention due to the possible use of abundant transition metals as electrocatalysts under the basic conditions.1−7 For this reason, a number of anion exchange membranes (AEMs) that exhibit high anion conductivity and durability have been explored.8−19 Particularly, the alkaline stability of AEMs is a critical issue for long-term, reliable operation of AEMFCs. A number of cationic groups including acyclic and cyclic ammonium, phosphonium, and other onium groups have been investigated. In addition, polymer main-chain structures and their high order structures (or membrane morphology) have also been explored.20,21 We have developed a series of semiblock poly(arylene ether sulfone ketone) copolymer [QPE-bl-11a(C1)] membranes containing benzylammonium groups derived from preaminated monomers as the hydrophilic component.22 Unlike the conventional postfunctionalization methods such as chloromethylation and bromination reactions which often accompany © XXXX American Chemical Society

unfavorable side reactions, the synthetic method provides copolymers without structural defects. The QPE-bl-11a(C1) membranes that had high ion exchange capacity, IEC = 2.47 mequiv g−1, exhibited high OH− conductivity (130 mS cm−1) in water at 80 °C. The QPE-bl-11a membrane showed reasonable stability in 1 M KOH for 1000 h at 60 °C. However, further improvement of the alkaline stability is required for practical fuel cell applications. The benzyltrimethylammonium groups in QPE-bl-11a membranes seemed unstable under strongly alkaline conditions. The decomposition of benzyltrimethylammonium groups was reported for a number of aromatic polymer based AEMs in the literature.23−25 The nucleophilic attack of hydroxide anions to the benzylammonium groups provides tertiary amines and alcohols. In addition, tertiary amines and water are generated via Sommelet−Hauser rearrangement or Stevens rearrangement. In order to mitigate these unfavorable decompositions, pendant-type quaternary ammonium groups attached to the polymer backbone via alkyl chain spacers have been investigated. For example, Hibbs et al. reported alkaline Received: February 6, 2018 Revised: April 18, 2018

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DOI: 10.1021/acs.macromol.8b00284 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

obtain 2,5-dichlorocinnamic acid (2a) as a white solid (12.0 g, 96% yield). 1H NMR (500 MHz, DMSO-d6): δ 6.73 (d, J = 16.0 Hz, 1H), 7.49 (dd, J = 2.2, 8.7 Hz, 1H), 7.57 (d, J = 8.7 Hz, 1H), 7.79 (d, J = 16.0 Hz, 1H), 8.02 (d, J = 2.2 Hz, 1H), 12.7 (brs, 1H). 13C NMR (125 MHz, DMSO-d6): δ 123.9, 127.7, 131.1, 131.5, 132.1, 132.5, 133.7, 137.3, 167.0. 2b was prepared from 1b in a manner similar to that of 2a. 3,5-Dichlorocinnamic acid (2b): white solid (92% yield). 1H NMR (500 MHz, DMSO-d6): δ 6.72 (d, J = 16.1 Hz, 1H), 7.55 (d, J = 16.1 Hz, 1H), 7.62 (s, 1H), 7.82 (s, 2H), 12.6 (brs, 1H). 13C NMR (125 MHz, DMSO-d6): δ 122.6, 126.7, 129.1, 134.6, 138.0, 140.9, 167.1. Synthesis of 3-(2,5-Dichlorophenyl)-N,N-dimethyl-2-propenamide (3a). To a 500 mL one-neck round-bottomed flask equipped with a magnetic stirrer bar, 2,5-dichlorocinnamic acid (2a; 12.0 g, 55.3 mmol) and dichloromethane (100 mL) were added. To the suspension, a solution of oxalyl chloride (7.72 g, 60.8 mmol) in dichloromethane (25 mL) was added followed by the addition of few drops of DMF. After stirring for 4 h, complete conversion of the carboxylic acid was confirmed by the 1H NMR spectrum. After cooling with ice bath, dimethylamine hydrochloride (9.01 g, 111 mmol) was added slowly to the mixture followed by the addition of trimethylamine (23.1 mL, 166 mmol). The ice bath was removed, and the mixture was stirred at room temperature for 16.5 h and diluted with large excess of deionized water. The aqueous layer was extracted with dichloromethane several times. The combined organic layers were washed with 1 M hydrochloric acid, saturated sodium bicarbonate aqueous solution, and deionized water and evaporated. By the addition of hexane to the residue, a light brown solid as a crude product was precipitated. The solid was collected by filtration, washed with hexane, and dried under reduced pressure at 60 °C overnight to obtain 3-(2,5dichlorophenyl)-N,N-dimethyl-2-propenamide (3a) as a white solid (23.8 g, 93% yield). 1H NMR (500 MHz, CDCl3): δ 3.08 (s, 3H), 3.18 (s, 3H), 6.86 (d, J = 15.5 Hz, 1H), 7.24 (dd, J = 2.3, 8.6 Hz, 1H), 7.34 (d, J = 8.6 Hz, 1H), 7.55 (d, J = 2.3 Hz, 2H), 7.92 (d, J = 15.5 Hz, 1H). 13C NMR (125 MHz, CDCl3): δ 35.9, 37.4, 121.7, 127.2, 130.0, 131.1, 132.73, 132.76, 135.2, 136.9, 165.8. 3b was prepared from 2b in a manner similar to that of 3a. 3-(3,5-Dichlorophenyl)-N,N-dimethyl2-propenamide (3b): white solid (92% yield). 1H NMR (500 MHz, CDCl3): δ 3.07 (s, 3H), 3.18 (s, 3H), 6.89 (d, J = 15.5 Hz, 1H), 7.33 (t, J = 1.7 Hz, 1H), 7.38 (d, J = 1.7 Hz, 2H), 7.52 (d, J = 15.5 Hz, 1H). 13 C NMR (125 MHz, CDCl3): δ 35.9, 37.4, 120.2, 125.8, 129.0, 135.3, 138.3, 139.3, 165.6. Synthesis of 3-(2,5-Dichlorophenyl)-N,N-dimethylpropanamide (4a). To a 500 mL three-neck round-bottomed flask equipped with a magnetic stirrer bar, 3-(2,5-dichlorophenyl)-N,N-dimethyl-2propenamide (3a, 13.0 g, 53.3 mmol), EtOAc (250 mL), and EtOH (50 mL) were placed. After the addition of 10% Pd/C (1.30 g), the mixture was stirred under atmospheric hydrogen. After 24 h, consumption of olefin was confirmed by the 1H NMR spectrum. The catalyst was removed by filtration through a Celite plug and washed with EtOAc. The filtrate was evaporated, and the residue was purified by silica gel column chromatography (eluent: EtOAc) to obtain 3-(2,5-dichlorophenyl)-N,N-dimethylpropanamide (4a) as a light yellow liquid (5.99 g, 46% yield). 1H NMR (500 MHz, CDCl3): δ 2.61 (t, J = 7.8 Hz, 2H), 2.963 (s, 3H), 2.965 (s, 3H), 3.05 (t, J = 7.8 Hz, 2H), 7.13 (dd, J = 2.3, 8.6 Hz, 1H), 7.27 (d, J = 8.6 Hz, 1H), 7.29 (d, J = 2.3 Hz, 1H). 13C NMR (125 MHz, CDCl3): δ 29.2, 32.8, 35.4, 37.0, 127.6, 130.4, 130.5, 132.1, 132.4, 140.7, 171.3. 4b was prepared from 3b in a manner similar to that of 4a. 3-(3,5-Dichlorophenyl)N,N-dimethylpropanamide (4b): light orange liquid (quant). 1H NMR (500 MHz, CDCl3): δ 2.59 (t, J = 7.9 Hz, 2H), 2.93 (t, J = 7.9 Hz, 2H), 2.95 (s, 3H), 2.96 (s, 3H), 7.09−7.12 (m, 2H), 7.17−7.20 (m, 1H). 13C NMR (125 MHz, CDCl3): δ 30.5, 34.3, 35.4, 37.0, 126.2, 126.9, 134.6, 144.8, 171.2. Synthesis of 5-(2,5-Dichlorophenyl)-4-pentenoic Acid (6a). To a 300 mL three-neck round-bottomed flask equipped with a magnetic stirrer bar and a nitrogen purge, (3-carboxypropyl)triphenylphosphonium bromide (9.06 g, 21.1 mmol) and THF (125 mL) were added to afford white suspension. After cooling with an ice bath, a solution of potassium tert-butoxide (5.18 g, 46.2 mmol) in THF (50 mL) was added to the suspension. During the addition of the

stability of poly(phenylene) backbone with trimetylalkylammonium cations with different pendant chain length.26 The polymer membrane with trimethylhexylammonium groups showed only 5% loss of conductivity with no loss in IEC after 14 days in 4 M KOH at 90 °C, while the membrane with benzyltrimethylammonium groups showed 33% loss of conductivity with 14% decrease in IEC. Bae et al. developed quaternary ammonium-tethered poly(biphenyl alkylene)s that did not contain alkaline labile C−O bonds.27 The resulting anion exchange membranes with pentyl spacer between the polymer main-chain and the ammonium groups showed high OH− conductivity up to 120 mS cm−1 and excellent alkaline stability in 1 M NaOH at 80 °C for 30 days. These results in addition to the others suggest that balanced properties of OH− conductivity and alkaline stability are likely to be achievable when the alkyl spacer length is ca. 4−6 carbons regardless of the differences in the polymer main-chain structure.28−30 The objective of the present research is to evaluate the effect of the side-chain length (C1, 3, 5) on the membrane properties of pendant-type ammonium-functionalized semiblock poly(arylene ether sulfone ketone) copolymers. We prepared structurally well-defined monomers containing pendant tertiary amines so as to produce defect-free AEMs unlike those prepared via other methods (e.g., chloromethylation and bromination reactions). On the basis of the results, we found that pendant alkyl spacers were effective in improving the properties of QPE-bl-11a membranes for fuel cell applications.

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EXPERIMENTAL SECTION

Materials. 2,5-Dichlorobenzaldehyde (>98.0%), 3,5-dichlorobenzaldehyde (>97.0%), malonic acid (>99.0%), pyrrolidine (>98.0%), dimethylamine hydrochloride (>99.0%), (3-carboxypropyl)triphenylphosphonium bromide (>95.0%), 4,4′-dichlorodiphenyl sulfone or bis(4-chlorophenyl)sulfone (ClPS, >98.0%), 4,4′dihydroxybenzophenone or bis(4-hydroxyphenyl) ketone (DHBP, >98.0%), and 2,2′-bipyridine (>99.0%) were commercial products from TCI Co., Ltd., and used as received. Pyridine (>99.5%), dichloromethane (>99.5%), N,N-dimethylformamide (DMF, >99.5%), triethylamine (>99.0%), ethyl acetate (EtOAc, >99.5%), ethanol (EtOH, >99.5%), sodium hydroxide (>97.0%), lithium aluminum hydride (LAH, >92.0%), tetrahydrofuran (THF, >99.5%), potassium tert-butoxide (>97.0%), bis(1,5-cycloocatadiene)nickel(0) (Ni(cod)2, >95.0%), N,N-dimethylacetamide (DMAc, >99.0%), toluene (>99.5%), potassium carbonate (>99.5%), 35−37 wt % hydrochloric acid, iodomethane (CH3I, >99.5%), dimethyl sulfoxide (DMSO, >99.0%), potassium hydroxide (>86.0%), chloroform-d1 with 0.03% TMS (CDCl3, 99.8% D), dimethyl sulfoxide-d6 with 0.03% TMS (DMSO-d6, 99.9% D), and potassium tetrachloroplatinate(II) (>95.0%) were commercial products from Kanto Chemical Co., Inc., and used as received. Palladium-activated carbon (10% Pd/C, 9.5− 10.4% Pd) and oxalyl chloride (>95.0%) were commercial products from Wako Pure Chem. Industries, Ltd., and used as received. 1,1,2,2Tetrachloroethane-d2 (TCE-d2, 99% D) was a commercial product from Acros Organics and used as received. Synthesis of 2,5-Dichlorocinnamic Acid (2a). To a 100 mL one-neck round-bottomed flask equipped with a magnetic stirrer bar and a condenser, 2,5-dichlorobenzaldehyde (1a, 10.1 g, 57.5 mmol), malonic acid (8.97 g, 86.2 mmol), pyridine (3.0 mL, 37.3 mmol), and piperidine (853 μL, 8.63 mmol) were added. When the mixture was heated at 80 °C, solid materials were dissolved gradually to afford a light yellow solution. Next, the reaction temperature was elevated to 155 °C, and white solid generated with evolution of gas. After 3.5 h, the mixture was cooled to room temperature, and 3 M hydrochloric acid was added. The resulting solid was collected by filtration using a membrane filter, washed with 3 M hydrochloric acid and deionized water, and then dried under reduced pressure at 60 °C overnight to B

DOI: 10.1021/acs.macromol.8b00284 Macromolecules XXXX, XXX, XXX−XXX

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dimethylpentan-1-amine (9b): colorless liquid (78% yield). 1H NMR (500 MHz, CDCl3): δ 1.34 (tt, J = 7.6, 7.6 Hz, 2H), 1.48 (tt, J = 7.6, 7.6 Hz, 2H), 1.61 (tt, J = 7.6, 7.6 Hz, 2H), 2.21 (s, 6H), 2.24 (t, J = 7.6 Hz, 2H), 2.56 (t, J = 7.6 Hz, 2H), 7.05 (s, 2H), 7.17 (s, 1H). 13C NMR (125 MHz, CDCl3): δ 26.9, 27.5, 35.4, 45.5, 59.7, 125.9, 126.9, 134.6, 146.0. Synthesis of PE-bl-11a(C3). A typical procedure for PE-bl11a(C3) is as follows (m:n = 1:6). To a 100 mL three-neck roundbottomed flask equipped with a magnetic stirrer bar, a condenser, a nitrogen purge, and a Dean−Stark trap, oligomer 1022 (504 mg, 0.231 mmol), dichloromonomer 5a (3281 mg, 1.41 mmol), 2,2′-bipyridine (600 mg, 3.84 mmol), toluene (5 mL), and DMAc (10 mL) were added. The mixture was stirred at 170 °C for 2 h for azeotropic removal of water. After removing toluene and cooling to 80 °C, Ni(cod)2 (1.00 g, 3.64 mmol) was added to the mixture. The polymerization reaction was carried out at 80 °C for 19 h. The mixture was cooled to room temperature, diluted with additional DMAc (10 mL), and poured into 200 mL of concentrated hydrochloric acid to obtain the light yellow powder as a crude product. The solid was recovered by filtration and washed with deionized water, potassium carbonate aqueous solution, deionized water, and methanol several times. After drying at 60 °C under reduced pressure overnight, PE-bl11a(C3) was obtained as a light yellow solid (601 mg, 84% yield). 1H NMR spectrum suggested the copolymer composition of m/n = 1.0/ 5.3. Preparation of QPE-bl-11a(C3) Membrane. A typical procedure for PE-bl-11a(C3) is as follows (m:n = 1:6). To a 20 mL vial equipped with a magnetic stirrer bar, PE-bl-11a(C3) (601 mg, 1.13 mmol; amount of dimethylamino groups) and DMAc (6 mL) were added. To the solution, CH3I (352 μL, 5.65 mmol) was added. The mixture was stirred for 48 h at room temperature and diluted with additional DMAc (6 mL). The mixture was poured into 200 mL of deionized water to obtain the light yellow precipitate as a crude product. The solid was washed with deionized water several times and dried at 60 °C under reduced pressure overnight to obtain QPE-bl-11a(C3) in I− form as a pale orange solid (863 mg). The obtained QPE-bl-11a(C3) was dissolved in DMSO (12 mL) and cast onto a flat glass plate. Drying the solution at 50 °C provided a QPE-bl-11a(C3) membrane (769 mg, 40−70 μm thick). The 1H NMR spectrum suggested the composition of m/n = 1.0/3.9. IEC value was estimated to be 1.42 mequiv g−1. The membrane was soaked with 1 M KOH aqueous solution at 40 °C for 48 h. The membrane was washed and soaked in a large excess of degassed deionized water for 1 day to obtain the QPEbl-11a(C3) membrane in OH− form. Other PE-bl-11s and QPE-bl-11s were synthesized using same standard procedure from monomers 5b, 9a, and 9b. Measurements. 1H and 13C NMR spectra were measured with a JEOL JNM-ECA/ECX500 using CDCl3, TCE-d2, or DMSO-d6 as solvents. Molecular weights were measured by gel permeation chromatography (GPC) equipped with a Jasco 805 UV detector and Shodex KF-805L and SB-803HQ columns. Standard polystyrene samples were used for calibration. DMF containing 0.01 M LiBr was used as eluent. Transmission electron microscopic (TEM) images were taken for membrane samples stained with tetrachloroplatinate ions. The stained samples were embedded in epoxy resin, sectioned to 50 nm thickness, and placed on a copper grid. Ion exchange capacity (IEC) was determined by 1H NMR spectra and titration (Mohr method). The hydroxide ion conductivity of the membranes was measured in degassed and deionized water using a four-probe conductivity cell attached to an ac impedance spectroscopy system. Water uptake was measured at room temperature. As mechanical properties, the storage moduli (E′), loss moduli (E″), and tan δ (E″/E′) of QPE-bl-11 membranes in I− forms were measured with an ITK DVA-225 dynamic viscoelastic analyzer at 60% relative humidity (RH) from room temperature to 95 °C at a heating rate of 1 °C min−1. Humidity dependence of the viscoelastic properties was measured out at 80 °C from 0 to 90% RH at a humidification rate of 1% RH min−1. Details on these measurements are described in the Supporting Information.

base, the reaction mixture turned to yellow and orange. After stirring for 2 h, a solution of 2,5-dichlorobenzaldehyde (1a, 2.63 g, 15.0 mmol) in THF (25 mL) was added. The mixture was then allowed to warm to room temperature. After stirring for further 24 h, the mixture was quenched by adding 1 M hydrochloric acid (75 mL) and extracted with EtOAc. The combined organic layers were washed with deionized water and evaporated. The obtained crude product was purified by silica gel column chromatography (eluent:hexane/EtOAc = 1/1) to obtain 5-(2,5-dichlorophenyl)-4-pentenoic acid (6a) as a light yellow solid (3.39 g, 92% yield). This product was mixture of E/Z isomers (E/Z = 9/1). 1H NMR (500 MHz, CDCl3): E isomer δ 2.53−2.61 (m, 4H), 6.20 (dt, J = 6.3, 15.8 Hz, 1H), 6.73 (d, J = 15.8 Hz, 1H), 7.10 (dd, J = 1.9, 8.6 Hz, 1H), 7.23 (d, J = 8.6, 1H), 7.44 (d, J = 1.9 Hz, 1H), 11.9 (brs, 1H). 13C NMR (125 MHz, CDCl3): E isomer δ 27.9, 33.4, 126.45, 126.51, 128.0, 130.6, 132.3, 132.6, 136.8, 179.4. 6b was prepared from 1b in a manner similar to that of 6a. 5-(3,5Dichlorophenyl)-4-pentenoic acid (6b): orange liquid (73% yield, E/Z = 9/1). E isomer δ 2.53−2.56 (m, 4H), 6.24 (m, 1H), 6.34 (d, J = 15.5 Hz, 1H), 7.19 (s, 3H), 10.8 (brs, 1H). 13C NMR (125 MHz, CDCl3): E isomer δ 27.7, 33.4, 124.4, 126.9, 128.8, 131.1, 130.9, 140.2, 179.2. Hydrogenation of 5-(Dichlorophenyl)-4-pentenoic Acids 6. 5-(Dichlorophenyl)pentanoic acids 7a and 7b were synthesized from 5-(dichlorophenyl)-4-pentenoic acids 6a and 6b in a manner similar to that of 4a besides without EtOH. 5-(2,5-Dichlorophenyl)pentanoic acid (7a): yellow liquid (92% yield). 1H NMR (500 MHz, CDCl3): δ 1.62−1.76 (m, 4H), 2.41 (t, J = 7.2 Hz, 2H), 2.71 (t, J = 7.2 Hz, 2H), 7.11 (dd, J = 2.3, 8.6 Hz, 1H), 7.20 (d, J = 2.3 Hz, 1H), 7.25 (d, J = 8.6 Hz, 1H), 11.7 (brs, 1H). 13C NMR (125 MHz, CDCl3): δ 24.2, 28.8, 33.1, 33.8, 127.3, 130.0, 130.5, 132.1, 132.4, 141.2, 180.1. 5-(3,5Dichlorophenyl)pentanoic acid (7b): orange liquid (97% yield): δ 1.63−1.71 (m, 4H), 2.39 (m, 2H), 2.59 (m, 2H), 7.05 (d, J = 1.8 Hz, 2H), 7.19 (t, J = 1.8 Hz, 1H), 11.2 (brs, 1H). 13C NMR (125 MHz, CDCl3): δ 24.1, 30.2, 33.6, 35.0, 126.1, 126.9, 134.7, 145.3. Amidation of 5-(Dichlorophenyl)pentanoic Acids 7. 5(Dichlorophenyl)-N,N-dimethylpentanamides 8a and 8b were synthesized from 5-(dichlorophenyl)pentanoic acids 7a and 7b in a manner similar to that of 3a. 5-(2,5-Dichlorophenyl)-N,N-dimethylpentanamide (8a): orange liquid (88% yield). 1H NMR (500 MHz, CDCl3): δ 1.61−1.76 (m, 4H), 2.35 (t, J = 7.2 Hz, 2H), 2.72 (t, J = 7.5, 2H), 2.95 (s, 3H), 3.00 (s, 3H), 7.10 (dd, J = 2.4, 8.4 Hz, 1H), 7.21 (d, J = 2.4 Hz, 1H), 7.25 (d, J = 8.4 Hz, 1H). 13C NMR (125 MHz, CDCl3): δ 24.6, 29.1, 32.9, 33.2, 35.2, 37.1, 127.1, 130.0, 130.3, 132.0, 132.2, 141.5, 172.5. 5-(3,5-Dichlorophenyl)-N,N-dimethylpentanamide (8b): orange liquid (69% yield). 1H NMR (500 MHz, CDCl3): δ 1.60−1.71 (m, 4H), 2.32 (t, J = 6.9 Hz, 2H), 2.60 (t, J = 7.2, 2H), 2.94 (s, 3H), 2.98 (s, 3H), 7.06 (d, J = 1.7, 1H), 7.17 (brs, 1H). 13C NMR (125 MHz, CDCl3): δ 24.6, 30.7, 33.0, 35.3, 35.4, 37.2, 126.0, 126.9, 134.6, 145.7, 172.6. Reduction of Amides 4 and 8. While the reduction of the amides was carried out according to the previous report,22 alumina column chromatography was employed for purification of dichloromonomers 5 and 9 owing to their adsorbability onto silica gel. 3-(2,5Dichlorophenyl)-N,N-dimethylpropan-1-amine (5a): light orange liquid (87% yield). 1H NMR (500 MHz, CDCl3): δ 1.77 (tt, J = 7.6, 7.6 Hz, 2H), 2.24 (s, 6H), 2.31 (t, J = 7.6 Hz, 2H), 2.72 (t, J = 7.6 Hz, 2H), 7.11 (dd, J = 2.3, 8.6 Hz, 1H), 7.22 (d, J = 2.3 Hz, 1H), 7.26 (d, J = 8.6 Hz, 1H). 13C NMR (125 MHz, CDCl3): δ 27.4, 31.2, 45.3, 58.9, 127.1, 130.1, 130.3, 132.0, 132.2, 141.5. 3-(3,5-Dichlorophenyl)N,N-dimethylpropan-1-amine (5b): light yellow liquid (82% yield). 1H NMR (500 MHz, CDCl3): δ 1.75 (tt, J = 7.6, 7.6 Hz, 2H), 2.22 (s, 6H), 2.26 (t, J = 7.6 Hz, 2H), 2.60 (t, J = 7.6 Hz, 2H), 7.07 (d, J = 1.9 Hz, 2H), 7.18 (t, J = 1.9 Hz, 1H). 13C NMR (125 MHz, CDCl3): δ 28.9, 33.0, 45.4, 58.7, 126.0, 126.9, 134.6, 145.6. 5-(2,5-Dichlorophenyl)-N,N-dimethylpentan-1-amine (9a): colorless liquid (90% yield). 1H NMR (500 MHz, CDCl3): δ 1.39 (tt, J = 7.6, 7.6 Hz, 2H), 1.51 (tt, J = 7.6, 7.6 Hz, 2H), 1.62 (tt, J = 7.6, 7.6 Hz, 2H), 2.21 (s, 6H), 2.25 (t, J = 7.6 Hz, 2H), 2.69 (t, J = 7.6 Hz, 2H), 7.10 (dd, J = 2.5, 8.4 Hz, 1H), 7.20 (d, J = 2.5 Hz, 1H), 7.25 (d, J = 8.4 Hz, 1H). 13 C NMR (125 MHz, CDCl3): δ 27.1, 27.5, 29.4, 33.4, 45.5, 59.7, 127.1, 130.0, 130.4, 132.1, 132.3, 141.9. 5-(3,5-Dichlorophenyl)-N,NC

DOI: 10.1021/acs.macromol.8b00284 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of Dichloromonomers 5

Scheme 2. Synthesis of Dichloromonomers 9

Preparation of Catalyst Coated Membrane (CCM) and Fuel Cell Operation. A catalyst paste was prepared from commercial 50 wt % Pt-loaded catalyst (Pt/CB, TEC10E50E, Tanaka Kikinzoku Kogyo K. K.) and QPAF-4 ionomer31 (2.1 mequiv g−1). The mass ratio of the ionomer to carbon was adjusted to 0.8. The catalyst paste was sprayed onto both sides of QPE-bl-11a(C5) (1.86 mequiv g−1, 90 μm thick) membrane using a pulse-swirl-spray apparatus. The Pt loading was 0.2 ± 0.02 mg cm−2, and the geometric area of the electrodes was 4.41 cm2. The obtained CCM was ion exchanged to hydroxide ion form by immersion in 1.0 M KOH for 48 h and then in deionized water for 24 h. The CCM was sandwiched between two gas diffusion layers at 1.0 MPa for 3 min. The membrane electrode assembly (MEA) was placed in a single cell consisting of two carbon separator plates. The fuel cell was operated at 60 °C under an ambient pressure with fully humidified hydrogen and oxygen. The flow rates of hydrogen and oxygen were 100 mL min−1. The high frequency resistance of the fuel cell was measured with a Kikusui FC impedance meter at 5.0 kHz.

following reactions with dimethylamine hydrochloride. After hydrogenation of the olefins 3 using Pd/C under atmospheric hydrogen, the resulting 3-(dichlorophenyl)-N,N-dimethylpropanamides 4 were reduced with LAH at reflux to obtain dichloromonomers 5 in good yields. On the other hand, for the synthesis of dichloromonomers 9, Wittig reaction was employed in the first step to afford 5(dichlorophenyl)-4-pentenoic acids 6. Unsaturated carboxylic acids 6 could be applied to hydrogenation directly because of its high solubility. Then, following amidation and reduction of amide were conducted similarly to the case of dichloromonomers 5. 1H NMR and 13C NMR spectra characterized well these compounds (Figures S1−S16). Synthesis of QPE-bl-11s. Scheme 3 shows the overall synthetic route for the quaternized semiblock copolymers QPEbl-11a(C3): (i) synthesis of the ClPS-terminated telechelic hydrophobic oligomer 10 according to our previous report,22 (ii) Yamamoto coupling33 reaction of the oligomer 10 and dichloromonomer 5a to obtain semiblock precursor copolymers [PE-bl-11a(C3)], and (iii) quaternization of PE-bl11a(C3) using iodomethane in homogeneous system to obtain QPE-bl-11a(C3) (I− form). Oligomer 10 was prepared by nucleophilic substitution polymerization reaction from DHBP with slight excess of ClPS using K2CO3 as a base in DMAc solution. The number of repeat unit (X = 4) was controlled by the feed comonomer ratio. The chemical structure of 10 was characterized by the 1H NMR spectrum (Figure S17a). The

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RESULTS AND DISCUSSION Synthesis of Dichloromonomers 5 and 9. Four types of dichloromonomers 5 and 9 with different substitution position and alkyl chain length were synthesized from commercially available benzaldehydes (Schemes 1 and 2). In the case of dichloromonomers 5, initially, dichlorocinnamic acids 2 were synthesized by Knoevenagel condensation (Doebner modification)32 from commercially available benzaldehydes such as 2,5dichlorobenzaldehyde (1a) and 3,5-dichlorobenzaldehyde (1b). Then, the obtained acids 2 were converted to 3-(dichlorophenyl)-N,N-dimethylpropenamides 3 by chlorination and the D

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Macromolecules Scheme 3. Synthesis of Oligomer 10, PE-bl-11a(C3), and QPE-bl-11a(C3)

Table 1. Properties of PE-bl-11 and QPE-bl-11 PE-bl-11 no.

monomer

m:na

m:nb

yield (%)

Mnc (kDa)

Mwc (kDa)

Mw/Mn

QPE-bl-11 m:nb

a(C3)-1 a(C3)-2 a(C3)-3 b(C3)-1 b(C3)-2 b(C3)-3 a(C5)-1 a(C5)-2 a(C5)-3 b(C5)-1

5a 5a 5a 5b 5b 5b 9a 9a 9a 9b

1:6 1:9 1:12 1:3 1:6 1:9 1:6 1:9 1:12 1:6

1:5.3 1:6.4 1:7.8 1:2.1 1:4.5 1:5.9 1:4.3 1:6.7 1:8.4 1:3.5

84 80 66 94 84 78 83 85 84 34

21.0 17.6 16.2 18.1 16.9 15.6 15.9 12.3 10.7 14.0

103 77.1 65.1 187 114 83.7 77.1 92.9 163 50.6

4.9 4.4 4.0 10.3 6.8 5.4 4.8 7.5 15.2 3.6

1:3.9 1:5.8 1:6.4 1:2.2 1:3.9 1:4.8 1:3.8 1:6.7 1:8.9 1:3.6

IECb,d (mequiv g−1) 1.42 1.80 1.92 0.95 1.49 1.70 1.28 1.86 2.18 1.24

(1.58)e (1.80) (1.93) (1.21) (1.65) (1.97) (1.46) (1.80) (2.22) (1.35)

a

Calculated from the feed comonomer ratios. bDetermined by the 1H NMR spectra. cDetermined by GPC analyses. dCalculated as QPE-bl-11 (in hydroxide ion form). eValues in parentheses were determined by titration.

Figure 1. Structure of PE-bl-11a(C1) and QPE-bl-11a(C1) containing similar hydrophobic structure as that of QPE-bl-11s.

number of repeat units of 10 calculated from the peak integral ratios in the 1H NMR spectrum was in fair agreement with that calculated from the feed comonomer ratio. In Ullmann coupling reaction, several types of PE-bl-11(C3) were prepared by changing the reaction conditions, such as monomers 5a,b and comonomer feed ratio (Table 1). For the synthesis of PE-bl-11(C5), monomers 9a,b were used. PE-bl11s were obtained in reasonable yields except for the case of 9b due to the dissolution of the resulting PE-bl-11b(C5) in

methanol during the purification procedure. The molecular weights of PE-bl-11s were measured by gel permeation chromatography (GPC) to be Mn = 10.7−21.0 kDa and Mw = 50.6−163 kDa. Polydispersity indexes (PDI) were Mw/Mn = 3.6−15.2, relatively high for these kinds of polycondensation polymers. The high PDI values were due to the formation of oligomers containing no or little hydrophobic oligomer component. Mw’s of the PE-bl-11(C3) and -(C5) were smaller than those of our previously reported PE-bl-11a(C1) (Figure E

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Figure 2. TEM images of (a) QPE-bl-11a(C3) (IEC = 1.42 mequiv g−1), (b) QPE-bl-11b(C3) (IEC = 1.49 mequiv g−1), (c) QPE-bl-11a(C5) (IEC = 1.28 mequiv g−1), and (d) QPE-bl-11b(C5) (IEC = 1.24 mequiv g−1) membranes stained with tetrachloroplatinate ions.

1).22 It is considered that the reaction intermediates including Ni(0) would be stabilized by coordination of more basic and flexible amines (as the monomers for C3 and C5) compared with dimethylbenzylamine (as the monomer for C1) decreasing their reactivities. In fact, as the amount of monomers 5 or 9 increased, the average molecular weights (Mn and Mw) decreased (Figure S21). Particularly, when monomer 9a was used, copolymers with lower molecular weight and higher PDI were obtained. The quaternization reactions of PE-bl-11(C3) and PE-bl11(C5) were carried out using iodomethane in DMAc solution. In all cases, the light yellow precipitates of the QPE-bl-11s were recovered by adding the reaction mixture into a large excess of deionized water. The obtained PE- and QPE-bl-11s were characterized well by 1H NMR spectra (Figures S17b,c and S18−S20). Complete quaternization reaction was suggested in all cases by the shift of methylene and methyl protons on sidechains to the lower magnetic field. The quaternized QPE-bl-11s were soluble in polar organic solvents such as DMSO and DMAc and insoluble in THF, 1,4-dioxane, and methanol and provided yellow and transparent QPE-bl-11 membranes by casting from DMSO solution (Figure S22). The IECs of the quaternized membranes determined by titration were generally in fair agreement with those calculated from the 1H NMR spectra (Table 1).

Morphology and Properties of QPE-bl-11 Membranes. Figure 2 shows TEM images of some selected membranes, QPE-bl-11a(C3) (IEC = 1.42 mequiv g−1), -11b(C3) (IEC = 1.49 mequiv g−1), -11a(C5) (IEC = 1.28 mequiv g−1), and -11b(C5) (IEC = 1.24 mequiv g−1) stained with tetrachloroplatinate ions. The QPE-bl-11 membranes exhibited phase-separated morphology based on hydrophilic (dark areas) and hydrophobic (bright areas) differences in the components. While QPE-bl-11a(C1) membrane reported previously contained hydrophilic domains with ca. 5−10 nm in width (Figure S23),22 QPE-bl-11a(C3) and QPE-bl-11a(C5) membranes with longer alkyl side-chains exhibited more developed phase separation containing somewhat greater hydrophilic domains (ca. 10−20 nm in width, Figures 2a and 2c). The phase-separated morphology was more distinct and larger for the QPE-bl-11a membranes (Figures 2a and 2c and Figure S23) linked with p-phenylene groups in the hydrophilic part than for the QPE-bl-11b membranes (Figures 2b and 2d) with m-phenylene groups. In particular, QPE-bl-11b(C5) membrane with low IEC (1.24 mequiv g−1) exhibited considerably smaller hydrophilic/hydrophobic domains compared to those of the other membranes. Figure 3 shows water uptake (at room temperature) and OH− conductivity (at 30 °C in water) of the QPE-bl-11 membranes as a function of IEC. As expected, both water F

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phenylene groups showed higher hydroxide ion conductivity. More distinct and developed morphology with larger hydrophilic (ammonium-containing) channels in QPE-bl-11a were presumably contributing to the better water utilization for the hydroxide ion conduction. QPE-bl-11 membranes exhibited good Arrhenius-type temperature dependence of the hydroxide ion conductivity in water at least up to 80 °C (Figure 5). The apparent activation energies calculated from the slopes of the linear lines were ca. 11−14 kJ mol−1, which were similar to those of our aromatic polymer based AEMs and other AEMs reported in the literature.14−20,34−41 The mechanical stability was evaluated by viscoelastic properties, the storage moduli (E′), loss moduli (E″), and tan δ (E″/E′) of QPE-bl-11 membranes in I− forms at 60% RH (Figure 6) in order to avoid the effect of carbon dioxide from the air. Although the E′ and E″ of QPE-bl-11 membranes showed minor changes from room temperature to 95 °C, these values decreased as increasing the side alkyl chain length. QPEbl-11a(C3) showed higher E′ and E″ values than those of QPEbl-11b(C3) with comparable IEC value, indicating that the pphenylene linkage in the hydrophilic component improved mechanical properties. The pendant-type QPE-bl-11 membranes investigated in this study did not show detectable peaks in the E″ and tan δ curves, suggesting no thermal transitions under the tested conditions, while QPE-bl-11a(C1) showed a minor and broad peak at ca. 70−80 °C.22 The thermal transition, therefore, was likely to be associated with the microBrownian motion of the side-chains. The alkaline stability of the QPE-bl-11a(C1), -11a(C3), -11b(C3), and -11a(C5) (IEC = 1.29, 1.42, 1.49, and 1.28 mequiv g−1, respectively, in I− forms) membranes was investigated in 1 M KOH at 60 °C (Figure 7). All membranes showed initial jump of the conductivity within ca. 20 h because of the ion exchange from I− to OH− ions and then gradual decrease with testing time. The remaining conductivity (conductivity after 1000 h divided by the maximum conductivity) improved as the side-chain became longer: 34% for QPE-bl-11a(C1), 54% for QPE-bl-11a(C3), and 72% for QPE-bl-11a(C5). The results are not contradictory to the reported AEMs, as mentioned above in the Introduction, in which C4 to C6 in the side-chains stabilized the pendant ammonium groups in the strongly alkaline solution.26−30 Furthermore, alkaline stability of QPE-bl-11a(C3) and -11a(C5) (IEC = 1.42 and 1.28 mequiv g−1, I− forms) membranes was also tested at 80 °C (see Figure S24 for the time course of the ion conductivity). The remaining conductivities were 57% for QPE-bl-11a(C3) and 52% for QPE-bl-11a(C5), comparable to or somewhat lower than those at 60 °C. The 1H NMR spectra of the QPE-bl-11a(C3) and -(C5) membranes after the test implied partial chemical degradation with a decrease in methyl proton peaks of the trimethylammonium groups and an appearance of phenolic proton peaks derived from cleavage of the ether bonds in the hydrophobic part (Figure 8). From these results, the major degradation mechanisms of QPE-bl-11a(C3) and -(C5) membranes in alkaline solution seemed to include demethylation of trimethylammonium groups and cleavage of ether linkages with electron-withdrawing groups in the main-chain by nucleophilic substitution of hydroxide ions. Since no olefin peaks were detected between 5 and 6 ppm in the 1H NMR spectra, Hofmann elimination was unlikely. The steric shielding caused by flexible hydrophobic alkyl chains around the

Figure 3. (a) Water uptake (at room temperature) and (b) hydroxide ion conductivity (in water 30 °C) of QPE-bl-11 and Tokuyama A201 membranes as a function of IEC.

uptake and ion conductivity increased as increasing the IEC value. These properties were unlikely to be dependent on the substitution position in the hydrophilic phenylene unit and the pendant alkyl chain length. Among the QPE-bl-11 membranes, QPE-bl-11b(C5) membrane (IEC = 1.24 mequiv g−1) showed very low water uptake (8.5 wt %) and ion conductivity (0.89 mS cm−1). As shown in the TEM image (Figure 2d), less developed phase-separated morphology with narrow hydrophilic channels would be responsible. It is considered that the developed phase separation is one of the important factors for obtaining high ion conductivity. Similar results have been previously reported.22,34 Compared to Tokuyama A201 membrane, QPE-bl-11 membranes exhibited higher water uptake and hydroxide ion conductivity. In Figure 4, hydroxide ion conductivity of the QPE-bl-11 membranes at 30 °C is replotted as a function of water

Figure 4. Hydroxide ion conductivity (30 °C) of QPE-bl-11 membranes as a function of λ.

molecules absorbed per ammonium group (λ). QPE-bl-11 membranes showed nearly linear increase of the ion conductivity to the λ for each series, para-linked (circles) and meta-linked (triangles), respectively. The conductivity was not dependent on the pendant alkyl chain length. In contrast, the substitution position in the hydrophilic phenylene unit was more influential and QPE-bl-11a membranes linked with pG

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Figure 5. Temperature dependence of hydroxide ion conductivities of QPE-bl-11 membranes in water.

Figure 7. Retention of hydroxide ion conductivity (40 °C) of QPE-bl11a(C1), -11a(C3), -11b(C3), and -11a(C5) (IEC = 1.29, 1.42, 1.49, and 1.28 mequiv g−1) membranes after 1000 h stability test in 1 M KOH aqueous solution at 60 °C.

fuel cell as a function of the current density. The ohmic resistance was nearly constant at 0.24 mΩ cm2, which was 2 times higher than that expected from the OH− conductivity (74 mS cm−1) and the thickness of the membrane (see Figure 5 for the conductivity) probably because of the contact resistance with the catalyst layer and somewhat lower conductivity of the membrane in humidified conditions than in water. Similar results were obtained with our previously reported membranes.15,18 The open circuit voltage was 1.00 V, suggesting low gas permeability of the membrane. The fuel cell was operable up to 0.28 A cm−2, at which the power density reached 139 mW cm−2.

Figure 6. DMA analysis of QPE-bl-11 membranes in I− forms of (a) storage elastic modulus (E′), (b) loss elastic modulus (E″), and (c) tan δ temperature dependence at 60% RH.

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ammonium centers may alleviate the attack by hydroxide ions more than benzyltrimethylammonium groups resulting in improved stability. Fuel Cell Performance of QPE-bl-11a(C5) Membrane. A catalyst-coated membrane (CCM) was prepared from QPEbl-11a(C5) (IEC = 1.86 mequiv g−1, 90 μm thick) and Pt/C catalyst. For the binder in the catalyst layer, our previous anion conductive polymer, QPAF-4 (IEC = 2.1 mequiv g−1) was used.31 Fuel cell was operated with hydrogen and oxygen under fully humidified conditions (100% RH) at 60 °C. Figure 9 shows cell voltage, power density, and ohmic resistance of the

CONCLUSIONS We have synthesized a series of structurally well-defined aromatic semiblock copolymer AEMs [QPE-bl-11(C3) and QPE-bl-11(C5)] containing pendant ammonium groups, which were composed of oligophenylene moieties as scaffold for the ammonium groups and oligo(arylene ether sulfone ketone)s as the hydrophobic block. The synthetic strategy was to design several hydrophilic monomers with different amine groups, side-chain lengths, and substitution positions, which were H

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Figure 8. 1H NMR spectra of (a) QPE-bl-11a(C3) (IEC = 1.42 mequiv g−1) and (b) QPE-bl-11a(C5) (IEC = 1.28 mequiv g−1) membranes in DMSO-d6 before conductivity measurement (black line, I− form) and after 1000 h stability test in 1 M KOH at 80 °C (blue line, Cl− form).

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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.8b00284. 1 H and 13C NMR spectra of compounds 2−9, 1H NMR spectra of oligomer 10, PE-bl-11, and QPE-bl-11, GPC profiles of PE-bl-11a(C3) and PE-bl-11a(C5), photos of QPE-bl-11a(C3) membrane and QPE-bl-11a(C5) membrane, TEM image, alkaline stability of QPE-bl-11a(C1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +81 552208707; Fax +81 552208707 (K.M.).

Figure 9. Fuel cell performance (cell potential, power density, and ohmic resistance as a function of current density) of QPE-bl-11a(C5) membrane (IEC = 1.86 mequiv g−1) with fully humidified hydrogen and oxygen at 60 °C.

ORCID

Kenji Miyatake: 0000-0001-5713-2635 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by CREST (JPMJCR12C3), Japan Science and Technology Agency (JST), and the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Japan through a Grant-in-Aid for Scientific Research (18H02030). K.M. acknowledges the Ogasawara Foundation for the Promotion of Science and Engineering for financial support.

converted to the copolymers capable of tuning structures and properties by copolymerizing with hydrophobic oligomers. From TEM images, it was found that the QPE-bl-11a membranes having p-phenylene linked hydrophilic part showed more developed phase-separated morphology compared with QPE-bl-11b membranes linked with m-phenylene groups. QPEbl-11a membranes also exhibited better water utilization for ion conduction and better mechanical properties. The QPE-bl11a(C5) membrane showed higher alkaline stability compared with the membranes with shorter pendant alkyl chain length. The results are in agreement with those of other AEMs reported in the literature. The post alkaline stability test analyses suggested that the better alkaline stability of QPE-bl11a(C5) membrane would be originated from the steric and hydrophobic effect of the alkylene groups in the side-chains to mitigate the attack by the hydroxide ions to the ammonium groups.

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REFERENCES

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DOI: 10.1021/acs.macromol.8b00284 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules block-poly(ethylene-ran-butylene)-block-polystyrene for direct methanol alkaline fuel cells. J. Membr. Sci. 2010, 349, 237−243. (41) Ren, X.; Price, S. C.; Jackson, A. C.; Pomerantz, N.; Beyer, F. L. Highly Conductive Anion Exchange Membrane for High Power Density Fuel-Cell Performance. ACS Appl. Mater. Interfaces 2014, 6, 13330−13333.

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DOI: 10.1021/acs.macromol.8b00284 Macromolecules XXXX, XXX, XXX−XXX