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Materials 2021, 14, 1617 3 of 20 2. Experimental 2.1. Materials 3-Sulfopropyl methacrylate potassium salt (SPM, 98%), acrylonitrile (AN, ≥99%), gly- cidyl methacrylate (GMA, 97%), copper(I) bromide (CuIBr, 98%), 2,2′-bipyridyl (bpy, 98%), 2-bromopropionitrile (BPN, 98%), ethylene diamine (EDA), ethylene carbonate (EC, 98%), dimethylsulfoxide (DMSO, HPLC-grade), diethyl ether (98%, AR-grade), methanol (99%, AR-grade), and butanol (HPLC-grade) were supplied by Sigma-Aldrich Pte Ltd., Singa- pore, Singapore. Sulfuric acid (95–97%, pro-analysis) was obtained from Merck, Singapore, Singapore. The inhibitors in AN and GMA monomers were removed by passing them through an alumina column from Sigma-Aldrich Pte Ltd. CuIBr was purified according to the Keller–Wycoff procedure [12]. All other chemicals were used without further purifi- cation. For the comparative experiments, Nafion® 112 and Nafion® 117 films (Chemours, Wilmington, DE, USA) were purchased from Sigma-Aldrich Pte Ltd, St. Louis, MO, USA. The following fuel cell electrodes from BASF Fuel Cell were also used: unsupported Pt:Ru alloy (1:1) anode with 5.0 mg/cm2 of total metal loading and unsupported Pt cathode with 5.0 mg/cm2 of metal loading. 2.2. Synthesis of Aliphatic Diblock Ionomers by a One-Pot Atom Transfer Radical Polymerization (ATRP) Technique The diblock ionomers were synthesized by an ATRP technique [13] with the minor modification that two solvents were used successively according to the reaction scheme in Figure 2. A mixture of 28.69 mg (0.2 mmol) of CuIBr, 93.71 mg (0.6 mmol) of bpy and 15 g of EC (0.17 mol) was introduced to a 50-mL Schlenk flask. After vacuum degassing, the mixture was heated at 45 ◦C with stirring until it was completely molten. The solution was purged with Ar for 30 min to remove oxygen, which could interfere with the polymerization by oxidizing the radical species and the Cu(II) catalyst. The AN and GMA monomers were also separately purged and introduced to the Schlenk flask together with a specified amount of BPN initiator by a syringe. The hydrophobic block polymerization was carried out at 65 ◦C for 6 h. The P[(AN-co-GMA)-b-SPM] diblock ionomers were prepared as follows: after the completion of hydrophobic block polymerization, a predetermined amount of Argon- purged SPM solution (10 mmol, 2463.2 mg in 10 mL DMSO) was added to the mixture with a syringe. Hydrophilic block polymerization was then allowed to proceed at 65 ◦C for 20 h. The ionomer was then separated by precipitation in an excess of a diethyl ether/methanol mixture (40/60), filtered and dried in vacuum at 40 ◦C for 24 h. The diblock ionomers prepared as such are identified as AxGyS-10, where x and y refer to the millimole of AN and GMA used in the starting mixture. For example, A50G4S-10 refers to the P[(AN-co-GMA)-b- SPM] ionomer synthesized from a starting mixture of 50 mmole of AN, 4 mmole of GMA, and 10 mmole of SPM. In the all compositions studied only x was varied with respect to G4S-10. To demonstrate the criticality of the GMA monomer, a series of P(AN-b-SPM) diblock ionomers were also similarly prepared and identified as AxS-10. For example, A50S- 10 refers to the P(AN-b-SPM) ionomer synthesized from a starting mixture of 50 mmole of AN and 10 mmole of SPM. 2.3. Membrane Formation and Pretreatment A specified amount of the ionomer powder was dissolved in DMSO and stirred for 4 h at room temperature. A small amount (0.5 mmole, 33.3 μL) of ethylene diamine (EDA) cross-linker was added to the solution and stirred vigorously for 5 h at room temperature to initiate cross-linking by the ring-opening reaction of the epoxide group of GMA. The solution was then decanted into a glass dish and cured at 80 ◦C for 48 h. A uniform transparent membrane was formed by this procedure which could easily be separated from the dish surface. The membrane was then equilibrated in 0.5 M sulfuric acid (H2SO4) for 24 h at room temperature.PDF Image | Hydrophilic Cross-Linked Aliphatic Hydrocarbon Diblock Copolymer
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