Synthesis and Evaluation of Polyphenylene-Based Copolymers with Phosphonium Group (III) – Relationship between Copolymerization Ratio and Film Formability

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Polymer electrolyte fuel cells (PEFCs) are limited to operate at ~80oC because of the requirement of water absorption for the proton conduction and the low thermal properties of the proton exchange membrane. Operation of PEFCs in the middle and high temperature range (> 100oC) without humidification has many advantages such as simplify of water management system, improvement of catalytic activities, and reduction of catalytic poisoning. Ionic liquids with active protons (protic ionic liquids, PILs) are attracting attention as new proton conductors that can be used without humidification. Diethylmethylammonium triflate ([dema][TfO]) has high ionic conductivity (4.33 × 10- 2 S cm- 1 at 120oC) without humidification, excellent thermal stabilities, and wide potential windows among PILs. 1 ）In order to use PILs as electrolyte materials, it is necessary to fabricate solid membranes by complexing with a matrix polymer. In this study, poly(4-(trimethylphosphonium)propyl)benzyl)-1,4-phenylene-random-poly(4-phenoxybenzophenone) (PPr-r-PPBP) with quaternary phosphonium groups, which has excellent mechanical properties and thermal stabilities as a matrix polymer was synthesized, and these film formability and electrolyte properties were evaluated.Random copolymers PPr-r-PPBPx:y having different copolymerization ratios was synthesized by Ni (0) coupling reaction with (3-(4-(2,5-dichlorobenzoyl)phenyl)propyl)trimethylphosphonium (P-PrDBP) and 2,5-dichloro-4’-phenoxybenzophenone (DPBP). Since it was difficult to measure the molecular weight of these cationic copolymers by GPC, the approximate molecular weight of these copolymers was determined by comparing the viscosity of the homopolymer, PPBP with a known molecular weight. The 5% weight loss temperatures were 416oC for PPr-r-PPBP1.0:3.0 and 453oC for PPr-r-PPBP1.0:6.9, indicating that these copolymers have sufficient thermal stabilities for materials used in the medium and high temperature ranges. Composite membranes PPr-r-PPBP/PIL-z were obtained by casting the mixed DMSO solution of PPr-r-PPBP and [dema][TfO] on glass substrates, where z is the weight ratio of [dema][TfO] to the composite membrane (wt%). FT-IR measurements showed that all samples had absorption bands for the stretching vibration of S-O bonds at 1023 and 1154 cm-1 derived from [dema][TfO], the stretching vibration of C-F bond at 1225 cm-1, and the stretching vibration of N-H bond at 3054 cm-1. The S-O stretching vibration of PPr-r-PPBP/PIL-z shifted to the lower wavenumber than the S-O stretching vibration of [dema][TfO] before the composition. This result suggests that the cationic groups of PPr-r-PPBP interact with [dema][TfO] in PPr-r-PPBP/PIL-z. The 5% weight loss temperatures of the composite membrane were 362oC for PPr-r-PPBP1.0:3.0/PIL-60 and 344oC for PPr-r-PPBP1.0:3.0-PIL30, which decreased the thermal stability compared with the copolymer before being composited with [dema][TfO]. Proton conductivity was evaluated in the non-humidified condition from 50oC to 100oC. The proton conductivity increased with increasing the content of [dema][TfO]. The proton conductivity of PPr-r-PPBP1.0:3.0-PIL60 was 4.3 × 10-2 S cm- 1 at 100oC, which showed sufficient proton conductivity as electrolyte membranes for the use of high-temperature and non-humidified operation.