Effects of the N-alicyclic cation and backbone structures on the performance of poly(terphenyl)-based hydroxide exchange membranes

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Abstract

Hydroxide ion conducting poly(terphenyl alkylene)s functionalized with piperidine-based quaternary ammonium cations were synthesized via superacid-catalyzed polyhydroxyalkylations. By employing different synthetic strategies, we have systematically varied the structures of the cation and the backbone polymer to study the effects on morphology, stability and hydroxide conductivity. Two monomers were initially prepared by attaching 4-benzylpiperidine groups to trifluororacetophenone and m-terphenyl, respectively, through Suzuki coupling reactions. Polymerizations followed by quaternizations were then carried out to obtain poly(terphenyl alkylene)s with approximately the same ionic contents. These contained either m- or p-terphenyl backbone units, and were tethered with monocyclic N,N-dimethylpiperidinium (DMP) or spirocyclic 6-azonia-spiro[5,5]undecane-6-ium (ASU) cations placed on either the stiff terphenyl or the more flexible alkylene units along the backbone. Polymer chain flexibility and functionalization with DMP cations were found to promote ionic clustering and conductivity. Hence, a membrane based on a m-terphenyl backbone tethered with DMP on pendant phenyl groups achieveda hydroxide conductivity of 146 mS cm-1 at 80 oC. While the thermal stability was significantly higher forASU-functionalized HEMs, the alkaline stability was highest for the ones carrying DMP cations, which showed less than 5% ionic loss after 720 h in 2 M NaOH at 90 oC. After 168 h at 120 oC, 1H NMR analysis suggested that the DMP cation degraded by a combination of β-Hofmann elimination and methyl substitution. Overall, the results of the study demonstrated that the structural features of the present polymers provided high alkaline stability, most probably due to aryl ether-free backbones, and that all the β-protons of the DMP and ASU cations were placed in 6-membered rings. 

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  • Polymer Technologies
  • Polymer Chemistry
Original languageEnglish
Pages (from-to)15895-15906
Number of pages12
JournalJournal of Materials Chemistry A
Volume7
Issue number26
Publication statusPublished - 2019
Publication categoryResearch
Peer-reviewedYes

Bibliographic note

The article was received on 24 May 2019, accepted on 06 Jun 2019 and first published on 06 Jun 2019