The inhibiting water uptake mechanism of main-chain type N-spirocyclic quaternary ammonium ionene blended with polybenzimidazole as anion exchange membrane

Abstract

Molecular dynamics (MD) and density functional theory (DFT) were utilized to evaluate the effect of deprotonation degree and flexibility of polybenzimidazole (PBI) on the inhibiting water uptake mechanism of main-chain type N-spirocyclic quaternary ammonium ionene (SI) as anion exchange membranes (AEMs). Results show that the electrostatic interactions play a key role in inhibiting water uptake of the blend membranes, which is mainly originated from the N+ of SI and the N- of deprotonated PBI and partly from the inter-molecular weak hydrogen bonding C-H···N and C-H···π. The Eele between SI and PBI of SI/NP-50 and SI/NP-100 increases by 745.6% and 1291.8% compared with SI/NP-0 at λ=40, respectively, which exhibits the PBI deprotonation enhances the electrostatic interactions. And the Eele between SI and PBI increases as the flexibility of PBI decreases. Furthermore, the π-π stacking interactions contribute to the inhibiting water uptake of SI AEMs. The AEM of SI blended with low flexible poly[2,2′-(1,4-naphthalene)-5,5′-bibenzimidazoles] (NPBI) exhibits excellent dimensional stability due to strong electrostatic interactions. Our work deepens the understanding of the inhibiting water uptake mechanism of SI blended with PBI as AEMs from a microscopic viewpoint, and provides guidance for the selection of PBI structure.