Synthesis of soluble copolymers bearing ionic graft for alkaline anion exchange membrane

Abstract

In this study, we report firstly the synthesis of a soluble quarternary ammonium salt and the corresponding graft copolymer P(VDF-g-QVBC) (poly(vinylidene fluoride-graft-quaternary ammonium-functionalized 4-vinylbenzyl chloride)), via atom transfer radical polymerization (ATRP) of QVBC from a PVDF macroinitiator. ¹H NMR spectra indicate the successful synthesis of a series of copolymers with different graft ratios by varying the amount of monomers. The soluble quarternary ammonium-containing copolymers are composed of hydrophobic backbones and hydrophilic graft chains, which allow for favorable nano-scale phase separation during the solvent casting process. As a consequence, the resultant membrane exhibits excellent ionic conductivity as high as 45 mS cm⁻¹, which suggests its potential application in alkaline anion exchange membrane fuel cells. In addition, semi-crystalline nature of PVDF macromolecules imparts the graft membrane excellent thermal and mechanical stability which are required for fuel cell application. Different from available studies of AEMs based on insoluble quarternary ammonium-containing copolymers, this study served as a model system for improving hydroxide conductivity from the viewpoint of self-generated nano-phase separation, and gives pioneering understanding of the morphology–property relationship in novel graft copolymer AEMs.