An attempt to generate anion exchange membranes by amination of the perfluorinated 3M precursor leads to the hydrolysis of the precursor

Abstract

We report attempts to synthesize and to study the stability of anion exchange membranes (AEMs) from the perfluorinated 3M membrane precursor having terminal sulfonyl fluoride, –SO2F, groups in the side chains of the backbone. The precursor was obtained in powder form from which flexible films were prepared using hot pressing; the films were reacted with trimethylamine (TMA), 1,4-dimethylpiperazine (DMP), and quaternized 1,4-diazabicyclo[2.2.2]octane (DABCO) amines, following published protocols. The chemical stability of the generated membranes was determined by FTIR, and by 1H and 19F NMR, first after the reaction between the precursor and the amines, and then after addition of KOH. The FTIR spectra indicated the presence of the C–H bands of the amines after reaction, and their disappearance after addition of KOH, a result consistent with 1H NMR spectra of the membranes swollen by deuterated dimethylsulfoxide (DMSO-d6). The 19F NMR spectra indicated the presence of SO3− anions, in the membranes after the initial amination reaction and also after addition of KOH. This deduction was consistent with the detection of the band at 1052cm−1 in the FTIR spectra of membranes generated after amination, as well as after the addition of KOH. The main conclusion is that no AEMs were generated by the reaction of the 3M precursor with TMA, DMP, and DABCO: The amines acted as catalysts for the hydrolysis of the sulfonyl fluorides and led to the generation of the SO3− anions. The conclusions of this study are expected to be valid for synthetic efforts to obtain AEMs starting with other sulfonyl fluoride polymers, such as the Nafion™ precursor.