Chemical and thermal stability of poly(phenylene oxide)-based anion exchange membranes containing alkyl side chains

The influence of an alkyl side chain attached to the nitrogen-centered cationic group on the properties and chemical stability of poly(phenylene oxide) based anion exchange membrane is the focus of this study. Two different cationic groups were investigated: one is the common benzyl trimethyl ammonium (BTMA) cationic group, and the other is benzyl dimethyldecyl ammonium with a C10 alkyl chain pendant to the cation-centered quaternary nitrogen. Swelling measurements were performed over a range of temperatures for both types of samples. Water uptake was found to be lower for the C10 (21 wt%) membrane than for the BTMA (53 wt%) membrane, due to the hydrophobicity of the long alkyl side chain and hydrophilic/hydrophobic phase separation. Degradation measurements under alkaline conditions indicated that the C10 membrane degraded more slowly than the BTMA sample at 60°C. At 80°C the C10 sample showed degradation likely due to the irreversible swelling and poor mechanical properties of the C10 membrane at high temperature due to the weakening of the poly(phenylene oxide) backbone interchain interactions by the C10 pendant group.