Study of Polyphenylene Oxide Membranes Containing Long and Short Alkyl Side Chains

Investigation on the subject of structure and chemical stability was carried out on two anion exchange membranes that have a great potential for being candidates in alkaline fuel cells. Both of the membranes contain poly(2,6-dimethylphenylene oxide) (PPO) backbone, one has benzyltrimethyl ammonium cation (BTMA) while the other has long (10 carbons) alkyl side chain pendant to the nitrogen-centered cation (C10) as can be seen in Scheme.  Membrane with the long alkyl side chain contributes to better hydrophilic–hydrophobic separation which significantly influence on the ionic channels of the membranes.Membrane morphology and in particular hydrophilic-hydrophobic separation was investigated using tapping-mode AFM. Both of the membranes exhibit phase-separated morphology and connectivity among the hydrophilic domains. AFM measurements show that long alkyl side chain contributed to better phase separation in the C10 membrane, where the hydrophilic domain size was ∼8 nm compared to ∼5 nm in BTMA. In addition, chemical stabilities of the membranes were evaluated under accelerated-aging conditions. Degradation process was determined by measuring FTIR, Elemental analysis and EDS during the aging. It was found that long side chain contributes to its chemical stability under basic and high temperature conditions.