(594e) Positively Charged Membranes with Flexible Side Chains to Improve Conductivity

Positively charged membranes have superior selectivity of anions, anti-fouling ability and water flux due to Donnan effect and hydrophilicity, therefore exhibit promising applications as separation membrane for food and bioprocessing. Ion conductivity, though is an important indicator of Donnan effect, usually shows low values in most charged membranes with aromatic polymer materials because of the rigid benzene ring in backbone and short linkage with the quaternized functional groups. Here in this work, two types of flexible side chains with pendent quaternized cations are developed to promote hydrogen bond networks and flexible aggregation of ionic clusters in the charged membranes and thus improve ion conductivity. By means of molecular dynamic simulation, both the N-spirocyclic side chain with -NH-(CH₂)_x-NH- spacer and the tri-cations with -O- spacer could induce hydrogen bond networks with water, and the latter side chain could also form hydrogen bond networks within the PBI polymer backbone, which will enhance hydrophilicity and broaden the hydrophilic pathways for selected ion conduction. The trade-off between diffusion coefficient and solubility parameter demonstrates well of the effect of -NH-(CH₂)_x-NH- spacer length on the aggregation of ionic clusters, which goes a step further from literatures by merely experimental investigation for the optimal spacer length. TEM and SAXS investigations exhibit good hydrophilic-hydrophobic microphase separated structure in the positively charged membranes with both kinds of functional side chains. High ion conductivity of above 100 mS cm^-1 at 80 °C is achieved with around 90 % conductivity retained after concentrated KOH treatment for over 600 h. This work provides a universal method to prepare positively charged membranes with high ion conductivity.