Phosphonium-Based Poly(ionic liquid)/Ionic Liquid Ion Gel Membranes: Influence of Structure and Ionic Liquid Loading on Ion Conductivity and Light Gas Separation Performance

The influence of free IL structure and loading level in phosphonium-based poly(ionic liquid)/ionic liquid (PIL/IL) ion gel composite membranes was studied with regards to their ion conductivity and light gas separation performance. The resulting series of ion gels demonstrated ion conductivity values of 10–7–10–5 S/cm at 25 °C and 10–5–10–3 S/cm at 110 °C. It was found that shorter alkyl chain lengths on the phosphonium cation of the IL resulted in increased thermal stability of the ionic conductivity for the resulting PIL/IL membrane materials with negligible changes in ionic conductivity performance at temperatures up to 110 °C. Further, an increase of free IL loading resulted in an increase in overall ion conductivity and membrane mechanical stability. Similar structure/composition/function relationships were observed for the light gas separation performance of these phosphonium-based PIL/IL ion gel membranes. However, the resulting performance is lower than PIL/IL ion gel membranes based on imidazolium cations reported in the literature.