Heterogeneous anion-selective membranes: Influence of a water-soluble component in the membrane on the morphology and ionic conductivity

Heterogeneous anion-selective membranes were prepared by blending anion-exchange particles, linear polyethylene, low density polyethylene or poly(ethylene-co-methacrylic acid) and a water-soluble additive poly(ethylene glycol-ran-propylene glycol) at 140 °C with subsequent compression moulding into the shape of a flat membrane. SEM micrographs showed that the membranes based on linear polyethylene or low density polyethylene had voids around the anion-exchange particles, while the membranes comprising poly(ethylene-co-methacrylic acid) had the particles completely encapsulated with the matrix polymer. The membranes with the voids around the particles were ion conductive, whereas the membranes with encapsulated particles were nonconductive. The presence of the water-soluble additive in the system resulted after extraction with water in a microporous membrane skin and in a considerable increase in conductivity. The conductivity of the membrane consisting (before extraction) of 66 wt.% anion-exchange particles, 3.4 wt.% water-soluble additive and 30.6 wt.% linear polyethylene was (70 °C) 7.1 S/m, whereas the conductivity of a similar membrane prepared without the water-soluble additive was only 3.5 S/m.

► Heterogeneous membrane conductivity depends on the structures of skin and core.
► A water-soluble additive increases the porosity of the skin.
► Conductivity is increased with the increased porosity of the skin.