Fabrication of soft-etched nanoporous polyimide membranes for ionic conduction and discrimination

Abstract

Ionic selectivity in nanopores is usually based either on steric or charge exclusion mechanisms. By simultaneously incorporating both mechanisms into a functionalized membrane, an improved control over selectivity can be achieved. We describe the fabrication and experimental characterization of alkali metal cation-selective nanopores in heavy ion-tracked polyimide (PI) membranes using the soft-etching (SE) technique. The latent ion tracks in the PI membrane are selectively dissolved by an organic solvent to form tiny pores without affecting the bulk material. The ionic transport properties of SE-PI membranes are characterized using different electrolyte solutions containing alkali metals, divalent metals, and organic cations under symmetric and asymmetric electrolyte conditions. Under symmetric conditions, the pores exhibit ohmic behavior when exposed to alkali metal chlorides and ammonium chloride solutions while divalent cations cannot pass through the pores as evidenced from the current-voltage curves. For the case of asymmetric electrolyte conditions, current rectification suggests pore blockage from the side of membrane exposed to divalent cations and tetraalkylammonium (TAA) cations. The experimental data show that the SE-PI membranes efficiently discriminate alkali cations from divalent metal cations and ammonium cation from TAA cations. The ionic conduction of the membranes is also sensitive to the mole fraction of Ca2+ in multi-ionic solutions. The ionic transport experiments further confirm that the nanopores allow significant alkali cation fluxes while rejecting divalent cations. Based on the good stability and high selectivity, the solvent treated PI membranes constitute remarkable candidates to be employed in applications concerning a wide range of electrolyte solutions.