Fabrication of an Inverse Size-Selective Membrane Using an Electrospun Nanofiber Mat as a Template.

Size-selective membranes have applications in water purification and other areas. Although many methods have been developed for the fabrication of nanoporous membranes, each method has its limitations. For example, thin films containing uniform nanochannels can be prepared from block copolymer precursors. However, block copolymers are expensive. The electrospinning of a polymer solution can yield long nanofibers that fold into mats and the diameters of these fibers can be tuned from nanometers to micrometers. Infusing another polymer into the voids between these fibers and subsequently removing the nanofiber template should yield an inverse porous membrane, complementary in pore structure to the original nanofiber mat membrane. In this paper, we report on the fabrication of these membranes. We discovered that the flux across such membranes increased by thermally annealing the nanofiber mats under pressure before the infusion of the second polymer and by etching the surfaces of the final membrane with plasma to expose the encapsulated nanochannels. We further discovered that the size of the pores formed at the junction of the fused nanofibers and, eventually, the nanotubes governed the size selectivity of the final membrane. The pore size at the junctions increased by increasing the thermal annealing temperature and, thus, the extent of fiber fusion. The developed methodology is general and should be useful for the fabrication of nanoporous membranes from different materials possessing pore diameters that are governed by the diameter of the templating nanofibers and the extent of their fusion.