Nanofabricated perforated membranes have great potential in molecular separation applications. The spatial structuring has been achieved by ion bombardment, self-assembly of block copolymers, replica molding using various kinds of templates such as surfactant arrays, etc. However, most of the porous materials produced by these methods have three-dimensional structures or thicknesses significantly larger than the pore sizes. Complicated three-dimensional pore structure and a comparatively large membrane thickness easily lead to the trapping or adsorption of particles and large molecules within the membranes, and increase the diffusion time of permeating substances. It is highly desirable to reduce the thickness of the porous material and to make it comparable to the size of the pores. This limitation can be addressed by a new class of ultrathin nanostructured membranes where the membrane is roughly as thick (10-20 nm) as the molecules being separated, but membrane fragility and complex fabrication have prevented the use of ultrathin membranes for molecular separations. In this study we demonstrate possibility of fabrication of ultrathin (thickness 20-30 nm) polymeric membranes with pore sizes comparable with membrane thickness using layer-by-layer (LbL) assembly on polymeric substrates.
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