Actuatable Membranes based on Polypyrrole-Coated Vertically Aligned Nanostructures

Abstract

Nanoporous membranes are an enabling technology in a wide variety of applications because of their ability to efficiently and selectively separate molecules. A great deal of effort is concentrated on developing methods of externally controlling membrane selectivity and on integrating the membranes within multi-scale systems. In this dissertation, synthetic nanoporous membranes that fit the described needs are constructed from vertically aligned nanostructures. Vertically aligned carbon nanofibers and anisotropically etched silicon posts are aligned perpendicular to the substrate and act as obstacles to material flow parallel to the surface. The distances between the outer edges of the nanostructures define the pores of the membranes. Transport through the membranes is controlled by physically selecting species as they pass between the vertically aligned nanostructures. Membrane properties such as permeability and porosity are specified by defining the spatial locations of the membrane components. Subsequent physical and chemical modification of the nanostructures enables further tuning of pore sizes and opens up new methods to controllably modulate the permeability of the membranes. In this dissertation, permeability is externally controlled by electrochemical actuation of the conductive polymer, polypyrrole. Vertically aligned membrane components are coated with the actuatable polymer. Upon electrochemical reduction, the polypyrrole coatings swell in volume, increasing the diameters of the membrane components and decreasing the pore sizes of the membranes. Modulating the physical size of the membrane pores enables size selective transport of species and gating of the nanoscale pores