Surface pore structures of micro- and ultrafiltration membranes imaged with the atomic force microscope

Abstract

In this paper we present images of the surface of various microfiltration (MF) and ultrafiltration (UF) membranes obtained with an atomic force microscope (AFM) operating in air. The AFM works with an optical lever system and uses microfabricated cantilevers to minimize forces between tip and membrane surface. Three track-etched MF membranes with pores between 100 nm and 10 nm were used to demonstrate that the AFM can image openings in this range of size. The influence of the tip shape on the measured pore structure is discussed. Eight UF membranes with cutoff between 5,000 and 100,000 show wide variations in surface roughness, size and shape of pores, and homogenity. Most surfaces were found to have a network-like fine structure, in good agreement with field emission scanning electron micrographs. Surface pore characteristics like pore diameter, mean pore distance and pore density have been determined from image analysis. Pore sizes of the UF membranes were found to be between 7.2 and 36.6 nm, pore densities range from 88 to 482 pores/μm 2 and are much higher than for the MF membranes. The porosity is less than 7% for both MF and UF membranes.