The Measurement of Membrane Filter Pore Size by a Gas Permeability Technique

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Abstract
A differential low pressure gas flow technique has been developed for the measurement of the mean pore size of membrane filters. The method is here applied to the pore size determination in a range of commercial micro-porous polymer films with pore diameters ranging from 0.03 to 0.8 μm (‘Nuclepore’). The polymer films were also examined using scanning electron microscopy so that structural and physical parameters could be evaluated. Two flow regimes could be demonstrated for the permeability data with clear separation between diffusional (Knudsen) flow and viscous or transitional flow occurring at a pore size of 0.2 μm. Alternative theoretical treatments are required for the two regimes but mean pore diameters could be calculated with reasonable precision and agreed with the results obtained by microscopy. In the case of viscous flow it is necessary to assume that the membranes have a tortuosity of less than unity and this is shown to be justified from a consideration of the structure of the pore in the plastic film.