Comparison Of Different Characterisation Techniques To Evaluate The Determination Of The Pore Size Of Polymeric Membranes

Keywords:Capillary Flow Porometry, polymeric membranes, pore size distribution, Scanning Electron Microscopy, Molecular Weight Cut-Off.

Abstract:

Multiple characterization techniques  are commonly used both in industry and in academic research to measure the pore size of membranes. This leads often to discussion and confusion about which technique provides the most relevant and actual information of porous materials. The aim of this work is to compare some of these characterization techniques to understand better the differences between them and to find a correlation between the results obtained.

The principle of measurement in Capillary Flow Porometry (CFP) is the displacement of a wetting liquid from the through pores of a sample by applying an inert gas at increasing pressure. The pressure (P) required to expel the wetting liquid is then used to calculate a pore diameter (D), based on the Young-Laplace equation P=4*γ/D, where (γ) is the surface tension at the interface gas and liquid. The diameter measured in CFP corresponds to the most constricted part of the pore, since this is the most challenging part and offers the highest resistance to evacuate the liquid. CFP is generally used for measuring the bubble point pressure, which is the largest through pore diameter in the membrane. Other results that can be obtained  are the minimum and mean flow pore size and the full pore size distribution.

In the present work CFP was used to characterize microfiltration and ultrafiltration polymeric membranes of polyethersulfone and  polyvinylidene fluoride, among others. These include  both commercial and research stage membranes used in aqueous and solvent filtration. The membranes were also characterized by Scanning Electron Microscopy (SEM) and Molecular Weight Cut-off (MWCO), based on the retention of Dextran and/or polyethylene glycol solutes of different molecular weights evaluated by gel permeation chromatography (GPC).  The results obtained by CFP, SEM and MWCO seem to be in good agreement, if the differences in the measured parameters by the different techniques are taken into account.

In conclusion, CFP appears to be a simple, non-destructive, rapid technique for the  accurate and reproducible measurement of both the bubble point pore-size and the full pore size distributions of through-pores in polymeric membranes. It makes it a very powerful tool for the selection of a suitable membrane for a particular application and for further membrane research and development.