Membrane characterization via evapoporometry (EP) and liquid-liquid displacement porosimetry (LLDP) techniques

To comparatively assess the Evapoporometry (EP) technique vis-a-vis the Liquid-Liquid Displacement Porosimetry (LLDP) technique, the pore size distributions, mean pore diameters (d(avg)) and porosities of five polymeric (namely, nylon, PES, PTFE, PET and PVDF) and one inorganic (namely, alumina) UF/tight MF membranes were quantified by both techniques. For all the membranes, the pore size ranges were generally narrower and the pore size distributions had distinctive peaks for the LLDP technique. For the nylon, PES and PTFE membranes, the d(avg) values obtained from the two techniques agreed well. However, for the PET and PVDF membranes, the differences were twofold due to the higher pressure needed for the LLDP tests. Specifically, for PET, the d(avg) value obtained via EP was half that via LLDP, because the higher pressure compacted the lower mechanical strength polymer, leading to pore closure. On the other hand, for PVDF, due to the rubber nature, the higher pressure caused the pores to be stretched, leading to larger pores. As for the alumina membrane, because of the more ideal cylindrical pores, the d(4)-weighting of the LLDP measurement gave a greater d(avg) value than that of the d(2)-weighting of the EP measurement. Also, porosity measurements were erroneous for LLDP if the active layer cannot be precisely quantified. With respect to MWCO, while EP does not explicitly quantify this, the LLDP generally over-estimated the values, because of the errors associated with the measurement of the first (largest) pores at the lowest pressures.