Charting the quantitative relationship between two-dimensional morphology parameters of polyamide membranes and synthesis conditions

Abstract

Polyamide membranes serve as the active layer in thin-film composites used for nanofiltration and reverse osmosis, and their surface morphology strongly impacts separation performance. However, because these surface morphologies are highly irregular and heterogeneous, linking morphology parameters to membrane synthesis conditions quantitatively is challenging. Here we utilize a quantitative morphometry approach, together with the surface feature classification scheme reported in our earlier work, to image and analyse the surface morphologies of polyamide membranes synthesized with a range of monomer concentrations. From transmission electron micrographs of polyamide membranes, we measure projected morphology parameters of “dome” and “dimple” crumples in the membrane, including surface curvature, Feret dimensions, thickness, circularity, perimeter, and area. All features except circularity, which remains constant, exhibit opposite trends when charted against the concentrations of m-phenylene diamine or trimesoyl chloride monomers used in synthesis suggesting competing roles of these two monomers in shaping crumples. Surprisingly, mathematical fittings (linear, logarithmic, or exponential) relate these morphology parameters quantitatively to the monomer concentration ratio, despite the apparent irregularity of crumples. Our highly quantitative approach sheds insight into predictive design of membrane materials with desirable properties.