Importance of Variability in Surface Morphology of Polyamide Selective Layers on Thin-Film Composite Membranes

The polyamide active layer of thin-film composite membranes critically influences performance in reverse osmosis, forward osmosis, and pressure retarded osmosis. The dependence of membrane perm-selectivity and fouling propensity on polyamide layer structure is an active area of research. Most of this work assumes that polyamide morphology is uniform across a given membrane sample. A small area (~0.1 – 1 mm²) is characterized via scanning electron microscopy or atomic force microscopy, and the results are taken to represent the entire membrane surface.

To evaluate this assumption, we developed a method for capturing the variability in polyamide morphology across larger areas (~0.1 to >1 cm²), and we applied it to three thin-film composite membranes: DOW SW30HR, DOW XLE, and a hand-cast seawater reverse osmosis membrane. Significant variability in morphology exists in all three membranes, and we demonstrate the influence of this variability on membrane characterization data through its strong correlation with contact angle measurements. Further, these membranes all contain portions of smooth morphology that are drastically different from what is considered ‘typical’ ridge-and-valley polyamide. These regions of smooth morphology were isolated and their perm-selectivity was compared with that of ‘typical’ polyamide. In addition, the roughness of each distinct morphology was quantified via atomic force microscopy. These data were combined with results from quantification of surface morphology variability to obtain a weighted-average roughness for a given membrane surface area that accounts for local variability in polyamide morphology. This method for more representative characterization of polyamide surface structure will support robust investigation of structure – performance relationships for polyamide active layers.