Abstract: This study aims to explore the structural characteristics of the inhomogeneous top layer of thin-film composite membranes when pretreated by different methods: room temperatura-oven, etanol-hexane in a solvent exchange process, and freeze-drying. An evaluation of the nano-order free-volume pore size of the polyamide samples was carried out by nanopermporometry (NPP) and was quantitatively compared with the free-volume pore estimated from normalized Knudsen-based permeance (NKP) and with positron annihilation characterization (PALS). NPP results denoted a bimodal polyamide membrane structure described by a dense matrix and highly permeable regions. The application of different condensable vapors (water, hexane, and isopropanol) resulted in a free-volume pore size smaller than dp = 0.6 nm for dense regions, which was confirmed after NKP and PALS. In addition, the influence of highly permeable regions on permeance decreased in the following order: ethanol?hexane > freeze-drying > room temperatura-oven samples, demonstrating an effective membrane structure alteration after different pretreatments.

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