Effect of free volume and formation mechanisms of polyamide layers on nanofiltration membrane

Amine monomers of different chemical structures and multiple reactive functional groups, namely tris(2-aminoethyl)amine (TAEA), triethylenetetramine (TETA), and 1,3-diaminopropane (DAP) was reacted with acyl chloride monomers such as 1,3,5-benzenetricarbonyl chloride (TMC) and suberoyl chloride (SC) to synthesize polyamide thin film composite membranes through interfacial polymerization. The effect of membrane formation mechanisms on nanofiltration was investigated. A variable monoenergy slow positron beam was used to explore the differences in the free volume diameters of polyamide layers composed of different chemical structure. The results revealed that the polymerization degree, free volume diameter, and selective layer thickness of the polyamide layers were strongly correlated with each other. Specifically, a high polymerization degree induced a small free volume diameter and a thin selective layer. The cross-linked TAEA-TMC polyamide layer exhibited a free volume diameter of 3.9 Å and was the thinnest (168 nm) among the synthesized membranes, whereas the linear DAP-SC polyamide layer was the thickest (873 nm) with a free volume diameter of 4.7 Å. Finally, a stability test on the TAEA-TMC/PAN showed a slight fluctuation during the 36 h filtration, salt rejection maintained stability at around 97% and water flux of 22.3 L m−2 h−1.