The monomer concentration is a key factor affecting the diffusion rate and reaction rate of the monomers and determines the physicochemical property of polyamide nanofiltration membranes. However, the mechanism of the monomer concentration effect on the polyamide nanofiltration membrane remains unclear. The properties of nascent membranes fabricated with different monomer concentrations with a fixed ratio presented completely new results in the absence of heat treatment. The results reveal that the polyamide layer thickness increases linearly with the monomer concentration when the concentration is high. The nanofiltration membranes prepared at a low concentration have a larger pore size and a lower cross-linking degree, which contributed to high water permeability. The optimal membrane presented a high pure water permeability of 36.1 L h–1 m–2 bar–1, with the Na2SO4 rejection maintained at 98.4%. This work provides a novel insight into the relationship between the properties of the nanofiltration membrane and monomer concentration.

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Recent studies show that heat curing has a great influence on the performance of polyamide nanofiltration (NF) membranes. In this work, nascent polypiperazine-amide NF membranes were pretreated before heat curing to reveal the change in permeation resistance caused by heat curing. Aqueous solutions containing a series of organic molecules (glycerol, raffinose, α-cyclodextrin, and polyvinylpyrrolidone) with different molecular weights were used to treat the polypiperazine-amide NF membrane via different immersion approaches before heat curing. These organic molecules play a role in filling and alleviating the collapse and shrinkage of the membrane structure during heat curing and in further protecting the membrane from permeance loss. Furthermore, the organic molecules were used to regulate the shrinkage of the membrane structure in different immersion approaches to clarify the proportion of permeability change attributed to the shrinkage in different membrane layers during heat curing. Through a resistances-in-series model calculation, the permeation resistance of the polyamide layer was found to have decreased from 0.0523 to 0.0329 h bar m–1 after being pretreated with glycerol and that of the polyether sulfone substrate decreased from 0.1651 to 0.0033 h bar m–1. This work proves that protecting the ultrafiltration membrane from drying during heat curing is the key to maintaining the performance of the thin-film composite membrane. This research is instructive for a better understanding of the permeance loss of the NF membrane during heat curing and the optimization of the heat treatment procedure in industrial processes.