Exploration of Permeation Resistance Change of the Polyamide Nanofiltration Membrane during Heat Curing by Using Organic Molecules as Functional Fillers

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, alpha-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.