Impact of Solvent Activation Before Heat Treatment on the Structure of Aromatic Polyamide Reverse Osmosis Membrane

Solvent activation or solvent rinse of aromatic polyamide reverse osmosis membrane before heat treatment typically improves membrane permeance. Generally，during solvent activation before heat treatment，the residual polyamide oligomers in the polyamide layer dissolve in the solvent，which can swell and rearrange the polyamide layer，thus changing the structure and performance of the membrane. However，the solvent used for solvent activation before heat treatment is typically a non-polar solvent whose solubility parameter is substantially different from aromatic polyamide. Consequently，the swelling and dissolution of the aromatic polyamide layer should be weak with these non-polar solvents；thus，the change in the membrane structure caused by solvent activation before heat treatment cannot be completely attributed to the interaction between the non-polar solvent and membrane. Recent studies have shown that residues in the aromatic polyamide layer can be dissolved in water. The solvent activation before heat treatment may affect the dissolution of residues in water and later change the structure of the membrane. To further explore how solvent activation before heat treatment affects the membrane structure and properties，the cases before and after water immersion were compared. The ultraviolet-visible spectra of the substances dissolved in water from the activated and inactivated membranes were analyzed，and their structures and properties were compared. The results indicate that numerous water-soluble hydrophilic residues remained in the aromatic polyamide layer after heat treatment. Solvent activation of the membrane with hexane before heat treatment removed unreacted monomers and oligomers，weakening further the cross-linking of the polyamide layer during the heat treatment and accelerating the dissolution of residues in the aromatic polyamide layer in water. Therefore，the thickness，roughness，and polyamide content of the separation layer of the activated membrane are reduced，and the membrane permeance is improved. In this study，the effect of solvent activation before heat treatment on the structure and desalination performance of the aromatic polyamide membrane is further elucidated. © 2023 Tianjin University. All rights reserved.