The porous structure of the fully-aromatic polyamide film in reverse osmosis membranes

• Revealed complex porous structure in the polyamide layer of RO membrane.
• The dense layer forming the polyamide top surface is the key barrier layer.
• Proposed a new structure model of FAPA film in RO membrane.
• Performed “inverse” interfacial polymerization with MPD and TMC.
• Proved the correctness of Cadotte׳s assumption on pores׳ forming mechanism.

In this study, the porous structures of two commercial fully-aromatic polyamide (FAPA) reverse osmosis (RO) membranes as well as homemade RO membranes were thoroughly characterized and revealed by using SEM and TEM. With a designed “inverse” interfacial polymerization experiment, it was confirmed that pores always form on the surface of the FAPA film facing the aqueous phase. Based on the SEM observation on the cross-section of FAPA film and the TEM results, a new structure model of FAPA film in RO membrane was proposed. The dense layer of the flat area and the dense layer near the top surface of the protuberances and ridges connect together to form a complete dense discriminating layer as the key barrier layer of the RO membrane. Protuberances and ridges have irregular interconnecting cavities and tunnels, through which the dense layer and the pores on the back surface of the FAPA film are connected. All these findings proved the correctness of Cadotte׳s assumption about the pores׳ forming mechanism during interfacial polymerization process.