The morphology of fully-aromatic polyamide separation layer and its relationship with separation performance of TFC membranes

TFC membranes with various morphologies of fully aromatic polyamide (PA) separation layer were prepared by just changing the monomer concentrations in a wide range. The morphologies of PA top surface, cross-section and back surface were systematically studied by using SEM and TEM. The PA layer thickness and its chemical structure were also characterized by using FTIR and XPS. A corresponding relationship of PA layer morphology was found, i.e. if there are obvious pores on the back surface of PA layer, there must be void structure on the PA cross-section and rough leaf-like structure on the PA top surface. The relationship between PA layer morphology and membrane separation performance, especially water flux, was elucidated. The porous structure on the back surface of PA layer is a necessary condition for TFC membranes to achieve high separation performance. For PA layer with back surface pores and single-layer void structure, its effective thickness is only the thickness of the top surface of PA layer. A three-stage formation mechanism of PA layer was proposed, which can explain the variation of pore size of PA back surface with the change of monomer concentration very well.