Enhanced RO Performance of Polyamide Bi-Layer Membranes Prepared by Sequential Interfacial Polymerization

A novel thin film composite (TFC) membrane having polyamide bilayer on a porous polysulfone support was prepared by a process we call sequential interfacial polymerization. A polyamide membrane prepared by interfacial polymerization of m-phenylenediamine (MPD) in water with trimesoly chlorides (TMS) in hexane was subsequently immersed into an aqueous basic solution of hexafluoroalcohol (HFA)-containing aromatic diamine (HFA-MDA) to form HFA-substituted aromatic polyamide top-layer (HFAPA) on a conventional polyamide under-layer (REFPA). Water contact angle measurement indicated that the surface of membrane becomes much more hydrophobic (140 ^o) after forming the second layer, HFAPA, onto REFPA (78 ^o) although cross-sectional SEM/TEM images showed no noticeable enhancement in the film thickness. The HFAPA-REFPA bilayer membrane with hydrophobic surface nature have showed significant improvement in salt rejection (ca. 50 % reduction in salt passage) with very minor loss in water flux (ca. 8 % reduction) compared to REFPA, resulting in excellent combination of salt rejection and water flux. Efficient removal of toxic substances such as arsenic and boron was also demonstrated. This new approach to fabricate layered polyamide membranes resulted in the synergy of the characteristic properties of two layers, thus we think it has promise as a new design strategy for improving membrane performance.  In the presentation, we will discuss the rational for the performance enhancement.