High flux reverse osmosis membranes fabricated with hyperbranched polymers via novel twice-crosslinked interfacial polymerization method

In this study, two kinds of carboxyl-terminated hyperbranched polyesters (HBPs) with special three-dimensional structure were proposed to act as the substrates to synthesize two new kinds of acyl chloride-terminated hyperbranched polyesters (HBPACs) that were used as the key functional materials to participate in the interfacial polymerization reaction to fabricate the novel thin film composite membranes (TFC) with high water flux and monovalent salt rejection. First, the two HBPACs were synthesized from two HBPs with different molecule weights (Mn=3289 g/mol and 1964 g/mol) through acylation reaction with thionyl chloride. Then, the resultant HBPACs were used to modify the conventional reverse osmosis (RO) membrane by a new twice-crosslinked interfacial polymerization method. That is, the m-phenylenediamine (MPD) coated on the polysulfone supporting was successively reacted with HBPAC and trimesoyl chloride (TMC) which were solved in different organic solutions, respectively. The resultant HBPAC-based TFC membranes presented smoother surface and showed few change in surface hydrophilicity. Especially, one of the HBPAC-based membrane fabricated with the mixture of two HPBACs (a mass ratio of 1:1), showed a similar to 50% increase in water flux without sacrificing the salt rejection and exhibited excellent long-term stability compared with the conventional TFC RO membrane. Obviously, the HBPAC-based composite PA layer showed a certain cross-linking network, which not only maintained the salt retention rate but also enhanced the compatibility of HBPAC in PA main body, and as a result promoted the membrane stability. Furthermore, the residual carboxyl groups of HBPAC can act as hydrophilic sites between polyamide chains, which contribute to the rapid diffusion of water molecules through the PA layer of the membrane. Thus, this work provides a new approach to enhance the flux and stability of the membrane via the incorporation of new acyl chloride-terminated hyperbranched polymer.