Incorporating hyperbranched polyester into cross-linked polyamide layer to enhance both permeability and selectivity of nanofiltration membrane

To prepare thin film composite (TFC) nanofiltration (NF) membranes with improved flux and rejection performance at low pressure, hyperbranched polyester (HPE) was selected as the nano-sized macromonomer and successfully incorporated into the skin layers on PVC hollow fiber substrates via interfacial polymerization between trimesoyl chloride and the mixtures of hydroxyl-terminated HPE/piperazine (PIP). The resultant NF membranes were characterized using ATR-FTIR, FESEM, AFM and surface zeta potential, while their performance was evaluated in terms of permeate flux and rejection rates of different inorganic salts and neutral solutes. When the HPE content was lower than 60 wt% in HPE/PIP mixtures, the increased membrane surface roughness and the HPE embedded in skin layer effectively enhanced the permeate flux without sacrificing salt rejection for the prepared TFC NF membranes. Further increasing HPE content in HPE/PIP mixtures yielded looser polyamide skin layer, which led to the decline of solute rejection. Due to the larger size and good interfacial compatibility with cross-linked polyamide matrix, the incorporated HPE could exist stably in the skin layer. And thus, the prepared TFC NF membranes showed good flux and rejection stability in a long-term filtration process.