Regulating the aqueous phase monomer balance for flux improvement in polyamide thin film composite membranes
Polyamide thin film composite (PA TFC) membranes are synthesized from interfacial polymerization using two amines in the aqueous phase. The conventional monomer, mphenelynediamine (MPD), is partially replaced by a linear monomer, 1,3–diamino-2- hydroxypropane (DAHP). The water permeability of the membranes improves by around 22 % (to 2.67 ± 0.09 L.m-2.h-1.bar-1) while keeping the same high salt rejection (96-98%) at an optimum DAHP/MPD ratio of 12.8 %. While developing the control PA TFC membrane we introduce a washing step and show that the support surface should be free from surface protective coatings to achieve high water flux (2.18 ± 0.08 L.m-2.h-1.bar-1). Incorporating DAHP units into the polyamide network improves the water flux through the membranes fabricated on both original and washed supports. The surface morphologies of polyamide films change significantly with introduction of DAHP, from large ridge-and-valley structure to enlarged nodular structures. High resolution SEM images show an ultrathin polyamide thin film with a thickness that is reduced with addition of DAHP. These influences of DAHP, namely a reduction in the selective layer thickness, an alteration in surface morphology, changes in internal molecular packing and hydrophilicity, are suggested as factors behind the improved water permeability.