Vacuum-assisted diamine monomer distribution for synthesizing polyamide composite membranes by interfacial polymerization

Polyamide composite membranes are still required to promote their structure uniformity and desalination performance. Herein, a vacuum-assisted strategy was used to deal with this challenge by improving the distribution homogeneity of diamine monomer in porous substrates for typical interfacial polymerization. Laser scanning confocal microscope analyses demonstrate that the piperazine solution can be adsorbed in the microporous substrates with a very homogeneous and stable way. This uniform diamine distribution shows great advantages for synthesizing the selective layers of polyamide on the substrate surfaces via the interfacial polymerization of piperazine and trimesoyl chloride. The as-prepared polyamide layers with a circular size of 30 cm exhibit uniform structures including thickness less than 20 nm, cross-linking degree up to 100%, surface roughness lower than 20 nm and pore diameter about 0.41 nm. Therefore, the polyamide composite membranes present nanofiltration performances of ultrahigh rejection above 99.6% to Na2SO4 and high water permeability of 20 L/m(2) h bar. These membranes also show a promising selectivity for NaCl/Na2SO4 up to 196, which is higher than most of the commercial and the reported nanofiltration membranes in recent years.