Molecular dynamic simulations of pressure-driven water transport through polyamide nanofiltration membranes at different membrane densities

Abstract

In the real membrane manufacture of polyamide (PA) thin-film composite membranes, the membrane density is unevenly distributed in the PA active layer, and the membrane performance may be influenced by the membrane density. Due to the limitation of traditional experimental approaches, molecular dynamics (MD) simulations are carried out to investigate the effect of PA density on membrane structures and transport properties. The amorphous aromatic PA membranes studied in this work were modeled as nanofiltration (NF) membranes, and the water flows passing through the membranes were driven by 1.0 to 100 MPa pressure difference. Each membrane was a cube composed of several linear PA polymeric molecules, and the number of linear PA polymers in the cube was varied to give different densities. The simulation results show that the fractional free-volume and average free-volume pore size of the PA membrane both decrease with increasing membrane density, leading to a decrease in water permeability. When the membrane is denser than 1368 kg m⁻³, the membrane is nonporous and the water flow is blocked even at 100 MPa pressure difference. The structures and the transport properties of the modeled PA membranes are compared with the PA top layer of a commercial NF membrane; the FilmTec® NF 90 membrane. This comparison shows that the simulation results of the modeled PA membrane are similar to the experimental results of the real NF 90 membrane. The MD simulations are thus feasible as a means of studying pressure-driven water transport through an amorphous polymeric membrane on a molecular scale, and provide a direct way to reveal the relationship between the free-volume property of the polymeric membrane and the pressure-driven water transport.