Transport Mechanism of Water Molecules Passing through Polyamide/Cof Mixed Matrix Membranes

The mixed matrix membranes (MMMs) have gained a great deal of attention due to their higher water permeability without cost of salt rejection. However, the mechanism of water transporting through MMMs is still controversial. Herein, a kind of two dimensional covalent organic framework (COF), TpPa-1, was selected as blending additives mixed with polyamide (PA) due to its good compatibility with polymeric matrices. We performed nonequilibrium molecular dynamics simulations to investigate the transport properties of water molecules passing through MMMs. The permeance of MMMs is obviously enhanced compared to the pure PA membrane. However, such permeance enhancement does not originate from the intrinsic pore of COFs. The PA atoms confined in pores have extremely low diffusivities, the water molecules are thus trapped inside COF pores. Outside COFs pores, there is an interfacial region between COFs and the PA, in which the water molecules aggregates to form a high-density region. In this region, the average flow velocities of water molecules are much higher than those in pure PA or PA outside the mixed region of MMMs. The water transport mechanism discovered in this work offers an alternative and more likely explanation for the high permeance results observed in PA/COF MMMs, and these understandings can be helpful to design more MMMs.