Pressure-Induced Diffusion of Water in Dense Polymer Membranes: What is the Role of NEMD Simulations?

Penetrant transport through polymer membranes is generally described by two distinct models: solution-diffusion and pore-flow. The former describes transport in non-porous polymers, in which penetrants partition into the membrane phase and diffuse down the thermodynamic activity gradient, while the latter describes transport in porous polymers as convection through permanent pores induced by a pressure gradient. While the applicability of these models to certain applications is clear, the physical model for pressure-driven liquid permeation in solvent swollen polymers has been intensely debated. In recent years, non-equilibrium molecular dynamics (NEMD) simulations have been applied in support of both the solution diffusion and pore-flow mechanisms for reverse osmosis applications. In this study, we apply pressure-driven NEMD simulations to probe the mechanisms underlying water transport in crosslinked polyamide membranes. Our results suggest that evidence for both the solution-diffusion and pore-flow mechanisms may be observed depending on the methods applied to restrain the membrane to prevent diffusion under pressure. Therefore, a careful evaluation of the assumptions underlying both models, and the physical picture of reverse osmosis experiments is necessary to appropriately evaluate the transport of water in reverse osmosis using NEMD simulations.