Polyamide Reverse Osmosis Membrane Compaction and Relaxation: Mechanisms and Implications for Desalination Performance

This work investigates the compaction and relaxation behavior of composite reverse osmosis (RO) polyamide (PA) selective layers, utilizing non-equilibrium molecular dynamic (NEMD) simulations and well-controlled permeation experiments. Composite PA-RO membranes are prepared by interfacial polymerization of different amine monomer ratios to vary PA film crosslinking degree (CD). Wet-testing results suggest that “tighter” (more highly crosslinked) composite RO membranes with a higher PA layer CD undergo 65% less compaction and recover 17% more of their initial permeability (termed “relaxation”) when the pressure is relieved compared to lower CD PA layers. NEMD simulations provide a visual and quantitative characterization of PA layer’s free volume changes in operando. NEMD simulations also corroborate experimental findings and elucidate the viscoelastic properties of the PA layer that govern compaction and relaxation behavior. The mechanisms of compaction under water permeation derived from NEMD simulations further support the notion of viscous flow of water through interconnected free volume elements (i.e., “pores”) within crosslinked PA films.