Understanding Aromatic Motif Structure and Filtration Relationships of Polyamide Barrier Layers in Reverse Osmosis Membranes

Reverse osmosis membranes include a thin, nanoporous polymer (polyamide) barrier layer that is responsible for separating ions from water in desalination processes. The polyamide barrier layer is typically formed through a chemical reaction process called interfacial polymerization. The molecular structure of the polyamide layer depends on the experimental conditions under which the interfacial polymerization occurs. Understanding how various process conditions influence the polyamide layer microstructure and, thus, the filtration performance can lead to the development of better reverse osmosis membrane materials. For example, more permeable polyamide barrier layers with an optimal degree of cross-linking and controlled microstructures have the potential to improve the permeance, maintain high selectivity, and reduce the costs of freshwater generation. Despite the technical importance of the barrier layer, the molecular level structure-property-process relationships have yet to fully emerge. This project integrates experimental and computational methods to describe how reacting monomers, reaction and processing conditions, and post-treatments change the structure of the polyamide thin films. The effects of ultrahigh-pressure operation on the polyamide layer molecular structure will also be evaluated. The project is expected to lead to the development of new energy-efficient nanostructured materials for water desalination, which will reduce the environmental and economic burden of reverse osmosis processes and mitigate the threat of global water scarcity. The objective of this research is to investigate fundamental issues, concerning atomic- and molecular-scale interfacial phenomena and engineering, in interfacially polymerized aromatic polyamide barrier layers of reverse osmosis membranes. Three research activities will be completed. Task 1 will investigate aromatic molecular motif structure and property relationships in polyamide thin films as functions of different reacting monomers and reaction and processing conditions in interfacial polymerization. Task 2 will test the hypothesis that certain solvents can more effectively influence the cross-linked structure of the polyamide scaffold, i.e., in a different manner between chain ends and backbone, thus creating a different water-channel distribution. Task 3 will characterize structural changes in polyamide barrier layers by ultrahigh-pressure reverse osmosis processes. A suite of experimental and computational approaches will be combined to reveal new insights into the anisotropic steric interactions between the functional aromatic rings and the formation of inhomogeneous cross-linked network structures. State-of-the-art characterization methods will be employed, including grazing-incident wide-angle and small-angle X-ray scattering and atomic pair distribution function using synchrotron radiations. The molecular modeling tasks will apply mesoscopic dissipative particle dynamics, Monte Carlo with stochastic reaction model, and atomistic molecular dynamic simulations methods. This project is also expected to reveal new approaches to produce energy-efficient interfacial polymerization barrier layers through solvent activation post-treatment and improve the desalination efficiency of ultrahigh-pressure reverse osmosis processes. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.