National Institute of Standards and Technology,
Keywords: Polyamide Membranes, desalination
Summary:The state-of-the-art membranes for desalination are comprised of thin film composites where the permselective layer is a thin but highly crosslinked, interfacially polymerized polyamide. Although effective, the rapid polymerization rate and reaction conditions produce films with rough surface structures and chemical heterogeneity, which precludes some advanced characterization techniques. In 2011, we proposed a paradigm shift in how these types of membranes are fabricated, where the selective layer is created layer-by-layer through a reactive deposition process. By doing so, we can create membranes that are smooth, tailorable, and exceptionally thin (10’s nm). In this presentation, I will highlight the development of our automated deposition system for making tailored model polyamide membranes and present recent measurements of performance of these membranes through dead-end filtration. I will also present some recent advanced measurements of network structure and dynamics on these types of membranes that challenge our understanding of the operative transport mechanism in these materials. In addition to employing model membrane materials, we have also studied a series of polyamide thin film composite (TFC) membranes that exhibit variations in desalination performance. Here, we use solid-state NMR to quantify purity and composition of the polyamide selective layers after harvesting from the TFC membranes. We show that the carbonyl 13C intensity is useful for quantifying composition in these samples, allowing acid:amine monomer ratios to be determined. Structural characterization of these samples via grazing incidence wide angle x-ray scattering (GI-WAXS) also provides insight into the packing efficiency of this series of polyamide membranes, and we correlated the packing efficiency to the compositional and membrane performance data.