(595d) 3D Analysis of Nanostructured Polyamide Membranes Using Quantitative Electron Tomography

State-of-the-art reverse osmosis, nanofiltration, and desalination membranes are constructed as a thin film composite: they consist of crosslinked polyamide selective layer with an extremely thin thickness (i.e., <100 nm). These thin film membranes are generally prepared via interfacial polymerization between aromatic or aliphatic diamine and aromatic acyl chloride. In general, their performance of polyamide membranes is governed by their morphology. However, the interfacial polymerization process leads to highly irregular and complex crumpled nanostructures that make the quantitative measurement of their physiochemical properties extremely difficult. Here, we demonstrated 3D electron tomographic imaging and quantitative morphometry of polyamide films to understand their synthesis-structure-property relationship. We employed a series of interfacially polymerized membranes with varying monomers concentrations to determine how complex nanoscale morphology emerge from different synthesis conditions. We reconstructed the crumpled morphology in 3D with a voxel of 0.68 nm using low-dose electron tomography. Using our imaging-quantitative morphometry platform, we observed that the effective thickness films increased and the film structure became more crumpled with decreasing the concentration of an acyl chloride monomer and/or increasing that of a diamine monomer. We observed multimodal thicknesses in each reaction condition. This observation supported a crumpling mechanism in which uniform pieces of polyamide film are formed in the early stage of polymerization and then a local collapsing process serves as the basis for the emergence of 3D crumple structures. Furthermore, various parameters governing film transport properties, such as surface-to-volume ratio and mass-per-area, were measured directly from the reconstructed membrane structures. We conducted local curvature mapping and metal ion adsorption tests to understand how reaction condition affects the microenvironment of the crumples. We anticipate that this imaging−morphometry platform can be applicable to other nanoscale soft materials and provides engineering strategies based directly on synthesis−morphology−function relationships.