Molecular orientation in polyamide reverse osmosis membranes revealed by polarized resonant soft x-ray scattering*

Despite their broad application for water desalination, little is known about the fundamental interplay of synthesis, structure, and water transport in polyamide thin film composite membranes. The active layers of these materials are produced by interfacial polymerization atop a porous support resulting in films of typical thickness 100 nm with > 50 nm RMS roughness, making characterization by conventional techniques extremely challenging. Using polarized resonant soft x-ray scattering, a uniquely sensitive probe of molecular orientation and structure, we observe a striking degree of polarization-induced anisotropy in both commercial and model polyamide films. This anisotropy is likely due to local molecular orientation of C=C and C=O pi* electron systems. We compare orientation distributions and optical constants from P-RSoXS with those obtained by angle-dependent NEXAFS and demonstrate a new strategy for signal enhancement of interfacially oriented scattering. This previously-unreported molecular orientation may play a key role in connecting the processing and structure in polyamide thin film composite membranes to their water transport mechanisms, enabling enhanced performance in these important devices.