In Situ Ptychographic X-ray Computed Tomography of Fully Hydrated Polyamide Membranes

Understanding the morphology of membranes under operational conditions is essential for advancing membrane material development for water purification and industrial applications. Traditional characterization methods, such as scanning and transmission electron microscopy, require vacuum environments and sample modifications that could alter membrane structures susceptible to hydration and swelling. Here, we present the first application of synchrotron-based ptychographic X-ray computed tomography (PXCT) for direct in situ nanoscale 3D imaging of hydrated thin-film composite polyamide membranes immersed in water. This method requires no staining, vitrification, or layer separation from the porous support by exposure to organic solvents. It preserves the polyamide membrane's native state. We studied lab-fabricated membranes containing 2-hydroxy-N-(diphenylmethyl) acetamide artificial water channels, revealing significant hydration-induced changes in structure, including up to 51% volume swelling, and increased surface roughness, attributed to the presence of artificial water channels in the polyamide matrix. Morphological features of the ridge-and-valley surface, such as bowl-shaped nodule structures, notably expanded in water. PXCT data were compared to atomic force microscopy measurements, highlighting both the capabilities and limitations of each technique. The in situ PXCT method demonstrated here offers invaluable insight into the hydrated polyamide membrane morphology. The demonstrated high-resolution PXCT method can be applied across disciplines in materials science and life sciences, enabling a detailed analysis of hydrated nanostructures under realistic conditions.