Quantitative Nanoscale Structure Determination in Polymer Desalination Membranes by Correlated Electron Tomography and Spectroscopy

A complete and thorough understanding of the complicated heterogeneous structure of polyamide separation membranes is crucial to improving their performance. Electron tomography has been used to study density variations in dense polymer membranes; however, the nonuniformity of membrane thickness and surface morphology present major challenges to the accuracy of that method. In this article, we show that nanoscale 2D electron energy loss spectroscopy (EELS) maps can be correlated with 3D scanning transmission electron microscopy (STEM) tomography to improve the quantitative mapping of density. We reveal quantitative nanoscale structural differences between commercial seawater and brackish water polyamide thin film composite reverse osmosis membranes and compare them to thin uniform printed membranes. To reduce electron beam damage, we employ a high-speed direct electron detector for low-dose EELS, which allows for membrane thickness and electron scattering measurements to be spatially correlated to improve the measurement of density. We resolve nanoscale differences between three polyamide membranes which have distinctly different separation performances. Our work provides a framework for the use of STEM and EELS to extract heterogeneous density variations in structurally complex membranes.