Letter Cite This: ACS Macro Lett. 2018, 7, 927−932
pubs.acs.org/macroletters
Probing the Internal Microstructure of Polyamide Thin-Film Composite Membranes Using Resonant Soft X‑ray Scattering Tyler E. Culp,† Dan Ye,† Mou Paul,‡ Abhishek Roy,‡ Michael J. Behr,§ Steve Jons,‡ Steve Rosenberg,§ Cheng Wang,∥ Esther W. Gomez,†,⊥ Manish Kumar,† and Enrique D. Gomez*,†,#
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Department of Chemical Engineering, ⊥Department of Biomedical Engineering, and #Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ Dow Water and Process Solutions, Edina, Minnesota 55439, United States § Analytical Sciences, The Dow Chemical Company, Midland, Michigan 48667, United States ∥ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: Characterization of the internal morphology of thin film composite membranes used in reverse osmosis (RO) is a prerequisite for understanding the connection between microstructure and water transport properties and is necessary for the design of membranes with improved performance. Here, we examine a series of fully aromatic polyamide active layers of RO membranes that vary in crosslinking using a combination of resonant soft X-ray scattering (RSoXS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Analysis of RSoXS profiles reveals a correlation between membrane structure and crosslinking density. Through a combination of scattering contrast calculations, TEM, and AFM micrographs, we assign the dominant contribution to RSoXS data as either surface roughness or chemical heterogeneity, depending on the X-ray energy used. Altogether, our results demonstrate the utility of soft X-ray scattering to examine the microstructure of water filtration membranes.
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layer.4 As a result of the interfacial polymerization reaction, a rough outer surface, commonly known as a ridge-and-valley structure, is formed for RO membranes.5 Despite the vast commercial relevance and availability of RO membranes, little quantitative information about the internal structure of the active layer exists.6−17 Small angle X-ray scattering (SAXS) has been applied to study the polyamide active layer, where the film was broken up into particles and suspended in water to enable a long enough path length (∼1 mm) for experiments with 12 keV photons.18 Scattering profiles were interpreted in terms of globular features with a few hundred nanometers in size, but the sample preparation procedures may have affected the microstructure. Ideally, X-ray scattering experiments would take place in a transmission geometry of free-standing polyamide films. Film thicknesses near 200 nm, however, prohibit transmission scattering experiments with X-rays near 10 keV due to insufficient scattering.19 Although scattering experiments in the “soft” Xray regime (
