Microscopy and Microanalysis of Reverse-Osmosis and Nanofiltration Membranes
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Microanalysis of Reverse-Osmosis and Nanofiltration Membranes
David G. Cahill, Viatcheslav Freger, and Seung-Yeop Kwak Abstract The polyamide active layers of commercial reverse-osmosis and nanofiltration membranes are examples of nanoscale functional materials that challenge the state of the art of materials characterization. The active layer is only ~100 nm thick, and because the active layer is formed by a process of interfacial polymerization, the structure and composition of the membrane is highly inhomogeneous. Even such basic physical and chemical properties of the membrane as the atomic density, swelling in water, distribution of charged species, and the mobility of water and ions, are poorly understood. In this article, we briefly review progress in the characterization of polyamide separation membranes using transmission electron microscopy, atomic force microscopy, vibrational spectroscopy, positron annihilation, nuclear magnetic resonance, and Rutherford backscattering spectrometry. Advances in the microanalysis methods applicable to these complex materials will advance fundamental understanding of the structure–property relationships of polymer membranes and further the long-term goal of synthesizing membranes with improved performance.
averaged composition and chemistry. Studies of the molecular motion and molecular-level free-volume characteristics of the thin-film polymers are enabled by solid-state nuclear magnetic resonance (NMR) spectroscopy and positron annihilation lifetime spectroscopy (PALS).
Interfacial Polymerization Since the work by Cadotte and coworkers,1,2 interfacial polymerization has been used to manufacture the polyamide top layers in most commercial RO and nanofiltration thin-film composite membranes.3 In this method, the film is formed in a very fast polycondensation reaction between two multifunctional monomers A and B at the interface between the fluid phases,4,5 as depicted in Figure 1. The remarkable success of interfacial polymerization is due to the unique combination of the self-limiting thickness and selfhealing character of the film formation, resulting in extremely thin (typically 15–300 nm) films with very few defects.3 Unlike coating approaches that produce uniform films of predetermined thickness, the characteristics of films spontaneously formed by interfacial polymerization are a complex function of the composition of the monomer solutions, reaction conditions, and reaction time.4,6,7 A recent analysis8,9 reveals that film formation proceeds in a succession of two kinetically distinct regimes. At the first stage, the polymer evolves non-uniformly inside a narrow and constantly shrinking reaction zone (incipient film), kinetically limited by monomer diffusion in a free solvent, until
Reaction zone (incipient film)
Aqueous phase soaking
Introduction Understanding the relationships between processing, structure, and properties of thin-film composite membranes used in reverse-osmosis (RO) and nanofiltration processes is hindered by limited knowledge of the chemica
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