Microscopic characterization of the nanostructure of polyamide thin films in reverse osmosis and nanofiltration membranes [electronic resource]

The overall objective of this dissertation is to provide a thorough characterization of the nanostructure of polyamide thin films to achieve a better understanding of transport and rejection mechanisms in reverse osmosis (RO) and nanofiltration (NF) membranes. The active layer in state-of-the-art RO membranes consists of cross-linked networks of fully aromatic polyamide, with an overall average thickness of approximately 200 nm and a heterogeneous structure that confers the membrane a relatively rough surface described in the field as the ridge-and-valley structure (root-mean-square (RMS) roughness of about 100 nm). The ability to separate these polyamide films from the underlying polysulfone support was used in combination with advanced microscopy methods to investigate the nanostructure of the thin films. Two new methods for transmission electron microscopy (TEM) were developed for this purpose. The first technique produced cross-sectional images of isolated polyamide thin films by removing the polysulfone support from TEM cross-sections of regular membranes. In the second technique called "projected area" TEM (PA-TEM), isolated polyamide thin films were placed with their surface perpendicular to the electron beam. The resulting images captured the thickness, morphology and mass density of the entire thin film. Cross-sectional and PA-TEM images of isolated films revealed the existence of a 20--60 nm compact base of nodular polyamide that forms the interface with the polysulfone support and from which the more open ridge-and-valley structure extends outward. In contrast, the semi-aromatic polyamide active film in NF membranes is approximately 40 nm thick with an extremely smooth surface (RMS roughness of about 7 nm). The ridge-and-valley structure is absent in these films. To further examine which sections of the film are actively involved in the transport and rejection mechanisms (i.e., to look for specific areas of higher water permeability that could be more sensitive to initial fouling), dead-end filtration experiments with 10 nm gold nanoparticles were performed. The working hypothesis was that the nanoparticles would follow the flow streamlines as water enters the membrane, thereby serving as conservative markers to pinpoint areas of the surface where enhanced water transport occurs. Surveying relatively small membrane areas of about 7 square microns revealed that for the nanofiltration membrane NF270, nanoparticles did not deposit uniformly over the entire surface of the membrane but instead formed networks of nanoparticle clusters leaving some regions of the membrane completely uncovered. In addition to the local permeability differences, nanoparticle deposition also reflected regional permeability variations at significantly larger scales (of several thousand square microns) that produced leopard-skin patterns. The formation of these nanoparticle deposition patterns at various scales verified the existence of high water permeability sites or hot spots on the membrane surface. Although no flux decline was detected in these experiments, it is likely that fouling would first occur at these sites.

Advisor: Leckie Jim
Reinhard Martin
Luthy Richard G. advisor. Thesis advisor
Pinnau I. (Ingo)