Investigating the void structure of the polyamide active layers of thin-film composite membranes

The potential presence of voids in the fully-aromatic polyamide active layers of thin-film composite (TFC) membranes for water purification was studied in a selection of commercial membranes with a broad range of performance levels. The membranes were characterized for their potential void fractions using three independent methods: (i) analysis of transmission electron microscopy (TEM) images of membrane cross-sections, (ii) water uptake measurements by quartz crystal microbalance (QCM), and (iii) estimates of the effective refractive indices of active layers by spectroscopic ellipsometry. Results revealed that voids having tens of nanometers in diameter exist in the fully-aromatic polyamide active layers of TFC membranes, the voids fill up with water when immersed in it, and the voids account for a significant volume fraction of the active layers (i.e., 15-32% for the membranes studied). It was concluded that the voids in polyamide active layers do not form passageways connecting the feed and permeate sides, but rather are cavities disconnected from the feed side. In addition, it was also concluded that the globular features observable in TEM images of membrane cross sections that had been previously identified as voids or nodules are indeed voids, and not nodules. The finding that a significant volume fraction of fully-aromatic polyamide active layers corresponds to water-filled voids has deep implications on various aspects of TFC membrane science and technology. For example, we illustrate how the presence of voids can potentially increase the effective water permeability of the active layer by as much as a factor of ≈5 compared with the case of an equivalent active layer without any voids. The methods developed in this study to measure void volume fraction represent useful tools for future membrane characterization studies, and the void fractions measured can be used as input or calibration parameters in future modeling studies of active layer formation or water and solute transport

Identifying facile and accurate methods to measure the thickness of the active layers of thin-film composite membranes - A comparison of seven characterization techniques

Despite the important role played by active layer thickness in the performance of thin-film composite (TFC) membranes, and in the understanding of the intrinsic transport properties (i.e., permeability, and water and solute partition and diffusion coefficients) of active layers, there is no study in the peer-reviewed literature evaluating whether existing measurement techniques provide consistent results among each other. Thus, we compared active layer thickness results obtained for each of six commercial TFC membranes with seven measurement techniques, including four techniques commonly used in the literature (scanning electron microscopy - SEM, transmission electron microscopy - TEM, atomic force microscopy - AFM, Rutherford backscattering spectrometry - RBS) and three non-commonly used techniques (quartz crystal microbalance - QCM, profilometry and ellipsometry). The six membranes tested covered performance levels ranging from nanofiltration to seawater reverse osmosis. Our results showed that AFM, RBS, QCM, profilometry and ellipsometry produced consistent results among each other, and thus likely provide the most accurate thickness results. SEM and TEM produced thickness results that were greater than those obtained with the five non-electron microscopy techniques, thus suggesting that SEM and TEM should only be used for rough estimates of active layer thickness. On the basis of nine different factors used to evaluate the advantages and disadvantages of the measurement techniques, ellipsometry was found to be the most advantageous technique for measuring active layer thickness. Results for active layer thickness and mass were used to obtain experimental estimates of the density of polyamide active layers, which for uncoated polyamide layers was found to be 1.26±0.21gcm-3