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

Abstract

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.