Correlating Chlorine-Induced Changes in Mechanical Properties to Performance in Polyamide-Based Thin Film Composite Membranes
Junghyun Lee, Jun Y. Chung, Edwin P. Chan, Christopher M. Stafford
Understanding the degradation of polyamide-based (PA) thin-film composite membranes by chlorine is imperative to develop chlorine-resistant membranes, an important characteristic that is needed to provide the desired long-term performance of membrane-based desalination and water purification modules. In this study, chlorine-induced structural changes of fully-aromatic and semi-aromatic PA active layers used in reverse osmosis (RO) and nanofiltration (NF) membranes were investigated by a combination of chemical, mechanical, and performance analyses. Out results indicated that chlorination causes different structural alterations depending on the chemical nature of the PA active layers, as identified by increased brittleness for the fully-aromatic PA and improved ductility for the semi-aromatic PA. Moreover, the data also revealed that the mechanical responses of the PA active layers after chlorine exposure were directly correlated to the overall membrane performance. A significant increase in water flux and a large decrease in salt rejection were observed for the RO membrane after chlorination, which can be ascribed to the increased fragility and resultant defects of the oxidized fully-aromatic PA network. In sharp contrast, the chlorination of the NF membrane resulted in a slightly reduced water flux accompanied with improved salt rejection, which was attributed to the structural compaction and densification of the semi-aromatic PA network under high operating hydraulic pressure since chlorination enhances its chain flexibility.