Characterizing the Network Properties of Polyamide Pressure-Induced Membranes Via Swelling

Polyamide (PA) thin film composite (TFC) represent the state-of-the-art pressure-induced membranes used in water desalination. The performance of these materials, such as permselectivity, is intimately linked with network properties of the PA active layer. Traditionally, the network properties of a swollen crosslinked polymer are determined by describing its swelling behavior with the appropriate thermodynamic network swelling model. However, quantifying the swelling behavior of commercial PA active layers is quite challenging due to their nanoscale thickness and confinement to the TFC geometry. In this work, we present out recent approaches that overcome these measurement challenges and enable characterization of the network properties of the PA active layer. Specifically, we study the swelling behavior of isolated commercial and synthesized PA active layers using X-ray reflectivity and quartz-crystal microbalance as a function of water hydration. By applying the Painter-Shenoy blob swelling model to describe the swelling behavior of a trifunctional polymer network, we quantify the PA network properties including Flory interaction parameter and the monomer units between crosslinks. We apply this approach to characterize the network properties of different forms of PA active layers relevant to water purification and discuss the relationships between network and transport properties.