Mechanical Stability of Polymer Active Layers Used In Membranes for Clean Water

Thin polymer membranes are at the core of a diverse set of emerging technologies critical to our global health and security, ranging from water purification to energy production to carbon capture.  These engineered membranes must maintain performance over many years and under harsh conditions, such as oxidizing chemical environments, thermal cycling, and mechanical loading.  Although there are recognized methods for monitoring performance variations over time, changes in the mechanical properties of these types of membranes have largely been ignored.  In this presentation, we describe a measurement approach based on a combination of surface wrinkling and thin-film cracking to probe the elastic modulus, strength, and fracture strain of nanoscale polymer membranes.  As a demonstration, we apply this approach to the crosslinked aromatic polyamide thin active layer of reverse osmosis membranes before and after chlorination.  Our measurements reveal that treatment with acidic hypochlorite solution causes significant mechanical degradation of the polyamide active layers, most notably a marked decrease in both fracture strength and onset fracture strain with increasing chlorine exposure.  These results allow us to provide a better understanding of their oxidative degradation mechanism, together with complementary chemical analysis of Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy.