Aqueous Degradation of Polyamide Membrane Materials in Halogenated Environments
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Aqueous Degradation of Polyamide Membrane Materials in Halogenated Environments Logan T. Kearney1, John A. Howarter2 1
School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States 2 Division of Environmental and Ecological Engineering, Purdue University, West Lafayette, Indiana 47907, United States ABSTRACT Model polyamide thin films were prepared through a controlled interfacial polymerization route known as molecular layer by layer (mLbL). Films were synthesized directly onto quartz crystals and subjected to halogenated aqueous environments that are known to cause degradation of the amide network. A quartz crystal microbalance (QCM) was used as the detection platform to ascertain mass loss due to degradation in real time. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements were also performed at various stages of the degradation sequence to elucidate the chemical and morphological changes at the surfaces respectively. Appropriate strategies for accurately comparing material degradation resistance are proposed along with modifications to the crosslinked polyamide chemistry to produce more halogen tolerant polymeric surfaces. INTRODUCTION Accurately ascertaining the failure modes is perhaps the most critical component to the improvement of any highly engineered materials system in adverse environments where material degradation occurs. Polymeric thin films are classes of materials that have found increasing use in applications including energy storage, membrane separations, and protective coatings1,2. Correlation of the chemical and morphological contributions to the film properties is of the utmost importance to evolve the understanding of device performance and inspire next generation thin film technologies. Unfortunately, detection of the relevant nanoscale events is difficult, resulting in the primary use of macroscale performance3 as the means to determine the thin film condition. These approaches often require longer and more complicated experimental sets and ultimately fall short of directly clarifying the degradation mechanisms. As a result of this methods for rapidly and robustly probing the degradation behavior of thin films are needed. Reverse osmosis (RO) membranes are widely used in industry as a highly selective separation technique due to established membrane production routes and relatively low operating costs4. To accurately exclude unwanted dissolved ionic species, a highly-crosslinked polyamide thin film (~100nm) is used. In this application chlorine is often added to compromised waters as a disinfectant in order to deter the formation of biofilms at the exchange interface. The added chlorine has been shown to rapidly degrade the polymer network structure, eventually resulting in intolerable losses in membrane performance5,6. Increasing both membrane service lifetimes and the process efficiency is an essential breakthrough needed to satisfy the water production demands of the coming years4. Metrological techniques capable of mo
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