Studies of oxidative degradation in polyamide RO membrane barrier layers using pendant drop mechanical analysis

A major concern with the use of polyamide thin film composite (TFC) membranes is their loss of performance when exposed to oxidizing agents such as aqueous chlorine that is used in membrane cleaning and water disinfection. A better understanding of this oxidative degradation process would facilitate the development of improved TFC membranes. This study utilizes pendant drop mechanical analysis (PDMA) to measure chlorine-exposure-induced changes in the behavior of polyamide barrier-layers formed by the interfacial polymerization (IP) of m-phenylenediamine (MPD) and trimesoyl chloride (TMC). PDMA enables the measurement of mechanical and transport properties of unsupported MPD–TMC thin films. Based upon the PDMA results as well as those obtained from experiments conducted on commercial TFC RO membranes, a hypothesis is presented describing a two-pathway mechanism for the chlorine degradation of polyamide TFC membranes. At a constant pH, a morphology change, not polymer chain scission, occurs leading to separation of the barrier layer from the support. A second degradation pathway is observed when chlorine exposure under acidic conditions is followed by exposure of the polyamide thin-film to caustic (pH >11) conditions. The results of this investigation provide a strong rationale for further refinement of the PDMA methodology for use in basic studies as well as for screening applications that could aid in the development of chlorine-resistant IP thin films.