Ammonia permeation and plasticization of glassy polymeric membranes

Abstract

Ammonia (NH3) is a vital industrial chemical and a potential clean fuel, whose production is increasing globally. The permeation of ammonia through glassy polymeric membranes demonstrates considerable variability, due to hydrogen bonding with both the polymer and residual water. This study reveals the underlying mechanisms for ammonia permeability within membranes, which is strongly correlated with the presence of water and the polymer's chemistry. Three polymers with varying water/ammonia affinity were studied: poly (vinyl alcohol) (PVA), polysulfone (PSU), and Teflon AF 1600. Ammonia was found to readily plasticize PVA, with sorption isotherms that follow Henry's law at conditions below the nominal glass transition temperature. Residual water within the PVA membranes increased ammonia permeability by an order of magnitude greater than that observed for well-dried PVA, due to water-induced swelling. PSU, being hydrophobic, was unaffected by residual water, though ammonia permeability was clearly time-dependent with a 50 % reduction over 4 days. This was attributed to ammonia clustering within the ammonia phobic PSU, which resulted in pore-blocking. The superhydrophobic Teflon AF 1600 membrane demonstrated the same time dependent permeability behaviour, but at a much faster timescale of minutes rather than days. It is suggested that this relates to the rate of formation of ammonia clusters, which is dependent on the degree of ammonia sorption within the polymer.