Elucidating the Reverse Osmosis Polyamide Formation Mechanism from Pseudo-Stop-Motion Imaging

The formation mechanism of reverse osmosis polyamide membranes has long eluded the scientific community, primarily due to an inability to observe the formation, which occurs both too quickly and on too small a scale.  Extensive data has been gathered over the years to understand the effects of recipe and processing conditions on the performance of these membranes, but as the technology approaches performance limits, understanding the intricacies of film synthesis may be the key to fine-tuning an established chemistry to maximize separation efficiency.

For the first time, methods of freezing in time polyamide at various stages of formation have been developed.  Custom-designed casting and imaging methods provide a pseudo-stop-motion view of polyamide film growth and insight into the forces that lead to the typical polyp structures of RO membranes.  The structures have been shown to be much less random than they appear, and through well-controlled pilot-scale membrane manufacturing have been manipulated to illustrate a new mechanistic theory. 

Evidence of a relationship between structure and parameters such as interfacial tension, amine pK_a and temperature will be presented.  Further evidence based on manipulation of downstream process conditions solidifies the argument for a “balloon inflation” growth mechanism.  This evidence will be presented in the context of three common polyamide structures along with application to the future of reverse osmosis membrane development.