|M.Sc Student||Ukrainsky Ben|
|Subject||Direct Microscopic Observation of Membrane Formation In a|
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Guy Ramon|
|Full Thesis text|
Polymeric membranes are finding increasing application in separation processes, ranging from biomedical, pharmaceutical, chemical separations to gas separation and water purification and desalination. Over the past few decades, membranes have been studied extensively on aspects of molecular structure, morphology, formation kinetics, and transport mechanisms, in order to improve performance. It has become apparent that membrane morphology and performance are intimately linked. Phase separation (PS) and Interfacial polymerization (IP) are both well-established methods for membrane fabrication; however, both comprise complex mechanisms, involving fluid-fluid interfaces at which the reactions take place, driven by mass transfer and reaction kinetics that are not yet fully understood. Furthermore, synthesizing and characterizing membranes fabricated by these techniques is often a lengthy procedure, and so new testing new materials is time consuming.
In this study, a microfluidic platform was developed, and used for examination of membrane formation. Such a system, if successfully deployed, would not only provide novel morphological and kinetical insight, but can also act as a rapid prototyping tool for testing new materials and potential membrane formulations. Further, the microfluidic method, compared with previous imaging platforms, better represents commercial "roll-to-roll" fabrication of PS membranes, since it accounts for flow at the interface.
First, a system for visualizing morphology formation during PS was designed and tested on a well-known formula for making polysulfone membranes. Here, the focus was on stabilization of the fluid-fluid interface as a required condition for controllable and trackable dynamic behavior of the rapid formation process. It was observed that when applying 3 different flow rates of the water on the polymer solution, 3 different morphologies were obtained. Under low flow rates, spongy-like morphology characterized by slow kinetics was observed, whilst a porous 'macrovoid' morphology with much faster kinetics were observed for the high flow rate.
Next, visualization experiments were conducted on IP, where the focus was placed on evaluating the exothermic character of the reaction. The temperature field was measured using an image analysis method based on temperature-dependent fluorescence intensity of a dye added to the reactant solution. The temperature field was measured for different conditions that represent formulations reported in the literature. Increasing flow rate, monomer concentration and addition of a co-solvent to the basic system, resulted in significant increases to the observed temperature, especially at the interface, reaching values close to 90ºC after 50 s of reaction. Interestingly, the morphologies observed exhibited a striking resemblance to typical polyamide films reported in the literature, despite the dramatically larger length scales involved (microns vs nm’s), which suggests that there might be scale similitude between nano and micro features. This would be of great value for further studies of morphological features as affected by reaction conditions, and their impact on transport.
These findings suggest that the exothermal nature of the IP reaction likely plays a significant role in determining the structure of the formed film. Including such aspects, and use of our direct observation methodology offers a new direction for investigating the actual cause for the formation of the unique IP morphology.