Polyimide Organic Solvent Nanofiltration Membranes-Formation and Function

Abstract

This thesis offers a comprehensive study that analyses the relationship between polyimide (PI) organic solvent nanofiltration (OSN) membrane formation parameters, membrane structure, and membrane functional performance. The dissertation starts by addressing the structure-related problem of macrovoid formation, which arises when more open membranes are prepared. Incorporation of TiO2 nanofillers into the membrane matrix results in macrovoid-free, organic/inorganic PI/TiO2 mixed matrix membranes without compromising rejection. Subsequently, a detailed analysis of the membrane formation process, considering the dope solution composition, evaporation step, and structural properties of polyimides, was conducted. The effect of the choice of polymer/solvent/co-solvent/non-solvent was found to be very profound and qualitatively predictable through introduction of a complex solubility parameter. Increasing value of complex solubility parameter can predict higher rejections. The study of the evaporation in PI OSN membrane formation has shown that this optional step is undesirable, as its presence results in unaltered rejection and significantly lower flux. Nevertheless, the presence of a co-solvent, regardless of whether it is volatile or not, was found to be required as it promotes formation of a dense membrane top layer. We have also studied sensitivity of PI OSN membranes to small perturbations in polymer characteristics, such as: molecular weight, alternating diisocyanates to form the PI chain, and copolymerisation method (block vs random). Finally, we proposed a less hazardous route for the PI OSN membrane formation process, which would reduce environmental impact without compromising the separation performance of the existing membranes.