Thin-film composite membranes for organophilic nanofiltration based on photo-cross-linkable polyimide

This work demonstrates that it is possible to prepare new, competitive thin-film composite (TFC) membranes with a polyolefin ultrafiltration membrane as support and with a non-porous photo-cross-linked polyimide as separation layer for organic solvent nanofiltration. The commercial polyimide Lenzing P84® was modified by a polymer-analogous reaction to introduce side groups with carbon–carbon double bonds to increase its photo-reactivity with respect to cross-linking. Polymer characterization revealed that this was successfully achieved at acceptable level of main chain scission. The higher reactivity of the photo-cross-linkable polyimide had been confirmed by comparison with the original polymer; i.e., shorter gelation times upon UV irradiation, higher suppression of swelling by solvents and complete stability in strong solvents for not cross-linked polyimide such as dimethylformamide (DMF) had been obtained. For films from unmodified and modified polyimide, the degree of swelling in various solvents could be adjusted by UV irradiation time. Photo-cross-linking of the original polyimide did not lead to stability in DMF. TFC membranes had been prepared by polymer solution casting on a polyethylene ultrafiltration membrane, UV irradiation of the liquid film and subsequent solvent evaporation. Polyimide barrier film thicknesses between 10 and 1 μm were obtained by variation of cast film thickness. Performance in organic solvent nanofiltration was analyzed by using hexane, toluene, isopropanol and DMF as well as two dyes with molar masses of ∼300 and ∼1000 g/mol. Permeances of TFC membranes from unmodified polyimide were low (<0.1 L/hm² bar) while rejections of up to 100% for the dye with ∼1000 g/mol could be achieved. TFC membranes from modified and photo-cross-linked polyimide had adjustable separation performance in DMF with a trade-off between permeance and selectivity, in the same range (e.g.: 0.3 L/hm² bar and 97% rejection for the dye with ∼1000 g/mol) as a commercial conventional polyimide membrane tested in parallel. The established membrane preparation method is promising because by tuning the degree of cross-linking of the polymeric barrier layer, the membrane separation performance could be tailored within the same manufacturing process.