Molecular weight resolution of solvent resistant nanofiltration (SRNF) membranes

Engineering membranes for molecular separations in organic media is still a big challenge, especially when two closely resembling solutes have to be separated by allowing one to pass with the solvent and the other to be rejected. The so-called "resolution" of the membrane then becomes critical. Resolution reflects the difference in rejection values of the two (or more) solutes for that solvent-membrane system. Recently developed crosslinked poly(vinylidene difluoride) (XL-PVDF) and well-known crosslinked polyimide (XL-PI) membranes were prepared and tested for solvent resistant nanofiltration (SRNF). Rejections of 7 dyes (i.e. Rose Bengal (RB), Acid fuchsin (AF), Rhodamin B (RhB), Sudan Black (SB), Zinc tetraphenylporphyrin (ZTPP), and 55,10,15,20-Tetrakis(3,5-di-tert-butyl phenyl)porphyrin (TBPP) in a broad range of solvents, i.e. a set of polar solvents (water, ethanol, isopropanol), and a set of non-polar ones (butyl acetate, chloroform, xylene and toluene) were studied. As anticipated, the molecular weight cut-offs (MWCOs) of both membrane-types varied with the applied solvent type. Butyl acetate and chloroform filtrations respectively yielded the sharpest MWCO for XL-PVDF and XL-PI. The varying MWCO-curve per solvent is explained by the changing affinity of solvents and solutes for the membrane, as indicated by swelling and sorption experiments respectively.