Performance of a composite membrane of a perfluorodioxole copolymer in organic solvent nanofiltration

Solvent swelling of polymers is of major importance in organic solvent nanofiltration (OSN) affecting membrane rejection of 200–1000+ Da solutes. A selected perfluoro type amorphous glassy copolymer, perfluoro-2,2-dimethyl-1,3-dioxole copolymerized with tetrafluoroethylene (PDD-TFE), is completely insoluble in all solvents except perflourinated solvents; further it showed very little swelling in solvents that are not of the perfluorinated type. Pure component permeation flux for a particular variety of this copolymer, CMS-7, was measured with a wide variety of solvents for two membrane thicknesses, 1.67 µm and 0.6 µm, in a composite form supported on a porous ePTFE substrate. The feed pressure range was 1000–3000 kPa. The solvents included methanol, ethanol, ethyl acetate, n-heptane, tetrahydrofuran (THF), toluene, dimethylsulfoxide (DMSO); n-heptane showed much higher flux than the rest. Successful OSN was demonstrated with high solute rejection in methanol solutions of two dye solutes Safranin O (MW, 351 Da) and Brilliant Blue R (MW, 826 Da). Rejection for Safranin O varied between 94% and 97% depending on the pressure and membrane thickness; that for Brilliant Blue R varied in the range of 97–99.8%. Successful OSN was also demonstrated with Safranin O in THF with solute rejection varying between 94% and 97%. Permeation behaviors of various mixtures of two solvents, toluene and n-heptane, were studied. The measured values of very limited extent of sorption of a variety of organic solvents in 25 µm thick dense samples of this and a related perfluoro type copolymer, CMS-3, were employed to explain the observed behavior.