Investigation of phase inversion behavior of cellulose- ionic liquid solutions in relationship with membrane formation

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2017
Durmaz, Elif Nur
Cellulose membranes were produced from ionic liquid solutions by phase inversion technique and thermodynamic and kinetic aspects of the process were investigated to relate these to membrane morphology and performance. In thermodynamics part, polymer-solvent, polymer-nonsolvent and polymer-solvent-nonsolvent interactions were examined experimentally, together with Hansen solubility parameter estimations. Kinetics part consisted of measuring phase inversion rate. Obtained membranes were characterized by their morphology, crystallinity and separation performances. Performances and SEM images of cellulose, cellulose acetate and poly(ether sulfone), PES, membranes show that when amount of ionic liquid (1-ethyl-3- methylimidazolium acetate, EMIMAc) increased in the polymer solution and when the coagulation media is water rather than ethanol, membranes have denser selective layer. Bromothymol Blue retentions of non-dried cellulose membranes drop from 40% to 20% as dimethyl sulfoxide (DMSO) is added to the polymer solution in 1:1 ratio with ionic liquid and when it is added in 1:2 ratio retention reduced to approximately 10%. Rejections are not more than 7% for membranes obtained from coagulation in ethanol. Additionally when membranes were dried, Bromothymol Blue retentions increased to ca. 80% for all cellulose membranes while pure ethanol permeance drops to approximately 1 L/m2.h.bar from around 70 L/m2.h.bar. XRD measurements of cellulose membranes indicate the highest crystallinity in membranes prepared from EMIMAc only and coagulated in water. Solubility parameters estimate that solvent quality of EMIMAc is higher for cellulose than EMIMAc-DMSO mixtures, while it is opposite for cellulose acetate and PES. The nonsolvent quality of water is found to be lower for cellulose solutions indicated by cloud point measurements and swelling tests. Cloud point and swelling test measurements and solubility parameter estimations agree that water is a stronger nonsolvent for cellulose acetate and PES. Phase inversion rates from microscope observations are always lower when the nonsolvent is ethanol. However not seeing higher front rates in lower viscosity DMSO containing solutions, imply that phase inversion front rates do not only depend on viscosities for these solutions. Light transmission measurements, on the other hand, show more reliable results on membrane porosity rather than precipitation rate of polymer solution. 

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Citation Formats
E. N. Durmaz, “Investigation of phase inversion behavior of cellulose- ionic liquid solutions in relationship with membrane formation,” M.S. - Master of Science, Middle East Technical University, 2017.