Coagulation Bath

Abstract

Phase Separation (PS) or Phase Inversion (PI) is among the principal techniques for membrane preparation. The membrane matrix and the membrane pores are formed, respectively, from the polymer-rich and the polymer-lean phases originated by the phase separation of an initially homogeneous polymeric dope. Immersion of a cast (or spun) polymeric dope in a coagulation bath is normally used in nonsolvent (or diffusion)-induced phase separation (NIPS or DIPS), also called immersion precipitation (IP), to achieve such separation, often referred to as demixing or precipitation as well. Upon immersion in the coagulation bath, the initial composition of the dope changes as the solvent diffuses in the bath and is gradually replaced by the nonsolvent. Polymer precipitation due to sol-vent/nonsolvent (S/NS) exchange is exploited both in flat sheet and hollow fiber preparation via NIPS. However, for flat sheet membranes, the cast polymer film is immersed in the coagu-lation bath and phase inversion starts at the top surface of the film. Regarding hollow fibers, prepared via wet or dry/wet spinning, phase inversion takes place both at the inner and at the outer surfaces. While the term "coagulation bath" is normally used when referring to the media for the coagulation of the outer surface, the inner coagulant is often referred as bore fluid. In any case, the solvent/nonsolvent exchange is at the basis of the membrane preparation mechanism via immersion precipitation. Phase separation will occur only when the system composition, in terms of polymer/solvent/nonsolvent (P/S/NS) concentrations, reaches the miscibility gap, surrounded by the spinodal curve in the ternary phase diagram (at a selected temperature). Membrane morphology will be strongly affected by the coagulation bath composition and temperature , since they both influence the solvent/ nonsolvent exchange rate and the polymer precipitation. In particular, the exchange rate depends on the mutual affinity between the solvent contained in the dope and the nonsolvent in the bath as well as on temperature. On the other hand, diverse nonsolvents usually display different coagulation power; polymer precipitation is affected by the nonsolvent nature and, obviously, by temperature. The mutual affinities between S/NS and P/NS are often described in terms of Hildebrand's solubility parameters (d). In general , it is known that the closer the values of the solubility parameters of two species, the higher the affinity between the two. Therefore, the difference between the S and the NS solubility parameters (d S-NS) can be used as a reference to estimate the S/NS exchange rate during coagula-tion: it will be faster if they have high mutual affinity, indicated by a small d S-NS. Furthermore,