Definition of Phase Inversion Membrane
Phase inversion is a process of controlled polymer transformation from a liquid phase to solid phase. There are four basic techniques used to create phase inversion membranes: precipitation from vapor phase, precipitation by controlled evaporation, thermally induced phase separation, and immersion precipitation. Out of the four, immersion precipitation is the most widely-used technique for preparing polymeric membranes.
Precipitation from the Vapor PhaseOnce a solvent-polymer mixture is cast on the film, it is placed in a vapor atmosphere that contains a nonsolvent saturated with the same solvent. Due to the high concentration of solvent in the vapor atmosphere, the solvent from the cast film stays instead of evaporates into the atmosphere. Membrane forms by diffusion of nonsolvent into the cast film. This process results in a porous membrane.
Precipitation by Controlled EvaporationThe polymer in this case is dissolved in a solvent and nonsolvent mixture. The evaporation of the solvent due to high volatility occurs, causing the composition to have a higher nonsolvent and polymer content. The polymer eventually precipitates and forms a skinned membrane.
Thermally Induced Phase SeparationA mixed or single solvent polymer solution is cooled down to achieve phase separation. Solvent evaporation induces membrane formation. This method is often used in preparing microfiltration membranes.
Immersion PrecipitationPhase inversion via immersion precipitation is the most widely-used membrane preparation method. A polymer plus solvent (polymer solution) is cast on a proper supporting layer and then submerged in a coagulation bath containing nonsolvent. Due to the solvent and nonsolvent exchange, precipitation takes place. The polymer must be soluble in solvent mixture. The combination of phase separation and mass transfer affects the membrane structure.
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