Phase inversion (also called phase separation or demixing) refers to two related phenomena in chemistry and materials science: (1) a controlled phase-separation process widely used to fabricate porous polymer membranes, and (2) the inversion of continuous and dispersed phases in an emulsion.
In membrane fabrication, phase inversion converts an initially homogeneous polymer solution into a solid, porous structure by inducing phase separation and solidification.
Phase inversion is one of the most common routes for producing polymeric membranes used in microfiltration, ultrafiltration, nanofiltration and related separations. In practice, a polymer is dissolved in a solvent (sometimes with additives), shaped as a film or hollow fiber, and then phase-separated to form a polymer-rich phase (matrix) and polymer-lean phase (pores).
DIPS is driven by mass transfer between the cast polymer solution and a contacting vapor or liquid, causing local composition changes that induce demixing.
Common DIPS variants include:
In TIPS, phase separation is triggered primarily by changing temperature (e.g., cooling a polymer solution prepared at elevated temperature), causing demixing and solidification.
The pore structure produced by phase inversion depends on thermodynamics and mass transfer during demixing and solidification, which are influenced by polymer concentration, solvent/non-solvent choice, additives, and process conditions (e.g., bath composition, temperature, exposure time to vapor/air).
In emulsions, phase inversion occurs when the dispersed and continuous phases exchange roles (e.g., water-in-oil to oil-in-water), which can be driven by formulation changes (such as surfactant affinity) or by changing phase volume fractions.