Phaseninversion

Phaseninversion is a fabrication method for polymeric membranes in which a homogeneous polymer solution is transformed into a porous solid through phase separation. The phase separation is typically induced by contact with a non-solvent (solvent-induced phase separation or immersion precipitation) or by changing temperature (temperature-induced phase separation, TIPS). An evaporation-induced variant (EIPS) uses solvent loss during casting to trigger demixing.

In a typical solvent-induced process, a polymer is dissolved in a solvent to form a casting solution, which is spread as a thin film on a support and then immersed in a non-solvent bath such as water. Exchange of solvent and non-solvent lowers the polymer solubility and precipitates the polymer, forming an asymmetric membrane with a dense surface layer and a porous substructure. After phase inversion, residual solvent is removed and the membrane is dried.

During phase inversion, the final pore structure is governed by the thermodynamics of mixing and the kinetics of solvent–non-solvent exchange, polymer concentration, molecular weight, and temperature. Rapid demixing tends to generate finger-like macrovoids, whereas delayed demixing can produce sponge-like or highly micro-porous structures. The choice of polymer, solvent system (for example N-methyl-2-pyrrolidone, NMP; dimethylformamide, DMF; dimethylacetamide, DMAc), and non-solvent (commonly water) enables control over pore size and permeability.

Phaseninversion membranes are widely used in water treatment, ultrafiltration, microfiltration, nanofiltration, and gas separation. Common polymers include polyvinylidene fluoride (PVDF), polysulfone (PPSU), polyethersulfone (PES), cellulose acetate, and polyacrylonitrile (PAN). The method offers scalable production of asymmetric membranes but is sensitive to environmental conditions and composition, requiring careful control of casting, immersion parameters, and post-treatment.