A QUALITATIVE SKIN LAYER FORMATION MECHANISM FOR MEMBRANES MADE BY DRY WET PHASE INVERSION

Optimized membranes made by a dry/wet phase inversion process using forced-convective evaporation consist of an ultrathin and pore-free skin layer supported by a nodular transition layer and an underlying open-cell, sponge-like substructure. It is suggested that the outer-most region of such cast membranes undergo phase separation by spinodal decomposition in the initial stages of the evaporation process. As a result of the small scale of phase separation present in polymer-solvent systems, an appreciable capillary pressure occurs in the interstitial space filled with the polymer-poor phase in contact with air. The resulting force acts normal to the membrane-air interface, which tends to consolidate the polymer-rich phase to form a nonporous surface layer.