Analysis of the structure-determining process of phase inversion membranes

Abstract The properties of phase inversion membranes are mainly determined by their structure obtained during the actual membrane formation process. Depending on the system polymer, solvent and nonsolvent, the resulting membrane may exhibit five typical structures: a closed cell foam, an open cell foam, a lacy or net structure, a granular or packed sphere structure and an unconnected powder. Based on the ternary phase diagram, the tie-lines and some kinetic considerations, a new approach is introduced to explain membrane formation. It can be shown that the five different structures are the result of the same continuously transformed formation mechanism. The analysis can be applied to vapor-, diffusion- and thermal-induced phase separation processes. It is, however, necessary to introduce an incubation time and to define points of state inside the miscibility gap of a ternary phase diagram. The new defined kinetic spinodal region on the composition course for one membrane volume element together with the extension of the critical point to a critical line allows the explanation of structure formation.