Modeling of pore formation in phase inversion processes: Model and numerical results

Various theories about the mechanism of pore formation in porous polymer membranes have been proposed in the past. Here, we predict pore structure formation, based on density-gradient theory, where surface tension is included as part of the gradient of the chemical potential. A simplified thermodynamic model, assuming a fluid mixture with symmetric miscibility gap, is used. Based on this model we conclude that the development of finger pores as well as of sponge pores is due to diffusive mass transport. The evolving picture of pore formation is as follows: At contact of polymer solution and coagulation bath, a counter-diffusion of polymer, solvent and non-solvent occurs until the miscibility gap is reached. Then, nuclei are formed and the polymer solution phase separates, while the locus of nucleation moves in the direction of the polymer solution by continuing counter-diffusion of polymer and non-solvent, when solvent concentration is constant. As a result, we observe a moving precipitation front. Different morphology develop, depending on polymer mass fraction and velocity of the precipitation front. The results are in qualitative agreement with experimental observations.