A phase-field study on polymerization-induced phase separation occasioned by diffusion and capillary flow—a mechanism for the formation of porous microstructures in membranes

The performance and the application of membranes, which are usually produced from polymer solutions, are strongly determined by their porous microstructures. One important mechanism for producing the porous microstructures of membranes is polymerization-induced phase separation (PIPS). Here, we scrutinize PIPS by employing a Cahn–Hilliard–Navier–Stokes method coupling with the Flory–Huggins model. We focus on the formation of membranes via diffusion as well as capillary flow. We report several morphological evolution characteristics of PIPS: (1) an asynchronous effect, where the polymer-rich phase and the polymer-lean phase reach their equilibrium concentrations at different times, (2) a center-to-center movement and collision-induced collision of polymer-rich particles, (3) transition of network structures into polymer particles and rebuilding of network structures from polymer particles, (4) polymer ring patterns. We expect that these findings would shed light on complex microstructures of membranes and provide guidance for the fabrication of desired membranes. Highlights We model polymerization induced phase separation in membranes by considering diffusion and capillary flow. We find that the polymer-rich and polymer-lean phases reach the equilibrium state asynchronously. We observe several typical morphological transitions of polymerization induced phase separation, such as percolationto-cluster, cluster-to-percolation, tiny droplets and polymer ring-patterns.