Diffusion Driven Nonsolvent Induced Phase Separation

Nonsolvent induced phase separation (NIPS) occurs when a polymer solution is brought into contact with a miscible nonsolvent, leading to the precipitation of a polymer-rich phase. Because of its simplicity, NIPS processes are widely used to generate a variety of microstructures in polymer materials such as membranes and micro/nanoparticles. Despite its prevalence, predicting and controlling the microstructure generated by NIPS remains a difficult challenge, owing to the complex interactions between the diffusive transport, hydrodynamics and phase-separation kinetics in the process. In our approach, we use simulations of a "multi-fluid" phase-field model of a ternary polymer solution that incorporates all of these kinetic processes. In the case of NIPS driven by purely diffusive solvent/nonsolvent exchange, we find two regimes capable of generating microstructure via spinodal decomposition: one at times much shorter than the diffusion time of the nonsolvent and one at much longer times. We then use our model to predict (i) which compositions of polymer solution will lead to microstructure formation at both short and long times and (ii) what microstructures emerge as composition is varied.