The production of polymeric particles via nonsolvent-induced phase separation

Nonsolvent-induced phase separation (NIPS) is frequently employed for producing porous microstructures such as polymer membranes. Because of their ease of manufacture, NIPS-like processes have recently become a method of choice for producing polymeric micro/nanoparticles. Despite their popularity, many questions still surround the process kinetics and its effect on the resultant structure of such particles. At its most basic level, NIPS involves an exchange of a good solvent with a bad one (a nonsolvent) that eventually produces phase separated structures. While simple, predicting microstructure is nontrivial due to the presence of multiple dynamic modes across a wide range of length and time scales. In our study, we overcome this challenge by using a phase-field model of the NIPS kinetics of polymer droplets that phase separate when brought into contact with a bath of nonsolvent. We investigate two separate cases: (i) "rapid precipitation" behavior where the kinetics are determined by the initial mixture, and (ii) "delayed precipitation" behavior where the kinetics involve long-time exchange of solvent and nonsolvent. We perform parameter studies in both cases to investigate the effect of droplet shape and size and of solvent/nonsolvent miscibility on microstructural evolution.