Hollow Polymer Droplets and Capsules Formed by Nonsolvent‐Induced Phase Separation

Abstract Using large‐scale particle‐based simulations, the formation of hollow liquid droplets and solid capsules during nonsolvent‐induced phase separation (NIPS) in polymer‐solvent droplets immersed in a nonsolvent bath is investigated. As solvent and nonsolvent exchange, nonsolvent gradually diffuses into the droplet, and the droplet surface retracts, leading to the formation of a dense polymer skin. This skin slows down further solvent‐nonsolvent exchange. When the polymer‐nonsolvent incompatibility is high, macrophase separation commences within the polymer skin, creating a porous droplet surface. These pores facilitate solvent‐nonsolvent exchange. If the polymer remains liquid, these pores eventually close, trapping nonsolvent inside the cavity enclosed by the polymer skin. As a result, these hollow droplets become quasi‐stable due to the protracted exchange of nonsolvent between the cavity and the external environment through the polymer skin. If the polymer vitrifies upon macrophase separation, the porous structure is permanently preserved, leading to the formation of porous microparticles. The findings align with experimental observations, and an outlook on the behavior of copolymer droplets is provided.