Multilayer Poly(ionic liquid) Microcapsules Prepared by Sequential Phase Separation and Subsequent Photopolymerization in Ternary Emulsion Droplets

We report a simple microfluidic process to prepare multilayer poly(ionic liquid) (PIL) microcapsules via sequential liquid-liquid phase separation within ternary emulsion droplets followed by the photopolymerization of ionic liquid (IL) monomer-rich phases. Emulsion droplets, consisting of a hydrophobic IL monomer, water, and N,N-dimethylformamide (DMF), are first formed in a microfluidic device, and the droplets are then carried by a continuous aqueous phase. Subsequently, DMF diffuses from the droplets into the continuous aqueous phase, resulting in the sequential internal phase separation of the IL-rich and water-rich phases, generating multilayer emulsion droplets comprising alternating IL-rich and water-rich phases. The number of droplet layers was controlled from one to five by varying the initial composition of the dispersed phase. Furthermore, under the conditions where higher-order emulsion droplets were formed, the time scale between the onset of phase separation and the formation of each layer became shorter. Additionally, the IL-rich phases in the multilayer emulsion droplets were easily solidified via photopolymerization, resulting in PIL microcapsules with multilayer structures. Anion exchange of the obtained PIL microcapsules effectuated their transition from a hydrophobic to a hydrophilic nature, resulting in PIL microcapsules with diverse swelling properties and PIL layer permeability across various solvents. We believe that the sequential phase separation system observed in the ternary emulsion droplets can pave the way for the design of PIL-based colloidal materials with thermodynamically nonequilibrium structures, thereby extending their application in functional materials.