Interfacial Polycondensation to Fabricate Polyurethane Microcapsules: Unraveling the Mechanism of Shell Formation and Achieving Desired Shell Thickness Based on Hansen Solubility Parameter Theory

The self-limitation nature of interfacial polymerization can lead to thin-shell thicknesses and pores in the shell layer, as well as rugged surfaces of microcapsules prepared from interfacial polycondensation of polyurethane. This study systematically investigates the microcapsule shell formation process and mechanism of interfacial polycondensation of polyurethane at the oil-in-water (O/W) emulsion droplet interface, wherein self-limitation arises from phase separation that generates polyurethane nanoparticles, gradually aggregating to form the microcapsule shell layer, thereby creating barriers to prevent monomers in each phase from direct contact. Based on the Hansen solubility parameter (HSP) theory, the distance between the solvent in the oil phase and the polyurethane polymer (Ra) in the Hansen solubility sphere was employed to reveal the swelling effect of the solvent such as ethyl acetate on in situ formed polyurethane oligomers. The swelling effect of solvents can delay phase separation of in situ interfacially formed oligomers, overcoming self-limitation of polyurethane interfacial polycondensation. It is demonstrated that the good solvents of polyurethane with the Hansen solubility parameter closer to the center of the Hansen solubility sphere of polyurethane can result in decreased core/shell ratios with thicker microcapsule shells. It is demonstrated that Ra is a crucial factor in regulating the shell thickness of microcapsules from interfacial polycondensation, even with the mixed solvent. The establishment of the microcapsule shell growth mechanism allowed us to prepare polyurethane microcapsules with thick and desired shell thickness that could not be achieved in previous research.