Interfacial Polymerization Engineering of Polyurethane Shells via Controlled Swelling toward Robust Microcapsules

Abstract Self-limitation of interfacial polymerization for the fabrication of microcapsules can easily lead to thin and inferior shells, which negatively impact the mechanical properties of the microcapsules. This study explores the polyurethane (PU)/polyurea (PUA) microcapsule shell growth during interfacial polymerization and strategies to produce intact and robust microcapsule shells. Introducing solvents into the oil phase, a controlled swelling strategy was employed on the basis of the Hansen solubility parameter (HSPs) theory to retard phase separation and extend the reaction duration, thus overcoming the self-limitation of interfacial polymerization. Experimental results show that the controlled swelling effect can increase the shell thickness from 0.5 to 3 μm with high yield. By using the swelling effect, cross-linked and composite PU/PUA shell formation with adjustable thickness (ranging from 1 to 3 μm) was achieved by introducing cross-linking groups and highly reactive monomers. Nanomechanical testing results show that the maximum yield load of the microcapsules with a cross-linked shell exceeds 6 mN, while the yield deformation rate of the microcapsules with a composite shell reaches 30%. In high-speed milling tests, the microcapsule breakage rate is decreased by 80%.