Polymer Phase Separation in a Microcapsule Shell

Phase separation has been used for engineering microscale fluids and particles with designed structures. But it is challenging to use phase separation to create complicated microcapsules because phase separation in the shell correlates with applied osmotic pressure and affects capsule stability significantly. Here we employ two biodegradable polymers to study the phase separation in microcapsule shells and its effect on the mechanical stability. The dynamic process reveals that phase separation creates a patchy shell with distinct regions transiently, then transports the discrete domains across the shell, and coalesces them at the surface. The equilibrium structure with balanced osmotic pressure is a Janus shell, where one component forms the shell and the other component dewets on the surface. Under slight osmotic pressure to the shell, phase separation reaches a different Janus shape, which consists of two partial shells of each component. We can in further take advantage of phase separation and osmotic pressure to rupture microcapsules at specific locations. Phase separation in the shell provides a facile approach to create versatile capsule structures and affords a reliable strategy to harness the shell mechanics.