Microcapsules having liquid cores and polymer shells were prepared by a simple physical process. The morphology of the resulting particles depends upon materials, and can be controlled to yield spherical core-shell type microcapsules. The thickness of the shell is readily varied from 1% to 10% of the microcapsule radius.
A three-component solution containing a polymer, a good solvent for the polymer (dichloromethane, DCM) and poor solvent (hexadecane) was emulsified into an aqueous phase containing an appropriate emulsifier to yield droplets of ~10µm mean diameter. By controlled evaporation of the DCM, the polymer-solvating power of the solvent mixture within the droplets is reduced until eventually the polymer phase-separates and forms a complete shell around each droplet. A typical phase diagram for the 3-component droplet phase is shown in figure 1. The region where the ternary mixture is a single phase is denoted I and is in the dichloromethane rich corner of the diagram. The line separating this region from the region where the polymer is in a separate phase (denoted II in the figure) is called the bimodal line and was determined experimentally by observation of the turbidity of various mixtures (clear = one phase, cloudy = two phases). The initial polymer solution used to make the present microcapsules was formulated to have the composition indicated by the dot in the one-phase region. By removing DCM from the droplets of an emulsion made with oil of this composition, the oil droplet composition moves toward one that is on the bimodal line, at which point the polymer in solution begins to separate as a phase rich in solvent and so quite fluid and mobile. This mobile fluid appears in the droplets as smaller droplets which move to the droplet surface to form capsules as illustrated in figure 2 [ref]).