Oil core/polymer shell microcapsules by internal phase separation from emulsion droplets. II: Controlling the release profile of active molecules

Microcapsules with oil cores and solid polymer shells have been prepared by precipitation of the polymer from the internal phase of an oil-in-water emulsion. The dispersed phase consists of a polymer, a good solvent for the polymer (dichloromethane), and a poor solvent for the polymer (hexadecane). Removal of the good solvent results in phase separation of the polymer within the emulsion droplet, leading to the formation of a polymeric shell surrounding the poor solvent. A UV-active organic molecule is added to the oil phase prior to emulsification. Provided this molecule has some water solubility, the release profile of the molecule from the capsule can be determined. While the microcapsule size was kept approximately constant, the influence of a wide range of factors on the release profile has been studied. These include the type and molecular weight of the shell-forming polymer, the molecular weight of the active ingredient molecule, the shell thickness, the use of copolymers or polymer blends to form the shell, the effect of cross-linking the shell or heating the capsule to temperatures above the T_g value of the polymer after the shell has been formed, and the effect of changes in the pH of the release solution in the case when a weak polyelectrolyte is used as the shell polymer. The differences in behavior are discussed in terms of the properties of the polymer shell, in particular the thickness, the polymer/release molecule interaction, and the free volume/porosity. Variation of these parameters allows one to control both the final release yield and the rate of release for time periods between a few hours and days.