Microcapsules with an oil core surrounded by a polymeric shell have been prepared by the controlled phase separation of polymer dissolved within the oil droplets of an oil-in-water emulsion. The dispersed oil phase consists of the shell polymer (polystyrene), a good solvent for the polymer (dichloromethane), and a poor solvent for the polymer (typically hexadecane). Removal of the good solvent results in phase separation of the polymer within the oil droplets. If the three interfacial tensions between the core oil, the shell-forming polymer, and the continuous phase are of the required relative magnitudes, a polymer shell forms surrounding the poor solvent. A UV-responsive organic molecule was added to the oil phase, prior to emulsification, to investigate the release of a model active ingredient from the microcapsules. This molecule should be soluble in the organic core but also have some water solubility to provide a driving force for release into the continuous aqueous phase. As the release rate of the active ingredient is a function of the thickness of the polymeric shell, for controlled release applications, it is necessary to control this parameter. For the preparative method described here, the thickness of the shell formed is directly related to the mass of polymer dissolved in the oil phase. The rate of volatile solvent removal influences the porosity of the polymer shell. Rapid evaporation leads to cracks in the shell and a relatively fast release rate of the active ingredient. If a more gentle evaporation method is employed, the porosity of the polymer shell is decreased, resulting in a reduction in release rate. Cross-linking the polymer shell after capsule formation was also found to decrease both the release rate and the yield of the active ingredient. The nature of the oil core also affected the release yield.

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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.

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