| Interfacial polycondensation is an efficient process for the fabrication of polymer thin film and the microencapsulation of active organic agents such as drugs and pesticides. The microencapsulation is often used to minimize the environmental impact of toxic substances such as pesticides and herbicides, and/or to achieve a controlled-release of the active substances that are encapsulated. In the polymerization process, an organic phase containing the organic-soluble monomer and the substance to be encapsulated is dispersed in an aqueous phase containing the other monomer. Polymerization can only occur at the interface between the two phases, and hence is controlled by the diffusion of one or both monomers to this interface. The time-release diffusion properties of the microcapsule depend on the thickness of their shells, as well as the molecular weight and structure of the polymer. Previous modelling studies have only concentrated on predicting the time of encapsulation necessary to achieve a certain shell thickness, but did not address the polymeric properties of the shell. A dynamic model based on the method of moments is proposed for the molecular weight development in the microcapsule shell. This model not only takes into account the polymerization reaction at the inner interface of the capsule shell, but also accounts for the simultaneous reaction and diffusion of the aqueous-phase monomer, and of the transient development of the molecular weight of the polymer in the shell as it reacts further after its formation. The model also applies to the unstirred system to make polymer film. The model is solved using a finite difference technique to describe the transient behavior of the concentration and molecular weight profiles in the shell. Results illustrate the effect of various operating variables (monomer concentrations, monomer diffusivity, and specific reaction rates) on the development of these profiles across the shell. Methods for controlling the molecular weight distribution profile are also developed and presented. |
