Clay-Coated Polyurea Microcapsules for Controlled Release

Abstract

<p> Polyurea microcapsules are micron-scale, hollow polymer spheres commonly used in agriculture to encapsulate pesticides for controlled diffusive release onto target crops. Diffusion of these active materials through a protective polymer wall offers a safer and more effective method of delivery compared to the direct spraying of crops with toxicants. The approach we are taking to control the release rate is to coat pre-formed porous polyurea capsules with a separate release-controlling outer layer. This allows us to separately optimize the load-bearing capsule wall and the release control layer, an approach commonly used in other membrane diffusion systems.</p> <p> Montmorillonite clay incorporation into polymer matrices can reduce membrane permeability by forcing diffusants to take a tortuous path around the stacked silicate sheets. Effective formation of clay-polyurea composites requires the delamination of clay particles into thin sheets with high aspect ratios, and their incorporation into polyurea microcapsules either during interfacial polymerization, or post-polymerization. The net negative surface charge of the silicate sheets should facilitate their initial binding to the cationic polyurea surfaces, as well as subsequent binding of polycations to the clay-coated polyurea capsules to create layer-by-layer (LbL) capsule assemblies with decreasing release rates of internal materials.</p> <p> The main focus of this project is to gain a fundamental understanding of montmorillonite clay and polyurea microcapsules, and the development of a model polyurea composite capsule for release rate analysis. Emphasis will be placed on the reduced permeability of microcapsules coated with clay by LbL assembly post-polymerization, followed by an exploration of further layering with polycations.</p>