Layer-by-layer deposition of clay and a polycation to control diffusive release from polyurea microcapsules

Polyurea microcapsules containing model organic fills were coated with clay and polycation layers to control diffusive release. This control process allows post-modification of the release rate without interfering with capsule formation. Microcapsules prepared by interfacial polyaddition of polymethylene polyphenyl isocyanate and diethylenetriamine were chosen as representative polyurea capsules having cationic surface charges. The microcapsules were coated in sequence with anionic delaminated montmorillonite clay, and a polycation, poly(N-[3-aminopropyl]-methacrylamide hydrochloride). Deposition of clay was mapped and quantified using transmission electron microscopy and thermal gravimetric analysis, respectively, while deposition of the polycation was followed using fluorescently labeled polymers. Analysis by TEM and TGA indicated that the first coating resulted in a homogeneous clay layer approximately 15 nm thick corresponding to 4.2 ± 1 wt% relative to the polyurea. Release of volatile fills was found to be reduced by about 50% after deposition of this first layer. Surprisingly, the increased barrier properties of the wall were lost when the capsules were coated with the polycation and then restored again when a second clay coating was applied.