Modification of microcapsules for controlled release

Doctoral thesis, 2012

Fouling of marine organisms such as algae and barnacles on the boat hull is an enormous problem for the shipping industry. The negative consequences for the society are both economical and environmental. To prevent fouling in general, biocides are typically incorporated directly into the paint. Premature leakage of the biocides is a drawback which reduces the lifetime of the coating and pollutes the surrounding ecosystems. Microencapsulation is an efficient way of encapsulating active substances for controlling the release and thereby prolonging the antifouling properties of the coating. The microcapsules used in this work consist of an oil core and a hydrophobic polymer shell. The rate of release into the marine environment may be further tailored by modifying the microcapsules. Triggered release is achieved by rendering the microcapsule shell water sensitive. This may be accomplished by incorporating salt into the shell using imidazole coordination chemistry. On the other hand, extended release is achieved by improving the barrier properties of the microcapsule. This may be realized by providing the microcapsule with an additional shell, such as a highly charged polyelectrolyte multilayer or a lipid bilayer. The objective of this thesis is subsequently twofold: 1) To synthesize and characterize imidazole containing shell materials with a view to obtain triggered release. 2) To surface modify microcapsules with polyelectrolyte multilayers and lipid bilayers toward extended release. Imidazole containing polymers were synthesized using vinyl and maleimide radical polymerization, as well as grafting techniques comprising maleimide bond formation and epoxide ring opening. The imidazole-containing polymeric materials, with and without the salts CuCl2 or ZnCl2, were characterized using differential scanning calorimetry, electron paramagnetic resonance (EPR) and vibrational spectroscopy. The coordination chemistry of the imidazole-metal ion complex was investigated using vibrational spectroscopy, EPR and ab initio calculations. The imidazole coordination to the transition metal ions Cu2+ and Zn2+ in polymeric materials generates cross-links. The interaction between the imidazole moiety and the transition metal ions is very strong and specific. As a consequence, the coordinating polymer is rendered insoluble in conventional solvents, excluding only strongly coordinating solvents. The specificity and strength of the imidazole-transition metal ion interaction may be used for a variety of applications. However, with respect to the microencapsulation route used in this project, the limited solubility of the coordinating polymer material is unfortunate. The use of strongly coordinating solvents during the microencapsulation results in aggregation and phase separation instead of microcapsule formation. Routes for synthesising highly charged microcapsules for further surface modification were investigated using three types of ionic dispersants; a weak polyacid, a small set of amphiphilic block copolymers and a hydrophobic anionic surfactant in combination with a polycation. The charged microcapsules were subsequently modified with polyelectrolyte multilayers using the Layer-by-Layer technique and with lipid bilayers using lipid vesicle spreading. The microcapsules and model systems thereof were characterized mainly using micro-electrophoresis, light microscopy, optical tensiometry and quartz crystal microbalance with dissipation (QCM-D). The release behaviour in aqueous suspension of a hydrophobic model compound was investigated using UV-Vis spectroscopy. The use of the ionic dispersants facilitated the formation of highly charged microcapsules and the subsequent polyelectrolyte multilayer assembly and lipid bilayer formation were also successful. In particular, the block copolymer based microcapsules displayed excellent properties with respect to high and stable surface charge, as well as long term colloidal stability through electrostatic and steric stabilization. The release of the hydrophobic model compound was considerably reduced after modification with polyelectrolyte multilayers. In addition, the type of dispersant had a significant impact on the release. The block copolymer based microcapsules with a higher charge density had a much lower release compared to the weak polyelectrolyte based microcapsules. The polyelectrolyte multilayer is an efficient barrier against hydrophobic molecules and the low permeability is clearly a result of the high charge density. As of yet, the effect of the lipid bilayers on the release has not been investigated but has a large potential since the permeability may be altered by the lipid composition. A microcapsule consisting of an oil core, a hydrophobic polymer shell, a polyelectrolyte multilayer and a lipid bilayer is a complex release system with large degrees of freedom for tailoring the release behaviour.