Protected sulphonate nanoparticles, thiol-Michael functionalisation of nanogel decorated nanoparticles, and thiol-Michael microcapsules for agricultural mobility and adhesion applications

Targeted delivery of an active ingredient to plant systems required extensive studies in areas of soil mobility, plant root adhesion and encapsulation. To accomplish the aims of mobility and adhesion to roots, specific surface charges and colloidal forces are required. These charges are required to provide sufficient repulsive forces preventing adhesion to soil and attractive forces allowing adhesion to roots. To accomplish the aims of synthesising colloids for mobility, adhesion and also encapsulation we formed a specific strategy. We aimed to increase the anionic surface charge on colloidal nanoparticles as a means to aid mobility studies of colloids through soil by providing increased repulsive forces to prevent adhesion to both air interfaces and soil. We also designed a way to decorate colloidal nanoparticles with microgel particles thus forming a soft adhesive surface layer with which further functionality could be incorporated i.e. surface charges. Finally we designed a simple methodology for the encapsulation of oil with post functionalisation in mind. We synthesised colloidal nanoparticles, less than 200 nm, using soap-free emulsion polymerisation of styrene where up to 50 wt% of protected ethyl styrene sulphonate monomer was incorporated. The nanoparticles were hydrolysed to release the sulphonic charge using thermolysis in an autoclave. Soap-free emulsion polymerisation was used to synthesise raspberry-like and core-shell morphology decorated nanoparticles by the adhesion of crosslinked nanogel particles to a range of hydrophobic and hydrophilic seeds. Poly(acrylic) and poly(styrenic) seeds were synthesised with a range of hydrophobicities whereby more decoration of nanogel particles was achieved for hydrophilic seeds. The nanogel decoration was studied using di-, tri- and multi-functional acrylates and the morphologies changed from raspberry-like to core-shell as the functionality of the crosslinker increased. The presence of Vinyl functional groups was proved using Raman microscopy. Post functionalisation with different thiols was carried out using nucleophilic thiol-Michael addition chemistry and a loss of vinyl groups was shown. Encapsulation of aromatic oil was carried out using an interfacial thiol-Michael addition reaction between a tetra- functional thiol and a penta-/hexa- functional ene using an amine nucleophilic catalyst. A microfluidic device was used to control the capsule sizes and dispersity and light microscopy was used to study capsule sizes and dry-down properties. No rupture was observed for the millimetre and microcapsules produced but unfortunately size scale-down could not be accomplished for the current system using high shear homogenisation. In conclusion we increased the loading of sulphonate charge for colloidal poly(styrene) nanoparticles and we successfully increased the surface area of styrenic and acrylic seed nanoparticles using nanogel particle adhesion. We post functionalised decorated nanoparticles using thiol-Michael addition reactions and synthesised capsules using thiol-Michael addition reactions confined in a microfluidic device.