High resolution characterisation of microcapsule structure, adhesion and release properties
This thesis aims to characterise the structural, adhesion and release properties of polymeric microcapsules which are used ina griculture for t he controlled delivery of pesticides to crops. An adaptation of atomic force microscopy (AFM) known as chemical force microscopy (CFM) has been used to investigate the adhesion characteristics of functionalised polyurea microcapsules to surfaces. The adhesion properties of microcapsules (attached to AFM cantilevers) to self-assembled monolayers (SAMs) of alkyl thiols on gold have been investigated at the single capsule level, and as a function of polymer wall surface functionality. Measurements have been made to OHterminated, CH3-terminated and mixed CH3: COOH-terminated SAMs. The importance of microcapsule topography and elasticity on adhesion measurements has been discussed. The surface pK1I2 of a capsule with a surface modified with a sulfonateterminated molecule has been estimated using force titration methodology. The adhesion properties of functionalised microcapsules to the leaf cuticle of Prunus laurocerasus have been examined, highlighting areas of hydrophilicity on the surface of the leaf. The release properties of a ctive ingredients from p olyurea microcapsules into solution using confocal laser scanning microscopy (CLSM) have been investigated. Preliminary studies obtained the optimum species to use inside the microcapsule and the solution into which the species should be released. The release characteristics 0 f six varieties of microcapsule, differing in the wall thickness and cross linking density were examined and from the release profiles obtained, the product of the mass transfer coefficient and solubility coefficient could be calculated. The high resolution imaging capabilities of the confocal microscope have been demonstrated, providing detail on the structure of the leaf of the Prunus Laurocerasus plant and t he caterpillars Heliothis virescens and P lutella xylostella, two of t he most common pests, which the microcapsules of interest are aimed at controlling. The release properties to leaf surfaces of two types of polyurea microcapsules, which have different release mechanisms, were investigated using CLSM. Detailed release studies to model surfaces showed that one type of capsule (lambda cyhalothrin) released in the dry state and the other (emamectin benzoate) did not. A release rate was determined in the former case. A technique has been developed which is capable of visualising the release and diffusion of a species (Cd2+) from a hemisphere. Voltammetry has been used in conjunction with CLSM to monitor quantitatively and visually the preconcentration and release of Cd2+ into and away from a mercury hemisphere UME. The fluorescent indicator used was Calcium Green-5N, which is a cadmium-sensitive fluorophore that fluoresces upon binding Cd2+and can thus be detected by CLSM. A delay was observed between the release of C d2+ from t he hemisphere (determined voltammetrically) and binding to the fluorophore (visualised using CLSM).