In this Thesis, reversible addition-fragmentation chain transfer (RAFT) polymerization has been used to prepare various types of sterically-stabilized diblock copolymer nanoparticles via polymerization-induced self-assembly (PISA). These nanoparticles are then evaluated as putative dispersants for a specific agrochemical formulation known as a suspension concentrate (SC).
Firstly, RAFT aqueous emulsion polymerization of methyl methacrylate (MMA) was conducted at 70 °C using poly(glycerol monomethacrylate) (PGMA) as a water-soluble precursor block. Targeting a mean degree of polymerization (DP) of 20 to 100 for the PMMA block led to colloidal dispersions of kinetically-trapped spheres with z-average diameters ranging from 17 nm to 31 nm. However, targeting DPs above 100 only produced aggregates comprising flocculated spheres. This unexpected constraint appears to be related to the relatively high glass transition temperature (Tg) of the PMMA block. In contrast, colloidally stable dispersions could be obtained when targeting similar (or higher) PMMA DPs simply by switching from a non-ionic PGMA stabilizer to either an anionic poly(methacrylic acid) stabilizer or a cationic poly(2-(methacryloyloxy)ethyl trimethylammonium chloride) stabilizer. RAFT end-groups from PGMA50-PMMA80 nanoparticles were cleaved by visible light irradiation using a blue LED source (λ = 405 nm). UV GPC studies indicated that 87% dithiobenzoate end-groups were removed within 12 h at 80 °C. This approach proved to be more effective than using excess H2O2 under the same conditions, presumably because such water-soluble reagents suffer from restricted access to the hydrophobic PMMA cores.
PGMA50-PMMA80 nanoparticles were evaluated as a dispersant for the preparation of organic crystalline microparticles of a widely-used fungicide, azoxystrobin, via ball milling. Stable SCs were readily obtained at 20% w/w solids after milling for 10 min. Laser diffraction and optical microscopy studies indicate the formation of ~2 μm azoxystrobin microparticles, which is comparable to that obtained using commercial water-soluble dispersants such as Morwet D-425. Nanoparticle adsorption onto the surface of the azoxystrobin microparticles was confirmed by electron microscopy studies. An adsorbed amount of approximately 5.5 mg m–2 was estimated using a supernatant assay based on solution densitometry. Moreover, further evidence for nanoparticle adsorption was provided by aqueous electrophoresis and X-ray photoelectron spectroscopy (XPS) studies. Similar data were obtained when changing the nature of the core-forming block from PMMA to poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA). Fractional nanoparticle surface coverages were 0.24 to 0.28 by XPS.
Follow-up studies examined the effect of varying the nature of the steric stabilizer block, the mean nanoparticle diameter and the Tg of the core-forming block on the particle size and colloidal stability of the nanoparticle-coated azoxystrobin microparticles. Diblock copolymer nanoparticles prepared using a non-ionic steric stabilizer – rather than a cationic or an anionic steric stabilizer – were demonstrated to be more effective dispersants. Furthermore, nanoparticles of up to 51 nm diameter enabled efficient milling and produced stable SCs, whereas larger nanoparticles proved to be less effective. Moreover, crosslinking the nanoparticle cores and lowering the Tg of the core-forming block had little effect on the formation of azoxystrobin microparticles. This versatile approach was also shown to be applicable to five other organic crystalline agrochemicals, suggesting generic behaviour.
Finally, ball milling was used to produce anthracene microparticles of 2 to 4 μm diameter in the presence of an anionic commercial polymeric dispersant (Morwet D-425) using two different ball milling techniques. These anthracene microparticles were then coated with a thin overlayer of polypyrrole (PPy), which is an air-stable organic conducting polymer. The uncoated and PPy-coated anthracene microparticles were characterized using optical microscopy, scanning electron microscopy, laser diffraction, aqueous electrophoresis, FT-IR spectroscopy, Raman microscopy, and XPS. Moreover, our collaborations with space scientists at U. Kent confirmed that both types of microparticles can be accelerated up to hypervelocities (~ 6 km s-1) using a light gas gun. Thus, these PPy-coated anthracene microparticles are expected to serve as the first useful synthetic mimic for understanding the behaviour of polyaromatic hydrocarbon-based cosmic dust.
