Pickering Emulsifiers Based on Block Copolymer Nanoparticles Prepared by Polymerisation-Induced Self-Assembly

This Thesis evaluates the use of a range of block copolymer nanoparticles prepared by polymerisation-induced self-assembly (PISA) as putative Pickering emulsifiers. Firstly, reversible addition-fragmentation chain transfer (RAFT) solution polymerisation was used to prepare well-defined poly(glycerol monomethacrylate) (PGMA) chains bearing either carboxylic acid, tertiary amine or neutral end-groups. Each of these PGMA precursors was then chain-extended in turn via RAFT aqueous emulsion polymerisation of 2,2,2-trifluoroethyl methacrylate (TFEMA) to form spherical nanoparticles. High-shear homogenisation of n-dodecane in the presence of such sterically-stabilised nanoparticles led to the formation of oil-in-water Pickering macroemulsions. High-pressure microfluidisation was then used to prepare the three corresponding Pickering nanoemulsions. The nanoparticle adsorption efficiency at the oil/water interface was assessed for each type of nanoparticle at both pH 3 and pH 7 by gel permeation chromatography (GPC) using a UV detector. Nanoparticles with charged end-groups exhibited relatively low adsorption efficiency, whereas up to 90% of the neutral nanoparticles were adsorbed onto the oil droplets. This was confirmed using small-angle X-ray scattering (SAXS) experiments, which indicated that the packing efficiency of neutral nanoparticles around oil droplets was higher than that of nanoparticles bearing charged end-groups. Moreover, Pickering nanoemulsions stabilised with nanoparticles bearing charged end-groups proved to be significantly less stable than those prepared using neutral end-groups.
Sterically-stabilised diblock copolymer nanoparticles were prepared in n-dodecane using RAFT dispersion polymerisation. In the absence of any salt dissolved in the aqueous phase, high-pressure microfluidisation of precursor water-in-oil macroemulsions led to the formation of relatively large aqueous droplets, with dynamic light scattering (DLS) measurements indicating a mean diameter of more 600 nm. However, systemically increasing the salt concentration prior to microfluidisation produced finer aqueous droplets, until a limiting diameter of around 250 nm was obtained at 0.11 M NaCl. SAXS studies conducted on a nanoemulsion confirmed that the water droplets are coated with a loosely-packed monolayer of adsorbed nanoparticles. The effect of varying the amount of NaCl dissolved in the aqueous droplets on their initial rate of Ostwald ripening was investigated using DLS. Finally, the long-term stability of these water-in-oil Pickering nanoemulsions was examined using analytical centrifugation. The rate of droplet ripening can be substantially reduced by using 0.11 M NaCl instead of pure water. However, increasing the salt concentration up to 0.43 M provided no discernible improvement in long-term stability.
RAFT aqueous emulsion polymerisation of hydroxybutyl methacrylate (HBMA) has been conducted using a relatively short PGMA41 precursor as a steric stabiliser block. 1H NMR studies indicated that conversions of more than 99% were achieved within 2 h at 50 °C using a low-temperature VA-044 initiator. GPC analysis confirmed that high blocking efficiencies and relatively low dispersities (Mw/Mn < 1.37) could be achieved. A pseudo-phase diagram was constructed by systematically increasing the PHBMA target degree of polymerisation from 10 to 120 while varying the copolymer concentration between 5 and 20% w/w. The evolution in copolymer morphology when targeting PGMA41-PHBMA120 vesicles was monitored using TEM. This technique revealed intermediate morphologies that are similar to those reported during the preparation of PGMA47-PHPMA200 vesicles via RAFT aqueous dispersion polymerisation. Linear PGMA41-PHBMA110 vesicles survive high-shear homogenisation to act as genuine Pickering emulsifiers for n-dodecane droplets.
Finally, the first example of thermoresponsive diblock copolymer nano-objects prepared via RAFT aqueous emulsion polymerisation of HBA is reported. More specifically, a poly(ethylene glycol) (PEG) precursor is chain extended with HBMA. The resulting PEG45-PHBMA20 diblock copolymer exhibited three thermoreversible transitions in aqueous solution, as confirmed by DLS, SAXS, and rheology studies. Variable temperature 1H NMR studies suggest that these transitions are initially driven by dehydration of the PEG45 stabiliser block and subsequently facilitated by uniform hydration of the hydrophobic PHBMA block at higher temperatures.