A near-monodisperse monohydroxy-terminated polydimethylsiloxane (PDMS; mean degree of polymerization = 66) was esterified using a carboxylic acid-functionalized trithiocarbonate to yield a PDMS66 precursor with a mean degree of functionality of 92 ± 5 % as determined by 1 H NMR spectroscopy. This PDMS66 precursor was then chain-extended in turn using nine different methacrylic monomers in a low-viscosity silicone oil (decamethylcyclopentasiloxane, D5). Depending on the monomer type, such PISA syntheses proceeded via either RAFT dispersion polymerization or RAFT emulsion polymerization. In each case the target DP of the core-forming block was fixed at 200 and the copolymer concentration was 25 % w/w. Transmission electron microscopy studies indicated that kinetically-trapped spheres were obtained in almost all cases. The only exception was 2-(dimethylamino)ethyl methacrylate (DMA), which enabled access to spheres, worm or vesicles. This striking difference is attributed to the relatively low glass transition temperature for this latter block. A phase diagram was constructed for a series of PDMS66-PDMAx nano-objects by systematically increasing the PDMA target DP from 20 to 220 and varying the copolymer concentration between 10 and 30 % w/w. Higher copolymer concentrations were required to access a pure worm phase, whereas only spheres, vesicles or mixed phases were accessible at lower copolymer concentrations. Gel permeation chromatography studies indicated a linear evolution of number-average molecular weight (Mn) with PDMA DP while dispersities remained below 1.39, suggesting relatively well-controlled RAFT polymerizations. Small angle x-ray scattering (SAXS) was used to characterize selected examples of spheres, worms and vesicles. PDMS66-PDMA100-112 worms synthesized at 25-30 % w/w formed freestanding gels at 20 °C. Oscillatory rheology studies performed on a 30 % w/w PDMS-PDMA105 worm dispersion indicated a storage modulus (gel strength) of 1057 Pa and a critical gelation concentration (CGC) of 12 % w/w. Finally, PDMS66-PDMAx worms could also be prepared in n-dodecane, hexamethyldisiloxane or octamethylcyclosiloxane. Rotational rheometry studies indicate that such worms are efficient viscosity modifiers for these non-polar oils.

RAFT dispersion polymerization of 2-hydroxypropyl methacrylate produces diblock copolymer spheres, worms or vesicles in mineral oil; the Pickering emulsifier performance of the spheres is examined.

Sterically stabilized diblock copolymer nanoparticles are prepared in n -dodecane using polymerization-induced self-assembly. Precursor Pickering macroemulsions are then prepared by the addition of water followed by high-shear homogenization. In the absence of any salt, high-pressure microfluidization of such precursor emulsions leads to the formation of relatively large aqueous droplets with DLS measurements indicating a mean diameter of more than 600 nm. However, systemically increasing the salt concentration produces significantly finer droplets after microfluidization, until a limiting diameter of around 250 nm is obtained at 0.11 M NaCl. The mean size of these aqueous droplets can also be tuned by systematically varying the nanoparticle concentration, applied pressure, and the number of passes through the microfluidizer. The mean number of nanoparticles adsorbed onto each aqueous droplet and their packing efficiency are calculated. SAXS studies conducted on a Pickering nanoemulsion prepared using 0.11 M NaCl confirms that the aqueous droplets are coated with a loosely packed monolayer of nanoparticles. The effect of varying the NaCl concentration within the droplets on their initial rate of Ostwald ripening is investigated using DLS. Finally, the long-term stability of these water-in-oil Pickering nanoemulsions is assessed 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 further improvement in the long-term stability of such nanoemulsions.

Reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization-induced self-assembly (PISA) is used to prepare epoxy-functional PGMA48-P(HPMA90-stat-GlyMA15) diblock copolymer worms, where GMA, HPMA and GlyMA denote glycerol monomethacrylate, 2hydroxypropyl methacrylate and glycidyl methacrylate, respectively and the subscripts indicate the mean degrees of polymerization. The epoxy groups on the GlyMA residues were ring-opened using 2aminopropyltriethoxysilane (APTES) in order to cross-link the worm cores via a series of hydrolysiscondensation reactions. Importantly, the worm aspect ratio can be adjusted depending on the precise conditions selected for covalent stabilization. Relatively long cross-linked worms are obtained via APTES crosslinking conducted at 20 °C, whereas much shorter worms with essentially the same copolymer composition are formed by cooling the linear worms from 20 °C to 4 °C prior to APTES addition. Smallangle X-ray scattering (SAXS) studies confirmed that the mean aspect ratio for the long worms is approximately eight times greater than that for the short worms. Aqueous electrophoresis studies indicated that both types of cross-linked worms acquired weakly cationic surface charge at low pH as a result of protonation of APTES-derived secondary amine groups within the nanoparticle cores. These cross-linked worms were evaluated as emulsifiers for the stabilization of n-dodecane-in-water emulsions via highshear homogenization at 20 °C and pH 8. Increasing the copolymer concentration led to a reduction in mean droplet diameter, indicating that APTES cross-linking was sufficient to allow the nanoparticles to adsorb intact at the oil/water interface and hence produce genuine Pickering emulsions, rather than undergo in situ dissociation to form surface-active diblock copolymer chains. In surfactant challenge studies, the relatively long worms required a thirty-fold higher concentration of a non-ionic surfactant (Tween 80) to be displaced from the n-dodecane-water interface compared to the short worms. This suggests that the former nanoparticles are much more strongly adsorbed than the latter. In contrast, colloidosomes prepared by reacting the hydroxyl-functional adsorbed worms with an oil-soluble polymeric diisocyanate remained intact when exposed to high concentrations of Tween 80.

Block copolymer nanoparticles prepared via polymerization-induced self-assembly (PISA) represent an emerging class of organic Pickering emulsifiers. Such nanoparticles are readily prepared by chain-extending a soluble homopolymer precursor using a carefully selected second monomer that forms an insoluble block in the chosen solvent. As the second block grows, it undergoes phase separation that drives in situ self-assembly to form sterically stabilized nanoparticles. Conducting such PISA syntheses in aqueous solution leads to hydrophilic nanoparticles that enable the formation of oil-in-water emulsions. Alternatively, hydrophobic nanoparticles can be prepared in non-polar media (e.g., n -alkanes), which enables water-in-oil emulsions to be produced. In this review, the specific advantages of using PISA to prepare such bespoke Pickering emulsifiers are highlighted, which include fine control over particle size, copolymer morphology, and surface wettability. This has enabled various fundamental scientific questions regarding Pickering emulsions to be addressed. Moreover, block copolymer nanoparticles can be used to prepare Pickering emulsions over various length scales, with mean droplet diameters ranging from millimeters to less than 200 nm.