How do charged end-groups on the steric stabilizer block influence the formation and long-term stability of Pickering nanoemulsions prepared using sterically stabilized diblock copolymer nanoparticles?

Reversible addition–fragmentation chain transfer (RAFT) solution polymerization is used to prepare well-defined poly(glycerol monomethacrylate) (PGMA) chains bearing carboxylic acid, tertiary amine, or neutral end-groups. Each of these PGMA precursors was then chain-extended in turn via RAFT aqueous emulsion polymerization of 2,2,2-trifluoroethyl methacrylate to form spherical nanoparticles as confirmed by transmission electron microscopy (TEM) analysis. Dynamic light scattering studies indicated an intensity-average diameter of approximately 25 nm. Aqueous electrophoresis measurements confirmed that the amine-functional nanoparticles became cationic at low pH owing to end-group protonation. In contrast, carboxylic acid-functional nanoparticles became appreciably anionic at pH 10 owing to end-group ionization. Finally, nanoparticles bearing neutral end-groups exhibited zeta potentials close to zero over a range of solution pH. High-shear homogenization of n-dodecane in the presence of such sterically stabilized nanoparticles led to the formation of oil-in-water Pickering macroemulsions with volume-average diameters of 20–30 μm. High-pressure microfluidization was then used to prepare the three corresponding Pickering nanoemulsions. Each Pickering nanoemulsion was characterized by analytical centrifugation and TEM studies of the dried nanoemulsion droplets confirmed their original nanoparticle superstructure. The nanoparticle adsorption efficiency at the oil–water interface was assessed by gel permeation chromatography (using a UV detector) for each nanoparticle type at both pH 3 and 7. 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 observation was supported by small-angle X-ray scattering experiments, which indicated that the packing efficiency of neutral nanoparticles around oil droplets was higher than that of nanoparticles bearing charged end-groups. Analytical centrifugation was used to evaluate the colloidal stability of the aged Pickering nanoemulsions. Pickering nanoemulsions stabilized with nanoparticles bearing charged end-groups proved to be significantly less stable than those prepared using neutral end-groups

Synthesis of diblock copolymer spheres, worms and vesicles via RAFT aqueous emulsion polymerization of hydroxybutyl methacrylate

RAFT aqueous emulsion polymerization of hydroxybutyl methacrylate using a poly(glycerol monomethacrylate) precursor leads to diblock copolymer spheres, worms or vesicles. A pseudo-phase diagram is constructed and the vesicles are briefly evaluated as a Pickering emulsifier

Synthesis of thermoresponsive diblock copolymer nano-objects via RAFT aqueous emulsion polymerization of hydroxybutyl methacrylate

We recently reported that the reversible addition–fragmentation chain transfer (RAFT) aqueous emulsion polymerization of hydroxybutyl methacrylate (HBMA) using a relatively short non-ionic poly(glycerol monomethacrylate) (PGMA) precursor enables convenient preparation of diblock copolymer nano-objects with spherical, worm-like, or vesicular morphologies. We postulated that the relatively high aqueous solubility of HBMA (∼25 g dm–3 at 50 °C) was likely to be a key parameter for overcoming the problem of kinetically trapped spheres that is observed for many RAFT aqueous emulsion polymerization formulations. In this study, we revisit the RAFT aqueous emulsion polymerization of HBMA using a poly(ethylene glycol) (PEG) precursor as a steric stabilizer block. Remarkably, the resulting PEG45–PHBMA20 diblock copolymer nanoparticles exhibit thermoreversible morphological transitions in aqueous solution. More specifically, transmission electron microscopy and small-angle X-ray scattering studies confirmed that spheres are formed at 25 °C, worms at 58 °C, and vesicles at 65 °C. This is the first time that such behavior has been reported for nano-objects prepared by RAFT aqueous emulsion polymerization. Moreover, variable temperature dynamic light scattering and oscillatory rheology studies confirmed that these transitions are highly reversible at 0.1 and 10% w/w, respectively. Variable temperature 1H NMR studies indicated that (i) the PEG stabilizer block undergoes dehydration on heating and (ii) the apparent degree of hydration of the hydrophobic PHBMA block increases on heating from 25 to 65 °C. This suggests that the change in copolymer morphology is best explained in terms of a uniform plasticization mechanism

Track D Social Science, Human Rights and Political Science

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138414/1/jia218442.pd

Effect of salt on the formation and stability of water-in-oil Pickering nanoemulsions stabilized by diblock copolymer nanoparticles

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