Synthesis, Characterization, and Pickering Emulsifier Performance of Anisotropic Cross-Linked Block Copolymer Worms: Effect of Aspect Ratio on Emulsion Stability in the Presence of Surfactant

Abstract

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.

Synthesis of poly(stearyl methacrylate)-poly(2-hydroxypropyl methacrylate) diblock copolymer nanoparticles via RAFT dispersion polymerization of 2-hydroxypropyl methacrylate in mineral oil

“…According to the literature, high-shear homogenization of diblock copolymer nano-objects can cause in situ dissociation in some cases, which leads to the formation of emulsion droplets stabilized by individual diblock copolymer chains acting as a polymeric surfactant. 52,53,55,75 Moreover, such instances typically involve PHPMA-core nanoparticles, albeit those prepared via aqueous PISA formulations rather than the present non-aqueous formulation. 52,53,55 Fortunately, whether such nano-object dissociation actually occurs in practice can be assessed by determining the variation in mean droplet diameter with copolymer concentration.…”

“…51 Since 2012, various types of diblock copolymer nano-objects have been evaluated as Pickering emulsifiers. [52][53][54][55] For example, hydrophilic diblock copolymer nanoparticles prepared via aqueous PISA enable the formation of oil-in-water (o/w) Pickering emulsions. [53][54][55] In contrast, hydrophobic nanoparticles prepared in non-polar solvents such as n-alkanes or mineral oil favour the formation of water-in-oil (w/o) emulsions.…”

“…[52][53][54][55] For example, hydrophilic diblock copolymer nanoparticles prepared via aqueous PISA enable the formation of oil-in-water (o/w) Pickering emulsions. [53][54][55] In contrast, hydrophobic nanoparticles prepared in non-polar solvents such as n-alkanes or mineral oil favour the formation of water-in-oil (w/o) emulsions. 56,57 Interestingly, Thompson et al utilized hydrophobic and hydrophilic worms in turn to prepare waterin-oil-in-water (w/o/w) Pickering emulsions.…”

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

“…71,86,93,94 In contrast, Thompson and coworkers reported essentially no change in the mean droplet diameter with copolymer concentration when using linear PGMA 45 -PHPMA 200 vesicles, which is consistent with the in situ break-up of these more delicate nano-objects during high-shear homogenization. 71,81,92 TEM studies of the dried emulsion droplets (see Fig. 6a) confirmed that linear PGMA 41 -PHBMA 110 vesicles do indeed remain intact after high-shear homogenization.…”

“…Preparation of Pickering emulsions using PGMA-PHBMA vesicles RAFT-mediated PISA enables the design of a wide range of surface-active block copolymer nanoparticles that can stabilize either oil 42,71,[81][82][83] or water droplets, [84][85][86][87] as summarized in a recent review. 88 Particular attention has been paid to vesicles since they were the initial diblock copolymer nanoparticles to be utilized as Pickering emulsifiers.…”