Are block copolymer worms more effective Pickering emulsifiers than block copolymer spheres?

Abstract

RAFT-mediated polymerisation-induced self-assembly (PISA) is used to prepare six types of amphiphilic block copolymer nanoparticles which were subsequently evaluated as putative Pickering emulsifiers for the stabilisation of n-dodecane-in-water emulsions. It was found that linear poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) (PGMA-PHPMA) diblock copolymer spheres and worms do not survive the high shear homogenisation conditions used for emulsification. Stable emulsions are obtained, but the copolymer acts as a polymeric surfactant; individual chains rather than particles are adsorbed at the oil-water interface. Particle dissociation during emulsification is attributed to the weakly hydrophobic character of the PHPMA block. Covalent stabilisation of these copolymer spheres or worms can be readily achieved by addition of ethylene glycol dimethacrylate (EGDMA) during the PISA synthesis. TEM studies confirm that the resulting cross-linked spherical or worm-like nanoparticles survive emulsification and produce genuine Pickering emulsions. Alternatively, stabilisation can be achieved by either replacing or supplementing the PHPMA block with the more hydrophobic poly(benzyl methacrylate) (PBzMA). The resulting linear spheres or worms also survive emulsification and produce stable n-dodecane-in-water Pickering emulsions. The intrinsic advantages of anisotropic worms over isotropic spheres for the preparation of Pickering emulsions are highlighted. The former particles are more strongly adsorbed at similar efficiencies compared to spheres and also enable smaller oil droplets to be produced for a given copolymer concentration. The scalable nature of PISA formulations augurs well for potential applications of anisotropic block copolymer nanoparticles as Pickering emulsifiers.

A Critical Appraisal of RAFT-Mediated Polymerization-Induced Self-Assembly

“…35,73,120 Similarly, hydrophobic diblock copolymer nanoparticles can be designed to stabilize water-in-oil emulsions. 121 By judiciously combining these two types of nanoparticles (and optimizing the homogenization conditions), Thompson and co-workers exploited PISA to produce Pickering double emulsions .…”

Polymerization-induced self-assembly of block copolymer nanoparticles via RAFT non-aqueous dispersion polymerization

“…In essence, hydrophilic particles typically form oil-in-water (o/w) emulsions, whereas hydrophobic particles usually stabilize water-in-oil (w/o) emulsions. The development of robust PISA formulations provided a timely opportunity to compare the performance of hydrophilic block copolymer spheres, worms or vesicles synthesized via RAFT aqueous dispersion polymerization for the preparation of o/w Pickering emulsions [20,40,108]. One interesting question in this context is whether worms offer any advantages over spheres.…”

“…One interesting question in this context is whether worms offer any advantages over spheres. A rudimentary analysis suggests that worms should be more effective Pickering emulsifiers since they possess a relatively high specific surface area yet are much more strongly adsorbed at the oil/water interface [40,109] [109]. These worms undergo a worm-to-sphere transition on heating to 150 °C [96].…”

“…Perhaps the most intensively studied formulations involve the chain extension of poly(glycerol monomethacrylate) (PGMA) [22,32,33], poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) [12] or poly(ethylene glycol) (PEG) [34] macro-CTAs using HPMA to generate the core-forming PHPMA block. Such sterically-stabilized nanoparticles have several potential applications, including coatings [37], drug delivery [38], sterilizable gels [39] and novel Pickering emulsifiers [40]. More complex morphologies have also been reported for certain RAFT aqueous dispersion polymerization formulations, including so-called 'lumpy rods' [41], 'framboidal' vesicles [23], and oligolamellar vesicles [34].…”

Effect of fullerene on the dispersibility of nanostructured lipid particles and encapsulation in sterically stabilized emulsions

“…Some surfactant emulsions are co-stabilized by more than one type of mechanisms [39,40]. In our recent work we stabilized lipid based emulsions using a range of CNTs.…”