Polymer architecture dictates thermoreversible gelation in engineered emulsions stabilised with branched copolymer surfactants

Abstract

The generation of materials that switch from a liquid to gel state upon warming can enable new healthcare technologies with improved functionality, such as in situ gel-forming materials for drug delivery to topical or parenteral sites. The majority of these materials are aqueous polymer solutions, which then suffer from an inability to solubilise hydrophobic drugs. This study investigates the generation of thermoresponsive “engineered emulsions” which are low-viscosity emulsions at low temperature and switch to a gel state upon warming. This is achieved by the synthesis of novel branched copolymer surfactants (BCS) containing di(ethylene glycol) methyl ether methacrylate (DEGMA) as a thermoresponsive component giving a lower critical solution temperature (LCST). The copolymers were employed as emulsifiers to prepare 1 : 1 dodecane:water emulsion systems. The effect of polymer architecture is shown to be intimately linked to the rheology of these systems, where branching, elevation of molecular weight, and the presence of hydrophobic end groups is demonstrated to be commensurate with gel formation upon heating. Mechanisms of gel formation were probed by small-angle neutron scattering, which demonstrated that the branched copolymer surfactants formed oblate ellipsoids in solution that grew anisotropically with temperature, forming larger disk-like nanoparticles. The formation of these elongated particles leads to thickening of the emulsions, whilst connectivity of the aggregates and BCS at the oil–water interface is required for gel formation to occur. Overall, the study provides design principles for this novel class of thermoresponsive material with great potential in healthcare, cosmetic, and energy applications.