Reports: AC747430-AC7: Miktoarm Star Copolymer Interfacial Activity and Emulsion Stabilization

Robert D. Tilton , Carnegie Mellon University

This project produced star copolymers that function as highly efficient emulsifiers, meaning that they stabilize oil/water emulsions at extremely low emulsifier concentrations. These star copolymers have insoluble polymer cores surrounded by extended polymer chains, in essence representing nanoparticles stabilized by polymer brushes.  Emulsification entails the generation of extremely large interfacial areas as droplets of the dispersed phase are generated within the continuous phase liquid.  The individual droplet surfaces must be stabilized by adsorption of the emulsifier in order to stabilize the overall emulsion against droplet coalescence and macroscopic phase separation. Thus, the star copolymers are designed to adsorb with high affinity to the oil/water interface to ensure that the vast majority of the copolymers reside on droplet surfaces.  Given their resemblance to nanoparticles, this mode of emulsification by adsorption of nanoparticulate star copolymers can be considered as Pickering emulsification: emulsification by adsorption of particles rather than conventional molecular surfactants.  Typically, Pickering emulsions require on the order of 1 wt% particle concentrations for stability, sometimes as low as several tenths of a wt%, Once formed, Pickering emulsions tend to be extremely stable, resisting coalescence for many months. The objective for this research has been to achieve that level of emulsion stability using emulsifier concentrations of hundredths or thousandths of a wt %.

The star copolymers were synthesized by atom transfer radical polymerization (ATRP).  In addition, inorganic silica nanoparticles with polymer brushes grafted by ATRP from their surfaces were also synthesized for comparison.  Amphiphilic miktoarm star copolymers were synthesized with a mixture of hydrophilic poly(acrylic acid) (PAA) and hydrophobic poly(butylacrylate) (PBA) arms in a variety of size ratios.  These were used to stabilize emulsions of water with either cyclohexane or xylene, where the former is a poor solvent for PBA and the latter is a good solvent for PBA.  It was anticipated that the amphiphilicity of the stars would increase their adsorption affinity for the oil/water interface and enhance emulsification efficiency. This was not realized, as concentrations on the order to 1 wt% were required to produce stable emulsions for either xylene or cyclohexane.  Far greater emulsification efficiency was achieved by a star copolymer prepared by polymerization of poly(ethylene glycol) methacrylate macromonomer with divinylbenzene crosslinking agents.  The emulsion system again consisted of water with either cyclohexane (poor solvent for PEG) or xylene (good solvent for PEG). These “PEG stars” were extremely efficient emulsifiers, stabilizing xylene-in-water emulsions for several months using 0.05 wt% of the PEG stars and cyclohexane-in-water emulsions using just 0.008 wt% PEG stars. 

The thermal responsiveness of emulsions stabilized by PEG stars was investigated.  Thermal responsiveness, whereby a stable emulsion can be broken rapidly in response to a temperature increase, had recently achieved in this lab using silica nanoparticles with grafted poly(dimethylaminoethyl methacrylate) (Si-PDMAEMA).  Si-PDMAEMA stabilized cyclohexane-in-water and xylene-in-water emulsions at concentrations as low as 0.05 wt%, achieving 6 – 13 month stability against coalescence. These emulsions broke rapidly, within less than a minute, when the emulsions were heated to 50 C.  This temperature coincided with the critical flocculation temperature (CFT) of the Si-PDMAEMA nanoparticles in aqueous suspension. Like PDMAEMA, PEG has a lower critical solution temperature (LCST) in water, making PEG stars a potentially thermally responsive emulsifier.  Whereas PEG homopolymers have an LCST near 100 C, PEG stars aggregate at significantly lower temperatures due to the hydrophobicity of the divinylbenzene crosslinked cores. Using turbidity measurements, it was determined that the CFT of PEG stars could be lowered to approximately 50 C by adding phosphate salts to decrease PEG solubility in water.  Unlike Si-PDMAEMA stabilized emulsions that broke when heated to the CFT, emulsions stabilized by PEG stars did not break upon heating.  They remained stable, but microscopic observation showed that heating caused irreversible droplet flocculation.  This produced a lasting increase in the emulsion viscous and elastic moduli.  Thus, these emulsifiers produced a system with thermally responsive mechanical properties.   Emulsions stabilized by a PAA star copolymer were pH-responsive, breaking rapidly in response to acidification.

This grant provided partial financial support for the education of one Ph.D. student, Trishna Saigal.  It also provided materials and research projects for three undergraduate research students, Mana Ameri, Alex Yoshikawa and Masanari Kato.  These students investigated, respectively, the potential application of the materials developed here as foaming agents, emulsion rheology and the electrolyte dependence of PEG star critical flocculation temperatures.  This grant enabled the principal investigator to begin a new research thrust focusing on the applications of nanoscale polymer brush materials and their underlying fundamental interfacial behaviors.

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