AmericanChemicalSociety.com

The goals of this research project are to develop star copolymers that function as highly efficient emulsifiers, meaning that they stabilize oil/water emulsions at extremely low emulsifier concentrations, and to deduce the relationship between star copolymer architecture and emulsification efficiency. 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 are synthesized by atom transfer radical polymerization (ATRP). In addition, inorganic silica nanoparticles with polymer brushes grafted by ATRP from their surfaces are also being synthesized for comparison. Amphiphilic miktoarm star copolymers have been 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 greater than 13 weeks using 0.05 wt% of the PEG stars and cyclohexane-in-water emulsions for greater than 13 weeks using just 0.008 wt% PEG stars. At 0.005 wt% PEG stars cyclohexane-in-water and xylene-in-water emulsions were stabilized, but their shelf-lives were limited to 2.5 weeks. In all cases, emulsions have been oil-in-water. That is, the dispersed droplets were oil, and the continuous phase was water.

When the overall oil-to-water ratio was 1:1, the emulsion coexisted with a neat water phase. 100% of the oil was emulsified, and the emulsion phases were approximately 70 vol% oil. When mixed at an overall composition of 70 vol% oil and 30 vol% water it was possible to emulsify all components with no neat oil or water phase present.

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, was 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 the structure of the emulsions did change. While coalescence could not be triggered, the emulsion droplets flocculated irreversibly as a result of the temperature increase. This produced a lasting increase in the emulsion viscosity. Thus, these emulsifiers did produce a system with thermally responsive mechanical properties. Emulsion rheology is the subject of ongoing research.

Emulsification synergism between PEG stars and Si-PDMAEMA was investigated. By combining these two highly efficient emulsifiers, emulsions were stabilized at a total emulsifier concentration of 0.003 wt%, compared to 0.05 wt% or 0.008 wt% for either one acting alone. This demonstrates the possibility of exploiting synergism among brushy nanoparticulate emulsifiers to achieve even greater efficiency.

Microbubble tensiometry was used to measure interfacial tension lowering characteristics of these emulsifiers. Whereas bare nanoparticles produce extremely small interfacial tension reductions, often less than 1 mN/m, Si- PDMAEMA particles decrease interfacial tensions by approximately 30 mN/m and produce Gibbs elasticities that are comparable to ungrafted PDMAEMA homopolymers in solution.

Plans for the upcoming year include ellipsometry measurements of PEG star and Si-PDMAEMA adsorption isotherms at oil/water interface and tensiometry measurements of PEG star interfacial tension isotherms and Gibbs elasticities to help determine why these systems are so efficient and why PEG stars are notably more so than Si-PDMAEMA. In addition, the relative merits of the star copolymer morphology and the polymer grafted silica nanoparticle morphology will be compared. PDMAEMA star copolymers are to be synthesized and tested for emulsification efficiency, as well as PEG-grafted silica nanoparticles. This should indicate whether the efficiency is mainly determined by the polymer type or the overall morphology.