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Particle-stabilized “Pickering” emulsions are noted for their excellent stability and ability to disperse very large volume fractions of the discontinuous liquid phase, relative to conventional surfactant-stabilized emulsions. Under this research grant, novel star copolymers with well-controlled compositions and architectures are being synthesized by atom transfer radical polymerization (ATRP) for use as high efficiency nanoparticulate emulsifiers.  Emulsions are not thermodynamically stable.  They can be produced and engineered to be kinetically stable in the sense that the inevitable process of emulsion breaking can be made to be rather slow. Particulate emulsifiers are often extremely effective at kinetically stabilizing emulsions, and polymer-grafted nanoparticles have been shown to be particularly so.

To form an emulsion, some surface active agent is required to adsorb to the oil/water interface of the droplets.  This is typically a molecular surfactant or a polymer.  They decrease the interfacial tension to minimize the penalty for creating the highly dispersed oil/water mixture. They also may affect viscosity and provide repulsive colloidal forces that act across the thin liquid films that separate droplets near contact.   The combination of decreased interfacial tension, repulsive colloidal forces, and altered thin film viscosity decreases the rate of coalescence of emulsion droplets, resulting in kinetic stability.

When emulsion droplets coalesce, the total surface area decreases and some of the surface active agents must desorb.  Thus, the adsorption energy may strongly influence emulsion stability.  The key feature of particulate emulsifiers is that they often have extremely large adsorption energies, such that the barrier against particle desorption becomes prohibitively large and droplet coalescence becomes highly unfavorable.  The stability of particle-stabilized emulsions can be engineered by modifying particle surfaces, especially by amphiphilic polymers, to maximize the adsorption energy and also to control colloidal forces acting at the droplet surfaces.

In this research, star copolymers are used as nanoparticulate emulsifers with high adsorption affinity.   Star copolymers have multiple arms surrounding a dense, cross-linked polymer core.  Stars with a single type of arm, as well as miktoarm stars that have two or more types of polymer arms, are being synthesized.  The stars are designed to be amphiphilic to enhance adsorption to the oil/water interface and to cause interfacial tension lowering.  This combines the interfacial tension lowering characteristic of conventional surfactant emulsifiers with the extremely large adsorption energies characteristic of particulate emulsifiers.  The presence of adsorbed stars at the oil/water interface introduces strong steric colloidal repulsion forces. The emulsification efficacy of star copolymers is being examined, as is the related phenomenon of foam stabilization.

Two types of star copolymers and one type of miktoarm star copolymer have been synthesized.  Stars with polyethylene glycol (PEG) arms have been synthesized by ATRP of PEG-methacrylate macromonomer in the presence of divinylbenzene cross-linkers.  Dynamic light scattering indicates these have a hydrodynamic radius of approximately 20 nm.  Static light scattering work is in progress to determine the average number of arms per star.  The amphiphilicity of the PEG stars arises from the nonpolar core coupled to the hydrophilic arms.  PEG itself is somewhat amphiphilic and adsorbs readily to oil/water and air/water interfaces.  The second type of star synthesized thus far has polyacrylic acid arms, also with a cross-linked divinyl benzene core.  Three variants of poly(acrylic acid)/poly(butyl acrylate) (PAA/PBA) miktoarm stars have been synthesized by a macroinitiator route.  These variants have 1:3, 1:1 and 3:1 ratios of the number of PAA arms to PBA arms.  The arm lengths are equal in each case, and the amphiphilicity is dictated by the arm number ratio.

Efforts have also begun to synthesize stars whose arms are amphiphilic gradient copolymers of methyl methacrylate and acrylic acid.  Thus far, conditions required to reliably synthesize linear gradient copolymers with well-defined compositional gradients have been established.  These provide a gradient of amphiphilicity along the chains in a manner that is expected to enhance the multipoint penetration of the oil/water interface as the chains adsorb. The next step is to produce stars with such arms.

The PEG stars have been used to emulsify xylene and water. The preferred emulsion type is determined by identifying the type of emulsion formed at an overall 1:1 ratio of oil to water. PEG stars produced oil-in-water (xylene-in-water) emulsions. The preference for oil-in-water emulsions is consistent with the good water solubility of PEG stars. All of the available xylene was emulsified, and the resulting emulsion phase was 60% xylene by volume. This emulsion coexists with a water phase. Emulsions have remained stable over six months. Currently, the concentration dependence and temperature response of PEG star-stabilized emulsions are being examined.  Since PEG displays a lower critical solution temperature (and is therefore deemed a “thermally responsive” polymer), the stars will be less hydrophilic at elevated temperatures.  Work on silica nanoparticles grafted with thermally responsive polymers has shown that such emulsions can be thermally responsive, breaking abruptly in response to mild heating.

The PEG stars are also effective foaming agents in water.  PEG stars solutions were foamed on a wrist-action shaker, after which foam height was measured periodically.  Foam height half life varied from 75 min at 0.001 wt% to 200 min at 0.04 wt% PEG star concentrations.

One reasons that the PEG stars are good emulsifiers and foaming agents is that they are pre-formed brushes.  Adsorbing a monolayer of these pre-formed brushes assembles a dense polymer brush layer to provide strong steric repulsions.  Adsorption of homopolymers or even block copolymers typically does not produce such a highly extended brush layer. Parallel work has begun using ellipsometry to measure and compare adsorption isotherms for PEG stars or PEG homopolymers at various interfaces.  Because of the high density and stretching of the PEG arms, PEG stars produce several-fold higher surface concentrations than PEG homopolymers.

Work completed to date confirmed the effectiveness of star polymers as emulsifiers.  Now that several types of stars have been synthesized, the emulsification work needed to understand how star architecture controls emulsifying power is beginning to produce quantitative structure-activity relationships. Stars will be compared against comparable polymer-grafted silica nanoparticles in order to determine which types of nanoparticles are the most effective emulsifiers.