Date of Completion
Douglas H. Adamson
Thomas A.P. Seery
Field of Study
Doctor of Philosophy
Emulsions exhibit great utility in a wide variety of industries, ranging from food, cosmetics, drugs, polishes, and agriculture to paving materials, paints, coatings, photography, and electrically and thermally insulating materials. Surfactants stabilize these emulsions by creating a barrier to coalescence of the constituent droplets. In the case of molecular surfactants, this stabilization is achieved by lowering the interfacial tension in a mixture whereas colloidal or particle surfactants prevent coalescence via steric stabilization of the interface. The type of emulsion formed is dictated by the surfactant’s preference to be dispersed or the particle’s preference to be wetted by one phase over the other. Emulsions of both the surfactant- and particle-stabilized varieties can be very useful in providing templates for polymerization reactions.
Poly(dimethylsiloxane) (PDMS) (two-dimensional) and glass capillary (three- dimensional) microfluidic devices provide the means produce complex multiple emulsions. By taking advantage of well-defined channels and flow parameters, monodisperse multiple emulsions can be made that can later be converted into synthetic polymer vesicles, or polymersomes. These polymersomes can be used for flavor protection, drug delivery, or protein storage. This dissertation presents a new robust capillary microfluidic device that utilizes a polymer scaffold and fully interchangeable parts that make it extremely versatile for any parameter that an experiment dictates. The flow regimes in the device are characterized by analyzing drop size and the encapsulation profiles are investigated. Double emulsions encapsulating single or multiple droplets are produced and subsequently dialyzed into polymersomes with great levels of control. The emulsion-stabilizing ability of poly(vinyl alcohol) (PVA) is studied by analysis of the expulsion rates of the inner drop from the double emulsion. By observing double emulsions sinking through a column of water as well as suspended on a stationary pedestal, the stability and expulsion rates could be determined while the double emulsions continued to dialyze. It is shown that even by increasing the viscosity of the organic phase or better matching the densities of the organic and aqueous phases, the absence of PVA renders the double emulsions far less stable.
Graphene oxide (GO) is used as a solid surfactant to stabilize a styrene-in- water emulsion system to template the suspension polymerization of polystyrene particles. The effect of GO concentration and mixing time on the sphere size is analyzed, and it is shown that only the available amount of surfactant, and not the emulsification process, will dictate the final particle size. The importance of divinylbenzene (DVB) as a crosslinker in the suspension polymerization is also studied, elucidating its effect in stabilizing the emulsion by quickly increasing the viscosity and maintaining a spherical morphology in the mixture. Lastly, chemical and physical modifications are performed on the GO and the subsequent effects on the emulsification ability and suspension polymerization products are investigated. GO sheets are functionalized with trimethylsilyl (TMS) groups and analyzed by thermal gravimetric analysis (TGA) and Fourier Transform Infrared Spectroscopy (FTIR). By making the surface of the GO sheets hydrophobic, a phase-inversion is generated in the emulsion system. Subsequent polymerization of the styrene phase produces a porous solid composite rather than a GO-coated polystyrene powder. Lastly, a single batch of GO is fractionated according to degree of oxidation, and it is shown that the graphitic character dictates the shape of the emulsion droplets and subsequently the polystyrene particles that are produced.
Huang, Kevin Y.-S., "High Molecular Weight Surfactants for the Formation of Complex Emulsions" (2015). Doctoral Dissertations. 950.