Polymerization in emulsion microdroplet reactors
Please use this identifier to cite or link to this item: http://hdl.handle.net/1928/13127
Polymerization in emulsion microdroplet reactors
Carroll, Nick J.
University of New Mexico. Dept. of Chemical and Nuclear Engineering
Emulsion Polymerization, Microfluidics, Nanostructured materials
The goal of this research project is to utilize emulsion droplets as chemical reactors for execution of complex polymerization chemistries to develop unique and functional particle materials. Emulsions are dispersions of immiscible fluids where one fluid usually exists in the form of drops. Not surprisingly, if a liquid-to-solid chemical reaction proceeds to completion within these drops, the resultant solid particles will possess the shape and relative size distribution of the drops. The two immiscible liquid phases required for emulsion polymerization provide unique and complex chemical and physical environments suitable for the engineering of novel materials. The development of novel non-ionic fluorosurfactants allows fluorocarbon oils to be used as the continuous phase in a water-free emulsion. Such emulsions enable the encapsulation of almost any hydrocarbon compound in droplets that may be used as separate compartments for water-sensitive syntheses. Here, we exemplify the promise of this approach by suspension polymerization of polyurethanes (PU), in which the liquid precursor is emulsified into droplets that are then converted 1:1 into polymer particles. The stability of the droplets against coalescence upon removal of the continuous phase by evaporation confirms the formation of solid PU particles. These results prove that the water-free environment of fluorocarbon based emulsions enables high conversion. We produce monodisperse, cross-linked, and fluorescently labeled PU-latexes with controllable mesh size through microfluidic emulsification in a simple one-step process. A novel method for the fabrication of monodisperse mesoporous silica particles is presented. It is based on the formation of well-defined equally sized emulsion droplets using a microfluidic approach. The droplets contain the silica precursor/surfactant solution and are suspended in hexadecane as the continuous oil phase. The solvent is then expelled from the droplets, leading to concentration and micellization of the surfactant. At the same time, the silica solidifies around the surfactant structures, forming equally sized mesoporous particles. The procedure can be tuned to produce well-separated particles or alternatively particles that are linked together. The latter allows us to create 2D or 3D structures with hierarchical porosity. Oil, water, and surfactant liquid mixtures exhibit very complex phase behavior. Depending on the conditions, such mixtures give rise to highly organized structures. A proper selection of the type and concentration of surfactants determines the structuring at the nanoscale level. In this work, we show that hierarchically bimodal nanoporous structures can be obtained by templating silica microparticles with a specially designed surfactant micelle/microemulsion mixture. Tuning the phase state by adjusting the surfactant composition and concentration allows for the controlled design of a system where microemulsion droplets coexist with smaller surfactant micellar structures. The microemulsion droplet and micellar dimensions determine the two types of pore sizes (single nanometers and tens of nanometers). We also demonstrate the fabrication of carbon and carbon/platinum replicas of the silica microspheres using a “lost-wax” approach. Such particles have great potential for the design of electrocatalysts for fuel cells, chromatography separations, and other applications. It was determined that slight variations in microemulsion mixture components (electrolyte concentration, wt% of surfactants, oil to sol ratio, etc.) produces strikingly different pore morphologies and particle surface areas. Control over the size and structure of the smaller micelle-templated pores was made possible by varying the length of the hydrocarbon block within the trimethyl ammonium bromide surfactant and characterized using X-ray diffraction. The effect of emulsion aging was studied by synthesizing particles at progressive time levels from a sample emulsion. It was discovered surface pore size increases after just a few hours, with high number of hollow particles observed. After 3 days, the particles were irregular shaped with little surface porosity observed via scanning electron microscopy. This may indicate that the microemulsion in the standard synthesis is not at equilibrium and that the alkoxide monomer, tetraethylorthosilicate, may change surface activity over time as additional levels of hydrolysis are obtained. Monodisperse, microemulsion nanoporous particles were synthesized utilizing a microfluidic platform. Emulsification of silica precursor in a pure oil phase at the microfluidic orifice, with infusion of surfactant-laden oil phase into the device downstream of the orifice, allows for successful fluidic treatment of a low interfacial tension system and the formation of monodisperse particles. Temperate evaporation of the solvent from the droplets at ambient conditions preserves the excellent size distribution of the fluidic-formed precursor droplets during gelation. Successful synthesis of monodisperse silica particles with bimodal nanoporosity demonstrates engineering control at three different length scales: the nanoscale via surfactant molecular templating, tens of nanometers via spontaneous microemulsion templating and at the micron level through control of overall size distribution via a microfluidic platform.