(554d) Polymer Nanoparticle Formation at Flowing Fluid-Fluid Interfaces

Nanoparticles have broad industrial value in a variety of industries because of their size dependent properties. Thus, controlling nanoparticle size during the manufacturing process is a critical element. To obtain small sizes with narrow distributions, continuous or semi-continuous processes are often preferred over batch methods that display large gradients. Previously, we reported electrohydrodynamic mixing nanoprecipitation, a process in which a high voltage across an electrified needle is used to generate rapid electrohydrodynamic mixing of an aqueous continuous phase. An organic phase containing polymer is injected into this aqueous phase resulting in near instantaneous high supersaturation, leading to the rapid formation of nanoparticles. This process results in small nanoparticle size (~ 20-70 nm) with low polydispersity (< 0.2). In similar nanoprecipitation methods, particle size and polydispersity are controlled by mixing. If mixing time is below that of polymer aggregation, particle size approaches a constant value. To determine if electrohydrodynamic mixing nanoprecipitation exhibits similar behavior, we investigated nanoparticle size and polydispersity as a function of voltage (which controls mixing) using a well defined system of poly(ethylene oxide)-poly(caprolactone) (PEOPCL) block copolymers in water-miscible tetrahydrofuran solvents.

In these experiments, we observed a decline in particle size and polydispersity as a function of increasing voltage from +/- ~500-2500 V. However, we also identified an interesting and unexpected phenomenon at extremely low voltage (close to zero), where mixing is poor, in which small and uniform nanoparticles were nonetheless obtained (i.e., ~ 30-100 nm depending on polymer molecular weight with polydispersity similar to that obtained at high mixing). In this region, we observed that the injected phase, though miscible with water, remained as a unified fluid stream without mixing. This suggests that the solvent diffusion time was slower than the time required for solvent to separate from the aqueous phase due to density differences, resulting in a short duration in which the flowing stream was in interfacial contact with the aqueous phase. During this time, particle formation is possible at the fluid-fluid interface because of the high supersaturation. In this sense, this process mirrors formation through interfacial mechanisms, such as emulsion processes and the thin film hydration method, though no surfactant is present to stabilize the interface. Given these findings, we are exploring this phenomenon using water immiscible chloroform solvents with the same PEOPCL polymer system. These results indicate a possible scheme for continuous nanoparticle production at low voltage that depends solely on diffusion based mixing for rapid nanoparticle formation, which may reduce energy requirements. These data have implications for polymer nanomanufacturing industries ranging from healthcare to energy.