Nanoemulsion-like Polymersomes for Nanoreactors

Abstract

Self-assembly of block copolymers in various selective solvents provides a means to control nanostructures. Among selective solvents, ionic liquids (ILs) are of great interest as reaction media, with the possibility of replacing organic solvents. However, the implementation ILs is limited by their high viscosity and cost. Phase transfer of IL-filled polymer vesicles (polymersomes) from the IL phase to water produces a very stable kind of "nanoemulsion"�. Nanoemulsion-like polymersomes have great potential as they confine a catalyst within the interiors, thus mitigating the mass transfer limitations of ILs while simultaneously providing a facile route to quantitative catalyst recovery The issues in the nanoreactor system and the mechanism of the phase transfer in the biphasic system are discussed. First, a new reversible reaction process with the thermo-responsive shuttling of the IL-filled polymersomes between the phases was designed. In nanoreactor applications, a narrowly distributed, small vesicle size is required. The size of polymersomes having rubbery and glassy membranes was controlled through mechanical and kinetic approaches. In the mechanical approach, the extrusion method was employed. For the kinetic approach, the amount of co-solvent and the hydrophilic fraction of amphiphilic block copolymer were varied and its effects on the size and dispersity were studied. Transport phenomena across the glassy and rubbery bilayer membranes was elucidated by NMR techniques to quantify the mobility inside and outside the polymersomes, plus the rate of exchange through the membrane. The dependence of the membrane thickness, glass transition temperature of the membranes and the partition coefficient of tracer molecules in the IL/water were also examined. We demonstrated a general boundary for the phase transfer of polymersomes in terms of a reduced tethering density for poly(ethylene oxide) (PEO), and analyzed the phenomena thermodynamically. The tethering density can be increased by increasing the block length of PEO and the size of the polymersomes, and the increased tethering density induces the phase transfer. Interfacial tension-related phase transfer led to develop a novel separation method in the biphasic system of the IL and water. By controlling the interfacial tension between the hydrophobic membrane and water, worm-like micelles and polymersomes were successfully separated.